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Selectivity of anti-bacterial affect of oxygen

Selectivity of anti-bacterial affect of oxygen


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As far as I know, oxidizing agents (AKA reactive oxygen species or ROS) are potent antimicrobial agents that act on a broad range of bacteria and viruses, as well as inactivating certain toxins. Is this correct? How do ROS exert their effect on microbes? Do they inactivate certain toxins, and how?

Few quotes from this article:

  • Oxygen destroys pathogen.

  • Oxygen disrupts the integrity of the bacterial cell envelope through oxidation of the phospholipids and lipoproteins.

  • Aerobic organisms possess enzymes that deactivate oxygen so that reactive toxic molecules containing oxygen do not damage the cells.

Do ROS selectively kill pathogens but not non-pathogenic microbes or the multi-cellular organism's cells? If so, what causes ROS to only damage pathogens or damage them significantly more than it damages the organism's cells and non-pathogenic microbes?


First and foremost, the comments may be right that this question is being asked with the ulterior motive of promoting a product, but it is nevertheless true that hydrogen peroxide and other oxidants (AKA reactive oxygen species, or simply ROS) are potent antimicrobial agents, so I'm going to attempt to answer this question concisely.

I only study immunology tangentially, but my studies on aging usually overlap with immunology due to the role of reactive oxygen species (ROS) in diverse cellular pathways, including DNA damage, protein folding, and immunology.

The ROS produced by organisms (particularly hydrogen peroxide, peroxide radicals, and superoxide) aren't "selective" antimicrobial agents any more than bleach is. Both endogenous ROS and exogenous oxidants like bleach will attack (oxidize) any sufficiently strong electron donor:

https://en.wikipedia.org/wiki/Bleach#Antimicrobial_efficacy

ROS act both directly and indirectly to promote immunity, ROS are produced in many epithelial tissues constitutively (i.e. independent of infection) as well as in response to infection by pathogenic organisms. The detection of pathogenic organisms is cell-dependent, and immunological signaling pathways determine whether or not ROS production is appropriate; this is one mechanism by which ROS can be used "selectively" against microbes. On the other hand, innoculation with non-pathogenic microbes can be sensed through immunological signaling pathways at the cell/tissue level and can result in a reduction of extracellular ROS production below constitutive levels to allow colonization by symbiotic microbes:

Yang, H., Yang, M., Sun, J., Guo, F., Lan, J., Wang, X., &… Wang, J. (2015). Full length article: Catalase eliminates reactive oxygen species and influences the intestinal microbiota of shrimp. Fish And Shellfish Immunology, 4763-73. doi:10.1016/j.fsi.2015.08.021

The net oxidation/reduction potential of an organism has less to do with the effects of ROS and more to do with their resistance to it. Even if there are plenty of reducing agents (AKA antioxidants) around (NADH-dependent enzymes, reduced glutathione, thioredoxin, etc.), proteins and lipids can be temporarily or permanently damaged by oxidation. Even if antioxidants are available, this damage can overload the cell's capacity to quickly reverse the damage, or even temporary damage can send pro-death signals by disrupting proteins/lipids involved in cellular signaling (see wiki article on bleach). See the following section of the wikipedia article on antioxidants (and its references) for a more thorough understanding of the interplay between a cell's antioxidant capabilities, it's production of ROS, the accumulation of oxidative damage, and the effects of oxidative damage:

https://en.wikipedia.org/wiki/Antioxidant#Oxidative_challenge_in_biology

Ultimately, a microbe's ability to tolerate temporary oxidative damage (a complex phenotype to explain) combined with its ability to rapidly reverse oxidative damage (based on expression of antioxidant enzymes, accumulation of glutathione, and cellular oxidative metabolism) are the primary determinants of a microbe's resistance to ROS - which is the more accurate way to describe the concept that ROS are (weakly) "selective" for certain kinds of microbes.


Effect of Composition on Antibacterial Activity of Sequence-Defined Cationic Oligothioetheramides

Publication History

  • Received 22 March 2018
  • Published online 11 May 2018
  • Published in issue 10 August 2018
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Oxygen Self-Sufficient Nanoplatform for Enhanced and Selective Antibacterial Photodynamic Therapy against Anaerobe-Induced Periodontal Disease

The hypoxic microenvironment, continuous oxygen consumption, and poor excitation light penetration depth during antimicrobial photodynamic therapy (aPDT) tremendously hinder the effects on bacterial inactivation. Herein, a smart nanocomposite with oxygen-self-generation is presented for enhanced and selective antibacterial properties against anaerobe-induced periodontal diseases. By encapsulating Fe3O4 nanoparticles, Chlorin e6 and Coumarin 6 in the amphiphilic silane, combined light (red and infrared) stimulated aPDT is realized due to the increased conjugate structure, the corresponding red-shifted absorption, and the magnetic navigation performance. To address the hypoxic microenvironment problem, further modification of MnO2 nanolayer on the composites is carried out, and catalytical activity is involved for the decomposition of hydrogen peroxide produced in the metabolic processing, providing sufficient oxygen for aPDT in infection sites. Experiments in the cellular level and animal model proved that the rising oxygen content could effectively relieve the hypoxia in a periodontal pocket and enhance the ROS production, remarkably boosting aPDT efficacy. The increasing local level of oxygen also shows the selective inhibition of pathogenic and anaerobic bacteria, which determines the success of periodontitis treatment. Therefore, this finding is promising for combating anaerobic pathogens with enhanced and selective properties in periodontal diseases, even in other bacteria-induced infections, for future clinical application.


Plasma medicine research highlights antibacterial effects and potential uses

As interest in the application of plasma medicine -- the use of low-temperature plasma (LTP) created by an electrical discharge to address medical problems -- continues to grow, so does the need for research advancements proving its capabilities and potential impacts on the health care industry. Across the world, many research groups are investigating plasma medicine for applications including cancer treatment and the accelerated healing of chronic wounds, among others.

Researchers from Penn State's College of Engineering, College of Agricultural Sciences and College of Medicine say direct LTP treatment and plasma-activated media are effective treatments against bacteria found in liquid cultures. The researchers also say they have devised a unique way to create plasma directly in liquids.

The team, comprised of engineers, physicists, veterinary and biomedical scientists and medical professionals, is using an atmospheric-pressure plasma jet to use room temperature -- "cold" -- plasma to treat bacteria.

Plasma, the fourth state of matter, is typically very hot -- thousands to millions of degrees. By using plasma generated at atmospheric pressure or in liquids, the researchers can create molecules and atoms with antibacterial effects without burning anything. Sean Knecht, assistant teaching professor of engineering design at Penn State and leader of the Cross-disciplinary Laboratory for Integrated Plasma Science and Engineering, said this process creates many different types of reactive particles, making the likelihood of bacterial mutations to simultaneously combat all the particles almost nonexistent.

Knecht explained that the team's research results, published in Scientific Reports, show that plasma technology generates large amounts of reactive oxygen species or reactive particles created from molecules that contain oxygen atoms, including oxygen molecules in the air and water vapor. The plasma's effect on different bacteria such as E. coli and Staph. aureus is significant, resulting in many bacterial deaths through multiple generations.

"Over the course of four generations of bacteria, these bacteria do not acquire any form of resistance to the plasma treatment," he said.

Girish Kirimanjeswara, associate professor of veterinary and biomedical sciences at Penn State, said this is extremely important due to the typical way bacteria mutate, making them resistant to antibiotics.

Antibiotics target a specific metabolic pathway, essential protein or nucleic acids in bacteria. Because of this, antibiotics have to enter a bacterial cell to find and bind to that specific target. Any bacterial mutation that decreases an antibiotic's entry capabilities or increases its rate of exit makes the antibiotic less effective. Mutations happen naturally at a low rate but can rapidly accumulate by selection pressure when introduced to antibiotics aimed at fighting the bacteria.

According to Kirimanjeswara, the team's research results show that plasma treatment produces various reactive oxygen species at a concentration high enough to kill bacteria, but low enough to not have negative impacts on human cells. He explained that the oxygen species quickly target virtually every part of the bacteria including proteins, lipids and nucleic acids.

"One can call it a sledgehammer approach," Kirimanjeswara said. "It is difficult to develop resistance by any single mutation or even by a bunch of mutations."

The team also applied these findings to design a system that can create plasma directly in liquids. The researchers intend to create plasma in blood to address cardiovascular infections directly at the source. To do so, high electric voltage and large electric currents are typically used. In the plasma system created by the researchers, the electrical current and energy that might reach the patient are minimized by using dielectric, or electrically-insulating, materials. Materials that the team would typically use to create the plasma include glass and ceramic due to their capability to withstand high local temperatures. These materials tend to make blood clot and may not be very flexible, a necessity if they are to be used in the cardiovascular system. The team is investigating insulating coatings that are biocompatible, or acceptable by the human body, and flexible. Knecht said the team has identified a polymer called Parylene-C and reported the initial results in the journal IEEE Transactions on Radiation and Plasma Medical Sciences. The team is further pursuing this avenue, as polymers have low melting points and may not withstand repeated exposure to plasma.

"Biocompatible polymers can be used for plasma generation in biological liquids, but their lifetime is limited," Knecht said. "New unique plasma generation designs must be developed to produce lower intensity plasma discharges that can extend their lifetime. That is what we are continuing to work on."

Kirimanjeswara said scientists typically work to understand how different bacteria cause disease or how host immune responses eliminate bacteria to create new antibiotics and vaccines. Though these more traditional approaches are essential, they are often gradual and time-consuming. The team's innovative research highlights the importance of continuing to investigate new ways to combat bacteria.

"Transformative and cross-disciplinary approaches have the potential to speed up finding solutions to urgent global problems," he said. "It is important for the general public to be aware of and appreciate the fact that the scientific community is engaged in several approaches, some traditional and others nontraditional, to combat the growing problem of antibiotic resistance. We hope our research reinforces the idea of embracing nonantibiotic approaches to treat bacterial infections in the future."


Co-selection of AMR by using non-antimicrobial compounds

Widespread AMR is mostly attributed to the selective pressure by overuse and misuse of antimicrobials. However, concerns have been raised based on growing evidences regarding co-selection of AMR among bacteria exposed to biocides which are used as disinfectants, antiseptics, preservatives and various cationic heavy metals included in animal diets as nutritional supplements, growth promoters and therapeutic agents for livestock [6]. These metals can also be spread on pastures to support crop growth and protection.

Co-selection of AMR by heavy metals

Heavy metals occur everywhere in the environment, and on occasion at high concentrations in certain settings when they are used in agriculture production for various purposes. Heavy metals can continue to exist in the environment and remain stable for prolonged periods. While most veterinary antimicrobial compounds can be metabolized and cleared from the food-producing animals within weeks or months. The bioavailability of commonly feed-used minerals (mostly inorganic) is usually quite low in animals, and the unabsorbed heavy metals are excreted as fecal material in higher concentrations than in feeds [40].

The correlation between heavy metal tolerance and AMR had already been observed several decades ago. Copper (Cu) has been reported to be related to resistance against Ampicillin, Sulphanilamide [41], Erythromycin [42], Enrofloxacin [43], Vancomycin [44], and Glycopeptide [45]. Methicillin-resistant Staphylococcus aureus (MRSA) is often associated with Zinc (Zn) [45,46,47,48] and Cu [45]. There are positive correlations between Mercury (Hg) tolerant gene merA and transposon Tn21 [42]. sulA and sulIII were strongly correlated with levels of Cu, Zn and Hg [49]. Multidrug-resistant CTX-M-(15, 9, 2) and KPC-2-producing Enterobacter hormaechei and E. asburiae are found to possess a set of acquired Silver (Ag) resistance genes [50]. Other heavy metals including Nickel (Ni), Cadmium (Cd), and Chromium (Cr) are also reported to co-select certain AMR [42, 51,52,53]. A recent study showed that genes potentially conferring metal-resistance, including arsA (Arsenic compounds), cadD (Cd), copB (Cu) and czrC (Zn/Cd) were frequently present in livestock associated MRSA [54]. A Chinese study even found only a weak positive correlation between ARGs and their corresponding antimicrobials, while significant positive correlations were found between some ARGs (sulA and sulIII) and typical heavy metals such as Hg, Cu, and Zn [49].

The molecular mechanisms for the ability of bacteria to develop heavy metal resistance are similar to those for AMR since heavy metals have known antimicrobial effects [55]. Co-selection is achieved in two ways: (1) Co-resistance, whereby selection for one gene fosters the maintenance of another resistance gene and (2) Cross-resistance, whereby one resistance gene can offer protection from multiple toxic chemicals [56]. Co-resistance/Co-transfer for a heavy metal and an antimicrobial is often caused by the co-resident metal and antimicrobial- resistance genes, which can be physically localized to plasmids or chromosomes that also contain one or more ARGs [57, 58]. For example, MRSA from livestock have been described harboring plasmids carrying resistance genes for Cu and Cd (copA, cadDX and mco) and for multiple antimicrobials including Macrolides, Lincosamides, Streptogramin B, Tetracyclines, Aminoglycosides and Trimethoprim (erm(T), tet(L), aadD and dfrK) [59]. The link between Zn usage in animal feeds and the occurrence of MRSA is explained by the physical presence of the Zn resistance gene, czrC, on the methicillin resistance-encoding SCCmec element [60, 61]. Another example of co-resistance involved a number of resistance genes such as aadA2 (streptomycin R ), qacED1 (spectinomycin R ) and sul1 (sulfonamide R ) located to Tn5045 where chromate resistance genes chrBACF are found [62]. A Portuguese study found in monophasic S. Typhimurium variants of human and pig origin that ARGs in this multi-drug-resistant Pathovar were co-located with sil operon which encoded an efflux for Cu and Ag on the chromosome or a non-transferable plasmid [63]. A conjugation assay demonstrated co-transfer of tcrB and erm(B) genes between E. faecium and E. faecalis strains [64]. Genomic analysis of E. faecalis from Cu-supplemented Danish pigs revealed the presence of chromosomal Cu-insusceptibility genes, including the tcrYAZB operon and Tetracycline (tetM) and Vancomycin (vanA) resistance genes were present in one of the “Cu-insusceptible” isolates [65]. The genetic linkage of Cu, Zn and ARGs in bacteria has been comprehensively summarized in a recent review written by Keith Poole [57].

Like antimicrobials, metals are stressors that activate a variety of adaptive/protective responses in bacteria, and this can make co-regulation of metal and antimicrobial resistance resulting in cross-resistance [66]. In Gram-negative bacteria, The Membrane Stress Responsive Two Component System CpxRA which is linked to resistance against variety of cell envelope-targeting drugs [67] is also Cu-responsive and contributes to Cu tolerance [68]. In the presence of Zn, TCS CscRS in Pseudomonas aeruginosa influences the transcription of czcCBA operon encoding an RND-type efflux pump which confers resistance to Zn, Cd and cobalt (Co), meanwhile the CscRS system also reduces the expression of porin OprD through which imipenem enters the bacteria [69]. In Listeria monocytogenes, a Multidrug efflux pump MdrL confers resistance against a range of antimicrobials, and the same transport system also works for heavy metals such as Zn, Co and Cr [70]. Similarly, the Envelope Stress Response Sigma Factor RpoE activated by Polymyxin B and linked to Polymyxin B resistance in a number of Gram-negative bacteria [71] is also activated by Zn in E. coli and contributes to Zn and Cu tolerance [72]. Cu has also been shown to increase expression of the Oxidative Stress-responsive Regulatory Gene soxS that is linked to expression of the AcrAB efflux pump and multidrug resistance in E. coli [73].

Biofilms, in which bacteria are embedded in extra cellular polymeric substances, are more resistant to heavy metals than their planktonic counterparts [74]. In turn, the biofilm matrix may drive the frequency of mutation in the bacterial genomes, which is favorable for co-selection for AMR [75]. Many reports have described in several Gram-negative bacteria that Cu induces a Viable but Nonculturable (VNC) state, which is a stress-induced antimicrobial-resistant dormant state [76]. A Zn-linked VNC state has also been seen in Xylella Fastidiosa, and it appears to hasten the onset of the VNC state in this organism [77]. Moreover, the exposure of E. coli to Cu has been shown to increase the recovery of small colony variants, and the slow-growing variants are typically antimicrobial-resistant for a variety of bacteria [78].

Heavy metals can also facilitate the HGT. A recent study suggested that sub-inhibitory concentrations of heavy metals accelerate the horizontal transfer of plasmid-mediated ARGs in water environment by promoting conjugative transfer of genes between E. coli strains [79]. Another study showed that via Cu shock at 10 and 100 mg/L loading on bacteria from a drinking water bio-filter, bacterial resistance to Rifampin, Erythromycin, Kanamycin, and a few others was significantly increased. Furthermore, the relative abundance of most ARGs, particularly the mobile genetic elements (MGE) intI and transposons, were markedly enriched by at least one-fold [80].

Co-selection of AMR by biocides

Biocides can be used as antiseptics on body surfaces, as disinfectants on equipment and surfaces in many environments including farms and hospitals, as decontaminants on carcass surfaces following slaughter, and as preservatives in pharmaceuticals, cosmetics and food [81]. A possible cross-resistance between biocides and antimicrobials is still controversial. Some studies have reported that there is no cross-resistance between biocides and antimicrobials. For example, no cross-resistance between Chlorhexidine and five antimicrobials was found in 130 Salmonella spp. from two Turkey farms [82]. Among 101 genetically distinct isolates of Burkholderia cepacia, no correlation was found between the susceptibility to Chlorhexidine and 10 different antimicrobials [83]. On Enterococcus faecium, low doses of Peracetic Acid, usually used as disinfectant in wastewater treatments, promoted a bacterial adaptation but without affecting the abundance of the AGRs [84].

On the other hand, several surveys have been performed on the co-selection of AMR by biocides in bacterial isolates from food-animals and aquacultures. It has been indicated that the overall exposure to Chlorhexidine Digluconate increases the risk for resistance to a variety of antimicrobials [85]. When 310 Gram-positive isolates from milking cow teats were subjected to Iodine or Chlorhexidine antisepsis, a significant association among Streptococci between reduced susceptibility to Chlorhexidine and to Ampicillin, Tetracycline and three Aminoglycoside antibiotics [86]. In 87 isolates from seafoods, moderate positive correlations were detected for the biocides Cetrimide, Hexadecylpyridinium chloride and Triclosan with the antibiotic Cefotaxime, and also for Triclosan with Chloramphenicol and Trimethoprim/Solfamethoxazole and with the phenolic compound Thymol [87]. It was reported in E. coli O157 and various Salmonella serovars reductions in susceptibility to a panel of antimicrobials following stepwise training of Triclosan, Chlorhexidine and Benzalkonium chloride [88]. Exposure of veterinary field E. coli isolates to three quaternary ammonium compounds yielded elevations of MIC that were above the clinical breakpoints for Phenicol, Tetracycline, Fluoroquinolone, β-lactams and Trimethoprim [89]. Salmonella Enteritidis surviving a short exposure to in-use concentrations of Chlorine exhibited up to eight-fold increases in MIC values for Tetracycline, Nalidixic Acid and Chloramphenicol [90], similar to those observed with stepwise training procedures.

There are more surveys and investigations that have involved hospitals or other healthcare environments about the co-selection of AMR by biocides [6]. When the aerobic microbial communities were exposed to Benzalkonium Chloride, the community-wide MIC values for Benzalkonium Chloride, Ciprofloxacin, Tetracycline and Penicillin G were all increased [91]. Recent data showed that exposure of vancomycin-resistant E. faecium to Chlorhexidine for only 15 min up-regulates the vanA-type Vancomycin resistance gene (vanHAX) and genes associated with reduced Daptomycin susceptibility (liaXYZ) [92].

It has been demonstrated a role of efflux for the co-selection of AMR in some biocide training studies [93], and reduced susceptibility to biocides may follow from the development of AMR vice versa [94,95,96]. Under Benzalkonium Chloride exposure, the expression of two non-specific efflux pumps genes (lde and mdrL) in Listeria monocytogenes isolated from pork meat processing plants was evaluated [97]. The expression of lde was dose-dependent in the case of the post cleaning and disinfection procedure strain, while the expression of mdrL was inhibited under low biocidal stress (10 ppm) and enhanced in the presence of high stress (100 ppm). In a study of biofilm formation potential and efflux pump activity, E. coli isolates from dairy equipment that had reduced susceptibility to Benzalkonium Chloride and Ciprofloxacin proved to have superior biofilm capacity, in parallel with increased efflux activity [98]. Improved biofilm capability plus efflux has also been seen in Triclosan-adapted E. coli [99]. Genetic co-occurrences suggest that plasmids provide limited opportunities for biocides and metals to promote horizontal transfer of AMR through co-selection, whereas quite large possibilities exist for indirect selection via chromosomal biocide/metal resistance genes [100].

There are a lot of theoretical and experimental evidences that certain biocides may co-select for AMR, mainly by close link of biocide resistance determinants to AMR determinants. However, there is lack of empirical data to indicate that the use of biocides drives this co-selection of AMR in the food chain [101, 102].


Contents

Antimicrobial peptides are a unique and diverse group of molecules, which are divided into subgroups on the basis of their amino acid composition and structure. [3] Antimicrobial peptides are generally between 12 and 50 amino acids. These peptides include two or more positively charged residues provided by arginine, lysine or, in acidic environments, histidine, and a large proportion (generally >50%) of hydrophobic residues. [4] [5] [6] The secondary structures of these molecules follow 4 themes, including i) α-helical, ii) β-stranded due to the presence of 2 or more disulfide bonds, iii) β-hairpin or loop due to the presence of a single disulfide bond and/or cyclization of the peptide chain, and iv) extended. [7] Many of these peptides are unstructured in free solution, and fold into their final configuration upon partitioning into biological membranes. It contains hydrophilic amino acid residues aligned along one side and hydrophobic amino acid residues aligned along the opposite side of a helical molecule. [3] This amphipathicity of the antimicrobial peptides allows them to partition into the membrane lipid bilayer. The ability to associate with membranes is a definitive feature of antimicrobial peptides, [8] [9] although membrane permeabilization is not necessary. These peptides have a variety of antimicrobial activities ranging from membrane permeabilization to action on a range of cytoplasmic targets.

The modes of action by which antimicrobial peptides kill microbes are varied, [10] and may differ for different bacterial species. [11] Some antimicrobial peptides kill both bacteria and fungi, e.g., psoriasin kills E. coli and several filamentous fungi. [12] The cytoplasmic membrane is a frequent target, but peptides may also interfere with DNA and protein synthesis, protein folding, and cell wall synthesis. [10] The initial contact between the peptide and the target organism is electrostatic, as most bacterial surfaces are anionic, or hydrophobic, such as in the antimicrobial peptide Piscidin. Their amino acid composition, amphipathicity, cationic charge and size allow them to attach to and insert into membrane bilayers to form pores by ‘barrel-stave’, ‘carpet’ or ‘toroidal-pore’ mechanisms. Alternately, they may penetrate into the cell to bind intracellular molecules which are crucial to cell living. [13] Intracellular binding models includes inhibition of cell wall synthesis, alteration of the cytoplasmic membrane, activation of autolysin, inhibition of DNA, RNA, and protein synthesis, and inhibition of certain enzymes. However, in many cases, the exact mechanism of killing is not known. One emerging technique for the study of such mechanisms is dual polarisation interferometry. [14] [15] In contrast to many conventional antibiotics these peptides appear to be bactericidal [2] instead of bacteriostatic. In general the antimicrobial activity of these peptides is determined by measuring the minimal inhibitory concentration (MIC), which is the lowest concentration of drug that inhibits bacterial growth. [16]

AMPs can possess multiple activities including anti-gram-positive bacterial, anti-gram-negative bacterial, anti-fungal, anti-viral, anti-parasitic, and anti cancer activities. A big AMP functional analysis indicates that among all AMP activities, amphipathicity and charge, two major properties of AMPs, best distinguish between AMPs with and without anti-gram-negative bacterial activities. [17] This implies that being AMPs with anti-gram-negative bacterial activities may prefer or even require strong amphipathicity and net positive charge.

In addition to killing bacteria directly they have been demonstrated to have a number of immunomodulatory functions that may be involved in the clearance of infection, including the ability to alter host gene expression, act as chemokines and/or induce chemokine production, inhibiting lipopolysaccharide induced pro-inflammatory cytokine production, promoting wound healing, and modulating the responses of dendritic cells and cells of the adaptive immune response. Animal models indicate that host defense peptides are crucial for both prevention and clearance of infection. It appears as though many peptides initially isolated as and termed "antimicrobial peptides" have been shown to have more significant alternative functions in vivo (e.g. hepcidin [18] ). Dusquetide for example is an immunomodulator that acts through p62, a protein involved in toll like receptor based signalling of infection. The peptide is being examined in a Phase III clinical trial by Soligenix (SGNX) to ascertain if it can assist in repair of radiation-induced damage to oral mucosa arising during cancer radiotherapy of the head and neck. [19]

Antimicrobial peptides generally have a net positive charge, allowing them to interact with the negatively charged molecules exposed on bacteria and cancer cell surfaces, such as phospholipid phosphatidylserine, O-glycosylated mucins, sialylated gangliosides, and heparin sulfates. The mechanism of action of these proteins varies widely but can be simplified into two categories: membranolytic and non-membranolytic antimicrobial peptides. [20] The disruption of membranes by membranolytic antimicrobial peptides can be described by four models: [21]

  • toroidal model
  • disordered toroidal-pore model
  • carpet model
  • barrel stave model

Although the specifics of each mechanism differ, all propose peptide-induced membrane rupture, allowing cytoplasmic leakage that ultimately leads to death.

Recent work has painted a more complex picture of antimicrobial peptide activity. The non-membranolytic antimicrobial peptides may also function as metabolic inhibitors, directly interacting with DNA, RNA, protein synthesis, and inhibitors of cell wall synthesis or septum formation. They are also known to cause ribosomal aggregation and delocalize membrane proteins.

Adding a further layer of complexity, many natural antimicrobial peptides possess weak bactericidal activity. Rather than directly inhibit bacterial growth, they are now known to act in concert with the host immune system through mechanisms including chemokine induction, histamine release, and angiogenesis modulation. These immunomodulatory effects have only recently begun to receive attention.

Several methods have been used to determine the mechanisms of antimicrobial peptide activity. [11] [13] In particular, solid-state NMR studies have provided an atomic-level resolution explanation of membrane disruption by antimicrobial peptides. [22] [23] In more recent years, X-ray crystallography has been used to delineate in atomic detail how the family of plant defensins rupture membranes by identifying key phospholipids in the cell membranes of the pathogen. [24] [25] Human defensins have been thought to act through a similar mechanism, targeting cell membrane lipids as part of their function. In fact human beta-defensin 2 have now been shown to kill the pathogenic fungi Candida albicans through interactions with specific phospholipids. [26] From the computational point of view, the molecular dynamics simulations can shed light in the molecular mechanism and the specific peptide interactions with lipids, ions and solvent. [27]

Methods Applications
Microscopy to visualize the effects of antimicrobial peptides on microbial cells
Atomic emission spectroscopy to detect loss of intracellular potassium (an indication that bacterial membrane integrity has been compromised)
Fluorescent dyes to measure ability of antimicrobial peptides to permeabilize membrane vesicles
Ion channel formation to assess the formation and stability of an antimicrobial-peptide-induced pore
Circular dichroism and orientated circular dichroism to measure the orientation and secondary structure of an antimicrobial peptide bound to a lipid bilayer
Dual polarization interferometry to measure the different mechanisms of antimicrobial peptides
Solid-state NMR spectroscopy to measure the secondary structure, orientation and penetration of antimicrobial peptides into lipid bilayers in the biologically relevant liquid-crystalline state
Neutron and X-ray diffraction to measure the diffraction patterns of peptide-induced pores within membranes in oriented multilayers or liquids
Molecular dynamics simulations to study the molecular behaviour and search for specific peptide-lipid interactions
Mass spectrometry to measure the proteomic response of microorganisms to antimicrobial peptides

Antimicrobial peptides have been used as therapeutic agents their use is generally limited to intravenous administration or topical applications due to their short half-lives. As of January 2018 the following antimicrobial peptides were in clinical use: [28]

    for pneumonia, topical , Hepatitis C (oral, cyclic peptide) , bacterial infections, IV , bacterial infections, IV , HIV, subcutaneous injection , bacterial infections, IV , bacterial infections, IV , Hepatitis C, oral cyclic peptide , bacterial infection, IV , bacterial infection, IV. , bacterial infection against Gram-positive and Gram-negative also.

Activity beyond antibacterial functions Edit

AMPs have been observed having functions other than bacterial and fungal killing. These activities include antiviral effects, but also roles in host defence such as anticancer functions and roles in neurology. [29] This has led to a movement for re-branding AMPs as "Host-defence peptides" to encompass the broad scope of activities AMPs can have. [30]

Anticancer properties Edit

Some cecropins (e.g. cecropin A, and cecropin B) have anticancer properties and are called anticancer peptides (ACPs). [31] : 3 Hybrid ACPs based on Cecropin A have been studied for anticancer properties. [31] : 7.1 The fruit fly Defensin prevents tumour growth, suspected to bind to tumour cells owing to cell membrane modifications common to most cancer cells, such as phosphatidylserine exposure. [32]

Cecropin A can destroy planktonic and sessile biofilm-forming uropathogenic E. coli (UPEC) cells, either alone or when combined with the antibiotic nalidixic acid, synergistically clearing infection in vivo (in the insect host Galleria mellonella) without off-target cytotoxicity. The multi-target mechanism of action involves outer membrane permeabilization followed by biofilm disruption triggered by the inhibition of efflux pump activity and interactions with extracellular and intracellular nucleic acids. [33]

Other research Edit

Recently there has been some research to identify potential antimicrobial peptides from prokaryotes, [34] aquatic organisms such as fish, [35] [36] and shellfish, [37] and monotremes such as echidnas. [38] [39]

In the competition of bacterial cells and host cells with the antimicrobial peptides, antimicrobial peptides will preferentially interact with the bacterial cell to the mammalian cells, which enables them to kill microorganisms without being significantly toxic to mammalian cells. [40] Selectivity is a very important feature of the antimicrobial peptides and it can guarantee their function as antibiotics in host defense systems.

With regard to cancer cells, they themselves also secrete human antimicrobial peptides including defensin, and in some cases, they are reported to be more resistant than the surrounding normal cells. Therefore, we cannot conclude that selectivity is always high against cancer cells. [41] [42]

Factors Edit

There are some factors that are closely related to the selectivity property of antimicrobial peptides, among which the cationic property contributes most. Since the surface of the bacterial membranes is more negatively charged than mammalian cells, antimicrobial peptides will show different affinities towards the bacterial membranes and mammalian cell membranes. [43]

In addition, there are also other factors that will affect the selectivity. It's well known that cholesterol is normally widely distributed in the mammalian cell membranes as a membrane stabilizing agents but absent in bacterial cell membranes and the presence of these cholesterols will also generally reduce the activities of the antimicrobial peptides, due either to stabilization of the lipid bilayer or to interactions between cholesterol and the peptide. So the cholesterol in mammalian cells will protect the cells from attack by the antimicrobial peptides. [44]

Besides, the transmembrane potential is well known to affect peptide-lipid interactions. [45] There's an inside-negative transmembrane potential existing from the outer leaflet to the inner leaflet of the cell membranes and this inside-negative transmembrane potential will facilitate membrane permeabilization probably by facilitating the insertion of positively charged peptides into membranes. By comparison, the transmembrane potential of bacterial cells is more negative than that of normal mammalian cells, so bacterial membrane will be prone to be attacked by the positively charged antimicrobial peptides.

Similarly, it is also believed that increasing ionic strength, [44] which in general reduces the activity of most antimicrobial peptides, contributes partially to the selectivity of the antimicrobial peptides by weakening the electrostatic interactions required for the initial interaction.

Mechanism Edit

The cell membranes of bacteria are rich in acidic phospholipids, such as phosphatidylglycerol and cardiolipin. [40] [46] These phospholipid headgroups are heavily negatively charged. Therefore, the outmost leaflets of the bilayer which is exposed to the outside of the bacterial membranes are more attractive to the attack of the positively charged antimicrobial peptides. So the interaction between the positive charges of antimicrobial peptides and the negatively charged bacterial membranes is mainly the electrostatic interactions, which is the major driving force for cellular association. In addition, since antimicrobial peptides form structures with a positively charged face as well as a hydrophobic face, there are also some hydrophobic interactions between the hydrophobic regions of the antimicrobial peptides and the zwitterionic phospholipids (electrically neutral) surface of the bacterial membranes, which act only as a minor effect in this case.

In contrast, the outer part of the membranes of plants and mammals is mainly composed of lipids without any net charges since most of the lipids with negatively charged headgroups are principally sequestered into the inner leaflet of the plasma membranes. [43] Thus in the case of mammalian cells, the outer surfaces of the membranes are usually made of zwitterionic phosphatidylcholine and sphingomyelin, even though a small portion of the membrane's outer surfaces contain some negatively charged gangliosides. Therefore, the hydrophobic interaction between the hydrophobic face of amphipathic antimicrobial peptides and the zwitterionic phospholipids on the cell surface of mammalian cell membranes plays a major role in the formation of peptide-cell binding. [47] However, the hydrophobic interaction is relatively weak when compared to the electrostatic interaction, thus, the antimicrobial peptides will preferentially interact with bacterial membranes. [ citation needed ]

Dual polarisation interferometry has been used in vitro to study and quantify the association to headgroup, insertion into the bilayer, pore formation and eventual disruption of the membrane. [48] [49]

Control Edit

A lot of effort has been put into controlling cell selectivity. For example, attempts have been made to modify and optimize the physicochemical parameters of the peptides to control the selectivities, including net charge, helicity, hydrophobicity per residue (H), hydrophobic moment (μ) and the angle subtended by the positively charged polar helix face (Φ). [45] Other mechanisms like the introduction of D-amino acids and fluorinated amino acids in the hydrophobic phase are believed to break the secondary structure and thus reduce hydrophobic interaction with mammalian cells. It has also been found that Pro→Nlys substitution in Pro-containing β-turn antimicrobial peptides was a promising strategy for the design of new small bacterial cell-selective antimicrobial peptides with intracellular mechanisms of action. [50] It has been suggested that direct attachment of magainin to the substrate surface decreased nonspecific cell binding and led to improved detection limit for bacterial cells such as Salmonella and E. coli. [51]

Bacteria use various resistance strategies to avoid antimicrobial peptide killing. [13] Some microorganisms alter net surface charges. Staphylococcus aureus transports D-alanine from the cytoplasm to the surface teichoic acid which reduces the net negative charge by introducing basic amino groups. [52] S. aureus also modifies its anionic membranes via MprF with L-lysine, increasing the positive net charge. [52] The interaction of antimicrobial peptides with membrane targets can be limited by capsule polysaccharide of Klebsiella pneumoniae. [53] Alterations occur in Lipid A. Salmonella species reduce the fluidity of their outer membrane by increasing hydrophobic interactions between an increased number of Lipid A acyl tails by adding myristate to Lipid A with 2-hydroxymyristate and forming hepta-acylated Lipid A by adding palmitate. The increased hydrophobic moment is thought to retard or abolish antimicrobial peptide insertion and pore formation. The residues undergo alteration in membrane proteins. In some Gram-negative bacteria, alteration in the production of outer membrane proteins correlates with resistance to killing by antimicrobial peptides. [54] Non-typeable Hemophilus influenzae transports AMPs into the interior of the cell, where they are degraded. Furthermore, H. influenzae remodels its membranes to make it appear as if the bacterium has already been successfully attacked by AMPs, protecting it from being attacked by more AMPs. [55] ATP-binding cassette transporters import antimicrobial peptides and the resistance-nodulation cell-division efflux pump exports antimicrobial peptides. [56] Both transporters have been associated with antimicrobial peptide resistance. Bacteria produce proteolytic enzymes, which may degrade antimicrobial peptides leading to their resistance. [57] Outer membrane vesicles produced by Gram-negative bacteria bind the antimicrobial peptides and sequester them away from the cells, thereby protecting the cells. [58] The outer membrane vesicles are also known to contain various proteases, peptidases and other lytic enzymes, which may have a role in degrading the extracellular peptide and nucleic acid molecules, which if allowed to reach to the bacterial cells may be dangerous for the cells. Cyclic-di-GMP signaling had also been involved in the regulation of antimicrobial peptide resistance in Pseudomonas aeruginosa [59]

While these examples show that resistance can evolve naturally, there is increasing concern that using pharmaceutical copies of antimicrobial peptides can make resistance happen more often and faster. In some cases, resistance to these peptides used as a pharmaceutical to treat medical problems can lead to resistance, not only to the medical application of the peptides, but to the physiological function of those peptides. [60] [61]

The ‘Trojan Horse’ approach to solving this problem capitalizes on the innate need for iron by pathogens. “Smuggling” antimicrobials into the pathogen is accomplished by linking them to siderophores for transport. While simple in concept, it has taken many decades of work to accomplish the difficult hurdle of transporting antimicrobials across the cell membranes of pathogens. Lessons learned from the successes and failures of siderophore-conjugate drugs evaluated during the development of novel agents using the ‘Trojan horse’ approach have been reviewed. [62]

Antimicrobial peptides are produced by species across the tree of life, including:

  • bacteria (e.g.bacteriocin, and many others)
  • fungi (e.g.peptaibols, plectasin, and many others)
  • cnidaria (e.g.hydramacin, aurelin)
  • many from insects and arthropods (e.g.cecropin, attacin, melittin, mastoparan, drosomycin) [63]
  • amphibia, frogs (magainin, dermaseptin, aurein, and others) [64][65]
  • birds (e.g. avian defensins) [66]
  • and mammals (e.g.cathelicidins, alpha- and beta-defensins, regIII peptides)

Research has increased in recent years to develop artificially-engineered mimics of antimicrobial peptides such as SNAPPs, in part due to the prohibitive cost of producing naturally-derived AMPs. [67] An example of this is the facially cationic peptide C18G, which was designed from the C-terminal domain of human platelet factor IV. [68] Currently, the most widely used antimicrobial peptide is nisin being the only FDA approved antimicrobial peptide, it is commonly used as an artificial preservative. [69] [ citation needed ]

Several bioinformatic databases exist to catalogue antimicrobial peptides such as ADAM (A Database of Anti-Microbial peptides), [70] APD (Antimicrobial Peptide Database), BioPD (Biologically active Peptide Database), CAMP (Collection of sequences and structures of antimicrobial peptides), [71] DBAASP (Database of Antimicrobial Activity and Structure of Peptides) and LAMP (Linking AMPs).

The Antimicrobial peptide databases may be divided into two categories on the basis of the source of peptides it contains, as specific databases and general databases. These databases have various tools for antimicrobial peptides analysis and prediction. For example, CAMP contains AMP prediction, feature calculator, BLAST search, ClustalW, VAST, PRATT, Helical wheel etc. In addition, ADAM allows users to search or browse through AMP sequence-structure relationships. Antimicrobial peptides often encompass a wide range of categories such as antifungal, antibacterial, and antituberculosis peptides.

dbAMP: [72] Provides an online platform for exploring antimicrobial peptides with functional activities and physicochemical properties on transcriptome and proteome data. dbAMP is an online resource that addresses various topics such as annotations of antimicrobial peptides (AMPs) including sequence information, antimicrobial activities, post-translational modifications (PTMs), structural visualization, antimicrobial potency, target species with minimum inhibitory concentration (MIC), physicochemical properties, or AMP–protein interactions.

Tools such as PeptideRanker, [73] PeptideLocator, [74] and AntiMPmod [75] [76] allow for the prediction of antimicrobial peptides while others have been developed to predict antifungal and anti-Tuberculosis activities. [77] [78]


MAJOR GROUPS OF ANTIMICROBIAL COMPOUNDS FROM PLANTS

Plants have an almost limitless ability to synthesize aromatic substances, most of which are phenols or their oxygen-substituted derivatives (76). Most are secondary metabolites, of which at least 12,000 have been isolated, a number estimated to be less than 10% of the total (195). In many cases, these substances serve as plant defense mechanisms against predation by microorganisms, insects, and herbivores. Some, such as terpenoids, give plants their odors others (quinones and tannins) are responsible for plant pigment. Many compounds are responsible for plant flavor (e.g., the terpenoid capsaicin from chili peppers), and some of the same herbs and spices used by humans to season food yield useful medicinal compounds (Table ​ (Table1). 1 ).

TABLE 1

Plants containing antimicrobial�tivity a

Common nameScientific nameCompoundClassActivity d Relative toxicity b Reference(s) c
AlfalfaMedicago sativa? Gram-positive organisms2.3
AllspicePimenta dioicaEugenolEssential oilGeneral2.5
AloeAloe barbadensis, Aloe veraLatexComplex mixtureCorynebacterium, Salmonella, Streptococcus, S. aureus2.7136
AppleMalus sylvestrisPhloretinFlavonoid derivativeGeneral3.0101
AshwagandhaWithania somniferumWithafarin ALactoneBacteria, fungi0.0
AvelozEuphorbia tirucalli? S. aureus0.0
Bael treeAegle marmelosEssential oilTerpenoidFungi 179
Balsam pearMomordica charantia? General1.0
BarberryBerberis vulgarisBerberineAlkaloidBacteria, protozoa2.0140, 163
BasilOcimum basilicumEssential oilsTerpenoidsSalmonella, bacteria2.5241
BayLaurus nobilisEssential oilsTerpenoidsBacteria, fungi0.7
Betel pepperPiper betelCatechols, eugenolEssential oilsGeneral1.0
Black pepperPiper nigrumPiperineAlkaloidFungi, Lactobacillus, Micrococcus, E. coli, E. faecalis1.078
BlueberryVaccinium spp.FructoseMonosaccharideE. coli 158
Brazilian pepper treeSchinus terebinthifoliusTerebinthoneTerpenoidsGeneral1.0
BuchuBarosma setulinaEssential oilTerpenoidGeneral2.0
BurdockArctium lappa Polyacetylene, tannins, terpenoidsBacteria, fungi, viruses2.3
ButtercupRanunculus bulbosusProtoanemoninLactoneGeneral2.0
CarawayCarum carvi CoumarinsBacteria, fungi, viruses 24, 26, 83, 193
Cascara sagradaRhamnus purshianaTanninsPolyphenolsViruses, bacteria, fungi1.0
Anthraquinone
CashewAnacardium pulsatillaSalicylic acidsPolyphenolsP. acnes
Bacteria, fungi 91
Castor beanRicinus communis? General0.0
Ceylon cinnamonCinnamomum verumEssential oils, othersTerpenoids, tanninsGeneral2.0
ChamomileMatricaria chamomillaAnthemic acidPhenolic acidM. tuberculosis, S. typhimurium, S. aureus, helminths2.326, 83, 193
Coumarins Viruses 24
ChapparalLarrea tridentataNordihydroguaiaretic acidLignanSkin bacteria2.0
Chili peppers, paprikaCapsicum annuumCapsaicinTerpenoidBacteria2.042, 107
CloveSyzygium aromaticumEugenolTerpenoidGeneral1.7
CocaErythroxylum cocaCocaineAlkaloidGram-negative and -positive cocci0.5
CockleAgrostemma githago? General1.0
ColtsfootTussilago farfara? General2.0
Coriander, cilantroCoriandrum sativum? Bacteria, fungi
CranberryVaccinium spp.FructoseMonosaccharideBacteria 17, 158, 159
Other
DandelionTaraxacum officinale? C. albicans, S. cerevisiae2.7
DillAnethum graveolensEssential oilTerpenoidBacteria3.0
EchinaceaEchinaceae angustifolia? General 153
EucalyptusEucalyptus globulusTanninPolyphenolBacteria, viruses1.5
Terpenoid
Fava beanVicia fabaFabatinThioninBacteria
GambogeGarcinia hanburyi ResinGeneral0.5
GarlicAllium sativumAllicin, ajoeneSulfoxideGeneral 150, 187, 188
Sulfated terpenoids 250
GinsengPanax notoginseng SaponinsE. coli, Sporothrix schenckii, Staphylococcus, Trichophyton2.7
Glory lilyGloriosa superbaColchicineAlkaloidGeneral0.0
GoldensealHydrastis canadensisBerberine, hydrastineAlkaloidsBacteria, Giardia duodenale, trypanosomes2.073
Plasmodia 163
Gotu kolaCentella asiaticaAsiatocosideTerpenoidM. leprae1.7
Grapefruit peelCitrus paradisa TerpenoidFungi 209
Green teaCamellia sinensisCatechinFlavonoidGeneral2.0
Shigella 235
Vibrio 226
S. mutans 166
Viruses 113
Harmel, ruePeganum harmala? Bacteria, fungi1.0
HempCannabis sativaβ-Resercyclic acidOrganic acidBacteria and viruses1.0
HennaLawsonia inermisGallic acidPhenolicS. aureus1.5
HopsHumulus lupulusLupulone, humulonePhenolic acidsGeneral2.3
(Hemi)terpenoids
HorseradishArmoracia rusticana TerpenoidsGeneral
HyssopHyssopus officinalisTerpenoidsViruses
(Japanese) herbRabdosia trichocarpaTrichorabdal ATerpeneHelicobacter pylori 108
LantanaLantana camara? General1.0
LawsoniaLawsoneQuinoneM. tuberculosis
Lavender-cottonSantolina chamaecyparissus? Gram-positive bacteria, Candida1.0213
Legume (West Africa)Millettia thonningiiAlpinumisoflavoneFlavoneSchistosoma 175
Lemon balmMelissa officinalisTanninsPolyphenolsViruses 245
Lemon verbenaAloysia triphyllaEssential oilTerpenoidAscaris1.5
?E. coli, M. tuberculosis, S. aureus
LicoriceGlycyrrhiza glabraGlabrolPhenolic alcoholS. aureus, M. tuberculosis2.0
Lucky nut, yellowThevetia peruviana? Plasmodium0.0
Mace, nutmegMyristica fragrans? General1.5
MarigoldCalendula officinalis? Bacteria2.7
MesquiteProsopis juliflora? General1.5
Mountain tobaccoArnica montanaHelaninsLactonesGeneral2.0
OakQuercus rubraTanninsPolyphenols
Quercetin (available commercially)Flavonoid 113
Olive oilOlea europaeaHexanalAldehydeGeneral 120
OnionAllium cepaAllicinSulfoxideBacteria, Candida 239
Orange peelCitrus sinensis?TerpenoidFungi 209
Oregon grapeMahonia aquifoliaBerberineAlkaloidPlasmodium2.0163
Trypansomes, general 73
Pao d𠆚rcoTabebuiaSesquiterpenesTerpenoidsFungi1.0
PapayaCarica papayaLatexMix of terpenoids, organic acids, alkaloidsGeneral3.034, 168, 191
Pasque-flowerAnemone pulsatillaAnemoninsLactoneBacteria0.5
PeppermintMentha piperitaMentholTerpenoidGeneral
PeriwinkleVinca minorReserpineAlkaloidGeneral1.5
PeyoteLophophora williamsiiMescalineAlkaloidGeneral1.5
PoinsettiaEuphorbia pulcherrima? General0.0
PoppyPapaver somniferumOpiumAlkaloids and othersGeneral0.5
PotatoSolanum tuberosum? Bacteria, fungi2.0
Prostrate knotweedPolygonum aviculare? General2.0
Purple prairie cloverPetalostemumPetalostemumolFlavonolBacteria, fungi 100
QuinineCinchona sp.QuinineAlkaloidPlasmodium spp.2.0
Rauvolfia, chandraRauvolfia serpentinaReserpineAlkaloidGeneral1.0
RosemaryRosmarinus officinalisEssential oilTerpenoidGeneral2.3
SainfoinOnobrychis viciifoliaTanninsPolyphenolsRuminal bacteria 105
SassafrasSassafras albidum? Helminths2.0
SavorySatureja montanaCarvacrolTerpenoidGeneral2.06
SennaCassia angustifoliaRheinAnthraquinoneS. aureus2.0
Smooth hydrangea, seven barksHydrangea arborescens? General2.3
SnakeplantRivea corymbosa? General1.0
St. John’s wortHypericum perforatumHypericin, othersAnthraquinoneGeneral1.7
Sweet flag, calamusAcorus calamus? Enteric bacteria0.7
TansyTanacetum vulgareEssential oilsTerpenoidHelminths, bacteria2.0
TarragonArtemisia dracunculusCaffeic acids, tanninsTerpenoidViruses, helminths2.5
Polyphenols 245
ThymeThymus vulgarisCaffeic acidTerpenoidViruses, bacteria, fungi2.5
ThymolPhenolic alcohol
TanninsPolyphenols
Flavones
Tree bardPodocarpus nagiTotarolFlavonolP. acnes, other gram-positive bacteria 123
NagilactoneLactoneFungi 121
122
Tua-TuaJatropha gossyphiifolia? General0.0
TurmericCurcuma longaCurcuminTerpenoidsBacteria, protozoa 14
Turmeric oil
ValerianValeriana officinalisEssential oilTerpenoidGeneral2.7
WillowSalix albaSalicinPhenolic glucoside
TanninsPolyphenols
Essential oilTerpenoid
WintergreenGaultheria procumbensTanninsPolyphenolsGeneral1.0
WoodruffGalium odoratumCoumarinGeneral3.026, 83, 193
Viruses 24
YarrowAchillea millefolium? Viruses, helminths2.3
Yellow dockRumex crispus? E. coli, Salmonella, Staphylococcus1.0

Useful antimicrobial phytochemicals can be divided into several categories, described below and summarized in Table ​ Table2. 2 .

TABLE 2

Major classes of antimicrobial compounds from plants

ClassSubclassExample(s)MechanismReference(s)
PhenolicsSimple phenolsCatecholSubstrate deprivation174
EpicatechinMembrane disruption226
Phenolic acidsCinnamic acid 66
QuinonesHypericinBind to adhesins, complex with cell wall, inactivate enzymes58, 114
FlavonoidsChrysinBind to adhesins175, 182
Flavones Complex with cell wall
AbyssinoneInactivate enzymes32, 219
Inhibit HIV reverse transcriptase164
FlavonolsTotarol?122
TanninsEllagitanninBind to proteins196, 210
𠀻ind to adhesins192
𠀾nzyme inhibition87, 33, 35
 Substrate deprivation
𠀼omplex with cell wall
 Membrane disruption
 Metal ion complexation
CoumarinsWarfarinInteraction with eucaryotic DNA (antiviral activity)26, 95, 113, 251
Terpenoids, essential oils CapsaicinMembrane disruption42
Alkaloids BerberineIntercalate into cell wall and/or DNA15, 34, 73, 94
Piperine
Lectins and polypeptides Mannose-specific agglutininBlock viral fusion or adsorption145, 253
FabatinForm disulfide bridges
Polyacetylenes 8S-Heptadeca-2(Z),9(Z)-diene-4,6-diyne-1,8-diol?62

Phenolics and Polyphenols

Simple phenols and phenolic acids.

Some of the simplest bioactive phytochemicals consist of a single substituted phenolic ring. Cinnamic and caffeic acids are common representatives of a wide group of phenylpropane-derived compounds which are in the highest oxidation state (Fig. ​ (Fig.1). 1 ).

Structures of common antimicrobial plant chemicals.

The common herbs tarragon and thyme both contain caffeic acid, which is effective against viruses (245), bacteria (31, 224), and fungi (58).

Catechol and pyrogallol both are hydroxylated phenols, shown to be toxic to microorganisms. Catechol has two −OH groups, and pyrogallol has three. The site(s) and number of hydroxyl groups on the phenol group are thought to be related to their relative toxicity to microorganisms, with evidence that increased hydroxylation results in increased toxicity (76). In addition, some authors have found that more highly oxidized phenols are more inhibitory (192, 231). The mechanisms thought to be responsible for phenolic toxicity to microorganisms include enzyme inhibition by the oxidized compounds, possibly through reaction with sulfhydryl groups or through more nonspecific interactions with the proteins (137).

Phenolic compounds possessing a C3 side chain at a lower level of oxidation and containing no oxygen are classified as essential oils and often cited as antimicrobial as well. Eugenol is a well-characterized representative found in clove oil (Fig. ​ (Fig.1). 1 ). Eugenol is considered bacteriostatic against both fungi (58) and bacteria (224).

Quinones.

Quinones are aromatic rings with two ketone substitutions (Fig. ​ (Fig.1). 1 ). They are ubiquitous in nature and are characteristically highly reactive. These compounds, being colored, are responsible for the browning reaction in cut or injured fruits and vegetables and are an intermediate in the melanin synthesis pathway in human skin (194). Their presence in henna gives that material its dyeing properties (69). The switch between diphenol (or hydroquinone) and diketone (or quinone) occurs easily through oxidation and reduction reactions. The individual redox potential of the particular quinone-hydroquinone pair is very important in many biological systems witness the role of ubiquinone (coenzyme Q) in mammalian electron transport systems. Vitamin K is a complex naphthoquinone. Its antihemorrhagic activity may be related to its ease of oxidation in body tissues (85). Hydroxylated amino acids may be made into quinones in the presence of suitable enzymes, such as a polyphenoloxidase (233). The reaction for the conversion of tyrosine to quinone is shown in Fig. ​ Fig.2. 2 .

Reaction for the conversion of tyrosine to quinone.

In addition to providing a source of stable free radicals, quinones are known to complex irreversibly with nucleophilic amino acids in proteins (210), often leading to inactivation of the protein and loss of function. For that reason, the potential range of quinone antimicrobial effects is great. Probable targets in the microbial cell are surface-exposed adhesins, cell wall polypeptides, and membrane-bound enzymes. Quinones may also render substrates unavailable to the microorganism. As with all plant-derived antimicrobials, the possible toxic effects of quinones must be thoroughly examined.

Kazmi et al. (112) described an anthraquinone from Cassia italica, a Pakistani tree, which was bacteriostatic for Bacillus anthracis, Corynebacterium pseudodiphthericum, and Pseudomonas aeruginosa and bactericidal for Pseudomonas pseudomalliae. Hypericin, an anthraquinone from St. John’s wort (Hypericum perforatum), has received much attention in the popular press lately as an antidepressant, and Duke reported in 1985 that it had general antimicrobial properties (58).

Flavones, flavonoids, and flavonols.

Flavones are phenolic structures containing one carbonyl group (as opposed to the two carbonyls in quinones) (Fig. ​ (Fig.1). 1 ). The addition of a 3-hydroxyl group yields a flavonol (69). Flavonoids are also hydroxylated phenolic substances but occur as a C6-C3 unit linked to an aromatic ring. Since they are known to be synthesized by plants in response to microbial infection (56), it should not be surprising that they have been found in vitro to be effective antimicrobial substances against a wide array of microorganisms. Their activity is probably due to their ability to complex with extracellular and soluble proteins and to complex with bacterial cell walls, as described above for quinones. More lipophilic flavonoids may also disrupt microbial membranes (229).

Catechins, the most reduced form of the C3 unit in flavonoid compounds, deserve special mention. These flavonoids have been extensively researched due to their occurrence in oolong green teas. It was noticed some time ago that teas exerted antimicrobial activity (227) and that they contain a mixture of catechin compounds. These compounds inhibited in vitro Vibrio cholerae O1 (25), Streptococcus mutans (23, 185, 186, 228), Shigella (235), and other bacteria and microorganisms (186, 224). The catechins inactivated cholera toxin in Vibrio (25) and inhibited isolated bacterial glucosyltransferases in S. mutans (151), possibly due to complexing activities described for quinones above. This latter activity was borne out in in vivo tests of conventional rats. When the rats were fed a diet containing 0.1% tea catechins, fissure caries (caused by S. mutans) was reduced by 40% (166).

Flavonoid compounds exhibit inhibitory effects against multiple viruses. Numerous studies have documented the effectiveness of flavonoids such as swertifrancheside (172), glycyrrhizin (from licorice) (242), and chrysin (48) against HIV. More than one study has found that flavone derivatives are inhibitory to respiratory syncytial virus (RSV) (21, 111). Kaul et al. (111) provide a summary of the activities and modes of action of quercetin, naringin, hesperetin, and catechin in in vitro cell culture monolayers. While naringin was not inhibitory to herpes simplex virus type 1 (HSV-1), poliovirus type 1, parainfluenza virus type 3, or RSV, the other three flavonoids were effective in various ways. Hesperetin reduced intracellular replication of all four viruses catechin inhibited infectivity but not intracellular replication of RSV and HSV-1 and quercetin was universally effective in reducing infectivity. The authors propose that small structural differences in the compounds are critical to their activity and pointed out another advantage of many plant derivatives: their low toxic potential. The average Western daily diet contains approximately 1 g of mixed flavonoids (124) pharmacologically active concentrations are not likely to be harmful to human hosts.

An isoflavone found in a West African legume, alpinumisoflavone, prevents schistosomal infection when applied topically (175). Phloretin, found in certain serovars of apples, may have activity against a variety of microorganisms (101). Galangin (3,5,7-trihydroxyflavone), derived from the perennial herb Helichrysum aureonitens, seems to be a particularly useful compound, since it has shown activity against a wide range of gram-positive bacteria as well as fungi (2) and viruses, in particular HSV-1 and coxsackie B virus type 1 (145).

Delineation of the possible mechanism of action of flavones and flavonoids is hampered by conflicting findings. Flavonoids lacking hydroxyl groups on their β-rings are more active against microorganisms than are those with the −OH groups (38) this finding supports the idea that their microbial target is the membrane. Lipophilic compounds would be more disruptive of this structure. However, several authors have also found the opposite effect i.e., the more hydroxylation, the greater the antimicrobial activity (189). This latter finding reflects the similar result for simple phenolics (see above). It is safe to say that there is no clear predictability for the degree of hydroxylation and toxicity to microorganisms.

Tannins.

“Tannin” is a general descriptive name for a group of polymeric phenolic substances capable of tanning leather or precipitating gelatin from solution, a property known as astringency. Their molecular weights range from 500 to 3,000 (87), and they are found in almost every plant part: bark, wood, leaves, fruits, and roots (192). They are divided into two groups, hydrolyzable and condensed tannins. Hydrolyzable tannins are based on gallic acid, usually as multiple esters with d -glucose, while the more numerous condensed tannins (often called proanthocyanidins) are derived from flavonoid monomers (Fig. ​ (Fig.1). 1 ). Tannins may be formed by condensations of flavan derivatives which have been transported to woody tissues of plants. Alternatively, tannins may be formed by polymerization of quinone units (76). This group of compounds has received a great deal of attention in recent years, since it was suggested that the consumption of tannin-containing beverages, especially green teas and red wines, can cure or prevent a variety of ills (198).

Many human physiological activities, such as stimulation of phagocytic cells, host-mediated tumor activity, and a wide range of anti-infective actions, have been assigned to tannins (87). One of their molecular actions is to complex with proteins through so-called nonspecific forces such as hydrogen bonding and hydrophobic effects, as well as by covalent bond formation (87, 210). Thus, their mode of antimicrobial action, as described in the section on quinones (see above), may be related to their ability to inactivate microbial adhesins, enzymes, cell envelope transport proteins, etc. They also complex with polysaccharide (247). The antimicrobial significance of this particular activity has not been explored. There is also evidence for direct inactivation of microorganisms: low tannin concentrations modify the morphology of germ tubes of Crinipellis perniciosa (33). Tannins in plants inhibit insect growth (196) and disrupt digestive events in ruminal animals (35).

Scalbert (192) reviewed the antimicrobial properties of tannins in 1991. He listed 33 studies which had documented the inhibitory activities of tannins up to that point. According to these studies, tannins can be toxic to filamentous fungi, yeasts, and bacteria. Condensed tannins have been determined to bind cell walls of ruminal bacteria, preventing growth and protease activity (105). Although this is still speculative, tannins are considered at least partially responsible for the antibiotic activity of methanolic extracts of the bark of Terminalia alata found in Nepal (221). This activity was enhanced by UV light activation (320 to 400 nm at 5 W/m 2 for 2 h). At least two studies have shown tannins to be inhibitory to viral reverse transcriptases (111, 155).

Coumarins.

Coumarins are phenolic substances made of fused benzene and α-pyrone rings (161). They are responsible for the characteristic odor of hay. As of 1996, at least 1,300 had been identified (95). Their fame has come mainly from their antithrombotic (223), anti-inflammatory (177), and vasodilatory (152) activities. Warfarin is a particularly well-known coumarin which is used both as an oral anticoagulant and, interestingly, as a rodenticide (113). It may also have antiviral effects (24). Coumarins are known to be highly toxic in rodents (232) and therefore are treated with caution by the medical community. However, recent studies have shown a “pronounced species-dependent metabolism” (244), so that many in vivo animal studies cannot be extrapolated to humans. It appears that toxic coumarin derivatives may be safely excreted in the urine in humans (244).

Several other coumarins have antimicrobial properties. R. D. Thornes, working at the Boston Lying-In Hospital in 1954, sought an agent to treat vaginal candidiasis in his pregnant patients. Coumarin was found in vitro to inhibit Candida albicans. (During subsequent in vivo tests on rabbits, the coumarin-spiked water supply was inadvertently given to all the animals in the research facility and was discovered to be a potent contraceptive agent when breeding programs started to fail 񛈥].) Its estrogenic effects were later described (206).

As a group, coumarins have been found to stimulate macrophages (37), which could have an indirect negative effect on infections. More specifically, coumarin has been used to prevent recurrences of cold sores caused by HSV-1 in humans (24) but was found ineffective against leprosy (225). Hydroxycinnamic acids, related to coumarins, seem to be inhibitory to gram-positive bacteria (66). Also, phytoalexins, which are hydroxylated derivatives of coumarins, are produced in carrots in response to fungal infection and can be presumed to have antifungal activity (95). General antimicrobial activity was documented in woodruff (Galium odoratum) extracts (224). All in all, data about specific antibiotic properties of coumarins are scarce, although many reports give reason to believe that some utility may reside in these phytochemicals (26, 83, 193). Further research is warranted.

Terpenoids and Essential Oils

The fragrance of plants is carried in the so called quinta essentia, or essential oil fraction. These oils are secondary metabolites that are highly enriched in compounds based on an isoprene structure (Fig. ​ (Fig.1). 1 ). They are called terpenes, their general chemical structure is C10H16, and they occur as diterpenes, triterpenes, and tetraterpenes (C20, C30, and C40), as well as hemiterpenes (C5) and sesquiterpenes (C15). When the compounds contain additional elements, usually oxygen, they are termed terpenoids.

Terpenoids are synthesized from acetate units, and as such they share their origins with fatty acids. They differ from fatty acids in that they contain extensive branching and are cyclized. Examples of common terpenoids are methanol and camphor (monoterpenes) and farnesol and artemisin (sesquiterpenoids). Artemisin and its derivative α-arteether, also known by the name qinghaosu, find current use as antimalarials (237). In 1985, the steering committee of the scientific working group of the World Health Organization decided to develop the latter drug as a treatment for cerebral malaria.

Terpenenes or terpenoids are active against bacteria (4, 8, 22, 82, 90, 121, 144, 197, 220, 221), fungi (18, 84, 122, 179, 180, 213, 221), viruses (74, 86, 173, 212, 246), and protozoa (78, 237). In 1977, it was reported that 60% of essential oil derivatives examined to date were inhibitory to fungi while 30% inhibited bacteria (39). The triterpenoid betulinic acid is just one of several terpenoids (see below) which have been shown to inhibit HIV. The mechanism of action of terpenes is not fully understood but is speculated to involve membrane disruption by the lipophilic compounds. Accordingly, Mendoza et al. (144) found that increasing the hydrophilicity of kaurene diterpenoids by addition of a methyl group drastically reduced their antimicrobial activity. Food scientists have found the terpenoids present in essential oils of plants to be useful in the control of Listeria monocytogenes (16). Oil of basil, a commercially available herbal, was found to be as effective as 125 ppm chlorine in disinfecting lettuce leaves (241).

Chile peppers are a food item found nearly ubiquitously in many Mesoamerican cultures (44). Their use may reflect more than a desire to flavor foods. Many essential nutrients, such as vitamin C, provitamins A and E, and several B vitamins, are found in chiles (27). A terpenoid constituent, capsaicin, has a wide range of biological activities in humans, affecting the nervous, cardiovascular, and digestive systems (236) as well as finding use as an analgesic (47). The evidence for its antimicrobial activity is mixed. Recently, Cichewicz and Thorpe (42) found that capsaicin might enhance the growth of Candida albicans but that it clearly inhibited various bacteria to differing extents. Although possibly detrimental to the human gastric mucosa, capsaicin is also bactericidal to Helicobacter pylori (106). Another hot-tasting diterpene, aframodial, from a Cameroonian spice, is a broad-spectrum antifungal (18).

The ethanol-soluble fraction of purple prairie clover yields a terpenoid called petalostemumol, which showed excellent activity against Bacillus subtilis and Staphylococcus aureus and lesser activity against gram-negative bacteria as well as Candida albicans (100). Two diterpenes isolated by Batista et al. (23) were found to be more democratic they worked well against Staphylococcus aureus, V. cholerae, P. aeruginosa, and Candida spp. When it was observed that residents of Mali used the bark of a tree called Ptelopsis suberosa for the treatment of gastric ulcers, investigators tested terpenoid-containing fractions in 10 rats before and after the rats had ulcers chemically induced. They found that the terpenoids prevented the formation of ulcers and diminished the severity of existent ulcers. Whether this activity was due to antimicrobial action or to protection of the gastric mucosa is not clear (53). Kadota et al. (108) found that trichorabdal A, a diterpene from a Japanese herb, could directly inhibit H. pylori.

Alkaloids

Heterocyclic nitrogen compounds are called alkaloids. The first medically useful example of an alkaloid was morphine, isolated in 1805 from the opium poppy Papaver somniferum (69) the name morphine comes from the Greek Morpheus, god of dreams. Codeine and heroin are both derivatives of morphine. Diterpenoid alkaloids, commonly isolated from the plants of the Ranunculaceae, or buttercup (107) family (15), are commonly found to have antimicrobial properties (163). Solamargine, a glycoalkaloid from the berries of Solanum khasianum, and other alkaloids may be useful against HIV infection (142, 200) as well as intestinal infections associated with AIDS (140). While alkaloids have been found to have microbiocidal effects (including against Giardia and Entamoeba species 學]), the major antidiarrheal effect is probably due to their effects on transit time in the small intestine.

Berberine is an important representative of the alkaloid group. It is potentially effective against trypanosomes (73) and plasmodia (163). The mechanism of action of highly aromatic planar quaternary alkaloids such as berberine and harmane (93) is attributed to their ability to intercalate with DNA (176).

Lectins and Polypeptides

Peptides which are inhibitory to microorganisms were first reported in 1942 (19). They are often positively charged and contain disulfide bonds (253). Their mechanism of action may be the formation of ion channels in the microbial membrane (222, 253) or competitive inhibition of adhesion of microbial proteins to host polysaccharide receptors (202). Recent interest has been focused mostly on studying anti-HIV peptides and lectins, but the inhibition of bacteria and fungi by these macromolecules, such as that from the herbaceous Amaranthus, has long been known (50).

Thionins are peptides commonly found in barley and wheat and consist of 47 amino acid residues (45, 143). They are toxic to yeasts and gram-negative and gram-positive bacteria (65). Thionins AX1 and AX2 from sugar beet are active against fungi but not bacteria (118). Fabatin, a newly identified 47-residue peptide from fava beans, appears to be structurally related to γ-thionins from grains and inhibits E. coli, P. aeruginosa, and Enterococcus hirae but not Candida or Saccharomyces (253).

The larger lectin molecules, which include mannose-specific lectins from several plants (20), MAP30 from bitter melon (128), GAP31 from Gelonium multiflorum (28), and jacalin (64), are inhibitory to viral proliferation (HIV, cytomegalovirus), probably by inhibiting viral interaction with critical host cell components. It is worth emphasizing that molecules and compounds such as these whose mode of action may be to inhibit adhesion will not be detected by using most general plant antimicrobial screening protocols, even with the bioassay-guided fractionation procedures (131, 181) used by natural-products chemists (see below). It is an area of ethnopharmacology which deserves attention, so that initial screens of potentially pharmacologically active plants (described in references 25, 43, and 238) may be made more useful.

Mixtures

The chewing stick is widely used in African countries as an oral hygiene aid (in place of a toothbrush) (156). Chewing sticks come from different species of plants, and within one stick the chemically active component may be heterogeneous (5). Crude extracts of one species used for this purpose, Serindeia werneckei, inhibited the periodontal pathogens Porphyromonas gingivalis and Bacteroides melaninogenicus in vitro (183). The active component of the Nigerian chewing stick (Fagara zanthoxyloides) was found to consist of various alkaloids (157). Whether these compounds, long utilized in developing countries, might find use in the Western world is not yet known.

Papaya (Carica papaya) yields a milky sap, often called a latex, which is a complex mixture of chemicals. Chief among them is papain, a well-known proteolytic enzyme (162). An alkaloid, carpaine, is also present (34). Terpenoids are also present and may contribute to its antimicrobial properties (224). Osato et al. (168) found the latex to be bacteriostatic to B. subtilis, Enterobacter cloacae, E. coli, Salmonella typhi, Staphylococcus aureus, and Proteus vulgaris.

Ayurveda is a type of healing craft practiced in India but not unknown in the United States. Ayurvedic practitioners rely on plant extracts, both “pure” single-plant preparations and mixed formulations. The preparations have lyrical names, such as Ashwagandha (Withania somnifera root) (54), Cauvery 100 (a mixture) (133), and Livo-vet (125). These preparations are used to treat animals as well as humans (125). In addition to their antimicrobial activities, they have been found to have antidiarrheal (134), immunomodulatory (54, 133), anticancer (59), and psychotropic (201) properties. In vivo studies of Abana, an Ayurvedic formulation, found a slight reduction in experimentally induced cardiac arrhythmias in dogs (75). Two microorganisms against which Ayurvedic preparations have activity are Aspergillus spp. (54) and Propionibacterium acnes (170). (The aspergillosis study was performed with mice in vivo, and it is therefore impossible to determine whether the effects are due to the stimulation of macrophage activity in the whole animal rather than to direct antimicrobial effects.)

The toxicity of Ayurvedic preparations has been the subject of some speculation, especially since some of them include metals. Prpic-Majic et al. identified high levels of lead in the blood of adult volunteers who had self-medicated with Ayurvedic medicines (178).

Propolis is a crude extract of the balsam of various trees it is often called bee glue, since honeybees gather it from the trees. Its chemical composition is very complex: like the latexes described above, terpenoids are present, as well as flavonoids, benzoic acids and esters, and substituted phenolic acids and esters (9). Synthetic cinnamic acids, identical to those from propolis, were found to inhibit hemagglutination activity of influenza virus (199). Amoros et al. found that propolis was active against an acyclovir-resistant mutant of HSV-1, adenovirus type 2, vesicular stomatitis virus, and poliovirus (9). Mixtures of chemicals, such as are found in latex and propolis, may act synergistically. While the flavone and flavonol components were active in isolation against HSV-1, multiple flavonoids incubated simultaneously with the virus were more effective than single chemicals, a possible explanation of why propolis is more effective than its individual compounds (10). Of course, mixtures are more likely to contain toxic constituents, and they must be thoroughly investigated and standardized before approved for use on a large-scale basis in the West.

Other Compounds

Many phytochemicals not mentioned above have been found to exert antimicrobial properties. This review has attempted to focus on reports of chemicals which are found in multiple instances to be active. It should be mentioned, however, that there are reports of antimicrobial properties associated with polyamines (in particular spermidine) (70), isothiocyanates (57, 104), thiosulfinates (215), and glucosides (149, 184).

Polyacetylenes deserve special mention. Estevez-Braun et al. isolated a C17 polyacetylene compound from Bupleurum salicifolium, a plant native to the Canary Islands. The compound, 8S-heptadeca-2(Z),9(Z)-diene-4,6-diyne-1,8-diol, was inhibitory to S. aureus and B. subtilis but not to gram-negative bacteria or yeasts (62). Acetylene compounds and flavonoids from plants traditionally used in Brazil for treatment of malaria fever and liver disorders have also been associated with antimalarial activity (29).

Much has been written about the antimicrobial effects of cranberry juice. Historically, women have been told to drink the juice in order to prevent and even cure urinary tract infections. In the early 1990s, researchers found that the monosaccharide fructose present in cranberry and blueberry juices competitively inhibited the adsorption of pathogenic E. coli to urinary tract epithelial cells, acting as an analogue for mannose (252). Clinical studies have borne out the protective effects of cranberry juice (17). Many fruits contain fructose, however, and researchers are now seeking a second active compound from cranberry juice which contributes to the antimicrobial properties of this juice (252).


Inhibitory Effects of Antimicrobial Peptides on Lipopolysaccharide-Induced Inflammation

Antimicrobial peptides (AMPs) are usually small molecule peptides, which display broad-spectrum antimicrobial activity, high efficiency, and stability. For the multiple-antibiotic-resistant strains, AMPs play a significant role in the development of novel antibiotics because of their broad-spectrum antimicrobial activities and specific antimicrobial mechanism. Besides broad-spectrum antibacterial activity, AMPs also have anti-inflammatory activity. The neutralization of lipopolysaccharides (LPS) plays a key role in anti-inflammatory action of AMPs. On the one hand, AMPs can readily penetrate the cell wall barrier by neutralizing LPS to remove Gram-negative bacteria that can lead to infection. On the contrary, AMPs can also inhibit the production of biological inflammatory cytokines to reduce the inflammatory response through neutralizing circulating LPS. In addition, AMPs also modulate the host immune system by chemotaxis of leukocytes, to promote immune cell proliferation, epithelialization, and angiogenesis and thus play a protective role. This review summarizes some recent researches about anti-inflammatory AMPs, with a focus on the interaction of AMPs and LPS on the past decade.

1. Introduction

Inflammation is the part tissue defense against the damage factors, and it is an important component of the innate immune system. Innate immune system is a functional and physical barrier against microorganisms which is naturally stimulated by pathogenic organisms through pattern recognition receptors (PRRs) on host cells [1]. The host cells such as monocytes and macrophages are important for innate immune that can be used as the first line and be recruited to the site of infection to defend against the pathogenic bacteria. Some proinflammation cytokines are the main molecule in macrophage-mediated innate immune responses [2].

LPS plays a crucial role in the pathophysiology of inflammation sepsis and shock [3] caused by Gram-negative bacteria. LPS is a major component of the cell wall of Gram-negative bacteria, which can be released during bacterial cell division or death. Once LPS is released into the blood system, it will cause monocytes and phagocytic cells to produce large amounts of cytokines such as tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), and interleukin-8 (IL-8). The overexpression of such cytokines can cause multiple organ damage, such as sepsis. Sepsis is considered to be the most common disease causing mortality in Intensive Care Unit (ICU), and there is no effective, safe drug against it. Antibiotics used in the clinical treatment of inflammation have been very common however, there are many side effects from the use of it. Antibiotics will speed up the release of bacterial LPS by killing bacteria in order to activate the immune system to secrete cytokines and produce endotoxin shock reaction. For this reason, looking for novel anti-inflammatory drugs that can have both antibacterial and neutralizing LPS is very urgent.

Recent studies have found that AMP not only is a broad-spectrum bactericidal agent but also can directly interact with LPS to inhibit the release of inflammatory cytokines and thus induce an anti-inflammatory effect. AMP may be the best choice for new anti-inflammatory drugs. For this reason, AMP is especially attractive which can be considered as a candidate for inflammation therapeutics, because of their potent activity of antibacterial and high affinity for LPS.

So far, there are more than 1000 natural AMPs that have been discovered and isolated [9, 10]. Although these AMPs are derived from various species and differed in sequence, most of them display a net positive charge and have well-defined secondary structures, like α-helical or β-strand structures [5–8, 11–14]. Amphipathicity and hydrophobicity of AMPs make them easy to interfere with the bacterial cell membrane stability, then causing leakage of bacteria contents. On the other hand, it is found that many antibacterial peptides can also directly neutralize LPS and inhibit the production of inflammatory cytokines, such as TNF-α, IL-6, and IL-8, control immune responses, and reduce inflammatory injury through the different immune regulation. This review is mainly focused on the interaction of the anti-inflammatory AMPs and LPS. It also reviewed how AMPs inhibit LPS-induced inflammation.

2. LPS-Induced Inflammation

LPS is a major structural and functional component of the Gram-negative bacterial outer membranes. It covers more than 90% of bacterial cell surface. It protects bacteria from antibiotics as a physical barrier. LPS consists of three parts as shown in Figure 1: (i) hydrophobic lipid A that consists of two glucosamines, phosphate, and an amount of fatty acid (ii) hydrophilic O-antigen which constitutes a polymer of oligosaccharide and (iii) polysaccharide core which is the connection between the two parts. Lipid A, the conserved portion of LPS, is also the active component of LPS, expressing the endotoxic activity. Lipid A consists of bisphosphorylated glucosamine disaccharide backbone containing six to seven fatty acyl chains per molecule. The core oligosaccharide and the phosphate group of LPS show negative charge, meaning that LPS will exhibit a strong affinity for cation [15].

LPS single molecular weight is about 10 kDa. However, above the critical micellar concentration, LPS can form supramolecular aggregates in aqueous environments, and the molecular weight of this complex can reach 1000 kDa [16].

For the function of the outer membrane (especially LPS), it plays a major role in protecting bacteria from antibiotics. The drug tolerance of bacteria is related to the thickness of LPS layer, which can prevent toxic molecules from entering the bacteria and allow the bacteria to survive in different environments. Meanwhile, the LPS barrier is believed to be stabilized by LPS-associated cations (e.g., Mg 2+ ) through salt bridges that neutralize the repulsive forces to link adjacent LPS molecules [17]. It protects bacteria from a variety of host-defense hydrophobic molecules by the oriented and tightly cross-linked leaflet. Second, bacteria adhesion on the surface of the host cell is necessary for the bacteria to infect the host. LPS as an adhesion molecule plays an important role in the pathogenesis of inflammation. Third, LPS can protect bacteria from phagocytes of host cell. Last but not least, LPS is also one of the efficient initiators of innate inflammatory response [18].

The important role of LPS in Gram-negative bacteria-induced inflammation has been widely recognized. LPS can interact with several types of host cells and induce inflammation. It is one of the highly conserved pathogen-associated molecular patterns (PAMPs) which is recognized by pattern recognition receptor, inducing the innate immune response. As a result of the antibiotic treatment against bacterial infection, LPS is released from the bacteria during cell death, cell division, or the treatment with antibiotics [19]. Once LPS is released into the blood circulation, it can be recognized by serum protein called LPS-binding protein (LBP) and formed LBP-LPS complexes. LBP is an essential protein that stimulates and amplifies the LPS-induced inflammatory response that is responsible for identifying monocytes. It can recognize LPS and transfers LPS to the cell surface receptor CD14 (mCD14) of mononuclear or phagocytic cell, forming LPS-CD14 complex to activate cells [20, 21]. As CD14 has no transmembrane domain, the LPS-CD14 complex initiates intracellular signaling by interacting with another membrane protein Toll-like receptor 4 (TLR4). TLR4 is a transmembrane protein, which can recognize specific ligands LPS. TLR4 combines with LPS with the help of the MD-2. The TLR pathway activates several different signaling molecules, such as nuclear factor κB (NF-κB) and extracellular signal-regulated kinase (ERK)/c-Jun-NH2-kinase (JNK)/p38 (as shown in Figure 2). The signaling elements induce the secretion of proinflammatory cytokines, including TNF-α, IL-1β, IL-6, IL-8, NO, and reactive oxygen species (ROS) [22], which can further release the second batch inflammatory cytokines, such as platelet activating factor (PAF), and leukotrienes (LT) [23–25]. However, unbalanced overproduction leads to multiple organ damage and eventually to death.

3. The Property of AMP Inhibiting Inflammation

The first antimicrobial peptide, Cecropins, was discovered from the giant silk moth Hyalophora cecropia by Swedish scientist G. Boman in early 70s of last century. Until now, more than one thousand of antimicrobial peptides have been characterized in plants and animals, even in bacteria and virus [26–29]. According to their secondary structure, antimicrobial peptides can be divided into four main groups: (i) amphipathic α-helices, (ii) β-sheet molecules stabilized by two or three disulphide bonds, (iii) extended molecules, and (iv) loop or disordered structures (as shown in Figure 3), with the first two classes being the most common in nature. Despite the different structures and sequences ofantimicrobial peptides, they have some common characteristics: (i) Most of the antimicrobial peptides exhibit amphiphilic structure with hydrophobic surface comprised of nonpolar amino acids and a hydrophilic face containing polar and positively charged residues. (ii) Antimicrobial peptides possess positive charges and have a high content of hydrophobic residues. The structural characteristics of antimicrobial peptides make the interaction with bacterial membranes easy. Cationic antimicrobial peptides could bind to the negatively net charged bacterial cell membranes under the action of electrostatic force and then insert the cell membrane through the amphiphilic and hydrophobic interaction force, by forming an ion channel, eventually causing the cell death [30]. Because of this function, some antimicrobial peptides may protect from infection and inflammation by targeting pathogens directly. Besides, antimicrobial peptides are important components of the innate defense systems of all species, forming the first line of defense with a broad spectrum of biological activity against the pathogenic microorganisms. They can be produced in large amounts at the site of infection or inflammation and quickly eradicate the microorganisms. Alalwani et al. found that, by stimulating with LPS, neutrophils had expressively increased the release of TNF-α in cationic AMP CRAMP-deficient animals [31]. Similarly, a deficiency of the sole human cathelicidin LL-37 (consisting of 37 amino acid residues) has increased susceptibility to infections [32, 33]. In addition, the relationship between the expression of AMPs with states of infection and inflammation was found. Lars et al. reported that the expression of many human defense peptides increases during infection and inflammation and decreases the levels of defense that prove the role of antimicrobial peptides in the innate immune system. Some host-defense peptides, which exhibited immune-stimulating activity, have been reported. Neeloffer et al. found that LL-37 can promote the generation of chemokines and inflammatory cytokines IL-1β by suppressing small interfering RNA (siRNA) in the presence of GAPDH. GAPDH was identified as a direct binding partner for LL-37 in monocytes. Except for the antibacterial activity and immunoregulation activity, antimicrobial peptides possess anti-inflammatory effect, inhibit the release of proinflammatory cytokines, and alleviate inflammation. Aaron et al. suggested that the human cathelicidin LL-37 inhibits LPS-induced IL-8 from THP-1 monocyte cells. Using enzyme-linked immunosorbent assay (ELISA), B. Fatoumata et al. found that antimicrobial peptide hepcidin inhibits the generation of proinflammation cytokines (such as IL-6, IL-1β). Using RT-PCR, Nagaoka et al. showed that human defensin-2 reduced the production of inflammatory cytokines TNF-α. However, compared with traditional antibiotics, the capacity of neutralizing LPS of antibacterial peptides made them the candidate of the therapeutic agent against infection or inflammation without side effect. How can AMPs suppress the inflammatory response by interacting with LPS? The interactions between them are divided into three parts.

4. The Interaction between AMP and LPS

4.1. AMP Binding to LPS

Binding of AMP with LPS plays an important role in both antibacterial activity and anti-inflammatory activity. Li et al. used several approaches including ELISA-based assay, fluorescence correlation spectroscopy (FCS), and surface plasmon resonance (SPR) and found that peptide S3, which was derived from Sushi3 domain of Factor C, could directly bind with LPS. This work demonstrated that antimicrobial peptide S3 dimer has stronger binding capacity to LPS, with 50% LPS-neutralizing capability at a concentration of 1 μM. Magainin 2 binding with LPS by its α-helical structure made the leakage of liposomes containing LPS possible, and this effect is weaker in liposome without LPS [34]. rBPI21 is a fragment of neutrophils BPI protein in N-terminal. It is a selective inhibiting Gram-negative bacteria and has a strong affinity for LPS. rBPI21 can cause the leakage of Gram-negative bacteria membrane that is rich in phosphatidylglycerol with the interaction of LPS [35, 36]. How does AMP bind to LPS and what is the key property that influences the binding activity of AMP to LPS?

Hydrophobicity and charge are important factors affecting the bactericidal activity of antimicrobial peptides. These properties determine the interaction between antimicrobial peptides and bacterial phospholipid membrane. As LPS is a content of the phospholipid membrane, hydrophobicity and positive polarity may affect the combination of the antimicrobial peptides and LPS. First, cationic AMPs perform strong electrostatic interactions with the negatively charged LPS in the membrane of Gram-negative bacteria. This enables them to get closer and neutralize the negative charge. Second, the hydrophobicity of AMPs made them easy to embed in LPS micelles. The hydrophobicity and positive charge of antibacterial peptides can increase the ability of binding to LPS. For example, Y. Rosenfeld designed a series of peptides contain twelve amino acids and the fatty acid-conjugated analogues of them consisting of both D- and L-isomers of Leu and Lys at various ratios. He found that adding fatty acid to N-terminus of antimicrobial peptides or using hydrophobic amino acid to replace hydrophilic amino acid can increase their ability to bind with LPS. The different proportions of hydrophobic residues and positively charged residues affect their ability to combine with LPS. The higher the ratio, the stronger the ability to bind with LPS. In addition, removal of hydrophobic residues of antimicrobial peptides significantly weakens their ability to neutralize LPS. Saurabh et al. used Lys to replace Glu in Temporin L showing that hydrophilic amino acid replaced by cationic amino acid can enhance the neutralization of the LPS. Scott et al. reported that antimicrobial peptide CEMA is an analog of CEME (a cecropin-melittin hybrid) with two additional positive charges. He demonstrated that the increased positive charge can strength the ability of CEMA to combine with the LPS [37].

In addition, the distance between the positively charged amino acids is also significant for the binding of LPS. In fact, in LPS-binding peptides, such as Pa4, a member of antibacterial peptide Pardaxins from the mucous glands of sole fishes, the distances between charged amino groups of Lys and Arg range from 12 Å to 15 Å, which is consistent with the interphosphate distance of the lipid A moiety in LPS. There may be a geometrical compatibility between AMPs and LPS conformation [38]. Similarly, Bhunia et al. used NMR in studying the structure and interaction with LPS of AMP MSI-594 (the analog of magainins) and found that the positively charged ammonium (H3N+) groups of Lys residues across the two helices maintain a typical distance range of 12–15 Å [39]. Domadia et al. found that Phe replaced by Ala in MSI-594 made the peptide structure loose, affecting its affinity of LPS [40]. For this reason, the positively charged residues in the peptide can neutralize the negative charge in the lipid A portion of LPS while the hydrophobic residues are inserted into the lipophilic core region with the assistance of the fit structure of AMPs.

4.2. AMP Effects on LPS Aggregate Structure

The effects on LPS aggregate structure of AMP are closely related to the capacity of LPS neutralization. Heinbockel and coworkers investigated the effects on LPS aggregate conformations of AMPs, Hbγ-35 and Pep19-2.5, by using light scattering technique. It is found that the two peptides interact differently with LPS. In the presence of Hbγ-35, LPS aggregates are disaggregated to a cubic form. And Hbγ-35 increases the secretion of LPS-induced TNF-α in human MNC. Conversely, Pep19-2.5 converted LPS from cubic to a multilamellar form, which brings about the inhibition of TNF-α production [41]. Kaconis et al. used a variety of biophysical techniques, like freeze-fracture electron microscopy and synchrotron radiation small-angle X-ray scattering (SAXS), to study LPS neutralization of a series of synthetic peptides. Their work suggests that the capacity of forming LPS multilamellar directly correlates with the inhibition of cytokines production stimulated by LPS [42]. Similarly, by using Cryo-Transmission Electron Microscopy (Cryo-TEM), Chen et al. observed that pure LPS exhibits fibrils with cylindrical forms. However, in the presence of peptide G12.21, which can neutralize LPS efficiently, the LPS structure changes into tightly arranged multilamellar structures [43]. These AMPs can promote LPS forming massive aggregation, which may facilitate the phagocytosis by macrophages, avoiding the activation of cell receptors and preventing cytokines secretion.

However, some AMPs induce disaggregation on LPS aggregates. This property may favor the antibacterial activity against Gram-negative bacteria and may promote the disruption to Gram-negative bacteria cell wall. For example, Domadia et al. explored the disturbance of LPS aggregates by the interactions with peptide MSI-594 and analogue MSI-594F5A, using dynamic light scattering (DLS). It is found that when LPS was dispersed in the phosphate buffer, the diameter is mainly centered at 7000 nm. However, there is a dramatic shift in the distribution of LPS aggregate sizes in the presence of the peptides [40], which is in accordance with their antimicrobial activity against Gram-negative bacteria. Similarly, the effect of disaggregating LPS aggregates of antimicrobial peptide chensinin-1 is weaker than its analog chensinin-1b, as same as their bacterial activity against Gram-negative bacteria [44].

4.3. The Flexible Structures of AMP in LPS and Effect on the LPS Phosphate Groups

Finally, the structure of AMPs also affects their combination with LPS. They exist in different structural forms in LPS environment. It is found that many of the antimicrobial peptides exhibit the random coil structure in aqueous solutions, after interacting with LPS. The secondary structure of antimicrobial peptides changes from random coil to α-helix, such as NRC-16 and magainin, and this may be because the amphiphilic structure is more likely to interact with the lipid. Some peptides are in β-turn, β-chain structure, and β-hairpin structure [45]. Bhattacharjya et al. designed a linear peptide YW12 with 12 residues. In aqueous solution, YW12 exists as conformation in the absence of LPS. However, the secondary structure of YW12 transforms from random coil to a well-folded structure in the presence of LPS. N-terminal of YW12 is extended conformation or loop, and C-terminus has two consecutive β-turns in LPS. This property makes YW12 easily displaceable in aqueous environment in order to get closer to LPS layer. In addition, the flexible structure is conducive for the interaction with LPS. Tan et al. reported that the S3 peptide goes through conformational changes in the presence of a disulfide bridge, transitioning from a random coil to β-sheet structure, with a β-sheet conformation binding to the bisphosphorylated glucosamine disaccharide head group of LA, primarily by ion-pair formation between anionic phosphates of LA and the cationic side chains by circular dichroism spectrometry [46]. The β-sheet secondary structure of S3 can prolong and continue the interaction with and disruption of LPS micelles [47]. NMR techniques further confirm that cationic C-terminus of melittin uses local coil hydrophobic N-terminal is unstructured and dynamic in LPS environment. Folded C-terminus acts as the anchor element and disrupts LPS structure. The MSI-594 helix-loop-helix or helix hairpin structure, the compact conformation, can help the AMP translocation across double endotoxin [28]. In conclusion, the random coil structure of AMP is conducive for the movement in aqueous solution, and the well-folded structure in the presence of LPS makes for the further interaction with LPS. One of the target sites is the phosphate groups inside LPS. Raquel Conde and his colleagues found that there are considerable changes in the phosphate as well as the sugar modes of LPS R595 in the presence of PMB. Regarding the phosphates, a drastic decrease of the band intensities at 1257 and 1221 cm −1 takes place the former corresponds to phosphate with low hydration, mainly due to the 4′-phosphate, and the latter band corresponds to phosphate with high hydration, primarily due to the 1-phosphate [48]. The decrease of the intensities can be attributed to a strong reduction of the mobility of both phosphate groups, illustrating that PMB interacts with the phosphate groups of LPS. Since the phosphate groups deeply embed in LPS, the interaction with the phosphate groups is regarded as the index of penetration to LPS barrier, contributing to the effect of antibacterial and anti-inflammation.

5. Mechanism of Antimicrobial Peptides Inhibited LPS-Induced Inflammation

LBP plays an important role in LPS-induced inflammation, and it is the trigger for LPS-induced inflammation. The activity of LPS is enhanced by combination with serum LBP. However, AMPs bind to LPS, inhibiting the LPS binding to LBP. Cathelicidins, CAP18 (cationic antibacterial proteins of 18 kDa) and CAP11 (cationic antibacterial polypeptide of 11 kDa), were investigated by Isao Nagaoka et al. These AMPs can bind to LPS and suppress LPS-induced TNF-α expression by macrophage cell line RAW264.7. Peptides such as CP29 and Indolicidin block the LPS inflammatory signal transmission by competing with LBP for LPS binding, which reduce, reducing the LPS mediated cytokines TNF-α release significantly [49]. When antimicrobial peptides, LPS and LBP, are incubated together, an AMP can successfully prevent LPS combining with LBP but rarely decompose the LPS-LBP complexes. G. Monisha et al. found that, with antimicrobial peptide MBI-28 pretreatment with phagocytic cells for one hour and addition of new culture medium of LPS after removing the supernatant, MBI-28 still suppresses the TNF-α expression by macrophages, suggesting that there is another mechanism that inhibits LPS-induced inflammation. MBI-28 may directly interact with immune cell. Yosef Rosenfeld et al. had confirmed that peptide LL37 competes with LPS for immune cell membrane receptor CD14 binding, preventing the binding of LPS and CD14 and inhibiting the release of cytokines [47, 50–52]. This shows that AMP can not only bind to LPS but also interact with immune cell membrane receptor CD14 and therefore inhibit the LPS-induced inflammation. These properties make AMPs the attractive drug candidates for treatment of endotoxin shock and sepsis caused by Gram-negative bacterial infection.

6. AMP Function as Innate Immune Regulators

The expression of AMPs is mainly induced by PRR, which can recognize nonspecific, highly conserved PAMPs. LPS of Gram-negative bacteria is one of the most active PAMPs and can promote a resilient induction of the innate immune system. When PRR interacts with PAMP, immune cells secrete chemokines, such as IL-8, monocyte chemotactic protein-1 (MCP-1/3), activating neutrophils, mast cells, and epithelial cells that secrete AMP, like defensins α and LL-37 [53, 54]. LL-37 can interact with formyl peptide receptor-like 1 (FPR1) making monocytes, neutrophils, and T lymphocytes chemotaxis. Another research undertaken by Hiemstra et al. showed that after activating FPR1, LL-37 makes leukocyte chemotaxis and enhances the adhesion, phagocytic ability, the release of oxygen, and antibacterial activity, thus strengthening the immune system [55, 56]. AMPs can also induce degranulation of mast cells, prompting the release of histamine and causing vasodilation followed by the release of immune cells in the blood. Consequently, apoptosis of macrophages and activation of lymphocytes were induced. In addition, AMP can enhance the chemotaxis of fibroblasts and proliferation of endothelial cells and lymphocyte and promote wound healing. Niyonsaba et al. found that β-defensin-2 cell activation and degranulation of mast cells, followed by the release of histamine and prostaglandin D2, increased permeability of the blood vessel wall [57].

7. Conclusion

LPS plays an important role in Gram-negative bacteria-induced inflammation. AMPs not only are intended to kill pathogens through their antimicrobial activity but also have a high affinity for LPS or membrane receptors. Besides, certain AMPs have the essential function of regulating and balancing the inflammatory response of the innate immune system. Though AMPs have the potential to neutralize the endotoxin of LPS to treat infection or inflammation, few of them used for clinical purposes have the stability problem and this needs to be further studied in the future.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This work is supported by the National Natural Science Foundation of China (31272314), the Natural Science Foundation of Liaoning (201202121), and the Program for Liaoning Innovative Research Team in University (LT2015015).

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Copyright

Copyright © 2015 Yue Sun and Dejing Shang. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


Plasma medicine research highlights antibacterial effects and potential uses

UNIVERSITY PARK, Pa. — As interest in the application of plasma medicine — the use of low-temperature plasma (LTP) created by an electrical discharge to address medical problems — continues to grow, so does the need for research advancements proving its capabilities and potential impacts on the health care industry. Across the world, many research groups are investigating plasma medicine for applications including cancer treatment and the accelerated healing of chronic wounds, among others.

Researchers from Penn State’s College of Engineering, College of Agricultural Sciences and College of Medicine say direct LTP treatment and plasma-activated media are effective treatments against bacteria found in liquid cultures. The researchers also say they have devised a unique way to create plasma directly in liquids.

The team, comprised of engineers, physicists, veterinary and biomedical scientists and medical professionals, is using an atmospheric-pressure plasma jet to use room temperature — “cold” — plasma to treat bacteria.

An atmospheric-pressure plasma jet is used for sterilization of antibiotic-resistant bacteria. The plasma is non-thermal and can be applied to living tissue without thermal damage.

Plasma, the fourth state of matter, is typically very hot — thousands to millions of degrees. By using plasma generated at atmospheric pressure or in liquids, the researchers can create molecules and atoms with antibacterial effects without burning anything. Sean Knecht, assistant teaching professor of engineering design at Penn State and leader of the Cross-disciplinary Laboratory for Integrated Plasma Science and Engineering, said this process creates many different types of reactive particles, making the likelihood of bacterial mutations to simultaneously combat all the particles almost nonexistent.

Knecht explained that the team’s research results, published in Scientific Reports, show that plasma technology generates large amounts of reactive oxygen species or reactive particles created from molecules that contain oxygen atoms, including oxygen molecules in the air and water vapor. The plasma’s effect on different bacteria such as E. coli and Staph. aureus is significant, resulting in many bacterial deaths through multiple generations.

“Over the course of four generations of bacteria, these bacteria do not acquire any form of resistance to the plasma treatment,” he said.

Girish Kirimanjeswara, associate professor of veterinary and biomedical sciences at Penn State, said this is extremely important due to the typical way bacteria mutate, making them resistant to antibiotics.

Antibiotics target a specific metabolic pathway, essential protein or nucleic acids in bacteria. Because of this, antibiotics have to enter a bacterial cell to find and bind to that specific target. Any bacterial mutation that decreases an antibiotic’s entry capabilities or increases its rate of exit makes the antibiotic less effective. Mutations happen naturally at a low rate but can rapidly accumulate by selection pressure when introduced to antibiotics aimed at fighting the bacteria.

According to Kirimanjeswara, the team’s research results show that plasma treatment produces various reactive oxygen species at a concentration high enough to kill bacteria, but low enough to not have negative impacts on human cells. He explained that the oxygen species quickly target virtually every part of the bacteria including proteins, lipids and nucleic acids.

“One can call it a sledgehammer approach,” Kirimanjeswara said. “It is difficult to develop resistance by any single mutation or even by a bunch of mutations.”

The team also applied these findings to design a system that can create plasma directly in liquids. The researchers intend to create plasma in blood to address cardiovascular infections directly at the source. To do so, high electric voltage and large electric currents are typically used. In the plasma system created by the researchers, the electrical current and energy that might reach the patient are minimized by using dielectric, or electrically-insulating, materials. Materials that the team would typically use to create the plasma include glass and ceramic due to their capability to withstand high local temperatures. These materials tend to make blood clot and may not be very flexible, a necessity if they are to be used in the cardiovascular system. The team is investigating insulating coatings that are biocompatible, or acceptable by the human body, and flexible. Knecht said the team has identified a polymer called Parylene-C and reported the initial results in the journal IEEE Transactions on Radiation and Plasma Medical Sciences. The team is further pursuing this avenue, as polymers have low melting points and may not withstand repeated exposure to plasma.

“Biocompatible polymers can be used for plasma generation in biological liquids, but their lifetime is limited,” Knecht said. “New unique plasma generation designs must be developed to produce lower intensity plasma discharges that can extend their lifetime. That is what we are continuing to work on.”


Summary

In an era of increasing MDR, in which bacteria are developing resistance to many types of antibiotics, it is becoming very difficult to fight infectious diseases and cure patients, resulting in serious morbidity and mortality. NPs are a viable alternative to antibiotics and appear to have high potential to solve the problem of the emergence of bacterial MDR. The current in-depth review of the antibacterial mechanisms may contribute to the development of efficient antibacterial NPs and to the prevention of NP cytotoxicity.