Information

Lab 9: Fungi, Part 1 - The Yeasts - Biology

Lab 9: Fungi, Part 1 - The Yeasts - Biology


We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

Fungi are eukaryotic organisms and include the yeasts, molds, and fleshy fungi. Yeasts are microscopic, unicellular fungi; molds are multinucleated, filamentous fungi (such as mildews, rusts, and common household molds); the fleshy fungi include mushrooms and puffballs.

All fungi are chemoheterotrophs, requiring organic compounds for both an energy and carbon source, which obtain nutrients by absorbing them from their environment. Most live off of decaying organic material and are termed saprophytes. Some are parasitic, getting their nutrients from living plants or animals.

The study of fungi is termed mycology and the diseases caused by fungi are called mycotic infections or mycoses. In general, fungi are beneficial to humans. They are involved in the decay of dead plants and animals (resulting in the recycling of nutrients in nature), the manufacturing of various industrial and food products, the production of many common antibiotics, and may be eaten themselves for food. Some fungi, however, damage wood and fabrics, spoil foods, and cause a variety of plant and animal diseases, including human infections.

YEASTS

Yeasts are unicellular, oval or spherical fungi which increase in number asexually by a process termed budding (see Fig. 1). A bud forms on the outer surface of a parent cell, the nucleus divides with one nucleus entering the forming bud, and cell wall material is laid down between the parent cell and the bud. Usually the bud breaks away to become a new daughter cell but sometimes, as in the case of the yeast Candida, the buds remain attached forming fragile branching filaments called hyphae (see Fig. 10). Because of their unicellular and microscopic nature, yeast colonies appear similar to bacterial colonies on solid media. It should be noted that certain dimorphic fungi (see Lab 10) are able to grow as a yeast or as a mold, depending on growth conditions.

  • Scanning electron micrograph of Saccharomyces; courtesy of Dennis Kunkel's Microscopy.
  • Transmission electron micrograph of Candida albicans (see Fig. 3).
  • Movie of Saccharomyces cerevisiae reproducing by budding. Movie of Growth and Division of Budding Yeast (Saccharomyces cerevisiae) . © Phillip Meaden, author. Licensed for use, ASM MicrobeLibrary.

Yeasts are facultative anaerobes and can therefore obtain energy by both aerobic respiration and anaerobic fermentation. The vast majority of yeasts are nonpathogenic and some are of great value in industrial fermentations. For example, Saccharomyces species are used for both baking and brewing.

The yeast Candida is normal flora of the gastrointestinal tract and is also frequently found on the skin and on the mucous membranes of the mouth and vagina. Candida is normally held in check in the body by:

1. normal immune defenses, and

2. normal flora bacteria.

However, Candida may become an opportunistic pathogen and overgrow an area of colonization if the host becomes immunosuppressed or is given broad-spectrum antibiotics that destroy the normal bacterial flora. (Since Candida is eukaryotic, antibiotics used against prokaryotic bacteria do not affect it.)

Any infection caused by the yeast Candida is termed candidiasis. The most common forms of candidiases are oral mucocutaneous candidiasis (thrush; see Fig. 7A), vaginitis (see Fig. 7B), balantitis (infection of the penis), onychomycosis (infection of the nails), and dermatitis (diaper rash and other infections of moist skin). In addition, Candida can cause urinary tract infections. However, antibiotic therapy, cytotoxic and immunosuppressive drugs, and immunosuppressive diseases such as diabetes, leukemias, and AIDS can enable Candida to cause severe opportunistic systemic infections involving the skin, lungs, heart, and other organs. In fact, Candida now accounts for 10% of the cases of septicemia. Candidiasis of the esophagus, trachea, bronchi, or lungs, in conjunction with a positive HIV antibody test, is one of the indicator diseases for AIDS.

The most common Candida species causing human infections is C. albicans, causing 50-60% of all Candida infections. Candida glabrata is second, causing 15-20% of Candida infections; Candida parapsilosis is third, responsible for 10-20%.

Candida is said to be dimorphic, that is it has two different growth forms. It can grow as an oval, budding yeast, but under certain culture conditions, the budding yeast may elongate and remain attached producing filament-like structures called pseudohyphae. C. albicans may also produce true hyphae similar to molds. In this case long, branching filaments lacking complete septa form. The pseudohyphae and hyphae help the yeast to invade deeper tissues after it colonizes the epithelium. Asexual spores called blastoconidia (blastospores) develop in clusters along the hyphae, often at the points of branching. Under certain growth conditions, thick-walled survival spores called chlamydoconidia (chlamydospores) may also form at the tips or as a part of the hyphae (see Fig. 2A and Fig. 2B)

A lesser known but often more serious pathogenic yeast is Cryptococcus neoformans. Like many fungi, this yeast can also reproduce sexually and the name given to the sexual form of the yeast is Filobasidiella neoformans. It appears as an oval yeast 5-6 µm in diameter, forms buds with a thin neck, and is surrounded by a thick capsule. It does not produce pseudohyphae and chlamydospores. The capsule enables the yeast to resist phagocytic engulfment. The yeast is dimorphic. In its sexual form, as well as in its asexual form under certain conditions, it can produce a hyphal form.

Cryptococcus infections are usually mild or subclinical but, when symptomatic, usually begin in the lungs after inhalation of the yeast in dried bird feces. It is typically associated with pigeon and chicken droppings and soil contaminated with these droppings. Cryptococcus, found in soil, actively grows in the bird feces but does not grow in the bird itself. Usually the infection does not proceed beyond this pulmonary stage. However, in an immunosuppressed host it may spread through the blood to the meninges and other body areas, often causing cryptococcal meningoencephalitis. Any disease by this yeast is usually called cryptococcosis.

Dissemination of the pulmonary infection can result in severe and often fatal cryptococcal meningoencephalitis. Cutaneous and visceral infections are also found. Although exposure to the organism is probably common, large outbreaks are rare, indicating that an immunosuppressed host is usually required for the development of severe disease. Extrapulmonary cryptococcosis, in conjunction with a positive HIV antibody test, is another indicator disease for AIDS. People with AIDS-associated cryptococcal infections account for 80%-90% of all patients with cryptococcosis.

Cryptococcus can be identified by preparing an India ink or nigrosin negative stain of suspected sputum or cerebral spinal fluid in which the encapsulated, budding, oval yeast cells (see Fig. 4A) may be seen. It can be isolated on Saboraud Dextrose agar and identified by biochemical testing. Direct and indirect serological tests (discussed in Labs 17 & 18) may also be used in diagnosis.

Pneumocystis jiroveci, (formerly called Pneumocystis carinii), causes an often-lethal disease called Pneumocystis pneumonia (PCP). It is seen almost exclusively in highly immunosuppressed individuals such as those with AIDS, late stage malignancies, or leukemias. PCP and a positive HIV-antibody test is one of the more common indicators of AIDS.

P. jiroveci can be found in 3 distinct morphologic stages:

  • The trophozoite (trophic form), a haploid amoeboid form 1-4 µm in diameter that replicates by mitosis and binary fission. The trophic forms are irregular shaped and often appears in clusters.
  • A precystic form or early cyst. Haploid trophic forms conjugate and produce a diploid precyst form or sporocyte.
  • The precyst form matures into a cyst form, which contains several intracystic bodies or spores are 5-8 µm in diameter. It has been postulated that in formation of the cyst form (late phase cyst), the zygote undergoes meiosis and subsequent mitosis to typically produce eight haploid ascospores (sporozoites) See Fig. 9. As the haploid ascospores are released the cysts often collapse forming crescent-shaped bodies (see Fig. 5). P. jiroveci is usually transmitted by inhalation of the cyst form. Released ascospores then develop into replicating trophic forms that attach to the wall of the alveoli and replicate to fill the alveoli.
    • Proposed life cycle for Pneumocystis jiroveci; from dpd.cdc.gov

In biopsies from lung tissue or in tracheobronchial aspirates, both a trophic form about 1-4 µm in diameter with a distinct nucleus and a cyst form between 5-8 µm in diameter with 6-8 intracystic bodies (ascospores) can be seen.

When viewing cysts of P. jiroveci in lung tissue after utilizing the Gomori methenamine silver stain method, the walls of the cysts are stained black and often appear crescent shaped or like crushed ping-pong balls. The intracystic bodies are not visible with this stain.

  • P. jiroveci cysts from bronchoalveolar lavage (see Fig. 5)
  • P. jiroveci cysts from the lungs (see Fig. 9)

Malassezia globosa

Malassezia globosa is a dimorphic yeast that is the most frequent cause of a superficial skin infection called tinea versicolor that commonly appears as a hypopigmentation or hyperpigmentation of the infected skin. M. globosa is also the most common cause of dandruff and seborrheic dermatitis. The yeast is naturally found on the skin.

For a decription of antifungal agents used to treat fungal infections, see section IIE: Chemotherapeutic Control of Fungi in you lecture E-text.

Today we will use three agars to grow our yeast: Saboraud Dextrose agar (SDA), Mycosel agar, and Rice Extract agar. Saboraud Dextrose agar (SDA)is an agar similar to trypticase soy agar but with a higher sugar concentration and a lower pH, both of which inhibit bacterial growth but promote fungal growth. SDA, therefore, is said to be selective for fungi.

Another medium, Mycosel agar, contains chloramphenicol to inhibit bacteria and cycloheximide to inhibit most saprophytic fungi. Mycosel agar, therefore, is said to be selective for pathogenic fungi.

Rice Extract agar with polysorbate 80 stimulates the formation of hyphae, blastoconidia, and chlamydoconidia (see Fig. 2B), structures unique to C. albicans, and may be used in its identification. The speciation of Candida is based on sugar fermentation patterns.

MATERIALS

Coverslips, alcohol, forceps, and one plate each of Saboraud Dextrose agar, Mycosel agar, and Rice Extract agar.

ORGANISMS

Trypticase Soy broth cultures of Candida albicans and Saccharomyces cerevisiae.

PROCEDURE (to be done in pairs)

1. With a wax marker, divide a Saboraud Dextrose agar and a Mycosel agar plate in half. Using a sterile swab, inoculate one half of each plate with C. albicans and the other half with S. cerevisiae. Incubate the two plates upside down and stacked in the petri plate holder on the shelf of the 37°C incubator corresponding to your lab section until the next lab period.

2. Using a sterile swab, streak two straight lines of C. albicans into a plate of Rice Extract agar plate. Pick up a glass coverslip with forceps, dip the coverslip in alcohol, and ignite with the flame of your gas burner. Let the coverslip cool for a few seconds and place it over a portion of the streak line so that the plate can be observed directly under the microscope after incubation. Repeat for the second steak line and incubate the plate upside down at room temperature until the next lab period.

3. Observe the following demonstrations:

RESULTS

1. In the table below, describe the appearance of Candida albicans and Saccharomyces cerevisiae on Saboraud Dextrose agar.

Also in the table below, describe the appearance of Candida albicans and Saccharomyces cerevisiae on Mycosel agar.

YeastSDAMycosel agar
Candida albicans
Saccharomyces cerevisiae

2. Remove the lid of the Rice Extract agar plate and put the plate on the stage of the microscope. Using your yellow-striped 10X objective, observe an area under the coverslip that appears "fuzzy" to the naked eye. Reduce the light by moving the iris diaphragm lever almost all the way to the right. Raise the stage all the way up using the coarse focus (large knob) and then lower the stage using the coarse focus until the yeast comes into focus. Draw the hyphae, blastoconidia, and chlamydoconidia. See lab 1 for focusing instructions using the 10X objective.

3. Observe and make drawings of the demonstration yeast slides.

PERFORMANCE OBJECTIVES FOR LAB 9

After completing this lab, the student will be able to perform the following objectives:

INTRODUCTION

1. Define mycology and mycosis.

2. State three ways fungi may be beneficial to humans and three ways they may be harmful.

DISCUSSION

1. Describe the typical appearance of a yeast cell and its usual mode of reproduction.

2. Describe yeasts in terms of their oxygen requirements.

3. State two ways the yeast Saccharomyces is beneficial to humans.

4. Name three yeasts that commonly infect humans.

5. Name four common forms of candidiasis.

6. Describe two conditions that may enable Candida to cause severe opportunistic systemic infections.

7. Describe pseudohyphae, hyphae, blastoconidia (blastospores), and chlamydoconidia (chlamydospores).

8. State the usefulness of Saboraud Dextrose agar, Mycosel agar, and Rice Extract agar.

9. State how Cryptococcus neoformans is transmitted to humans, where in the body it normally infects, and possible complications.

10. State the primary method of identifying Cryptococcus neoformans when causing cryptococcal meningoencephalitis.

11. State what disease is caused by Pneumocystis jiroveci and indicate several predisposing conditions a person is normally seen to have before they contract the disease.

12. Name an infection caused by Malassezia globosa.

RESULTS

1. Describe the appearance of Saccharomyces cerevisiae and Candida albicans on Saboraud Dextrose agar and on Mycosel agar. 2. When given a plate of Mycosel agar showing yeast-like growth and a plate of Rice Extract agar showing hyphae, blastosconidia (blastospores), and chlamydoconidia (chlamydospores), identify the organism as Candida albicans. 3. Recognize the following observed microscopically:

a. Saccharomyces cerevisiae and Candida albicans as yeasts in a direct stain preparation
b. A positive specimen for thrush by the presence of budding Candida albicans
c. Cryptococcus neoformans in an India ink preparation

d. Pneumocystis jiroveci in lung tissue

SELF-QUIZ


Study Notes on Fungi

Fungi (Lat. fungus—mushroom) are eukaryotes with a distinct nucleus and rigid chitinous cell wall and were formerly regarded as plants without chlorophyll and are now grouped with protozoa slime moulds and most algae as Higher Prostita. Mycoses are infections caused by true fungi.

Eumycetes contains more than 80,000 species and can be classified morphologically into:

(1) Phycomycetes:

They are fungi with an unicellular, non-septate mycelium (500 species). The spores (endospores) are enclosed in special sporangia. Reproduction is sexual and asexual. Atypical representative of Mucor (bread mould) is Mucor mucedo. Pathogenic species of this Mucor (mould) may cause infection of lungs, middle ear and general severe infectious process in man.

The mould (filamentous) or mycelial fungi grow as long filaments (hyphae) and reproduce by formation of spores. The major part of the mycelium the vegetative mycelium grows and penetrates into the substrate absorbing nutrients for growth other hyphae form aerial mycelium and protrude from the vegetative mycelium into the air. They form various kinds of spores and disseminate them in the air.

(2) Ascomycetes:

Ascomycetes of sac fungi (35,000 species) have a multicellular mycelium. They reproduce sexually by means of ascospores (spores which develop in spherical spore cases), asci—ascus (Gr. askos—sac) and asexually by conidia (exospores which have the function of asexual reproduction in many fungi).

The genus Aspergillus belongs to the class Ascomycetes. These fungi have divided separate mycelium and an unicellular conidiophore which terminates in a fan-like row of short sterigmata from which the spores are pinched off in chains-conidia (Gr. konia—dust).

The fruiting part of the aspergillus resembles a jet of water from a watering can and hence the name “Sprinkler” mould. Aspergillus niger is a representative of Aspergilla and is widespread in nature, certain species may cause aspergillosis of the lungs, ear and eye of man and may infect the whole body.

The genus Penicillium belongs to the class Ascomycetes. The mycelium and conidiophore are multicellular. The fruiting body is in the shape of a brush. The conidiophore branches towards its upper part and terminates into sterigmata from which even-rowed chains of conidia are pinched off (Fig. 99.1).

This genus of fungi is widespread in nature (fodder, milk products, moist objects, old leather, ink, jam). The type species is Penicillium glaucum. Certain species (Penicillium notatum, Penicillium chrysogenum) are used for the production of penicillin which is widely used in the treatment of many infectious diseases.

Some species of this genus of fungi are pathogenic and cause infection of the skin, nails, upper respiratory tract, lungs and other organs of man.

Yeasts belong to the class Ascomycetes. They are large, oval, round, rod shaped cells. They have a double cell wall and well defined nucleus. The cytoplasm is homogeneous—sometimes of a fine granular structure. It contains inclusions (glycogen, volutin, lipid) and also filamentous bodies-chondriosomes which are responsible for synthetic process of the cell.

Yeasts multiply by budding, fission, sporulation and some of them reproduces asexually. Daughter cells produced by budding from the parent cell transform into independent individuals. The yeasts can also reproduce by sporulation. When there is lack of nutrition, 2, 4, 8 or 16 endospores are formed inside the cells of some species of yeast.

Yeast cell forming the ascospores is called the ascus (sac), while sporulating yeasts are known as Ascomycetes, since the yeasts ferment various carbohydrates, they are widely used in brewing beer, in wine making and in baking bread. Saccharomyces cerevisiae, S. elipsoides are typical representatives of the yeasts.

Among the asporogenic yeasts (family Saccharomycetaceae), there are species pathogenic to man they are called as Candida (Fig. 99.4) which cause grave disease known as candidiasis. They occur as a result of the growth inhibition of the normal micro flora by antibiotics used for treating a number of infectious diseases and inflammatory processes.

Fungi with a multicellular mycelium. These organisms predominantly reproduce asexually by basidiospores (basidia reproductive organs) in which a certain number of spores develop, usually 4. Certain species are free parasites. Two hundred species of mushrooms are used as food. Twenty-five species of mushrooms are poisonous.

Smut fungi invade grains crops causing disease known as smut. Rust fungi affect sunflowers and other plants producing orange coloured spots on infected plants. Imperfect fungi (Fungi imperfecti) are a rather large group of fungi consisting of a multicellular mycelium without either asco- or basidiosporangiophore, but only with conidia.

Reproduction is asexual but the sexual reproduction is unknown. To this class belong the orders Hyphomycetes, Melanconiales and Sphaeropsidales.

Among the hyphomycetes, which may be of great interest to the physicians are:

Fusarium graminearum causing intoxication in human (drunken bread), and Fusarium sporotrichioides causing intoxication in man and domestic animals who had eaten the grain crops which had remained in the fields during the winter.

Pathogenic species of imperfect fungi are causative agents of dermatomycoses (superficial mycoses):

Favus (Achorion schoenleini) trichophytosis (Trichophyton violaceum), microsporosis (Microsporum lanosum), epidermophytosis (Epidermophyton inguinale).


Incr-Edible Science: Yeast (Micro Fungi – part 2)

After covering mold type fungi in Part 1, we moved onto the unicellular yeast:

  1. This is Leeuwenhoek, the man responsible for the discovery of micro-organisms through his very simple ‘microscope’
  2. A labelled diagram of a yeast cell
  3. Yeast as we know it

To find out a bit more about yeast we mainly used the same resources as before:

We also spent some time on the internet researching the properties of yeast. Yeasts, much like their fungal relatives mold, rust and mildew are eukaryotic organisms. However, unlike their relations they are unicellular, that is each one is made up of only a single cell. Most yeasts reproduce asexually by mitosis, with many doing so by an asymmetrical division known as budding:

Here are the single yeast cells as shown under a light then an electron microscope with increasing magnification. Although budding can be seen in picture 2 and 3, it is shown most clearly in picture 4.

We attempted to look at yeast under our own microscope. Each child prepared their own yeast slide, even A5, and looked on the TV microscope screen:

T12 had the most success at seeing the yeast cells, and that was with our other standard microscope and, although he tried, he couldn’t get a decent photo of them.

One reason that yeasts are considered a fungus is due to their inability to make their own nutrition, thereby feeding off organic matter whenever they can. Thus they are known as decomposers, much like their fellow fungi mould and rust. We tested the yeasts ability to decompose by carrying out a simple experiment adapted from one in Apologia’s General Science:

  1. We cut up a banana into three equal pieces and run them under a tap. One was immediately placed in a baggy labelled ‘banana and water’. The second one was handled by all the children to see if the bacteria from their hands had the same decomposing effect. This was placed in a baggy labelled ‘banana, water and handled’
  2. The final piece of banana was placed in the baggy and a teaspoon of yeast was added.
  3. The three baggies were left to sit in a warm kitchen for a day
  4. Approximately four hours later there was little change to the baggy containing the handled banana or to the baggy holding the untouched plain banana. However, as can be seen in the baggy on the left hand side of picture 4 the banana which had been mixed with the yeast was separating, soggy and squishy, with a pool of thick liquid surrounding it. This suggested that the presence of yeast had a decomposing or breaking down action on the banana.

And these were the results the next day:

I asked the children what they knew about yeast and they said it rose bread by giving off a carbon dioxide gas. It didn’t surprise me that they were able to deduce this as we had done many experiments to illustrate gasses being used in the rising of various breads in our past Incr-Edible Science:

Click here to read about how bicarb works when mixed with an acid to rise soda bread and then Click here to read about buttermilk substitutes in soda bread:

Click here to read about the experiments we did making our own home made baking powder to rise banana chocolate muffins:

Click here to read about the experiments we did with sour dough, using natural yeasts found in the air:

I thought it would be fun to demonstrate this using a bottle and a balloon and replicating a similar demonstration we did with bicarb and vinegar. I asked them if they thought the reaction would be as fast as the bicarb and vinegar and if not why not?

The children thought slower because it wasn’t a chemical reaction, rather a biological one, and you don’t often see explosions in nature. L thought it was interesting that it took a full 60 minutes to inflate the balloon, which ‘is about the time we leave the dough to rise when we are making bread!’ Sometimes I just love home school!

Following this demonstration, I wanted us to investigate the best possible conditions in which yeast was able to maximise its fermentation, thereby producing the most CO2. This information would be invaluable to L11, who loves to bake as well as useful knowledge for the other three.

Basically we sourced as many test tubes as we could get our hands on and set up two experiment stations. The first shown is in the red test tube holder. This station was manned by T12 and L11. They were testing the effect that heat has on the respiration rates of yeast. They also tested to see if an absence of food in the form of sugar would have any effect. (Picture 1 below)

The second station was manned by C11 and A5, and contained the yellow test tube holder. They were testing to see if the pH effected the respiration rates of the yeast. They used a strongly acidic solution (vinegar), a basic solution (water and salt), and a weakly acidic solution (water and lemon juice).

The respiration of the yeast was demonstrated by the production of CO2 which was collected in the balloons tightly sealed around the test tube lips:

According to our experiments, yeast grows best in warm conditions, needs a source of energy (sugar) to grow and prefers a slightly acidic environment, although will grow perfectly well in a neutral environment. None of our results were a surprise, except maybe the boiling water which we thought would kill the yeast. Either yeast is far more robust than we first thought or the water had cooled down a great deal between boiling the kettle and doing the experiment.

One variable which I hadn’t considered was the moisture level. If I did this again, I would include a test tube with no liquid, one with minimal liquid and one with an excess, observing the amount of CO2 produced in each test tube.

This was a simple experiment which was largely visual. A more accurate experiment could be carried out by taking measurements of the balloons at certain time intervals and plotting a graph. Doing so would have lost all three of my girl’s interest, so I chose the more simple but less scientific visual method!

From their research on-line the children found out that yeast do not require sunlight to grow, using organic compounds (mainly sugars) as their energy source. They require oxygen for respiration and thrive in a neutral or slightly acidic environment.

As the yeast grows, it converts its food (in the form of sugar or starch) into alcohol and carbon dioxide through the process of fermentation. This property has led to yeast being used widely in the making of wine and beer, as well as the process of baking.

In the same way as the reactions of bicarb with vinegar, I wanted the children to see and understand the equation of the reaction of yeast with a sugar solution. It is the enzyme, invertase, which is present in yeast which acts as a catalyst to convert the sucrose into glucose and fructose:

invertase
C12H22O11 + H2O ==> C6H12O6 + C6H12O6
Sucrose + water = Glucose + Fructose

The glucose, C6H12O6, and fructose, C6H12O6, formed are then converted into ethanol and carbon dioxide by another enzyme, zymase, which is also present in yeast.

zymase
C6H12O6 ==> C2H5OH + 2CO2

Simple sugar =>Ethanol + carbon dioxide

The addition of yeast can make bread rise because the yeast produces carbon dioxide from sugar (C6H12O6). Ethanol, the other product of the reaction, evaporates from the bread dough while it rises or it boils away during cooking.

We’ve been making bread on and off for years and all of the older children know how to do it, C11 at one point could probably have done it without consulting a recipe. I wanted them to see the difference between a loaf made with yeast, compared with one made with no yeast:

The difference was obvious and expected. No surprises here….that is until we cooked it:

It was interesting to see that whilst both loaves were put in the same oven on the same baking tray, and the non yeast one was significantly smaller, it was the larger yeast bread which cooked in the allotted time, whilst the non yeast bread remained practically uncooked. So the air (CO2) in the yeast bread actually has two roles – giving the bread a lovely light texture, but also reducing the time it takes to cook a loaf. Who knew? (Obviously not me!)

I also wanted them to look over their other Inc-Edible Science experiments done with soda bread and baking powder and answer the question – Why do you have to leave yeast to rise for an hour whereas both soda bread and muffins made with baking powder can be and should be cooked immediately? They had answered this question, unknowingly whilst we were comparing the balloon experiment done with yeast with the same experiment done with bicarb and vinegar. I wondered though whether they would be able to transfer this knowledge and understanding to the cooking process, which is the reason I write this curriculum.

It was a bit like pulling teeth, to be honest. I had all sorts of (ridiculous) answers until a light bulb suddenly went on in T’s head and they suddenly got what I was after! It was much easier once they had remembered that yeast was living whilst bicarb was not (!). It took a bit of prodding before T answered the query of why we allowed the dough to rise before putting it in the oven (ie. how come it had to sit at room temperature for an hour whilst the soda bread was put straight in the oven) Finally I got the answer I was looking for – that the heat would denature the yeast and stop it from reproducing. But then again, as T pointed out, our prior experiments did not demonstrate that at all, with the yeast in the boiling water working as well as those in the luke warm water. We surmised that the water must have cooled enough before putting it into the test tube for death not to occurred.

I’m debating whether or not to take yeast a new level and experiment with fermented goods. We’ll see, after the sour bread stench episode, I’m at all sure I’m up for it!


Reproduction in Fungi- Part-1: Vegetative Reproduction (Lecture Notes & PPT)

Ø Fungi reproduce by vegetative, asexual and sexual methods
Ø This post describes different types of Vegetative reproduction methods in fungi
Ø Vegetative reproduction helps to increase the number of individuals in the population
Ø Vegetative reproduction in fungi occurs by:

(1). Fragmentation
(2). Fission
(3). Bud fission
(4). Budding
(5). Gemmae
(6). Sclerotia
(7). Rhizomorphs
(8). Mycelial cords

(1). Fragmentation:

Ø Mycelium gets fragmented into small fragments, each of which is able to develop into new individual

Ø Fragmentation is common in filamentous fungi such as Rhizopus and Aspergillus

Ø Fission occurs in unicellular fungi such as Yeasts

Ø Mature cells divided mitotically into two and the two daughter cells separates and give rise to two individuals

(3). Bud fission

Ø It is a modified type of fission

Ø In bud fission cross wall are developed near the base of the bud to separate bud from mother cell

Ø Here only the buds undergo fission not the mother cell

Ø Bud like growth emerges out from the mature cells

Ø Budding is commonly occurs in unicellular forms such as Yeast

Ø Budding of Yeast may be of different types:

(a). Multilateral budding: buds arise at any point on the mother cell, but never again at the same site

(b). Uni-polar budding: budding repeated at same site on mother cell surface

(c). Bi-polar budding: budding restricted to both poles of the cell

(d). Monopolar budding: buds originate at only one pole of the mother cell

(5). Formation of Gemmae:

Ø Gemmae are specialized thick walled aggregation of chlamydospres like structures

Ø They are formed in un-favourable conditions (Example: Saprolegnia)

(6). Sclerotia:

Ø Sclerotia (sclerotium) are pseudo-parencymatous mycelial aggregations

Ø Sclerotia can also overcome unfavourable conditions

Ø Sclerotia can survive in the substratum (Eg. soil) for many years

Ø Sclerotia are commonly produced by plant pathogenic fungi

Ø The size and shape of sclerotia varies in different fungal groups

(7). Rhizomorphs:

Ø They are root like mycelial aggregations found in some fungi

Ø They are pseudo-parenchymatous hyphal modifications

Ø They can also overcome unfavourable conditions

Ø Rhizomorphs have high penetration capacity than individual hyphae and hence they have more pathogenic potential

Ø Bits of rhizomorps that survive in the soil, can act as inoculum for the next round of infection during the onset of favourable conditions

(8). Mycelial cords

Ø They are also thread like mycelial aggregations

Ø Formed by the more or less parallel aggregation of hyphal strand in some fungi

Ø Mycelial cords can also acts as bidirectional transport channels for nutrients

Ø Mycelial cords also helps the fungi to establish and colonize new areas from a food rich area


Watch the video: Zauberpilze Einführung, Christian Rätsch 14 (October 2022).