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Term Paper on Fungi
Term Paper Contents:
- Term Paper on the Introduction to Fungi
- Term Paper on the Characteristics of Fungi
- Term Paper on the Structure of the Fungi
- Term Paper on the Growth of the Fungi
- Term Paper on the Reproduction in Fungi
- Term Paper on the Classification of Fungi
- Term Paper on the Variety of Fungi
- Term Paper on the Fungi in Clinical Laboratories
- Term Paper on the Commercial Uses of Fungi
- Term Paper on the Life Cycle Pattern of Fungi
Term Paper # 1. Introduction to Fungi:
The fungi are a group of organisms so unlike any others that, although they were long classified with the plants, it has come to seem appropriate to assign them to a separate kingdom. Except for some one-celled forms, such as the yeasts, the fungi are basically coenocytic organisms composed of masses of filaments.
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A fungal filament is called a hypha, and all the hyphae of a single organism are collectively called a mycelium. The mycelium may appear as a mass on the surface of the nutrient or may be hidden beneath the surface. A fungus is essentially a multinucleate mass of cytoplasm enclosed within a rigid, much-branched system of tubes (the hyphae). The cytoplasm, organelles, and nuclei flow within those tubes.
A mycelium normally arises by the germination and outgrowth of a single cell, with growth taking place only at the tips of hyphae. The complex, spore-producing structures of the fungi, such as mushrooms, are tightly packed hyphae.
In most groups of fungi, the cell walls are composed primarily of chitin, a polysaccharide that is never found in the kingdom Plantae (it is, however, the principal component of the exoskeletons-the hard outer coverings-of insects).
In some groups, the hyphae are septate-divided by cell walls-but the walls, or septa, are perforated, and the cytoplasm and even the nuclei are able to flow through the septa. Only the reproductive structures are separated by cell membranes.
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All fungi are heterotrophs with a highly characteristic means of nutrition. Because of their filamentous form, each fungal cell is no more than a few micrometers from the soil, water, or other substance in which the fungus lives, and is separated from it only by a thin cell wall. Because of their rigid walls, fungi are unable to engulf small microorganisms or other particles.
They obtain food by absorbing dissolved inorganic and organic materials. Typically a fungus will secrete digestive enzymes onto a food source and then absorb the smaller molecules released.
The only motile cells of fungi are reproductive cells which may travel through water or air. Growth of the mycelium substitutes for motility, bringing the organism into contact with new food sources and different mating strains.
Under favourable conditions, a fungus can expand very rapidly, as evidenced by the overnight appearance of a lawnful of mushrooms, produced by the sudden transport of material from the underground mycelium into the fruiting bodies, or mushrooms.
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The fungi, together with the bacteria, are the principal decomposers of the world. Their activities are as vital to the continued survival of higher forms of life as are those of the food producers.
Some are also destructive; they may interfere with human activities by attacking our foodstuffs, our domestic plants and animals, our shelter, our clothing, and even our persons.
Term Paper # 2. Characteristics of Fungi:
The fungi (sing., fungus) are a diverse group of eukaryotic microorganisms, with over 80,000 identifiable species. For many decades, fungi were classified as plants, but laboratory studies have revealed a set of four properties that distinguish fungi from plants – fungi lack chlorophyll, while plants have this pigment; the cell walls of fungal cells contain a carbohydrate called chitin not found in plant cell walls; though generally filamentous, fungi are not truly multicellular like plants, because the cytoplasm of one fungal cell mingles through pores with the cytoplasm of adjacent cells; and fungi are heterotrophic eukaryotes, while plants are autotrophic eukaryotes. Mainly for these reasons, fungi are placed in their own kingdom Fungi, in the Whittaker classification of organisms.
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Fungi generally are saprobes with complex life cycles usually involving spore formation. A major sub-division of fungi, the molds, grows as long, tangled strands of cells that give rise to visible colonies. Another subdivision, the yeasts, are unicellular organisms whose colonies resemble bacterial colonies.
Term Paper # 3. Structure of the Fungi:
With the notable exception of yeasts, the fungi consist of masses of intertwined filaments of cells called hyphae (sing., hypha). Each cell of the hypha is eukaryotic, with a distinct nucleus surrounded by a nuclear membrane and other eukaryotic organelles. The cell wall is composed of small amounts of cellulose and large amounts of chitin.
Cellulose is a polysaccharide composed of glucose units linked together in such a way that most organisms cannot digest it. Chitin is a polymer of acetylglucosamine units, that is, glucose molecules containing amino and acetyl groups. Chitin gives the cell wall rigidity and strength, a function it also performs in the exoskeletons of arthropods.
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Fungal cells lack chlorophyll, and photosynthesis is therefore impossible. Since they consume preformed organic matter, fungi are described as heterotrophic organisms. They are saprobic, except for the parasitic fungi, which cause disease, Together with the bacteria, fungi decompose vast quantities of dead organic matter that would otherwise accumulate and make the earth uninhabitable.
In many species of fungi the individual cells are separated by cross walls, or septa (sing., septum). The septa are not complete, however, and pores allow a mixing of adjacent cytoplasms.
In other fungal species, the cells have no septa, and the cytoplasms and organelles of neighbouring cells mingle freely. These fungi are said to be coenocytic. The common bread mold Rhizopus stolonifer is coenocytic, while the blue-green mold that produces penicillin. Penicillium notatum, has septa.
The hypha is the morphological unit of the fungus and is seen only with the aid of a microscope. Hyphae have a broad diversity of forms, and many are highly branched with reproductive structures called fruiting bodies.
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A thick mass of hyphae is called a mycelium (pl., mycelia). This mass is usually large enough to be seen with the unaided eye, and generally it has a rough, cottony texture. The study of fungi is called mycology; and the individual who studies the fungi is a mycologist. Invariably, the prefix “myco-” will be part of a word referring to fungi, since is Greek for fungus.
Term Paper # 4. Growth of the Fungi:
In nature, the fungi are important links in ecological cycles because they rapidly digest animal and vegetable matter. In doing so, they release carbon and minerals back to the environment and make them available for recycling in plants. However, fungi may be a bane to industries because they also contaminate leather, hair products, lumber, wax, cork, and polyvinyl plastics.
Many fungi live in a harmonious relationship with other plants in nature, a condition called mutualism. In the southwestern Rocky Mountains, for instance, a fungus of the genus Acremonium thrives on the blades of a species of grass called Stipa robusta (“robust grass”).
The fungus produces a powerful poison that can put an animal such as a horse to sleep for about a week (the grass is called “sleepy grass” by the locals). Thus the grass survives where others are nibbled to the ground, reflecting the mutually beneficial interaction between plant and fungus.
Other fungi called mycorrhizal fungi also live harmoniously with plants. The hyphae of these fungi invade the roots of plants (and sometimes their stems) and plunge into their cells. Though poised to suck the plants dry the fungi are in fact gentle neighbours. Mycorrhizal fungi consume some of the carbohydrates produced by the plants, but in return they contribute certain minerals and fluids to the plant’s metabolism.
Mycorrhizal fungi have been found in plants from salt marshes, deserts, and pine forests. Indeed, in 1995, researchers from the University of Dayton reported that over 50 percent of the plants growing in the large watershed area of southwestern Ohio contain mycorrhizal fungi.
Most fungi grow best at approximately 25°C, a temperature close to normal room temperature (about 75°F). The notable exceptions are the pathogenic fungi, which thrive at 37°C, body temperature. Usually these fungi also grow on nutrient media at 25°C. Such fungi are described as biphasic (two phases) or dimorphic (two forms). Many have a yeast-like phase at 37°C and a mold-like phase at 25°C. Certain fungi grow at still lower temperatures, such as the 5°C found in the normal refrigerator.
Many fungi thrive under acidic conditions at a pH from 5 to 6. Acidic soil may therefore favour fungal turf diseases, and lime should be used to neutralize the soil. Mold contamination is also common in acidic foods such as sour cream, applesauce, citrus fruits, yogurt, and most vegetables. Moreover, the acidity in breads and cheese encourages fungal growth. Blue cheese, for example, consists of milk curds in which the mold Penicillium roqueforti is growing.
Fungi are aerobic organisms, with the notable exception of the fermentation yeasts that multiply in the presence or absence of oxygen. Normally, a high concentration of sugar is conducive to growth, and laboratory media for fungi usually contain extra glucose in addition to an acidic environment. Examples of such media are Sabouraud dextrose agar and potato dextrose agar.
Term Paper # 5. Reproduction in Fungi:
Reproduction in fungi may take place by asexual processes as well as by a sexual process. The principal structure of asexual reproduction is the fruiting body. This structure usually contains thousands of spores, all resulting from the mitotic divisions of a single cell and all genetically identical. Each spore has the capability of germinating to reproduce a new hypha that will become a mycelium.
Certain spores develop within a sac called a sporangium. Appropriately, these spores are called sporangiospores. Other spores develop on supportive structures called conidiophores. These spores are known as conidia (sing., conidium), from the Greek conidios, meaning dust.
The bread mold Rhizopus produces sporangiospores, while the blue-green mold Penicillium produces conidia. Fungal spores are extremely light and are blown about in huge numbers by wind currents. Many people suffer allergic reactions when they inhale spores, and communities therefore report the mold spore count to alert sufferers.
Some asexual modes of reproduction do not involve a fruiting body. For example, spores may form by fragmentation of the hypha. This process yields arthrospores, from the Greek stem arthro- for joint. The fungi that cause athlete’s foot multiply in this manner.
Another asexual process is called budding. Here, the cell becomes swollen at one edge, and a new cell called a blastospore, or bud develops from the parent cell and breaks free to live independently. Yeasts multiply in this way. Chlamydospores and oidia are other forms of spores produced without a fruiting body. Chlamydospores are thick-walled spores formed along the margin of the hypha, while oidia form at the tip of the hypha.
Many fungi also produce spores by a sexual process of reproduction. In this process, the cells of opposite mating types of fungi come together and fuse. A fusion of nuclei follows and the mixing of chromosomes temporarily forms a double set of chromosomes, a condition called diploid (from the Greek diploos for twofold).
Eventually the chromosome number is halved, and the cell returns to the condition where it has a single set of chromosomes, the so-called haploid condition (from the Greek haploos for single). Spores develop from cells in the haploid condition.
Sexual reproduction is advantageous because it provides an opportunity for the evolution of new genetic forms better adapted to the environment than the parent forms. For example, a fungus may become resistant to fungicides as a result of chromosomal changes during sexual reproduction.
Separate mycelia of the same fungus may be involved in sexual reproduction, or the process may take place between separate hyphae of the same mycelium. The process is essentially similar to that taking place in complex animals and plants.
Term Paper # 6. Classification of Fungi:
Variations in the sexual process of reproduction provide important criteria for the classification of fungi. The true fungi, such as we are discussing, belong to the division Eumycota in the kingdom Fungi, as set forth by Whittaker.
Other fungi in this kingdom are the slime molds in the division Myxomycota, and the lichens in the division Mycophycomycota. Slime molds are complex organisms having an ameba-like motile stage and a fungus-like spore-producing stage. Lichens consist of a fungal mycelium containing a number of unicellular algae or cyanobacteria that perform photosynthesis.
Members of the division Eumycota (true fungi) are divided into five classes, based mainly on the type of sexual spore produced. Generally the distinctions among the fungi are made on the basis of structural differences or physiological or biochemical patterns.
However, DNA analyses are becoming an important tool for drawing relationships among the fungi. Indeed, the first-place winner of the 1993 Westinghouse Science Talent Search was an Illinois student named Elizabeth M. Pine who showed that two structurally related mushrooms should probably be reclassified on the basis of their DNA content.
We shall briefly examine each of the five classes in the next paragraphs:
1. Oomycetes:
Fungi of the class Oomycetes are commonly called “water molds,” a reference to the fact that most species are aquatic fungi. During sexual reproduction, the members of this group form clusters of egg = like bodies at the tips of hyphae. Other nearby hyphae grows toward the bodies and fuse with them. Nuclear fusion leads to the formation of sexual spores called oospores, which germinate to produce new hyphae.
A notable feature of an oomycete is the zoospore, a flagellated spore formed in the asexual process of reproduction. No other fungi produce motile cells. Also, fungi of the oomycetes class have diploid cells during most of their life cycle, whereas most other fungal species have haploid cells. Moreover, the cell walls lack chitin. Because of these characteristics, some mycologists postulate that oomycetes may be the product of an evolutionary development entirely separate from other fungi.
Aquatic oomycetes are familiar as the molds that plague fish in an aquarium. Some terrestrial oomycetes are parasites of insects and plants, and certain ones cause downy mildew in grapes, white rust disease of cabbage, and the infamous late blight of potatoes.
2. Zygomycetes:
The second class of Eumycota is Zygomycetes, a group of terrestrial fungi with coenocytic hyphae. Sexual reproduction in these organisms results in zygospores from the mating of hyphae. Both sexually and asexually produced spores are dispersed on air currents.
The well-known member of the class Zygomycetes is the common bread mold, Rhizopus stolonifer. The hyphae of this fungus form a white or gray mycelium on bread, with upright sporangiophores each bearing globular sporangia. Thousands of sporangiospores are formed in each sporangium.
Occasional contamination of bread is compensated by the beneficial roles Rhizopus plays in industry. One species, for example, ferments rice to sake, the rice wine of Japan – another species is used in the production of cortisone, a drug that reduces inflammation in body tissues.
3. Ascomycetes:
Members of the class Ascomycetes are very diverse, varying from unicellular yeasts to powdery mildews, cottony molds, and large and complex “cup fungi.” The latter form a cup-shaped structure composed of hyphae tightly packed together. The hyphae of an ascomycete are septate, with large pores allowing a continuous flow of cytoplasm.
Though their mycelia vary considerably, all ascomycetes form a reproductive structure called an ascus during sexual reproduction. An ascus is a sac within which up to eight haploid ascospores form. Most of the ascomycetes also reproduce asexually by means of conidia, produced in chains at the end of a conidiophore.
Certain members of the Ascomycetes class are extremely beneficial. One example is the yeast Saccharomyces, used in brewing and baking. Another example is Aspergillus, which produces such products as citric acid, soy sauce, and vinegar and is used in genetics research. A third is Penicillium, various species of which produce the antibiotic penicillin as well as such cheeses as Roquefort and Camembert.
On the deficit side, some ascomycetes attack valuable plants. For instance, one member of the class parasitizes crops and ornamental plants, causing powdery mildew. Another species has almost entirely eliminated the chestnut tree from the American landscape.
Still another ascomycete is presently attacking elm trees in the United States (Dutch elm disease) and is threatening extinction of this plant. Two other ascomycete pathogens are Claviceps purpurea, which causes ergot disease of rye plants, and Aspergillus flams, which attacks a variety of foods and grains.
4. Basidiomycetes:
Members of the class Basidiomycetes are commonly called “club fungi.” They include the common mushroom, as well as the shelf fungi, puffball, and other fleshy fungi, plus the parasitic rust and smut fungi. The name basidiomycete refers to the reproductive structure on which sexual spores are produced. The structure, resembling a club, is called a basidium, the Latin term for “small pedestal.” Its spores are known as basidiospores.
Perhaps the most familiar member of the class is the edible mushroom. Indeed, the Italian “fungi” means mushroom. Its mycelium forms below the ground and after sexual fusion has taken place, the tightly compacted hyphae force their way to the surface and grow into the mushroom cap.
Basidia develop on the underside of the cap along the gills, and each basidium may have up to eight basidiospores. Edible mushrooms belong to the genus Agaricus, but one of the most potent toxins known to science is produced by another species of a visually similar genus, Amanita.
Sixteen outbreaks of mushroom poisoning, most related to this genus, were reported to the CDC in recent years. Another mushroom, the huge puffball, caused serious respiratory illness in eight persons when the spores were inhaled in an incident in Wisconsin in 1994.
Agricultural losses due to rust and smut diseases are considerable. Rust diseases are so named because of the orange-red colour of the infected plant. The diseases strike wheat, oats, and rye, as well as trees used for lumber, such as white pines.
Many rust fungi require alternate hosts to complete their life cycles, and local laws often prohibit the cultivation of certain crops near rust sensitive plants. For example, it may be illegal to raise gooseberries near white pine trees. Smut diseases give a black, sooty appearance to plants. They affect corn, blackberries, and a number of grains, and cause untold millions of dollars of damage yearly.
5. Deuteromycetes:
Certain fungi lack a known sexual cycle of reproduction and consequently are labeled with the botanical term “imperfect.” These imperfect fungi are placed in the fifth class, Deuteromycetes, where reproduction is only by an asexual method. It should be noted that a sexual cycle probably exists for these fungi, but it has thus far eluded mycologists.
When the sexual cycle is discovered, the deuteromycete is reclassified into one of the other four classes. A case in point is the fungus known as Histoplasma capsulatum. This fungus causes histoplasmosis, a disease of the human lungs and other internal organs. When the organism was found to produce ascospores, it was reclassified with the Ascomycetes and given the new name Emmonsiella capsulata.
However, some traditions die slowly, and certain mycologists insisted on retaining the old name because it was familiar in clinical medicine. Thus, mycologists decided to use two names for the fungus: the new name, Emmonsiella capsulata, for the sexual stage, and the old name, Histoplasma capsulatum, for the asexual stage.
Many fungi pathogenic for humans are classified as Deuteromycetes. These fungi usually reproduce by budding or fragmentation, and segments of hyphae are commonly blown about by dust or deposited on environmental surfaces. For example, fragments of the athlete’s foot fungus are sometimes left on towels and the shower room floor. Recently discovered fungi are also placed here until more is known about them.
Term Paper # 7. Variety of Fungi-Yeasts:
The word “yeast” refers to a large variety of unicellular fungi (as well as the single cell stage of any fungus). Included in the group are non-spore -forming yeasts of the class Deuteromycetes, as well as certain yeasts that form basidiospores or ascospores and thus belong to the Basidiomycetes or Ascomycetes classes. The yeasts we shall consider here are the species of Saccharomyces used extensively in brewing, baking, and as a food supplement. Pathogenic yeasts will be discussed presently.
Saccharomyces translates literally to “sugar-fungus,” a reference to the ability of the organism to ferment sugars. The most commonly used species of Saccharomyces are S. cerevisiae and S. ellipsoideus, the former used for bread baking and alcohol production, the latter for alcohol production.
Yeast cells are about 8 μm long and about 5 μm in diameter. They reproduce chiefly by budding, but a sexual cycle also exists in which cells fuse and form an enlarged cell (an ascus) containing smaller cells (ascospores). The organism is therefore an ascomycete.
The cytoplasm of Saccharomyces is rich in B vitamins, a factor that makes yeast tablets valuable nutritional supplements. One pharmaceutical company adds iron to the yeast and markets its product as Ironized Yeast, recommended for people with iron-poor blood.
The baking industry relies heavily upon S. cerevisiae to supply the texture in breads. Flour, sugar, and other ingredients are mixed with yeast, and the dough is set aside to rise. During this time, the yeasts break down glucose and other carbohydrates, and produce carbon dioxide through the chemistry of glycolysis and the Krebs cycle.
The carbon dioxide expands the dough, causing it to rise. Protein-digesting enzymes, also from the yeast, partially digest the gluten protein of the flour to give bread its spongy texture. To make bagels, the dough is boiled before baking – for sour dough bread, Lactobacillus species are added to give an acidic flavor to the bread; for rye bread, rye flour is substituted. In all these modifications, yeast remains an essential ingredient.
Yeasts are plentiful where there are orchards or fruits (the haze on an apple is a layer of yeasts). In natural alcohol fermentations, wild yeasts of various Saccharomyces species are crushed with the fruit; in controlled fermentations, S. ellipsoideus is added to the prepared fruit juice.
Now the chemistry is identical with that in dough – the fruit juice bubbles profusely as carbon dioxide evolves through the reactions of glycolysis and the Krebs cycle. When the oxygen is depleted, the yeast metabolism shifts to fermentation and the pyruvic acid from glycolysis changes to consumable ethyl alcohol.
The products of yeast fermentation depend on the starting material. For example, when yeasts ferment barley grains, the product is beer; if grape juice is fermented, the product is wine. Sweet wines contain leftover sugar, but dry wines have little sugar. Sparkling wines such as champagne continue to ferment in thick bottles as yeast metabolism produces additional carbon dioxide.
For spirits such as whiskey, rye, or scotch, some type of grain is fermented and the alcohol is distilled off. Liqueurs are made when yeasts ferment fruits such as oranges, cherries, or melons. Virtually anything that contains simple carbohydrates can be fermented by Saccharomyces. The huge share of the U.S. economy taken up by the wine and spirits industry is testament to the significance of the fermentation yeasts.
Term Paper # 8. Fungi in Clinical Laboratories:
There seems to be a widespread mystique about the hazards of handling fungi in clinical laboratories. Although there are risks associated with examining the filamentous phases of dimorphic human pathogens, the commonsense application of a few basic safety rules that are commonly followed in clinical microbiology laboratories will suffice to protect laboratory workers against infection.
It is a good practice to examine all molds within an enclosure such as a bacteriological glove box or laminar flow hood. This practice not only will protect workers from accidental infection with the systemic mycotic agents but also will reduce contamination of laboratory cultures and help to avoid introducing conidia into a hospital’s air-conditioning system. Yeast cultures can be handled somewhat less cautiously, in the same manner that bacterial cultures are routinely handled, but they should never be handled carelessly.
Basically, two types of activities can lead to laboratory infections with fungi:
(i) Accidental creation of aerosols containing conidia and
(ii) Accidental inoculation with sharp instruments such as hypodermic needles, dissecting needles, and scalpel blades.
Activities such as smoking, drinking, eating, application of cosmetics, and insertion of contact lenses are to be avoided in laboratory work areas. It is a good practice to clean laboratory benches daily with a good disinfectant-containing detergent, not only to prevent potential infections but also to reduce the potential of contaminating laboratory cultures with undesirable molds.
Serological testing with fungal antigens presents two major difficulties – (i) the lack of commercial availability of sensitive and specific antigens for all fungus diseases and (ii) the broad cross-reactivity of antigens, which makes interpretation of test results more complex than might be desired.
The complexities of mycoserology derive not from the kinds of tests that are employed but from the crudeness and complexity of the antigens that must be used in the tests.
In the past, individual clinical laboratories that wanted to provide serological tests to assist in diagnosis of fungus diseases often had to manufacture their own antigens. Such antigens were carefully standardized by using known positive reference antisera obtained from the Centers for Disease Control, Atlanta, Ga., or from other reference laboratories.
Within recent years, however, commercial supply of most of the antigens and control sera needed for mycoserological techniques has become a reality. Suppliers of these reagents and any laboratory wishing to carry out serological tests for fungus disease should be able to acquire the necessary materials without difficulty.
In my experience, the latex test for cryptococcal antigen that is currently available in kit form has been particularly reliable and useful as an aid to detection of cryptococcal infections. The exo-antigen test is a recently developed procedure that has proven to be quite useful for serological confirmation of the identification of systemic fungal pathogens.
Term Paper # 9. Commercial Uses of Fungi:
1. Production of Citric Acid:
Citric acid is an important organic acid produced commercially by using Aspergillus niger. Before 1917, the citric acid was obtained only from citrus fruit. The commercial production was started as early as in 1917 using Aspergillus niger. About 70% of the annual production is utilised for foods, beverages and 20% in pharmaceuticals.
Commercial production:
During this process selected strains of A. niger are grown in an open aluminum tray (2.5m x 2m) containing molasses.
Initially, the medium (molasses) should be free from any trace element by passing through ion-exchange resin. The medium is then added with required amount of salts of metal ions. The metal ions like Fe++, Mn++, Zn++ and phosphates are required in this procedure as co-factor, but above critical level they inhibit citric acid production and not the mycelium.
Molasses solution containing inorganic nitrogenous salts are taken in the tray and seeded with A. niger mycelium and incubated at 30°C for 8-11 days and pH is maintained at about 2.0. Aseptic condition is not essential as low pH prevents bacterial growth.
Citric acid is produced by the fungal mycelium during idiophase of growth. At this stage, citrate synthetase activity is much more and citric acid is initially accumulated in the mycelium and then released in the medium.
After completion of incubation period, the culture fluid is taken out from the base of the tray, without disturbing the upper mycelial growth of A. niger. Fresh medium may be added to continue further production i.e., the next batch of fermentation. The citric acid is then recovered from culture filtrate by precipitation.
2. Production of Ethanol:
Industrial production of alcohol (Beer):
Beer is a product of fermentation of barley grains by yeasts, containing about 4% alcohol. During this process, barley grains are allowed to germinate for the conversion of starch to sugar, mainly maltose, by the naturally developed amylases. The process is called malting and the digested grains is called malt. After that, the grains were washed with water and then the liquid portion, called wort, removed.
Hops (dried petal of Humulus lupulus, the vine) are then added to the wort to add colour, flavour, and stability and also to prevent contamination (due to presence of two antimicrobial substances). At that time, the fluid is filtered and yeast is mixed in large quantity. Out of many useful strains, any one strain of Saccharomyces cerevisiae is used.
Commonly yeast is collected from previous batch culture for its further use. The mixture is then incubated for 7 days. After 7 days young beer is transferred to vats for primary and secondary ageing for 2 weeks to 6 months. Some yeast remains with the beer that is to become keg beer and the product is refrigerated for preservation. The thick wall of the keg trap produced CO2 for continual fermentation. For bottling, the beer is either filtered to remove the yeast or pasteurised at 140°F (60°C) for 13 minutes to kill the yeasts.
Filtered yeasts are then used in different ways:
1. used to mixed with new wort,
2. pressed to tablets for consumption by human being as single cell protein (SCP).
3. Production of Mycoproteins:
I. Enzyme: α-amylase:
Both fungi (Aspergillus niger and A. oryzae) and bacteria (Bacillus subtilis and B. diastaticus) are used in the production of amylases. They are of different types like α-, β- and glucamylases. These are used in different purposes.
Such as:
1. Preparation of sizing agents in textile industry,
2. Removal of spots on cloths used in laundry,
3. Production of chocolate and corn syrup,
4. Production of bread, and
5. Used in alcohol industry.
II. Amino acid: L-tryptophan:
L-tryptophan is an amino acid with non- polar side chains, produced commercially by a mutant (Px-115-97) of Corynebacterium gluta- micum, 12gm/litre in the molasses medium.
Composition of medium:
(a) 10% reducing sugars as invert (as cane molasses).
(b) 0.05% KH2PO4.
(c) 0.05% K2HPO4.
(d) 0.025% MgSO4.
(e) 2% (NH4)2SO4.
(f) 1% corn-steep liquor.
(g) 2% CaCO3.
pH should be maintained at 7.2.
Function:
The amino acid has several uses in both plant and human beings.
In plants:
1. Tryptophan acts as precursor of IAA, which controls plant growth and development.
In human beings:
1. The L-tryptophan is converted into either Serotonin or Tryptamine. Both the compounds help in action of central nervous system and also in neurotransmission.
2. The Serotonin (50H tryptamine) may be converted into Melatonin (Pineal gland hormone), which regulates the seasonal breeding.
3. It also helps in normal growth.
III. Vitamin: Riboflavin:
Riboflavin is an important vitamin produced through microbial fermentation. It is produced by Ashbya gossypii, a member of Phylum Ascomycota. The growth medium contains glucose, soybean oil, glycine and inorganic salts, seeded with A. gossypii. The medium is incubated aerobically at 35°C which gives an yield of riboflavin at about 4.25 gms/litre.
IV. Antibiotic: Griseofulvin:
Griseofulvin is an antibiotic, obtained from Penicillium griseofulvin.
The antibiotic is used in the treatment of superficial fungal infections and also for systemic mycoses. The drug is used orally.
V. Recombinant protein: hepatitis B vaccine:
Production of recombinant vaccines requires the identification of genes for desired antigens and cloning into suitable vectors. Vectors are then introduced into the suitable host for genetic expression. Though this method has several advantages, the disadvantage lies with the low level of immunogenicity (i.e., recombinant proteins).
4. Production of Hepatitis B Vaccine:
After infection, the hepatitis B virus (HBV) fails to grow in host and also in cultured cells. This character has been explained to be due to inhibition of its molecular expression and development of vaccine. Plasma of human being contains antigens at different amounts.
Three types of viral proteins found to be antigenic are:
(a) Viral surface antigen (HBsAg),
(b) Viral core antigen (HBcAg), and
(c) e-antigen (HBeAg).
The gene HBsAg contains 6bp long sequence preceding the AUG that synthesises N- terminal methionine. During production of HBV vaccine, initially HBsAg gene of the virus is cloned and inserted into the PMA56 plasmid of yeast with the help of EcoRI. The yeast cells have the ability of secreting glycosylate protein.
The gene HBsAg is inserted near the alcohol dehydrogenase (ADH) I promoter. The recombinant plasmid is inserted into the yeast cells. The transformed yeast cells are then grown in the tryptophan-free medium. The transformed cells are selected and culture of cloned yeast cells are developed. By mass culturing and isolation, it is now possible to get the antigenic material in large scale and to use it as vaccine.
The inserted gene produces particle similar to 22 pm particle of HBV as these particles are produced in serum of HBV patients. The particles that developed by HBsAg gene and those isolated from HBV-infected cells of patients have similarity in structure and high immunogenicity, which made it possible to use the recombinant product as vaccine against hepatitis B virus infection.
Term Paper # 10. Life Cycle Pattern of Fungi:
Fungi show much variation in their life cycle patterns. Some are haploid, others may be mostly dikaryotic or share both haploid and diploid equally etc.
Observing the above variations, Raper (1954, 1966) recognised and reported the presence of 7 basic types of life cycle in fungi:
i. Asexual Cycle:
In this type, sexual reproduction does not take place or has not yet been observed. This type is found in all members of Fungi imperfect and in some members of other groups like Penicillium notatum.
In many sterile forms, the benefits of sexuality are provided through parasexual recombination as was reported by Pontecorvo and Roper (1952).
ii. Haploid Cycle:
The dominating part is haploid and the diploid phase is restricted to the zygote nucleus only. The meiosis immediately takes place after nuclear fusion i.e., zygote formation. This type of cycle is found in Mucorales, some members of Ascomycotina etc.
iii. Haploid Cycle with Limited Dikaryotic Phase:
This type is almost alike to haploid cycle but paired conjugant nuclei in haploid segment (i.e., the dikaryotic phase) undergo synchronous division for variable time’s i.e., either lesser or greater period of time. This type is found in higher Ascomycotina, e.g., Neurospora etc.
iv. Haploid Dikaryotic Cycle:
This type is like that of the previous one, but once the dikaryon is formed, it continues for unrestricted period of time. Thereby, the dikaryotic phase persists as the longest phase of their life cycle. This type is found in Basidiomycotina (e.g., Agaricus, Polyporus etc.), except many members of Ustilaginales i.e., smut fungi.
v. Dikaryotic Cycle:
In this type, the ascospores or basidiospores are formed after meiosis, that are fused immediately and form dikaryon. The dikaryotic phase continues till meiosis. This type is found in Ustilaginales (smut fungi) and sometimes in yeasts.
vi. Haploid-Diploid Cycle:
In this type, both haploid and diploid phase take place alternately and equally share with each other. This is found in section Eu-Allomyces of the genus AHomyces, Ascocybe grovesii of Endomycetales. This is a rare type in fungi.
vii. Diploid Cycle:
In this type, haploid phase is restricted only in gametes. This is found in Saccharomyces cerevisiae, true slime molds, and majority of Oomycetes.