In contrast, the to nm-thick yeast cell wall has three layers. Mannans, galactomannans, and, less frequently, rhamnomannans are responsible for the immunologic response to the medically important yeasts and molds. Structurally, mannan consists of an inner core, outer chain, and base-labile oligomannosides. The outer-chain region determines its antigenic specificity. Determination of mannan concentrations in serum from patients with disseminated candidiasis has proven a useful diagnostic technique.
Cryptococcus neoformans produces a capsular polysaccharide composed of at least three distinct polymers: glucuronoxylomannan, galactoxylomannan, and mannoprotein. On the basis of the proportion of xylose and glucuronic acid residues, the degree to which mannose has side-chain substituents, and the percentage of O-acetyl attachments of the capsular polysaccharides, isolates of C neoformans can be separated into four antigenic groups designated A, B, C, and D.
The capsule is antiphagocytic, serves as a virulence factor, persists in body fluids, and allows the yeast to avoid detection by the host immune system. The outer cell wall of dermatophytes contains glycopeptides that may evoke both immediate and delayed cutaneous hypersensitivity.
In the yeast Candida albicans , for example, the cell wall contains approximately 30 to 60 percent glucan, 25 to 50 percent mannan mannoprotein , 1 to 2 percent chitin located primarily at the bud scars in the parent yeast cell wall , 2 to 14 percent lipid, and 5 to 15 percent protein. The proportions of these components vary greatly from fungus to fungus. Table-M1 summarizes the relationship between cell wall composition and taxonomic grouping of the fungi.
Fungal plasma membranes are similar to mammalian plasma membranes, differing in having the nonpolar sterol ergosterol, rather than cholesterol, as the principal sterol. The plasma membrane regulates the passage of materials into and out of the cell by being selectively permeable. Membrane sterols provide structure, modulation of membrane fluidity, and possibly control of some physiologic events.
The plasma membrane contains primarily lipids and protein, along with small quantities of carbohydrates. The major lipids are the amphipathic phospholipids and sphingolipids that form the lipid bilayer. The hydrophilic heads are toward the surface, and the hydrophobic tails are buried in the interior of the membrane.
Proteins are interspersed in the bilayer, with peripheral proteins being weakly bound to the membrane. In contrast, integral proteins are tightly bound. The lipoprotein structure of the membrane provides an effective barrier to many types of molecules. Molecules cross the membrane by either diffusion or active transport. The site of interaction for most antifungal agents is the ergosterol in the membrane or its biosynthetic pathway.
Polyene antifungal agents such as amphotericin B bind to ergosterol to form complexes that permit the rapid leakage of the cellular potassium, other ions, and small molecules. The loss of potassium results in the inhibition of glycolysis and respiration. Several antifungal agents interfere with ergosterol synthesis. The first step in the synthesis of both ergosterol and cholesterol is demethylation of lanosterol.
The necessary enzymes are associated with fungal microsomes, which contain an electron transport system analogous to the one in liver microsomes. This results in plasma membrane permeability changes and inhibition of growth. Ergosterol may also be involved in regulating chitin synthesis. Inhibition of ergosterol synthesis by antifungal agents can result in a general activation of chitin synthetase zymogen, leading to excessive chitin production and abnormal growth.
Fungi possess microtubules composed of the protein tubulin. This protein consists of a dimer composed of two protein subunits.
Microtubules are long, hollow cylinders approximately 25 nm in diameter that occur in the cytoplasm as a component of larger structures. These structures are involved in the movement of organelles, chromosomes, nuclei, and Golgi vesicles containing cell wall precursors. Microtubules are the principal components of the spindle fibers, which assist in the movement of chromosomes during mitosis and meiosis. When cells are exposed to antimicrotubule agents, the movement of nuclei, mitochondria, vacuoles, and apical vesicles is disrupted.
Griseofulvin, which is used to treat dermatophyte infections, binds with microtubule-associated proteins involved in the assembly of the tubulin dimers. By interfering with tubulin polymerization, griseofulvin stops mitosis at metaphase. The destruction of cytoplasmic microtubules interferes with the transport of secretory materials to the cell periphery, which may inhibit cell wall synthesis.
The fungal nucleus is bounded by a double nuclear envelope and contains chromatin and a nucleolus. Fungal nuclei are variable in size, shape, and number. The DNA and associated proteins occur as long filaments of chromatin, which condenses during nuclear division.
The number of chromosomes varies with the particular fungus. Within the cell, 80 to 99 percent of the genetic material occurs in chromosomes as chromatin, and approximately 1 to 20 percent in the mitochondria.
In some isolates of Saccharomyces cerevisiae , up to 5 percent of their DNA can be found in nuclear plasmids. Yeasts are fungi that grow as solitary cells that reproduce by budding see ch. Yeast taxa are distinguished on the basis of the presence or absence of capsules, the size and shape of the yeast cells, the mechanism of daughter cell formation conidiogenesis , the formation of pseudohyphae and true hyphae, and the presence of sexual spores, in conjunction with physiologic data. Morphology is used primarily to distinguish yeasts at the genus level, whereas the ability to assimilate and ferment various carbon sources and to utilize nitrate as a source of nitrogen are used in conjunction with morphology to identify species.
Yeasts such as C albicans and Cryptococcus neoformans produce budded cells known as blastoconidia. The formation of blastoconidia involves three basic steps: bud emergence, bud growth, and conidium separation. During bud emergence, the outer cell wall of the parent cell thins. Concurrently, new inner cell wall material and plasma membrane are synthesized at the site where new growth is occurring. New cell wall material is formed locally by activation of the polysaccharide synthetase zymogen.
The process of bud emergence is regulated by the synthesis of these cellular components as well as by the turgor pressure in the parent cell. Mitosis occurs, as the bud grows, and both the developing conidium and the parent cell will contain a single nucleus. A ring of chitin forms between the developing blastoconidium and its parent yeast cell.
This ring grows in to form a septum. Separation of the two cells leaves a bud scar on the parent cell wall. The bud scar contains much more chitin than does the rest of the parent cell wall. When the production of blastoconidia continues without separation of the conidia from each other, a pseudohypha, consisting of a filament of attached blastoconidia, is formed.
In addition to budding yeast cells and pseudohyphae, yeasts such as C albicans may form true hyphae. Candida albicans may form a budding yeast, pseudohyphae, germ tubes, true hyphae, and chlamydospores. A number of investigators are interested in germ tube formation because it represents a transition between a yeast and a mold.
Generally, either low temperature or pH favors the development of a budding yeast. Other substances such as biotin, cysteine, serum transferrin, and zinc stimulate dimorphism in this yeast. Approximately 20 percent of the C albicans yeast cell wall is mannan, whereas the mycelial cell wall contains a substantially smaller amount of this sugar. Candida albicans has three serotypes, designated A, B, and C. These are distinguished from each other on the basis of their mannans. The antigenic determinant for serotype A is its mannoheptaose side chain.
In serotype B, it is the mannohexaose side chain. Serotype B tends to be more resistant to 5-fluorocytosine than is serotype A. It has been suggested that these glucans may impede the access of amphotericin B to the plasma membrane. Molds are characterized by the development of hyphae see ch.
Hyphae elongate by a process known as apical elongation, which requires a careful balance between cell wall lysis and new cell wall synthesis. Because molds are often differentiated on the basis of conidiogenesis, structures such as conidiophores and conidiogenous cells must be carefully evaluated.
Some molds produce special sac-like cells called sporangia, the entire protoplasm of which becomes cleaved into spores called sporangiospores. Sporangia are typically formed on special hyphae called sporangiophores. A number of medically important fungi express themselves phenotypically as two different morphologic forms, which correlate with the saprophytic and parasitic modes of growth. Such fungi are called dimorphic fungi. In contrast, others use dimorphic for any fungus that can exist as two different phenotypes, regardless of whether it is pathogenic.
Examples of medically important dimorphic fungi include Blastomyces dermatitidis hyphae and yeast cells and Coccidioides immitis hyphae and spherules. A number of external factors contribute to the expression of dimorphism. Increased incubation temperature is the single most important factor. Increased carbon dioxide concentration, which probably affects the oxidation-reduction potential, enhances the conversion of the mycelial form to the tissue form in C immitis and Sporothrix schenckii.
Some fungi require a combination of these factors to induced dimorphism. The conversion of the mycelial form of Blastomyces dermatitidis to the large, globose, thick-walled, broadly based budding yeast form requires only increased temperature. Hyphal cells enlarge and undergo a series of changes resulting in the transformation of these cells into yeast cells.
The cells enlarge, separate, and then begin to reproduce by budding. Increased temperature, nutrition, and increased carbon dioxide are important for the production of sporulating spherules. A uninucleate arthroconidium begins to swell and undergo mitosis to produce additional nuclei.
Once mitosis stops, initiation of spherule septation occurs. The spherule is segmented into peripheral compartments with a persistent central cavity. Uninucleate endospores occurring in packets enclosed by a thin membranous layer differentiate within the compartments. As the endospores enlarge and mature, the wall of the spherule ruptures to release the endospores Fig. Pairs of closely appressed endospores that have not completely separated from each other may resemble the budding yeast cells of B dermatitidis.
Development of the spherule of Coccidioides immitis from an arthroconidium. Academic Press, San Diego, , with permission. Dimorphism in Histoplasma capsulatum involves three stages. In the first stage, induced by an increase in temperature, respiration ceases and the level of cytochromes decreases. During the second stage of the mycelial-to-yeast conversion, cysteine or other sulfhydryl-containing compounds are required.
Shunt pathways are initiated that restore the appropriate cytochrome levels, which supply the needed ATP. Cysteine is also required for the yeast form to grow. The final stage is characterized by normal cytochrome levels and respiration as the yeast grows and reproduces. The conversion of terminal or intercalary hyphal cells to a yeast form requires 3 to 14 days.
In tissue, H capsulatum proliferates within giant cells. Yeast cells of H capsulatum have been divided into two chemotypes. Chemotype 2 correlates with serotypes 1 and 4. It is difficult to assess the importance of the chemotypes, because only a few isolates have been studied. A great deal of work has been done with the mycelial-to-yeast conversion of Paracoccidioides brasiliensis. In tissue the yeast is characterized by multiple budding. Series of smaller yeast daughter cells attached by narrow tubular necks are formed around a large central cell.
Hyphal cells first swell and then separate from each other. The separated cells begin to bud, resulting in a yeast growth. As with H capsulatum , growth stops briefly as a result of increased temperature. The yeast cell wall of P brasiliensis has three layers and is approximately to nm thick. Only the yeast form has this glucan. Penicillium marneffei is a dimorphic fungus that is becoming extremely important as a pathogen in AIDS patients living in Southeast Asia.
The fungus has been recovered from soil associated with plants such as bamboo. In tissue, the fungus forms yeast cells that divide by fission. Like H. The last dimorphic fungus to be considered is Sporothrix schenckii. In this species, mycelial-to-yeast conversion is enhanced by increased carbon dioxide, increased temperature, and nutrition.
It has been suggested that some product of carbon dioxide fixation may be required for development of the yeast form. Unlike the other dimorphic fungi capable of producing a yeast form, S schenckii initially produces yeast cells by direct budding from hyphae. Skip to content. Fungi have been utilized for thousands of years and their importance in agriculture, medicine, food production and the environmental sciences is well known.
New advances in genomic and metabolomic technologies have allowed further developments in the use of fungi in industry and medicine, increasing the need for a compilation of new applications, developments and technologies across the mycological field.
Applied Mycology brings together a range of contributions, highlighting the diverse nature of current research. Chapters include discussions of fungal associations in the environment, agriculture and forestry, long established and novel applications of fungi in fermentation, the use of fungi in the pharmaceutical industry, the growing recognition of fungal infections, current interests in the use fungal enzymes in biotechnology and the new and emerging field of myconanotechnology.
They have immense potential and comprise a myriad of useful bioactive compounds. Fungi feature in a wide range of diverse processes and applications in modern agriculture, the food science industry, and the pharmaceutical industry. In the food and drink arena, the role of fungi is historically important in the form of mushrooms and in fermented foods as yeasts for baking and brewing. These roles are supplemented by the use of fungal food processing enzymes and additives, and more recently in the development of protein-based foodstuffs from fungi.
Additionally, they are used in the formulation of biofertilizers and biopesticides used as biostimulants and bioprotectants of crops.
The practical use of newer techniques such as genetic recombination and robotics have revolutionized the modem agricultural biotechnology industry, and have created an enormous range of possible further applications of fungal products. Myco-materials created from mycelia the root-like parts of fungi are gaining attention as a sustainable alternative for a wide range of materials.
They are being used as insulation, sustainable packaging, foam inserts, and even "eco-leather. In addition, medicinal uses of fungal species have been historically recorded as important agents in the pharmaceutical sciences. The potential for myco-materials seems limitless.
The field of mycology and its application has become an increasingly important component in the education of industrial biotechnology. This volume explains both the basic science and the applications of mycology and bio-resource technology with special emphasis on entrepreneurial applications. It offers a complete, one-stop resource for those interested in microbiology, food and agricultural science, medical mycology, and for those in industrial biotechnology.
Also covered is the use of molds to biologically control insects that yearly cause enormous crop losses and a consequent drain in the economy of the nations of the world.
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