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Basic Mycology by Abid...................
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- 2. Mycology Study offungi called Mycology Fungi or fungus kingdom of usually multicellular eukaryotic organisms that are heterotrophs (cannot make their own food) and have important roles in nutrient cycling in an ecosystem.
- 3. Common characteristics offungi Eukaryotic Lack chlorophyll Heterotrophic Most are Multicellular, Unicellular (yeast) cell wall contain chitin . Most are saprophytes some are parasites
- 4. Fungus cell structure The fungal cell wall consists primarily of chitin. Chitin is a polysaccharide composed of long chains of N- acetylglucosamine. The fungal cell wall contains other polysaccharides as well, the most important of which is β-glucan, a long polymer of D-glucose. The fungal cell membrane contains ergosterol, in contrast to the human cell membrane, which contains cholesterol.
- 5. Fungus cell Thereare two types of fungi: yeasts and molds. Yeasts grow as single cells that reproduce by asexual budding. Molds grow as long filaments (hyphae) and form a mat (mycelium). Some hyphae form transverse walls (septate hyphae), whereas others do not (non septate hyphae). Non septate hyphae are multinucleated .
- 6. Metabolism and Habitat Most fungi are obligate aerobes; some are facultative anaerobes; but none are obligate anaerobes. All fungi require a preformed organic source of carbon—hence their frequent association with decaying matter that’s why call saprophytes The natural habitat of most fungi is, therefore, the environment. An important exception is Candida albicans, which is part of the normal human flora.
- 7. Reproduction Some fungireproduce sexually by mating and forming sexual spores (e.g., zygospores, ascospores, and basidiospores). Zygospores are single large spores with thick walls. Most fungi of medical interest propagate asexually by forming conidia (asexual spores) from the sides or ends of specialized structures. The shape, color, and arrangement of conidia aid in the identification of fungi.
- 8. Example of someimportant conidia Arthrospores,which arise by fragmentation of the ends of hyphae and are the mode of transmission of Coccidioides immitis; Chlamydospores, which are rounded, thick-walled, and quite resistant (the terminal chlamydospores of C. albicans aid in its identification); Blastospores, which are formed by the budding process by which yeasts reproduce asexually (some yeasts, e.g., C. albicans, can form multiple buds that do not detach, thus producing sausage like chains called pseudo hyphae, which can be used for identification); Sporangiospores, which are formed within a sac (sporangium) on a stalk by molds such as Rhizopus and Mucor.
- 9. PATHOGENESIS The responseto infection with many fungi is the formation of granulomas. Granulomas are produced in the major systemic fungal diseases (e.g., coccidioidomycosis, histoplasmosis, and blastomycosis, as well as several others). The cell-mediated immune response is involved in granuloma formation. Acute suppuration, characterized by the presence of neutrophils in the exudate, also occurs in certain fungal diseases such as aspergillosis and sporotrichosis. Fungi do not have endotoxin in their cell walls and do not produce bacterial-type exotoxins
- 10. Pathogenesis cont…. Intactskin is an effective host defense against certain fungi (e.g., Candida, dermatophytes), but if the skin is damaged, organisms can become established. Fatty acids in the skin inhibit dermatophyte growth, and hormone-associated skin changes at puberty limit ringworm of the scalp caused by Trichophyton. The normal flora of the skin and mucous membranes suppress fungi. When the normal flora is inhibited (e.g., by antibiotics), overgrowth of fungi such as C. albicans can occur. In the respiratory tract, the important host defenses are the mucous membranes of the nasopharynx, which trap inhaled fungal spores, and alveolar macrophages. Circulating IgG and IgM are produced in response to fungal infection, but their role in protection from disease is uncertain. The cell-mediated immune response is protective; its suppression can lead to reactivation and dissemination of asymptomatic fungal infections and to disease caused by opportunistic fungi.
- 11. Transmission and GeographicLocation of Some Important Fungi
- 12. FUNGAL TOXINS &ALLERGIES In addition to mycotic infections, there are two other kinds of fungal disease: Mycotoxicoses, caused by ingested toxins and Allergies to fungal spores. The best-known mycotoxicosis occurs after eating Amanita mushrooms. These fungi produce five toxins, two of which—amanitin and phalloidin—are among the most potent hepatotoxins. Another mycotoxicosis, ergotism, is caused by the mold Claviceps purpurea, which infects grains and produces alkaloids. Other ingested toxins, aflatoxins, are coumarin derivatives produced by Aspergillus flavus that cause liver damage and tumors in animals and are suspected of causing hepatic carcinoma in humans. Aflatoxins are ingested with spoiled grains and peanuts and are metabolized by the liver to the epoxide, a potent carcinogen. Aflatoxin B1 induces a mutation in the p53 tumor suppressor gene, leading to a loss of p53 protein and a consequent loss of growth control in the hepatocyte.
- 13. LABORATORY DIAGNOSIS Thereare four approaches to the laboratory diagnosis of fungal diseases: Direct microscopic examination, culture of the organism, DNA probe tests, serologic tests. Direct microscopic examination of clinical specimens such as sputum, lung biopsy material, and skin scrapings depends on finding characteristic asexual spores, hyphae, or yeasts in the light microscope. The specimen is either treated with 10% potassium hydroxide (KOH) to dissolve tissue material, leaving the alkali-resistant fungi intact, or stained with special fungal stains. Methenamine silver stain is also useful in the microscopic diagnosis of fungi in tissue.
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