Mycology

Mycology    Dr Fungus   

Mycology is the study of fungi. Fungi are eucaryotes with a cell membrane that has ergosterol as a key component rather than cholesterol. This is an importance difference since many antifungal agents are directed towards ergosterol. Fungi also have a rigid cell wall that contains chitin as well as hyphae which are branching, threadlike tubular filaments.

Most fungi are not pathogens but they can produce toxins and cause allergies (primarily IgE mediated). For example, spoiled peanuts and other grains contain coumarin derivatives produced by Aspergillus flavus and cause liver damage. Claviceps purpura also infects grains and produces lysergic acid diethylamide, commonly known as "LSD".

Fungi can be detected by 1) microscopic examination with the use of fungal stains (ex.India ink) or with KOH which dissolves host tissue leaving alkali resistant fungi or by the unique morphology of fungi (e.g., hyphae), 2) culture (media that inhibits bacterial growth like Sabouraud's agar), 3) DNA probes or 4) serology (detection of specific antibodies).

Fungi are sometimes classified as follows:

• superficial mycoses which live on dead skin and hair and do not elicit an immune response. Superficial mycoses include Tinea versicolr, Tinea nigra, Black and White piedra which grow on hair shafts.

• cutaneous mycoses which are caused by dermatophytes and infect keratinized skin, hair and nails. Cutaneous mycoses invoke an immune response. Diseases are very common and include ringworm or tinea (e.g., Tinea pedis).

• subcutaneous mycoses are lesions in deeper skin layers usually caused by a trauma such as a thorn. For example, sporotrichosis ("Rose grower's disease") typically occurs when one is pricked by a thorn which has the Sporothrix schenckii fungus agent. Other subcutaneous infections are Chromoblastomycosis and Mycetoma.

• systemic mycoses are inheritently infectious and include (1) Histoplasmosis otherwise known as "Darling's" "cave" or "spelunker's" disease. Histoplasmosis occurs in the midwest. Infection is due to inhalation of conidia or hyphal fragments which are phagocytized by pulmonary macrophages. It is usually found on bird dropping and is a dimorphic fungus in that it can exist in the yeast (e.g., macophages)  or mold (e.g., soil) form. Treatment may include itraconazole and/or amphotericin B.  (2) Blastomycosis otherwise known as "Gilchrist's" or "Chicago" disease is endemic in Ohio and the Mississippi Valley. Treatment can again be amphotericin B. (3) Coccidioidomycosis otherwise known as "Posada-Wernicke" or "San Joaquin Valley vever" is endemic in California and Texas. Treatment can again include amphotericin B.

• opportunistic mycoses are pathogenic in immunocompetent individuals and include (1) Aspergillosis such as A. fumigatus, A. flavus, A. niger are ubiquitous in damp basements and vegetation. It can cause blockage of airways from fungal masses. Treatment can include surgery, amphotericin B or itraconazole. (2) Cryptococcosis neoformans (3) Candidiasis is a dimorphic fungus with yeast on mucosal surfaces. It is actually part of one's normal flora in skin, mouth and intestines and stays as yeast but it forms hyphae when invasive. Treatment includes cessation of antibiotic and can also include itraconzaole.