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Fungi are classified according to the appearance of microscopy and in culture, and by the method of reproduction.
Growing fungi have branched filaments called hyphae, which make up the mycelium (like branches are part of a tree). Some fungi are compartmented by cross-walls (called septae). Arthrospores are made up of fragments of the hyphae, breaking off at the septae. Asexual spores (conidia) form on conidiophores. The sexual reproductive phase of many fungi is unknown; these are fungi imperfecta.
Superficial fungal infections affect the outer layers of the skin, the nails and hair. Fungi causing superficial fungal infections are dermatophytes of the genera Trichophyton, Microsporum and Epidermophyton.
Dermatophyte infections (tinea) are named according to the site affected.
Skin scrapings and nail clippings are taken to establish or confirm the diagnosis of a fungal infection by microscopy and culture (mycology).
Scrapings of scale are best taken from the leading edge of the rash after the skin has been cleaned with alcohol. Gently remove the surface skin using a blade or curette and place in a sterile container or a black paper envelope.
In the mycology laboratory, the material is examined using potassium hydroxide (KOH) to dissolve keratinocytes, then staining the preparation with blue or black ink to identify KOH-resistant mycelium and arthrospores by direct microscopy. Fungal elements are sometimes difficult to find, especially if the tissue is very inflamed, so a negative result does not rule out fungal infection.
Fungal culture may take several weeks, incubated at 25-30C. The specimen is inoculated into a medium such as Sabouraud's dextrose agar containing cycloheximide and chloramphenicol. A negative culture may arise because:
Repeat negative scrapings if fungal infection appears likely, preferably prior to treatment. Consider a wide differential diagnosis – there are other reasons for ring-shaped or scaly rashes.
Yeasts form a subtype of fungus characterised by clusters of round or oval cells. These bud out similar cells from their surface to divide and propagate. The main yeasts causing skin infections are:
Yeast infection can be confirmed by swabs and by scrapings.
Think about the implications of the following results on mycology:
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