Medically Important Fungi

An accurate taxonomic scheme of the major fungal pathogens and contaminants encountered in medicine is not presented here; instead, a simpler but perhaps more useful organization will be applied. Morphology is particularly helpful in speciating filamentous fungi and Pneumocystis carinii (see Other fungi, below), but may also play a role in identifying certain yeasts.

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Zygomycetes

A class of fungi that have characteristically broad, usually aseptate hyphae; zygomycosis is usually characterized by opportunism, invasiveness, and involvement of nasal cavity, paranasal sinuses, and orbit with invasion into the brain (so-called rhinoorbitocerebral mucormycosis), involvement of the lungs, gastrointestinal tract, or skin (the latter especially in burn patients), and occasional dissemination. Its hyphae are generally broader than the hyalohyphomyces and they lack the pigment found in dematiaceous fungi. Their gross appearance is characterized by rapid, plate-covering growth.

Hyalohyphomyces

A large, heterogeneous group of fungi characterized by narrow, septate hyphae that are colorless on microscopic examination. This is a morphologically diverse group.

Dematiaceous fungi

A large, heterogeneous group of fungi characterized by dark colonies grossly and pigmented fungal elements seen on microscopic examination of involved biopsy material.

Dermatophytes

Fungi infecting stratum corneum, hair, and nails. Grossly, colonies often display fluffy or fine texture and are pale colored or white. Grow moderately rapidly to slowly and have narrow, septate hyphae.

Dimorphic fungi

Fungi that characteristically grow as a mold under certain environmental conditions (usually 25-30°C) and as a yeast under other conditions (usually at 35-37°C). Medically important dimorphic fungi can be highly pathogenic; special caution is warranted when handling fungal cultures largely because of the risk of culturing one of these organisms.

Yeasts

Unicellular fungi that reproduce by budding (with rare exceptions). Unlike many of the other fungi presented here, biochemical tests and carbohydrate or nitrate assimilation are disproportionately important for identification.

Other

Pneumocystis carinii is now considered a fungus by many authors. Diagnosis depends upon its characteristic morphology.