Diagnosis and Identification of Dermatophytes

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Diagnosis and Identification of Dermatophytes

Many of these species can be detected by direct microscopic exam, but are best identified through culturing the organism. Often the initial growth must be subcultured to a sporulation medium such as cornmeal agar or potato dextrose agar to induce sporulation. Please see the following table for microscopic and macroscopic appearance, as well as other defining features:
OrganismClinical CommentsMacroscopic AppearanceMicroscopic AppearanceOther Comments
Epidermatophyton floccosumCause of tinea cruris and pedisFolded, khaki green center with yellow periphery. Reverse yellow/brown with folds.Large, two to four-celled, club-shaped, multiseptate macroconidia are produced. No microconidia.Sensitive to cold; don’t refrigerate specimen; can be acquired by shared bath towels.
Microsporum canis
Zoonotic; acquired from dogs Center white to buff, membranous with feathery periphery, yellow periphery. Reverse yellow/orange.
(image courtesy of University of Adelaide)
Rough, thick walled, multi-septate, echinulate macroconidia called “aleurospores” (detached by the lysis or fracture of the wall of the attachment cell.)
Microconidia are present but rare.
Rapid growing
M. gypseumFree-living in soilCinnamon color and powdery; reverse light tan. Rough, thick, multi-septate, spindle shape macroconidia. Rare microconidia.
Rapid growing. Usually acquired from the soil, not other humans.
Trichophyton mentagrophytes
Common isolateVary from white granular to fluffy; reverse buff or reddish brown.Round microconidia usually in grape-like clusters. Thin walled, club shaped macroconidia.Urease- positive
T. tonsurans
Cause of epidemic tinea capitisWhite-tan-rust suede-like or powdery. Reverse yellow to tan or rust-red.Microconidia tear drop or club shaped with flat bottoms. Macroconidia are rare.Slow growing
T. verrucosum
Often acquired from cattleWhite velvety with rugal folds.Chlamydoconidia in chains; microconidia are tear drop shape but rare.Slow growing
T. rubrum
Common isolate; cause of feet & nail infection (Tinea Pedis)White/pink; rugal folds. Reverse side becomes wine or red color with age. Tiny microconidia along sides of hyphae. Resemble “birds on a fence”:
Macroconidia usually absent
Slow growing Urease- negative