Dermatophytes

General Comments

• Microsporum: multicelled macroconidia with thick, rough walls.
• Epidermophyton: microconidia absent; macroconidia typically club-shaped, with thin, smooth walls. Chlamydoconidia are usually also present in older cultures.
• Trichophyton: macroconidia absent or present in small numbers; when present, they are pencil-shaped and thin, with smooth walls. 1-2 μ microconidia are usually abundant

Dermatophytes: Direct Examination of Hyphae

• Narrow, 2-3 μ thick
• Regular
• Rectangular arthrospores sometimes seen
• Found only in skin, nail scrapings, and hair

Microsporum canis

• Colonies grow at moderate rates and are cottony or wooly colonies with lemon-yellow pigment around the periphery or on the underside; radial grooves may also be present. The reverse may become yellow-brown with age
• Barrel-shaped, rough macroconidia have pointed ends which turn slightly to one side at the tip and have greater than 6 cells

FIG. 1. Microsporum canis in slide culture.

Microsporum gypseum

• Colonies grow moderately rapidly and darken from buff to cinnamon brown, often with granular to sugary texture imparted by heavy sporulation
• Macroconidia more numerous than M. canis, are less barrel-shaped, and have rounded tips
• Unlike M. canis, M. gypseum macroconidia have 6 or fewer cells

FIG. 1. Microsporum gypseum in slide culture. Multiple macroconidia are present.

FIG. 2. Microsporum gypseum in slide culture. Note that macroconidia are more numerous, less barrel-shaped, and contain fewer cells than M. canis.

Microsporum audouinii

• Colonies grow at moderate rates and are gray-white, cream, or tan; flat, spreading, or velvety; and light pink to brown on the reverse of agar plate
• Usually no conidia present; microconidia, if present, resemble those of other dermatophytes
• Macroconidia fusiform, beaked and often have a septae at irregular intervals and a narrowed portion
• Hyphae may be pectinate, irregular
• Apiculate terminal chlamydospores are the most characteristic feature, but are not found in all slide cultures

FIG. 1. Microsporum audouinii in slide culture. Specific features can be difficult to obtain with this species. Final diagnosis may require consultation with a reference laboratory or expert.

Trichophyton verrucosum

• Colonies grow slowly, are cream, tan, or ochre; shield-shaped or convoluted; glabrous or velvety; the reverse may be colorless or yellow
• Microconidia uncommon, pyriform or subglobose
• Macroconidia rare and shaped like a rat tail
• Chlamydospores usually numerous, symmetrical, and in chains

FIG. 1. Trichophyton verrucosum in slide culture.

FIG. 2. Trichophyton verrucosum in slide culture.

Trichophyton tonsurans

• Colonies mature slowly and have variable color, texture, and shape; usually produces a shade of yellow-brown
• Clavate to elongate microconidia; some are usually swollen and attached to conidiophores by short stalks
• Macroconidia are rare, but may occur
• Chlamydoconidia are frequently present in older cultures

FIG. 1. Trichophyton tonsurans in slide culture.

FIG. 2. Trichophyton tonsurans in slide culture. Macroconidia, such as this one, are rare but may be present.

Trichophyton rubrum

• White colony with wine red reverse; red pigment is best seen in corn meal agar and diffuses into culture medium; colonies mature slowly
• Pyriform microconidia are rare to abundant and usually are found along unbranched hyphae or directly from macroconidia
• Macroconidia are absent to rare and are thin and cylindrical or clavate; may form directly on ends of thick hyphae
• Arthroconidia may be present
• Simple biochemical tests may be helpful in distinguishing T. rubrum from T. mentagrophytes. While T. mentagrophytes grows profusely on Dermatophyte milk agar (containing bromcresol purple), T. rubrum growth is restricted. T. mentagrophytes is also strongly urease positive within 7 days whileT. rubrum is urease negative.

FIG. 1. Trichophyton rubrum in slide culture. Pyriform microconidia are abundant in this culture.

Trichophyton mentagrophytes

• Colony of variable appearance; may impart red pigment to agar, similar to T. rubrum; reverse may be tan-brown, colorless, yellow, or red
• Globose to pyriform microconidia are found in clusters or singly along hyphae
• Clavate macroconidia may be present
• Helically coiled hyphal appendages and antler-like hyphae may be present

FIG. 1. Trichophyton mentagrophytes in slide culture.

FIG. 2. Trichophyton mentagrophytes in slide culture; macroconidia as seen here may occasionally be found. Also scattered within this photomicrograph are microconidia.

Epidermophyton floccosum

• Colonies khaki or green-yellow with low aerial hyphae imparting a "suede" appearance
• Microconidia absent
• Macroconidia clavate, with smooth walls, and contain 2-6 cells
• Chlamydoconidia also may be present, especially in older cultures

FIG. 1. Epidermophyton floccosum in slide culture.

Trichophyton equinum

• Colonies cream white to yellow, with reddish to tan centers; reverse deep yellow or red-brown
• Requires nicotinic acid for growth; does not require L-histidine

FIG. 1. Trichophyton equinum in slide culture.