The dermatophytes break down and utilize keratin as a source of nitrogen but are usually incapable of penetrating the subcutaneous tissue, unless the host is immunocompromised; even in this situation, however, penetration into the subcutis is rare. The genus Trichophyton is capable of invading the hair, skin, and nails, whereas the genus Microsporum involves only the hair and skin; the genus Epidermophyton involves the skin and nails. Common species of dermatophytes recovered from clinical specimens, in order of frequency, include Trichophyton rubrum, Trichophyton mentagrophytes, Epidermophyton floccosum, Trichophyton tonsurans, Microsporum canis, and Trichophyton verrucosum,151 The frequency of recovery of these species may differ by geographic local. Other geographically limited species are described elsewhere.
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