The relationship between the downy and granular forms of Trichophyton mentagrophytes.

Mycologists and practicing dermatologists frequently have designated as distinct species the downy forms of Trichophyton mentagrophytes that are commonly isolated from chronic ringworm of the feet or toenails and the granular forms of this fungus usually isolated from suppurative ringworm of the beard or scalp. The chief difference between the two types of culture is one of gross colonial form. Microscopic structures appear to be identical, although the granular form produces a relatively higher percentage of spores, microconidia and macroconidia, than the downy form. Differences in pathogenicity towards experimental animals have also been noted in studies with cultures of these two colonial types. In general, it is easier to produce characteristic ringworm lesions in the guinea pig with a granular than with a downy strain.