Microbiology of tinea versicolor.

Lesions and nearby normal skin in 31 patients with tinea versicolor were studied for (1) quantity and composition of microbial flora, (2) quantity of desquamating cells in the horny layer (corneocyte count), and (3) quantity of fungus and proportion of yeasts and filamentous forms. The bacterial species in the lesions were the usual cutaneous residents. Bacterial density was increased fourfold because of the increased surface. Corneocyte count was elevated and an increased rate of turnover was revealed by an eightfold increase in parakeratotic cells. In the lesions, filamentous forms were twice as numerous as yeast forms; on normal skin, yeasts were five times as numerous as filaments. Normal sites yielded more filaments than similar sites in normal subjects. This supports the concept that the cause of tinea versicolor is Pityrosporum orbiculare which becomes pathogenic when it changes from a yeast to a filamentous habitat.