Experimental penetration of Trichophyton mentagrophytes into human stratum corneum.

We present confirmation of the experimental penetration of Trichophyton mentagrophytes into human stratum corneum under designated conditions of temperature and humidity. When stratum corneum, obtained from healthy human heel region, was incubated at 100% humidity, mycelium was observed in the corneum layer on day 2 at 35 degrees C and 27 degrees C, and on day 4 at 15 degrees C. At 90% humidity, the hyphae penetrated into the stratum corneum on day 4 at 35 degrees C, and on day 6 at 27 degrees C. Whereas, at 80% humidity, no fungal elements were observed in the stratum corneum at both 27 degrees C and 35 degrees C for up to 7 days. These data suggested that humidity was a more important environmental factor for penetration than temperature, and that at least 90% humidity is necessary for dermatophytes to penetrate into the stratum corneum within a few days. Mean humidity in the interdigital space between the fourth and fifth toes was found to be approximately 98%.