Experimental studies on the penetration of dermatophytes into the human stratum corneum and the effects of washing

Abstract

After several types of dermatophytes were applied to the stratum corneum obtained from a healthy human heel, we evaluated the penetration speed of fungal elements into the stratum corneum and the effect of washing its surface. We designed the following two environments, assuming tinea pedis after applying fungal elements to the surface of the stratum corneum. The samples were incubated under conditions simulating the daily life of those who wear socks in a house: (1) 90% humidity for 8 hours, and 100% humidity for 16 hours, and those who have bare feet in a house: (2) 80% humidity for 8 hours and 100% humidity for 16 hours. We took the samples out every 24 hours and made observations by PAS stain and scanning electron microscope before and after washing them. Although fungal elements were not removable in (1), even if washed one day later, they could be removable one and two days later in (2). We suggest that fungal elements were easily removable even if dermatophytes had begun to penetrate the surface of the stratum corneum, because the soles retained a low humidity when the shoes were removed, and the soles and interdigital regions were washed every day. Moreover, in an experiment assuming tinea corporis, fungal elements were applied to the cutting side of the stratum corneum, incubated at 80% humidity, and observed after PAS stain. The penetration of Trichophyton tonsurans to the cutting side of the stratum corneum began in 0.5 days, which was sooner than other dermatophytes. We believe this is one factor of the latest expansion of T. tonsurans infection in Japan.