Effect of temperature, humidity and minor injury to the penetration of dermatophytes into human stratum corneum

Abstract

We evaluated the minimum period for penetration of Trichophyton mentagrophytes into human stratum corneum using an experimental model of tinea pedis. Fungal elements were applied to the surface of stratum corneum which were obtained from a healthy human heel, and samples were incubated under a designated condition of temperature and humidity. Thirty-five degrees, which is the approximate temperature of the surface of human skin, seemed more suitable for penetration than 27C. The result also indicated that humidity was a more significant factor than temperature. In the following examination using T. mentagrophytes and T. rubrum, the minimum period for penetration at 100% humidity was 1 day with the former and 1.5 days with the latter. At 95% humidity, T. mentagrophytes penetrated into human stratum corneum on day 1.5 and T. rubrum penetrated on day 4. Humidity of the 4th interdigital space in daily life was calculated to be below 95% using a hygro-thermometer. Therefore, to wash the feet daily can prevent development of tinea pedis. When we applied fungal elements to a cut side of stratum corneum, fungi penetrated within a day at 100% and 95% humidity. In addition, fungi were able to penetrate at below 85% humidity, which they were unable to do when applied them to the surface side. We stress that minor injury of stratum corneum is also a significant factor of the development of tinea pedis.