Antifungal Susceptibility and Mutations in the Squalene Epoxidase Gene in Dermatophytes of the Trichophyton mentagrophytes Species Complex

During the past decade, a prolonged and serious outbreak of dermatophytosis due to a terbinafine-resistant novel species in the Trichophyton mentagrophytes-T. interdigitale complex has been ongoing in India, and it has spread to several European countries. The objective of this study was to investigate the molecular background of the squalene epoxidase (SQLE) gene in order to understand the risk of emergence and spread of multiresistance in dermatophytes. Antifungal susceptibility to fluconazole, griseofulvin, itraconazole, ketoconazole, miconazole, naftifine, sertaconazole, and terbinafine was tested in 135 isolates from India, China, Australia, Germany, and The Netherlands. Based on the latest taxonomic insights, strains were identified as three species: T. mentagrophytes sensu stricto (n = 35), T. indotineae (n = 64, representing the Indian clone), and T. interdigitale sensu stricto (n = 36). High MICs of terbinafine (>16 mg/liter) were found in 34 (53%) T. indotineae isolates. These isolates showed an amino acid substitution in the 397th position of the SQLE gene. Elevated MICs of terbinafine (0.5 mg/liter) were noted in 2 (3%) T. indotineae isolates; these isolates lead to Phe(415)Val and Leu(393)Ser of the SQLE gene. The stability of the effect of the mutations was proven by serial transfer on drug-free medium. Lys(276)Asn and Leu(419)Phe substitutions were found in susceptible T. mentagrophytes strains. The Phe(377)Leu/Ala(448)Thr double mutant showed higher MIC values for triazoles. High MICs of terbinafine are as yet limited to T. indotineae and are unlikely to be distributed throughout the T. mentagrophytes species complex by genetic exchange.