Antifungal susceptibility and mutations in the squalene epoxidase gene in dermatophytes of the Trichophyton mentagrophytes species complex.

Background: 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 is ongoing in India, and it spreads to several European countries.Objective: To investigate the molecular background of the squalene epoxidase (SQLE) gene in order to understand the risk of emergence and spread of multi-resistance in dermatophytes.Methods: Antifungal susceptibility for 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 s. str (n=35), T. indotineae (n=64 representing the Indian clone) and T. interdigitale s. str. (n=36).Results: High minimum inhibitory concentrations (MICs) of terbinafine (>16 mg/L) 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/L) were noted in 2 (3%) T. indotineae isolates; these isolates lead to Phe415Val and Leu393Ser of the SQLE gene. Stability of the effect of the mutations was proven by serial transfer on drug-free medium. Substitutions of Lys276Asn and Leu419Phe were found in susceptible T. mentagrophytes strains. The double mutant Phe377Leu/Ala448Thr showed higher MIC values for triazoles.Conclusions: High MICs of terbinafine are as yet limited to T. indotineae, and are unlikely to be distributed through the T. mentagrophytes species complex by genetic exchange.