Antifungal susceptibility testing of dermatophytes: Development and evaluation of an optimised broth microdilution method

Background: Dermatophytosis is one of the most common infections affecting 3%-17% of the population. Resistance to antifungals so far was not of concern in the therapeutic management. However, recent reports of terbinafine-resistant strains in several countries are worrisome making antifungal susceptibility testing inevitable. Objectives: We aimed to develop and evaluate an optimised broth microdilution assay for antifungal drug susceptibility testing of dermatophytes. Methods: We first studied the effect of different inocula, incubation temperatures and incubation times to establish an optimised assay. Subsequently, we tested 79 clinical strains of 11 dermatophyte species with 13 antifungals. Results: We found inoculating with 0.5-5 × 104 colony forming units (CFU) and incubating at 29°C ± 1°C for 4 days to be appropriate. Terbinafine was the most active antifungal agent with minimum inhibitory concentration (MIC) values ≤ 0.06 µg/mL, expect for one resistant T mentagrophytes strain, which was isolated from an Indian patient. Also, a majority of MICs of other antifungals that are commonly used to treat dermatophytosis were low, except those of fluconazole. Fluconazole MICs do not correlate with the good efficacy in the clinical management. Conclusions: Our assay enables fast and reliable susceptibility testing of dermatophytes with a large panel of different antifungals. This helps to improve the therapeutic management of dermatophytosis by detecting resistant strains. Keywords: antifungal susceptibility testing; broth microdilution; dermatophytes; dermatophytosis; susceptibility tests; terbinafine resistance