Comparison between the Standardized Clinical and Laboratory Standards Institute M38-A2 Method and a 2,3-Bis(2-Methoxy-4-Nitro-5-[(Sulphenylamino)Carbonyl]-2 H -Tetrazolium Hydroxide- Based Method for Testing Antifungal Susceptibility of Dermatophytes

Abstract

In this study, we determined the utility of a 2,3-bis(2-methoxy-4-nitro-5-[(sulfenylamino)carbonyl]-2 H -tetrazolium hydroxide (XTT)-based assay for determining antifungal susceptibilities of dermatophytes to terbinafine, ciclopirox, and voriconazole in comparison to the Clinical and Laboratory Standards Institute (CLSI) M38-A2 method. Forty-eight dermatophyte isolates, including Trichophyton rubrum ( n = 15), Trichophyton mentagrophytes ( n = 7), Trichophyton tonsurans ( n = 11), and Epidermophyton floccosum ( n = 13), and two quality control strains, were tested. In the XTT-based method, MICs were determined spectrophotometrically at 490 nm after addition of XTT and menadione. For the CLSI method, the MICs were determined visually. With T. rubrum , the XTT assay revealed MIC ranges of 0.004 to >64 μg/ml, 0.125 to 0.25 μg/ml, and 0.008 to 0.025 μg/ml for terbinafine, ciclopirox, and voriconazole, respectively. Similar MIC ranges were obtained against T. rubrum by using the CLSI method. Additionally, when tested with T. mentagrophytes , T. tonsurans , and E. floccosum isolates, the XTT and CLSI methods resulted in comparable MIC ranges. Both methods revealed similar lowest drug concentrations that inhibited 90% of the isolates for the majority of tested drug-dermatophyte combinations. The levels of agreement within 1 dilution between both methods were as follows: 100% with terbinafine, 97.8% with ciclopirox, and 89.1% with voriconazole. However, the agreement within 2 dilutions between these two methods was 100% for all tested drugs. Our results revealed that the XTT assay can be a useful tool for antifungal susceptibility testing of dermatophytes.