EUCAST Susceptibility testing of microconidia-forming dermatophytes
Antifungal drug resistance in dermatophytes was first reported shortly after the turn of the millennium and has today been reported in Trichophyton and occasionally in Microsporum, but not in Epidermophyton species. Although drug resistance in dermatophytes is not routinely investigated, resistance in Trichophyton spp. is increasingly reported worldwide. The highest rates are observed in India (36% and 68% for terbinafine (MIC ≥4 mg/L) and fluconazole (MICs ≥16 mg/L), respectively), and apparently involve the spread of a unique clade related to the T. mentagrophytes/T. interdigitale complex.
Objectives: The EUCAST-AFST (European Committee on Antimicrobial Susceptibility Testing Subcommittee on Antifungal Susceptibility Testing) has developed a new method (E.Def 11.0) for the antifungal susceptibility testing against microconidia-forming dermatophytes including tentative MIC ranges for QC control strains and tentative breakpoints against T. rubrum and T. interdigitale. The method is based on the EUCAST microdilution method for moulds but significant differences include: a) an altered test medium selective for dermatophytes; b) an altered incubation time and temperature; and c) a different endpoint criterion (spectrophotometric determination) of fungal growth. The method can easily be implemented in laboratories already performing EUCAST microdilution methods. The method was recently validated for terbinafine, voriconazole, itraconazole and amorolfine against wildtype as well as molecularly characterised terbinafine resistant mutants of T. rubrum and T. interdigitale in a multicentre study (1). Modes for wildtype and mutant populations were ≥7 two-fold-dilutions apart in all cases. Excluding one I121M/V237I T. rubrum mutant and two mixed wildtype/mutant T. interdigitale specimens, the number of VMEs were: T. rubrum: CC-visual, 1/67 (1.5%); CC-spec-90%, 3/59 (5.1%); CC-spec-50%, 1/67 (1.5%), and T. interdigitale: none. Voriconazole and amorolfine MICs were quite uniform but trailing growth complicated itraconazole visual and spec-90% MIC determination.
Conclusions: Although none of the laboratories were experienced in dermatophyte testing, error rates were low. We recommend the CC-spec-50% method and provide QC-ranges and WT-ULs for wildtype/non-wildtype classification. This standardised procedure with automated endpoint reading will allow broader implementation of susceptibility testing of dermatophytes and thus facilitate earlier appropriate therapy. This is important, as resistance is rapidly emerging and largely underdiagnosed.
Implications: This standardised procedure with automated endpoint reading will allow broader implementation of susceptibility testing of dermatophytes and thus facilitate earlier appropriate therapy. This is important, as resistance is rapidly emerging and largely underdiagnosed.