S6.2b In vitro susceptibility testing of dermatophytes: toward standardization

Abstract

Abstract S6.2 Resurgence of dermatophytic infections, September 22, 2022, 4:45 PM - 6:15 PM Antifungal treatment-resistant dermatophytosis has been known for years1. It has mainly been reported as sporadic cases with clinical failure to a specific antifungal confirmed by in vitro resistance to antifungal compounds determined by antifungal susceptibility testing (AFST). However, in vivo AFST of dermatophytes is not routinely available in most countries, and, therefore, many clinicians solve the problem by changing the antifungal treatment to another drug class hoping that it will result in clinical response. Unfortunately, cross-resistance revealing concomitantly reduced sensitivity to different classes of drugs including terbinafine and azoles have been reported2,3. Furthermore, an increase of antifungal resistant dermatophytosis has been noted mainly in India and other Asian countries4 but sporadic cases have also been registered in the Middle East, Europe, and North and South America suggesting that this may be the top of the iceberg5-8. This stress the need for a standardized AFST, which can be used routinely in order to surveil the disease spread and implement targeted antifungal treatment.   Molecular-based methods are able to detect a genetic mutation known to cause antifungal resistance (e.g., mutation in the squalene epoxidase gene)2 whereas culture-based AFST methods are able to determine the minimum inhibitory concentration (MIC) of a given drug for a specific clinical isolate. This should ideally enable to classify the isolate as sensitive, intermediate, or resistant to a specific antifungal agent, but unfortunately, it may be difficult to compare results across studies as the interpretations of MIC results are depending on the AFST method used. Following AFST methods have been used to determine the MIC of dermatophytes: E-test, agar dilution, agar disc diffusion, and macro- and microbroth dilution methods9. They differ in several ways including inoculum concentration, incubation temperature, incubation time, different culture media, and end-point criterion of fungal growth (percentage of growth inhibition)9,10. Standardization is therefore important, and currently, two standardized guidelines for in vitro AFST of dermatophytes exist. One is from the European Committee on Antimicrobial Susceptibility Testing (EUCAST)11 and the other is from the Clinical Laboratory Standards Institute (CLSI)12. Both are using microtiter plates and the EUCAST (E.Def 11.0) guideline has included MIC breakpoints for the classification of whether an isolate is susceptible or resistant to a given drug. Unfortunately, breakpoints are only available for a limited number of dermatophytes and antifungals11.   Standardized validated AFST methods will enable us to target the antifungal treatment thereby reducing the risk of ineffective and unnecessary exposure to inappropriate antifungals with potential side effects and reducing the risk of disease spreading. Sources