P041 Preliminary evaluation of gradient concentration strips for detection of terbinafine resistance in Trichophyton spp.

Abstract

Objectives Dermatophytosis is the most common superficial fungal infection. Trichophyton rubrum and T. mentagrophytes are the most frequently isolated species, but their incidence varies according to geographical regions. Terbinafine is the main molecule used to treat this type of infection. In recent years, a high incidence of chronic infections, reinfections, and treatment failures due to a newly described specie, T. indotineae, have been reported in India and recently described in Europe. It is currently a public health problem for the management of these infections in this country. Until now, the monitoring of dermatophyte susceptibility to antifungals was rarely performed due to the lack of standardized in vitro tests. Since then, an in vitro technique has been standardized by the European Committee for Antimicrobial Susceptibility Testing (EUCAST) to test terbinafine and other antifungals. Recently, a gradient concentration strip method has been marketed. The aim of this study was to compare terbinafine susceptibility testing by the gradient concentration strip (GCS) method and the EUCAST standardized method. Methods A panel of 47 molecularly identified isolates of T. interdigitale, T. mentagrophytes, and T. indotineae was used. The panel included 39 terbinafine- susceptible isolates and 8 terbinafine resistant isolates for which the squalene epoxidase gene was sequenced. Minimum inhibitory concentration (MIC) of terbinafine was determined using EUCAST microdilution broth method for dermatophytes. Inoculum was supplemented with cycloheximide and chloramphenicol. Final drug concentrations ranged from 0.008 to 8 μg/ml and microtiter plates were incubated at 25°C for 5 days. The MIC was determined spectrophotometrically with a 90% growth inhibition endpoint. MIC of terbinafine was also determined using GCS (Terbinafine Ezy MIC™ Strip, HiMedia, India) on RPMI agar. The plates were incubated for 5 days at 25°C. After incubation, MIC was read by using a complete inhibition endpoint. Isolates were considered wild-type when MIC was ≤ 0.125 μg/ml. Results EUCAST MIC values ranged from 0.008 to 0.0625 μg/mL and from 0.25 to 16 μg/ml for susceptible and resistant isolates, respectively. GCS MIC values ranged from 0.002 to 0.03 μg/ml and 0.125 to >32 for susceptible and resistant isolates, respectively. The categorical agreement (percentage of strains found in the same category) by the two techniques was 98%. Conclusion These preliminary results show that GCS can detect resistance to terbinafine and could be used as a screening method. These results must be confirmed on a larger panel of isolates.