Evaluation of Gradient Concentration Strips for Detection of Terbinafine Resistance in Trichophyton spp.

The number of dermatophytosis cases resistant to terbinafine is increasing all over the world. Therefore, there is a need for antifungal susceptibility testing of dermatophytes for better management of the patients. In the present study, we have evaluated a gradient test (GT) method for testing the susceptibility of dermatophytes to terbinafine. MIC values to terbinafine determined by the EUCAST reference technique and by gradient test were compared for 79 Trichophyton spp. isolates. Overall, MICs were lower with gradient test (MIC50 of 0.002 µg/mL) than with EUCAST (MIC50 of 0.016 µg/mL). Good categorical agreement (>90%) between the 2 techniques was obtained but the essential agreement was variable depending on the batch of gradient test.

Reliability of a terbinafine agar containing method for the screening of dermatophyte resistance.

The increase in terbinafine resistance worldwide due to Trichophyton indotineae underlies the need for surveillance networks, deploying easy to perform methods to correctly identify resistant isolates and thereby reduce their spread. In the present study, we evaluated the performances of the terbinafine containing agar method (TCAM). Different technical parameters, such as culture medium (RPMI agar [RPMIA] or Sabouraud dextrose agar [SDA]) and inoculum size, were evaluated. Our study showed that terbinafine susceptibility determined using the TCAM was reliable and independent of the inoculum or medium used. We then performed a multicenter, blinded study. 5 isolates of T. indotineae and 15 of genotype I or II of T. interdigitale, including 5 terbinafine-resistant isolates (4 T. indotineae and 1 T. interdigitale), were sent to eight clinical microbiology laboratories. Each laboratory analyzed the 20 isolates' terbinafine susceptibility by the TCAM using both culture media. TCAM allowed all participants to correctly determine the terbinafine susceptibility of analyzed isolates without prior training. All participants agreed that the dermatophyte tested, regardless of species or genotype, grew better on SDA than on RPMIA medium but accumulated fungal growth after 14 days eventually minimized the effect of this difference. In conclusion, TCAM is a reliable, easy to perform screening method for assessing terbinafine resistance. However, despite good performances, TCAM is a qualitative method and minimal inhibitory concentrations must be determined by the European Committee for Antimicrobial Susceptibility Testing standardized method to follow the evolution of terbinafine resistance levels.

The increase in terbinafine resistance worldwide due to Trichophyton indotineae underlies the need for surveillance networks. The terbinafine containing agar method is a reliable and easy-to-use tool in clinical microbiology laboratories. It can be used to rapidly screening resistant isolates, thereby reducing their spread.

Terbinafine Resistance in Dermatophytes: A French Multicenter Prospective Study.

In recent years, we have moved from the sporadic description of terbinafine-resistant (TerR) Trichophyton spp. isolates to the Indian outbreak due to T. indotineae. Population flows have spread TerR worldwide, altering local epidemiology. We conducted a prospective multicentric study to determine the relative frequency of TerR isolates in France (Paris area) and of the newly introduced T. indotineae species. TerR isolates were screened by the terbinafine-containing-agar-medium (TCAM) method and confirmed by EUCAST. Sequencing methods were used to identify isolates to the species/genotype level and to analyze substitutions in the squalene epoxidase gene (SQLE). In total, 3 isolates out of 580 (T. rubrumn = 1; T. interdigitalen = 1; T. indotineaen = 1) grew on TCAM, showed terbinafine resistance by EUCAST and harbored the Phe397Leu (n = 2) or Leu393Ser (n = 1) substitution in the SQLE. ITS-sequencing of isolates of the T. mentagrophytes/interdigitale complex (n = 125) revealed a relative frequency of 4.8% for T. indotineae and the presence of T. mentagrophytes genotype VII. Despite the detection of terbinafine resistance, isolates from this complex remained susceptible to itraconazole, voriconazole and amorolfine. Terbinafine resistance is present in France and the dermatophyte epidemiology is changing. Efficient systems must be implemented to survey the evolution of newly introduced species and to identify TerR isolates.