Easy and rapid screening method to detect the resistance to terbinafine in dermatophytes

Objectives:
Treatment resistant dermatophytoses caused by Trichophyton rubrum (T. rubrum) or Trichophyton indotineae (T. indotineae) have recently emerged as a global public health issue. This phenomenon is spreading, and is particularly important in endemic areas such as India. In Europe and other industrialized countries, several cases have been reported. Most dermatophyte resistance concerns terbinafine (TERB) and is characterized by point mutations in the squalene epoxidase gene. We here designed a rapid screening method to detect TERB resistance in dermatophytes.
Material and methods:
35 strains of T. rubrum and T. indotineae were selected and were previously characterized for their resistance profile to TERB (Arendrup et al, 2020). 12-wells plates have been designed to screen simultaneously four dermatophytes strains by plate. Columns of these plates are filled with Sabouraud dextrose medium agar (Merck, Germany), containing respectively the following concentrations of TERB (Merck, Germany). 0, 0.05, 0.1 and 0.2 mg/L. 25µl of a 0.5 McFarland dermatophyte’s suspension is then putted into each well. Incubation of the plates at 30°C for 4 or 6 days has been evaluated.
Results:
Our study shows that incubation time of four days seems optimal to screen for TERB resistance among dermatophytes. Indeed we obtained a sensitivity of the method of 100% and a specificity of 95% under these conditions. Only one strain that seemed to have a susceptible profile previously, showed a positive growth even with a concentration of TERB of 0.2 mg/L. After six days of incubation, a higher rate of false positive were observed reducing the specificity of the method.
Conclusion:
As the situation in India is alarming, with the incidence of TERB resistance increasing yearly, the situation can rapidly become problematic in other countries, as in Europe or the USA, if no correct follow up of dermatophytoses is carried out. We here present a valuable tool to rapidly screen for dermatophytes resistance to TERB that can help laboratories to evaluate the current rate of TERB resistance. This will permit to adapt treatments in cases of suspected TERB resistance and rapidly improve the patient management.