Evaluation of microdilution and disk diffusion methods for antifungal susceptibility testing of dermatophytes

A total of 63 dermatophyte strains belonging to 6 species (Trichophyton rubrum (16), T. mentagrophytes (16), T. tonsurans (20), T. violaceum (2), Microsporum canis (7), and Epidermophyton floccosum (2)) were tested for their in vitro susceptibility to a range of antifungal drugs using disk diffusion and Clinical Laboratory Standards Institute M38-A (CLSI, formerly NCCLS) broth microdilution methods. The antifungals used were ciclopirox (CIC), terbinafine (TER), griseofulvin (GRE), fluconazole (FLC), itraconazole (ITR), posaconazole (POS), and ravuconazole (RAV). In the broth microdilution assay terbinafine was found to have the highest activity followed by ravuconazole, posaconazole, ciclopirox, itraconazole, griseofulvin and fluconazole. In the disk diffusion assay terbinafine produced the largest inhibition zone diameters (IZDs) on Dermasel agar media followed by ravuconazole, griseofulvin, posaconazole, and itraconazole. A significant correlation was not observed between the minimum inhibitory concentrations (MICs) and IZDs, but some correlations were observed for POS, RAV, and TER (correlation coefficients r=-0.507, -0.249, -0.267, P<0.05, respectively). MICs obtained by the microdilution method with ITR and GRE did not correlate with IZDs obtained in disk diffusion assays in this study. The ciclopirox (20 microg/disk) and fluconazole (25 microg/disk) did not produce well defined inhibition zone diameters on Dermasel agar medium.