Influence of incubation time in the in vitro antifungal activity of terbinafine against Trichophyton rubrum.

Antifungal susceptibility tests are influenced by a number of technical variables, including inoculum size, temperature, medium formulation and duration of incubation. In this study, we have compared the in vitro susceptibility of 20 strains de Trichophyton rubrum against clotrimazole and terbinafine, and studied the influence of incubation time on MICs of both drugs. The assay was performed by agar dilution, the medium used was Saboraud glucose agar without an antibiotic. The MIC was evaluated at 15, 30 and 45 days' incubation. The MICs ranges of terbinafine were 0.002 to 0.0975 microg/ml, 0.0975 to 0.39 microg/ml and 0.195 to 0.39 microg/ml at 15, 30 and 45 days' incubation, respectively. The MICs ranges of clotrimazole at 15, 30 and 45 days' incubation were 3.125 to 50 microg/ml. T. rubrum was markedly more susceptible to terbinafine than to clotrimazole (p<0.001). In addition, we observed that an increase of incubation time causing an increase in the MIC value of terbinafine (p<0.001), but MIC values for clotrimazole remained constant with time (p=0.464). In conclusion, the MIC is dependent on reading time and the antifungal compound.