Evaluation of the susceptibility of dermatophytes to antifungal drugs: a new technique.

The evaluation of the minimal inhibitory concentration (MIC) for pathogenic fungi is still a technically difficult assay. Insufficient standardization of the technique is often the basis of problems which appear. Culture characteristic of dermatophytes do not favour techniques usually used in bacteriology (Steers agar dilution method). A study was undertaken to compare the Steers agar dilution method and a new culture method to evaluate the minimal inhibitory concentration of antifungal compounds on several species of dermatophytes. The new method involves dilution of the antifungal drug in solid medium in a Petri dish. Standardized agar cylinders are cut from the plates and filled with inocula of the same size cut from plates of dermatophyte cultures. Such inocula facilitate analysis of the fungus in its natural growth conditions in vitro without being submitted to a disruptive preparative technique. The MIC values were similar for the two methods of evaluation in spite of important differences between the inocula. The new technique is reliable, quick, and highly reproducible. It is more efficient than the Steers agar dilution method because it enables assays to be run on several strains simultaneously and avoids labour-intensive procedures for the preparation of the inocula.