Antifungal susceptibility testing of dermatophytes: Establishing a medium for inducing conidial growth and evaluation of susceptibility of clinical isolates

A standardized reference method for dermatophyte in vitro susceptibility te sting is lacking, In a previous study, Norris et al, (H. A. Norris, B. E. E lewski, and M. A. Ghannoum, J. Am. Acad, Dermatol, 40(6, part 2):S9-S13) es tablished the optimal medium and other growth variables. However, the earli er study did not address two issues: (i) selection of an optimal medium for conidial formation by dermatophytes and (ii) validation of the method with a large number of dermatophytes, The present study addresses these two poi nts. To select which agar medium best supported conidial growth, representa tive isolates of dermatophytes were grown on different agars, Preliminary e xperiments showed that only oatmeal cereal agar supported the production of conidia by Trichophyton rubrum, We tested the abilities of 251 T. rubrum i solates to form conidia using three different cereal agars and potato dextr ose agar, Overall, oatmeal cereal and rice agar media were comparable in th eir abilities to support T. rubrum conidial growth. Next, we used the oatme al cereal agar for conidial formation along with the optimal conditions for dermatophyte susceptibility testing proposed by Norris ct al, and determin ed the antifungal susceptibilities of 217 dermatophytes to fluconazole, gri seofulvin, itraconazole, and terbinafine, Relative to the other agents test ed, terbinafine possessed the highest antifungal activity against all of th e dermatophytes. The mean +/- standard error of the mean. MICs of fluconazo le, itraconazole, terbinafine, and griseofulvin were 2.07 +/- 0.29, 0.13 +/ - 0.01, 0.002 +/- 0.0003, and 0.71 +/- 0.05 mu g/ml, respectively. This stu dy is the first step in the identification of optimal conditions that could be used for the standardization of the antifungal susceptibility testing m ethod for dermatophytes. Inter- and intralaboratory agreement as well as cl inical correlations need to be established.