Factors affecting griseofulvin susceptibility testing of Trichophyton rubrum in microcultures

A microculture broth assay system for griseofulvin susceptibility testing of Trichophyton rubrum was further characterized. The effects of mass and number of colony-forming units of a fragmented mycelial inoculum, 5- or 8-day incubation periods, 25 or 32 degrees C incubation temperatures, and the solvents used to dissolve griseofulvin on the minimal inhibitory concentration (MIC) of griseofulvin were determined. An inoculum density with an absorbance of 0.600 at 450 nm ensured successful inoculation of all microcultures. Reduction of the inoculum mass to an absorbance of 0.200 lowered the number of colony-forming units in the inoculum by 60 to 80%. This decreased the efficiency of inoculation but did not alter the resulting MIC. There was no correlation between MIC and the number of colony-forming units used to initiate growth. Neither incubation temperature nor the length of incubation affected the MIC. The use of either acetone or ethanol to solubilize griseofulvin likewise had no effect on the MIC. The mean reproducibility of the MICs determined with the microculture method was 96%.