Antifungal Activity of 5-Fluorocytosine as Measured by Disk Diffusion Susceptibility Testing

The susceptibilities of 216 isolates of pathogenic and commensal yeasts to 5-fluorocytosine were tested by a disk diffusion technique with use of 1- and 10-microgram disks. Minimal inhibitory concentrations (MICs) were determined by an agar dilution procedure with yeast morphology agar, pH 5.0, supplemented with 0.25 mg of thiamine/dl. Zones of inhibition produced by the two disks were correlated with paired MIC values. An MIC of 16 micrograms/ml, the upper limit of probable clinical susceptibility, correlated with zones of 14 mm and 25 mm, respectively, for the 1- and 10-microgram disks. With these values as interpretative breakpoints, the 1-microgram disk would have failed to predict clinical susceptibility with 25% of the suceptible isolates of Candida species other than Candida albicans, with 14% of the susceptible isolates of Cryptococcus neoformans, with 7% of the susceptible isolates of C. albicans, and with one of the 25 susceptible isolates of Torulopsis glabrata. The 10-microgram disk would have failed to predict susceptibility with only two isolates of Candida species.