Fluconazole disk diffusion susceptibility testing of Candida species

Abstract

We describe a simple procedure for detecting fluconazole-resistant yeasts by a disk diffusion method. Forty clinical Candida sp. isolates were tested on RPMI-glucose agar with either 25- or 50-microgram fluconazole disks. With 25-microgram disks, zones of inhibition of >/=20 mm at 24 h accurately identified 29 of 29 isolates for which MICs were </=8 microgram/ml, and with 50-microgram disks, zones of >/=27 mm identified 28 of 29 such isolates. All 11 isolates for which MICs were >8 microgram/ml were identified by using either disk. Disk diffusion may be a useful screening method for clinical microbiology laboratories.

FIG. 1

(A) A 25-μg fluconazole disk on a lawn of 10⁴ CFU of C. albicans after 24 h of incubation. (B) A 50-μg fluconazole disk on a lawn of 10⁴ CFU of C. albicans after 48 h of incubation. Inhibitory zone diameters were measured at the transitional point where growth abruptly decreased (interior edges of bars), as determined by a marked reduction in colony sizes.

FIG. 2

(A) Regression analysis correlating zones of inhibition (24 h) obtained with 25-μg fluconazole disks with NCCLS 24-h broth macrodilution MICs. (B) Regression analysis correlating zones of inhibition (24 h) obtained with 50-μg fluconazole disks with NCCLS 24-h broth macrodilution MICs.