Screening for Posaconazole Resistant Yeasts on Solid Agar Media

Objectives: Screening antifungal resistance in yeasts using the Clinical Laboratory Standards Institute (CLSI) method is time consuming and laborious. There is a need for a more rapid and practical method. Previously, we showed that chromogenic agar with fluconazole is an effective method for identifying resistant yeasts. Posaconazole (POS) is a newly introduced azole antifungal with anti-Candida activity. We blended solid agar media with POS to screen for resistant yeasts and evaluated a variety of pathogenic yeasts using this screening method for antifungal resistance comparing the results to the CLSI reference standard. Methods: To test media based drug resistance screening, mixtures of drug susceptible and resistant C. albicans and C. glabrata were plated on CHROMagar Candida with POS at 0, 1, 2, and 4 µg/ml. Also, 63 C. glabrata isolates were screened for POS resistance on CHROMagar with drug in a double blind fashion using isolates previously evaluated by the CLSI broth microdilution method. Results: Chromogenic agar exhibited abundant growth of susceptible yeasts on plates without drug, while only resistant yeasts grew to normal size on plates with drug. On CHROMagar, versus the CLSI standard, POS resistance was correctly predicted for 4 of 4 (100%) isolates and susceptibility was correctly predicted for 53 of 59 (90%) isolates. Our CHROMagar based screening method showed excellent agreement by correctly predicting POS susceptibility in 91% of isolates within one tube dilution of the CLSI standard method. Conclusion: Solid agar drug resistance screening with POS appears to rapidly and accurately identify yeasts with altered susceptibility. This project was funded in part by federal funds from the National Institute of Dental and Craniofacial Research, National Institutes of Health, under Contract No. NIH NIDCR 5R01-DE018096.