Screening for Drug-resistant Yeasts with Chromogenic Agar Containing Echinocandins

Objectives: We evaluated agar dilution screening to identify antifungal resistance in Candida yeasts using CHROMagar plates containing anidulafungin, micafungin, or caspofungin at different concentrations. We also compared this method of drug resistance screening with a reference standard method (CLSI) for several Candida isolates. Methods: CHROMagar Candida media was prepared using anidulafungin, micafungin or caspofungin at 0.5- and 1 µg/ml added independently to the agar solution prior to solidification. Control plates were similarly prepared without drug. Assessment of resistance was based on growth characteristics comparing colony sizes of yeasts on the various plates. Decreased colony size in the presence of antifungal drugs is used as indicator or measure of degree of resistance. An average of 93 yeast isolates, including C. glabrata, C. parapsilosis and C. tropicalis were screened for resistance and susceptibility to each drug. The MICs for each isolate were quantified using the CHROMagar plate screening method and compared with MICs by CLSI broth microdilution technique. Results: Compared to the CLSI standard method, our CHROMagar technique correctly predicted micafungin susceptibility for 78 of 78 (100%) isolates and resistance was correctly predicted for 8 of 12 (66%) isolates; for anidulafungin, susceptibility was correctly predicted for 76 of 78 (97%) isolates and resistance was correctly predicted for 11 of 13 (84%) isolates; and for caspofungin, susceptibility was correctly predicted for 78 of 81 (96%) isolates and resistance was correctly predicted for 12 of 22 (54%) isolates. Overall, anidulafungin, micafungin, and caspofungin were shown to be 96%, 95%, and 86% accurate, respectively, when compared to MICs from CLSI technique. Conclusion: Our results indicate that this technique is rapid, accurate, and reproducible when compared to the CLSI method and is highly useful for screening for yeasts with reduced drug susceptibility. Acknowledgment: This work was supported by NIDCR, Grant #DE14318 for the CO STAR Program.