Screening for drug-resistant Candida yeasts with chromogenic agar

We examined the utility of agar dilution to screen yeasts for reduced susceptibility to several newer antifungal drugs including echinocandins and azoles. We compared agar dilution susceptibility screening with the Clinical and Laboratory Standards Institute (CLSI) method for Candida isolates. We added echinocandins and azoles to CHROMagar Candida medium prior to its solidification. Assessment of resistance was based on growth characteristics, wherein decreased colony size in the presence of antifungal drugs was used as an indicator of susceptibility. Clinical Candida isolates of C. albicans, C. glabrata, C. parapsilosis, C. tropicalis, C. guilliermondii, C. lusitaniae, C. rugosa and C. dubliniensis were screened for drug susceptibility. Overall, antifungal susceptibility of the yeasts to anidulafungin, caspofungin, micafungin, posaconazole and voriconazole, determined using CHROMagar agar dilution, were shown to be 96, 80, 94, 90 and 97% as accurate, respectively, as those determined by the CLSI method, i.e., within one tube dilution of CLSI MICs. Categorical errors by percentage had a broader range. Major errors noted with anidulafungin, caspofungin and micafungin were 3, 6 and 0%, respectively, while very major errors were 15, 55 and 38%, respectively. Major errors with posaconazole and voriconazole were 12 and 0%, respectively, while very major errors were 0 and 22%, respectively, compared to CLSI standards. Most of the assessment errors were found with C. glabrata and C. parapsilosis. Agar dilution screening for drug susceptibility with the current panel of antifungal drugs is rapid, accurate and effective. However, the determination of resistance or non-susceptibility in yeasts may be more problematic, and may be species dependent.