Susceptibility testing of Cryptococcus diffluens against amphotericin B, flucytosine, fluconazole, itraconazole, voriconazole and posaconazole

Abstract

Cryptococcus diffluens is a recently re-established species that shares several phenotypic features with Cryptococcus neoformans. We evaluated the application of the Clinical Laboratory Standards Institute (CLSI, formerly NCCLS) macro- and microbroth dilution methods and the E-test agar diffusion method to determine the in vitro susceptibilities of known strains of C. diffluens against amphotericin B (AMB), flucytosine (5-FC), fluconazole (FLC), itraconazole (ITC) and the novel triazoles, voriconazole (VRC) and posaconazole (PSC). Seven strains were found to be resistant in vitro to AMB (MICs ≥2μg/ml), five were resistant to 5-FC (MICs of ≥32 μg/ml), four were resistant to FLC (MICs of FLC ≥32μg/ml) and nine were resistant to ITC (MICs of ITC >1 μg/ml). In contrast, VRC and PSC showed good in vitro activity against C.diffluens strains, even those with elevated MICs to amphotericin B and/or established azoles. Most of the isolates were inhibited by 0.5μg/ml of both VRC and PSC. A clinical isolate showing phenotypic switching exhibited elevated MICs to both agents, i.e., VRC (>16μg/ml) and PSC (>8μg/ml).