Clinical Candida krusei isolates remain susceptible during extensive exposure to antifungal drugs.

We have previously reported a Candida krusei outbreak during which a number of our patients were infected or colonized by several different closely related Candida krusei genotypes. The treatment response in many of our patients was at best modest and the patients remained positive for Candida krusei. We speculated that extended exposure to antifungals in patients with an incomplete treatment response might lead to the conditions for selection of drug resistance in the multiple Candida krusei clones. Therefore, we followed the in vitro susceptibility of the Candida krusei isolates taken from our patients before and during the antifungal treatment. A total of 28 Candida krusei isolates from 11 patients with prolonged exposure to antifungal medication were analyzed for their in vitro susceptibility to commonly used drugs. We found that MIC(50) values of all Candida krusei isolates was 12 microg/ml for fluconazole, 0.19 microg/ml for voriconazole, 1.0 microg/ml for amphotericin B, and 1.0 microgt/ml for caspofungin with the corresponding MIC(90) values being 16 microg/ml, 0.5 microg/ml, 2.0 microg/ml, and 1.0 micro/ml, respectively. Extended antifungal exposure did not change these MIC values. We conclude that resistance development in Candida krusei during prolonged antifungal treatment may not be common and the treatment failure of our patients was not likely due to the development of drug resistance by the etiologic agent.