In vitro susceptibility of Candida species to five antifungal agents in a German university hospital assessed by the reference broth microdilution method and Etest

Abstract

Objectives: The aim of this study is to evaluate the susceptibilities of Candida spp. to the common antifungal agents in a German university hospital. Since quick results of in vitro testing are desirable, Etest and the CLSI broth microdilution (BMD) method (reference method) were compared, focusing on the validity of early readings.

Methods: A total of 512 Candida spp. isolates, including 174 from primarily sterile sites, were collected in the clinical routine. The yeasts were differentiated by CHROMagar and verified by API 20C AUX if necessary. In vitro susceptibilities to amphotericin B, flucytosine, fluconazole, voriconazole and caspofungin were determined using the BMD method described in the CLSI (formerly NCCLS) M27-A2 document and Etest. MICs were noted after 24 and 48 h of incubation.

Results: The most frequently isolated species was Candida albicans. Among the non-albicans species, Candida glabrata was the most prevalent, followed by Candida tropicalis, Candida parapsilosis and Candida krusei. MICs (mg/L) at which 90% of the strains were inhibited were 1 for amphotericin B, 32 for flucytosine, 8 for fluconazole, 0.25 for voriconazole and 1 for caspofungin. Susceptibility to fluconazole was 85.0% for C. glabrata and 5.3% for C. krusei, almost all other isolates were susceptible in over 90% except very rare species. The 48 h MIC values of Etest and BMD were in agreement (no more than 2 log(2) dilutions) in 88.7% to 98.1% with categorical agreement rates of 91.6% to 98.2%, depending on the antifungal agent. Comparison of the 24 h MICs of both BMD and Etest with the 48 h MICs of the reference method showed categorical agreement in 94.9% to 99.2%. For caspofungin, however, a comparison of the categorical agreement was not possible due to the lack of interpretive breakpoints. The order of frequency and the resistance patterns of the isolates from primarily sterile sites and those of isolates from non-sterile sites did not differ.

Conclusions: No alarming resistances against the agents tested were found; however, owing to the relatively high frequency of C. glabrata with elevated fluconazole MICs, this species and, to a certain extent, C. krusei must be taken into consideration when choosing antifungal agents for calculated therapy. Etest is a reliable method for the susceptibility testing of Candida spp. and the 24 h readings of both Etest and BMD can serve as helpful preliminary results in most cases.