In vitro antifungal susceptibilities of Candida species isolated from the blood

In vitro susceptibility of 158 strains of Candida species isolated from blood cultures to amphotericin B, flucytosine and fluconazole was evaluated. The broth macrodilution reference method of the National Committee for Clinical Laboratory Standards (NCCLS, M27-P) was adapted to the microdilution method. For 43 tests of standard strains: there were excellent agreement (within one doubling dilution) of minimum inhibitory concentrations (MICs) of amphotericin B and flucytosine with macrodilution reference. For fluconazole, 100% agreement was found using MICs scored 2 (prominent decrease in turbidity; MIC-2s) at 24 h reading, and much lower agreement using MICs scored 1 or at 48 h reading. In testing of the clinical isolates, amphotericin B and flucytosine demonstrated a very narrow range of MICs. On the other hand, fluconazole MICs showed a broad range and shifted substantially from higher level to lower level when MIC endpoints changed from the most strict MICs (complete absence of growth, MIC-0s) to a less stringent MIC-2s. However, MIC-2s distinguished C. glabrata as resistant compared with the other species. Also, MICs increased after prolonged incubation, particularly for fluconazole. Almost all Candida blood culture isolates tested were very susceptible to flucytosine. However, higher prevalence of fluconazole resistance was noted in our blood culture isolates than those in previous studies. The present study validated that broth microdilution method is an adequate tool for antifungal susceptibility testing following the recommendations provided by the NCCLS, and the MIC results might be determined as early as 24 h after incubation.