In vitro susceptibility of yeasts for fluconazole and itraconazole. Evaluation of a microdilution test

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Abstract
In vitro susceptibilities were determined for a total of 159 clinical isolates and 12 reference strains of yeasts belonging to different Candida species including 94 Candida albicans strains, and further genera such as Cryptococcus, Trichosporon, Geotrichum and Saccharomyces. Minimum inhibitory concentration (MIC) values for fluconazole and itraconazole were assessed using a microdilution technique with the semisynthetic high resolution (HR) medium supplemented with glucose and asparagine but without sodium hydrogen carbonate (pH 7.0), according to a proposal of the working group ‘Clinical Mycology’ of the German Speaking Mycological Society. Fluconazole MIC values for C. albicans were between 0.125 and ≥128 μg ml−1. Thus, the median of 1 μg ml−1 showed that the overall fluconazole susceptibility was good. As expected, Candida krusei (seven strains) exhibited diminished in vitro susceptibility with MIC values for fluconazole of 8 to 128 μg ml−1 with a median of 64 μg ml−1. Some Candida kefyr strains seemed to be less susceptible against fluconazole which was indicated by a MIC90 of 64 μg ml−1. Surprisingly, no Candida glabrata isolate exhibited a MIC value greater than 16 μg ml−1. Other Candida species, Trichosporon cutaneum, Geotrichum candidum and Saccharomyces cerevisiae showed low MICs to fluconazole. In vitro susceptibility testing of itraconazole revealed that all Candida species except C. albicans, but also Trichosporon cutaneum, Geotrichum candidum, and Saccharomyces cerevisiae exhibited acceptable low MIC values against itraconazole (0.03–2 μg ml−1). Their MIC90 values for itraconazole were in the close range between 0.125 and 2 μg ml−1. MIC values between 0.125 and 2 μg ml−1 were obtained, even for C. krusei strains. On the other hand, the range of C. albicans MICs was between 0.0125 and ≥16 μg ml−1 with MIC50 and MIC90 values of 0.125 and ≥16 μg ml−1, respectively, indicating that a considerable number of yeast strains have high MICs. The comparative evaluation of different experimental conditions revealed that there exists a marked influence both of inoculum size and incubation time on the results of susceptibility testing. Therefore, for routine usage 102 CFU ml−1 and 18–24 h incubation time for this microdilution method with HR medium are recommended.

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