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

In vitro susceptibilities were determined for a total of 159 clinical isola tes and 12 reference strains of yeasts belonging to different Candida speci es including 94 Candida albicans strains, and further genera such as Crypto coccus, Trichosporon, Geotrichum and Saccharomyces. Minimum inhibitory conc entration (MIC) values for fluconazole and itraconazole were assessed using a microdilution technique with the semisynthetic high resolution (HR) medi um supplemented with glucose and asparagine but without sodium hydrogen car bonate (pH 7.0), according to a proposal of the working group 'Clinical Myc ology' of the German Speaking Mycological Society. Fluconazole MIC values f or C. albicans were between 0.125 and greater than or equal to 128 mu g ml( -1). Thus, the median of 1 mu g ml(-1) showed that the overall fluconazole susceptibility was good. As expected, Candida krusei (seven strains) exhibi ted diminished in vitro susceptibility with MIC values for fluconazole of 8 to 128 mu g ml(-1) with a median of 64 mu g ml(-1). Some Candida kefyr str ains seemed to be less susceptible against fluconazole which was indicated by a MIC90 of 64 mu g ml(-1). Surprisingly, no Candida glabrata isolate exh ibited a MIC value greater than 16 mu g ml(-1) Other Candida species, Trich osporon cutaneum Geotrichum candidum and Saccharomyces cerevisiae showed lo w MICs to fluconazole. In vitro susceptibility testing of itraconazole reve aled that all Candida species except C. albicans, but also Trichosporon cut aneum, Geotrichum candidum, and Saccharomyces cerevisiae exhibited acceptab le low MIC values against itraconazole (0.03-2 mu g ml(-1)). Their MIC90 va lues for itraconazole were in the close range between 0.125 and 2 mu g ml(- 1) MIC values between 0.125 and 2 mu g ml(-1) were obtained, even for C. kr usei strains. On the other hand, the range of C. albicans MICs was between 0.0125 and greater than or equal to 16 mu g ml(-1) with MIC50 and MIC90 val ues of 0.125 and greater than or equal to 16 mu g ml(-1), respectively, ind icating that a considerable number of yeast strains have high MICs. The com parative evaluation of different experimental conditions revealed that ther e exists a marked influence both of inoculum size and incubation time on th e results of susceptibility testing. Therefore, for routine usage 10(2) CFU ml(-1) and 18-24 h incubation time for this microdilution method with HR m edium are recommended.