Turbidimetric and visual criteria for determining the in vitro activity of six antifungal agents againstCandida spp. andCryptococcus neoformans

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Abstract
The drug concentration which inhibited 50% of growth (IC50), the lowest drug concentration at which growth was less than 30% of that in a positive control well (IC30), the visual minimal inhibitory concentration (MIC), and the minimum fungicidal concentration (MFC), were applied to study the effects of fluconazole, itraconazole, ketoconazole, miconazole, flucytosine, and amphotericin B against 36 isolates ofCandida spp. andCryptococcus neoformans by a broth microdilution technique. When the recommendations established by the NCCLS Subcommittee on Antifungal Susceptibility Tests were applied for the visual reading of the microplates, the results were comparable with those obtained by the turbidimetric methods. Differences between MICs and IC30s were observed with miconazole against strains ofC. glabrata (p=0.014) and with flucytosine against strains ofC. neoformans (p=0.041). Differences between MICs and IC50s were observed with fluconazole against strains ofC. albicans (p=0.027),C. tropicalis (p=0.046), andC. neoformans (p=0.041); with miconazole against strains ofC. glabrata (p=0.014); and with amphotericin B against strains ofC. parapsilosis (p=0.025). Ten additional isolates ofC. albicans from AIDS patients suffering from recurrent episodes of oral candidiasis and clinically resistant to fluconazole also were included in this study. The MICs of fluconazole of these strains were significantly higher than those of the control group (p=0.003). When the turbidimetric parameters were applied for testing the in vitro activity of fluconazole against the above isolates, both IC30 and IC50 were capable of discriminating the strains of the two groups (p=0.002,p=0.001, respectively). Only amphotericin B demonstrated fungicidal activity against all the tested yeasts.