Basic examinations of susceptibility testing for amphotericin-B and flucytosine against Candida species using an agar dilution method

We examined the appropriate conditions for measuring the susceptibility to amphotericin-B (AMPH-B) and flucytosine (5-FC) of Candida species. Yeast morphology agar (YMA, pH 7.0) was used as the testing medium and Sabouraud dextrose agar (SDA, pH 7.0) as control medium. Seventy-six strains clinically isolated from 1989 to 1990 in Nagasaki University Hospital were tested in this study to provide the following results. 1.Appropriate measuring conditions. (1) Inoculum size: 5×10³CFU in absolute values (or concentration of 1 x 10⁶CFU/ml using a multipoint inoculator), (2) Incubation temperature: 30°C, (3) Incubation time: 48 hours. These conditions were suitable for cell growth and determination of MIC values. 2.Antifungal activities against clinical isolates. MIC ranges of AMPH-B and 5-FC were 0.1— 1.56/μg/ml and 0.1—50μg/ml on YMA. Although AMPH-B had excellent activity also on SDA, activity of 5-FC was markedly inhibited on it, as stated in past reports. 3.Reproducibility of MIC values. In triple serial measurement by the method mentioned above, all results agreed within one twofold dilution. 4.Valid Period of ready-to-use susceptibility agar: 14 days for AMPH-B and 28 days for 5-FC under dark conditions at 4°C. 5.Our method was so efficient that twenty-seven strains were treated on one agar plate and visual evaluation of endpoints was easy.