Effect of pH and buffer system on the in-vitro activity of five antifungals against yeasts

We have compared the effect of various media on the in-vitro activity of amphotericin B, flucytosine, fluconazole, itraconazole and ketoconazole against 93 clinical yeast isolates by a micro-broth dilution technique. The media used were: RPMI 1640 with 2% glucose, buffered with 0.165 M MOPS at pH 7.0; the same medium, but buffered at pH 7.4; and the same medium, but buffered at pH 7.4 with 0.15% sodium bicarbonate. The three media gave similar results with azole antifungals and flucytosine, but the medium buffered at pH 7.0 failed to detect different populations of yeasts with respect to amphotericin B susceptibility. In the case of the media buffered at pH 7.4, Candida krusei was significantly less susceptible to amphotericin B than Candida albicans or Torulopsis glabrata. We could not evaluate the results obtained with Candida parapsilosis and Cryptococcus neoformans since these species did not grow adequately in all three media.