Susceptibility testing of Candida species for fluconazole: the role of buffering in the agar dilution assay.

The role of buffering in the determination of the minimal inhibitory concentration (MIC) of fluconazole was studied with Candida species. Agar dilution tests were performed on media (pH 7.25) buffered with either phosphate or morpholinopropane-sulfonic acid (MOPS) or endomethylene-tetrahydrophthalic acid (EMTA), 0.1 mol l-1 each, or on the unbuffered medium. It consisted of casitone and glucose supplemented with FeCl3 and MgSO4. The MICs recorded after 24 h at 37 degrees C extended from 0.1 mg l-1 to > or = 100 mg l-1 on the phosphate and EMTA medium, being concordant on both media. On the MOPS medium and the unbuffered medium the readings were also concordant; the MICs, however, were mostly 25 mg l-1 or higher. This increase of the values--up to six dilution steps--could not be correlated with the amount of acid secreted by the single strains. EMTA proved to be an alternative to phosphate in this system, and because it allows a faster growth of the yeasts it might be superior to phosphate. The concordance of the MIC values in the presence of such different buffer compounds tends to suggest that they indeed indicate the strongest inhibition attainable in vitro by fluconazole. MOPS was confirmed to be of no use in this system.