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 c oncentration (MIC) of fluconazole was studied with Candida species. Ag ar dilution tests were performed on media (pH 7.25) buffered with eith er phosphate or morpholinopropane-sulfonic acid (MOPS) or endomethylen e-tetrahydrophthalic acid (EMTA), 0. 1 mol l-1 each, or on the unbuffe red medium. It consisted of casitone and glucose supplemented with FeC l3 and MgSO4. The MICs recorded after 24 h at 37-degrees-C extended fr om 0.1 mg l-1 to greater-than-or-equal-to 100 mg l-1 on the phosphate and EMTA medium, being concordant on both media. On the MOPS medium an d the unbuffered medium the readings were also concordant; the MICs, h owever, were mostly 25 mg l-1 or higher. This increase of the values-u p to six dilution steps - could not be correlated with the amount of a cid secreted by the single strains. EMTA proved to be an alternative t o phosphate in this system, and because it allows a faster growth of t he yeasts it might be superior to phosphate. The concordance of the MI C values in the presence of such different buffer compounds tends to s uggest that they indeed indicate the strongest inhibition attainable i n vitro by fluconazole. MOPS was confirmed to be of no use in this sys tem.