Influence of different media, incubation times, and temperatures for determining the MICs of seven antifungal agents against paracoccidioides brasiliensis by microdilution

Abstract

MIC assays with Paracoccidioides brasiliensis, the etiological agent of paracoccidioidomycosis, had been conducted with variable protocols, employing both macrodilution and microdilution tests and including differences in inoculum preparation, media used, incubation periods, and temperatures. Twenty-one clinical and environmental isolates of Paracoccidioides were tested using amphotericin B, itraconazole, ketoconazole, fluconazole, sulfamethoxazole, sulfamethoxazole-trimethoprim, and terbinafine, according to the National Committee for Clinical Laboratory Standards (National Committee for Clinical Laboratory Standards, document M27-A2, 2002), with modifications such as three medium formulations (RPMI 1640 medium, McVeigh and Morton [MVM] medium, and modified Mueller-Hinton [MMH] medium), two incubation temperatures (room temperature [25 to 28°C] and 37°C), and three incubation periods (7, 10, and 15 days). The antifungal activities were also classified as fungicidal or fungistatic. The best results were obtained after 15 days of incubation, which was chosen as the standard incubation time. The MICs for most individual isolates grown for the same length of time at the same temperature varied with the different media used (P<0.05). Of the isolates, 81% showed transition from the yeast to the mycelial form in RPMI 1640 medium at 37°C, independent of the presence of antifungals.MMHmedium appears to be a suitable medium for susceptibility testing of antifungal drugs with P. brasiliensis, except for sulfamethoxazole and the combination of sulfamethoxazole-trimethoprim, for which theMVMmedium yielded better results. The incubation temperature influenced the MICs, with, in general, higher MICs at 25°C (mycelial form) than at 37°C (P<0.05). Based on our results, we tentatively propose a microdilution assay protocol for susceptibility testing of antifungal drugs against Paracoccidioides. Copyright © 2013, American Society for Microbiology.