Influence of DMSO on antifungal activity during susceptibility testing in vitro

Dimethylsulfoxide (DMSO) binds within the plasma membrane of cells and increases membrane permeability. DMSO is used in antifungal (AF) susceptibility assays for water insoluble agents. DMSO was observed to cause Candida albicans to express enhanced, diminished, or no change in growth compared to control medium, suggesting DMSO could influence the efficacy of water-insoluble AF agents. The activity of 4 water soluble AF agents against 6 yeast species was tested under conditions similar to the CSLI M27-A3 method. Growth response to 0.5% and 1% DMSO was variable. In 15 of 67 AF assays, DMSO resulted in a different MIC-2 (substantial inhibition of growth) value compared to control. Two of these involved differences of two doubling dilutions. The results imply that, in some cases of water-insoluble AF drug-yeast combinations, the MIC-2 may be skewed from the more clinically relevant MIC, providing one reason for possible discordance between MIC results and clinical outcomes.