Cytotoxic Effect of Dimethyl Sulfoxide in the Ames Test

Dimethyl sulfoxide (DMSO) is generally believed to be non-cytotoxic at the concentrations used in the routine Ames test. In the present study, we found that DMSO reduced the number of bacterial survivors at a concentration of 14% in the treatment mixture (0.1 mL of DMSO in 0.7 mL of treatment mixture) under some assay conditions. The cytotoxicity emerged even before preincubation, and it was largely dependent on the bacterial strains used and the cell density of the treated cells; a low toxicity was detected at a low cell density. The most potent cytotoxicity was observed in the TA100 and TA1535 strains with S9 mix, where the reduction in survivors was about 60-90%, before or after preincubation. This reduction was, however, not accompanied by a significant diminution in the bacterial background lawn. The reduction in survivors does not seem to affect the sensitivity of mutagen detection, because no significant decrease in the number of induced revertants was observed in the preincubation assay using a direct mutagen, methyl methanesulfonate.