Suppressive effects of chemicals in mixture on the Salmonella plate test response in the absence of apparent toxicity.

There are chemicals that affect the number of his+ revertant colonies of Salmonella in the plate test at doses that are apparently nontoxic, but may be causing nonlethal, toxic effects. When mixed with mutagens, these chemicals reduce the numbers of his+ revertant colonies on the plate with no accompanying visible toxic effect on the background lawn. Some of these plates are indistinguishable from spontaneous control plates, leading to the possibility that the mutagens under test would be evaluated as nonmutagenic, or that the mutagenic response would be underestimated. The reduction in mutagen-induced revertant colonies in most cases is equivalent to the reduction in spontaneous revertants in the absence of mutagen. A spot test that permits a rapid screen of chemicals for inhibitory effects has been developed; a plate incorporation assay is used to confirm the effect. Toxic effects can be seen in the background lawns of plates examined at magnifications of 100X or greater.