Microbial assays for mutagenicity: a modified liquid culture method compared with the agar plate system for precision and sensitivity.

A microbial assay system for mutagenicity was developed in which bacterial cells divided in liquid culture. The statistical and practical problems associated with dividing cells were avoided or reduced, whilst the advantages in precision and reliability resulting from the determination of mutation per colony-forming unit (survivor) and of separating the mutation and selection systems were retained. Seven mutagens, two of which required microsomal activation, were evaluated by this liquid-medium method and by the agar-plate method with two strains of Salmonella typhimurium to determine which assay system was the more sensitive. At highly mutagenic and/or very toxic concentrations of the test substance the liquid-medium assay was markedly more sensitive than the agar-plate assay, but at weakly mutagenic and less toxic concentrations the advantage of the liquid-medium test was reduced; however in only one case was the agar-plate assay obviously the more sensitive. There was a clear indication that the liquid-medium assay would be superior to the agar-plate assay for the detection of mutagenic agents active only at toxic concentrations, and also could be more easily and exactly quantified.