An examination of the quantitative suspension assay for mutagenesis with strains of Salmonella typhimurium.

The effects of cell toxicity on the mutagenic response obtained in a quantitative suspension assay using the strains of Salmonella typhimurium developed by Ames et al [Mutat Res 31:347--364, 1975] were examined. Cell toxicity was shown to produce false-positive results when the cells were plated immediately with trace amounts of histidine in the overlay agar. Replacement of the overlay agar with one lacking histidine produced false-negative results when the cells were plated immediately after exposure to a frame-shift-type mutagen. The inclusion of an overnight growth period in brain-heart infusion broth before plating the cells on minimal medium permitted incorporation of mutations into the genome and also yielded constant titers. The addition of the regrowth period is shown to be necessary for obtaining valid results in a suspension assay.