Selection of agar for use in Salmonella typhimurium and Escherichia coli mutation assays

The spontaneous and induced revertant frequency of four Salmonella typhimurium strains (TA1535, TA1537, TA98, and TA100) and Escherichia coli [WP2 uvrA (pKM101)] was evaluated using Vogel Bonner minimal plates prepared with ten different agars. In addition to the Difco Bacto agar originally recommended by Ames, Difco Noble, granulated and Bitek agars; BD grade A, BBL granulated and purified agars; Oxoid purified and No. 1 agars; and GIBCO select agar were tested. Several of these agars have been reported as acceptable alternatives for these Salmonella strains, but comparable studies with E. coli have not been done. The bacteria were treated with DMSO or an appropriate positive control in the presence or absence of an Aroclor 1254-induced rat liver activation system. With the exception of Noble agar in the presence of S9, there was little difference among the responses of the Salmonella strains on any of the agars. However, with E. coli the responses include either a reduction or an increase in spontaneous revertants numbers as well as a reduction in absolute and relative induced revertant frequency. Difco Bacto agar appears to be the most consistent agar for use with these strains. As an alternative, only BBL purified agar resulted in consistent results for all of these strains under all testing conditions. These results emphasize the need to evaluate the components of the standard mutation assay when incorporating additional bacterial strains. Suboptimal responses related to the agar or other components could compromise the detection of weak mutagens.