GENETIC-DIFFERENCES BETWEEN THE STANDARD AMES TESTER STRAINS TA100 AND TA98

The standard Ames tester strains of Salmonella typhimurium are separat ed by many steps in their pedigree, some involving mutagen treatments, and contain independently isolated uvrB-bio-gal deletions and rfa mut ations. In this work the araD531 mutation was introduced into the Ames tester strains TA100 and TA98. The responsiveness of the resulting st rains (BA15 and BA14) to a number of chemical mutagens was then assess ed by monitoring the induction of forward mutations to L-arabinose res istance (Ara test). Here we have shown that these two strains of the A mes test differ greatly in their responses to mutagens, in ways that a re not associated with the mutagenic specificities of the original his mutations. In general, the genetic background of strain TA100 appears to be more sensitive to the killing effects of chemicals than that of TA98. The greatest differences were found with nifurtimox (NFX) and i ts analogue, compound 1K. The Ara test responded to the mutagenic effe cts of these two nitrofurans when carried out in the genetic backgroun d of strain TA98 but not in that of TA100. A higher sensitivity to the lethal effects of NFX and 1K together with the greater nitro-reductio n capability of strain TA100 as compared with TA98 might explain the d ifferences. In conclusion, our results indicate that the standard Ames S.typhimurium tester strains are not isogenic and that genetic differ ences at loci other than his might be significant for mutagenicity tes ting. To this respect the routine use of the isogenic set of S.typhimu rium strains constructed by Popkin et al. (Mut. Res., 224, 453 - 464, 1989) and derived from strain hisD3052 (as the standard TA98) seems ad visable.