We described previously a simple test on petri plates for detecting chemical carcinogens as mutagens, using an especially sensitive set of bacterial strains to detect m4tagenic activity and a mammalian liver extract for carcinogen activation. We now extend the utility of the method by introducing two new bacterial strains which can detect with great sensitivity many carcinogens which we did not detect before or detected with less sensitivity. Among these carcinogens are aflatoxin B1, sterigmatocystin, benzyl chloride, benzojajpyrene, 7,12-dimethylbenzanthracene, F'-acetoxysafrole, and the nitrofuran food additive furylfuramide (AF-2).The new strains TA100 and TA98 contain an R factor plasmid, pKM101, in our standard tOeter strains TA1535 and TA1538. The R factor increases mutagenesis with certain mutagens, but not others. We present evidence that the mutagens that become more effective work through an error-prone recombinational repair.We have previously described a very sensitive and simple bacterial test for detecting chemical mutagens (1-4). The compounds are tested on petri plates with specially constructed mutants of Salmonella typhimurium as tester strains. Four tester strains have been selected, after screening hundreds of mutants, for sensitivity and specificity in being reverted from a histidine requirement back to prototrophy by a variety of mutagens. One strain (TA1535) can be used to detect mutagens causing base-pair substitutions and three (TA1536, TA1537, and TA1538) to detect various kinds of frameshift mutagens. In addition to the histidine mutation, we have added to the tester strains two additional mutations that greatly increase their sensitivity to mutagens: one causes loss of the excision repair system and the other loss of the lipopolysaccharide barrier that coats the surface of the bacteria (3).We have shown that by adding a microsomal activation system from rat (or human) liver to the petri plates, a wide variety of carcinogens can be activated to mutagens and detected easily. Thus, an important aspect of mammalian metabolism can be duplicated in an in vitro test. A large group of carcinogens-aflatoxin B1, benzo[a]pyrene, 2-acetylaminofluorene, etc., have been detected as reactive frameshift mutagens after liver activation (4). Each activated molecule contains a ring system capable of stacking interaction with DNA and an electrophilic group that can react with DNA (5-8).Other groups of carcinogens have been detected as mutagens causing base-pair substitutions: fl-propiolactone, propanesultone, etc. (1-4). Some carcinogens, such as nitroquinoline-1-oxide (NQNO), cause both types of mutations (3).We report here the development of two new bacterial strains which contain an R factor (plasmids carrying antibiotic resistance genes) and which greatly extend the usefulness of the test system. This work stemmed from the observation of MacPhee (9) that methyl methanesulfonate (MMS) and trimethyl phosphate werei more effective in reverting his646 (the histidine mutation in TA1535) when anoth...
