CHEMICALS MUTAGENIC IN SALMONELLA-TYPHIMURIUM STRAIN TA1535 BUT NOT IN TA100

The standard Salmonella mutagenicity test uses two strains of Salmonel la typhimurium (TA1535 and TA100) containing the same base pair substi tution mutation (hisG46). These strains differ only in that strain TA1 00 contains the plasmid pKM101, whose mucAB gene products enhance SOS mutagenesis. This makes strain TA100, in general, the more sensitive o f the two for mutagen detection, raising the question as to whether or not to include strain TA1535 in the core battery of strains in routin e testing. Out of 659 chemicals judged as mutagens in the S. typhimuri um assay when subjected to the National Toxicology Program's screening protocol, 36 (5%) were evaluated as positive in strain TA1535 but not in strain TA100. Of these, 23 were judged as negative and 13 as equiv ocal in strain TA100, and 5 were positive or equivocal in at least one other strain (TA97 or TA98). In general, the data on these chemicals indicate that the absolute increases in revertants per plate induced i n strain TA1535 were too small to have been judged as positive if simi lar increases occurred in strain TA100. which has a much higher sponta neous background. For three chemicals (acetaldehyde oxime, 6-mercaptop urine, and 1,3-butadiene) the absolute increases in revertants in stra in TA1535 greatly exceeded those in strain TA100. Evaluation of the re producibility of these findings and of the mechanisms and relevance of unique TA1535 positives should be useful when decisions are made as t o whether this strain should be kept as a part of the core battery of strains in the S. typhimurium assay. (C) 1998 Elsevier Science B.V.