Mutagenicity testing with Salmonella typhimurium strains. I. Unusual phenotypes of the tester strains

A protocol for the combined biochemical and serological identification of the Ames mutagen tester strains as Salmonella typhimurium

Previously published reports have noted biochemical reactions atypical of Salmonella among the Ames tester strains of Salmonella typhimurium, and an inability to assign the strains to a specific Salmonella O (heat-stable cell wall) antigen group. We studied the biochemistry and serology of strains TA97, 98, 100, 102, 104, 1535, 1537, and 1538 in an attempt to develop a protocol to correctly speciate the strains. Biochemical reactions of all eight strains using standard media supplemented with histidine and biotin were consistent with those of the genus Salmonella. Strains TA100, 104, and 1535 were assigned to Salmonella O groups using bacteria treated with hot ethanol (White schema). H (flagellar) antigen assignments were performed successfully with seven of the eight strains. Two H antigen assignments required the use of the Craigie tube test for selection of motile revertants. Combining our biochemical and serological results obtained by this protocol, we were able to correctly speciate TA100, 104, and 1535 as Salmonella typhimurium. Our results demonstrate that representatives of the tester strains can be correctly speciated provided that procedures are followed that allow for the unusual nutrient requirements, the deep rough cell wall mutation, and the variably deficient motility of these organisms.

The correlation between benzo[a]pyrene-induced mutagenicity and DNA adduct formation in Salmonella typhimurium TA100

In an attempt to stabilize the dose response in the Salmonella typhimurium test (STT), the use of DNA-bound products from BP was evaluated as a measure of the biologically effective dose. In addition to the previously documented interlaboratory variation, we observed a 3-fold difference in the dose response of TA100 to BP even when the assay was repeated with the same experimental conditions. When overall BP-DNA adduct formation was related to the level of His+ revertants, a series of responses emerged with two predominating. In the first type of response around 70 revertants per plate were generated for every BP molecule bound per 10(6) nucleotides of cellular DNA. The second response gave about 1400 revertants per plate for one BP bound in every 10(6) nucleotides. Several intermediates curves were also detected. The variation in the mutational response to binding levels occurred regardless of the source of S9 or the growth stage of the cells. These experiments indicate that there was no constant level of DNA damage that would lead to a specified number of revertants of TA100 by BP and that DNA modification was not solely responsible for mutagenic potency. It is possible that an induction of an error-prone repair function of the muc gene carried by the plasmid pKM101 in TA100 may be affecting the relationship between the measured adduct level and reversion frequency.

Phenotypic instability of Salmonella typhimurium tester strains: an example of a plasmid-enhanced genetic drift?

Strains derived from Salmonella typhimurium LT-2, which are used as tester strains in mutagenicity assays, show significant changes in biochemical phenotypes. The presence of plasmid pKM101 in these strains greatly increases both the frequency of these shifts as well as the spectrum of phenotypes involved. It is suggested that in plasmid-free strains these variations reflect the effects of endogeneously induced mutations which are amplified in the absence of a functional uvrB gene product. In plasmid-containing strains this genetic drift may be promoted further by the pKM101-coded error-prone DNA repair system. The observation of a plasmid-mediated genetic drift lends support to the suggestion that transposons may contribute to the carcinogenic process.

Measurement of drug-metabolizing systems in Salmonella typhimurium strains G46, TA15135, TA100, TA1538 and TA98

Salmonella typhimurium strains which are commonly used in the Ames test for screening potential carcinogens were examined for a number of drug-metabolizing systems. Neither cytochrome P-450 itself nor two activities catalyzed by the cytochrome P-450 system in mammalian cells, i.e., benzpyrene monooxygenase and ethoxycoumarin O-deethylation, could be detected. Nor do these bacterial strains demonstrate any ability to detoxify epoxides by hydrating them or to conjugate p-nitrophenol with glucuronic acid. On the other hand, S. tryphimurium strains G46, TA1535, TA100, TA1538 and TA98 contain considerable amounts of acid-soluble thiols, approx. 5--10% of which is glutathione. These bacteria can also enzymatically conjugate glutathione with 1-chloro-2,4-dinitrobenzene (CDNB) and can reduce oxidized glutathione using NADPH as cofactor. Thus, enzymatic and non-enzymatic reaction of immediate carcinogens with thiol groups in s. typhimurium may have a significant effect on the outcome of the Ames test in certain cases.

Substrate-specificity of uvr excision repair

The substrate specificity of the uvr endonuclease, the product of the uvrA, uvrB, and uvrC genes is reviewed. It is suggested that the relatively well-defined substrate specificity of this repair enzyme is useful as a guide in determining the nature of the DNA-lesion caused by a given mutagen.

Mutagenicity testing with Salmonella typhimurium strains. II. The effect of unusual phenotypes on the mutagenic response

The enhanced sensitivity of some Salmonella typhimurium strains to the mutagenic action of a number of chemicals appears to be due to the defect in the uvrB gene product and not to an inability to produce H-2-S or to the absence of formic acid hydrogenlyase which also characterizes these strains.