Increased sensitivity of the SOS-LUX-Test for the detection of hydrophobic genotoxic substances with Salmonella typhimurium TA1535 as host strain

The SOS-LUX-Test is a simple and fast prokaryotic bioassay for the detection and quantification of genotoxic substances by the measurement of the DNA-damage proportional intensity of bioluminescence. This bioluminescence is produced by bacteria carrying the plasmid pPLS-1 with a lux operon under the control of a SOS promoter. In previous experiments different Escherichia coli strains have been used as host bacteria. Depending on the degree of hydrophobicity of the genotoxic substances the sensitivity of the SOS-LUX-Test was lower than measured in other bacterial bioassays for genotoxicity. An initial improvement could be made by using the E. coli strain PB3 with a tolC mutation causing an increased cell membrane permeability for hydrophobic substances compared to a tol wild-type strain. The change of the bacterial species from E. coli to a Salmonella typhimurium strain with a defect in the synthesis of the outer cell membrane (rfa-, strain TA1535) has led to a further increase in sensitivity. After a minor adaptation and modification of the experimental procedure the results obtained with both strains were compared for the test substances NFO, ICR 191, MMC, NQO, NA, AA, 2-AF, B(a)P. With hydrophobic substances the sensitivity of TA1535 (pPLS-1) is higher than that of PB3(pPLS-1). This is correlated in most cases with a higher toxicity, whereas the sensitivity for water-soluble substances is not significantly increased, as expected. From these results we decided to use the strain S. typhimurium (also called S. choleraesuis ssp. choleraesuis) TA1535(pPLS-1) in the SOS-LUX-Test in future genotoxicity tests.