The presence of KCl in the exposure medium strongly influences the mutagenicity of metabolites of polycyclic aromatic hydrocarbons in Escherichia coli.

Previous studies demonstrated that the ion composition of the exposure medium may strongly influence the mutagenicity of many compounds in the liquid preincubation modification of the reversion assay with his- Salmonella typhimurium strains. Similar influences were now observed in the reversion assay with trp- Escherichia coli strain WP2 uvrA. The exposure medium was 8 mM sodium phosphate buffer (pH 7.4), containing no other ions or 125 mM KCl. Omission of KCl resulted in an about 10-fold enhancement of the mutagenic activity of 7-methylbenz[a]anthracene 5,6-oxide, but in a strong decrease in the mutagenicity of 1-hydroxymethylpyrene sulphate, close to the limit of detection. The findings with these two representative mutagens are very similar to those previously observed in S. typhimurium, suggesting that unobtrusive medium components may exert strong influences on the results with many test compounds in various bacterial test systems.