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Effect of aluminum on fluoride uptake by Salmonella typhimurium TA98; implications for the Ames mutagenicity assay.

The bacterium Salmonella typhimurium TA98 (TA98) is commonly used in the Ames test to detect frameshift mutations. Published Ames test results for fluoride mutagenicity are inconsistent. We considered that this might be due to an unnoticed variable in experimental conditions. Al and F, both present in finished drinking water, form a stable complex. The effect of Al and F uptake by TA98 cells was evaluated and any effect of Al on Ames test results for F was also determined. A known number of TA98 cells in 0.1 M potassium phosphate buffer (PPB, pH 7.4) was incubated with buffer only, and various concentrations of Al as AlCl3, F as NaF, or aluminum and fluoride (Al: F, 1: 62.5 w/w) for 1 h at 37 degrees C in a shaking incubator. Neither F (19 and 190 ppm), Al (0.3 and 3 ppm), nor Al:F (7.6 ppb-3 ppm: 0.45-190 ppm) was found to be mutagenic as assessed by a modification of the Ames standard plate incorporation assay. Intracellular Al accumulated in a concentration-dependent manner from 0.5 to 4.5 ppm, then decreased as Al was increased to 9.5 ppm. Intracellular F was below the limit of detection (0.2 ppm) even when the medium contained 589 ppm F. However, F was taken up from media containing greater than 200 ppm F, providing that aluminum was also present. Experiments carried out using N-2-hydroxyethyl-piperazine-N'-2-ethanesulfonic acid (HEPES) buffer (100 mM, pH 7.4) gave similar results for F uptake, suggesting the Al requirement for F uptake is not an artifact caused by buffer chelation. However, Al uptake was not biphasic from HEPES buffer, but increased with dose over the range studied (1.5-9.0 ppm). While F did not appear to cause mutations in the presence or absence of Al, it was concluded that the mutagenicity test, using TA98, is not suitable for the evaluation of F mutagenicity.