Investigation of chemicals with inconsistent Ames results using miniaturized Ames test systems

The Ames assay is a bacterial reverse gene mutation test that has been a cornerstone of mutagenicity assessment. The emphasis now is on developing miniaturized versions of the Ames test in Petri dish to require less chemicals, reagents, and liver microsomal S9 fraction, thus reducing the number of test animals needed and to better comply with 3R principles. Miniaturized Ames assay versions promote high throughput testing of multiple samples during compound screening and facilitate the early exclusion of genotoxic agents during the product development process. Existing experimental data shed light on a high concordance between results gained with miniaturized Ames tests and the Petri dish-based method, yet further testing is required to corroborate these findings. We selected compounds with previously reported inconsistent outcomes and assessed their mutagenic potential using two miniaturized Ames assay formats, an agar-based 6-well plate test, and a liquid microplate fluctuation format assay. Investigation of dose-response curves of known mutagens with varying bacterial cell density inputs revealed that the sensitivity of the 6-well agar plate format might be increased by applying the right bacterial cell density. Our analysis indicates an overall good correlation between the results acquired with the two miniaturized Ames assay formats despite the conceptual characteristic differences in the assay paradigms. Furthermore, the miniaturized Ames assay formats could detect several chemicals as positive at lower concentrations than the Petri dish-based assay. Our findings indicate that the miniaturized Ames assay variations show promise as a reliable method for assessing chemical mutagenicity, while also aligning with environmentally friendly testing strategies. Finally, our results show that the miniaturized assays may exhibit increased sensitivity to impurities, potentially contributing to the observed discrepancies in the obtained results.