Due to their unique properties, the use of nanoparticles (NPs) is expanding; these same properties may affect their potential risk to humans. However, standard methods for genotoxicity assessment may not be adequate for NPs; altered tests reported here have been developed to address perceived inadequacies. The bacterial reverse mutation assay is an essential part of the battery of tests to determine genotoxicity. The utility of this test for assessing NPs is currently questioned, due to negative results seemingly caused by failure of particle uptake. To probe uptake issues, we examined the physical state in different media, dose and time dependent association, uptake and mutagenicity of titanium dioxide (TiO2) NPs in Salmonella typhimurium and Escherichia coli. The NPs suspended in water were characterized using dynamic light scattering, NP tracking analysis and transmission electron microscopy. NP association with bacteria was assessed by flow cytometry. Association was found to be time and dose dependent, with maximal association by 60min. Therefore mutagenicity was assessed after a 60min pre-incubation in a miniaturized assay demonstrating enhanced sensitivity. To assess potential indirect effects on bacterial mutagenicity, the effect of TiO2 NPs on the action of standard mutagens or on DNA repair capability was also investigated. TiO2 NPs did not affect mutant yields in standard strains of S. typhimurium or E. coli, including those detecting oxidative damage, using the modified methods. Nor did TiO2 NPs affect the action of standard mutagens or DNA excision repair capability. Despite particle association with the bacteria, subsequent analysis using electron microscopy and energy dispersive x-ray spectroscopy indicated that the NPs were not internalized. This work demonstrates that additional studies, including flow cytometry, are valuable tools for understanding the action of NPs in biological systems.