Modifications of the bacterial reverse mutation test reveals mutagenicity of TiO2 nanoparticles and byproducts from a sunscreen TiO2-based nanocomposite

The bacterial reverse mutation test, recommended by the Organization for Economic Co-operation and Development (OECD) to determine genotoxicity of chemical compounds, has been recently used by several authors to investigate nanoparticles. Surprisingly, test results have been negative, whereas in vitro mammalian cell tests often give positive genotoxic responses. In the present study, we used the fluctuation test procedure with the Salmonella typhimurium strains TA97a, TA98, TA100 and TA102 to determine the mutagenic potential of TiO2 nanoparticles (NP-TiO2) and showed that, when it is used conventionally, this test is not suitable for nanoparticle genotoxicity assessment. Indeed, the medium used during exposure prevents electrostatic interactions between bacterial cells and nanoparticles, leading to false-negative responses. We showed that a simple pre-exposure of bacteria to NP-TiO2 in a low ionic strength solution (NaCl 10 mM) at a pH below the nanoparticle isoelectric points (pH 5.5) can strongly improve the accuracy of the test. Thus, based on these improvements, we have demonstrated the genotoxicity of the engineered NP-TiO2 tested and a NP-TiO2 byproduct from a sunscreen nanocomposite. It was also shown that strain TA102 is more sensitive than the other strains, suggesting an oxidative stress-mediated mechanism of genotoxicity.

► The Ames test is not suitable for nanoparticle (NP) genotoxicity assessment.
► The Ames test medium prevents electrostatic interactions between bacteria and NPs.
► The Ames test medium strongly promotes the aggregation of NPs.
► Simple pre-exposure step in an adequate medium improve the accuracy of the test.
► Modified Ames test showed mutagenicity of NP-TiO2 and NP-TiO2-based nanocomposite.