Toxicity of Food-Grade TiO2 to Commensal Intestinal and Transient Food-Borne Bacteria: New Insights Using Nano-SIMS and Synchrotron UV Fluorescence Imaging

Abstract

Titanium dioxide (TiO₂) is commonly used as a food additive (E171 in the EU) for its whitening and opacifying properties. However, a risk of intestinal barrier disruption, including dysbiosis of the gut microbiota, is increasingly suspected because of the presence of a nano-sized fraction in this additive. We hypothesized that food-grade E171 and Aeroxyde P25 (identical to the NM-105 OECD reference nanomaterial in the European Union Joint Research Centre) interact with both commensal intestinal bacteria and transient food-borne bacteria under non-UV-irradiated conditions. Based on differences in their physicochemical properties, we expect a difference in their respective effects. To test these hypotheses, we chose a panel of eight Gram-positive/Gram-negative bacterial strains, isolated from different biotopes and belonging to the species <i>Escherichia coli</i>, <i>Lactobacillus rhamnosus</i>, <i>Lactococcus lactis</i> (subsp. <i>lactis</i> and <i>cremoris</i>), <i>Streptococcus thermophilus</i>, and <i>Lactobacillus sakei</i>. Bacterial cells were exposed to food-grade E171 vs. P25 <i>in vitro</i> and the interactions were explored with innovative (nano)imaging methods. The ability of bacteria to trap TiO₂ was demonstrated using synchrotron UV fluorescence imaging with single cell resolution. Subsequent alterations in the growth profiles were shown, notably for the transient food-borne <i>L. lactis</i> and the commensal intestinal <i>E. coli</i> in contact with food-grade TiO₂. However, for both species, the reduction in cell cultivability remained moderate, and the morphological and ultrastructural damages, observed with electron microscopy, were restricted to a small number of cells. <i>E. coli</i> exposed to food-grade TiO₂ showed some internalization of TiO₂ (7% of cells), observed with high-resolution nano-secondary ion mass spectrometry (Nano-SIMS) chemical imaging. Taken together, these data show that E171 may be trapped by commensal and transient food-borne bacteria within the gut. In return, it may induce some physiological alterations in the most sensitive species, with a putative impact on gut microbiota composition and functioning, especially after chronic exposure.