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 Escherichia coli, Lactobacillus rhamnosus, Lactococcus lactis (subsp. lactis and cremoris), Streptococcus thermophilus, and Lactobacillus sakei. Bacterial cells were exposed to food-grade E171 vs. P25 in vitro 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 L. lactis and the commensal intestinal E. coli 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. E. coli 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.
Highlights
Engineered nanomaterials are increasingly used in numerous industrial sectors, due to their unique properties compared to their larger counterparts, provided by their nanometric dimensions and their high specific surface area (Nel et al, 2006)
Food-grade E171 particles in aggregated (A) and dispersed (D) forms, like their P25 counterparts, could be excited in the deep ultraviolet (DUV) range at 270 nm and displayed fluorescence emission spectra with characteristic peaks of TiO2 (Serpone et al, 1995), i.e., a maximum of fluorescence emission around 390 nm with fluorescence intensity remaining at high levels up to 470 nm (Figure 1A)
The signals in the [327–353 nm] range originate from cell autofluorescence, mainly due to tryptophan fluorescence [peaking at around 340 nm (Jamme et al, 2010; Kašèáková et al, 2012)], which is responsible for the characteristic optical properties of many proteins (Lakowicz, 2006), whereas, as seen above (Figure 1A), emission from TiO2-exposed bacterial cells in the [420–480 nm] range is attributed to the fluorescence of TiO2 particles, located inside and/or on the surface of the bacterial cells
Summary
Engineered nanomaterials are increasingly used in numerous industrial sectors, due to their unique properties compared to their larger counterparts, provided by their nanometric dimensions and their high specific surface area (up to several hundred m2/g of product) (Nel et al, 2006). A sizable fraction (17–55% of total particles) of nano-sized particles (
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