| Summary: | Food-grade titanium dioxide (TiO<sub>2</sub>) containing a nanoparticle fraction (TiO<sub>2</sub> NPs -nanoparticles) is widely used as a food additive (E171 in the EU). In recent years, it has increasingly been raising controversies as to the presence or absence of its harmful effects on the gastrointestinal microbiota. The complexity and variability of microbiota species present in the human gastrointestinal tract impede the assessment of the impact of food additives on this ecosystem. As unicellular organisms, bacteria are a very convenient research model for investigation of the toxicity of nanoparticles. We examined the effect of TiO<sub>2</sub> (three types of food-grade E171 and one TiO<sub>2</sub> NPs, 21 nm) on the growth of 17 strains of lactic acid bacteria colonizing the human digestive tract. Each bacterial strain was treated with TiO<sub>2</sub> at four concentrations (60, 150, 300, and 600 mg/L TiO<sub>2</sub>). The differences in the growth of the individual strains were caused by the type and concentration of TiO<sub>2</sub>. It was shown that the growth of a majority of the analyzed strains was decreased by the application of E171 and TiO<sub>2</sub> NPs already at the concentration of 150 and 300 mg/L. At the highest dose (600 mg/L) of the nanoparticles, the reactions of the bacteria to the different TiO<sub>2</sub> types used in the experiment varied.
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