الفهرس 
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Abstract
The use of metal/metal oxides nanoparticles (NPs) in consumer products has increased dramatically. Accordingly, human exposure to these NPs has increased. Lactobacillus reuteri, a member of beneficial gut microbiota, is crucial for the human health. In the present study, the toxic effects of three metal oxides (CuO, ZnO, and CdO) and one metal (Ag) NPs on L. reuteri were investigated in vitro. L. reuteri was susceptible to all NPs in a dose-dependent manner, visualized as an increase in the zones of inhibition and a significant reduction in the maximum specific growth rates. The minimal inhibitory concentrations (MICs) were 5.8, 26, 560 and 560 g/mL for CdO-, Ag-, ZnO- and CuO- NPs, respectively, and the respective minimal bactericidal concentrations (MBCs) were 60, 70, 1500 and 1500 g/mL. Electron microscopic examinations revealed the adsorption of the NPs on L. reuteri cell surface, causing cell wall disruption and morphological changes. These changes were accompanied by significant leakage of cellular protein content by 214, 191, 112, and 101% vs. the untreated control when L. reuteri was treated with CdO-, Ag-, CuO-, and ZnO NPs, respectively. NPs also induced oxidative damage, where the malondialdehyde level was significantly increased, and glutathione content was significantly decreased. Quantifying the DNA damage using Comet assay showed that CuONPs (560 g/mL) had the maximum DNA tail length (8.2 px vs. 2.1 px for the control). While CdONPs (5.8 g/mL) showed the maximum percentage of DNA in the tail (15.5% vs. 3.1%). This study provides a mechanistic evaluation of the NPs-mediated toxicity to a beneficial microorganism.