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The current research reports the antibacterial effects of silver (Ag) and citric acid coated iron oxide (Fe 3 O 4 ) NPs on Escherichia coli wild type and kanamycin-resistant strains, as well as on Salmonella typhimurium MDC1759. NPs demonstrated significant antibacterial activity against these bacteria, but antibacterial effect of Ag NPs is more pronounced at low concentrations. Ag NPs inhibited 60–90% of S. typhimurium and drug-resistant E. coli. The latter is more sensitive to Fe 3 O 4 NPs than wild type strain: the number of bacterial colonies is decreased ~ 4-fold. To explain possible mechanisms of NPs action, H + -fluxes through the bacterial membrane and the H + -translocating F O F 1 -ATPase activity of bacterial membrane vesicles were studied. N,N′- Dicyclohexylcarbodiimide (DCCD)-sensitive ATPase activity was increased up to ~ 1.5-fold in the presence of Fe 3 O 4 NPs. ATPase activity was not detected by Ag NPs even in the presence of DCCD, which confirms the bactericidal effect of these NPs. The H + -fluxes were changed by NPs and by addition of DCCD. H 2 yield was inhibited by NPs; the inhibition by Ag NPs is stronger than by Fe 3 O 4 NPs. NPs showed antibacterial effect in bacteria studied in concentration-dependent manner by changing in membrane permeability and membrane-bound enzyme activity. The F O F 1 -ATPase is suggested might be a target for NPs.

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