Synergistic bactericidal effect by combined exposure to Ag nanoparticles and UVA

•Combined treatment with AgNPs and UV achieved a remarkable antibacterial effect in E. coli.•For the antibacterial effect, it is necessary to satisfy the following requirements:•1) Translocation of nano-sized Ag particles inside E. coli.•2) Oxidation of AgNPs by UVA, and extensive and persistent release of Ag+ inside E. coli.•Ag+ released inside cells reacted with intercellular molecules having –SH groups such as GSH.

Broad and strong antimicrobial properties of silver (Ag) have been used for biomedical applications, water treatment, etc. In this study, a synergistic antibacterial effect between Ag nanoparticles (AgNPs) and ultraviolet (UV) light was examined. AgNPs (< 0.1 μm) with subsequent exposure to UVA (320–400 nm) showed pronounced toxicity in Escherichia coli, but micro-sized Ag particles (> 1 μm) with UVA and AgNPs with UVB (280–325 nm) did not. As significant bactericidal activity was also exhibited by hydrogen peroxide-treated AgNPs, the surface oxidation of AgNPs caused by UVA irradiation was considered to contribute to the enhanced antibacterial effect. Although no difference in NP-incorporation rates was observed with or without the surface oxidation of AgNPs, a particle size of less than 0.1 μm was a factor for AgNPs uptake and an essential requirement for the antimicrobial function of Ag particles. Incorporated AgNPs oxidized by UVA irradiation released larger amounts of Ag ion inside cells than reduced AgNPs, which reacted with intercellular molecules having –SH groups such as glutathione. The synergistic use of AgNPs and UVA could become a powerful tool with broad antimicrobial applications.

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