Antibacterial Action of Zn²⁺ Ions Driven by the In Vivo Formed ZnO Nanoparticles

Antibacterial formulations based on zinc oxide nanoparticles (ZnO NPs) are widely used for antibiotic replacement in veterinary medicine and animal nutrition. However, the undesired environmental impact of ZnO NPs triggers a search for alternative, environmentally safer solutions. Here, we show that Zn²⁺ in its ionic form is a more eco-friendly antibacterial, and its biocidal action rivals that of ZnO NPs (<100 nm size), with a minimal biocidal concentration being 41(82) μg mL^-1 vs 5 μg mL^-1 of ZnO NPs, as determined for 10³(10⁶) CFU mL^-1 E. coli. We demonstrate that the biocidal activity of Zn²⁺ ions is primarily associated with their uptake by E. coli and spontaneous in vivo transformation into insoluble ZnO nanocomposites at an internal bacterial pH of 7.7. Formed in vivo nanocomposite then damages E. coli membrane and intracellular components from the inside, by forming insoluble biocomposites, whose formation can also trigger ZnO characteristic reactions damaging the cells (e.g., by generation of high-potential reactive oxygen species). Our study defines a special route in which Zn²⁺ metal ions induce the death of bacterial cells, which might be common to other metal ions capable of forming semiconductor oxides and insoluble hydroxides at a slightly alkaline intracellular pH of some bacteria.