Responses of a soil bacterium, Pseudomonas chlororaphis O6 to commercial metal oxide nanoparticles compared with responses to metal ions

The toxicity of commercially-available CuO and ZnO nanoparticles (NPs) to pathogenic bacteria was compared for a beneficial rhizosphere isolate, Pseudomonas chlororaphis O6. The NPs aggregated, released ions to different extents under the conditions used for bacterial exposure, and associated with bacterial cell surface. Bacterial surface charge was neutralized by NPs, dependent on pH. The CuO NPs were more toxic than the ZnO NPs. The negative surface charge on colloids of extracellular polymeric substances (EPS) was reduced by Cu ions but not by CuO NPs; the EPS protected cells from CuO NPs-toxicity. CuO NPs-toxicity was eliminated by a Cu ion chelator, suggesting that ion release was involved. Neither NPs released alkaline phosphatase from the cells’ periplasm, indicating minimal outer membrane damage. Accumulation of intracellular reactive oxygen species was correlated with CuO NPs lethality. Environmental deposition of NPs could create niches for ion release, with impacts on susceptible soil microbes.

► Toxicity of metallic nanoparticles (NPs) was evaluated in a beneficial bacterium, Pseudomonas chlororaphis O6 (PcO6).
► Aggregated commercial CuO and ZnO NPs released Cu and Zn ions and changed bacterial surface charge, depending on pH.
► The NPs were toxic to PcO6 through NP-specific, but also ion release mechanisms.
► Reactive oxygen species were produced by CuO NP and Cu ion at lethal concentrations, but bacterial EPS protected against Cu.
► The periplasmic marker, alkaline phosphate, activity was increased by the NPs and ions.