a comparison of the mechanisms of toxicity of zinc oxide nanomaterials and zinc ions to the bacterium Pseudomonas putida. nanomaterial toxicity to Pseudomonas

Metal oxide nanomaterials are increasingly used in a range of applications in consumer electronics, personal health care and as antimicrobial agents. These nanomaterials show broad range toxicity to microorganisms (bacteria and fungi) but there is no consistent view regarding whether toxicity is mediated by reactive oxygen species or by metal ions resulting from partial nanomaterial dissolution. However a thorough understanding is necessary, not only to evaluate nanomaterials as antimicrobial agents but also to aid the prediction of outcomes of increasing environmental exposures to nanomaterials. We report that zinc oxide nanomaterials are equally as toxic as free zinc ions to the soil bacterium Pseudomonas putida KT2440 in batch culture. Global transcriptome sequencing identified a common response of cells exposed to ZnO nanomaterials or Zn ions: transcription of zinc import genes were reduced while transcription of zinc export genes were increased. Furthermore, a number of genes for which reduced transcription were identified coded for proteins with requirements for Zn2+ and other metal cations. We could identify no nanomaterial specific changes in the phospholipid fatty acid components of exposed cells and genes indicative of cellular responses to reactive oxygen species did not show altered levels of transcription compared to unexposed cells. This combined evidence suggests that antibacterial effects of ZnO nanomaterials are due to the production of free zinc ions and not by production of reactive oxygen species.