Dual impact of dissolved organic matter on cytotoxicity of PVP-Ag NPs to Escherichia coli: Mitigation and intensification.

Cytotoxicity of silver nanoparticles (Ag NPs) is attributed to silver internalization, which can be depressed by dissolved organic matter (DOM) to form Ag-DOM. Ag-DOM are bioavailable and then silver internalization could be increased. Herein, 3 and 48-h Escherichia coli viability bioassays were employed to evaluate effect of fulvic acid and humic acid (10 mg L) on cytotoxicity of 30 and 100 nm PVP-Ag NPs. Moreover, zebrafish embryos were used as reference model to understand silver internalization routes. Ag ions or Ag-DOM internalization varied in routes to Escherichia coli and zebrafish embryos. Cytotoxicity mechanisms of PVP-Ag NPs are dynamic. In 3-h bioassays, cytotoxicity of PVP-Ag NPs mainly involves Ag particle-related toxicity. DOM significantly mitigated cytotoxicity of PVP-Ag NPs (p < 0.05) through adhering on surface of PVP-Ag NPs. Ag ions or Ag-DOM were hardly internalized into Escherichia coli cells during 3-h exposures duration. In 48-h bioassays, cytotoxicity of PVP-Ag NPs is dependent on Ag ion-related and particle-related toxicity. Silver was internalized into Escherichia coli via Ag-DOM consumption as an indirect route. Thus, DOM promoted silver internalization into Escherichia coli but not into zebrafish embryos, significantly intensifying cytotoxicity (p < 0.05). The results suggested DOM has a dual impact on cytotoxicity of PVP-Ag NPs: mitigation and intensification. For risk assessment, it should be considered that cytotoxicity mechanisms of PVP-Ag NPs are dynamic and threats can be intensified by DOM during prolonged exposure.