Mitigation of CuO nanoparticle-induced bacterial membrane damage by dissolved organic matter

• CuO NPs caused bacterial membrane damage and DOM reduced this damage
• Damage of cell wall and membrane by CuO NPs was directly observed under SEM/TEM
• Fulvic acid reduced the leakage of intracellular K+ ions caused by CuO NPs
• Fulvic acid hindered direct contact by electrostatic repulsion and surface coating

This work investigated the toxicity of CuO nanoparticles (NPs) to Escherichia coli in the presence of dissolved organic matter (DOM). CuO NPs (2, 6 and 10 mg/L) exhibited high bacteria toxicity in distilled water prepared medium, with a 2 h median inhibitory concentration of 10 mg/L. The toxicity was significantly reduced by the presence of fulvic acid (FA, 5 and 40 mg/L) which is an important fraction of DOM. Bacteria membranes were highly damaged by CuO NPs, but FA mitigated this damage. Disruption of the bacteria wall and membrane was directly observed using transmission/scanning electron microscopy. Moreover, CuO NPs significantly caused the leakage of K+ ions from bacteria cells to extracellular medium, while this leakage was reduced by FA. The reduced toxicity was due to enhanced electrostatic repulsion and surface coating by FA, therefore, the physical contact between CuO NPs and bacteria membrane was hindered. These findings indicate that DOM is of importance for antibacterial activity of NPs, and more attention should be paid to the effect of DOM when assessing the environmental risk of NPs.

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