Mechanism of low concentrations of polystyrene microplastics influence the cytotoxicity of Ag ions to Escherichia coli

Polystyrene microplastics (PSMPs) with different sizes, surface charges and aging statuses simulated field PSMPs and were applied to understand their cytotoxicity to Escherichia coli. The PSMPs hardly affected the viability, membrane integrity, ROS generation and ATPase activity of E. coli, and the cytotoxicity of field PSMPs is marginal and assumed to be overestimated. Low concentrations (1.0 mg L−1) of PSMPs dynamically affect the cytotoxicity of Ag+ to E. coli through various toxic mechanisms. PSMPs likely mitigated the cytotoxicity of Ag+ during the initial 24 h of co-exposure by protecting the cell membrane, inhibiting ROS generation and/or recovering ATPase activity (p < 0.05 or p < 0.01). During prolonged co-exposure for 48 h, nonfunctionalized polystyrene (PS-NF) still mitigated the cytotoxicity of Ag+ by protecting the integrity of the cell membrane, and aged PS-NF slightly affected cytotoxicity. PS-NH2 and PS-COOH intensified the cytotoxicity of Ag+ because PS markedly promoted ROS generation and inhibited ATPase activity. Thus, field PSMPs were assumed to exhibit marginal cytotoxicity to E. coli and can combine with surrounding Ag+ to modify the E. coli population levels and even the structure of aquatic ecosystems. Accordingly, the environmental and health risks of field PSMPs require further intensive investigation, and the combined toxicity effects of field PSMPs with Ag+ should be considered carefully due to their dynamic toxic effects and mechanisms.