Charged nanoplastics differentially affect the conjugative transfer of antibiotic resistance genes

As nanoplastics (NPs) increasingly become a global pollutant, their impact on the spread of antibiotic resistance genes (ARGs) remains unclear. In this study, we investigated the effects of the surface charge and colloidal stability of NPs, and organic matter on the conjugative transfer of plasmid-borne ARGs using widely used polystyrene (PS) as a representative. Our findings demonstrated that negatively charged PS and PS-COOH enhanced ARG transfer at low concentrations, while transfer declined at higher concentrations (up to 500 mg L-1). Conversely, PS-NH2 increased the conjugative transfer of ARGs at all concentrations tested (50-500 mg L-1), with the highest promotion effect being 1.7 times greater than the control at 500 mg L-1. At low concentrations, ROS overproduction induced by charged NPs was found to enhance transfer by increasing membrane permeability and upregulating transfer-related regulatory genes. At high concentrations (500 mg L-1), the effects on ARG transfer were a combination of ROS production-induced promotion and NP agglomeration-caused inhibition. Additionally, environmental organic matter such as humic acid had limited influence on the interaction between NPs and bacteria. Our study highlights the risks associated with the co-pollution of NPs and ARGs in the environment and underscores the significance of nanoparticle surface charge in disseminating ARGs.