Synergistic effects of micro/nanoplastics and Cu(II) on horizontal transfer of antibiotic resistance genes: New insight targeting on cell surface properties

Microplastics (MPs) and nanoplastics (NPs) facilitate antibiotic resistance genes (ARGs) transfer through horizontal gene transfer (HGT). However, the combined effects of M-NPs and heavy metals on HGT remain poorly understood, and the effects of cell surface properties is neglected. In this study, an antibiotic co-existence heavy metal Cu was used to study its synergetic effect with M-NPs on HGT, with a specific focus on bacterial surface characteristics and physiological responses. Results reveal that NPs amplified Cu(II)'s effect on conjugative transfer of ARGs, while MPs showed mitigation effect. NPs+Cu(II) co-exposure yielded the highest conjugative transfer frequency (4.4-fold) and a 35-fold surge in transformation frequency compared to the control. These disparities stem from bacterial physiological responses, including 4-7-fold elevated reactive oxygen species (ROS), 3-4-fold increased membrane permeability, 1.5-1.8-fold enhanced ATP synthesis, altered drug-resistant efflux and metabolic pathways; Furthermore, cell surface property modulation-Cu(II) stimulated 1.2-fold lipopolysaccharide (LPS) production and M-NPs regulated outer membrane vesicles (OMVs) concentration/sizes, with extracellular polymeric substances (EPS) optimizing interbacterial aggregation for gene transfer. In addition, MPs+Cu(II) induced 49 % viable but non-culturable (VBNC) bacteria and high-dose M-NPs caused excessive bacterial injury/death, reducing gene transfer (VBNC ratio indicating stress severity). These findings highlight co-exposure impacts and offer novel insights into the environmental risks posed by M-NPs and ARGs.