Microplastics amplify Cr(VI)-mediated RP4 plasmid conjugative transfer through coupled interfacial, oxidative, and metabolic regulation.

Microplastics (MPs) can accumulate heavy metals and influence plasmid-mediated antibiotic resistance gene (ARGs) transfer, yet the regulatory mechanisms under metal-associated MP exposure remain poorly understood. Here, a polystyrene microplastic-Cr(VI) composite (PSMPs@Cr) was established to investigate RP4 plasmid conjugative transfer by integrating physicochemical characterization, physiological measurements, transcriptomic analysis, and multivariable modeling. PSMPs@Cr exhibited a non-monotonic effect on conjugative transfer, with the highest conjugative transfer frequency (9.4 × 10-2) observed at 20 ppm Cr loading, representing a 4.75-fold increase compared with the control. Cr adsorption reconstructed the MP interface by modifying surface charge, wettability, morphology, and colloidal stability, which was accompanied by altered bacterial adhesion behavior. Meanwhile, PSMPs@Cr exposure induced coordinated changes in ROS accumulation, antioxidant responses, ATP production, membrane-associated functions, and energy metabolism. Transcriptomic analysis further revealed treatment-dependent regulation of oxidative stress, membrane transport, cellular communication, and metabolic pathways. Random forest-SHAP analysis identified zeta potential, CAT activity, ATP, and ROS as relatively important variables associated with CTF variation. Overall, PSMPs@Cr-associated ARGs transfer was linked to coupled interfacial, redox, and energetic processes rather than a single regulatory pathway, providing new insights into how microplastic-metal interactions influence plasmid-mediated horizontal gene transfer under complex contaminant exposure scenarios.