Plasmid engineering reveals size-dependent effects of plastic particles on horizontal gene transfer via transformation in Escherichia coli: Critical roles of plasmid size and plastic particle-bacteria spatial configuration

Plastic particles impact antibiotic resistance genes (ARGs) dissemination majorly via horizontal gene transfer (HGT) in environmental media, yet how different ARGs respond to plastic particles during HGT is rarely studied, and size-dependent effects of plastic particles on HGT remain debated. Here, we investigated polystyrene (PS) particles (20 nm, 80 nm, 2000 nm, 20000 nm) mediating HGT via transformation in Escherichia coli, using engineered pUC19-derived plasmids differing in size (3.75, 5.00, 7.50 kb) and replication capacity. Nanoplastics (NPs) enhanced transformation of 3.75 kb and 5.00 kb plasmids at 0.5 mg/L but inhibited transformation at 18, 36, and 72 mg/L, while consistently inhibiting that of 7.50 kb plasmids. Meanwhile, 2000-nm microplastics (MPs) monotonously promoted HGT efficiencies, yet 20000-nm MPs decreased them (0-72 mg/L). PS particle effects on HGT were independent of plasmid replication capacity. Enhancing mechanisms for HGT majorly involved increased membrane permeability via forming bacterial surface pores (NPs, 2000-nm MPs). The inhibiting mechanism stemmed from size-dependent physical barriers on cell membranes, as observed through scanning electron microscopy and laser scanning confocal microscopy. Three-dimensional models further simulated PS particle-induced spatial barriers on cell surfaces. Our findings improve understanding of environmental ARG dissemination driven by plastic pollution.