UV-aging of microplastics increases proximal ARG donor-recipient adsorption and leaching of chemicals that synergistically enhance antibiotic resistance propagation

Despite growing attention to environmental pollution by microplastics (MP), the effects of MP aging on bacterial horizontal gene transfer (HGT) have not been systematically investigated. Here, we used UV-aged polystyrene microplastics (PS-MPs) to investigate how aging affects antibiotic resistance genes (ARGs) transfer efficiency from various ARG vectors to recipient bacteria. The adsorption capacity of MP (20-day UV-aged PS-MPs) towards E. coli (harboring plasmid-borne bla), plasmid pET29 (harboring bla) and phage lambda (carrying the aphA1 ARG) increased by 6.6-, 5.2- and 8.3-fold, respectively, relative to pristine PS-MPs (MP), due to increased specific surface area and affinity for these ARG vectors. Moreover, MP released more organic compounds (TOC 1.6 mg/g-MP, versus 0.2 mg/g-MP in 4 h) -possibly depolymerization byproducts (verified by GC-MS), which induced intracellular ROS generation, increased cell permeability and upregulated HGT associated genes. Accordingly, MP enhanced ARG transfer frequency from E. coli, plasmid pET29 and phage lambda (relative to MP) by 1.3-, 4.7- and 3.5-fold, respectively. The Bliss independence model infers that higher bacterial adsorption and exposure to chemicals released during MP aging synergistically enhanced ARG transfer. This underscores the need to assess the significance of this overlooked phenomenon to the environmental dissemination of antibiotic resistance and other HGT processes.