Effects of Aged Biodegradable Plastics and Antibiotics on the Conjugative Transfer of Antibiotic Resistance Genes Between Bacteria

The widespread use of plastics has caused severe environmental pollution, driving interest in biodegradable alternatives like polylactic acid (PLA). However, incomplete degradation of biodegradable plastics under natural conditions may generate micro/nanoplastics that could exacerbate ecological risks. This study investigated the combined effects of UV-aged microplastics from biodegradable PLA and conventional PET, along with sulfamethoxazole (SMX), on the conjugative transfer of antibiotic resistance genes (ARGs) between bacteria. Using UV aging to simulate environmental weathering, the microplastic morphology, adsorption behavior, and interaction with SMX were characterized. The study further evaluated the bacterial viability, ROS level, membrane permeability, and the expression of conjugative transfer-related genes to elucidate the underlying mechanisms. Results showed that aged PLA released significantly more nanoplastics and exhibited higher adsorption affinity for SMX than PET. Combined exposure to aged PLA and SMX significantly enhanced ARG transfer frequency by approximately 14.5-fold compared to the control. Mechanistic studies revealed that this promotion was associated with increased intracellular ROS levels, elevated membrane permeability, and upregulation of conjugative related genes. These findings underscore that biodegradable plastics, after environmental aging, may pose greater ecological risks than conventional plastics, and highlight the importance of considering environmental aging in the risk assessment of plastics.