Salinity-dependent effects of erythromycin on biofilm development and conjugative plasmid transfer on poly(vinyl chloride) microplastics.

Microplastics (MPs) and antibiotics frequently co-occur in aquatic environments and jointly influence the dissemination of antibiotic resistance genes (ARGs). However, the modulatory role of salinity, a highly dynamic factor in estuarine and coastal regions, remains underexplored. This study investigated the effects of erythromycin (ERY) on biofilm development and RP4 plasmid conjugative transfer on poly(vinyl chloride) (PVC) MPs across varying salinity levels (5‰, 10‰, and 15‰). Results demonstrated that MP hydrophobicity increased with salinity, while the ERY adsorption capacity followed the order: 5‰ > 15‰ > 10‰. A higher salinity (15‰) promoted biofilm biomass and extracellular polymeric substance (EPS) secretion on MPs. Notably, conjugative transfer frequencies within MP-associated biofilms were consistently higher than those in planktonic cultures. Under varying salinities, the conjugation frequencies in biofilms displayed a U-shaped trend, with higher rates observed at 5‰ and 15‰ compared to 10‰. Crucially, the impact of ERY on plasmid transfer was highly salinity-dependent, i.e., the addition of ERY promoted conjugation at 5‰ and 10‰ but significantly inhibited it at 15‰. Physiological assessments revealed that exposure to varying salinities and ERY induced non-lethal oxidative stress, evidenced by elevated reactive oxygen species and upregulated antioxidant enzymes (CAT and SOD), while lipid peroxidation (MDA) remained relatively low. Concurrently, intracellular proline accumulation and cell membrane permeability increased significantly with rising salinity. These findings provide direct experimental evidence that salinity fluctuations and antibiotic residues jointly alter bacterial physiological states and antibiotic resistance dissemination on MPs, which highlights the necessity of considering dynamic environmental variables in ecological risk assessments.