Absence of synergistic effects between microplastics and copper ions on the spread of antibiotic resistance genes within aquatic bacteria at the community level

Microplastics and copper ions (Cu) are favorable in accelerating the propagation of antibiotic resistance genes (ARGs) in the plastisphere, however, their combined effects on the ARG spread within the bacterial community of the natural environment were less understood. The influence of microplastic types and Cu concentrations on the horizontal gene transfer (HGT) of ARGs mediated by RP4 plasmid within natural bacterial communities in aquatic environments was investigated. Both biodegradable polybutylene succinate (PBS) and non-biodegradable polyvinyl chloride (PVC) microplastics significantly enhanced the transfer of ARGs, with PBS showing a significant higher effect compared to PVC. Cu also increased transconjugation rates at environmentally relevant concentrations (5 μg L), but higher levels (50 μg L) lead to decreased rates due to severe bacterial cell membrane damage. The transconjugation rates in the presence of both microplastics and Cu were lower than the sum of their individual effects, indicating no synergistic effects between them on transconjugation. Proteobacteria dominated the composition of transconjugates for all the treatment. Transmission electron microscope images and reactive oxygen species production in bacterial cells indicated that the increased contact frequency due to extracellular polymeric substances, combined with enhanced membrane permeability induced by microplastics and Cu, accounted for the increasing transconjugation rates. The study provides valuable insight into the potential effects of microplastics and heavy metals on the spread of ARGs from donors to bacterial communities in natural environments.