Bio-based microplastics as vectors of resistance genes under combined pressure of antibiotics and heavy metals in marine environment

In this study, we investigated the characteristics of biofilm formation on petroleum-based polyethylene (PE) and bio-based polylactic acid (PLA) microplastics, the structure of bacterial communities, and the enrichment and transfer of related resistance genes in marine environments. We examined these factors under varying concentrations of the heavy metal zinc (Zn) and the sulfadiazine (SDZ), both individually and in combination, and analyzed the underlying mechanisms and interrelationships. The results indicated that PE surface was more conducive to bacterial colonization and biofilm stabilization. Conversely, the prolonged combined exposure to SDZ and Zn promoted the growth of PLA biofilm. Bacterial communities within the biofilms responded to external stresses through oxidative stress responses, alterations in extracellular polymeric substances, shifts in the relative abundance of specific microbial taxa, and adjustments in metabolic pathways. These adaptations positively influenced the enrichment and transfer of resistance genes. Under experimental conditions, PLA microplastics were more likely than PE to serve as carriers of resistance genes in marine environments. Zn promoted the spread of resistance genes by enhancing horizontal gene transfer (HGT) in the short term, and in the later stages, shaped microbial community composition and co-selected with SDZ, thereby influencing the distribution and dissemination of resistance genes.