Do microplastic biofilms promote the evolution and co-selection of antibiotic and metal resistance genes and their associations with bacterial communities under antibiotic and metal pressures?

Microplastic (MP) biofilms with heterogeneous bacterial compositions and structure have become a hotspot of antibiotic resistance genes (ARGs) in aquatic environments. The evolutionary features of ARGs and their related factors including class 1 integron (intI1), metal resistance genes (MRGs), and bacterial communities in MP biofilms under exogenous pressures and how they compared with natural substrates (NS) are unclear. The individual and combined pressures of sulfamethoxazole, tetracycline, and zinc were used to drive the dynamic evolution of ARGs, intI1, MRGs, and bacterial communities in the MP and NS biofilms. The exogenous pressures from the combined selection of sulfamethoxazole, tetracycline, and zinc and their increasing concentrations both significantly enhanced the abundances of ARGs on the MP compared to the NS. Meanwhile, the selective pressures resulted in obvious dissimilarities between the MP and NS bacterial communities. The core bacterial taxa and the co-occurrence patterns of ARGs and bacterial genera in the biofilms of MP and NS were obviously different, and more potential ARG host bacteria selectively colonized the MP. Metal pressure also enhanced the enrichment of ARGs in the MP biofilms by promoting the spread of intI1 via the co-selection mechanism.