Combined Antibiotic and Herbicide Pollution Accelerates the Horizontal Transfer of Antibiotic Resistance Genes in Coastal Microbial Communities

The extensive use of herbicides and antibiotics in aquaculture has led to compounded pollution of coastal waters, marked by the co-occurrence of herbicides, antibiotic residues, and antibiotic resistance genes (ARGs). Plasmid-mediated conjugative transfer is a major driver of the dissemination and evolution of ARGs, yet the influence of herbicides alone or in combination with antibiotics on this process in aquatic bacterial communities remains unclear. In this study, we demonstrated that triazine herbicides, alone and in combination with a fluoroquinolone antibiotic, significantly promote the conjugative transfer of the broad-host-range RP4 plasmid. Using Escherichia coli DH5α as a donor, we observed increased plasmid transfer to multiple recipients, including E. coli HB101, Pseudomonas putida KT2440, and a natural coastal seawater microbial community. The enhanced transfer under co-exposure was associated with several interacting mechanisms, including elevated intracellular reactive oxygen species (ROS) and adenosine triphosphate (ATP) levels, increased cell membrane permeability, altered extracellular polymeric substance (EPS) composition, and upregulated expression of conjugation-related genes. Overall, this work underscores the potential role of combined herbicide and antibiotic contamination in shaping the microbial resistome of coastal ecosystems and provides insights to inform strategies for mitigating the spread of antibiotic resistance.