Combination-specific effects of antibiotic and disinfectant co-contamination on the enrichment of antibiotic resistance genes in sediments.

The environmental co-occurrence of disinfectants and antibiotics has emerged as a typical multi-pollutant issue, with river sediments serving as critical sinks and accumulation media. However, whether their combined exposure accelerate antibiotic resistance genes (ARGs) dissemination in sediments remains unclear. Here, we systematically evaluated the effects of single and combined exposures to sodium hypochlorite and representative antibiotics on the sediment resistome and microbial communities. The results demonstrated that combined exposure generally increased the relative abundances of ARGs (1.83- to 3.57-fold) and mobile genetic elements (MGEs, 2.39- to 22.28-fold). Notably, sodium hypochlorite-gentamicin combination exhibited the most pronounced synergistic effect, consistent with toxicological model prediction. Combined exposure also reshaped bacterial community structure and enriched potential ARG-host taxa, while co-occurrence network analysis identified Limosilactobacillus as a candidate genus associated with multiple ARGs. Mechanistically, partial least squares path modeling indicated that MGE-mediated horizontal gene transfer was the dominant pathway driving ARGs enrichment. Plasmid conjugation assays further showed that NaClO + gentamicin increased conjugation frequency by 3.81-fold, supporting enhanced horizontal gene transfer. Overall, this study elucidates the intrinsic mechanisms by which complex pollution drives ARGs propagation in sediments, highlighting the overlooked ecological risks associated with disinfectant-antibiotic co-contamination in aquatic environments.