Persistence and dynamics of antibiotic resistome in a drinking water supply system with booster chlorination

Due to the extensive use of antibiotics worldwide, the prevalence of antibiotic resistance genes (ARGs) in aquatic environments has become a major public health concern. This study investigated the ARGs in a drinking water supply system, with particular emphasis on booster chlorination in the distribution network. To elucidate the dynamics of the antibiotic resistome, environmental DNA was extracted from water collected from five different sections, and the resistome profiles were subsequently reconstructed with metagenome assembly. Our findings revealed that 35 core ARGs persisted but decreased in concentration during water treatment and early distribution, with genes resistant to bacitracin, multidrug, and rifamycin being the most prominent. However, a notable surge of ARGs was observed at the terminal distribution segment. This increase was linked to changes in the resistome structure, which were primarily associated with shifts in the microbial community and, within the DWDS specifically, also linked to horizontal transfer mediated by mobile genetic elements (MGEs) under chlorine stress from booster chlorination. Microbial communities within the drinking water distribution system (DWDS) shifted distinctly from those in the water treatment plant. Under re-chlorination pressure, the chlorine-tolerant Mycobacteriales and the biofilm-forming Hyphomicrobiales and Rhodobacterales became the predominant taxa. Additionally, metagenome-assembled genomes (MAGs) reconstruction further identified that Hyphomicrobium and Mycobacterium were the main ARG carriers in the DWDS, with the latter as the main putative host for the core ARGs. Overall, this study demonstrated that booster chlorination in the water distribution system while controlling microbial regrowth, may simultaneously facilitate ARG dissemination. These findings highlight the need to optimise re-chlorination practices to balance microbial growth control while minimising ARG proliferation in DWDS.