Low-dose chlorine disinfection poses a greater potential risk of antibiotic resistance genes and their pathogenic hosts.

Identifying the responses of antibiotic resistance genes (ARGs) and their hosts to chlorine disinfection is necessary because it has been paradoxically reported to both amplify and suppress antibiotic resistance in water treatment processes. In this study, our integrated metagenomic and metatranscriptomic analysis of sequencing batch reactors under different chlorine disinfection conditions (0, 2, 6, and 10 mg/L) in activated sludge reveals that low-dose chlorine obviously increases ARG abundance, diversity, and transcriptional activity, particularly for multidrug, β-lactam, and tetracycline types, while higher doses reduce transcriptional diversity and activity. Acinetobacter johnsonii, a pathogen abundant and active under chlorine-addition conditions, poses a high risk of ARG transmission due to its multiple mobile genetic elements and potential involvement in horizontal gene transfer with non-pathogens. Notably, chlorine disinfection may simultaneously promote the co-transfer of chlorine resistance and antibiotic resistance genes, such as the qacE gene, with the involvement of plasmids and integrons. Overall, this study demonstrates that low-dose chlorine may promote greater ARG enrichment, mobility, and pathogenic potential in activated sludge. The findings highlight overlooked risks of low-concentration residual chlorine, urging reconsideration of disinfection strategies to protect public health.