Inactivation and reactivation of antibiotic-resistant bacteria by chlorination in secondary effluents of a municipal wastewater treatment plant

Reports state that chlorination of drinking water and wastewater affects the proportions of antibiotic-resistant bacteria by potentially assisting in microbial selection. Studies on the effect of chlorination on like species of antibiotic-resistant bacteria, however, have shown to be conflicting; furthermore, few studies have inspected the regrowth or reactivation of antibiotic-resistant bacteria after chlorination in wastewater. To understand the risks of chlorination resulting from potentially selecting for antibiotic-resistant bacteria, inactivation and reactivation rates of both total heterotrophic bacteria and antibiotic-resistant bacteria (including penicillin-, ampicillin-, tetracycline-, chloramphenicol-, and rifampicin-resistant bacteria) were examined after chlorinating secondary effluent samples from a municipal wastewater treatment plant in this study.Our experimental results indicated similar inactivation rates of both total heterotrophic bacteria and antibiotic-resistant bacteria. Microbial community composition, however, was affected by chlorination: treating samples with 10 mg Cl₂/L for 10 min resulted in chloramphenicol-resistant bacteria accounting for nearly 100% of the microbial population in contrast to 78% before chlorination. This trend shows that chlorination contributes to selection of some antibiotic-resistant strains. Reactivation of antibiotic-resistant bacteria occurred at 2.0 mg Cl₂/L for 10 min; specifically, chloramphenicol-, ampicillin-, and penicillin-resistant bacteria were the three prevalent groups present, and the reactivation of chloramphenicol-resistant bacteria exceeded 50%. Regrowth and reactivation of antibiotic-resistant bacteria in secondary effluents after chlorination with a long retention time could threaten public health security during wastewater reuse.