Trait-mediated restructuring of gut microbiota under chlorinated drinking water exposure

Chlorine residuals in drinking water are environmentally relevant oxidants regulated within distribution systems and ingested during routine consumption. Here, we use longitudinal, within-subject designs in humans (0.5 mg/L chlorine exposure) and a parallel mouse model (10 mg/L) to assess the ecological impact of chlorine residuals on gut microbiota under realistic conditions. Crucially, overall diversity, total bacterial biomass, antibiotic resistance genes, and phage communities remained largely unaffected. However, we report a lineage-independent de-dominance effect, where initially dominant taxa decline following exposure. Genome-resolution analysis reveals that microbes with larger genomes and functional enrichment in energy metabolism and membrane biogenesis are more likely to increase, enabling accurate prediction of microbial responses to chlorination. These patterns can be interpreted within the Competitor-Stress-tolerator-Ruderal life-history framework, in which disturbance of chlorine residuals transiently reduces the advantage of competitive dominant taxa and favors stress-tolerant taxa. Our findings demonstrate that chlorination residuals act as subtle, trait-mediated ecological stressors in the gut microbiome, producing selective yet predictable shifts. These insights frame chlorine residuals as hazardous environmental agents and inform microbiome-aware optimization of water disinfection and residual control.