Coupled physicochemical and microbial recovery of aged premise plumbing following water-supply interruption

Water-supply interruptions can cause pronounced deterioration of premise-plumbing water quality, yet the coordinated responses of bacterial communities, antibiotic resistance, bacteriophages, and microbial functional potential during water-supply resumption remain poorly resolved. Here, a full-scale 24-h water-supply interruption in an approximately 25-year-old cast-iron premise-plumbing system was investigated by integrating physicochemical measurements, quantitative PCR, metagenomics, virus-host prediction, and functional analyses. At 1 min after water supply resumption, residual chlorine was undetectable, whereas turbidity and total iron ranged from 149.9 to 155.3 NTU and 13.4 to 13.8 mg/L, respectively. By 40 min, residual chlorine reached 0.52 to 0.56 mg/L, while turbidity and total iron declined to levels of 0.56 to 0.61 NTU and 0.028 to 0.041 mg/L, respectively. Concurrently, Legionella pneumophila decreased from 4.12 to 0.83 log₁₀ (gene copies/mL), and mexA from 5.55 to 1.77 log₁₀ (gene copies/mL). Metagenomic analyses revealed pronounced succession of bacterial communities, putative human pathogen-associated taxa, and the resistome. Caudoviricetes dominated the viral community and showed coordinated temporal changes with predicted bacterial hosts. Biofilm-associated functional genes, extracellular polymeric substances (EPS), and putative phage-encoded auxiliary metabolic genes were more abundant in the initially discharged water and generally decreased following water supply resumption. The elevated EPS may have reflected both microbial production and detachment of pre-existing biofilm-associated material. Collectively, the results identify the initially discharged water as a distinct post-stagnation physicochemical and microbiological state. Water-supply resumption represents a rapid ecological transition in which restoration of disinfectant residual and displacement of stagnant/pipe-associated material jointly restructure the microbiome, resistome, and virome.