Long-Term Environmental Surveillance of Pseudomonas aeruginosa in a Hospital Water Distribution System: Trends in Prevalence, Biofilm Formation, and Antimicrobial Susceptibility

Background: Hospital water systems are major environmental reservoirs of Pseudomonas aeruginosa, but evidence on the long-term impact of Water Safety Plans (WSPs) on environmental prevalence, antimicrobial resistance, and biofilm-forming ability remains limited. This study evaluated the long-term occurrence of P. aeruginosa in a hospital water distribution system over a 15-year surveillance period. Methods: A 15-year environmental surveillance study was conducted in a tertiary-care hospital in Northern Italy. Water samples were cultured for P. aeruginosa, and recovered isolates were assessed for antimicrobial susceptibility and biofilm-forming ability. Temporal trends were analysed using generalized linear and segmented logistic regression models. Results: Among 1696 water samples, the overall prevalence of P. aeruginosa was 2.93%, with a significant annual decline of 12% (OR 0.88, 95% CI 0.82–0.94; p < 0.001). Segmented regression identified a marked reduction after the fourth year (OR 0.18, 95% CI 0.06–0.55; p = 0.003). Over time, isolates shifted toward weak or non-biofilm-producing phenotypes while maintaining high antimicrobial susceptibility, with only one multidrug-resistant isolate detected. Conclusions: Long-term surveillance integrated within a comprehensive WSP was associated with reduced P. aeruginosa contamination, decreased biofilm-forming ability, and persistently low antimicrobial resistance.