Prevalence and Determinants of Pseudomonas aeruginosa Contamination in Tourist Swimming Pools: A Four-Year Surveillance Study in Andalusia, Spain

Tourist swimming pools are complex aquatic systems where operational failures can favour microbial growth and exposure to opportunistic pathogens. We conducted a four‑year surveillance (2016–2019) in hotel pools across Andalusia (Spain), analysing 2,053 water samples under Spanish regulation (Royal Decree 742/2013) with ISO methods (Pseudalert®/ISO 16266‑2 for Pseudomonas aeruginosa and Colilert‑18/ISO 9308‑2 for Escherichia coli). Overall non‑compliance reached 24.8%, and 2.0% of samples triggered immediate pool closure, most frequently due to P. aeruginosa. The bacterium was detected in 5.1% of samples, with heterogeneous distribution among installations: whirlpools (7.9%) > indoor pools (6.4%) > outdoor pools (4.5%) > cold wells (2.9%). No significant association was observed between use by children and P. aeruginosa detection (p > 0.05). Contamination occurred under both chlorine and bromine disinfection, with comparable prevalence (p = 0.18), and overlapping residual distributions indicated that single‑point disinfectant measurements alone did not predict contamination. Seasonality showed a bimodal pattern with winter (January) and summer (August) peaks, and prevalence markedly increased in 2019 compared with prior years. These findings highlight that P. aeruginosa contamination in tourist pools is driven less by momentary disinfectant levels than by structural and operational determinants (e.g., biofilm‑prone niches and hydraulic performance), underscoring the need for continuous surveillance, hydraulic optimization, routine mechanical cleaning, and robust monitoring across all seasons.