Competitive colonization of Legionella and Pseudomonas aeruginosa in water systems of residential facilities hosting closed communities Legionella versus Pseudomonas aeruginosa in water systems of residential facilities.

Legionella and Pseudomonas aeruginosa are pathogens that live and multiply in water systems and resist disinfection through biofilm formation. As suggested by some studies, we wanted to verify whether the presence of P. aeruginosa can lead to a less frequent presence of Legionella in water systems of retirement homes and group homes. We collected 123 hot water and biofilm samples from showerheads. Samples were tested by selective culture methods, and Heterotrophic Plate Counts were determined. Legionella and P. aeruginosa were found in 35% and 39% of distal points, respectively. The presence of P. aeruginosa was significantly inversely correlated with the presence of Legionella when considering both matrices together (p<0.01), water samples only (p=0.02), and biofilm samples only (p=0.04). The inverse relationship was strong for L. pneumophila sg1 (Odds Ratio 0.182, p=0.002), while it is no longer verified when only L. pneumophila serogroups 2-14 are considered. The implications for the development of environmental investigations and risk management are discussed.