RESUMEN
This paper describes the results of a five-year monitoring programme applied to the water distribution system of the University Hospital of Pisa (Italy). The purpose of the programme was to evaluate the efficacy of an integrated water safety plan in controlling Legionella spp. colonisation of the potable water system. The impact of the safety plan on the ecology of legionella in the water network was evaluated by studying the genetic variability and the chlorine susceptibility of the strains isolated prior to, and throughout, the application of continuous chlorine dioxide treatment. After 45 months of water hyperchlorination, Legionella spp. were still present but the positive supply points were reduced by 79.4%. The samples exceeding 10(3)cfu/L were reduced by 83.8% and the mean counts showed a decrease of 94.6%. The majority of the isolates belonged to Legionella pneumophila serogroup 1 (overall positivity rate: 161/423; 38%). Molecular typing was performed on 61 isolates (37.9% of the positive samples) selected on spatial and temporal criteria. This revealed the circulation and the persistence in the hospital environment of three prevalent types of L. pneumophila Wadsworth, demonstrating allelic and electrophoretic characteristic profiles and different chlorine susceptibility. Two of these, one predominant and pre-dating the sanitation regimen, and one other isolated after three years of water treatment, were chlorine tolerant. Despite the ineffectiveness of chlorine dioxide in eradicating L. pneumophila, the risk management plan adopted appeared to discourage further cases of nosocomial legionellosis.