Premise plumbing promotes the regrowth and survival of opportunistic pathogens, such as Legionella pneumophila (L. pneumophila), especially within biofilms. The purpose of this study was to investigate the effectiveness of chlorine disinfection against L. pneumophila serogroup 1 and serogroup 2–15 planktonic form and biofilms. Planktonic cells were able to survive during the study period in the presence of chlorine at recommended free chlorine levels (0.2–0.5 mg/L). Biofilms were developed on galvanized steel and polyvinyl chloride (PVC) for 18 and 30 days and exposed to 50, 100, 150, and 200 mg/L for 2 hours. No colony appeared immediately after chlorination; however, persistent cells were able to tolerate treatment and continue to grow on subsequent days. The biofilm formation was evaluated by atomic force microscopy. This study demonstrates that the biofilm formed on the surfaces of plumbing materials increases bacterial resistance against high levels of chlorination. A new approach towards monitoring and eradicating L. pneumophila from water systems is required.

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