Resistance of Legionella pneumophila serotype 1 biofilms to chlorine-based disinfection

SummaryThe presence of Legionella spp. in potable water systems is a major concern to municipal water providers and consumers alike. Despite the inclusion of chlorine in potable supplies and frequent chlorination cycles, the bacterium is a recalcitrant human pathogen capable of causing incidents of Legionnaires' disease, Pontiac fever and community-acquired pneumonia in humans. Using two materials routinely employed for the delivery of potable water as a substratum, copper and stainless steel, the development of Legionella pneumophila biofilms and their response to chlorination was monitored over a three-day and a three-month period, respectively. Preliminary in vitro studies using broth and sterile tap water as culture media indicated that the bacterium was capable of surviving in low numbers for 28 days in the presence of chlorine. Subsequently, biofilms were grown for three days, one month and two months, respectively, on stainless steel and copper sections, which are widely used for the conveyance of potable water. Immediately after exposure to 50 mg/L chlorine for 1 h, the biofilms yielded no recoverable colonies, but colonies did reappear in low numbers over the following days. Despite chlorination at 50 mg/L for 1 h, both one- and two-month-old L. pneumophila biofilms were able to survive this treatment and to continue to grow, ultimately exceeding 1 × 106 cfu per disc. This research provides an insight into the resistance afforded to L. pneumophila against high levels of chlorine by the formation of biofilms and has implications for the delivery of potable water.