Hypochlorous acid is a faster and more effective antimicrobial agent against Legionella pneumophila than sodium hypochlorite.

Legionella poses significant health risks in recreational waters and causes legionellosis. Although sodium hypochlorite (NaClO) is widely used for water disinfection, it has several disadvantages such as skin irritation, corrosion of surfaces and formation of harmful disinfection by-products. Hypochlorous acid (HClO) has emerged as an alternative disinfectant that may overcome these limitations. Here, we showed that 5.0 ppm highly purified HClO (iPOSH) reduced approximately 7.7 log CFU/mL of L. pneumophila to below the detection limit (10 CFU/mL) within 60 sec whereas 5.0 ppm NaClO required 180 sec to achieve the same level of reduction. Notably, iPOSH retained greater antibacterial activity than NaClO even in the presence of organic matters. iPOSH also exhibited comparable antibacterial efficacy across a pH range of 6.0-8.0. In Onsen water, 0.4 ppm iPOSH reduced approximately 7.7 log CFU/mL of L. pneumophila to below the detection limit within 60 min whereas 0.4 ppm NaClO achieved only less than 1 log CFU reduction after 60 min. These results demonstrate that 0.4 ppm iPOSH, the minimum chlorine concentration permitted for disinfection of public bathing facilities by the Japanese Ministry of Health, Labor and Welfare, exhibits effective antibacterial activity against L. pneumophila, and hence a promising alternative to NaClO.