The Contribution of Legionella anisa to Legionella Contamination of Water in the Built Environment

Legionella bacteria can proliferate in poorly maintained water systems, posing risks to users. All Legionella species are potentially pathogenic, but Legionella pneumophila ( L. pneumophila ) is usually the primary focus of testing. However, Legionella anisa ( L. anisa ) also colonizes water distribution systems, is frequently found with L. pneumophila , and could be a good indicator for increased risk of nosocomial infection. Anonymized data from three commercial Legionella testing laboratories afforded an analysis of 565,750 water samples. The data covered July 2019 to August 2021, including the COVID-19 pandemic. The results confirmed that L. anisa commonly colonizes water distribution systems, being the most frequently identified non- L. pneumophila species. The proportions of L. anisa and L. pneumophila generally remained similar, but increases in L. pneumophila during COVID-19 lockdown suggest static water supplies might favor its growth. Disinfection of hospital water systems was effective, but re-colonization did occur, appearing to favor L. pneumophila ; however, L. anisa colony numbers also increased as a proportion of the total. While L. pneumophila remains the main species of concern as a risk to human health, L. anisa ’s role should not be underestimated, either as a potential infection risk or as an indicator of the need to intervene to control Legionella ’s colonization of water supplies.