Biofilms as an early indicator of Legionella colonisation within evaporative cooling towers

Cooling towers are the source of over a quarter of Legionnaires’ disease outbreaks. They provide ideal conditions for planktonic and biofilm-associated microbial growth, including Legionella pneumophila. Legionella monitoring typically relies on culture or, less often, rapid qPCR based analysis of bulk-water samples only and hence may miss early colonisation. We propose that biofilms, recognised as microbial reservoirs, can be utilised as a sample type to provide early indicators in cooling towers. This study developed, optimised, and deployed a novel biofilm sampling regime across multiple cooling tower systems on a single site. Biofilm and planktonic cell concentrations, viability, and Legionella presence were quantified alongside cooling tower metadata. L. pneumophila was detected exclusively in biofilms, while Legionella spp. concentrations were more frequently detected in biofilms than bulk-water, with increases in biofilm-associated Legionella spp. often preceding elevations in bulk-water. Cell concentration and viability did not correlate with Legionella presence, highlighting the limits of non-specific metrics. These findings position biofilm monitoring as a more representative and reliable early-warning tool for Legionella detection, enabling more proactive and precise environmental health surveillance.