Cooling tower biofilm and water microbiomes

The number of Legionnaire's Disease (LD) cases is increasing worldwide. It is caused by the pathogen Legionella. which is transmitted to humans by aerosols generated by engineered water systems. Cooling towers (CT) are a primary source of large LD outbreaks. Growth of Legionella within CTs depends on the presence of phagocytic protists, amoebas and ciliates, which feed on other microbes. Previous studies have mostly focused on the microbes present in the water phase, and more studies are needed to fully understand the role of the biofilm microbiome in the growth of Legionella within CTs. In this study, water and biofilm samples from 5 CTs in the city of Montreal, Quebec, were used to quantify Legionella presence and determine the microbial communities using 16S and 18S rRNA gene amplicon sequencing. Our results showed that each CT harbors a unique microbiome, and within each CT, the water and biofilm microbiomes are different. Biofilm samples have a higher abundance of ascomycetes, suggesting that they must have an important role in CT biofilm ecology. Biofilm samples had a higher relative abundance of amoebae, with Vermamoeba being the dominant genus, and a higher diversity than water samples. Some ciliates amplicon sequence variants (ASVs) were more abundant in the water samples. Legionella presence was ubiquitous in biofilm samples across towers, suggesting that it could serve as a niche for Legionella growth, and highlighting the importance of amoebas for long-term colonization of cooling towers. Site- and phase-specific ASVs of Legionella suggest that the release of specific ASV from biofilm could explain periodic detection of specific Legionella in the water phase, although additional studies are necessary to confirm this. Based on these findings, biofilm control should be prioritized, and biofilm sampling should be considered for source identification of LD outbreaks.