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Abstract The disease burden from Legionella spp. infections has been increasing in many industrialized countries and, despite decades of scientific advances, ranks amongst the highest for waterborne diseases. We review here several key research areas from a multidisciplinary perspective and list critical research needs to address some of the challenges of Legionella spp. management in engineered environments. These include: (i) a consideration of Legionella species diversity and cooccurrence, beyond Legionella pneumophila only; (ii) an assessment of their environmental prevalence and clinical relevance, and how that may affect legislation, management, and intervention prioritization; (iii) a consideration of Legionella spp. sources, their definition and prioritization; (iv) the factors affecting Legionnaires’ disease seasonality, how they link to sources, Legionella spp. proliferation and ecology, and how these may be affected by climate change; (v) the challenge of saving energy in buildings while controlling Legionella spp. with high water temperatures and chemical disinfection; and (vi) the ecological interactions of Legionella spp. with other microbes, and their potential as a biological control strategy. Ultimately, we call for increased interdisciplinary collaboration between multiple research domains, as well as transdisciplinary engagement and collaboration across government, industry, and science as the way toward controlling and reducing Legionella-derived infections. 

Ion exchange (IX) systems are widely used in drinking water treatment to remove charged constituents such as metals, nitrates, and other dissolved ions. While effective for chemical contaminant removal, IX resins may also provide a niche for microbial colonization, including opportunistic pathogens such as Legionella pneumophila. Despite the growing prevalence of residential and commercial IX systems, limited research has examined their potential to support biofilm formation or pathogen persistence. This study developed a bench-scale experimental method to assess the behavior of L. pneumophila in cationic (sulfonic acid functional group) and anionic (quaternary ammonium functional group) exchange resins over a 14-day period. Resins were conditioned for two weeks in the vessels to foster biofilm growth, after which they were inoculated with L. pneumophila and monitored under static batch conditions. Quantification of L. pneumophila was performed using IDEXX Legiolert and mip gene-targeted qPCR, while total bacterial biomass was assessed using 16S rRNA gene qPCR. The results showed consistent detection of L. pneumophila on resin surfaces over time, with higher persistence observed in the resin phase compared to the liquid phase. qPCR results demonstrated relatively stable gene copy concentrations throughout the study, contrasting with declining cultivability observed in Legiolert assays. This discrepancy suggests potential transitions to viable but non-culturable (VBNC) states or persistence of non-viable DNA associated with resin biofilms. Total bacterial concentrations remained high in all experiments, including in both cation and anion resins. While antimicrobial effects of QA resins could not be ruled out, further analyses are needed to evaluate resistance selection or inhibition. 

Session co-presenters: Weir M, Mitchell J, Fung D, Ross K, Hannapel L, Edens C. Session: Legionella in the Built Environment: Emerging research, practices, and policies. American Public Health Association (APHA), Atlanta, GA, November 12, 2023.