They are predators of other microbes in the system and may also be reservoirs for bacterial pathogens such as Legionella and Mycobacterium spp., Pseudomonas aeruginosa, Francisella tularensis, Coxiella burnetii, and Vibrio cholerae ( Abd et al., 2003 La Scola and Raoult, 2001 Sandström et al., 2010 Thomas and McDonnell, 2007 Thom et al., 1992 Van der Henst et al., 2016). Protists, such as free-living amoebae, are common members of microbial communities in cooling towers ( Barbaree et al., 1986 Berk et al., 2006 Delafont et al., 2016 Pagnier et al., 2009). Other opportunistic bacterial pathogens detected in cooling towers include Mycobacterium spp., Pseudomonas spp., Burkholderia spp., and Pantoea spp. This respiratory tract infection is caused by Legionella pneumophila, a gammaproteobacterial pathogen acquired through inhalation of aerosols and is potentially fatal for immunocompromised patients ( Hamilton et al., 2018 Walser et al., 2014). One of the major public health risks associated with cooling towers is Legionnaires’ disease. Containing large semi-open water volumes at a rather constant temperature, cooling towers are suitable environments for microbial growth throughout the year and have been implicated in bacterial outbreaks ( Kurtz et al., 1982 Pagnier et al., 2009 Torvinen et al., 2013 Yamamoto et al., 1992). Most modern commercial, industrial, and residential buildings rely on cooling towers as cost-efficient measures to remove excess heat.
Together, this study provides an unbiased and comprehensive overview of microbial diversity of cooling tower water basins, establishing a framework for investigating and assessing public health risks associated with these man-made freshwater environments. Co-occurrence analysis of bacteria and protist taxa successfully captured known interactions between amoeba-associated bacteria and their hosts, and predicted a large number of additional relationships involving ciliates and other protists. Protists are important members of the cooling tower water microbiome and known reservoirs for bacterial pathogens. Although cooling towers represent a rather stable environment, microbial community composition was highly dynamic and subject to seasonal change. We also detected several groups related to known opportunistic pathogens, such as Legionella, Mycobacterium, and Pseudomonas species, albeit at generally low abundance. While each cooling tower had a pronounced site-specific microbial community, taxa shared among all locations mainly included groups generally associated with biofilm formation.
Bacterial diversity in all three towers was broadly comparable to other freshwater systems, yet less diverse than natural environments the most abundant taxa are also frequently found in freshwater or drinking water. In this study, we analysed the microbiome of the bulk water from the basins of three cooling towers by 16S and 18S rRNA gene amplicon sequencing over the course of one year. While measures to minimize public health risks are in place, the general microbial and protist community structure and dynamics in these systems remain largely elusive. They are recognized as a potential source of bacterial pathogens and have been associated with disease outbreaks such as Legionnaires’ disease. Often located on rooftops, their semi-open water basins provide a suitable environment for microbial growth. Cooling towers for heating, ventilation and air conditioning are ubiquitous in the built environment.
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