| Summary: | <i>Legionella pneumophila</i> is the causative agent of Legionnaires’ disease, a severe pneumonia. Cooling towers are a major source of large outbreaks of the disease. The growth of <i>L. pneumophila</i> in these habitats is influenced by the resident microbiota. Consequently, the aim of this study was to isolate and characterize bacterial species from cooling towers capable of inhibiting several strains of <i>L. pneumophila</i> and one strain of <i>L. quinlivanii</i>. Two cooling towers were sampled to isolate inhibiting bacterial species. Seven inhibitory isolates were isolated, through serial dilution plating and streaking on agar plates, belonging to seven distinct species. The genomes of these isolates were sequenced to identify potential genetic elements that could explain the inhibitory effect. The results showed that the bacterial isolates were taxonomically diverse and that one of the isolates may be a novel species. Genome analysis showed a high diversity of antimicrobial gene products identified in the genomes of the bacterial isolates. Finally, testing different strains of <i>Legionella</i> demonstrated varying degrees of susceptibility to the antimicrobial activity of the antagonistic species. This may be due to genetic variability between the <i>Legionella</i> strains. The results demonstrate that though cooling towers are breeding grounds for <i>L. pneumophila</i>, the bacteria must contend with various antagonistic species. Potentially, these species could be used to create an inhospitable environment for <i>L. pneumophila</i>, and thus decrease the probability of outbreaks occurring.
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