Opportunistic pathogens belonging to the genus Legionella are among the most reported waterborne-associated pathogens in industrialized countries. Legionella colonize a variety of engineered aquatic ecosystems and persist in biofilms where they interact with a multitude of other resident microorganisms. In this review, we assess how some of these interactions could be used to develop a biological-driven “probiotic” control approach against Legionella. We focus on: (1) mechanisms limiting the ability of Legionella to establish and replicate within some of their natural protozoan hosts; (2) exploitative and interference competitive interactions between Legionella and other microorganisms; and (3) the potential of predatory bacteria and phages against Legionella. This field is still emergent, and we therefore specifically highlight research for future investigations, and propose perspectives on the feasibility and public acceptance of a potential probiotic approach.

Legionella are natural inhabitants of building plumbing biofilms, where interactions with other microorganisms influence their survival, proliferation, and death. Here, we investigated the associations of Legionella with prokaryotic and eukaryotic microbiomes in biofilm samples extracted from 85 shower hoses of a multiunit residential building. Legionella spp. relative abundance in the biofilms ranged between 0 - 7.8%, of which only 0 - 0.46% was L. pneumophila. Our data suggest that some microbiome members were associated with high (e.g., Chthonomonas, Vrihiamoeba) or low (e.g., Aquabacterium, Vannella) Legionella relative abundance. The correlations of the different Legionella variants (30 Zero-Radius OTUs detected) showed distinct patterns, suggesting separate ecological niches occupied by different Legionella species. This study provides insights into the ecology of Legionella with respect to: 1) the colonization of a high number of real shower hoses biofilm samples; 2) the ecological meaning of associations between Legionella and co-occurring prokaryotic/eukaryotic organisms; 3) critical points and future directions of microbial-interaction-based-ecological-investigations.