Role of Amoeba in Legionella pathogenicity and Potential Impacts on Public Water Systems

Since legionellosis became a reported disease in 2001, Legionella is now the most commonly identified agent causing waterborne disease outbreaks in the US. Although Legionella spp. are natural freshwater inhabitants, it is the propagation of certain strains within biofilms, more specifically within protozoan species inhabiting biofilms of buildings and municipal water distribution systems that could account for its ability to cause disease. The purpose of this study was to investigate Legionella growth within amoebae and potential virulence of various environmental and clinical isolates. Virulence was assessed by determining Legionella lung clearance in A/J mice and by growth in Acanthamoeba polyphaga and in a murine and human macrophage-like cell lines, J774 and THP-1, respectively. The L. pneumophila clinical isolate displayed delayed clearance from lungs compared to the L. longbeacheae clinical isolate, and proliferated in J774 and THP-1 cells whereas L. longbeacheae did not. Additionally, both clinical strains multiplied in the presence of A. polyphaga. However, an environmental isolate of L. dumoffii was unable to proliferate in the presence of A. polyphaga, J774 and THP-1 cells and was rapidly cleared from the lungs of infected mice. These preliminary results suggest the ability to grow within amoebae can be used as a screening tool for potentially pathogenic Legionella strains, since both clinical isolates and not L. dumoffii in this study, grew in the presence of A. polyphaga. This study underscores the need to understand the role protozoa and biofilms play in selecting, propagating and releasing opportunistic pathogens into drinking and non-potable irrigation waters.