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The role of biofilms and protozoa in Legionella pathogenesis: implications for drinking water
LAU, H. Y. AND N. ASHBOLT. The role of biofilms and protozoa in Legionella pathogenesis: implications for drinking water. JOURNAL OF APPLIED MICROBIOLOGY. Blackwell Publishing, Malden, MA, 107(2):368-378, (2009).
We propose a model in which the propagation and dissemination of pathogenic Legionella spp. in drinking water systems occurs via their colonization and interactions with protozoa within biofilms present along surfaces in drinking water systems (Figure 1). The high surface area and amount of water within the distribution network and in buildings only serves to concentrate potentially pathogenic microbes and increase the chances for selection and development of those specialized survival mechanisms. The ability of strains of Legionella spp. to persist in biofilms and replicate via parasitization of amoeba cells can be seen as a survival mechanism, but one with potentially severe consequences for human health. Currently, the concentration of Legionella spp. in drinking water systems and the infectious dose to humans is poorly defined (Armstrong & Haas 2008), as is the diversity and quantity of Legionella spp.-propagating protozoa. Thus, future research should aim to understand the relationships between Legionella spp. and their natural host(s), such as Acanthamoeba spp. and the putative role the latter may play in selecting and releasing opportunistic pathogens into drinking waters. The results of this research may well change our views on the preferred disinfection strategies and procedural designs for drinking water systems in controlling exposure to Legionella spp. and similar pathogens.
Current models to study Legionella pathogenesis include the use of primary macrophages and monocyte cell lines, various free-living protozoan species and murine models of pneumonia. However, there are very few studies of Legionella spp. pathogenesis aimed at associating the role of biofilm colonization and parasitization of biofilm microbiota and release of virulent bacterial cell/vacuoles in drinking water distribution systems. Moreover, the implications of these environmental niches for drinking water exposure to pathogenic legionellae are poorly understood. This review summarizes the known mechanisms of Legionella spp. proliferation within Acanthamoeba and mammalian cells and advocates the use of the amoeba model to study Legionella pathogenicity because of their close relationship with Legionella spp. in the aquatic environment. The putative role of biofilms and amoeba in the proliferation, development and dissemination of potentially pathogenic Legionella spp. is also discussed. Elucidating the mechanisms of Legionella pathogenicity development in our drinking water systems will aid in elimination strategies and procedural designs for drinking water systems in controlling exposure to Legionella spp. and similar pathogens.