Free-living amoebae protecting Legionella in water: the tip of an iceberg?

Bacteria are a main food source for free-living amoebae inhabiting aquatic systems. Some bacteria however, have the ability to prevent intracellular destruction and can survive and grow in amoebic cells as endosymbionts. Free-living amoebae are well adapted to their hostile environmental conditions and are resistant to both desiccation, elevated temperatures and various disinfectants. For their endosymbionts, amoebae represent perfect vectors, providing both protection against adverse environmental conditions and transportation. There is increasing interest in the potential role of free-living amoebae as reservoirs and vectors of pathogenic bacteria. The best known of such pathogenic bacteria is Legionella, and several studies provide evidence for the importance of the amoeba-bacterium relationship in the biology and epidemiology of pneumonia caused by this pathogen. Although the relative importance of endosymbiosis of this kind is unknown when it comes to other human bacterial infections and the exact role of amoebic hosts in bacterial survival, multiplication and transmission in the environment is still poorly understood, naming free-living amoebae the "Trojan horses" of the microbial world is appropriate.