Legionella bacteria are Gram-negative inhabitants of freshwater sources commonly found in free-living amoebae. Upon inhalation by humans, the bacteria also parasitize alveolar macrophages and lung epithelial cells. Legionella pneumophila is the species most frequently causing Legionnaires' disease, a severe form of pneumonia, which in many cases leads to an inhibition of lung function. One of the virulence determinants is the type II protein secretion system Lsp, which exports hydrolytic enzymes, like the Zn-metalloprotease, acid phosphatases, and phospholipases A. The actual virulence determinants transported by the type II system have not yet been found. Phospholipase activity has been characterized as a major virulence factor of bacteria such as Pseudomonas aeruginosa, Listeria monocytogenes and Yersinia enterocolitica. 
 
Legionnaires’ disease is caused by bacteria that belong to the family Legionellaceae. This family now includes 48 species and over 70 serogroups. Approximately half of these species have been implicated in human disease. Legionella pneumophila is responsible for approximately 90% of infections. Most cases are caused by L. pneumophila, serogroup 1. Legionella species are small (0.3 to 0.9 μm in width and approximately 2 μm in length) faintly staining Gram-negative rods with polar flagella.
 
Legionella organisms are readily found in natural aquatic bodies and some species have been recovered from soil. The organisms can survive in a wide range of conditions, including temperatures of 0 to 63o C, pH of 5.0 to 8.5, and dissolved oxygen concentrations of 0.2 to 15 ppm in water. Temperature is a critical determinant for Legionella proliferation. Colonization of hot water tanks is more likely if tank temperatures are between 40 and 50oC (104 to 122o F). Legionella and other microorganisms become attached to surfaces in an aquatic environment forming a biofilm.
 
 

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