Legionella pneumophila is a thin, aerobic, pleomorphic, flagellated, non-spore-forming, Gram-negative bacterium of the genus Legionella. L. pneumophila is the primary human pathogenic bacterium in this group and is the causative agent of Legionnaires' disease, also known as legionellosis. In nature, L. pneumophila infects freshwater and soil amoebae of the genera Acanthamoeba and Naegleria. The mechanism of infection is similar in amoeba and human cells. Characterization L. pneumophila is a Gram-negative, non-encapsulated, aerobic bacillus with a single, polar flagellum often characterized as being a coccobacillus. It is aerobic and unable to hydrolyse gelatin or produce urease. It is also non-fermentative. L. pneumophila is neither pigmented nor does it autofluoresce. It is oxidase- and catalase-positive, and produces beta-lactamase. L. pneumophila colony morphology is gray-white with a textured, cut-glass appearance; it also requires cysteine and iron to thrive. It grows on yeast extract agar in "opal-like" colonies. Cell membrane structure While L. pneumophila is categorized as a Gram-negative organism, it stains poorly due to its unique lipopolysaccharide content in the outer leaflet of the outer cell membrane. The bases for the somatic antigen specificity of this organism are located on the side chains of its cell wall. The chemical composition of these side chains both with respect to components and arrangement of the different sugars, determines the nature of the somatic or O-antigenic determinants, which are important means of serologically classifying many Gram-negative bacteria. At least 35 different serovars of L. pneumophila have been described, as well as several other species being subdivided into a number of serovars. Detection Sera have been used both for slide agglutination studies and for direct detection of bacteria in tissues using immunofluorescence via fluorescent-labelled antibody. Specific antibody in patients can be determined by the indirect fluorescent antibody test. ELISA and microagglutination tests have also been successfully applied. Legionella stains poorly with Gram stain, stains positive with silver, and is cultured on charcoal yeast extract with iron and cysteine. Ecology and reservoirs L. pneumophila is a facultative intracellular parasite that can invade and replicate inside amoebae in the environment, especially species of the genera Acanthamoeba and Naegleria, which can thus serve as a reservoir for L. pneumophila. These hosts also provide protection from environmental stresses, such as chlorination. Legionella has been shown to proliferate on the walls of pipes in biofilms. Sloughed legionella from biofilms in plumbing systems can be aerosolized through faucets, showers, sprinklers, and other fixtures