L. pneumophila

Legionella pneumophila is a thin, aerobic, pleomorphic, flagellated, nonspore-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. L. pneumophila is a gram-negative, nonencapsulated, 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, which also requires cysteine and iron to thrive. It grows on yeast extract in "opal-like" colonies. In humans, L. pneumophila invades and replicates inside macrophages. The internalization of the bacteria can be enhanced by the presence of antibody and complement, but is not absolutely required. Internalization of the bacteria appears to occur through phagocytosis. However, L. pneumophila is also capable of infecting nonphagocytic cells through an unknown mechanism.