These are Gram-negative, non-sporing non-acid fast rods, 1-2µm long by 0.5µm wide. They are fastidious bacteria requiring iron and cysteine for primary isolation. They are generally motile with one to three polar flagellae. They are chemoorganotrophs and lack the ability to ferment any sugars. They do not have a glucose transport system. Some amino acids act as primary carbon and energy sources and are metabolised through the tricarboxylic acid cycle, while carbohydrate synthesis occurs along the Embden-Meyerhof pathway. Most strains produce catalase and liquify gelatin and many produce beta-lactamase and oxidase. A number of species are pigmented and some show autofluorescence. The optimum temperature for growth of human isolates is around 35°C.

Genera

Legionella: This is the only accepted genus and comprises over 30 species. Two other genera have been proposed, Tatlockia and Fluoribacter, for the species T. micdadei , F. bozemanii , F. dumoffii and F. gormanii, but generally theses species are accepted as members of the genus Legionella.

Classification

The traditional methods of classification cannot be used because of their general inertness. They have a unique cell wall among Gram-negative bacteria, containing large amounts of branched chain fatty acids. They are also subdivided on the basis of their unusual isoprenoid quinones in their respiratory chain. The GC content of the DNA is 38-52 mol%.

Species Profiles

L. pneumophila

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