Legionella

Abstract

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There are 59 species within the genus Legionella, which is the only genus in the family Legionellaceae. The majority of human infections are caused by Legionella pneumophila, Legionella micdadei, Legionella longbeachae, and Legionella dumoffii. Once inhaled, Legionella infects human macrophages, in particular alveolar macrophages. The organisms possess several genes, including those belonging to a type IV secretion system, that enable them to evade host defenses, multiplying and surviving in an intracellular host environment. Legionella spp. are difficult to see on a routine Gramstain and therefore can be better visualized with a carbol fuchsin rather than safranin as a counterstain. Legionella spp. are motile aerobic organisms that require l-cysteine for growth, exhibit enhanced growth with iron under 5 percent CO2, and are asaccharolytic and relatively inert biochemically.