The Genus Legionella

Members of the genus Legionella are Gram-negative, aerobic bacilli that belong to the gamma-subgroup of proteobacteria. Legionella are facultative, intracellular bacteria that are ubiquitously found in freshwater ponds and streams where they colonize phagocytic protozoa. The relevance of these organisms to human health was first recognized following the eponymous 1976 outbreak at a meeting of the American Legion in Philadelphia. Subsequent outbreaks of Legionella infection have been traced to several types of man-made water reservoirs, including cooling towers, industrial air conditioners, hot water systems and fountains. The diseases resulting from exposure to Legionella, collectively termed legionellosis, include Legionnaires’ disease and Pontiac fever. Legionnaires’ disease is a severe infection of the respiratory tract that is fatal in approximately 10% of cases. Legionella infection is one of the most common causes of community-acquired pneumonia (CAP) and is second only to bacteremic pneumococcal pneumonia in causing severe CAP that requires treatment in intensive care.