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Legionellosis refers to the two clinical syndromes caused by bacteria of the genus Legionella. Pontiac fever is an acute, febrile, self-limited illness that has been serologically linked to Legionella species, whereas Legionnaires′ disease is the designation for pneumonia caused by these species. Legionnaires′ disease was first recognized in 1976, when an outbreak of pneumonia took place at a Philadelphia hotel during an American Legion convention.


The family Legionellaceae comprises more than 50 species with more than 70 serogroups. The species L. pneumophila causes 80–90% of human infections and includes at least 16 serogroups; serogroups 1, 4, and 6 are most commonly implicated in human infections. To date, 18 species other than L. pneumophila have been associated with human infections, among which L. micdadei (Pittsburgh pneumonia agent), L. bozemanii, L. dumoffii, and L. longbeachae are the most common. Members of the Legionellaceae are aerobic gram-negative bacilli that do not grow on routine microbiologic media. Buffered charcoal yeast extract (BCYE) agar is the medium used to grow Legionella.

Ecology and Transmission

The natural habitats for L. pneumophila are aquatic bodies, including lakes and streams. L. longbeachae has been isolated from natural soil and commercial potting soil. Legionellae can survive under a wide range of environmental conditions; for example, the organisms can live for years in refrigerated water samples. Natural bodies of water contain only small numbers of legionellae. However, once the organisms enter human-constructed aquatic reservoirs (such as drinking-water systems), they can grow and proliferate. Factors known to enhance colonization by and amplification of legionellae include warm temperatures (25°–42°C) and scale and sediment. L. pneumophila can form microcolonies within biofilms; its eradication from drinking-water systems requires disinfectants that can penetrate the biofilm. The presence of symbiotic microorganisms, including algae, amebas, ciliated protozoa, and other water-dwelling bacteria, promotes the growth of legionellae. The organisms can invade and multiply within free-living protozoa. Rainfall and humidity have been identified as environmental risk factors.

Sporadic community-acquired Legionnaires′ disease has been linked to colonization of residential, hotel, and industrial water supplies. Drinking-water systems in hospitals and extended-care facilities have been linked to health care–associated Legionnaires′ disease.

Cooling towers and evaporative condensers have been overestimated as sources of Legionella. Early investigations that implicated cooling towers antedated the discovery that the organism could also exist in drinking water. In many outbreaks attributed to cooling towers, cases of Legionnaires′ disease continued to occur despite disinfection of the cooling towers; drinking water was the actual source. Koch's postulates have never been fulfilled for cooling tower–associated outbreaks as they have been for hospital-acquired Legionnaires′ disease. Nevertheless, cooling towers have occasionally been identified in community-acquired outbreaks, including an outbreak in Murcia, Spain, in which several hundred suspected cases of Legionnaires′ disease occurred over a 3-week period. As mentioned above, L. longbeachae infections have been linked to potting soil, but the mode of transmission remains ...

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