Walsh & Hoyt: Legionella

In 1976, an outbreak of pneumonia occurred in a hotel at the site of the American Legion convention in Philadelphia, Pennsylvania. A total of 221 persons contracted pneumonia, and 34 died. Despite intensive investigation, the cause of the outbreak was not identified until months later, when Dr. Joseph McDade from the Centers for Disease Control isolated a bacterium from autopsy lung specimens. A serologic test for the bacterium was developed, and high titers of antibody to this bacterium were found in the serum of most of the patients who survived the attack. The bacterium was found to be different from any other previously described. It therefore was placed in a new taxonomic family, the Legionellaceae, with a new genus, Legionella, and a new species, pneumophila (from the Greek words, pneuma, meaning ""lung,"" and philos, meaning ""fond of""). In retrospect, this organism had been isolated in 1947 but had been thought to be a rickettsia-like agent because it failed to grow on various bacteriologic media. More than 30 species of Legionella are known to exist. These bacteria are all aerobic, nonspore-forming, unencapsulated, gram-negative bacilli. Most strains have a single, polar flagellum and multiple pili. The clinical syndromes produced by members of the Legionellaceae family are collectively called ""legionellosis."" Legionnaire disease is pneumonia caused by Legionella pneumophila, whereas Pontiac fever is an acute febrile illness not associated withpneumonia that is linked serologically to L. pneumophila and to other species of Legionella.