Classification of the legionellae

This article reviews the classification and identification of the 23 described species, the 11 unnamed species, and the 49 serogroups presently known in the family Legionellaceae and the genus Legionella. The events leading to the isolation and classification of the first species, Legionella pneumophila, are summarized; these include the outbreak of legionnaires' disease at the 1976 American Legion Convention in Philadelphia, the five outbreaks preceding the Philadelphia outbreak, and the isolation of three Legionella species before 1976. The phenotypic characteristics of legionellae are described, including growth requirements, isolation media, biochemical tests, cellular fatty acids, quinones, and the guanine-plus-cytosine (G + C) content of DNA. Identification of legionellae by serology, monoclonal antibodies, and gene probes is described. Each species is listed with an explanation of its name, the source, location of its isolation, its involvement, if any, in human disease, its type strain, and the person who isolated it. The basis of taxonomy by DNA hybridization at the species level and evidence consistent with all species in a single genus and family are described. Problems with identification of legionellae at the species level and with their classification, especially at the genus level, are discussed.