What is Legionella?
Legionella spp. is a type of bacillary bacteria, gram negative, ubiquitous, aerobic, asporigenic and generally motile thanks to the presence of one or more flagella ranging in size from 0.3 to 0.9 mm in width and from 1.5 to 5 mm in length. They are characterized by the intracellular proliferation and by the inability to synthesize the amino acid cysteine, essential for its survival.
It is one of the etiological agents of bacterial pneumonia and owes its name to the pneumonia epidemic that occurred among the participants in an American Legion meeting in the summer of 1976 in Philadelphia. Among the over 4,000 veterans that took part in it (called "Legionnaires") 221 fell ill and 34 of them died. The cause of the disease was discovered later: a "new" bacterium, called Legionella, which was isolated in the air conditioning system of the hotel where the veterans had stayed.
More than 58 species have been identified by then, divided into over 70 serogroups. About half of these are opportunistic pathogens. The most dangerous one, linked to about 90% of legionellosis cases, is L. pneumophila (which includes 16 serogroups) of which serogroup 1 is responsible for over 84% of cases, followed by L. longbeachae (3.9%) and L. bozemanae (2.4%). Other species, less frequently isolated in clinical samples are L. micdadei, L. dumoffii, L. feeleii, L. wadsworthii and L. anisa (2.2 % in total).