LEGIONELLOSIS

Etiologic Agents

The genus name Legionella was derived from the fact that the first recognized outbreak of infection due to these microorganisms affected members of the American Legion. Pneumophila (“lung loving”) was the species designation for the first isolate. There are now 48 species in the family Legionellaceae and about half of these have caused disease in humans. Legionella pneumophila serogroup 1 accounts for 80% to 90% of cases of legionnaire's disease (LD) (Table 139.1). However, in Australia and New Zealand Legionella longbeachae accounts for 30% of the cases. Legionellaceae are gram-negative, aerobic, non–spore-forming bacilli that measure 0.3 to 0.9 μm wide and 2 to 20 μm long. These organisms require special media for growth. Charcoal yeast extract agar buffered to pH 6.9 and containing α-ketoglutarate along with cefamandole, polymyxin B, and anisomycin to prevent growth of other microorganisms is the primary medium used for isolation of these organisms. Addition of α-ketoglutaric acid to the medium promotes growth of Legionella likely by stimulation of oxygen-scavenging enzymes.