Legionella species


  • Gram-negative aerobe, cause of legionellosis or Legionnaires’ disease.
  • Common environmental organisms found in warm water sources (e.g., 32°- 45°C temperature range as in warm ponds, air chillers, industrial air conditioning systems, water systems include storage and hot water).
    • Commonly infect free-living amoeba and other ciliated protozoa as natural hosts for this bacteria.
  • Legionella spp. include >60 species and 70 serotypes.
    • Most are capable of causing human disease.
    • L. pneumophila serogroup 1: most common, accounting for 80-90% of cases of Legionnaires’ disease.
      • The appellation is based on the first recognition of the disease in a pneumonia epidemic at the American Legion Convention in Philadelphia in 1976.
    • Others: less frequent, occasional causes of legionellosis
      • L. pneumophila serogroups 2-15
      • L. micdadei
        • Sometimes referred to as the "Pittsburgh pneumonia agent."
        • Alternatively, the organism has been called Tatlockia micdadei
      • L. dumoffi
      • L. longbeachae
      • L. bozemanii
  • Legionella do not grow on the most commonly used microbiologic media; buffered charcoal yeast extract (BCYE) agar [Fig 1] is preferred.
  • Legionella are poorly visualized with Gram stain but may be seen as a Gram negative [Fig 2]. Silver stains such as Warthin-Starry or Dieterle stains [Fig 3] are preferred.
    • It may weakly stain acid-fast but loses this characteristic when grown in culture.
  • An intracellular pathogen, effective drugs in animal models include fluoroquinolones, macrolides, TMP/SMX, rifampin and doxycycline.

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Last updated: April 16, 2023