MICROBIOLOGY

• 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.