MICROBIOLOGY

• Gram-negative aerobe, cause of legionellosis or Legionnaires’ disease.
• Common environmental organism, 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 amoeoba 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 an epidemic of pneumonia 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, organism has been called Tatlockia micdadei
    • L. dumoffi 
    • L. longbeachae 
    • L. bozemanii
• Legionella do not grow on most commonly used microbiologic media; buffered charcoal yeast extract (BCYE) agar is preferred.
• Legionella are poorly visualized with Gram stain. Silver stains such as Warthin-Starry or Dieterle stains are preferred.
  • 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.