USMLE

Legionella pneumophila

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Bacteria - Gram Negative
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  7. Legionella pneumophila
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Legionella 

  • Characteristics
    • Gram negative rod
      • Gram stains poorly, and will often show neutrophils with no bacteria inside
    • Best visualized with silver stain 
    • Grows on charcoal yeast extract medium with iron and cysteine
    • Facultative intracellular
      • This is relatively high yield, as cell-mediated (T-cell) immunity is important in clearing the bacteria
  • Aerosol transmission from contaminated water sources
    • E.g. air conditioning systems, hot water tanks
    • Usually affects groups of people sharing a water reservoir (e.g. cruise ships)
    • No person-to-person transmission
  • Presentation
    • Legionnaires’ disease
      • Increased risk in smokers (chronic lung disease) or immunocompromised individuals
      • Severe pneumonia (often unilateral and lobar)
      • High fever
        • Fever can often exceed 103 C
      • GI symptoms
        • Nausea or vomiting 
        • Watery diarrhea
      • CNS symptoms
        • Contrast with Mycoplasma pneumoniae, which does not present w GI or CNS involvement 
        • Confusion 
        • Encephalopathy 
      •  Labs may show hyponatremia
        • May have relative bradycardia on physical exam 
    • Pontiac fever
      • Milder version of disease with flu-like symptoms
      • Seen in healthy people
  • Diagnosis
    • Urine antigen test
  • Treatment
    • Macrolide or respiratory fluoroquinolone (levoquinolone)