Legionella pneumophila
Also known as: legionella
Legionella pneumophila is a gram-negative bacterium that lives in warm water systems and causes Legionnaires' disease, a severe atypical pneumonia. It spreads by inhaled aerosols rather than person to person.
Legionella pneumophila is a facultative intracellular gram-negative rod that stains poorly on routine Gram stain and requires charcoal yeast extract agar supplemented with cysteine and iron to grow in the laboratory. It survives inside free-living amoebae in aquatic environments, which is why it colonizes air conditioning cooling towers, hot tubs, decorative fountains, humidifiers, and building water supplies. Transmission occurs by inhaling contaminated aerosols; there is no person-to-person spread.
The organism causes two syndromes. Legionnaires' disease is a severe atypical pneumonia with high fever, cough, and — characteristically — accompanying gastrointestinal symptoms such as diarrhea, neurologic findings including confusion, and laboratory hyponatremia, a combination that helps distinguish it from typical pneumonias. Pontiac fever is a milder, self-limited flu-like illness without pneumonia. Diagnosis usually rests on a urine antigen test, which detects the most common serogroup rapidly. Because Legionella is intracellular, macrolides or respiratory fluoroquinolones are used rather than beta-lactams.
Risk concentrates in older adults, smokers, patients with chronic lung disease, and the immunocompromised. Outbreaks are typically traced to a shared water source in a hotel, hospital, or large building, which makes Legionella a recurring subject in public health investigation and in facility water management.
USMLE Step 1 covers Legionella pneumophila among the other important bacteria, and question stems reliably signal it through the same cluster of clues: aerosol exposure from air conditioning or water systems, atypical pneumonia with diarrhea and confusion, low serum sodium, and a positive urine antigen. Contrast it with the other atypical pneumonia pathogens, particularly Mycoplasma pneumoniae.
Key takeaways
- Legionella pneumophila is a gram-negative rod that stains poorly and requires charcoal yeast extract agar with cysteine and iron.
- It lives in warm water systems and spreads by inhaled aerosols, never person to person.
- Legionnaires' disease is a severe atypical pneumonia often accompanied by diarrhea, confusion, and hyponatremia.
- Diagnosis is commonly made by urine antigen test, and treatment uses macrolides or respiratory fluoroquinolones.
- USMLE Step 1 tests the exposure history, the pneumonia-plus-diarrhea-plus-hyponatremia pattern, and the culture requirements.