Microbiology · Other Gram-negative rods
Legionella pneumophila !
- Bacterium
- Gram-negative
- Exam priority: medium
A bacterium in water from cooling towers, showers and hot tubs. Inhaled, it causes severe pneumonia (Legionnaires' disease) or a mild fever.
Updated 2026-11-01
At a glance
- Gram
- Gram-negative
- Shape
- Thin rod
- Oxygen
- Aerobic
- Family
- Legionellaceae
- Stain
- Stains poorly on Gram; silver stain
Morphology
- Thin Gram-negative rod that barely shows on Gram stain
- Facultative intracellular: lives inside water amoebae and alveolar macrophages
- Grows only on BCYE agar (charcoal yeast extract with cysteine and iron)
Transmission
- Inhalation of contaminated water aerosols: cooling towers, hot water systems, showers, humidifiers, hot tubs
- Aspiration of contaminated water in hospitalized patients
- No person-to-person spread: standard precautions only
Reservoir: Fresh water and artificial water systems, inside amoebae and biofilms
Isolation precautions
What the care team adds to standard precautions.
- Standard
Virulence factors
- Multiplies inside alveolar macrophages and blocks phagosome-lysosome fusion
- Type IV secretion system (Dot/Icm)
- Biofilms in plumbing
Diseases it causes
- Legionnaires' disease (pneumonic legionellosis) Legionnaires disease and Pontiac fever →
- Severe atypical community-acquired pneumonia, also hospital-acquired; Pontiac fever is the mild form without pneumonia Community-acquired pneumonia →
Clinical picture
- High fever, cough, shortness of breath, muscle aches and headache
- Diarrhea, confusion and hyponatremia, uncommon in other pneumonias
- Worse in people over 50, smokers, the immunosuppressed and transplant recipients
Diagnosis
- Urinary antigen (detects serogroup 1, the most common)
- Sputum culture on BCYE agar
- PCR of respiratory samples
- Labs: hyponatremia and raised transaminases
Treatment
Antibiotics that enter cells: macrolides (azithromycin) or fluoroquinolones (levofloxacin). Beta-lactams do not work because the bacterium is intracellular.
Drug class only; doses and regimens are set by the clinician from the current guideline.
In Pharmacology:
Prevention
- Maintenance, cleaning and disinfection of cooling towers and water systems
- Proper hot water temperature in hospitals and hotels
- No vaccine
Test yourself
Think of the answer, then tap the question to see it.
How is Legionnaires' disease acquired?
By inhaling contaminated water aerosols, such as those from cooling towers, showers or hot tubs; it does not spread between people.
Which rapid test is used to diagnose Legionella?
The urinary antigen test, which detects serogroup 1.
Which electrolyte disturbance suggests Legionella pneumonia?
Hyponatremia.
Why are beta-lactams not used against Legionella?
The bacterium lives inside cells, where beta-lactams do not reach; macrolides or fluoroquinolones are used.
Related
Related conditions
← Back to the group: Other Gram-negative rods
Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Legionellosis (WHO, 2022); About Legionnaires' Disease (CDC, 2025).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.