[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Microbiology](https://durgical.com/en/microbiology/) › [Other Gram-negative rods](https://durgical.com/en/microbiology/groups/other-gram-negative-rods/) › Legionella pneumophila

[Microbiology](https://durgical.com/en/microbiology/) · [Other Gram-negative rods](https://durgical.com/en/microbiology/groups/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

## Key points

- Pneumonia with diarrhea, confusion and hyponatremia after exposure to water aerosols: Legionella.
- Urinary antigen and BCYE agar (cysteine and iron).
- Stains poorly on Gram: silver stain.
- No spread between people: standard precautions only.
- Treated with macrolides or fluoroquinolones, not beta-lactams.

## 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 →](https://durgical.com/en/conditions/legionnaires-disease-and-pontiac-fever/)
- Severe atypical community-acquired pneumonia, also hospital-acquired; Pontiac fever is the mild form without pneumonia [Community-acquired pneumonia →](https://durgical.com/en/conditions/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:

- [Azithromycin](https://durgical.com/en/pharmacology/azithromycin/)
- [Levofloxacin](https://durgical.com/en/pharmacology/levofloxacin/)

## Prevention

- Maintenance, cleaning and disinfection of cooling towers and water systems
- Proper hot water temperature in hospitals and hotels
- No vaccine

## Memory aid

"Legionella rides the air conditioning and steals the salt": it arrives in cooling tower mist and drops blood sodium.

## 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.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/microbiology/legionella-pneumophila/)

## Related

- [Pseudomonas aeruginosa](https://durgical.com/en/microbiology/pseudomonas-aeruginosa/)

## Related conditions

- [Legionnaires disease and Pontiac fever](https://durgical.com/en/conditions/legionnaires-disease-and-pontiac-fever/)
- [Community-acquired pneumonia](https://durgical.com/en/conditions/community-acquired-pneumonia/)

## What the colours mean

**Gram-positive**
Thick peptidoglycan wall: stains purple.

**Gram-negative**
Thin wall with an outer membrane: stains pink.

**Acid-fast**
Lipid-rich wall (mycobacteria): Ziehl–Neelsen stain.

[← Back to the group: Other Gram-negative rods](https://durgical.com/en/microbiology/groups/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](https://www.who.int/news-room/fact-sheets/detail/legionellosis) (WHO, 2022); [About Legionnaires' Disease](https://www.cdc.gov/legionella/about/index.html) (CDC, 2025).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/microbiology/legionella-pneumophila/