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Gram-negativeBacilliRespiratory

Legionella pneumophila

Gram-negative, facultative intracellular rod transmitted via contaminated water aerosols, causing Legionnaires' disease (severe atypical pneumonia) and Pontiac fever.

Organism Card

DomainMust know
Identity
  • Gram-negative bacillus (thin, poorly staining) [1][2]
  • Family Legionellaceae; facultative intracellular pathogen
  • > 50 species; L. pneumophila serogroup 1 causes ~70% of clinical disease [3][4]
Lab discriminator
  • Poorly visualised on Gram stain (WBC++ but no organisms seen) — classic clue [3]
  • Requires BCYE agar (buffered charcoal yeast extract) with L-cysteine & iron; will NOT grow on standard media
  • Culture takes 3–5 days (up to 1–3 weeks); colonies appear ground-glass
  • cf. Mycoplasma: no cell wall, grows on Eaton agar; cf. Chlamydia: obligate intracellular, cannot culture on agar
Reservoir / transmission
  • Environmental aqueous sources: water cooling towers, hot water systems, spas, fountains [3][5]
  • Also garden soil, compost, potting mixes [3]
  • Inhalation of contaminated aerosols/mist from artificial water systems
  • No person-to-person transmission
  • HK relevance: cooling tower outbreaks reported; notifiable disease
Key virulence
  • Facultative intracellular survival within alveolar macrophages (Dot/Icm T4SS)
  • Inhibits phagosome–lysosome fusion → replicates in modified vacuole
  • β-lactams ineffective because organism is intracellular [4][6]
Clinical syndromes
  • Atypical pneumonia ("Legionnaires' disease") — classically severe CAP [1][5]
  • Prodromal flu-like illness → high fever, CNS (confusion), GI (diarrhoea) [3]
  • Rapidly deteriorating pneumonia despite β-lactam use — key vignette clue [3]
  • Pontiac fever: self-limited flu-like illness without pneumonia (milder form)
  • RFs: smoker, >50 y/o, chronic lung disease, immunocompromised, DM, CKD [3]
  • Bloods: lymphopenia without marked leukocytosis, hyponatraemia (SIADH), deranged LFT, type 1 RF [3]
Diagnosis
  • Urine Legionella antigen test — detects serogroup 1 only (~70%) [3][4]
  • Positive from day 3 to several weeks; not affected by antibiotics; rapid (few hours); high Sn & Sp [4]
  • Sputum/BAL PCR (broader serogroup coverage)
  • Culture on BCYE (gold standard but slow)
  • Serology: 4-fold rise in paired sera (retrospective)
  • CXR: lobar or multilobar consolidation (can show bulging fissure) but variable [3]
Treatment
  • Macrolides (azithromycin) or fluoroquinolones (levofloxacin) are first-line [3][6]
  • Doxycycline is alternative
  • Duration: 7–10 days (may extend to 14–21 days if immunocompromised)
  • β-lactams are ineffective (intracellular organism, no β-lactam target access) [1][3]
  • In severe CAP: always cover Legionella — add macrolide/doxycycline to empirical β-lactam [1][5]
Prevention
  • Statutory notifiable disease in Hong Kong [3]
  • Environmental control: water system disinfection, temperature regulation (>60°C storage, >50°C distribution)
  • No vaccine available for clinical use
  • No chemoprophylaxis; no isolation required (no person-to-person spread)
Classic traps
  • Gram stain shows plenty of WBCs but no organisms → think Legionella [3]
  • Pneumonia not responding to β-lactams + hypoNa + confusion + diarrhoea = Legionella until proven otherwise
  • Urine antigen only detects serogroup 1; negative urine Ag does NOT exclude non-serogroup 1 Legionella [4]
  • Distinguish from Mycoplasma (younger patients, milder, cold agglutinins) and Chlamydia psittaci (bird exposure, relative bradycardia)

Exam Intelligence

Bordetella pertussis

Gram-negative coccobacillus that causes pertussis (whooping cough), characterized by severe paroxysmal coughing with inspiratory whoop, primarily in unvaccinated children.

Campylobacter jejuni

Microaerophilic, curved Gram-negative rod that is the most common bacterial cause of acute gastroenteritis, transmitted via contaminated poultry, and associated with post-infectious Guillain-Barré syndrome.

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