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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

  1. "Need to cover Legionella pneumophila in SEVERE cases of CAP" — stated verbatim on GC 052 lecture slide 17 [5]. This is a direct-from-lecture exam point: in all CAP cover S. pneumoniae; in severe CAP additionally cover Legionella.

  2. Urine antigen detects only L. pneumophila serogroup 1 (~70% of cases); positive from day 3, persists for weeks, unaffected by antibiotics, rapid (few hours), high sensitivity and specificity — emphasised in multiple senior notes as a footnote-level must-know [4].

  3. Legionella is classified as an atypical pneumonia pathogen alongside Mycoplasma pneumoniae, Chlamydia pneumoniae, and Chlamydophila psittaci — consistently grouped in lecture and senior note "atypical pneumonia" lists [1][6]. Exam stems may ask you to list atypical CAP organisms.

  4. Legionella can cause atypical lymphocytosis / infectious mononucleosis-like syndrome — listed alongside EBV, CMV, HIV in the Block A lymphadenopathy lecture differential [7].

  5. Classic blood picture: lymphopenia (not leukocytosis), hyponatraemia (SIADH), deranged LFT [3]. An exam stem with a pneumonia patient showing Na 124 mmol/L should trigger Legionella on your differential.

  6. Source of Legionella: contaminated water systems (cooling towers, hot water tanks, spas) and garden compost/potting mix — directly examined in 2018 Minicase [8].

  • β-lactam non-response trap: A vignette showing worsening pneumonia despite Augmentin/ceftriaxone + hypoNa + confusion + diarrhoea = Legionella. The wrong answer is to escalate β-lactam coverage; the correct move is to add azithromycin or levofloxacin.
  • Urine antigen limitation: If the stem says "urine Legionella antigen is negative," this does NOT exclude Legionella if the infecting serogroup is not serogroup 1. PCR or culture on BCYE may still be needed.
  • Mycoplasma vs Legionella: Mycoplasma → younger patients, milder "walking pneumonia," extrapulmonary (erythema multiforme, cold agglutinins). Legionella → older/immunocompromised, severe, hypoNa, confusion, diarrhoea, rapid deterioration.
  • Normal WBC with pneumonia: A stem showing consolidation + normal WBC + no sputum culture growth should prompt you to consider urine antigen testing for Legionella (or S. pneumoniae) — this is the exact setup of the 2023 MCQ Q40 [9].

1. 2018 Fourth Summative Minicase Q9 [8]

"What was the likely source of Legionella pneumophila?" (10 marks)

Answer: Contaminated water systems — cooling towers, hot water tanks/systems, air conditioning systems, spa pools, fountains, or similar man-made aqueous environments. Garden soil/compost/potting mix is also an acceptable source.

Rationale: The question directly tests knowledge of the environmental reservoir. No person-to-person transmission. The preceding sections of this minicase showed a patient with hyponatraemia (Na 124 mmol/L) and CXR consolidation, consistent with Legionella pneumonia.

2. 2018 Fourth Summative Minicase Q4 [8]

"Give four organisms that are commonly involved in this disorder." (20 marks) — in context of a patient with chest consolidation on CXR.

Answer: S. pneumoniae, H. influenzae, Legionella pneumophila, Mycoplasma pneumoniae (or Staphylococcus aureus, Klebsiella pneumoniae, Chlamydia pneumoniae depending on context). Legionella is an expected inclusion in any list of CAP organisms.

3. 2023 Fourth Summative MCQ Q40 [9]

"A 65-year-old woman, with a history of chronic renal impairment, presented with fever, cough and yellowish sputum for 5 days. Physical examination showed consolidation signs over right lower chest. Initial investigations revealed normal white blood cell (WBC) count, chest X-ray showing right lower lobe consolidation and no growth on sputum aerobic culture. Which of the following further investigations may help in confirming the aetiology?"

A. Blood culture
B. Sputum acid-fast bacteria (AFB) culture
C. Urinary legionella antigen
D. Urinary pneumococcal antigen

Answer: D. Urinary pneumococcal antigen

Rationale: The stem describes a typical presentation (purulent sputum, lobar consolidation) with no growth on sputum culture — S. pneumoniae is the most common CAP pathogen and can be culture-negative (prior antibiotics, fastidious growth). Urinary pneumococcal antigen is the best next step. Option C (urinary Legionella antigen) is a distractor — Legionella classically presents with atypical features (confusion, diarrhoea, hypoNa, lymphopenia), not purulent sputum with normal WBC pointing to a typical organism. This question tests your ability to distinguish Legionella from S. pneumoniae presentations.

4. 2024 Fourth Summative MCQ Q37 [10]

"A 58-year-old man with pemphigus on long-term oral corticosteroid. He presented with low grade fever and weight loss for 8 weeks. Chest examination showed right lower zone stony dullness. Blood neutrophil count was normal. Sputum culture revealed no growth. Which investigation will likely provide an early diagnosis?"

A. Blood culture
B. Pleural biopsy
C. Sputum cytology
D. Urinary antigen test

Answer: B. Pleural biopsy

Rationale: 8-week history + immunosuppression + stony dullness (pleural effusion) + no growth on sputum = likely TB or malignancy, not acute Legionella. Pleural biopsy provides tissue for histology/AFB/culture. Option D (urinary antigen) is a trap — while urinary Legionella antigen is high-yield for acute Legionella pneumonia, this chronic presentation does not fit. This question tests discrimination of Legionella (acute severe pneumonia) from chronic pleural pathology.

[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.165–166) [2] Lecture slides: GC 101. Diagnosis of infections [Handouts].pdf [3] Senior notes: Ryan Ho Respiratory.pdf (p.62–63) [4] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.165, footnote 97); Ryan Ho Respiratory.pdf (p.63, footnote 45) [5] Lecture slides: GC 052. Fever and purulent sputum.pdf (slide 17) [6] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p.139, 142) [7] Senior notes: Block A - Generalised Lymphadenopathy_ Differential diagnosis and principle of management.pdf (p.3) [8] Past papers: 2018 Fourth Summative Minicase.pdf (p.3, p.7) [9] Past papers: 2023 Fourth Summative MCQ.pdf (Q40, p.15) [10] Past papers: 2024 Fourth Summative MCQ.pdf (Q37, p.15)

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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