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Gram-negativeBacilliCurved / comma-shaped

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.

Organism Card

DomainMust know
Identity
  • G−ve curved/spiral rod (comma- or S-shaped); microaerophilic
  • Genus Campylobacter; species jejuni is the most common cause of bacterial GE
  • Oxidase +, catalase +
  • Related to Helicobacter pylori (formerly classified as Campylobacter) [1]
Lab discriminator
  • Grows at 42 °C (thermophilic) on selective media (e.g. Skirrow's, Butzler) under microaerophilic conditions (5% O₂, 10% CO₂)
  • Darting/corkscrew motility on wet mount (single polar flagellum)
  • Oxidase + (vs Enterobacteriaceae oxidase −)
  • Hippurate hydrolysis + (distinguishes C. jejuni from C. coli)
Reservoir / transmission
  • Reservoir: poultry, cattle, dogs, cats, wild birds
  • Transmission: undercooked poultry (most common), unpasteurised milk, contaminated water
  • Faecal–oral; low infectious dose (~500 organisms)
  • Seasonal peak in warmer months; common cause of traveller's diarrhoea
Key virulence
  • Flagella-mediated motility → intestinal colonisation
  • Adhesins (CadF, FlpA) → epithelial attachment and invasion
  • Cytolethal distending toxin (CDT) → DNA damage, cell cycle arrest, cell death
  • Lipo-oligosaccharide (LOS) molecular mimicry of gangliosides (GM1, GD1a) → triggers Guillain-Barré syndrome (GBS) post-infection [4]
Clinical syndromes
  • Inflammatory (large-bowel type) diarrhoea: bloody mucoid stool, abdominal cramps, fever [2][3]
  • Prodrome of fever, headache, myalgia → then profuse diarrhoea (may mimic appendicitis if RIF pain predominant)
  • Usually self-limiting in 5–7 days
  • Post-infectious complications: Guillain-Barré syndrome (most important association), reactive arthritis, erythema nodosum [4]
  • Bacteraemia rare (mainly immunocompromised); C. fetus → systemic/penetrating infection [3]
Diagnosis
  • Stool culture on selective media at 42 °C under microaerophilic conditions — gold standard
  • Stool microscopy: fecal polymorphonuclear leukocytes (inflammatory pattern) [3]
  • Gram stain of stool: characteristic curved G−ve rods ("gull-wing")
  • Rapid antigen detection and multiplex PCR panels increasingly used
  • Pitfall: routine aerobic culture will NOT grow Campylobacter — must specifically request
Treatment
  • Most cases self-limiting → supportive care + rehydration only
  • Antibiotic if indicated: erythromycin (or azithromycin) is drug of choice [5]
  • Only of value if given early; in institutional settings to shorten bacterial excretion [5]
  • Alternative: fluoroquinolones — but rising fluoroquinolone resistance worldwide (especially Asia/HK)
  • Avoid antibiotics in uncomplicated cases (similar principle to non-typhoidal Salmonella)
Prevention
  • Notifiable disease in Hong Kong
  • Proper cooking of poultry (≥ 74 °C core temperature); avoid cross-contamination
  • Pasteurisation of milk; safe water supply
  • No vaccine available for clinical use
  • Standard + contact precautions for hospitalised patients with diarrhoea
Classic traps
  • Bloody diarrhoea followed 1–3 weeks later by ascending weakness/areflexia → think Campylobacter-triggered GBS [4]
  • Inflammatory diarrhoea DDx: Shigella (human-only reservoir, no animal), Salmonella (eggs/poultry, non-lactose fermenter), EHEC (no fever, HUS), C. jejuni (poultry, 42 °C growth)
  • C. jejuni = inflammatory colonic; **C. fetus = penetrating/systemic (distal small bowel, enteric-fever-like)*** [3]
  • Don't confuse with H. pylori: both spiral G−ve, but H. pylori lives in stomach, urease +++ [1]

Exam Intelligence

  1. Campylobacter jejuni is classified under "inflammatory (large-bowel type)" gastroenteritis alongside Shigella, non-typhoidal Salmonella, EHEC, C. difficile, and Entamoeba histolytica — characterised by mucosal invasion, cytotoxin production, fecal PMN leukocytes, and mucoid ± bloody diarrhoea [2][3]. This classification table is extremely high-yield and has appeared repeatedly in GC lecture frameworks.

  2. Campylobacter jejuni is the single most important infectious trigger of Guillain-Barré syndrome (GBS) via molecular mimicry between its lipo-oligosaccharide and human gangliosides (GM1, GD1a, GQ1b). The classic exam vignette: a patient with recent gastroenteritis developing progressive ascending symmetrical weakness with areflexia and albuminocytological dissociation in CSF [4].

  3. Drug of choice for Campylobacter when antibiotics are indicated is erythromycin (macrolide), per HKUMed paediatric and microbiology teaching [5]. This contrasts with Salmonella typhi (3rd-gen cephalosporin or ciprofloxacin), Shigella (cotrimoxazole), and C. difficile (metronidazole → vancomycin) — examiners love asking "match the organism to the antibiotic."

  4. C. fetus causes penetrating/systemic infection (enteric-fever-like) and appears in a different column from C. jejuni in the GE classification table [3]. This is a discriminator: if the vignette describes enteric fever features with Campylobacter, think C. fetus, not C. jejuni.

  5. Antibiotics for Campylobacter GE are generally NOT recommended for uncomplicated cases — only of value if given early and in institutional settings to shorten bacterial excretion [5]. This mirrors the principle that most acute GE is self-limiting and antibiotics are not routinely indicated.

  6. H. pylori was formerly classified as Campylobacter (called "Campylobacter pyloridis" historically) due to its spiral morphology — both are G−ve curved/spiral rods, but their clinical niches are completely different (stomach vs intestine) [1]. This is a favourite examiner trivia point.

  • Inflammatory vs non-inflammatory diarrhoea: Campylobacter = inflammatory (fecal WBC +, bloody mucoid stool). If the vignette describes profuse watery diarrhoea with NO blood or WBC, think non-inflammatory causes (rotavirus, norovirus, ETEC, cholera).
  • Campylobacter vs appendicitis: RIF pain with diarrhoea in a young patient after eating undercooked chicken → may mimic acute appendicitis. Key clue: preceding diarrhoea/fever + exposure history.
  • GBS trigger: The commonest wrong answer is "Salmonella" or "Shigella" — but C. jejuni is THE organism most associated with GBS.
  • Temperature for culture: 42 °C is a classic MCQ discriminator — most enteric organisms grow at 37 °C; Campylobacter requires the higher temperature.
  • Fluoroquinolone resistance: In Hong Kong/Asia, fluoroquinolone resistance rates in Campylobacter are high ( > 50% in some series), making macrolides the preferred empirical choice.

No directly relevant past paper questions referencing Campylobacter jejuni by name were identified in the indexed past paper context provided (2016–2025 Fourth Summative MCQ, SAQ, and Minicase papers). The 2023 Minicase Case 2 (paediatric vomiting with intussusception) involves GI pathology in a child but centres on surgical management rather than Campylobacter specifically [6].

Note: Campylobacter is more likely to appear as a distractor or part of an EMQ/MCQ option list in GE or GBS questions. Students should be prepared for it in the context of:

  • "Which organism is most associated with GBS?" → Campylobacter jejuni
  • "Match the antibiotic to the GE organism" → Erythromycin for Campylobacter
  • "Which organism causes inflammatory diarrhoea?" → Campylobacter among others

[1] Senior notes: Block A - Upper abdominal pain: peptic ulcer; pancreatitis and gallstone.pdf (H. pylori formerly called Campylobacter) [2] Senior notes: Ryan Ho Fundamentals.pdf (Classification of acute GE table) [3] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (GE classification: inflammatory vs non-inflammatory vs penetrating; antibiotic table) [4] Senior notes: learning_points_output.txt (GBS triggered by Campylobacter jejuni — molecular mimicry, albuminocytological dissociation) [5] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (Drug of choice for Campylobacter = erythromycin) [6] Past papers: 2023 Fourth Summative Minicase.pdf

Legionella pneumophila

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

Helicobacter pylori

Gram-negative, microaerophilic, spiral-shaped bacillus that colonizes the gastric mucosa, causing chronic gastritis, peptic ulcer disease, and predisposing to gastric adenocarcinoma and MALT lymphoma.

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