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Gram-negativeBacilliCurved / comma-shaped

Vibrio vulnificus

Halophilic, lactose-fermenting, curved Gram-negative rod found in warm saltwater that causes rapidly progressive wound infections and fulminant primary septicemia, especially in patients with liver disease or iron-overload states.

Organism Card

DomainMust know
Identity
  • G−ve curved rod (comma-shaped); halophilic Vibrio [1]
  • Oxidase +; facultatively anaerobic
  • Non-O1/non-O139 Vibrio (distinct from V. cholerae)
Lab discriminator
  • Grows on TCBS agar → green colonies (vs V. cholerae → yellow colonies) [1]
  • Halophilic: requires NaCl for growth (vs V. cholerae grows without added salt)
  • Oxidase + curved G−ve rod on blood culture → think Vibrio spp.
Reservoir / transmission
  • Warm seawater and raw/undercooked shellfish (especially oysters) — highly HK-relevant [1][2]
  • Two routes: (1) ingestion of contaminated seafood; (2) wound exposure to seawater
  • Hong Kong: raw oyster consumption and fishing/seafood handling are classic exam exposures [2]
Key virulence
  • Polysaccharide capsule → anti-phagocytic; essential for systemic virulence
  • Cytolysin/haemolysin → tissue destruction, haemorrhagic bullae
  • Rapid iron acquisition system → proliferates in iron-overload states (liver cirrhosis, haemochromatosis, thalassaemia)
Clinical syndromes
  • Primary septicaemia: ingestion of raw oysters in patient with chronic liver disease / cirrhosis / iron overload → fever, shock, haemorrhagic bullae on limbs; mortality 50% [2][3]
  • Necrotising fasciitis / wound infection: seawater wound exposure → rapidly progressive cellulitis with haemorrhagic bullae, pain out of proportion, systemic toxicity [3]
  • Gastroenteritis: self-limited watery diarrhoea after raw seafood (less severe)
Diagnosis
  • Blood cultures (primary septicaemia) or wound swab/tissue C/ST
  • TCBS agar: green colonies
  • LRINEC score ≥ 8 → high risk for necrotising fasciitis [3]
  • Finger probe test: +ve if minimal resistance to blunt dissection along fascial plane [3]
Treatment
  • Aggressive surgical debridement (for NF / wound infection) + IV broad-spectrum antibiotics [3]
  • Antibiotic: 3rd-gen cephalosporin (e.g. ceftriaxone) + doxycycline (or tetracycline)
  • NF: do NOT delay surgery for imaging — clinical diagnosis is sufficient
  • Resuscitation and ICU support for septic shock
Prevention
  • Avoid raw/undercooked shellfish in patients with liver disease, iron overload, immunosuppression
  • Wound care: avoid seawater exposure of open wounds
  • Notifiable disease in Hong Kong (invasive V. vulnificus infection)
  • No vaccine available
Classic traps
  • V. vulnificus vs V. cholerae: vulnificus = septicaemia/NF in liver disease patient + haemorrhagic bullae; cholerae = profuse rice-water diarrhoea, no bullae, yellow on TCBS
  • NF due to V. vulnificus vs Group A Strep (S. pyogenes): both cause NF, but seawater/oyster exposure + liver disease + haemorrhagic bullae → V. vulnificus [3]
  • Gas gangrene (C. perfringens): post-trauma + gas on XR + NO neutrophilia ≠ V. vulnificus NF [3]
  • Bullous skin lesions DDx: do not confuse with bullous pemphigoid (autoimmune, no sepsis)

Exam Intelligence

  1. GC 105 ("Medically important microbes") and GC 178 ("An ugly wound") both frame V. vulnificus as a key cause of necrotising soft tissue infection (NSTI) alongside S. pyogenes and polymicrobial causes [1]. The exam commonly presents a vignette of an immunocompromised patient with wound infection after seafood handling or seawater exposure.

  2. Maksim Surgery Notes explicitly list V. vulnificus alongside S. pyogenes and polymicrobial organisms as causes of necrotising fasciitis — with hallmark features of pain out of proportion, haemorrhagic bullae, systemic toxicity, and "dirty dishwater" discharge [3]. Know that LRINEC score > 8 = high risk, and finger probe test positivity indicates NF.

  3. The classic HKUMed vignette: middle-aged man with known hepatitis B cirrhosis (or alcoholic liver disease) presents with fever, shock, and haemorrhagic bullae on the lower limbs hours after eating raw oysters → V. vulnificus primary septicaemia. This ties directly to the "Jaundice after raw oysters" lecture block [2].

  4. TCBS agar colony colour is a favourite discriminator: V. vulnificus = green; V. cholerae = yellow. Both are oxidase-positive curved G−ve rods, so colony colour on TCBS is the rapid lab differentiator.

  5. Iron-overload connection: V. vulnificus has siderophore-independent iron uptake, making patients with haemochromatosis, thalassaemia major, or cirrhosis (high serum iron/transferrin saturation) uniquely susceptible. This is a common MCQ trap — the question asks "which underlying condition predisposes to fatal V. vulnificus septicaemia?"

  6. Treatment principle for NF: the exam expects "aggressive surgical debridement + IV broad-spectrum antibiotics" as a paired answer. For V. vulnificus specifically, the recommended combination is a 3rd-gen cephalosporin + doxycycline/tetracycline [1][3].

  • V. vulnificus vs V. parahaemolyticus: V. parahaemolyticus causes self-limited gastroenteritis from undercooked seafood (Kanagawa phenomenon — β-haemolysis on Wagatsuma agar); it does NOT cause the fulminant septicaemia/NF picture. If the stem mentions liver disease + bullae → V. vulnificus, not parahaemolyticus.
  • V. vulnificus vs Aeromonas hydrophila: both can cause wound infections from contaminated water, but Aeromonas is associated with freshwater, while V. vulnificus is associated with warm seawater/brackish water.
  • "Pain out of proportion": this phrase in a vignette points to either necrotising fasciitis or gas gangrene. Differentiate by exposure history (seawater/seafood = V. vulnificus; post-trauma/surgery + gas on XR = C. perfringens) and the presence/absence of neutrophilia (absent in C. perfringens due to leucocidin) [3].
  • Common wrong answer trap: selecting V. cholerae when the stem describes septicaemia with bullae. V. cholerae causes cholera (watery diarrhoea), not septicaemia or skin lesions.

No directly indexed past paper questions specifically naming or stem-matching Vibrio vulnificus were identified in the provided 2016–2025 Fourth Summative MCQ, SAQ, or Minicase papers. However, NF-related surgical vignettes (as per the Maksim Surgery Notes framing tested in past surgical SAQs) commonly draw on V. vulnificus as a key differential. Students should be prepared for this organism to appear in any stem involving:

  • Raw oyster ingestion + liver disease + septicaemia
  • Seawater wound + haemorrhagic bullae + NF
  • LRINEC score interpretation

[1] Lecture slides: GC 105. Medically important microbes what every doctor should know.pdf; GC 178. An ugly wound Wound healing; Wound infection; Anti-septic technique.pdf [2] Lecture slides: Block A - Jaundice after raw oysters_ acute hepatitis.pdf [3] Senior notes: Maksim Surgery Notes.pdf (Necrotising fasciitis section, p.274–275)

Vibrio cholerae

Curved, oxidase-positive Gram-negative rod (serogroups O1 and O139) that produces cholera toxin, causing profuse secretory ("rice-water") diarrhea and potentially fatal dehydration.

Pasteurella multocida

Gram-negative coccobacillus predominantly transmitted through animal bites and scratches, causing rapidly progressive soft-tissue infections, and less commonly pneumonia, septicemia, or meningitis.

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