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Gram-negativeBacilliEnterobacterales

Yersinia enterocolitica

Gram-negative, facultatively anaerobic coccobacillus of the family Yersiniaceae that causes enterocolitis, mesenteric lymphadenitis, and pseudoappendicitis, particularly in children.

Organism Card

DomainMust know
Identity
  • G−ve coccobacillus (small rod), family Enterobacteriaceae
  • Non-lactose fermenter; facultative anaerobe
  • Motile at 25 °C (peritrichous flagella), non-motile at 37 °C (classic exam point)
  • Three pathogenic Yersinia spp: Y. pestis, Y. enterocolitica, Y. pseudotuberculosis
Lab discriminator
  • Cold enrichment: grows well at 4 °C (can isolate from refrigerated blood products) [1]
  • Urease +, oxidase −; non-lactose fermenter on MacConkey
  • CIN (cefsulodin-irgasan-novobiocin) agar → "bull's-eye" colonies
  • Motile at 22–25 °C but non-motile at 37 °C (vs other Enterobacteriaceae: motile at 37 °C)
  • Differentiate from Y. pestis: Y. enterocolitica is motile & urease + (Y. pestis is non-motile & urease −)
Reservoir / transmission
  • Reservoir: pigs (main), dogs, rodents, contaminated water
  • Transmission: fecal-oral; contaminated food (esp. undercooked pork, chitterlings)
  • Contaminated blood products (grows at 4 °C in stored RBC units → transfusion sepsis) [1]
  • No major HK-specific endemic focus but transfusion-related cases examinable
Key virulence
  • Inv (invasin): binds β1-integrin → facilitates invasion of Peyer's patches / M cells
  • Ail (attachment-invasion locus): serum resistance, epithelial invasion
  • Yersinia outer proteins (Yops) injected via Type III secretion system → antiphagocytic
  • Iron-acquisition systems (no siderophore in non-pathogenic strains); virulence enhanced by iron overload (e.g. haemochromatosis, desferrioxamine Rx)
Clinical syndromes
  • Enterocolitis: most common; watery → bloody diarrhoea, esp. young children [2]
  • Mesenteric lymphadenitis / pseudoappendicitis: RLQ pain ± fever in older children/young adults — mimics appendicitis [2][3]
  • Terminal ileitis: may mimic Crohn's disease
  • Septicaemia: esp. in iron-overload states (haemochromatosis, thalassaemia, desferrioxamine use)
  • Post-infectious sequelae: reactive arthritis (a/w HLA-B27) [4], erythema nodosum [5]
  • Classified under "penetrating" enteric infections (distal small bowel) alongside typhoidal Salmonella and C. fetus [2]
Diagnosis
  • Stool culture on CIN agar or cold-enriched culture
  • Blood culture if septicaemia suspected
  • Serology (agglutination) — limited use; cross-reacts with Brucella
  • Pitfall: routine stool culture may miss it unless lab is specifically requested (cold enrichment / CIN agar)
Treatment
  • Enterocolitis: usually self-limiting; antibiotics reserved for severe/systemic disease
  • Septicaemia / invasive: fluoroquinolones, TMP-SMX, or 3rd-gen cephalosporins
  • Key resistance: intrinsically β-lactamase producing → resistant to ampicillin and 1st-gen cephalosporins
  • Combination with aminoglycosides in severe sepsis
Prevention
  • Cook pork thoroughly; avoid raw pork products
  • Screen / leukoreduction of blood products to reduce transfusion-transmitted risk
  • No vaccine available
  • Notifiable in HK as part of bacterial foodborne illness surveillance
Classic traps
  • RLQ pain + fever in adolescent → "pseudoappendicitis" = classic vignette for Y. enterocolitica mesenteric adenitis (vs true appendicitis)
  • Transfusion reaction with septic shock from refrigerated RBC unit → think Y. enterocolitica (grows at 4 °C)
  • Iron-overload patient on desferrioxamine + sepsis → Y. enterocolitica (desferrioxamine acts as siderophore)
  • Reactive arthritis post-diarrhoea: Yersinia is one of the classic enteric triggers alongside Salmonella, Shigella, Campylobacter [4]
  • Erythema nodosum: Yersinia listed alongside Streptococcus, TB, Salmonella, Campylobacter as infective causes [5]
  • Do not confuse with Y. pestis (plague): Y. pestis is non-motile, urease −, bipolar "safety-pin" staining, transmitted by flea bites

Exam Intelligence

  1. Penetrating enteric infection pattern: GC / Paeds lecture slides classify Y. enterocolitica under "penetrating" infections of the distal small bowel, alongside typhoidal Salmonella and Campylobacter fetus — expect fecal mononuclear leukocytes, not PMNs [2].

  2. Reactive arthritis trigger: Multiple senior notes and the Felix Lai Medicine notes [4] list Yersinia as one of the five classic enteric pathogens causing reactive arthritis (Reiter's): Salmonella, Shigella, Yersinia, Campylobacter, C. difficile — associated with HLA-B27, seronegative spondyloarthropathy.

  3. Erythema nodosum: Ryan Ho Rheumatology notes [5] explicitly list Yersinia alongside Strep, TB, Salmonella, and Campylobacter as infectious causes of EN — bilateral tender shin nodules with prodromal fever and arthralgia.

  4. Cold enrichment / 4 °C growth: This is the single most-tested lab feature. It explains both the CIN agar culture technique and the transfusion sepsis scenario (stored RBC bags at 4 °C).

  5. Iron-overload synergy: Patients with haemochromatosis or those on desferrioxamine chelation are uniquely susceptible because desferrioxamine acts as a xenosiderophore for Yersinia, dramatically increasing virulence.

  6. Pseudoappendicitis in exams: A young patient (adolescent/young adult) with RLQ pain, fever, and preceding diarrhoea who goes to surgery and has a normal appendix but inflamed mesenteric lymph nodes → Y. enterocolitica mesenteric adenitis is the expected answer.

  • Yersinia vs Appendicitis: Pseudoappendicitis from Y. enterocolitica typically has antecedent diarrhoea and mesenteric lymphadenopathy on imaging; true appendicitis has periumbilical-to-RLQ migration pain without prominent diarrhoea.
  • Yersinia vs Crohn's: Terminal ileitis on imaging may look identical; clinical history of acute onset + culture positivity differentiates infectious ileitis from IBD.
  • Y. enterocolitica vs Y. pestis: Both are Yersinia but vastly different. Y. pestis = plague (buboes, pneumonic plague, flea-bite transmission, non-motile, urease −); Y. enterocolitica = food-/water-borne enteritis (motile at 25 °C, urease +). The 2022 MCQ EMQ [6] lists Yersinia pestis as a distractor among opportunistic organisms — do not confuse the two species.
  • Transfusion sepsis differential: Organisms that contaminate stored blood products include Y. enterocolitica (RBC units, 4 °C) and Staphylococcus epidermidis / Serratia (platelet units, room temperature). Vignette of post-transfusion rigors + shock with G−ve rod in blood → Y. enterocolitica.
  • Reactive arthritis pathogen list: When asked for "enteric organisms causing reactive arthritis," give the classic five — do not list E. coli or Vibrio (common wrong answers).

  • 2022 Fourth Summative MCQ Q19–21 (EMQ) [6]: The option list includes "Yersinia pestis" as a distractor alongside other organisms in an immunocompromised-patient EMQ. While the correct answers for Q19–21 are Talaromyces marneffei, Streptococcus mitis, and CMV respectively, Y. pestis was placed as a distractor. This tests whether students can differentiate Y. pestis (plague, not opportunistic in HIV) from other choices. Y. enterocolitica was not listed but the inclusion of Yersinia in the option bank highlights genus-level awareness expected.

  • No other directly indexed past-paper questions specifically test Y. enterocolitica by name in the provided past-paper context. However, the organism is highly examinable via reactive arthritis triggers (Felix Lai Medicine notes [4]) and erythema nodosum causes (Ryan Ho Rheumatology [5]), both of which are testable as SAQ list-type questions.

[1] Lecture slides: GC 105. Medically important microbes what every doctor should know.pdf; GC 142. A child with loose stool.pdf [2] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf — p.328 (enteric infection classification table) [3] Lecture slides: GC 195. Lower and diffuse abdominal pain RLQ problems; pelvic inflammatory disease; peritonitis and abdominal emergencies.pdf [4] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf — p.1715 (Reactive arthritis etiology) [5] Senior notes: Ryan Ho Rheumatology.pdf — p.157 (Erythema nodosum causes) [6] Past papers: 2022 Fourth Summative MCQ.pdf — p.40 (EMQ Q19–21 option list including Yersinia pestis)

Shigella

Gram-negative, non-motile, facultatively anaerobic bacillus of the family Enterobacteriaceae that causes bacillary dysentery (shigellosis) characterized by invasive bloody, mucoid diarrhea.

Pseudomonas aeruginosa

Obligate aerobic, oxidase-positive, non-fermenting Gram-negative bacillus that is a leading cause of nosocomial infections, particularly ventilator-associated pneumonia, burn wound infections, and opportunistic infections in immunocompromised and cystic fibrosis patients.

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