Shigella
Gram-negative, non-motile, facultatively anaerobic bacillus of the family Enterobacteriaceae that causes bacillary dysentery (shigellosis) characterized by invasive bloody, mucoid diarrhea.
Organism Card
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| Lab discriminator |
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| Classic traps |
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Exam Intelligence
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GC 142 "A child with loose stool" — Shigella is a key organism in the differential of acute bloody diarrhoea in children; the lecture emphasises distinguishing invasive (Shigella, Salmonella, Campylobacter) from non-invasive (ETEC, Vibrio, viral) causes by the presence of faecal leucocytes and blood [1].
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GC 105 "Medically important microbes" — Shigella is listed as a key enteric pathogen; non-motile G−ve rod, human-only reservoir, very low infective dose (~10 organisms) making it the most contagious bacterial cause of diarrhoea [2].
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GC 103 "Fever after travelling" — Shigella dysentery is a major differential for returning travellers with bloody diarrhoea from endemic areas (South Asia, Southeast Asia); must distinguish from typhoid fever (Salmonella Typhi) which presents with constipation → then diarrhoea, relative bradycardia, and rose spots [3].
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AOS Microbiology — Shigella contrasted with Salmonella: mnemonic discriminators include motility (Shigella non-motile), H₂S (Shigella negative), reservoir (Shigella human-only), and infective dose (Shigella lowest among enteric bacteria) [4].
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GC 099 "Antimicrobial resistance" — Shigella highlighted as an organism with increasing multidrug resistance; ampicillin and TMP-SMX resistance now widespread; fluoroquinolone resistance emerging, making susceptibility testing essential [5].
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GC 142 — Anti-motility agents (loperamide) are contraindicated in invasive bacterial diarrhoea (Shigella, Salmonella, C. difficile) as they delay clearance and risk toxic megacolon [1].
- Bloody diarrhoea + fever + faecal WBCs → think invasive: Shigella, Salmonella, Campylobacter, enteroinvasive E. coli (EIEC). Shigella is the classic "bacillary dysentery" answer.
- Bloody diarrhoea + NO fever + NO faecal WBCs + HUS → think EHEC O157:H7 (non-invasive toxin-mediated).
- Traveller with fever + relative bradycardia + rose spots + constipation then diarrhoea → Salmonella Typhi (not Shigella). The 2019 minicase stem tests exactly this distinction [6].
- Exam trap: Students confuse Shiga toxin (produced by S. dysenteriae type 1) with Shiga-like toxin/verotoxin (produced by EHEC). Both cause HUS, but the organisms differ in invasiveness and clinical picture.
- Non-motile is the single most tested lab discriminator for Shigella vs Salmonella.
- Shigella dysenteriae type 1 is the only Shigella species producing Shiga toxin; other species cause dysentery by direct mucosal invasion without toxin-mediated HUS.
2019 Fourth Summative Mini Case, Case 1, Sections 1–4 [6]:
- Stem: "A 23-year-old female student presented with recurrent fever for 2 weeks upon return from Bangladesh. She also complained of constipation and abdominal pain during the first week upon return. Later in the second week, she developed non-bloody diarrhoea… Blood and stool culture confirmed the diagnosis of typhoid fever."
- Relevance to Shigella: This stem tests the differential between typhoid (Salmonella Typhi — constipation first, relative bradycardia, rose spots) and Shigella dysentery (bloody diarrhoea, high fever, tenesmus from the outset). The confirmed diagnosis is typhoid, but Shigella would be a key differential to exclude. Discriminators: typhoid has relative bradycardia, constipation preceding diarrhoea, and rose spots — features absent in shigellosis.
2021 Fourth Summative SAQ Q12(a) [7]:
- Stem: "A 40-year-old man had bone marrow transplant for chronic myeloid leukaemia… He developed high fever on day 10 after conditioning with high dose chemotherapy. His blood neutrophil count was less than 0.1 × 10⁹/L. List common microbes which could be found in his blood culture."
- Relevance: While Shigella itself would not typically appear in this context (rarely causes bacteraemia), the question tests knowledge of enteric G−ve rods as a category of bloodstream infection in neutropenic patients. Shigella's near-absence from blood cultures is itself a discriminating feature vs Salmonella.
No other directly Shigella-specific past paper question was identified in the indexed context. Students should be prepared for Shigella to appear in MCQ/EMQ format as a differential in "bloody diarrhoea" or "traveller's diarrhoea" stems.
[1] Lecture slides: GC 142. A child with loose stool.pdf [2] Lecture slides: GC 105. Medically important microbes what every doctor should know.pdf [3] Lecture slides: GC 103. Fever after travelling.pdf [4] AOS material: AOS - Microbio.pdf [5] Lecture slides: GC 099. Antimicrobial resistance.pdf [6] Past papers: 2019 Fourth Summative Mini Case.pdf [7] Past papers: 2021 Fourth Summative SAQ.pdf
Nontyphoidal salmonella
Flagellated, facultatively anaerobic Gram-negative bacilli of the family Enterobacteriaceae (chiefly *Salmonella enterica* serovars other than Typhi and Paratyphi) that typically cause self-limited gastroenteritis but can produce invasive bacteremia, particularly in immunocompromised hosts.
Yersinia enterocolitica
Gram-negative, facultatively anaerobic coccobacillus of the family Yersiniaceae that causes enterocolitis, mesenteric lymphadenitis, and pseudoappendicitis, particularly in children.