A 28-year-old woman from rural Maharashtra presents with acute onset bloody diarrhea and abdominal cramps for 3 days. She reports consuming contaminated water from a community well. On examination, she is febrile (38.5°C) with mild dehydration. Stool microscopy shows RBCs and WBCs; culture on MacConkey agar yields non-lactose-fermenting colonies. The isolate is oxidase-negative, catalase-positive, and produces acid without gas from glucose. Further testing shows it is Shiga toxin-positive. What is the most likely organism?

Explanation

## Identification of Shigella dysenteriae Type 1 ### Clinical Presentation The patient presents with the classic triad of acute bacillary dysentery: 1. Bloody diarrhea (mucoid, blood-stained stools) 2. High fever (38.5°C) 3. Severe abdominal cramps **Key Point:** Shigella dysenteriae type 1 causes the most severe form of shigellosis with the highest incidence of complications, including toxic megacolon and hemolytic uremic syndrome (HUS). ### Microbiological Features | Feature | Finding | Significance | |---------|---------|---------------| | Gram stain | Gram-negative rod | Typical morphology | | Lactose fermentation | Non-fermenter | Key differential from E. coli | | Oxidase | Negative | Confirms Enterobacteriaceae | | Catalase | Positive | Distinguishes from Vibrio | | Glucose fermentation | Acid, no gas | Shigella pattern | | Shiga toxin | Positive | Pathognomonic for S. dysenteriae type 1 | | Motility | Non-motile | No flagella | **High-Yield:** Shiga toxin production is the hallmark of S. dysenteriae type 1 and is responsible for systemic complications. This is the ONLY Shigella species that produces Shiga toxin (Stx1). ### Pathogenesis ```mermaid flowchart TD A[Ingestion of contaminated water/food]:::outcome --> B[Invasion of colonic epithelium] B --> C[Intracellular multiplication] C --> D[Cell-to-cell spread via actin polymerization] D --> E[Mucosal ulceration & inflammation] E --> F[Bloody diarrhea] C --> G[Shiga toxin production] G --> H[Systemic effects: HUS, encephalopathy]:::urgent ``` **Clinical Pearl:** The severity of disease correlates with Shiga toxin production. S. dysenteriae type 1 is the most virulent species, followed by S. flexneri, then S. boydii and S. sonnei (milder disease). ### Why S. dysenteriae Type 1? - Positive Shiga toxin production is pathognomonic - Causes the most severe dysentery with highest complication rate - Epidemiologically associated with epidemic dysentery in developing countries - Non-motile, non-lactose-fermenting Gram-negative rod fits perfectly **Mnemonic:** **SHIGA** = **S**evere **H**emolytic uremic syndrome, **I**nvasive **G**ram-negative, **A**cute dysentery (S. dysenteriae type 1 causes all three) [cite:Textbook of Microbiology by Ananthanarayan & Paniker 10e Ch 32]