A 6-year-old child from Delhi presents with acute onset bloody diarrhea, fever, and abdominal cramps for 3 days. Stool microscopy shows numerous RBCs and WBCs. Culture on MacConkey agar shows non-lactose fermenting colonies. Which is the most appropriate next investigation to confirm the diagnosis of Shigella?

Explanation

## Diagnosis of Shigella: Investigation Algorithm **Key Point:** Shigella identification requires a systematic biochemical approach after preliminary screening on selective media. ### Step-wise Diagnostic Approach 1. **Preliminary screening** (already done in this case): - Non-lactose fermenting (NLF) colonies on MacConkey agar - Gram-negative rod morphology 2. **Confirmatory biochemical tests** (next step): - **TSI (Triple Sugar Iron) agar**: Acid in butt, no gas, no H₂S (Shigella produces acid in butt only, not slant) - **MIU (Motility, Indole, Urease)**: Non-motile, indole variable (positive in S. flexneri), urease negative - **Motility test**: Shigella is **non-motile** (key differentiator from Salmonella) ### Why Biochemical Tests Are Confirmatory | Feature | Shigella | Salmonella | E. coli | |---------|----------|-----------|--------| | Lactose fermentation | Negative | Negative | Positive | | Motility | Non-motile | Motile | Motile | | Indole | Variable | Negative | Positive | | H₂S production | Negative | Positive | Negative | | TSI pattern | A/A (no gas) | A/A (with gas) | A/A (with gas) | **High-Yield:** The **non-motility** of Shigella is the single most important biochemical feature that distinguishes it from Salmonella in the field of NLF gram-negative rods. **Clinical Pearl:** Shigella's inability to ferment lactose combined with non-motility makes biochemical tests the gold standard for confirmation after culture isolation. ### Why Other Investigations Are Not Appropriate - **Oxidase test**: Shigella is oxidase-negative (like all Enterobacteriaceae), but this is not specific for Shigella identification - **Serological tests (VDRL, Widal)**: Not relevant for bacterial identification; Widal is for Salmonella typhi, not Shigella - **Electron microscopy**: Not a routine diagnostic tool; pili are not diagnostic for Shigella