## Distinguishing Shigella from E. coli ### Key Structural Differences **Key Point:** Shigella species are non-motile (lack flagella), whereas most E. coli strains are motile with peritrichous flagella. This is the primary and most reliable discriminator between these two closely related genera. ### Comparison Table | Feature | E. coli | Shigella | |---------|---------|----------| | **Motility** | Motile (H antigen present) | Non-motile (no H antigen) | | **Lactose fermentation** | Positive (acid + gas) | Negative or delayed | | **Indole production** | Positive | Variable | | **Capsule** | Variable (some strains) | Present (K antigen) | | **Pathogenesis** | ETEC, EAEC, EHEC, EIEC | Invasive dysentery | ### Clinical Pearl **Clinical Pearl:** On a motility test (semi-solid medium), E. coli shows diffuse turbidity (motile), while Shigella shows growth only along the line of inoculation (non-motile). This simple test is routinely used in diagnostic microbiology labs to differentiate these organisms. ### High-Yield Mnemonic **Mnemonic:** **SHIGELLA = SHy In Getting Extra Locomotion Lacking Ability** — Shigella is shy and lacks the ability to move (non-motile). ### Why Motility Matters 1. Shigella lost flagellar genes during evolution as an obligate intracellular pathogen 2. E. coli retained flagella for environmental survival and transmission 3. Loss of motility in Shigella correlates with its shift to invasive (dysenteric) pathogenesis rather than toxin-mediated diarrhea [cite:Jawetz, Melnick & Adelberg's Medical Microbiology 28e Ch 16]
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