Which feature best distinguishes Shigella from Salmonella in gram-negative enterobacterial infections?

Explanation

## Distinguishing Shigella from Salmonella ### Key Morphological Difference **Key Point:** Shigella is non-motile (lacks flagella), whereas Salmonella is motile with peritrichous flagella. This is the single most reliable biochemical and structural discriminator between these two closely related enterobacteria. ### Comparative Table | Feature | Shigella | Salmonella | |---------|----------|------------| | **Motility** | Non-motile (no flagella) | Motile (peritrichous flagella) | | Lactose fermentation | Non-fermenter | Non-fermenter | | H₂S production | Negative | Positive (most strains) | | Urease | Negative | Negative | | Indole | Variable | Negative | | Lysine decarboxylase | Negative | Positive | | Ornithine decarboxylase | Negative | Positive | ### Clinical Pearl **Clinical Pearl:** The non-motility of Shigella correlates with its pathogenic strategy—it relies on invasion and intracellular multiplication within colonic epithelium rather than systemic dissemination. Salmonella, being motile, can reach deeper tissues and cause bacteremia more readily. ### High-Yield Mnemonic **Mnemonic:** **SHIGELLA = SHy (non-motile, stays local)**. Shigella does not spread systemically; it causes dysentery confined to the colon. Salmonella, by contrast, is a "traveler"—motile and systemic. ### Laboratory Identification Motility is tested using: - Hanging drop preparation (direct observation) - Motility medium (semi-solid agar with triphenyltetrazolium chloride) - Flagellar antigen typing (H antigen in Salmonella; absent in Shigella) **High-Yield:** On TSI (Triple Sugar Iron) agar, both produce similar patterns (no gas, no lactose fermentation), but Salmonella produces H₂S (black butt), whereas Shigella does not. However, the most fundamental distinction remains motility.