Shigella species are differentiated biochemically from other Enterobacteriaceae primarily by their inability to ferment which sugar?

Explanation

## Biochemical Differentiation of Shigella **Key Point:** Shigella species are **lactose-fermenting negative** Gram-negative rods, a critical distinguishing feature from other Enterobacteriaceae. ### Shigella Fermentation Profile **High-Yield:** The classic biochemical pattern of Shigella is: - **Glucose:** Fermented (acid only, no gas) - **Lactose:** NOT fermented ✗ - **Mannitol:** Variable (depends on species) - **Sucrose:** NOT fermented (except S. sonnei) ### Comparison with Related Organisms | Organism | Glucose | Lactose | Mannitol | H~2~S | |----------|---------|---------|----------|-------| | Shigella | + | − | Variable | − | | E. coli | + | + | + | − | | Salmonella | + | − | + | + | | Klebsiella | + | + | + | − | **Clinical Pearl:** On MacConkey agar, Shigella appears as **colorless (non-lactose-fermenting) colonies**, whereas E. coli produces pink/red colonies due to lactose fermentation and acid production. **Mnemonic:** **SHIGELLA = SHI (Shigella Has no lactose fermentation)** — remember that the inability to ferment lactose is the hallmark biochemical feature. ### Species-Specific Variations - **S. flexneri, S. boydii, S. dysenteriae:** Mannitol-negative - **S. sonnei:** Mannitol-positive and lactose-positive (late fermenter) — the most common species in developed countries