## Distinguishing Shigella from Salmonella **Key Point:** Motility is the single most reliable and rapid discriminator between these two closely related enteroinvasive gram-negative bacilli. ### Comparison Table | Feature | Shigella | Salmonella | | --- | --- | --- | | **Motility (37°C)** | **Non-motile** (no flagella) | **Motile** (peritrichous flagella) | | Lactose fermentation | Non-fermenter | Non-fermenter | | H₂S production | Negative | Positive (most species) | | Ornithine decarboxylase | Positive (most) | Positive | | Lysine decarboxylase | Negative | Positive | | Invasion mechanism | Invades colonic epithelium | Invades terminal ileum | **High-Yield:** Motility testing at 37°C is the **gold standard** initial discriminator. Shigella lacks flagella entirely and is always non-motile, whereas Salmonella is motile at 37°C (though some Salmonella typhi may show reduced motility). **Clinical Pearl:** In routine clinical microbiology labs, a non-motile gram-negative rod fermenting glucose but not lactose, isolated from stool, is presumptively Shigella. Confirmation of non-motility on semi-solid media (Cragie's tube or motility agar) takes <4 hours and is faster than serotyping. **Mnemonic:** **SHIG** = **S**tationery (non-motile), **H**ighly invasive, **I**nflammatory, **G**ram-negative. ### Why Lactose & H₂S Are Not Discriminators - Both Shigella and Salmonella are **non-lactose fermenters** — this is a shared feature, not a discriminator. - H₂S production is **positive in most Salmonella** but **negative in Shigella** — however, some Salmonella species (e.g., *S. paratyphi A*) are H₂S-negative, making this less reliable than motility. - Ornithine decarboxylase is positive in both (with rare exceptions). **Tip:** On TSI agar, both organisms show K/A (alkaline slant/acid butt) or A/A patterns, so TSI alone cannot distinguish them. Motility testing must follow.
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.