What lab feature helps distinguish Salmonella from Shigella on stool culture?

• A. Shigella produces hydrogen sulfide (H2S) on TSI agar and is motile.
• B. Salmonella produces hydrogen sulfide (H2S) on TSI agar and is motile.
• C. Salmonella does not produce H2S and is non-motile.
• D. Shigella does not produce H2S and is non-motile.

Answer: (B)

On stool culture, the main discriminator between Salmonella and Shigella is hydrogen sulfide production on a TSI slant and the organism’s motility. Salmonella reduces sulfur to hydrogen sulfide, which reacts with iron in the medium to form a black precipitate, a positive H2S signal. It is also motile because of its peritrichous flagella. Shigella, on the other hand, does not produce hydrogen sulfide and is non-motile, so you don’t see blackening on TSI and there’s no movement in motility testing. Because Salmonella exhibits both H2S production and motility, this combination makes it the best distinction from Shigella on stool culture.