Which test is commonly used to identify the presence of Salmonella in a culture?

The Triple Sugar Iron (TSI) agar test is a crucial diagnostic tool used for identifying the presence of Salmonella in a culture. This medium contains three sugars (glucose, sucrose, and lactose) and iron, allowing for the differentiation of enteric bacteria based on their fermentation abilities and gas production.

When Salmonella is cultured on TSI agar, it typically ferments glucose, producing acid and causing the butt of the medium to turn yellow. However, it does not ferment lactose or sucrose, which results in the slant remaining red. Additionally, Salmonella can produce hydrogen sulfide (H2S), which reacts with the iron in the medium to form a black precipitate. This combination of acid production from glucose fermentation and H2S production is characteristic of Salmonella and helps confirm its presence in the sample.

In contrast, the other tests listed serve different purposes. The indole test is used to determine the ability of an organism to convert tryptophan to indole. The urease test checks for an organism's ability to hydrolyze urea, producing ammonia and increasing the pH of the medium. The gelatin hydrolysis test assesses whether an organism can liquefy gelatin through the action of gelatinase.