Biochemical Test of Salmonella Typhi

By Prof Walter Jaoko

Salmonella Typhi is the bacterium responsible for causing typhoid fever. There are various Biochemical tests that are commonly used in clinical laboratories to identify and confirm the presence of Salmonella Typhi in patient samples. These tests help differentiate Salmonella Typhi from other bacteria and assist in the diagnosis of typhoid fever.

Here are some of the key biochemical tests used for the identification of Salmonella Typhi:

1. Gram Staining: Gram staining is the initial step in bacterial identification. Salmonella Typhi appears as Gram-negative rods under the microscope.
2. Catalase Test: Salmonella Typhi produces the enzyme catalase, which breaks down hydrogen peroxide into water and oxygen. When a small amount of hydrogen peroxide is added to a colony of Salmonella Typhi, it will produce bubbles of oxygen, indicating a positive catalase test.
3. Oxidase Test: Salmonella Typhi is oxidase-negative, meaning it does not produce the enzyme oxidase. A negative result is characterized by no color change after adding the oxidase reagent.
4. Triple Sugar Iron (TSI) Agar: TSI agar is a differential medium that helps identify Salmonella Typhi based on its ability to ferment sugars and produce hydrogen sulfide gas. Salmonella Typhi typically shows the following reaction on TSI agar: alkaline/acid slant (red/yellow) with no gas production and no hydrogen sulfide production (no blackening).
5. Indole Test: The indole test is performed to determine if Salmonella Typhi can produce indole, a byproduct of tryptophan metabolism. Salmonella Typhi is indole-negative, meaning it does not produce indole. A negative test is indicated by the absence of a red color after adding Kovac’s reagent.
6. Urease Test: Salmonella Typhi is urease-negative, meaning it does not produce the enzyme urease. A negative result is characterized by no color change after incubation with urea agar.
7. Citrate Utilization Test: Salmonella Typhi is unable to utilize citrate as a carbon source. A negative citrate utilization test is indicated by the absence of a color change (green) in the Simmons citrate agar slant.

Facts Table