Question

Which of the following charecteristics colonies of Salmonella species will be appeared on Xylose Lysine Deoxycholate agar?

• A. Red colored colonies with black centres
• B. Blue colored colonies with black centres
• C. Green colored colonies with black centres
• D. Red colored colonies without black centres

Hint:

A "red colony with a black center" is the classic diagnostic signature of Salmonella on XLD agar, distinguishing it from Shigella (which appears as plain red colonies without black centers).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Xylose Lysine Deoxycholate (XLD) agar is a selective and differential growth medium used to isolate and identify enteric pathogens, particularly Salmonella and Shigella.

Step 2: Detailed Explanation:
The visual appearance of Salmonella colonies on XLD agar is due to three coordinated biochemical reactions:
1. Xylose Fermentation: Salmonella initially ferments xylose, which produces acid and temporarily turns the phenol red indicator yellow.
2. Lysine Decarboxylation: Once xylose is depleted, Salmonella decarboxylates the amino acid lysine to form alkaline cadaverine. This increases the pH, turning the colony color back to red.
3. \(\text{H}_2\text{S}\) Production: Salmonella reduces sodium thiosulfate to hydrogen sulfide (\(\text{H}_2\text{S}\)). The \(\text{H}_2\text{S}\) reacts with ferric ammonium citrate in the medium, forming a black precipitate of iron sulfide in the center of the colony.
This results in distinct red colonies with black centers.

Step 3: Final Answer
On XLD agar, Salmonella species appear as red colonies with black centers.