XLD agar

Xylose Lysine Deoxycholate Agar

Salmonella colonies on XLD agar

Xylose Lysine Deoxycholate Agar (XLD agar) is a selective growth medium used in microbiology for the isolation and differentiation of enteric gram-negative pathogens, particularly Salmonella and Shigella species. It is commonly used in clinical laboratories to identify these pathogens in stool samples and other clinical specimens.

Composition

XLD agar contains several key components that make it selective and differential:

• Xylose: A sugar that is fermented by most enteric bacteria except for Shigella. This fermentation results in acid production, which lowers the pH and changes the color of the medium.
• Lysine: An amino acid that is decarboxylated by Salmonella, leading to an alkaline reaction that can reverse the acidification caused by xylose fermentation.
• Deoxycholate: A bile salt that inhibits the growth of gram-positive bacteria, making the medium selective for gram-negative organisms.
• Phenol red: A pH indicator that changes color in response to the pH changes in the medium.
• Sodium thiosulfate and ferric ammonium citrate: These compounds are used to detect hydrogen sulfide production, which is characteristic of some Salmonella species, resulting in black-centered colonies.

Mechanism of Action

XLD agar works by exploiting the metabolic differences between enteric bacteria. When bacteria are inoculated onto the medium:

• Shigella species do not ferment xylose, resulting in red colonies due to the alkaline pH.
• Salmonella species initially ferment xylose, producing acid and turning the colonies yellow. However, they also decarboxylate lysine, which reverses the pH change, turning the colonies back to red. Additionally, hydrogen sulfide production results in black-centered colonies.
• Other enteric bacteria that ferment xylose without decarboxylating lysine will produce yellow colonies.

Applications

XLD agar is primarily used in the clinical laboratory setting for the isolation of Salmonella and Shigella from stool samples. It is also used in food microbiology to test for these pathogens in food products. The medium's ability to differentiate between lactose fermenters and non-fermenters, as well as its ability to detect hydrogen sulfide production, makes it a valuable tool in the identification of enteric pathogens.

Limitations

While XLD agar is effective for isolating Salmonella and Shigella, it is not without limitations. Some non-pathogenic bacteria can mimic the appearance of pathogenic colonies, leading to false positives. Additionally, some strains of Salmonella may not produce hydrogen sulfide, resulting in colonies that do not have the characteristic black centers.

Related Pages

• Salmonella
• Shigella
• Microbiological culture
• Selective medium
• Differential medium