Isolate Gram-Negative Enteric Pathogens With XLD Agar

 

Introduction

For the identification of Enterobacteriaceae and another microbiological testing, Xylose Lysine Deoxycholate (XLD) Agar is used. The Xylose Lysine Deoxycholate Agar was developed by Taylor for the isolation and differentiation of enteric pathogens such as Salmonella typhi from other Salmonella species found in foods, water, and dairy products. Other plating media, such as SS Agar, EMBAgar, and Bismuth Sulfate Agar, have lower selectivity and sensitivity than Xylose Lysine Deoxycholate Agar. The media formulation prevents the overgrowth of organisms other than Salmonella and Shigella. Initially, samples suspected of containing enteric pathogens and other mixed flora are enriched in Modified Semisolid RV Medium Base. Xylose Lysine Deoxycholate Agar is a medium that is both selective and differential. Because it employs sodium deoxycholate as a selective agent, it is inhibitory to gram-positive microorganisms. Xylose Lysine Deoxycholate Agar was created specifically to allow the growth of Shigella species and is a tried and true medium for the isolation of this organism. It has also been discovered to be an excellent medium for the isolation of Salmonella species. The USP microbial limit test includes Xylose Lysine Deoxycholate Agar for screening specimens for the presence or absence of Salmonella and is recommended for testing foods, dairy products, and water. To differentiate the target organisms, the medium employs xylose fermentation, lysine decarboxylation, and H2S production reactions.

Composition

Ingredients

Gms / Ltr

Yeast extract

3.000

L-Lysine

5.000

Lactose

7.500

Xylose

3.500

Sodium chloride

5.000

Sodium deoxycholate

2.500

Sodium thiosulphate

6.800

Ferric ammonium citrate

0.800

Phenol red

0.080

Agar

15.000

 

Principle

The medium contains yeast extract, which provides the necessary nitrogen and vitamins for growth. Though the sugars xylose, lactose, and sucrose provide fermentable carbohydrates, xylose is primarily incorporated into the medium because it is not fermented by Shigellae but is fermented by nearly all enterics. This aids in the identification of Shigella species.



The osmotic balance of the medium is maintained by sodium chloride. Lysine is used to distinguish the Salmonella group from non-pathogens.

Salmonellae rapidly degrade xylose and deplete the supply. Following that, the enzyme lysine decarboxylase decarboxylates lysine to form amines with a reversion to an alkaline pH that mimics the Shigella reaction. To prevent this reaction by lysine-positive coliforms, lactose and sucrose are added in excess to produce acid. The color of the phenol red indicator changes to yellow as xylose, lactose, and sucrose degrade to acid.

How to Use?

  • In 1000 ml purified or distilled water, dissolve 56.68 grams of TM Media XLD Agar.
  • Heat with frequent agitation until the medium boils. Do not heat it in an autoclave.
  • Transfer immediately to a water bath at 50°C.
  • After cooling, pour into sterile Petri plates.
  • In Petri dishes, a red-colored, clear to slightly opalescent gel forms.

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