Xylose lysine deoxycholate (XLD) agar: Composition Preparation, Results and Uses

Xylose lysine deoxycholate (XLD)  Agar was formulated by Taylor for the isolation and differentiation of enteric pathogens including Salmonella Typhi from other Salmonella species.

XLD Agar is both selective and differential medium for the isolation, cultivation and differentiation of gram-negative enteric microorganisms This media is primarily used for isolation and differentiation of Salmonella and Shigella from both clinical and non-clinical specimens.

Composition of XLD: Ingredients in Per litre formulations

Principle:

XLD agar contains sugars like Xylose, Lactose and Sucrose provide sources of fermentable carbohydrate.

Yeast Extract provides sources of nitrogen, carbon, and vitamins required for organism growth.

The indicator Phenol red imparts red colour to the prepared media which changes to yellow after sugar fermentation thus differentiating lactose fermenters from non-lactose fermenters .Most gut bacteria, including Salmonella, can ferment xylose  to produce acid; Shigella colonies cannot do this and therefore remain red.

After exhausting the xylose supply Salmonella colonies decarboxylate lysine, increasing the pH once again to alkaline and mimicking the red Shigella colonies. However, Salmonellae also metabolise thiosulfate to produce hydrogen sulfide, which leads to the formation of colonies with black centers and allows them to be differentiated from the similarly coloured Shigella colonies.Sodium Deoxycholate inhibits the growth of Gram positive organisms.Sodium Chloride maintains the osmotic balance in the medium. Agar is the solidifying agent.

Directions for preparation of media

1. Suspend 55 g of the medium in one liter of purified water.
2. Heat with frequent agitation until the medium reaches the boiling point.
3. AVOID OVERHEATING. DO NOT AUTOCLAVE.
4. Transfer immediately to a water bath at 50°C.
5. After cooling, pour into sterile Petri plates.

Note: It is advisable not to prepare large volumes that will require prolonged heating, thereby producing precipitate.

Quality Control Specifications

1. Dehydrated Appearance: Powder is homogeneous, free flowing, and light pink-beige.
2. Prepared Appearance: Prepared medium is bright red to reddish-orange, trace to slightly hazy; pH 7.20-7.60

Colony characteristics in XLD Agar:

1. Enterobacter aerogenes ATCC 13048: Yellow
2. Escherichia coli ATCC 25922: Yellow
3. Proteus vulgaris ATCC 13315: grey with black centers
4. Salmonella Paratyphi A ATCC 9150: Red
5. Salmonella Paratyphi B ATCC 8759, Salmonella Enteritidis ATCC 13076 & Salmonella Typhi ATCC 6539 : Red with Black centers
6. Salmonella Typhimurium ATCC 14028:  red  colonies with black centres (strong H2s producer)
7. Shigella dysenteriae ATCC 13313, Shigella flexneri ATCC 12022 & Shigella sonnei ATCC 25931: Red
8. Staphylococcus aureus ATCC 6538: No growth

Limitations of the Procedure:

1. Some strains may be encountered that grow poorly or fail to grow on this medium.
2. Red, false-positive colonies may occur with Proteus and Pseudomonas.
3. Incubation in excess of 48 hours may lead to false-positive results.

Uses:
XLD Agar has been recommended for the

1. identification of Enterobacteriaceae
2. microbiological testing of foods, water and dairy products