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XLD Agar
XLD (Xylose Lysine Desoxycholate) Agar is used for the isolation of Salmonella in pharmaceutical products. The typical composition corresponds to that defined in the American and European Pharmacopeia.
The agar can also be used as a second media of choice in the normalized methods for the detection of Salmonella in food products and water.
A second formulation of XLD agar exists and corresponds to the composition in the standards, in food microbiology and in water microbiology under the references BK168 & BM087.
Available as dehydrated medium : 500 g bottle.
In compliance with regulatory requirements, a USDA permit (VS 16-3) is mandatory for shipments from Canada to the USA due to the inclusion of peptone, an animal byproduct.
Sodium desoxycholate inhibits contaminating Gram-positive flora. Xylose is fermented by practically all enteropathogenic bacteria, except for Shigella which are thus differentiated from the other species. After exhausting xylose, Salmonella decarboxylate lysine (via lysine decarboxylase) to cadaverine, causing the pH to rise. Colonies of salmonellae resemble those of shigellae in the medium having become basic.
The colonies formed are red in the presence of the indicator, phenol red.
The addition of lactose and sucrose to the medium enable coliform bacteria to decarboxylate lysine and thereby produce excess acidity, making the indicator turn yellow, favoring their differentiation.
In basic medium, pathogenic H 2 S-producers reduce ferric ammonium citrate and cause a blackening due to the production of iron sulfide at the center of the colonies. Non-pathogenic bacteria which do not decarboxylate lysine acidify the medium, a result of sugar fermentation. The pH decrease prevents the colonies from blackening.
