Xylose Lysine Deoxycholate (XLD) Agar

Xylose Lysine Deoxycholate Agar is used for the isolation and differentiation of enteric pathogens. Conforms to Harmonized USP/EP/JP Requirements. XLD Agar is not intended for use in the diagnosis of disease or other conditions in humans. A medium recommended by the Harmonized Pharmacopeia for isolation and identification of Salmonella from non-sterile products. Conforms to USP/EP/JP performance specification. Originally formulated by Taylor to differentiate enteric pathogens, the agar is widely used as the preferred differential medium for Salmonella spp. The medium is void of peptones but instead uses yeast extract as a carbon, nitrogen and vitamin source and xylose, lactose and sucrose are fermentable carbohydrates. Salmonella are able to ferment xylose to produce acid but not lactose or sucrose. When the xylose is exhausted Salmonella will decarboxylate lysine shifting the pH back to neutral. At near neutral pH, Salmonella can reduce sodium thiosulfate producing hydrogen sulfide which creates a complex with ferric ammonium citrate to produce black or black centered colonies. Other organisms are able to decarboxylate lysine but acid production from the fermentation of lactose and sucrose keeps the pH too acidic for H2S production. Selectivity is achieved through the incorporation of sodium deoxycholate and phenol red acts as a pH indicator. According to the Harmonized Pharmacopeia, Rappaport Vassiliadis Salmonella Enrichment Broth (NCM0103) is used as a selective enrichment broth, with subculture performed onto Xylose Lysine Deoxycholate (XLD) agar.