XLD Agar, for Xylose-Lysine-Deoxycholate, is a selective culture medium suitable for the isolation of Salmonella and Shigella from food products or feces and other clinical samples. XLD Agar is both a selective and differential medium. It contains a yeast extract as a source of nutrients and vitamins. It uses sodium deoxycholate as a selective agent and is therefore inhibitory to Gram-positive microorganisms. Xylose is incorporated into the medium since it is fermented by virtually all enterococci except Shigella and this property allows differentiation of Shigella species. Lysine is included to allow the differentiation of the Salmonella group from non-pathogenic species because without lysine, Salmonella would rapidly ferment xylose and could not be distinguished from non-pathogenic species. Once Salmonella have exhausted their xylose supply, lysine is attacked by the enzyme lysine decarboxylase, returning to an alkaline pH that mimics the Shigella reaction.