Xylose Lysine Deoxycholate agar utilizes yeast extract as a nutrient and vitamin source. Sodium deoxycholate acts as the selective agent that inhibits Gram-positive organisms. Xylose is used to differentiate Shigella species, as Shigella does not ferment xylose. Salmonella may be differentiated from non-pathogens with the use of lysine; without lysine, Salmonella will ferment the xylose and become indistinguishable from non-pathogens. After using xylose, Salmonella utilize lysine through the use of the enzyme lysine decarboxylase; this action reverts to an alkaline pH, similar to the Shigella reaction. Lactose and sucrose are provided to produce excess acid to prevent reversion by lysinepositive coliforms. Sodium thiosulfate and ferric ammonium citrate are included for the visualization of H2S, which causes the formation of the black centers in Salmonella spp.