Yeast Extract provides sources of nitrogen, carbon, and vitamins required for organism growth. Xylose, Lactose, and Sucrose, provide sources of fermentable carbohydrate. Xylose is fermented by most enteric organisms except Shigella spp. and Providencia spp. Lysine is added to differentiate Salmonella. As Xylose is exhausted, Salmonella spp organisms decarboxylate lysine causing a reversion to alkaline conditions. Alkaline reversion by other lysine-positive organisms is prevented by excess acid production from fermentation of lactose and sucrose.