XL (Xylose Lysine) Agar Base is used for the isolation and differentiation of Gram-negative enteric bacteria. In this medium, Yeast Extract provides the nitrogenous sources and carbohydrates required for bacterial growth. Sodium Chloride supplies electrolytes and maintains the osmotic equilibrium. Xylose, Lactose and Sucrose are the fermentable carbohydrates, being the former metabolized by most enteric bacteria except for Shigella and Providencia spp. Upon Xylose depletion, Salmonella (but not non-pathogen xylose fermenters) decarboxylates L-Lysine and reverts the pH acidification due to carbohydrate fermentation. Phenol Red serves as pH indicator. Agar is the gelifying agent.