Enzymatic Digest of Animal Tissue provides nitrogen, carbon, and amino acids required for organism growth. Yeast Extract is a vitamin source. Bile Salts Mixture and Acid Fuchsin inhibit Gram-positive organisms. Lactose, Sucrose, and Salicin are fermentable carbohydrates. Sodium Chloride maintains the osmotic balance of the medium. Ferric Ammonium Citrate, a source of iron, allows production of hydrogen sulfide (H2S) present from Sodium Thiosulfate. H2S-positive colonies have black centers. Bromothymol Blue is added as the pH indicator. Agar is the solidifying agent.