The combination of peptone, tryptone and yeast extract makes the medium highly nutritious and provides amino acids and long peptide chains. Sodium chloride maintains the osmotic balance. Lactose and sucrose are the sources of fermentable carbohydrates. Phenol red serves as an acid-base indicator giving a yellow color to the bacteria fermenting lactose and/or sucrose. Non-lactose-fermenting bacteria develop white to pinkish red colonies within 18-24 hours after incubation. This medium also contains bright green, which inhibits the growth of most Gram-negative and Gram-positive bacteria, but not salmonella. Salmonella are present as red to pinkish white colonies surrounded by a red zone in this medium.