MacConkey Sorbitol Agar is recommended for the isolation and detection of pathogenicEscherichiacolistrains. Pancreatic Digest of Casein, Peptic Digest of Animal Tissue, and Enzymatic Digest of Gelatin provide nitrogenous sources. D-Sorbitol is the fermentable carbohydrate. Crystal Violet and Bile Salts inhibit Gram-positive organisms and allow Gram-negative organisms to grow. Due to the presence of the Neutral Red pH indicator, coliforms that ferment Lactose turn dark red to purple color while non-fermenters are colorless. Sodium Chloride maintains the osmotic balance and Agar is the gelifying agent.