Triple Sugar Iron Agar is used as a differential medium for the presumptive identification of enteric bacteria. The medium contains no inhibitors and most organisms, including Gram-positive organisms and yeasts, can grow on it. Phenol Red indicates fermentation of the carbohydrates Dextrose, Sucrose and Lactose when it shifts to yellow due to acidification of the medium. Ferric Ammonium Citrate indicates hydrogen disulfide production through the reduction of Sodium Thiosulfate. Upon reaction with this gas, it causes a blackening in the middle and upper portions of the slant.