MacConkey Agar is a selective and differential culture medium for bacteria. It is designed to selectively isolate Gram-negative and enteric bacilli (normally found in the intestinal tract) and differentiate them by lactose fermentation. Lactose-fermenting microorganisms turn red or pink on MacConkey agar, and non-fermenting ones do not change color. The medium inhibits the growth of Gram-positive microorganisms with crystalline violet and bile salts, allowing the selection and isolation of Gram-negative bacteria. The pancreatic degradation of gelatin and peptones (meat and casein) provides the essential nutrients, vitamins and nitrogen factors necessary for the growth of microorganisms. Lactose monohydrate is the fermentable source of carbohydrates. The selective action of this medium is attributed to crystal violet and bile salts, which are inhibitors for most species of gram-positive bacteria. Sodium chloride maintains the osmotic balance in the medium. Lactose fermentation by enteric bacteria in the medium is detected by a neutral red pH indicator.