MacCONKEY AGAR BASE is used to study carbohydrate fermentation reactions of coliforms by adding single or multiple carbohydrates. The basal medium consists of Gelatin Peptone, Proteose peptone, which provide nitrogen, carbon and vitamin source for the growth of bacteria. This medium does not contain carbohydrates. So for studying fermentation reaction, carbohydrate of interest has to be added while preparing medium. The selective action of this medium is attributed to bile salts and crystal violet, which is inhibitory to most species of gram-positive bacteria. The lactose fermenting enterobacteria lower the pH of the medium which is detected by the neutral red indicator, producing red or rose colonies. Gram-negative bacteria usually grow well on the medium and are differentiated by their ability to ferment carbohydrates. Lactose fermenting strains grow as red or pink colonies. Sodium chloride helps to maintain the osmotic balance.