MacConkey Agar Base is recommended for studying fermentation reactions of coliforms by adding carbohydrates either individually or in combination. It is used for studying carbohydrate fermentation reactions of coliforms by adding carbohydrates either individually or in combination. The selective action of this medium is attributed to bile salts and crystal violet, which are inhibitory to most of the species of gram-positive bacteria. Gram-negative bacteria usually grow well on the medium and are differentiated by their ability to ferment carbohydrates.