MacConkey Agar is a solid medium recommended for the cultivation of Gram-negative, lactose-fermenting enteric bacteria in samples of water, milk, and foods. MacConkey Agar is used for the isolation and differentiation of the coliform organisms and enteric pathogens based upon the fermentation of Lactose.

Principles of Alpha Biosciences Medium: This formulation is an improvement on the original. Modifications were made to improve the growth of *Salmonella* and *Shigella* strains. MacConkey Agar contains Crystal Violet and Bile Salts that 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.

Principles of BBL Medium: Peptones are sources of nitrogen and other nutrients. Lactose is a fermentable carbohydrate. When lactose is fermented, a local pH drop around the colony causes a color change in the pH indicator (neutral red) and bile precipitation. Bile salts, bile salts no. 3, oxgall and crystal violet are selective agents that inhibit growth of gram-positive organisms. Agar is the solidifying agent.