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. This formulation is an improvement on the original. Modifications were made to improve the growth ofSalmonellaandShigellastrains. 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.