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 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. Coliforms that ferment lactose turn dark red to purple color and non-fermenters are colorless.