<P> MacConkey agar is an indicator, a selective and differential culture medium for bacteria designed to selectively isolate Gram - negative and enteric (normally found in the intestinal tract) bacilli and differentiate them based on lactose fermentation . The crystal violet and bile salts inhibit the growth of gram - positive organisms which allows for the selection and isolation of gram - negative bacteria . Enteric bacteria that have the ability to ferment lactose can be detected using the carbohydrate lactose, and the pH indicator neutral red . </P> <P> It contains bile salts (to inhibit most Gram - positive bacteria), crystal violet dye (which also inhibits certain Gram - positive bacteria), neutral red dye (which turns pink if the microbes are fermenting lactose). </P>

What types of organisms will grow on macconkey agar