MacConkey agar is used for the isolation of gram-negative enteric bacteria and the differentiation of lactose fermenting from lactose non-fermenting gram-negative bacteria. It has also become common to use the media to differentiate bacteria by their abilities to ferment sugars other than lactose. In these cases, lactose is replaced in the medium by another sugar. These modified media are used to differentiate gram-negative bacteria or to distinguish between phenotypes with mutations that confer varying abilities to ferment particular sugars.
