MacConkey agar is a selective and differential medium used to differentiate enteric bacteria that ferment lactose. Bile salts prevent gram positive bacteria from growing while a pH indicator called neutral red differentiates lactose fermentation (fermentation produces acid, drops the pH and turns the indicator pink. the dye is then taken up by the bacteria so that the colonies themsleves appear pink on the plate). The plate above was inoculated with Staphylococcus epidermidis (top), E. coli (left), and a mixture of the two (bottom-right). Because S. epidermidis is gram positive, it did not grow while the E. coli, a gram negative bacterium, grew on the plate and also exhibited lactose fermentation, as can be seen with the bright pink growth. Strong lactose fermenters like E. coli will also produce a pink halo around the growth. Notice the mixture also indicates a gram negative, lactose fermenting bacterium, even though S. epidermidis was also present in the inoculum - just a reminder that these tests are useless if we do not use pure cultures!