MacConkey agar (MAC) is both selective and differential. MAC is selective for enteric bacteria. It contains ingredients like bile salts that inhibit most other organisms. In particular, very few Gram-positive organisms grow on MacConkey agar.

The differential property of MAC results from the inclusion of lactose and a pH indicator in the medium. Bacteria that can grow on MAC and ferment lactose produce acid, which changes the bacterial colonies and sometimes the surrounding agar medium a pink color. Bacteria that produce pink colonies on MAC (lactose fermentors) are called coliforms. The coliforms that we will see in lab are Escherichia coli, Klebsiella pneumoniae, and Enterobacter aerogenes. Bacteria that can grow on MAC but do not ferment lactose produce tan colonies. The rest of the enteric bacteria we will see in lab fall into this group.

Remember that growth on MAC and lactose fermentation are two separate things to observe. Shown here are two enteric organisms that grow on MacConkey agar: Escherichia coli, (on the right) which ferments lactose, and Citrobactor freundii, on the left, which does not.