MacConkey Agar (MAC) is a selective and differential medium designed to isolate and differentiate enterics based on their ability to ferment lactose. Bile salts and crystal violet inhibit the growth of Gram positive organisms. Lactose provides a source of fermentable carbohydrate, allowing for differentiation. Neutral red is a pH indicator that turns red at a pH below 6.8 and is colorless at any pH greater than 6.8.
Organisms that ferment lactose and thereby produce an acidic environment will appear pink because of the neutral red turning red. Bile salts may also precipitate out of the media surrounding the growth of fermenters because of the change in pH. Non-fermenters will produce normally-colored or colorless colonies.
Klebsiella pneumoniae ferments lactose and produces pink colonies on MAC. Micrococcus luteus does not grow in the presence of bile salts and crystal violet.