MacConkey agar contains bile salts and crystal violet to inhibit most gram + organisms.Lactose is a carbohydrate that may be used as a nutrient. The utilization of lactose is important in identifying gram negative rods. Lactose + means that the organism can use lactose as an energy source whilst lactose – means they cannot. Differential media is used to distinguish organisms from each other based on a reaction that occurs as they grow. Lactose + organisms ferment the lactose and produce acid whilst lactose – organisms do not produce any acid as they grow. MacConkey agar contains a pH indicator called neutral red that detects the acid production in lactose + organisms. As lactose + bacteria produce acid, it causes the pH to drop and where neutral red is absorbed and the colony turns red.
Lactose – bacteria remains colorless and translucent as they do not produce any acid.
“Identification of Lactose Positive and Lactose Negative Bacteria using MacConkey Agar,” Wisc-online, accessed on April 10, 2013, http://www.wisc-online.com/objects/ViewObject.aspx?ID=MBY701.