MacConkey Agar

This medium is both selective and differential. The selective ingredients are the bile salts and the dye, crystal violet which inhibit the growth of Gram-positive bacteria. The differential ingredient is lactose. Fermentation of this sugar results in an acidic pH and causes the pH indicator, neutral red, to turn a bright pinky-red color. Thus organisms capable of lactose fermentation such as Escherichia coli, form bright pinky-red colonies (plate pictured on the left here). MacConkey agar is commonly used to differentiate between the Enterobacteriaceae.

macconkey fermentation macconkey nonfermenter
Growth on MacConkey Agar with lactose fermentation Growth on MacConkey Agar without lactose fermentation