How does MacConkey agar differentiate lactose fermenters from nonfermenters?

How does MacConkey agar differentiate lactose fermenters from nonfermenters?

• A. Lactose fermenters produce acid, turning the medium pink/red; nonfermenters remain colorless or pale.
• B. Lactose fermenters produce blue colonies; nonfermenters red colonies.
• C. Both produce purple colonies; not differentiating.
• D. Lactose fermentation yields larger colonies.

Answer: (A)

MacConkey agar uses a pH indicator to reveal lactose fermentation. Lactose fermenters produce acid as they metabolize lactose, which lowers the pH and causes the indicator to turn pink/red, so their colonies appear pink or red. Nonfermenters do not acidify the medium from lactose, so the colonies stay colorless or pale and the background remains the baseline color. This combination of delayed or no acid production versus acid production from lactose is what lets you distinguish the two groups on this medium.

MacConkey agar uses a pH indicator to reveal lactose fermentation. Lactose fermenters produce acid as they metabolize lactose, which lowers the pH and causes the indicator to turn pink/red, so their colonies appear pink or red. Nonfermenters do not acidify the medium from lactose, so the colonies stay colorless or pale and the background remains the baseline color. This combination of delayed or no acid production versus acid production from lactose is what lets you distinguish the two groups on this medium.