Which media is used to differentiate between lactose-fermenting and non-lactose-fermenting Enterobacteriaceae?

• A. Nutrient agar
• B. Bile esculin agar
• C. MacConkey agar
• D. Sabouraud agar

Answer: (C)

MacConkey agar is specifically designed to differentiate between lactose-fermenting and non-lactose-fermenting bacteria, particularly within the Enterobacteriaceae family. The medium contains lactose and a pH indicator, usually neutral red, which reacts to the presence of acid produced by lactose fermentation. When lactose-fermenting organisms, such as E. coli, grow on this medium, they ferment lactose, leading to the production of acid, which causes a color change, typically resulting in pink colonies. In contrast, non-lactose fermenters, such as Salmonella and Shigella, do not utilize lactose and appear as colorless or pale colonies on the agar.

This selective and differential medium allows for clear visualization of the ability of an organism to ferment lactose, which is vital for identifying and distinguishing between various Enterobacteriaceae species in laboratory settings.