What color do lactose-fermenting bacteria appear on MacConkey agar?

What color do lactose-fermenting bacteria appear on MacConkey agar?

• A. Yellow
• B. Pale white
• C. Pink
• D. Red

Answer: (C)

Lactose-fermenting bacteria produce acid as they metabolize lactose, which is included in the composition of MacConkey agar. This acid lowers the pH of the medium, causing a color change in the pH indicator present in the agar. Specifically, the neutral red dye in the agar turns pink at a lower pH level, which occurs when lactose-fermenting bacteria proliferate. Hence, colonies of these bacteria appear pink on MacConkey agar, indicating the fermentation of lactose and the subsequent acid production. This characteristic is a key differentiator in microbiological diagnostics, particularly for identifying members of the Enterobacteriaceae family that are lactose fermenters, such as Escherichia coli and Enterobacter spp.

Lactose-fermenting bacteria produce acid as they metabolize lactose, which is included in the composition of MacConkey agar. This acid lowers the pH of the medium, causing a color change in the pH indicator present in the agar. Specifically, the neutral red dye in the agar turns pink at a lower pH level, which occurs when lactose-fermenting bacteria proliferate. Hence, colonies of these bacteria appear pink on MacConkey agar, indicating the fermentation of lactose and the subsequent acid production. This characteristic is a key differentiator in microbiological diagnostics, particularly for identifying members of the Enterobacteriaceae family that are lactose fermenters, such as Escherichia coli and Enterobacter spp.