What is the purpose of neutral red in McConkey agar?

Neutral red in MacConkey agar serves as a pH indicator that helps differentiate between lactose fermenters and non-fermenters. When lactose is fermented by bacteria, it produces acidic byproducts that lower the pH, causing the neutral red to change color, typically resulting in pink colonies. This allows for easy identification of lactose-positive bacteria, such as E. coli, while non-fermenters, like Salmonella and Shigella, remain colorless or pale. Thus, neutral red aids in the selective isolation and differentiation of gram-negative bacteria based on their lactose fermentation capability.