Why E coli is red colour at McCONKEY?

E. coli appears red on MacConkey agar due to its ability to ferment lactose. The agar contains lactose and a pH indicator, neutral red. When E. coli ferments lactose, it produces acid, lowering the pH and causing the neutral red indicator to change color, resulting in the characteristic red colonies. Non-lactose fermenters remain colorless or take on a pale hue, allowing for differentiation.