Final answer:
Lactose-fermenting bacteria appear as bright pink colonies on MacConkey agar due to the presence of lactose and pH indicators.
Step-by-step explanation:
On MacConkey agar, lactose-fermenting bacteria appear as bright pink colonies. This is because MacConkey agar contains lactose and pH indicators that turn pink in the presence of acid produced by lactose fermenters. Non-fermenters, on the other hand, will not produce acid and will not cause a change in color on the agar. This characteristic color change helps in the identification of lactose-fermenting bacteria.