Final answer:
Red or pink colonies on MacConkey agar indicate lactose fermentation by bacteria such as Escherichia coli, while non-fermenters like Serratia marcescens produce white or colorless colonies.
Step-by-step explanation:
The appearance of red or pink colonies on MacConkey agar is attributed to the fermentation of lactose. This bacteriological culture medium contains lactose as a fermentable sugar and a pH indicator that changes color in response to acid production from lactose fermentation. When lactose-fermenting bacteria, such as Escherichia coli (E. coli), grow on MacConkey agar, they produce acid as a byproduct, causing the pH indicator to change color and resulting in the characteristic pink or red appearance of the colonies. Non-lactose fermenting bacteria, such as Serratia marcescens or Salmonella spp., do not produce this color change, leading to colorless or white colonies instead. Thus, the pigmentation observed is a convenient way for microbiologists to distinguish between different types of Gram-negative bacteria on this selective and differential medium.