Final answer:
On EMB agar, lactose fermenters form metallic green or deep purple colonies; while on MacConkey agar, they produce bright pink colonies. They possess the enzyme β-galactosidase, which breaks down lactose into glucose and galactose.
Step-by-step explanation:
Lactose-fermenting bacteria appear as colonies of distinct colors on both EMB (eosin methylene blue) agar and MacConkey agar plates. On EMB agar, lactose fermenters produce metallic green or deep purple colonies, while nonlactose fermenters form colorless colonies. Similarly, on MacConkey agar, colonies of lactose fermenters, such as E. coli, become bright pink due to the pH change in the medium, whereas non-lactose fermenters like Serratia marcescens appear as cream-colored or colorless streaks.
The enzyme that lactose fermenters possess is called β-galactosidase. This enzyme is responsible for the hydrolysis of lactose into glucose and galactose, which can then be metabolized by the bacteria. The gene encoding β-galactosidase, lacZ, is part of the lactose operon in bacteria like E. coli, allowing for the coordinated regulation of lactose metabolism when lactose is present.