Final answer:
In MacConkey agar, lactose fermenting organisms like E. coli produce pink colonies, while non-fermenters like Serratia marcescens form cream-colored streaks on the medium.
Step-by-step explanation:
When lactose is present, the medium turns a specific color depending on the type of organism and the medium used. In MacConkey agar, which is a selective and differential medium commonly used to differentiate between lactose fermenters and non-fermenters, lactose-fermenting organisms like E. coli produce pink colonies, while organisms that do not ferment lactose, like Serratia marcescens, form cream-colored streaks on the tan medium.
The color change is due to the presence of a pH indicator in the medium. MacConkey agar contains the pH indicator neutral red, which turns to a hot pink color at low pH. When lactose is fermented, it produces acid, causing the pH of the medium to decrease and the indicator to change color. The color change is a visual indication of the organism's ability to ferment lactose.