Which colony appearance indicates lactose fermentation on MacConkey medium?

MacConkey agar is a selective and differential culture medium designed to isolate gram-negative bacteria and differentiate lactose fermenters from non-fermenters. When lactose is fermented by bacteria on MacConkey agar, it produces acid, which changes the pH of the medium. This pH change results in a color change in the pH indicator (neutral red) contained in the medium.

Lactose fermenters typically produce colonies that appear brick red due to this acid production, which also leads to the precipitation of bile salts and causes the lactose-containing medium to develop a reddish hue around the colonies. This distinct coloration is a key feature of lactose fermenters, such as Escherichia coli, while non-fermenters will remain colorless since they do not produce acid and thus do not cause a change in the pH of the medium.

Other colony appearances, such as colorless, black, or pale yellow colonies, do not indicate lactose fermentation. Colorless colonies suggest that the bacteria do not ferment lactose. Black colonies are indicative of hydrogen sulfide production, typically seen in organisms like Salmonella. Pale yellow colonies can occur with some non-lactose fermenting bacteria but do not reflect the fermentation of lactose. Thus, brick red colonies are