How does Acinetobacter baumannii appear on MAC agar?

Acinetobacter baumannii appears as colorless colonies on MacConkey agar (MAC agar) because it is a non-lactose fermenter. MAC agar is used primarily to isolate Gram-negative bacteria and differentiate those that can ferment lactose from those that cannot. Lactose fermenters will produce acid, leading to a pH change that results in pink or red colonies due to the neutral red indicator in the medium. Since Acinetobacter baumannii does not ferment lactose, it does not produce this acid and thus does not alter the color of the agar, resulting in colorless (or pale) colonies. This characteristic helps in the identification of Acinetobacter baumannii in a laboratory setting.