Which statement about Acinetobacter baumannii is true?

• A. Requires cysteine
• B. Is oxidase negative
• C. Ferments glucose
• D. Does not grow on MacConkey agar

Answer: (B)

The test hinges on the oxidase reaction. Acinetobacter baumannii lacks cytochrome c oxidase, so it is oxidase negative. In the lab, the oxidase test uses a reagent that turns purple if oxidase is present; since this organism doesn’t have the enzyme, there’s no color change. That makes oxidase negativity the best descriptor for distinguishing it in this context.

Regarding the other statements: Acinetobacter baumannii does grow on MacConkey agar, but as a non–lactose fermenter, its colonies are colorless or pale rather than pink. It also does not ferment glucose, fitting the non-fermenter profile. It does not have a special cysteine requirement that would make that statement true.