Colonies of Neisseria species turn color when a redox reagent is applied. The color change is indicative of the activity of which bacterial enzyme?

• A. Beta-galactosidase
• B. Urease
• C. Cytochrome oxidase
• D. Phenylalanine deaminase

Answer: (C)

The key idea is the oxidase test, which detects the presence of cytochrome c oxidase in bacteria. When a redox reagent is applied to a colony, organisms that have cytochrome c oxidase transfer electrons from the reagent to oxygen. This rapid electron transfer reduces the dye, causing a visible color change—usually a deep purple—within seconds. Neisseria species possess this enzyme, so their colonies turn color with the oxidase reagent, indicating a positive test.

The other enzymes mentioned are detected by different methods that don’t rely on this redox dye. Beta-galactosidase activity is shown by cleavage of a substrate to release color, not by oxidative color change. Urease activity is detected by a pH change in urea broth, usually with a color indicator shifting color as ammonia is produced. Phenylalanine deaminase is detected by a reaction that yields a colored product with ferric chloride. These distinct tests don’t involve the redox reaction used in the oxidase assay.