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

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

Answer: (C)

The color change with the redox reagent reflects the activity of cytochrome oxidase, the enzyme that transfers electrons to oxygen in the bacterial electron transport chain. In the oxidase test, the reagent is reduced by cytochrome c oxidase and then oxidized to form a colored compound, so oxidase-positive bacteria like Neisseria produce a rapid (within seconds) purple color. This distinguishes them from oxidase-negative groups, which do not cause the reagent to change color. The other enzymes listed — beta-galactosidase, urease, and phenylalanine deaminase — are identified by separate tests and do not interact with the oxidase reagent, so they would not produce the observed color change.