What color change indicates that the oxidase reagent is oxidized?

The correct answer reflects a specific chemical reaction involving the oxidase reagent, which is crucial in identifying the presence of certain bacteria, particularly those that produce cytochrome c oxidase. When the oxidase reagent is applied to a bacterial culture containing this enzyme, a color change occurs, indicating a positive reaction. Specifically, the reagent transforms from colorless to dark-blue or maroon, demonstrating that oxidation has occurred.

This reaction is significant because it differentiates oxidase-positive bacteria—like Pseudomonas aeruginosa and Neisseria species—from oxidase-negative ones, which do not exhibit this enzyme's activity. The development of this dark-blue or maroon color is a clear visual confirmation that the enzyme is actively catalyzing the oxidation process, thus making this observation vital in microbiological diagnostics.