What does the oxidase test detect and what is a typical positive color change?

What does the oxidase test detect and what is a typical positive color change?

• A. Presence of cytochrome c oxidase; turns purple/blue within seconds
• B. Presence of catalase; bubbles
• C. Urease activity; pink
• D. Nitrate reduction; red

Answer: (A)

The oxidase test detects the presence of cytochrome c oxidase, an enzyme in the bacterial electron transport chain that transfers electrons to oxygen. If cytochrome c oxidase is present, the redox dye used in the test is reduced and changes color to purple or blue within seconds. This quick color change indicates oxidase-positive organisms, such as Pseudomonas, Neisseria, and Vibrio, while oxidase-negative bacteria (like many Enterobacteriaceae) do not produce the color change. Other tests described detect different enzymes or reactions—catalase produces bubbles with hydrogen peroxide, urease turns pink due to pH change, and nitrate reduction yields a red color after reagents are added—so they do not reflect the oxidase reaction.

The oxidase test detects the presence of cytochrome c oxidase, an enzyme in the bacterial electron transport chain that transfers electrons to oxygen. If cytochrome c oxidase is present, the redox dye used in the test is reduced and changes color to purple or blue within seconds. This quick color change indicates oxidase-positive organisms, such as Pseudomonas, Neisseria, and Vibrio, while oxidase-negative bacteria (like many Enterobacteriaceae) do not produce the color change. Other tests described detect different enzymes or reactions—catalase produces bubbles with hydrogen peroxide, urease turns pink due to pH change, and nitrate reduction yields a red color after reagents are added—so they do not reflect the oxidase reaction.