If a culture yields no color change within the oxidase test time, what is the result?

If a culture yields no color change within the oxidase test time, what is the result?

• A. Oxidase-negative.
• B. Oxidase-positive.
• C. Weakly positive.
• D. Not determinable.

Answer: (A)

The oxidase test determines whether an organism has cytochrome c oxidase, an enzyme in the electron transport chain. If the enzyme is present, the reagent (a colorless dye) is oxidized and changes to a purple/blue color within the prescribed incubation period. If no color change occurs during that time, the organism lacks cytochrome c oxidase activity, so the result is oxidase-negative. A faint color change would indicate a weakly positive result, and a test that can’t be interpreted due to improper procedure would be not determinable.

The oxidase test determines whether an organism has cytochrome c oxidase, an enzyme in the electron transport chain. If the enzyme is present, the reagent (a colorless dye) is oxidized and changes to a purple/blue color within the prescribed incubation period. If no color change occurs during that time, the organism lacks cytochrome c oxidase activity, so the result is oxidase-negative. A faint color change would indicate a weakly positive result, and a test that can’t be interpreted due to improper procedure would be not determinable.