When performing an oxidase test, which statement is correct?

• A. Bacteria should be taken from media with a high glucose content
• B. Pseudomonas aeruginosa can be used as a negative control
• C. Plates incubated anaerobically should be exposed to room air for about 15 minutes before testing
• D. Formation of a purple color after 1 minute is considered positive

Answer: (C)

The test hinges on detecting cytochrome c oxidase by using a dye that turns purple when the enzyme is present. Fresh, properly grown colonies are tested quickly because oxidase activity can wane in aged cultures or under low-oxygen conditions. If a plate was incubated without oxygen (anaerobically), reintroducing oxygen allows the cells to express and demonstrate oxidase activity, so exposing the plate to room air for about 15 minutes before testing helps ensure a reliable result.

That’s why this statement is the best choice: it reflects the need to reoxygenate anaerobically grown plates so the oxidase reaction can be observed when the reagent is applied.

In practice, the other points don’t fit: the oxidase test isn’t about the glucose content of the medium; Pseudomonas aeruginosa is a classic oxidase-positive organism and is used as a positive control, not negative; and color development should occur promptly (often within seconds, not reliably at a full minute), so waiting a full minute is not the standard way to interpret the result.