The oxidase test identifies organisms that produce the enzyme cytochrome oxidase. Cytochrome oxidase participates in the electron transport chain by transferring electrons from a donor molecule to oxygen.
The oxidase reagent contains a chromogenic reducing agent, which is a compound that changes color when it becomes oxidized. If the test organism produces cytochrome oxidase, the oxidase reagent will turn blue or purple within 15 seconds.
You can find the test protocol here.
We use sterile cotton swabs as a substrate for the oxidase test; we do not perform the test as described in your lab manual. Refer to the link above for the test protocol.
Once the vial has been opened, oxidase reagent will turn purple over time due to oxygen in the air. To avoid false positives, be sure to: 1) use oxidase reagent that is still clear and 2) to read your test quickly once you have performed it.