An oxidase is an enzyme which is responsible for the reduction of oxygen. Cytochrome oxidase is the terminal enzyme that produces adenosine triphosphate in the electron transport system by forming water through the transport of protons to oxygen. Thus many bacteria which live in an oxygenated atmosphere are producers of cytochrome oxidase. These bacteria are able to gain energy from the final stage of cellular metabolism, the ETS chain. NADH and FADH₂ ,which are produced in the krebs cycle and in glycolysis are oxidized, and the energy released is used to form ATPs. Once an unknown bacterium is isolated and diluted in liquified agar medium, cytochrome oxidase production can be ascertained. Using aseptic technique, a needle innoculated with bacteria is touched against a small circle of absorbance paper that has been treated with a drop of oxidase reagent. Oxidase reagent contains tetramethyl-p-phenylenediamine, a colorless compound. In the presence of cytochrome oxidase, the tetramethyl-p-phenylenediamine turns purple. Therefore, the spot where the needle contacted the absorbance paper will turn purple within 15 seconds after contact if the bacteria is positive for cytochrome oxidase production. If the bacteria is negative for oxidase production, there will be no change in color.