The identification of some bacteria is aided by detecting their ability to produce the enzyme cytochrome c oxidase (more commonly referred to simply as "oxidase"). Pseudomonas aeruginosa , Neisseria gonorrhoeae and Campylobacter jejuni are oxidase-positive pathogens frequently encountered. During cellular respiration, electrons are transferred through a series of oxidation-reduction reactions to a terminal electron acceptor such as oxygen. The terminal link in the electron transport chain is cytochrome oxidase, an enzyme that mediates the transfer of electrons from the reduced cytochrome c to molecular oxygen.

Oxidase positive organisms are detected by the use of oxidase reagent (N,N,N',N'-tetramethyl-p-phenylenediamine dihydrochloride). Oxidase reagent is colorless in its reduced state and dark purple in its oxidized state. Applying oxidase reagent directly to an oxidase-positive colony will cause it to change color. Commercial products such as swabs or plastic slides are also available that contain oxidase reagent and will turn purple when a heavy inoculum of an oxidase-positive organism adheres to the product. The images below show a negative (no color change) and positive (dark purple) oxidase reaction using Difco's DrySlide oxidase test. The color change occurs within 20 seconds if the test bacteria is oxidase positive.

oxidase test