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This laboratory test is based on detecting the production of the enzyme cytochrome oxidase by Gram-negative bacteria. It is a hallmark test for the Neiserria. It is also used to discriminate between aerobic Gram-negative organisms like Pseudomonas aeruginosa and other Enterobacteriaciae Cytochrome oxidase, part of an intracellular system that catalyzes the oxidation of cytochrome c by molecular oxygen, the terminal electron acceptor in the electron transport chain.

Pick a colony for testing using a sterile wooden toothpick. Transfer the colony to the surface of one of the four grid areas on the Oxidase Test slide. The test slide surface is impregnated with the reagent tetramethyl-p-phenylenediamine dihydrochloride. This reagent causes a dark purple color to appear in the presence of cytochrome oxidase. Observe the color change. The reaction color will change from pink to maroon to dark purple. Read the test results within 10 seconds. Some organisms may show slight positive reactions after this period and such results are NOT considered definitive.

Don't use a Nichrome inoculating loop to transfer cells; this will cause false positives.

The oxidase test is negative for all the Enterobacteriaceae. It is positive for the oxidative non-fermenters, Pseudomonas and Burkholderia, Neisseria, Hemophilus and Moraxella,Vibrio, Campylobacter, and Helicobacter are other oxidase- positive bacteria.

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