What's the significance of oxidase tests?

The significance of oxidase tests is that they help to identify microorganisms that contain the enzyme cytochrome oxidase. The oxidase test involves adding a colorless reagent tetra-methyl- p-phenylenediamine dihydrochloride (TMPD) to a sample. If the cytochrome c oxidase enzyme is present, TMPD gets oxidized and changes from colorless to a purple or dark blue compound. If cytochrome c is absent, TMPD remains reduced and colorless.