Some bacteria like Pseudomonas, Pasteurellaceae, Campylobacter, Neisseria, Moraxella, can produce cytochrome C oxidase located in their membrane which can catalyse the transport of electrons from donor compounds to electron acceptors (oxygen). This respiratory system is present in aerobic bacteria which are capable to use oxygen as the final hydrogen receptor.
In the presence of oxygen the cytochrome C enzyme oxidizes the phenylendiamine reagent, resulting indophenol, a purple-blue compound.
There are several ways to perform oxidase test: using filter paper, using a swab or adding the reactive directly on plate. Filter paper method: soak a piece of filter paper in N, N, N', N'-tetra-methyl-p-phenylenediamine dihydrochloride. Pick a colony and dispense on filter paper. Swab method: harvest culture from a solid media then add 1-2 drops of reagent. Direct method: add 1-2 drops of reagent to one colony from an agar plate. For all methods a blue color appears within 1 minute if reaction is positive.