Friday, 27 February 2015

Oxidase Test – an introduction

The oxidase test is used to determine if an organism possesses the cytochrome oxidase enzyme. The test is used as an aid for the differentiation of Neisseria, Moraxella, Campylobacter and Pasteurella species (oxidase positive). It is also used to differentiate pseudomonads from related species.

Oxidase positive bacteria possess cytochrome oxidase or indophenol oxidase (an iron containing haemoprotein). Both of these catalyse the transport of electrons from donor compounds (NADH) to electron acceptors (usually oxygen).

The test reagent, N, N, N’, N’-tetramethyl-p-phenylenediamine dihydrochloride acts as an artificial electron acceptor for the enzyme oxidase. The oxidised reagent forms the coloured compound indophenol blue.

The cytochrome system is usually only present in aerobic organisms which are capable of utilising oxygen as the final hydrogen receptor. The end product of this metabolism is either water or hydrogen peroxide (broken down by catalase).

There are many method variations to the oxidase test. These include, but are not limited to, the filter paper test, direct plate method, swab method, impregnated oxidase test strip method and test tube method. All times and concentrations are based upon the original recommendations.

Positive Result
Development of a deep purple-blue/blue colour indicates oxidase production.

Negative Result
No purple-blue colour/No colour change.

In relation to the oxidase test, Public Health England has issued a technical report, including safety information. The report can be accessed here.

  Posted by Tim Sandle