Oxidase test

Principle

Organisms with cytochrome c oxidase oxidise tetramethyl-p-phenylenediamine to a purple compound.

Procedure

• Moisten filter paper with fresh 1% oxidase reagent (or use a commercial strip).
• Smear a colony with a wooden stick or platinum loop; read within 10 seconds.

Results

• Positive (purple): Pseudomonas, Neisseria, Vibrio, Campylobacter, Aeromonas
• Negative: Enterobacterales, Acinetobacter, Stenotrophomonas

Pitfalls

• Nichrome loops, colonies from media containing glucose, and colonies from dye-containing selective media (e.g. MacConkey, EMB) can give wrong results; test growth from non-selective media such as blood agar.
• Controls: P. aeruginosa (positive), E. coli (negative).