When performing the oxidase test, which statement would be appropriate?

• A. The reagent used is o-nitrophenyl-β-D-galactopyranoside
• B. A nichrome wire loop should be used to acquire inoculum for testing
• C. Colonies from sheep blood agar can be used
• D. A positive colony turns dark purple after 30 seconds following application of the reagent

Answer: (C)

The oxidase test works by applying a reagent that becomes a deep purple if the bacterium has cytochrome c oxidase. Fresh colonies are best because aging can dull the reaction. Colonies taken from a sheep blood agar plate are appropriate to test because this medium is non-inhibitory and yields clean, well-isolated, fresh colonies suitable for the oxidase reaction.

The other statements aren’t correct for this test. The reagent used is not o-nitrophenyl-β-D-galactopyranoside (that compound relates to a different assay). Using a nichrome wire loop isn’t recommended for inoculum transfer, as metal loops can interfere with the reagent or give unreliable results; a sterile swab or wooden stick is preferred. And while a rapid color change to purple is characteristic of a positive oxidase result, relying on a 30-second threshold alone doesn’t define the appropriate source and method of testing—the key is using a fresh colony from a non-inhibitory medium like sheep blood agar.