Which statement about the oxidase test is not appropriate?

Which statement about the oxidase test is not appropriate?

• A. The reagent used is o-nitrophenyl-(3-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 within 10 seconds after application of the reagent

Answer: (B)

In the oxidase test, the principle is that organisms with cytochrome c oxidase transfer electrons to the reagent, turning the reagent a dark purple if the organism is oxidase-positive. The color change is rapid, typically within seconds, so a quick purple result indicates a positive test.

The not-appropriate statement is about how you obtain the colony. Using a nichrome wire loop is not recommended because metal loops can interfere with the reagent or promote unintended oxidation, which can give unreliable results. A sterile swab or a wooden applicator to transfer a tiny amount of colony to the reagent surface minimizes this risk and yields more trustworthy results.

Colonies from sheep blood agar can be used for the test since blood agar is a common, nonselective medium that supports growth of many oxidase-positive and -negative bacteria, allowing you to pick a representative colony for testing.

A positive oxidase reaction is characterized by a dark purple color appearing quickly, around 10 seconds or less, after the reagent is applied. If the color change is absent or delayed, the result is not positive.

(For completeness, the reagent used in the oxidase test is not a substrate for beta-galactosidase testing; techniques like ONPG are used for different purposes, such as assessing lactose metabolism, not oxidase activity.)