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, you’re looking for cytochrome c oxidase activity by applying a colorless reagent to a colony and watching for a rapid color change to dark purple as the reagent is oxidized. Using a nichrome (metal) wire loop to pick up and inoculate the test is not appropriate because metal tools can introduce metal ions or cause unwanted reactions with the reagent, potentially giving false results. The recommended approach is to transfer a small amount of colony with a sterile wooden stick or swab to the reagent-treated area, avoiding metal contamination. Colonies from sheep blood agar are suitable sources for oxidase testing, provided the colonies are fresh and handled properly. A rapid dark purple color within about 10 seconds after applying the reagent is consistent with a positive oxidase reaction. The reagent described as o-nitrophenyl-(3-D-galactopyranoside) is not used for oxidase testing (that substrate is associated with beta-galactosidase activity), so that statement would not fit the test’s methodology.

In the oxidase test, you’re looking for cytochrome c oxidase activity by applying a colorless reagent to a colony and watching for a rapid color change to dark purple as the reagent is oxidized.

Using a nichrome (metal) wire loop to pick up and inoculate the test is not appropriate because metal tools can introduce metal ions or cause unwanted reactions with the reagent, potentially giving false results. The recommended approach is to transfer a small amount of colony with a sterile wooden stick or swab to the reagent-treated area, avoiding metal contamination.

Colonies from sheep blood agar are suitable sources for oxidase testing, provided the colonies are fresh and handled properly.

A rapid dark purple color within about 10 seconds after applying the reagent is consistent with a positive oxidase reaction.

The reagent described as o-nitrophenyl-(3-D-galactopyranoside) is not used for oxidase testing (that substrate is associated with beta-galactosidase activity), so that statement would not fit the test’s methodology.