Which organism will turn a dark purple when tetramethyl-p-phenylenediamine hydrochloride is applied?

Which organism will turn a dark purple when tetramethyl-p-phenylenediamine hydrochloride is applied?

• A. Acinetobacter baumannii
• B. Stenotrophomonas maltophilia
• C. Moraxella catarrhalis
• D. Yersinia enterocolitica

Answer: (C)

This test detects the presence of cytochrome c oxidase using tetramethyl-p-phenylenediamine hydrochloride. If an organism has this enzyme, it oxidizes the reagent and the surface turns a dark purple color promptly. If the enzyme is absent, the reagent remains colorless or only lightly colored. Moraxella catarrhalis is oxidase-positive, so it will produce the dark purple color. The other organisms listed are oxidase-negative and would not show the purple result.

This test detects the presence of cytochrome c oxidase using tetramethyl-p-phenylenediamine hydrochloride. If an organism has this enzyme, it oxidizes the reagent and the surface turns a dark purple color promptly. If the enzyme is absent, the reagent remains colorless or only lightly colored. Moraxella catarrhalis is oxidase-positive, so it will produce the dark purple color. The other organisms listed are oxidase-negative and would not show the purple result.