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)

The test checks for the presence of cytochrome c oxidase using the chemical tetramethyl-p-phenylenediamine hydrochloride (TMPD). If the organism has this enzyme, it will transfer electrons from TMPD to oxygen, oxidizing TMPD to a deep purple color within seconds. If the organism lacks cytochrome c oxidase, the reagent stays colorless. Moraxella catarrhalis is oxidase positive, so applying TMPD yields a dark purple result. Acinetobacter baumannii and Stenotrophomonas maltophilia are oxidase negative and do not produce this purple color. Yersinia enterocolitica is also oxidase negative, so it remains colorless.

The test checks for the presence of cytochrome c oxidase using the chemical tetramethyl-p-phenylenediamine hydrochloride (TMPD). If the organism has this enzyme, it will transfer electrons from TMPD to oxygen, oxidizing TMPD to a deep purple color within seconds. If the organism lacks cytochrome c oxidase, the reagent stays colorless.

Moraxella catarrhalis is oxidase positive, so applying TMPD yields a dark purple result. Acinetobacter baumannii and Stenotrophomonas maltophilia are oxidase negative and do not produce this purple color. Yersinia enterocolitica is also oxidase negative, so it remains colorless.