A semisolid medium containing resazurin appears pink. What does this indicate?

A semisolid medium containing resazurin appears pink. What does this indicate?

• A. Presence of oxygen
• B. Acid environment
• C. Alkaline environment
• D. Motility

Answer: (A)

Resazurin acts as an oxygen indicator in semisolid culture media. When oxygen is present, resazurin stays in its oxidized form and appears pink. As bacteria consume the oxygen, the indicator is reduced and turns colorless (or very pale). So a pink color shows that the environment at that location is oxygenated, typically at the surface of the medium. The other options don’t fit because acid- or alkaline conditions are detected by separate pH indicators, not resazurin, and motility is inferred from how growth spreads through the medium rather than from a color change.

Resazurin acts as an oxygen indicator in semisolid culture media. When oxygen is present, resazurin stays in its oxidized form and appears pink. As bacteria consume the oxygen, the indicator is reduced and turns colorless (or very pale). So a pink color shows that the environment at that location is oxygenated, typically at the surface of the medium. The other options don’t fit because acid- or alkaline conditions are detected by separate pH indicators, not resazurin, and motility is inferred from how growth spreads through the medium rather than from a color change.