What indicator is commonly used to confirm that anaerobic conditions have been achieved in culture?

What indicator is commonly used to confirm that anaerobic conditions have been achieved in culture?

• A. Bromcresol purple
• B. Methylene blue
• C. Methyl red
• D. Phenol red

Answer: (B)

Redox indicators reveal the oxygen state of the culture environment. Methylene blue is blue when oxidized (present oxygen) and becomes colorless when reduced (no oxygen). In anaerobic setups, the medium is reduced, so methylene blue turns colorless, signaling that anaerobic conditions have been achieved. The other choices are pH indicators and change color with acidity or basicity rather than with the redox state, so they don’t reliably indicate the absence of oxygen.

Redox indicators reveal the oxygen state of the culture environment. Methylene blue is blue when oxidized (present oxygen) and becomes colorless when reduced (no oxygen). In anaerobic setups, the medium is reduced, so methylene blue turns colorless, signaling that anaerobic conditions have been achieved. The other choices are pH indicators and change color with acidity or basicity rather than with the redox state, so they don’t reliably indicate the absence of oxygen.