Which oxidation-reduction indicator is commonly used in anaerobic culture systems and turns from blue to white when reduced?

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

Answer: (B)

Methylene blue serves as a redox indicator whose color tracks the oxygen level of the environment. In the presence of oxygen, it stays in its blue, oxidized form. When the environment is reduced and oxygen is absent, it is converted to leucomethylene blue, which is colorless (appears white). This property makes it especially useful in anaerobic culture systems and their gas packs, because a color change to colorless signals that the atmosphere is reduced and suitable for anaerobic growth. The other indicators listed are pH indicators, not redox indicators, so they change color with acidity or basicity rather than with oxygen status. Bromcresol purple, methyl red, and phenol red would indicate pH shifts, not the presence or absence of oxygen, so they aren’t used to verify anaerobic conditions.