To ensure anaerobic conditions are achieved in anaerobic jars or chambers, which indicator is employed?

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

Answer: (B)

Verifying anaerobiosis relies on an indicator that responds to the presence or absence of oxygen. Methylene blue is blue when oxidized and becomes colorless in the reduced, oxygen-free environment created by the gas-pack system. In a properly sealed anaerobic jar or chamber, the indicator stays colorless, confirming anaerobiosis; if air leaks in and oxygen is present, it reverts to blue, signaling oxygen exposure. The other indicators are pH indicators and do not report oxygen levels: bromcresol purple, methyl red, and phenol red change color with acidity or basicity rather than with redox state, so they don’t reliably indicate anaerobic conditions.