To ensure anaerobic conditions in jars or chambers, which oxygen-sensitive indicator is used?

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

Answer: (B)

The main idea is to use a redox-sensitive indicator that reports the presence or absence of oxygen in an anaerobic setup. Methylene blue works best here because it changes color based on the redox state: it appears blue when oxygen is present (oxidized) and becomes colorless when reduced in an anaerobic environment. In GasPak or jar systems, a methylene blue indicator turns colorless once the atmosphere is effectively devoid of oxygen, signaling that anaerobiosis has been achieved; if oxygen leaks in, it reverts to blue. The other options are pH indicators and do not reliably reflect oxygen levels, so they aren’t used to confirm anaerobic conditions.