Which group cannot grow in the presence of oxygen and relies on fermentation or anaerobic respiration using a final electron acceptor other than oxygen?

• A. Aerotolerant anaerobes
• B. Obligate anaerobes
• C. Facultative anaerobes
• D. Microaerophiles

Answer: (B)

The concept being tested is how oxygen availability shapes microbial energy production and which groups must avoid oxygen. Obligate anaerobes cannot tolerate oxygen because they lack the protective enzymes needed to neutralize reactive oxygen species. In oxygen-free environments they generate energy by fermentation or by anaerobic respiration that uses electron acceptors other than oxygen (such as nitrate, sulfate, or carbon dioxide). This makes them unable to grow in the presence of oxygen, which is exactly why they’re classified this way.

Aerotolerant anaerobes can tolerate oxygen and still rely on fermentation; they don’t require it and aren’t harmed by it. Facultative anaerobes can switch between aerobic respiration when oxygen is available and fermentation or anaerobic respiration when it isn’t. Microaerophiles need oxygen too, but only at low, limited concentrations.