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

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

Answer: (A)

The key idea is how different microbes handle oxygen and how they generate energy without it. Obligate anaerobes cannot tolerate oxygen at all; exposure to oxygen is toxic for them because they often lack protective enzymes needed to detoxify reactive oxygen species. To grow, they rely on energy pathways that do not use oxygen, such as fermentation or anaerobic respiration that ends with an electron acceptor other than oxygen (for example nitrate or sulfate). This is why they flourish in oxygen-free environments like deep soil, sediments, or the gut, and not in places where oxygen is present.

In contrast, aerotolerant anaerobes don’t utilize oxygen but can tolerate its presence; facultative anaerobes can switch between aerobic respiration when oxygen is available and fermentation or anaerobic respiration when it isn’t; and microaerophiles require only small amounts of oxygen.