Which term describes organisms that do not use oxygen but can tolerate its presence and are not enhanced by it?

• A. Obligate aerobe
• B. Facultative anaerobe
• C. Aerotolerant anaerobe
• D. Obligate anaerobe

Answer: (C)

This item tests how microbes handle oxygen exposure. Organisms that do not use oxygen to generate energy but can tolerate its presence are aerotolerant anaerobes. They rely on fermentation or other anaerobic pathways for energy, so oxygen doesn’t boost their growth. Their ability to tolerate oxygen comes from enzymes that detoxify reactive oxygen species, allowing survival even when air is present. In other words, they grow just as well with or without oxygen, unlike those that actually benefit from or require it. Obligate aerobes need oxygen to grow because they rely on aerobic respiration; facultative anaerobes can use oxygen when it’s available but don’t require it and can switch to anaerobic methods; obligate anaerobes are harmed by oxygen and must be grown in oxygen-free conditions.