What describes an aerotolerant anaerobe?

• A. An organism that requires oxygen for growth
• B. An organism that uses oxygen but is harmed by it
• C. An organism that does not use oxygen but can tolerate its presence; growth not enhanced by oxygen
• D. An organism that grows faster with oxygen

Answer: (C)

Oxygen tolerance in microbes varies with how they get energy. An aerotolerant anaerobe does not use oxygen to generate energy (it relies on fermentation), but it can tolerate the presence of oxygen. Its growth proceeds the same with or without oxygen, so oxygen does not enhance growth. That exact idea—does not use oxygen for energy, but can tolerate its presence, with growth not boosted by oxygen—is why this description fits best. The other scenarios describe organisms that either need oxygen, are harmed by it, or grow better with it, which don’t match aerotolerant anaerobes.