What is a microaerophile?

• A. An organism that requires high oxygen levels
• B. An organism that requires reduced oxygen levels, lower than atmospheric O2, for optimal growth
• C. An organism that tolerates oxygen but does not require it
• D. An organism that grows only in the absence of oxygen

Answer: (B)

Microaerophiles need oxygen to grow, but only in reduced amounts. They thrive best in environments where O2 is lower than what’s present in the air (about 21%). In practice, many microaerophiles prefer around 5% O2 and can be harmed by higher oxygen levels because extra oxygen can generate reactive oxygen species that they aren’t as well equipped to detoxify.

That’s why the correct description is an organism that requires reduced oxygen levels, lower than atmospheric O2, for optimal growth. It’s not about needing high oxygen, which would describe aerobes; it’s not about tolerating oxygen without needing it (aerotolerant organisms); and it’s not about growing only without oxygen (obligate anaerobes).