Which organism category requires oxygen and grows best at lower-than-atmospheric oxygen levels?

• A. Optimum oxygen concentration
• B. Aerotolerant anaerobes
• C. Obligate anaerobes
• D. Microaerophiles

Answer: (D)

Organisms differ in how they handle oxygen. Microaerophiles need oxygen to grow but prefer levels lower than what’s in the air, because high oxygen can produce damaging reactive oxygen species. They typically flourish around a few percent of O2 (often about 5%), and this reduced oxygen helps them avoid stress that would occur at atmospheric levels. Culturing them usually requires a gas mix with less oxygen and more CO2. Examples include Campylobacter and Helicobacter species. This sets them apart from obligate aerobes, which require atmospheric oxygen and tolerate it well; and from aerotolerant anaerobes, which don’t rely on oxygen and are not harmed or particularly helped by its presence. The label describing an “optimum oxygen concentration” isn’t a type of organism, so the category that fits the description is microaerophiles.