Which group tests negative for catalase and does not perform aerobic respiration yet can grow in the presence of oxygen?

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

Answer: (D)

Oxygen tolerance and how a microbe harvests energy are what determine where it can grow and how it handles reactive oxygen species. The catalase test shows whether an organism has the enzyme catalase to detoxify hydrogen peroxide, a byproduct of oxygen use. Organisms that rely on fermentation and do not use oxygen for energy production often lack catalase and can survive in the presence of oxygen because they don’t trigger aerobic metabolism.

This combination—not using oxygen for growth, yet tolerating oxygen, and lacking catalase—fits aerotolerant anaerobes. They ferment for energy regardless of oxygen and aren’t harmed by oxygen because they can tolerate it, even though they don’t perform aerobic respiration.

Obligate anaerobes, in contrast, cannot tolerate oxygen and generally lack catalase, so they’re harmed by oxygen. Facultative anaerobes can switch to aerobic respiration when oxygen is available and are typically catalase positive. Microaerophiles require small amounts of oxygen and also usually possess detoxifying enzymes like catalase.