Which statement defines an anaerobe?

• A. Bacteria that grows without oxygen.
• B. Bacteria that grows in the presence of oxygen.
• C. Bacteria that dies in oxygen.
• D. Bacteria that grows at high temperatures.

Answer: (A)

Anaerobes are organisms that do not require oxygen to grow, and many cannot tolerate oxygen at all. The statement that best defines an anaerobe is bacteria that grows without oxygen, because it directly describes the organism’s growth environment in relation to oxygen. In practice, these bacteria are cultured in oxygen-free conditions, such as specialized anaerobic jars or chambers, to imitate their natural, oxygen-free habitats. Some anaerobes may be harmed by exposure to oxygen, but that sensitivity stems from their metabolism rather than the basic definition. The other descriptions point to different categories—growth in the presence of oxygen refers to aerobes, and growth at high temperatures describes thermophiles—neither of which define an anaerobe.