Which statement about obligate anaerobes is true?

• A. They require oxygen to grow
• B. They can grow only in oxygen-free environments
• C. They can grow with or without oxygen
• D. They cannot tolerate oxygen and must be cultured in oxygen-free environments

Answer: (D)

Obligate anaerobes cannot survive in the presence of oxygen because they lack the protective enzymes needed to neutralize reactive oxygen species. Oxygen is toxic to them, so they must be grown in oxygen-free environments. That’s why the statement describing their complete intolerance to oxygen and the need for an oxygen-free culture setting best captures their biology. In practice, these organisms are cultivated using anaerobic chambers or reducing media to remove oxygen. Clostridium species and many gut bacteria are classic examples. The idea that they require oxygen or can grow with oxygen doesn’t fit how obligate anaerobes live, since oxygen actively harms them.