Which term describes organisms that can grow with or without oxygen but prefer oxygen when available?

• A. Aerotolerant anaerobe
• B. Obligate anaerobe
• C. Facultative anaerobe
• D. Obligate aerobe

Answer: (C)

Organisms that can switch metabolic lifestyles depending on whether oxygen is present. They use aerobic respiration when oxygen is available to maximize energy yield, and can switch to anaerobic metabolism (like fermentation or anaerobic respiration) when oxygen is absent. Because aerobic respiration produces much more ATP, these microbes grow more efficiently in the presence of oxygen, yet they’re able to survive without it. This description fits facultative anaerobes. In contrast, aerotolerant anaerobes don’t rely on oxygen for growth and merely tolerate its presence, obligate anaerobes are harmed by oxygen, and obligate aerobes require oxygen for growth.