Which group shows better growth in the presence of oxygen but can also grow without it?

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

Answer: (B)

Metabolic flexibility with oxygen: organisms in this group use oxygen to generate energy when it’s available, but can also switch to fermentation or anaerobic respiration when oxygen is absent. With oxygen, aerobic respiration yields much more ATP, so growth is faster and more robust. If oxygen runs out, they don’t quit; they continue to grow, albeit more slowly, by alternative pathways that don’t require oxygen. This dual capability explains why they grow best in the presence of oxygen yet can survive and still proliferate without it.

Other groups don’t fit this pattern. Obligate aerobes need oxygen and won’t grow without it. Aerotolerant anaerobes don’t gain growth benefits from oxygen—they tolerate it but don’t use it for growth. Microaerophiles require only small amounts of oxygen and don’t thrive when oxygen is plentiful or absent.