Which statement best defines facultative bacteria?

• A. They require oxygen to grow
• B. They can grow with or without oxygen
• C. They require anaerobic conditions only
• D. They cannot grow in activated sludge

Answer: (B)

Facultative bacteria are versatile microbes that can grow with or without oxygen. When oxygen is present, they use aerobic respiration to generate energy efficiently. If oxygen is limited or absent, they switch to anaerobic processes such as fermentation or anaerobic respiration with other electron acceptors, allowing continued growth. This flexibility makes them well-suited to environments with fluctuating oxygen levels, like activated sludge in wastewater treatment, where some zones are aerated and others are more anaerobic. The other statements don’t fit: requiring oxygen to grow describes obligate aerobes, requiring anaerobic conditions only describes obligate anaerobes, and the idea that they cannot grow in activated sludge is incorrect since these bacteria are common there and contribute to digestion and stability.