______ bacteria can thrive in the complete absense of DO.

• A. Aerobic
• B. Anaerobic
• C. Facultative
• D. Microaerophilic

Answer: (B)

Oxygen availability drives where bacteria can grow. Bacteria that can thrive in a complete absence of dissolved oxygen are anaerobes, specifically obligate anaerobes. They lack the enzymes (like superoxide dismutase and catalase) that detoxify reactive oxygen species, so oxygen is toxic to them and they rely on anaerobic metabolism (fermentation or anaerobic respiration) to generate energy. Other types either require oxygen (aerobic), or can grow with or without oxygen (facultative), or need only very low levels of oxygen (microaerophilic), so they wouldn’t flourish in a totally oxygen-free environment. Hence, the correct description is anaerobic bacteria.