Which type of bacteria utilizes oxygen when present but can resort to fermentation without it?

Facultative anaerobes are capable of utilizing oxygen when it is available, enabling them to perform aerobic respiration, which is generally more efficient in energy production. However, when oxygen is scarce or absent, these bacteria have the metabolic flexibility to switch to fermentation or anaerobic respiration. This ability to thrive in both oxygen-rich and oxygen-poor environments provides facultative anaerobes with a significant survival advantage in varying conditions.

In contrast, obligate anaerobes cannot tolerate oxygen and rely exclusively on fermentation or anaerobic pathways for energy. Microaerophiles require limited amounts of oxygen to survive but are not capable of fermentation. Aerotolerant anaerobes do not utilize oxygen but can survive in its presence without it affecting their metabolism. The distinctive metabolic flexibility of facultative anaerobes makes them well-suited for diverse ecological niches.