Which type of bacteria preferentially uses oxygen but can also survive without it?

The correct choice highlights facultative anaerobes, which are a unique group of bacteria that have the ability to utilize oxygen for respiration when it is available, while also being able to switch to anaerobic metabolic pathways when oxygen is scarce or absent. This flexibility allows them to thrive in a variety of environments and make efficient use of available resources.

Facultative anaerobes can perform aerobic respiration, which is more energy-efficient thanks to the use of oxygen, but when conditions change—such as in low-oxygen environments—they can shift to anaerobic respiration or fermentation processes. This adaptability is crucial for survival in diverse ecological niches where oxygen levels may fluctuate.

In contrast, aerobic bacteria strictly require oxygen to grow and often cannot survive in its absence, while anaerobic bacteria are exclusively reliant on environments without oxygen. Obligate anaerobes are particularly sensitive to oxygen, as it can be toxic to them, making facultative anaerobes distinct in their metabolic versatility.