Which type of bacteria grows only in the absence of free dissolved oxygen?

Anaerobic bacteria are specifically adapted to live and grow in environments lacking free dissolved oxygen. These microorganisms can thrive in conditions where oxygen is absent, employing fermentation or anaerobic respiration to obtain energy. This characteristic is crucial in various natural processes, such as those occurring in deep-sea environments, swamps, and certain digestive systems.

The adaptation of anaerobic bacteria to oxygen-free conditions is significant in environmental health and microbiology, as they can play essential roles in nutrient cycling, waste decomposition, and even human health, when present in the gut microbiome.

In contrast, aerobic bacteria require oxygen for growth and metabolism, while facultative bacteria can switch between aerobic and anaerobic conditions, showing flexibility based on environmental availability. Pathogenic bacteria can be either aerobic or anaerobic, depending on the species, but their classification isn't solely based on their oxygen requirements.