Which type of bacteria operates in environments devoid of free or dissolved oxygen?

Anaerobic bacteria are specifically adapted to thrive and function in environments where there is no free or dissolved oxygen. These bacteria perform their metabolic processes through fermentation or anaerobic respiration, utilizing alternate electron acceptors like nitrate, sulfate, or carbon dioxide instead of oxygen. This capability allows them to break down organic matter in environments such as sewage and sludge, which are typically oxygen-depleted conditions.

In contrast, aerobic bacteria require the presence of oxygen to metabolize organic matter, making them unsuitable for environments without oxygen. Facultative bacteria can switch between aerobic and anaerobic processes depending on the availability of oxygen, while commensal bacteria refer to a type of bacteria that exist in a relationship with their host but are not classified according to their oxygen needs. Hence, anaerobic bacteria are the correct answer, as they are the type of organisms that specifically thrive in oxygen-free environments.