Anaerobes are microorganisms that can survive without oxygen. They are commonly found in environments that lack oxygen, such as soil, water, and the human gut. There are several synonyms for the term anaerobes, including anaerobic bacteria, obligate anaerobes, and facultative anaerobes. Anaerobic bacteria are those that require a lack of oxygen to grow and reproduce. Obligate anaerobes are microorganisms that are unable to survive in the presence of oxygen. Facultative anaerobes can survive in both the presence and absence of oxygen, and are also capable of switching between aerobic and anaerobic metabolism. These synonyms are often used interchangeably, though there are subtle differences in their meaning.