Bacteria that do not require oxygen for growth.

A facultative anaerobic organism can grow without oxygen when necessary but uses oxygen when it is present. Obligate anaerobic organisms cannot use oxygen for growth; they are tolerant to oxygen to varying degrees.

In general, aerobic bacteria are more clinically relevant than anaerobic bacteria. Anaerobic bacteria are major components of the microbial flora of multiple mucosal sites in the body, particularly the gastrointestinal tract, causing disease when mucosal barriers are breached. They are typically harder to culture and the cause of infections can be difficult to identify. Patients undergoing abdominal surgery often receive anti-anaerobe drugs to prevent peritoneal infections.