anaerobe

An anaerobe is an organism that does not require oxygen for growth. Many anaerobes are harmed by atmospheric oxygen, and exposure can inhibit growth or be lethal. In microbiology, anaerobes are primarily discussed with bacteria, though archaea and some eukaryotes also include anaerobic species. Anaerobes are categorized by their oxygen tolerance: obligate (strict) anaerobes cannot survive in the presence of oxygen; facultative anaerobes can grow with or without oxygen, often preferring aerobic metabolism when oxygen is available but switching to fermentation or anaerobic respiration otherwise; aerotolerant anaerobes do not use oxygen but can survive and grow in its presence.

Metabolism in anaerobes relies on fermentation or anaerobic respiration using alternative electron acceptors such as nitrate, sulfate, or carbon dioxide. This allows energy production without molecular oxygen. Ecologically, anaerobes occupy a wide range of environments, including the gastrointestinal tract, oral cavity, deep soil and sediments, and anaerobic reactors used in industry. Some anaerobes, such as Clostridium and Bacteroides species, are notable examples in medicine and ecology; many archaea also live anaerobically as methanogens or sulfate-reducers.

Culturing anaerobes requires oxygen-free conditions, achieved with anaerobic chambers, gasPak systems, and reducing culture media. Clinically, anaerobes are common members of the normal flora but can cause disease when they invade normally sterile sites or form abscesses, including dental, intra-abdominal, gynecological, and aspiration-related infections. Because of their oxygen sensitivity, they often grow slowly and may be resistant to antibiotics that rely on oxygen-dependent uptake.