Anaerobes are bacteria that are either capable of growing in the absence of oxygen (referred to as facultative anaerobes) or that absolutely require the absence of oxygen (these are also called obligate anaerobes). Among the oxygen-free environments in which such bacteria can grow are deep wounds and tissues in the body. Growth in these niches can produce infections.

Examples of infections are gas gangrene (which is caused by Streptococcus pyogenes) and botulism (which is caused by Clostridium botulinum). Other anaerobic bacteria that are frequently the cause of clinical infections are members of the genus Peptostreptococcus and Bacteroides fragilis.

There are a number of different types of anaerobic bacteria. Two fundamental means of differentiation of these types is by their reaction to the Gram stain and by their shape. The genus Clostridium consists of Gram-positive rod-shaped bacteria that form spores. Gram-positive rods that do not form spores include the genera Actinomyces, Bifidobacterium, Eubacterium, Propionibacterium, and Lactobacillus. Gram-positive bacteria that are spherical in shape includes the genera Peptostreptococcus, Streptococcus, and Staphylococcus. Rod-shaped bacteria that stain Gram-negative include Bacteroides, Campylobacter, and Fusobacterium. Finally, Gram-negative spherical bacteria are represented by the genus Veillonella.

The word anaerobic means "life without air." In the human body, regions that can be devoid of oxygen include the interior of dental plaque that grows on the surface of teeth and gums, the gastrointestinal tract, and even on the surface of the skin. Normally the anaerobic bacteria growing in these environments are benign and can even contribute to the body's operation. Most of the bacteria in the body are anaerobes. However, if access to underlying tissues is provided due to injury or surgery, the bacteria can invade the new territory and establish an infection. Such bacteria are described as being opportunistic pathogens. That is, given the opportunity and the appropriate conditions, they are capable of causing an infection. Typically, anaerobic bacteria cause from five to ten per cent of all clinical infections.

Anaerobic infections tend to have several features in common. The infection is usually accompanied by a foul-smelling gas or pus. The infections tend to be located close to membranes, particularly mucosal membranes, as the infection typically begins by the invasion of a region that is bounded by a membrane. Anaerobic infections tend to involve the destruction of tissue, either because of bacterial digestion or because of destructive enzymes that are elaborated by the bacteria. This type of tissue damage is known as tissue necrosis. The tissue damage also frequently includes the production of gas or a fluid.

There are several sites in the body that are prone to infection by anaerobic bacteria. Infections in the abdomen can produce the inflammation of the appendix that is known as appendicitis. Lung infections can result in pneumonia, infection of the lining of the lung (empyema) or constriction of the small air tubes known as bronchi (bronchiectasis). In females, pelvic infections can inflame the lining of the uterus (endometritis). Mouth infections can involve the root canals or gums (gingivitis). Infections of the central nervous system can lead to brain and spinal cord infections. Infection of the skin, via bites and other routes of entry, causes open sores on the skin and tissue destruction. An example is that massive and potentially lethal tissue degradation, which is known as necrotizing fascitis, and which is caused by group A b-hemolytic Streptococcus. Finally, infection of the bloodstream (bacteremia) can prelude the infection of the heart (endocarditis).

The diagnosis of anaerobic infections is usually based on the symptoms, site of the infection and, if the infection is visible, on both the appearance and smell of the infected area. Most of the bacteria responsible for infection are susceptible to one or more antibiotics. Treatment can be prolonged, however, since the bacteria are often growing slowly and since antibiotics rely on bacterial growth to exert their lethal effect. In the case of infections that create tissue destruction, the removal of the affected tissue is an option to prevent the spread of the infection. Amputation of limbs is a frequent means of dealing with necrotizing fascitis, an infection that is inside of tissue (and so protected from antibiotics and the host's immune response) and is exceptional in that it can swiftly spread.

See also Bacteria and bacterial infections