Anaerobic Infections

Definition

An anaerobic infection is an infection caused by bacteria (called anaerobes) which cannot grow in the presence of oxygen. Anaerobic bacteria can infect deep wounds, deep tissues, and internal organs where there is little oxygen. These infections are characterized by abscess formation, foul-smelling pus, and tissue destruction.

Description

Anaerobic means "life without air." Anaerobic bacteria grow in places which completely, or almost completely, lack oxygen. They are normally found in the mouth, gastrointestinal tract, and vagina, and on the skin. Commonly known diseases caused by anaerobic bacteria include gas gangrene, tetanus, and botulism. Nearly all dental infections are caused by anaerobic bacteria.

Anaerobic bacteria can cause an infection when a normal barrier (such as skin, gums, or intestinal wall) is damaged due to surgery, injury, or disease. Usually, the immune system kills any invading bacteria, but sometimes the bacteria are able to grow and cause an infection. Body sites that have tissue destruction (necrosis) or a poor blood supply are low in oxygen and favor the growth of anaerobic bacteria. The low oxygen condition can result from blood vessel disease, shock, injury, and surgery.

Anaerobic bacteria can cause infection practically anywhere in the body. For example:

• Mouth, head, and neck. Infections can occur in the root canals, gums (gingivitis), jaw, tonsils, throat, sinuses, and ears.
• Lung. Anaerobic bacteria can cause pneumonia, lung abscesses, infecton of the lining of the lung (empyema), and dilated lung bronchi (bronchiectasis).
• Intraabdominal. Anaerobic infections within the abdomen include abscess formation, peritonitis, and appendicitis.
• Female genital tract. Anaerobic bacteria can cause pelvic abscesses, pelvic inflammatory disease, inflammation of the uterine lining (endometritis), and pelvic infections following abortion, childbirth, and surgery.
• Skin and soft tissue. Anaerobic bacteria are common causes of diabetic skin ulcers, gangrene, destructive infection of the deep skin and tissues (necrotizing fascitis), and bite wound infections.
• Central nervous system. Anaerobic bacteria can cause brain and spinal cord abscesses.
• Bloodstream. Anaerobic bacteria can be found in the bloodstream of ill patients (a condition called bacteremia).

— Belinda Rowland, PhD

An infection caused by anaerobic bacteria (organisms that are intolerant of oxygen). Most such infections are mixed, involving more than one anaerobe and often aerobic or facultative bacteria as well. Anaerobes are prevalent throughout the body as indigenous flora, and virtually all anaerobic infections arise endogenously, the principal exception being Clostridium difficile colitis. Factors predisposing to anaerobic infection include those disrupting mucosal or other surfaces (trauma, surgery, and malignancy or other disease), those lowering redox potential (impaired blood supply, tissue necrosis, and growth of nonanaerobic bacteria), drugs inactive against anaerobes (such as aminoglycosides), and virulence factors produced by the anaerobes (toxins, capsules, and collagenase, hyaluronidase, and other enzymes). Anaerobic gram-negative bacilli (Bacteroides, Prevotella, Porphyromonas, Fusobacterium) and anaerobic gram-positive cocci (Peptostreptococcus) are the most common anaerobic pathogens. Clostridium (spore formers) may cause serious infection. The prime pathogen among gram-positive nonsporulating anaerobic bacilli is Actinomyces. Of the infections commonly involving anaerobes, the oral and dental pleuropulmonary, intraabdominal, obstetric-gynecologic, and skin and soft tissue infections are most important in terms of frequency of occurrence. To document anaerobic infection properly, specimens for culture must be obtained so as to exclude normal flora and must be transported under anaerobic conditions. Therapy includes surgery and antimicrobial agents. See also Antibiotic; Gangrene; Gas; Infection.