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Anaerobic bacteria are the greatest component of the human body's normal bacterial flora colonizing the skin, oral cavity, and genitourinary and lower gastrointestinal tracts and generally do not cause infection. Their presence is important for vitamin and other nutrient absorption and in preventing infection with pathogenic bacteria.
When usual skin and mucosal barriers are penetrated and in an anaerobic environment, these bacteria can behave as pathogens. Typical anaerobic infections include periodontitis, abdominal or pelvic abscesses, endometritis, pelvic inflammatory disease, aspiration pneumonia, empyema and lung abscesses, sinusitis, brain abscesses, gas gangrene, and other soft tissue infections.
Anaerobes grow aggressively in the body under anaerobic conditions and may possess a variety of virulence factors including capsules and extracellular enzymes. They also can develop resistance to antimicrobials by producing beta-lactamase and other modifying enzymes and by alterations in membrane permeability and structure of penicillin-binding proteins. Because anaerobic bacteria are a significant cause of human infection and they are often resistant to commonly used antimicrobials, susceptibility testing results are useful to clinicians. Many Bacteroides species produce beta-lactamases. Imipenem, metronidazole, and clindamycin are effective agents although resistance to clindamycin, and occasionally imipenem, is increasing.
Diagnosing anaerobic bacterial infections
Isolation of anaerobes in significant numbers from well-collected specimens including blood, other normally sterile body fluids, or closed collections of purulent fluid indicates infection with that (those) organisms.
Specimens should be collected by needle and syringe aspiration or surgical drainage to avoid contamination with normal-flora anaerobes; such contamination would make interpretation of culture results impossible.
Specimens must be transported in anaerobic transport vials.
Identification of probable pathogens
1. Summanen P, Baron EJ, Citron DM, et al. Wadsworth Anaerobic Bacteriology Manual. Sixth edition. Belmont CA, Star Publishing Co, 2002
2. Chapters 50-54; Anaerobic bacteria. In Manual of Clinical Microbiology, 11th edition. Edited by J Jorgensen, M Pfaller. Washington DC, ASM Press, 2015
3. Hall GS, Section 4. Anaerobic bacteriology, In Clinical Microbiology Procedures Handbook. Third edition. Edited by LS Garcia. Vol 1. ASM Press, Washington, DC. 2007