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Anaerobic bacteria 

Anaerobic bacteria

Anaerobic bacteria

Anilrudh A. Venugopal

and David W. Hecht


May 30, 2013: This chapter has been re-evaluated and remains up-to-date. No changes have been necessary.


Gastrointestinal infections—update on the management of Clostridium difficile and enterotoxigenic Bacteroides fragilis infections.

Updated data for the Bacteroides fragilis group provided for antibiotic resistance rates in the United States of America and Europe.

New section on the work-up and treatment of complicated intra-abdominal infections.

Additions to Further reading.

Updated on 31 May 2012. The previous version of this content can be found here.
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date: 28 April 2017

Anaerobic bacteria will not grow when incubated with 10% CO2 in room air, but vary in their tolerance of different levels of oxygen. They are important commensal flora of the skin and oral, intestinal, and pelvic mucosae, and are classified according to their Gram staining characteristics and ability to produce spores: (1) Gram positive—cocci, non-spore-forming bacilli, and spore-forming bacilli (notably clostridium); (2) Gram negative—cocci and bacilli. Many anaerobic bacteria possess virulence factors that facilitate their pathogenicity, e.g. histiolytic enzymes and various toxins.

Clinical features—anaerobes typically cause clinically significant infections when there is tissue compromise, ischaemia or mucosal injury. These infections are often polymicrobial in nature and include (1) bacteraemia; (2) central nervous system infection—intracranial abscesses by contiguous spread, e.g. from chronic otitis media, or haematogenous spread, e.g. from tooth abscess; (3) head and neck infections—periodontal and pharyngeal infections from spread of gingival disease; (4) pleuropulmonary infections—e.g. lung abscess from aspirated oropharyngeal flora; (5) intra-abdominal infections—often caused by mixed colonic flora that have been displaced by bowel injury; (6) gastrointestinal infections; (7) genitourinary infections; (8) skin and soft-tissue infections—ranging from cellulitis to necrotizing fasciitis; should be considered in cases of infected animal and human bites, and in intravenous drug users; diabetic foot ulcers often have polymicrobial infections that include anaerobes.

Diagnosis—a putrid odour of the affected tissue or discharge is very suggestive of anaerobic infection, as is the presence of gas in tissues. Care must be taken when collecting specimens for anaerobic cultures because many of the organisms are very sensitive to oxygen, and some cannot tolerate more than a few minutes at ambient oxygen levels. However, anaerobic spores are aerotolerant, can survive in harsh oxygen-laden environments, and will germinate under appropriate conditions.

Treatment and prevention—aside from supportive care, treatment requires (1) drainage of abscesses and resection of devitalized tissue; and (2) antibiotics—agents that are active against anaerobes include clindamycin, metronidazole, vancomycin, β‎-lactam/β‎-lactamase combinations, carbapenems, moxifloxacin, and tigecycline but resistance patterns are changing and the choice of empirical therapy is best guided by knowledge of local susceptibility testing. Prophylaxis against anaerobic bacteria significantly reduces postoperative infection rates following intra-abdominal surgery.

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