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Anaerobic infections

  • Common

  • May be serious and life-threatening

  • Anaerobes are the predominant components of bacterial flora of normal human skin and mucous membranes

  • Difficult to isolate from infectious sites and are often overlooked. Isolation requires appropriate methods of collection, transportation and cultivation of specimens

  • Treatment is complicated by slow growth of these organisms, by the commonly polymicrobial nature of the infection and by the growing resistance of anaerobic bacteria to antimicrobial agents

Risk factors

  • Exposure of sterile body sites to large innoculum of indigenous mucous membrane flora

  • Poor blood supply and tissue necrosis

    • Trauma

    • Foreign body

    • Malignancy

    • Surgery

    • Oedema

    • Shock

    • Colitis

    • Vascular disease

  • Diabetes mellitus

  • Splenectomy

  • Immunocompromise

  • Collagen vascular disease

  • Previous infection with aerobic or facultative organisms

Clinical features

Features suggestive of anaerobic infection include:

  • Clinical condition predisposing to anaerobic infection

  • Infection after bites, adjacent to a mucosal surface, related to use of antibacterials that are only effective against aerobes, related to tumours or other destructive processes

  • Infected thrombophlebitis

  • Foul smelling discharge

  • Necrotic gangrenous tissue and abscess formation

  • Free gas in tissue

  • Black discolouration of exudates (due to Bacteroides melaninogenicus)

  • “Sulphur granules” in discharge (due to Actinomyces spp.)

  • Bacteraemia or endocarditis with no growth on aerobic blood cultures


  • In order to obtain an accurate microbiological picture of the infection great care must be taken with the collection, transport and processing of specimens

  • Even minimal contamination of a specimen with normal flora can result in misleading results due to the large numbers of anaerobic organisms present on skin and mucous membranes. Specimens that are appropriate for anaerobic culture are those that can be obtained without contamination:

    • Blood

    • Aspirates of body fluids

    • Urine collected by suprapubic bladder aspiration

    • Abscess contents (preferably pus rather than a swab)

    • Deep aspirates of wounds

    • Transtracheal aspirate, lung aspirate

    • Protected specimen brush or bronchoalveolar lavage specimen combined with quantitative microbiology

  • Specimen should be transported to the laboratory promptly, preferably in an anaerobic transporter



Site of infection

Antibacterial therapy

Gram +ve cocci

Peptostreptococcus spp.

Respiratory tract, intra-abdominal & soft tissue infections


Alternatives: clindamycin, chloramphenicol, cephalosporins

Microaerophilic streptococci (not true anaerobes)

Sinusitis, brain abscess


Gram +ve non spore-forming bacilli

Actinomyces spp.


Intracranial abscess, chronic mastoiditis, aspiration pneumonia, head & neck infections


Propionibacterium spp.

Shunt infections, infections associated with foreign body

Bifidobacterium spp.

Chronic otitis media, cervical lymphadenitis, intra-abdominal infection

Arachnia propionica

Eubacterium lentum


Gram +ve spore-forming organisms

Clostridium perfringens

Soft tissue infection, food poisoning


Alternatives: metronidazole, cefoxitin, clindamycin, chloramphenicol

Clostridium septicum

Neutropaenic enterocolitis

Clostridium ramosum

Soft tissue infection

Clostridium botulinum



Clostridium tetani



Alternatives: penicillin

Clostridium difficile

Colitis, antibiotic-associated diarrhoea


Alternative: vancomycin

Gram –ve bacilli

Bacteroides spp.


Intra-abdominal infection, gynaecological infection, neonatal infection

Non-beta lactamase producing organisms


Alternatives: metronidazole, chloramphenicol, clindamycin

Beta-lactamase producing organisms

Metronidazole, carbapenem, penicillin with beta-lactamase inhibitor or clindamycin.

Alternative: chloramphenicol

Prevotella spp

Orofacial infection, aspiration pneumonia, periodontitis, intra-abdominal infection, gynaecological infection

Fusobacterium spp.

Aspiration pneumonia

Porphyromonas spp.



Site of infection




Metronidazole (with a penicillin)




Metronidazole (with a penicillin), chloramphenicol

Upper respiratory tract


Chloramphenicol, metronidazole


Clindamycin, cefoxitin or metronidazole (all with an anti-aerobic bacilli agent)

Carbapenem, penicillin plus b lactamase inhibitor


Cefoxitin (plus doxycycline), clindamycin (with an anti-aerobic bacilli agent),

Penicillin plus b lactamase inhibitor, metronidazole (plus doxycycline)


Clindamycin or cefoxitin

Metronidazole (with a penicillin and an anti-Staphyloccal penicillin)

Bone and joint

Clindamycin or a carbapenem

Chloramphenicol, metronidazole (with a penicillin) or penicillin plus b lactamase inhibitor

Bacteraemia with b lactamase producing organisms

Carbapenem or metronidazole

Cefoxitin or penicillin plus b lactamase inhibitor

Bacteraemia with non b lactamase producing organisms


Clindamycin, metronidazole or cefoxitin



©Charles Gomersall, April, 2014 unless otherwise stated. The author, editor and The Chinese University of Hong Kong take no responsibility for any adverse event resulting from the use of this webpage.
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