Anaerobic Infections

Anaerobic Infections 

An anaerobic infection is caused by one of a group of bacteria that requires a reduced oxygen tension for growth.

Physical Findings & Clinical Presentation

  • •May occur at any site, but most are anatomically related to mucosal surfaces
  • •Should be suspected when there is foul-smelling tissue, soft tissue gas, necrotic tissue, or abscesses
  • •Head and neck:
    • 1.Odontogenic infections from dental or soft tissue possibly progressing to periapical abscesses, at times extending to bone
    • 2.Both anaerobic and aerobic pathogens in chronic sinusitis, chronic mastoiditis, peritonsillar abscess, and chronic otitis media
    • 3.Complications: Deep neck space infections, brain abscesses, mediastinitis
    • 4.Specific examples of anaerobic infections in head and neck:
      • a.Ludwig angina: Bilateral infection of sublingual and submandibular spaces that causes swelling of the base of the tongue with potential airway compromise. Usually mixed aerobic and anaerobic flora
      • b.Lemierre syndrome: Jugular vein suppurative thrombophlebitis caused by anaerobic bacteria: Fusobacterium necrophorum
  • •Pleuropulmonary:
    • 1.May involve anaerobes present in the oropharynx
    • 2.Aspiration more common in persons with altered mental status or seizures
    • 3.Anaerobic bacteria more likely in those with gingivitis or periodontitis
    • 4.Manifestations: Necrotizing pneumonia, empyema, lung abscess
  • •Intraabdominal:
    • 1.Disruption of intestinal integrity leading to infection involving anaerobic bacteria
    • 2.Bacteria from colonic neoplasm, perforated appendicitis, diverticulitis, or bowel surgery, causing bacteremia, peritonitis, at times intraabdominal abscesses
    • 3.Resulting infections usually mixed, containing both anaerobes and aerobes
  • •Female genital tract:
    • 1.Anaerobes in bacterial vaginosis, salpingitis, endometritis, pelvic abscesses, septic abortion; infections tend to be mixed
    • 2.Possible pelvic thrombophlebitis when resolving pelvic infection is accompanied by new or persistent fever
  • •Other anaerobic infections:
    • 1.Skin and soft tissue infection at any site
    • 2.More commonly associated infections: Synergistic gangrene, bite wound infections, infected decubitus ulcers
    • 3.Clinical significance of anaerobes in diabetic foot infections unclear
    • 4.Anaerobic bacteremia is not common with source usually intraabdominal, followed by female genital tract, pleuropulmonary, and head and neck infections
    • 5.Osteomyelitis especially when associated with decubitus ulcers or vascular insufficiency
    • 6.Facial bone osteomyelitis from adjacent infections of the teeth or sinuses


  • •Most commonly endogenous, arising from bacteria that normally line mucosal surfaces
  • •Disruption of mucosal barriers resulting from various conditions (trauma, ischemia, surgery, perforation), with infection occurring when organisms gain access to normally sterile sites, causing tissue destruction and abscess formation
  • •Synergy between different anaerobes or between anaerobes and aerobes important
  • •Examples of anaerobic bacteria include gram-negative bacteria such as Bacteroides species, Fusobacterium and Prevotella species; and gram-positive bacteria such as Peptostreptococcus, Clostridium species, Finegoldia magna, and Actinomyces species


Differential Diagnosis

  • •Primary differential possibility is an aerobic bacterial infection without the presence of anaerobic bacteria.
  • •Ischemic necrosis without accompanying anaerobic infection (or “dry” gangrene [noninfected necrosis] vs. “wet” gangrene [infected tissue with anaerobic infection]).


  • •Specimens submitted for anaerobic culture should be processed within 30 min and may take up to 5 to 7 days to grow
  • •Large volume of material more likely to have significant growth; swabs less efficient for transporting infected material
  • •Blood cultures—preferably before antibiotic administration

Laboratory Tests

  • •Elevated white blood cell (WBC) count, with extremely high WBC counts sometimes seen with pseudomembranous colitis
  • •Positive stool C. difficile by polymerase chain reaction (PCR) or nucleic acid amplification test (NAAT)
  • •Increased lactate levels in ischemia or perforation
  • •Possible positive blood or wound cultures, but failure to grow anaerobes in culture may be common, attributed to inadequate culturing techniques or fastidious organisms

Imaging Studies

  • •Plain film of an affected area to show gas in tissues, free air resulting from a perforated viscus, or an air/fluid level inside an abscess
  • •Ultrasound, computed tomography (CT) scan, or MRI to reveal abscesses or tissue destruction


Nonpharmacologic Therapy

  • •Removal of necrotic tissue
  • •Drainage of abscesses (accomplished by CT scan–guided percutaneous drainage)

Acute General Rx

Oral antibiotics with anaerobic activity: Clindamycin, metronidazole, and chloramphenicol:

  • •Broader spectrum of activity with amoxicillin/clavulanate
  • •Penicillin VK in odontogenic infections
  • •Oral vancomycin is now the preferred agent for C. difficile–associated diarrhea, and metronidazole is now considered a second line agent. Use oral vancomycin for subsequent episodes and for pulse or taper regimens of C difficile–associated diarrhea treatment

Parenteral antibiotics for more serious illness:

  • •IV clindamycin, metronidazole, and chloramphenicol
  • •Cephalosporins (with anaerobic or mixed infections coverage): Cefoxitin and cefotetan
  • •Extended-spectrum penicillins (e.g., piperacillin) and combination beta-lactamase plus beta-lactamase inhibitor drugs (e.g., clavulanic acid, sulbactam, tazobactam):
    • 1.Significant anaerobic activity, plus various degrees of broad-spectrum coverage
    • 2.Include ampicillin/sulbactam, ticarcillin/clavulanate, and piperacillin/tazobactam
  • •Imipenem or other carbapenems, such as meropenem, doripenem, or ertapenem, which are broad-spectrum agents with extensive anaerobic activity
  • •Actinomycosis treated with penicillin for 6 to 12 mo
  • •SMX/TMP and fluoroquinolones are generally ineffective, but some newer quinolones (e.g., moxifloxacin) have inhibitory activity against anaerobes


It is essential that all necrotic debris be removed when treating an anaerobic infection or it will recur; follow-up is critically important to ensure resolution of the process.


Refer to a surgeon if drainage is required; infectious disease consultation may be useful in complicated patients or if treatment regimen is failing or slow to respond.

Comments: Resistance rates to antibiotics used to treat anaerobes are low: Piperacillin/tazobactam: 0.5%, ampicillin/sulbactam: 3%. However one study showed a resistance rate of over 50% of Bacteroides fragilis to clindamycin.

Suggested Readings

  • Bartlett J.G.: Anaerobic bacterial infection of the lung. Anaerobe 2012; 18: pp. 235-239.
  • Brook I.: Spectrum and treatment of anaerobic infections. J Infect Chemother 2016; 22: pp. 1-13.
  • Cobo F., et al.: Clinical findings and antimicrobial susceptibility of anaerobic bacteria isolated in bloodstream infections. Antibiotics(Basel) 2020; 9 (6): pp. 345.
  • Nagy E.: Anaerobic infections: update on treatment considerations. Drugs 2010; 70 (7): pp. 841-858.
  • Ogle O.E.: Odontogenic infections. Dent Clin North Am 2017; 61: pp. 235-252.

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