Skip to Main Content

Anaerobic bacteria comprise the majority of normal human flora. Normal microbial flora of the mouth (anaerobic spirochetes, Prevotella, fusobacteria), the skin (anaerobic diphtheroids), the large bowel (bacteroides, anaerobic streptococci, clostridia), and the female genitourinary tract (bacteroides, anaerobic streptococci, fusobacteria) produce disease when displaced from their normal sites into tissues or closed body spaces.

Anaerobic infections tend to be polymicrobial and abscesses are common. Pus and infected tissue often are malodorous. Septic thrombophlebitis and metastatic infection are frequent and may require incision and drainage. Diminished blood supply that favors proliferation of anaerobes because of reduced tissue oxygenation may interfere with the delivery of antimicrobials to the site of anaerobic infection. Cultures, unless carefully collected under anaerobic conditions, may yield negative results.

Important types of infections that are most commonly caused by anaerobic organisms are listed below. Treatment of all these infections consists of surgical exploration and judicious excision in conjunction with administration of antimicrobial drugs.


Prevotella melaninogenica (formerly Bacteroides melaninogenicus) and anaerobic spirochetes are commonly involved in periodontal infections. These organisms, fusobacteria, and peptostreptococci may cause chronic sinusitis, peritonsillar abscess, chronic otitis media, and mastoiditis. F necrophorum has been recognized as a common cause of pharyngitis in adolescents and young adults. F necrophorum infection has been associated with septic internal jugular thrombophlebitis (Lemierre syndrome) and causes septic pulmonary embolization. Hygiene, drainage, and surgical debridement are as important in treatment as antimicrobials. Penicillin alone is inadequate treatment for infections from oral anaerobic organisms because of penicillin resistance, usually due to beta-lactamase production. Therefore, ampicillin/sulbactam 1.5–3 g intravenously every 6 hours (if parenteral therapy is required), or amoxicillin/clavulanic acid 875 mg/125 mg orally twice daily, or clindamycin can be used (600 mg intravenously every 8 hours or 300 mg orally every 6 hours). Antimicrobial treatment is continued for a few days after symptoms and signs of infection have resolved. Indolent, established infections (eg, mastoiditis or osteomyelitis) may require prolonged courses of therapy, eg, 4–6 weeks or longer.


Usually in the setting of poor oral hygiene and periodontal disease, aspiration of saliva (which contains 108 anaerobic organisms per milliliter in addition to aerobes) may lead to necrotizing pneumonia, lung abscess, and empyema. Polymicrobial infection is the rule, and anaerobes—particularly P melaninogenica, fusobacteria, and peptostreptococci—are common etiologic agents. Most pulmonary infections respond to antimicrobial therapy alone. Percutaneous chest tube or surgical drainage is indicated for empyema.

Clindamycin, 600 mg intravenously once, followed by 300 mg orally every 6–8 hours, is the treatment of choice for these infections. Metronidazole does not cover facultative streptococci, which often are present, and if used, a second agent that is active against streptococci, such as ceftriaxone 1 g intravenously or intramuscularly daily, should be added. Penicillin, 2 million units intravenously every 4 hours, followed by amoxicillin, 500 mg orally every ...

Pop-up div Successfully Displayed

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.