Anaerobic Infections
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Direct answer
Anaerobic infections are caused by bacteria that grow without oxygen, most often in abscesses, aspiration pneumonia and devitalised tissue where mucosal flora spill into sterile sites. Bacteroides fragilis is the classic gram-negative anaerobe, while Clostridium perfringens causes gas gangrene. Foul-smelling pus, gas in tissue and failure to grow organisms on aerobic culture are the main clues, and metronidazole or clindamycin are the usual choices.
What you must remember
- Gram-negative anaerobe: Bacteroides fragilis is the commonest anaerobe in intra-abdominal and pelvic abscesses, while Fusobacterium is the usual cause of Lemierre syndrome.
- Gram-positive anaerobes: Clostridium, Peptostreptococcus and Actinomyces, the last producing sulphur granules in actinomycosis.
- Site clues: dental abscess, lung abscess, aspiration pneumonia, brain abscess following sinusitis or otitis, and pelvic abscess.
- Odour and gas: foul-smelling pus and gas in tissue reflect anaerobic fermentation, and radiology may show gas within the abscess cavity.
- Aspiration: the right lung is favoured because the right main bronchus is more vertical; supine patients aspirate into the posterior segment of the upper lobe or the superior segment of the lower lobe, and upright patients into the basal lower lobe.
- Metronidazole: active against nearly all obligate anaerobes, but poor against Actinomyces and microaerophilic streptococci, and it has no activity against aerobes.
- Clindamycin: good anaerobic coverage including Bacteroides, and it suppresses toxin production in gas gangrene, although resistance is rising and it carries a risk of C difficile colitis.
- Gas gangrene: Clostridium perfringens alpha toxin drives tissue necrosis, and treatment combines urgent debridement with penicillin and clindamycin.
A stepwise approach to a suspected anaerobic abscess
Anaerobes are missed when samples are poor or the culture request is incomplete. The steps below keep the diagnosis and the treatment on track.
- Recognise the setting: think of anaerobes with abscesses near mucosal surfaces, after aspiration, in bite wounds, or in devitalised tissue such as necrotic bowel.
- Collect the right sample: aspirate pus or take a surgical specimen, because swabs from mucosal surfaces grow commensals and tell little. Transport the specimen anaerobically.
- Look for gas and odour: note a foul smell at aspiration and check imaging for gas within the collection.
- Request anaerobic culture: a negative aerobic culture does not exclude anaerobes, so the request should ask for anaerobic culture in mixed infections.
- Drain and debride: surgical drainage matters as much as antibiotics, because pus and necrosis shelter the organisms from drug penetration.
- Choose empirical therapy: metronidazole or clindamycin for pure anaerobic infection, and amoxicillin with a beta-lactamase inhibitor for mixed aerobic and anaerobic infection.
Where the exam tries to catch you
Anaerobe questions usually turn on the sampling error, the drug limitations and the classic site clues.
- Trap: assuming penicillin covers all anaerobes Correct: Bacteroides fragilis is often beta-lactamase producing, so a beta-lactamase inhibitor combination or metronidazole is preferred.
- Trap: using metronidazole for actinomycosis Correct: penicillin is the drug of choice for actinomycosis, and metronidazole has poor activity against Actinomyces.
- Trap: sending a mouth or vaginal swab for anaerobic culture Correct: normal mucosal flora make the result uninterpretable, so aspirate or biopsy material is required.
- Trap: treating tetanus or gas gangrene with antibiotics alone Correct: these need wound debridement, along with tetanus immunoglobulin for tetanus and surgical control for gas gangrene.
Frequently asked questions
Which gram-negative anaerobe is most often isolated from intra-abdominal abscesses?
Bacteroides fragilis is the most often isolated gram-negative anaerobe from intra-abdominal and pelvic abscesses. It is a common member of colonic flora and is often mixed with facultative organisms such as Escherichia coli.
What is the drug of choice for actinomycosis?
Penicillin is the drug of choice for actinomycosis, given intravenously at high dose and then orally for a prolonged course. Actinomyces produces sulphur granules in pus, a feature that points towards this diagnosis.
Why does aspiration pneumonia usually involve the right lung?
The right main bronchus is more vertical and wider than the left, so aspirated material tends to enter the right lung, into the posterior upper-lobe or superior lower-lobe segments when supine and the basal segments when upright. Anaerobes from the oral flora are the usual cause of resulting lung abscesses.
Why is clindamycin useful in gas gangrene?
Clindamycin inhibits bacterial protein synthesis, which suppresses toxin production by Clostridium perfringens even as the organism is killed. It is usually combined with penicillin, and urgent surgical debridement remains essential.
Why must anaerobic culture be requested in a mixed infection?
Standard aerobic culture cannot grow obligate anaerobes, so a negative result does not exclude an anaerobic component. Requesting anaerobic culture on a properly collected aspirate avoids missing organisms that drive abscess formation.