Anaerobes in Clinical Practice
On this page
Direct answer
Foul-smelling pus, gas in the tissues, infection next to a mucosal surface and failure of aminoglycosides — that cluster points to anaerobes, which are killed by oxygen because they lack superoxide dismutase and catalase. Clinically the group divides into spore-forming Clostridium species with their toxin diseases (gas gangrene, tetanus, botulism, pseudomembranous colitis) and the non-sporing anaerobes of the mouth, gut and female genital tract, of which the Bacteroides fragilis group is the most important because it is capsulated and beta-lactamase producing. Specimens must be aspirated, never swabbed, transported in Robertson's cooked meat broth, and metronidazole covers nearly all of them.
What you must remember
- Clostridium perfringens: alpha toxin (lecithinase) causes gas gangrene; identified by the Nagler reaction, in which antitoxin inhibits opacity on egg-yolk agar, and by stormy fermentation of milk.
- Food poisoning timings: C. perfringens type A gives cramps and diarrhoea 8-16 hours after meat meals (no vomiting); staphylococcal poisoning vomits at 2-6 hours; type C causes enteritis necroticans (pigbel).
- Tetanus: drumstick terminal spores; tetanospasmin blocks glycine and GABA release from inhibitory neurons; manage with wound debridement, tetanus immunoglobulin, metronidazole and spasm control.
- Botulism: toxin blocks acetylcholine release, giving descending flaccid paralysis with clear sensorium; infant botulism follows honey ingestion (avoid honey under one year); give antitoxin early.
- Clostridioides difficile: toxins A and B after clindamycin, cephalosporins or fluoroquinolones; diagnose by glutamate dehydrogenase screening plus toxin enzyme immunoassay or nucleic acid amplification; oral vancomycin or fidaxomicin is first-line.
- Bacteroides fragilis: the commonest anaerobe in intra-abdominal sepsis, capsulated (a virulence fact), grows on 20% bile agar and resists penicillin through beta-lactamase; treat with metronidazole, carbapenems or piperacillin-tazobactam.
- Non-sporing anaerobes: Fusobacterium necrophorum causes Lemierre syndrome (post-anginal septic jugular thrombophlebitis); Actinomyces causes draining sinuses with sulphur granules (actually bacterial clumps) after weeks of penicillin.
Approach to suspected clostridial infection
A man steps on a nail while working in a field and 12 hours later has severe thigh pain out of proportion to a small wound, with crepitus and a sweetish odour. This is clostridial myonecrosis until proven otherwise: proceed to wide surgical debridement immediately — surgery is the treatment; antibiotics (penicillin G plus clindamycin) and hyperbaric oxygen are adjuncts, and antitoxin is no longer part of standard care. A Gram film of wound exudate showing rectangular Gram-positive rods with absent spores fits C. perfringens, which rarely sporulates in vivo.
Contrast the chronic presentations. An elderly woman on repeated antibiotics for urinary infection develops watery diarrhoea, fever and a leukocytosis of 18,000; colonoscopy shows yellow pseudomembranes. Test the stool rather than treating empirically forever: a two-step algorithm (glutamate dehydrogenase antigen, then toxin enzyme immunoassay) or nucleic acid amplification confirms C. difficile, oral vancomycin for ten days is first-line, and metronidazole is reserved for situations where the preferred drugs are unavailable — an Indian reality, since fidaxomicin is often cost-prohibitive.
For the non-sporing anaerobes, remember the specimen rule above all: pus aspirated with a syringe, capped with the needle bent or plugged, and sent immediately in Robertson's cooked meat medium; a routine swab in air destroys the diagnosis before the lab opens the box.
The smell is the clue
Anaerobic pus smells characteristically putrid because of short-chain fatty acids, and this bedside sign should change prescribing immediately — add metronidazole, suspect a gutter abscess or a bite wound, and ask the surgeon for a proper specimen. Another exam-worthy nuance: aminoglycosides fail against anaerobes because their uptake across the bacterial membrane requires oxygen-dependent transport, which is why combination regimens for mixed infections always pair the aminoglycoside with an antianaerobic agent.
Frequently asked questions
What is the Nagler reaction?
A plate test in which Clostridium perfringens lecithinase produces opacity on egg-yolk agar on one half of the plate, while the half pretreated with perfringens antitoxin shows inhibition — a specific identification of the alpha toxin.
Which anaerobe is the commonest cause of intra-abdominal sepsis?
The Bacteroides fragilis group, capsulated and beta-lactamase producing, usually in synergy with facultative organisms such as E. coli; it grows on bile-aesculin containing media.
How is C. difficile infection diagnosed and treated?
Glutamate dehydrogenase plus toxin enzyme immunoassay or nucleic acid amplification testing of unformed stool; first-line treatment is oral vancomycin or fidaxomicin, with metronidazole only when these cannot be used.
Why do aminoglycosides not work on anaerobes?
Aminoglycoside uptake requires an oxygen-dependent electron transport system on the bacterial membrane, which obligate anaerobes lack; the drug simply never reaches its ribosomal target.
Which anaerobe causes Lemierre syndrome?
Fusobacterium necrophorum, producing septic thrombophlebitis of the internal jugular vein with septic pulmonary emboli in young adults after a sore throat; treat with metronidazole or a beta-lactam-beta-lactamase inhibitor.