Microbiology · Other clinically important bacteria
Bacteroides fragilis !
- Bacterium
- Gram-negative
- Exam priority: medium
An anaerobe of the colon that, once it escapes through a perforation or surgery, forms foul-smelling abdominal and pelvic abscesses.
Updated 2026-11-01
Also called: mixed anaerobes
At a glance
- Gram
- Gram-negative
- Shape
- Pleomorphic rod
- Oxygen
- Obligate anaerobe (tolerates some oxygen)
- Family
- Bacteroidaceae
- Stain
- Pale Gram-negative rods in pus
Morphology
- Short Gram-negative rod with pale, irregular staining
- Polysaccharide capsule
- Grows on anaerobic media; bile stimulates its growth
Transmission
- Endogenous infection: it comes from the patient's own colonic flora
- Escapes the bowel through perforation, appendicitis, diverticulitis, surgery or abdominal trauma
- Not spread from person to person
Reservoir: Human colon, where it is a small fraction of the normal anaerobes
Isolation precautions
What the care team adds to standard precautions.
- Standard
Virulence factors
- The polysaccharide capsule alone can induce abscess formation
- Its lipopolysaccharide is far less toxic than that of Enterobacterales, so shock is less typical
- Produces beta-lactamases that destroy penicillins and many cephalosporins
- Works in synergy with aerobes: they use up oxygen and create the anaerobic niche
Diseases it causes
- Secondary peritonitis after bowel perforation Peritonitis →
- Intra-abdominal and pelvic abscess Intra-abdominal abscess →
- Anaerobic bacteraemia, with high mortality in older adults and patients with cancer Sepsis and septic shock →
Clinical picture
- Fever and abdominal pain days after surgery or a perforation
- Pus with a faecal odour, gas in tissues and necrosis
- Almost always a mixed infection with Enterobacterales such as Escherichia coli
Diagnosis
- Sample a normally sterile site (pus, peritoneal fluid, blood) and transport it anaerobically
- Foul odour and gas are strong clues to anaerobes
- Anaerobic culture; isolates can take several days
- CT locates the abscess for drainage
Treatment
Abscess drainage and source control, plus antibiotics active against anaerobes: metronidazole, carbapenems or a beta-lactam with a beta-lactamase inhibitor (piperacillin-tazobactam, ampicillin-sulbactam). It resists penicillin and many cephalosporins; clindamycin is no longer reliable.
Drug class only; doses and regimens are set by the clinician from the current guideline.
In Pharmacology:
Prevention
- Antibiotic prophylaxis with anaerobic cover before colorectal surgery
- Prompt surgical care of appendicitis and bowel perforation
Test yourself
Think of the answer, then tap the question to see it.
Which structure of Bacteroides fragilis promotes abscess formation?
The polysaccharide capsule.
Why is a B. fragilis infection usually mixed?
It leaves the bowel with the colonic flora: aerobes consume oxygen and let the anaerobes grow.
Which antibiotics are reliably active against B. fragilis?
Metronidazole, carbapenems and beta-lactam/beta-lactamase inhibitor combinations.
Which physical clue in abscess pus points to anaerobes?
A foul odour, especially together with gas in the tissues.
Related
Related conditions
← Back to the group: Other clinically important bacteria
Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Mixed Anaerobic Infections - Infectious Disease - MSD Manual Professional Edition (MSD Manual Professional Edition, 2025); Jawetz, Melnick & Adelberg's Medical Microbiology, 26th ed., chapter 21: Infections Caused by Anaerobic Bacteria (McGraw-Hill, 2013).
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Representative model, for educational purposes only.