[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Microbiology](https://durgical.com/en/microbiology/) › [Other clinically important bacteria](https://durgical.com/en/microbiology/groups/other-clinically-important-bacteria/) › Bacteroides fragilis [Microbiology](https://durgical.com/en/microbiology/) · [Other clinically important bacteria](https://durgical.com/en/microbiology/groups/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 ## Key points - The Gram-negative anaerobe most often isolated from intra-abdominal infections. - The polysaccharide capsule is the abscess-forming factor. - Clinical clue: foul pus, gas and a mixed infection after perforation or surgery. - Cover anaerobes with metronidazole, a carbapenem or a beta-lactam/inhibitor; without drainage it does not heal. ## 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 →](https://durgical.com/en/conditions/peritonitis/) - Intra-abdominal and pelvic abscess [Intra-abdominal abscess →](https://durgical.com/en/conditions/intra-abdominal-abscess/) - Anaerobic bacteraemia, with high mortality in older adults and patients with cancer [Sepsis and septic shock →](https://durgical.com/en/conditions/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: - [Metronidazole](https://durgical.com/en/pharmacology/metronidazole/) - [Meropenem](https://durgical.com/en/pharmacology/meropenem/) - [Piperacillin + tazobactam](https://durgical.com/en/pharmacology/piperacillin-tazobactam/) ## Prevention - Antibiotic prophylaxis with anaerobic cover before colorectal surgery - Prompt surgical care of appendicitis and bowel perforation ## Memory aid "Fragilis is far from fragile": it survives outside the colon, hides in its capsule and builds an abscess that smells of where it came from. ## 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. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/microbiology/bacteroides-fragilis/) ## Related - [Escherichia coli](https://durgical.com/en/microbiology/escherichia-coli/) - [Clostridioides difficile](https://durgical.com/en/microbiology/clostridioides-difficile/) ## Related conditions - [Peritonitis](https://durgical.com/en/conditions/peritonitis/) - [Intra-abdominal abscess](https://durgical.com/en/conditions/intra-abdominal-abscess/) - [Sepsis and septic shock](https://durgical.com/en/conditions/sepsis-and-septic-shock/) ## What the colours mean **Gram-positive** Thick peptidoglycan wall: stains purple. **Gram-negative** Thin wall with an outer membrane: stains pink. **Acid-fast** Lipid-rich wall (mycobacteria): Ziehl–Neelsen stain. [← Back to the group: Other clinically important bacteria](https://durgical.com/en/microbiology/groups/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](https://www.msdmanuals.com/professional/infectious-diseases/anaerobic-bacteria/mixed-anaerobic-infections) (MSD Manual Professional Edition, 2025); Jawetz, Melnick & Adelberg's Medical Microbiology, 26th ed., chapter 21: Infections Caused by Anaerobic Bacteria (McGraw-Hill, 2013). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/microbiology/bacteroides-fragilis/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/microbiology/bacteroides-fragilis/