Bacteroides fragilis

Bacteroides fragilis is a Gram-negative, anaerobic bacterium commonly found in the human intestine. It can cause abscesses and other infections when it enters normally sterile body sites.

Polysaccharide capsule: A bacterial surface layer made of repeating sugar units that can protect cells and shape host immune responses. Its capsule promotes abscess formation and helps B. fragilis evade immune clearance.
Intra-abdominal abscess: A localized collection of pus within the abdominal cavity or an abdominal organ. B. fragilis is a frequent anaerobic isolate from infections arising after intestinal leakage.
Gram-negative bacteria: Bacteria whose cell envelopes have a thin peptidoglycan layer and an outer membrane. B. fragilis has the outer membrane and staining behavior characteristic of this group.
Bacteroides thetaiotaomicron: A prominent gut Bacteroides species known for metabolizing complex dietary carbohydrates. It shares the intestinal niche but is less strongly associated with abscess-forming infections.
Bacteroides fragilis toxin: A secreted metalloprotease toxin produced by enterotoxigenic strains of B. fragilis. This toxin disrupts intestinal epithelial barriers and is linked to diarrheal disease.
Bacteremia: The presence of bacteria in the bloodstream. Bloodstream infection can follow translocation of B. fragilis from damaged intestinal sites.
Anaerobic organism: An organism that grows without requiring oxygen; some are harmed by oxygen exposure. B. fragilis is an obligate anaerobe adapted to oxygen-poor sites.
Parabacteroides distasonis: An anaerobic gut bacterium formerly classified within the genus Bacteroides. Its reclassification shows why older clinical reports may use a different genus name.
Anaerobic respiration: Energy generation using an electron acceptor other than oxygen, or using fermentation when respiration is unavailable. B. fragilis survives in oxygen-poor intestinal habitats through anaerobic energy metabolism.
Antibiotic susceptibility testing: Laboratory methods that determine whether a microorganism is inhibited by particular antimicrobial drugs. Testing can guide therapy because B. fragilis resistance patterns vary between isolates.

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BacteroidotaBroader topic: It illustrates how a common gut resident can act as a pathogen outside its usual niche.