A small share of the gut community that helps ferment carbohydrates, and the anaerobe most often isolated from infections. Explore its illustrative 3D model, human context, and sources.
Wexler’s review describes the human colon as the largest bacterial population in the body, in excess of 10 to the 11th organisms per gram of wet weight, of which about 25% are Bacteroides species. B. fragilis itself accounts for only 0.5% of the colonic flora yet is the most commonly isolated anaerobic pathogen. In the gut, carbohydrate fermentation by Bacteroides produces volatile fatty acids that the host reabsorbs and uses as an energy source. When it escapes the gut it causes abscesses and bacteraemia; PHAC notes that spread to the bloodstream is more common for B. fragilis than for any other anaerobe.
The same organism is a beneficial commensal in the colon and a significant pathogen when the gut barrier is breached. Wexler reports that anaerobic infections are usually polymicrobial, that B. fragilis is found in most of them, and that the associated mortality is more than 19%. PHAC adds that abdominal isolates almost always produce beta-lactamase, which complicates treatment. Presence in stool is normal and is not infection.
Original schematic 3D phenotype. PHAC describes Bacteroides as capsulated, obligately anaerobic, non-spore-forming Gram-negative bacilli, with most species immobile; the motile exceptions it names are other species, so no flagella should be assigned to B. fragilis. A capsule is supported and an endospore is not. Colours, dimensions, layer thicknesses and displayed arrangements are diagram choices, not a measured individual cell. Independent scientific review remains pending.
Source check: 2026-09-19. Educational content; independent scientific review remains pending.