Explore the recorded discovery and history of Bacteroides fragilis, with source links and stated limits.
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.
LPSN records Bacillus fragilis Veillon and Zuber 1898 as the basonym for this species.
LPSN records the current name as Castellani and Chalmers 1919, from their Manual of Tropical Medicine, on the Approved Lists of 1980, with type strain ATCC 25285 / NCTC 9343.
This profile describes a normal member of the human colonic community. It does not identify a first human case; infection follows displacement from the gut rather than acquisition from another person.
Source check: 2026-09-19. Educational content; independent scientific review remains pending.