Bacteroides Fragilis

Bacteroides fragilis is a Gram-negative bacillus bacterium species, and an obligate anaerobe of the gut.

B. fragilis group is the most commonly isolated Bacteroidaceae in anaerobic infections, especially those that originate from the gastrointestinal flora. B. fragilis is the most prevalent organism in the B. fragilis group, accounting for 41% to 78% of the isolates of the group. The B. fragilis group is the species of Bacteroidaceae isolated with greatest frequency in clinical specimens. These organisms are resistant to penicillin by virtue of production of beta-lactamase, and by other unknown factors.

This organism was formerly classified as subspecies of B. fragilis (i.e. ssp. fragilis, ssp. distasonis, ss. ovatus, ss. thetaiotaomicron, and ss. vulgatus). They have been reclassified into distinct species on the basis of DNA homology studies. B. fragilis (formerly known as B. fragilis ssp. fragilis, one of the subspecies of B. fragilis) is often recovered from blood, pleural fluid, peritoneal fluid, wounds and brain abscesses.

Although B. fragilis group is the most common species found in clinical specimens, it is the least common Bacteroides present in fecal flora, comprising only 0.5% of the bacteria present in stool. The pathogenicity of this group of organisms probably results from its ability to produce capsular material, which is protective against phagocytosis.