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.
It acts primarily at the surface of the mucosa. It predominates in bacteremia associated with intraabdominal infections, peritonitis and abscesses following rupture of viscus, and subcutaneous abscesses or burns near the anus.
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Working with lab cultures and mice, Johns Hopkins scientists have found a strain of the common gut pathogen Bacteroides fragilis causes colon inflammation and increases activity of a gene for the enzyme superoxide dismutase in the intestine. The effect is to expose the gut to hydrogen peroxide – the caustic, germ-fighting substance found in many medicine cabinets—and cause DNA damage, contributing to the formation of colon tumors, say the scientists.
In general, B. fragilis is susceptible to metronidazole, carbapenems, tigecycline, beta-lactam/beta-lactamase inhibitor combinations (e.g., Unasyn, Zosyn), and certain antimicrobials of the cephamycin class, including cefoxitin. The bacteria have inherent high-level resistance to penicillin. Production of beta lactamase appears to be the main mechanism of antibiotic resistance in B. fragilis. Clindamycin is no longer recommended as the first-line agent for B. fragilis due to emerging high-level resistance (>30% in some reports).
Bacteriophages infecting B. fragilis are commonly used as tracers of human faecal material.
Polysaccharide A (PSA) from these bacteria is reported to be involved in the protection of experimental colitis induced by Helicobacter hepaticus. Further research into B. fragilis PSA has shown it intermediates in several markers of a healthy mammalian immune system: the levels of CD4 T cells, the balance of T-helper cytokines, the presence of well-defined follicular structures in the spleen, and in the inflammatory gut response to pathogens. It is also used for separating carbohydrate groups that classify the group type of the blood cells. The enzyme GalNAC-ase cleaves the A blood cells into O type blood cells, which gives opportunity to produce universal blood units.
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