Bacteroides fragilis
Bacteroides fragilis is a Gram-negative bacillus bacterium species, and an obligate anaerobe
Obligate anaerobe
Obligate anaerobes are microorganisms that live and grow in the absence of molecular oxygen; some of these are killed by oxygen. -Metabolism:...

 of the gut.

B. fragilis group is the most commonly isolated bacteriodaceae in anaerobic infections
Anaerobic infection
Anaerobic infections are caused by anaerobic bacteria. Anaerobic bacteria do not grow on solid media in room air ; facultative anaerobic bacteria can grow in the presence as well as in the absence of air. Microaerophilic bacteria do not grow at all aerobically or grow poorly, but grow better under...

 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 that is 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. ss. fragilis, ss. 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 ss. fragilis, one of the subspecies of B. fragilis) is the anaerobe most frequently isolated from infections.

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.

Clinical significance

It is involved in 90% of anaerobic peritoneal infections.

Bacteroides fragilis 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 burn
A burn is an injury to flesh caused by heat, electricity, chemicals, light, radiation, or friction.Burn may also refer to:*Combustion*Burn , type of watercourses so named in Scotland and north-eastern England...

s near the anus.

Working with lab cultures and mice, Johns Hopkins scientists have found that a strain of the common gut pathogen Bacteroides fragilis causes colon inflammation and increases activity of a gene called spermine oxidase (SMO) 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
Metronidazole is a nitroimidazole antibiotic medication used particularly for anaerobic bacteria and protozoa. Metronidazole is an antibiotic, amebicide, and antiprotozoal....

, carbapenems, tigecycline
Tigecycline is a glycylcycline antibiotic developed by Francis Tally and marketed by Wyeth under the brand name Tygacil. It was given a U.S. Food and Drug Administration fast-track approval and was approved on June 17, 2005...

, beta-lactam/beta-lactamase inhibitor combinations (e.g., Unasyn, Zosyn), and certain antimicrobials of the cephamycin class, including cefoxitin
Cefoxitin is a cephamycin antibiotic developed by Merck & Co., Inc., often grouped with the second−generation cephalosporins. It is sold under the brand name Mefoxin.- Microbiology :...

. The bacteria have inherent high-level resistance to penicillin
Penicillin is a group of antibiotics derived from Penicillium fungi. They include penicillin G, procaine penicillin, benzathine penicillin, and penicillin V....

. Production of beta lactamase appears to be the main mechanism of antibiotic resistance in B. fragilis. Clindamycin
Clindamycin rINN is a lincosamide antibiotic. It is usually used to treat infections with anaerobic bacteria but can also be used to treat some protozoal diseases, such as malaria...

 is no longer recommended as the first-line agent for B. fragilis due to emerging high-level resistance (>30% in some reports).

Other use

Bacteriophages infecting B. fragilis are commonly used as tracers of human faecal material. See work undertaken by the University of Barcelona and EPHRU (Environment and Public Health Research Unit) at the University of Brighton.

Polysaccharide A (PSA) from this bacteria is reported to be involved in the protection of experimental colitis induced by Helicobacter hepaticus. Further research into B. Fragilis PSA has shown that 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's also used for separating carbohydrate groups that classifies the group 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.

External links

  • Bacteroides references in Baron's Medical Microbiology (online at the NCBI
    National Center for Biotechnology Information
    The National Center for Biotechnology Information is part of the United States National Library of Medicine , a branch of the National Institutes of Health. The NCBI is located in Bethesda, Maryland and was founded in 1988 through legislation sponsored by Senator Claude Pepper...

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