ISSN 2398-2977      

Bacteroides fragilis





  • Obligate anaerobic, Gram-negative, rod-shaped bacteria within the Bacteroidaceae family.
  • Kingdom: Bacteria.
  • Phylum: Bacteroidetes.
  • Class: Bacteroidetes.
  • Order: Bacteroidales.
  • Family: Bacteroidaceae.
  • Genus: Bacteroides.
  • Species: fragilis.

Active Forms

This article is available in full to registered subscribers

Sign up now to obtain ten tokens to view any ten Vetlexicon articles, images, sounds or videos, or Login

Clinical Effects

Pathological effects

  • B. fragilis is the most commonly isolated Bacteroidaceae in clinical specimens but is one of the least common in fecal flora. Their increased pathogenicity is partly a result of their ability to produce a capsule containing two specific polysaccharides, which gives increased protection against phagocytosis within the body, and also seem to stimulate abscess formation. There may also be synergism with other non-encapsulated bacteria, leading to enhanced survival of the latter and increased pathogenicity.
  • B. fragilis is the most commonly isolated anaerobic bacteria isolated from lower respiratory tract infections, including bacterial pneumonia Lung: pnuemonia - bacterial, pleuritis Lung: pleuropneumonia - bacterial (pleuritis), and lung abscesses Lung: abscess, often in the presence of a variety of other aerobic and facultatively anaerobic bacteria. There may be synergy between the different organisms, including protection from phagocytosis, production of essential growth factors, and lowering of local oxygen concentrations.
  • Obligate anaerobic bacteria are the most frequently isolated bacteria from cases of equine peritonitis Abdomen: peritonitis, and B. fragilis was found in 10-20% of cases in one survey. Mixed infections of Gram-negative aerobic and anaerobic infections are common and there is evidence of synergism between the various bacteria. The presence of B. fragilis infection tends to result in chronic abscessation often with a slower chronic progression to the disease.
  • The role of anerobic bacteria in equine endometritis Uterus: endometritis - bacterial is not always clear but they are commonly isolated alone or in mixed populations. B. fragilis is the most commonly isolated of the anaerobic bacteria. Samples taken post partum, and especially at the foal heat, are the most likely to lead to anaerobe bacterial culture.
  • There are occasional reports in the equine scientific literature of culture B. fragilis from cases of septicemia Foal: neonatal septicemia syndrome, cholangiohepatitis Liver: cholangiohepatitis and cholelithiasis, ulcerative keratitis Keratitis: traumatic - ulcerative, and peri-oral/endodontical/apical molar dental infections Teeth: dental disease - overview.
  • Enterotoxigenic strains of B. fragilis have been associated with the incidence of diarrhea in man and domestic animals. In one survey, strains were isolated from the feces of 25% of Thoroughbred foals of up to 7 days old which were affected with naturally occurring diarrhea. Up to 10% of these foals died. Other infectious agents were also isolated in some of these cases.


Control via chemotherapies

  • In general, B. fragilis is susceptible to metronidazole Metronidazole at 15-25 mg/kg PO BID and beta lactam/beta-lactamase inhibitor Therapeutics: beta-lactam antibacterials combinations.
  • It is inherently resistant to penicillin, ampicillin/amoxicillin, and first generation cephalosporins due to the production of beta lactamase.
  • There has been increased evidence in recent years of resistance, particularly in human medicine.


This article is available in full to registered subscribers

Sign up now to obtain ten tokens to view any ten Vetlexicon articles, images, sounds or videos, or Login

Further Reading


Refereed Papers

  • Recent references from PubMed and VetMedResource.
  • Myers L L et al (1987) Diarrhea associated with enterotoxigenic B. fragilis in foals. Am J Vet Res 48 (11), 1565 PubMed.

Other sources of information

  • Sellon D C & Long M (2014) Eds. Equine Infections Disease. Elsevier, USA.
  • van der Kolk J H & Veldhuis Kroeze E J B (2013) Infectious Diseases in the Horse: Diagnosis, Pathology, Management, and Public Health. CRC Press.
  • McVey D S, Kennedy M & Chengappa M M (2013) Eds Veterinary Microbiology. 3rd edn. Wiley-Blackwell.

Can’t find what you’re looking for?

We have an ever growing content library on Vetlexicon so if you ever find we haven't covered something that you need please fill in the form below and let us know!


To show you are not a Bot please can you enter the number showing adjacent to this field

 Security code