Felis ISSN 2398-2950

Bacteroides fragilis

Synonym(s): B. fragilis

Contributor(s): Richard Walker




  • Family: Bacteroidaceae.
  • Genus: Bacteroides.
  • Species: fragilis.


  • Gr: bacterum - staff or rod; idus - form or shape.
  • L: fragilis - fragile (referring to the brittle colonies that may form).

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Clinical Effects



  • Part of normal flora of gastrointestinal tract and mucous membranes.


  • Multiples in anaerobic environments such as devitalized tissue.
  • Does not form spores.


  • Opportunists - invade skin, mucosa and underlying tissues following damage.

Pathological effects

  • Trauma, necrosis and ischemia   →   decreased redox potential   →   multiplication of B. fragilis and possibly   →   focal multiplication of facultative anaerobes.

Other Host Effects

  • Part of the normal flora of gastrointestinal tract and mucous membranes.


Control via chemotherapies

  • Some strains are resistant to penicillin and some cephalosporins.
  • Clavulanic acid - amoxicillin Amoxicillin or metronidazole Metronidazole usually effective.
    All obligate anaerobes are resistant to aminoglycosides.
  • Metronidazole Metronidazole  active against most B. fragilis.


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Further Reading


Refereed papers

  • Recent references from VetMed Resource and PubMed.
  • Rocha E R & Smith C J (1998) Characterization of peroxide-resistant mutant of the anaerobic Bacterium bacteroides fragilis. J Bacteriol 180(22), 5906-5912.
  • Jang S S, Brecher J E, Dabaco L A & Hirsh D C (1997) Organisms isolated from dogs and cats with anaerobic infections and susceptibility to selected antimicrobial agents. JAVMA 210(11), 1610-1614.
  • Alexander C J, Citron D M, Gerardo S H, Claros M C, Talan D & Goldstein E J (1997) Characterization of saccharolytic Bacteroides and Prevotella isolated from infected dog and cat bite wounds in humans. J Clin Microbiol 35(2), 406-411.
  • Collings S & Love D N (1992) Further studies on some physical and biochemical characteristics of asaccharolytic pigmented Bacteroides of feline origin. J Appl Bacteriol 72(6), 529-535.
  • Love D N, Vekselstein R & Collings S (1990) The obligate and facultatively anaerobic bacterial flora of the normal feline gingival margin. Vet Microbiol 22(2-3), 267-275.
  • Clemetson L L & Ward A C (1990) Bacterial flora of the vagina and uterus of healthy cats. JAVMA 196(6), 902-906.
  • Mallonee D H, Harvey C E, Venner M & Hammond B F (1988) Bacteriology of periodontal disease in the cat. Arch Oral Biol 33(9), 677-683.
  • Dow S W & Jones R L (1987) Anaerobic infections Part 1 - Pathogenesis and clinical significance. Comp Cont Ed Pract Vet 9, 711.
  • Dow S W et al(1986) Anaerobic bacterial infections and response to therapy in dogs and cats - 36 cases (1983-1985). JAVMA 189, 930.
  • Hirsch D C et al(1985) Changes in prevalence and susceptibility of obligate anaerobes in clinical veterinary practice. JAVMA 186, 1086.