Bacteroides fragilis

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In microbiology, Bacteroides fragilis is a "gram-negative bacteria occurring in the lower intestinal tracts of man and other animals. It is the most common species of anaerobic bacteria isolated from human soft tissue infections."[1] They are variously described as part of the anaerobic gram negative bacilli (AGNB) or Bacteroidaceae family. While B. fragilis is most common clinically, other closely related pathogens include includes B. distasonis, B. ovatus, B.s thetaiotaomicron, and Ba. vulgatus.[2]


Bacteroides commonly present on mucous membranes, becoming part of infections after the tissue is damaged. Such infections often involve multiple aerobic and anaerobic species. Specialized culture techniques, however, are needed to confirm Bacteroides and they may be ignored. [3]

Clinical manifestations

B frag may cause bacteremia [4][5][6] and soft tissue infections.


Certainly in soft tissue infection, and where a source for bacteremia can be found, surgical drainage is important. Both due to the varying sensitivities of Bacteroides, and the likelihood of mixed-flora infections, therapy with multiple antibiotics is usually desirable. Pending the availability of specific culture and sensitivity, reasonable approaches for the anaerobes alone include:[3]


  1. Anonymous (2021), Bacteroides fragilis (English). Medical Subject Headings. U.S. National Library of Medicine.
  2. Brook I (29 October 2009), Bacteroides Infection, eMedicine
  3. 3.0 3.1 Brook I (2007), "Treatment of anaerobic infection (abstract)", Expert Rev Anti Infect Ther 5 (6): 991-1006
  4. Redondo MC, Arbo MD, Grindlinger J, Snydman DR (1995). "Attributable mortality of bacteremia associated with the Bacteroides fragilis group.". Clin Infect Dis 20 (6): 1492-6. PMID 7548498.
  5. Hung MN, Chen SY, Wang JL, Chang SC, Hsueh PR, Liao CH et al. (2005). "Community-acquired anaerobic bacteremia in adults: one-year experience in a medical center.". J Microbiol Immunol Infect 38 (6): 436-43. PMID 16341345.
  6. Cheng CW, Lin HS, Ye JJ, Yang CC, Chiang PC, Wu TS et al. (2009). "Clinical significance of and outcomes for Bacteroides fragilis bacteremia.". J Microbiol Immunol Infect 42 (3): 243-50. PMID 19812858.