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." 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.
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. 
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:
- carbapenems (imipenem, meropenem and ertapenem)
- combinations of a penicillin and a beta-lactamase inhibitor (ampicillin or ticarcillin plus clavulanate, amoxicillin plus sulbactam, and piperacillin plus tazobactam)
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