A species of obligately anaerobic, Gram-negative, rod shaped bacteria assigned to the phylum Bacteroidetes. This species is nonmotile, non-spore forming, indole positive, produces hydrogen sulfide, does not reduce nitrate, hydrolyzes esculin and starch, does not ferment trehalose, melezitose or arabinose, and is penicillin resistant. B. fragilis is commensal and part of the normal human gastrointestinal tract flora, but may become pathogenic in other body regions.

Any bacterial organism that can be assigned to the species Bacteroides fragilis.

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.