Anaerobic gram negative rod. B. fragilis, B. distatonis, B. ovatus, B. thetaiotaomicron (these last three put the d-o-t- in DOT), B. gracilus, B. urolyticus, B. vulgatus.
Bacteroides spp are the most common organism in the colon at 1010–1011 cells per gram of human feces. To put that into perspective, you are 3.7x1013 cells.
B. fragilis is minor anaerobe (30 to 70% of people have it) but a major pathogen from the colon and really lousy teeth.
Usually part of a mixed infection where abscesses are found anywhere in the body. They require a lack of oxygen and/or dead meat to grow.
B. fragilis can cause "marked abdominal pain and nonfebrile inflammatory diarrhea" due to an enterotoxin (PubMed).
Enterotoxigenic Bacteroides may be causal for bowel cancer (too early to be definitive) and Bacteroides in the blood may be associated with an underlying bowel adenocarcinoma (PubMed).
MDR B. fragilis is starting to rear its ugly head (PubMed), resistant to metronidazole, imipenem, piperacillin/tazobactam, clindamycin, cefotetan, ampicillin/sulbactam, and moxifloxacin also was observed..