Bacteroides are thin, pleomorphic, gram-negative bacilli that are nonmotile and nonsporulating.
All are obligate anaerobes, but the dominant pathogen, B. fra gilis, is relatively aerotolerant and is able to survive in the presence of oxygen for up to 8 hours. Bacteroides species are present in large numbers as part of the normal human flora, principally in the oropharynx and colon. Many Bacteroides species can participate in mixed infections that arise from insults to mucosal integrity, and like other anaerobes, they can infect devitalized tissues, producing further necrosis, abscess formation, and gas. Although other members of the group, such as B. ovatus, B. thetaiotaomicron , B. distasonis , and B. vulgatus , are much more numerous in human feces, B. fragilis is far more likely to cause human disease. The virulence of B. fragilis is explained by several factors.