Bacteroides spp are non-spore forming gram-negative bacilli that are part of the human resident flora. The Bacteroidesfragilis group consists of ten species:
B. fragilis (the most frequent isolate),
B. distasonis,
B.thetaiotaomicron,
B. vulgatus,
B. ovatus,
B. eggerrthii,
B. merdae,
B. stercoris, B. uniformis, and
B. caccae.
Some species are beneficial to humans as they crowd out potential pathogens. They form a considerable part of the normal human flora. As such many strains are opportunistic human pathogens, and can cause infections in several parts of the body including the peritoneal cavity and the female urogenital tract.