Bacteroides is a genus of Gram-negative, bacillus bacteria. Bacteroides species are non-endospore-forming, anaerobes, and may be either motile or non-motile, depending on the species. The DNA base composition is 40-48% GC. Unusual in bacterial organisms, Bacteroides membranes contain sphingolipids. They also contain meso-diaminopimelic acid in their peptidoglycan layer.
Bacteroides are normally mutualistic, making up the most substantial portion of the mammalian gastrointestinal flora, where they play a fundamental role in processing of complex molecules to simpler ones in the host intestine. As many as 1010-1011 cells per gram of human feces have been reported. They can use simple sugars when available; however, the main sources of energy for Bacteroides species in the gut are complex host-derived and plant glycans .
One of the most important clinically is Bacteroides fragilis.
Bacteroides melaninogenicus has recently been reclassified and split into Prevotella melaninogenica and Prevotella intermedia.