Bacteroides is a genus of Gram-negative, obligate anaerobic bacteria. Bacteroides species are non endospore-forming bacilli. Bacteroides species are normally mutualistic, making up the most substantial portion of the mammalian gastrointestinal microbiota, where they play a fundamental role in processing of complex molecules to simpler ones in the host intestine.
Bacteroides, has been suggested because they make up a significant portion of the fecal bacterial population. One of the most important clinically is Bacteroides fragilis.