Bacteroides are gram-negative, nonsporeforming, anaerobic, and rod-shaped bacteria. They have an outer membrane, a peptidoglycan layer, and a cytoplasmic membrane. They are commonly found in the human intestine where they have a symbiotic host-bacterial relationship with humans. They assist in breaking down food and producing valuable nutrients and energy that the body needs. However, when Bacteriodes are introduced to parts of the body other than the gastrointestinal area, they can cause or exacerbate abscesses and other infections.

Bacteroides are involved in many important metabolic activities in the human colon including fermentation of carbohydrates, utilization of nitrogenous substances, and biotransformation of bile acids and other steroids. Most intestinal bacteria are saccharolytic, which means that they obtain carbon and energy by hydrolysis of carbohydrate molecules. It is estimated that only about 2% of simple sugars make it past the upper gastrointestinal tract and to the Bacteroides. Thus, simple sugars are probably not Bacteroides main source of energy. However, polysaccharides from plant fibers, such as cellulose, xylan, arabinogalactan, and pectin, and vegetable starches such as amylose and amylopectin, are much more prevalent in the colon. These polysaccharides have also been shown to induce a variety of glucosidase activities from Bacteroides including a b-1,3-glucosidase activity responsible for laminarin degradation and a variety of a and b-1,4 and -1,6 xylosidase and glucosidase activities. A large part of the Bacteroides 4779-member proteome includes proteins that hydrolyze these polysaccharides (Jian et al. 2003). The ability to convert complex polysaccharides into useable compounds might allow Bacteroides to be more competitive than bacteria that must rely on other sources of energy. Furthermore, Bacteroides actually stimulates the gut lining to produce fucosylated glycans. The bacteria also stimulate angiogenesis (formation of blood vessels) in the newborn epithelium, enhancing human uptake of nutrients. Thus, Bacteroides bacteria have a complex and generally beneficial relationship with their host--so long as they are retained within the gut lumen.

When Bacteroides escape the gut, they are responsible for many types of infections and abscesses that can occur all over the body including the central nervous system, the head, the neck, the chest, the abdomen, the pelvis, the skin, and the soft tissues. The widely accepted model for abdominal infections goes as follows: disruptions of the intestinal wall, bacterial flora infiltrate the cavity, aerobes (most active part in infection) like E. coli start the preliminary tissue destruction and reduces the oxidation-reduction potential of the oxygenated tissue (low oxidation-reduction potential favors anaerobe growth), anaerobic Bacteroides start to replicate, and then Bacteroides dominate the infection. Along with diarrhea and abscesses, Bacteroides have been known to be involved in cases of meningitis and shunt infections, especially in children. These bacteria are prevalent in the gastrointestinal area and are responsible for most intra-abdominal infections, such as perirectal abscesses and decubitus ulcers. In addition, Bacteroides present a huge problem as a source of infection during gastrointestinal surgeries.

• PMID 15933020

• PMID 12663928

Bacteroides are found in colon. They help their host digest otherwise indigestible carbohydrates.

• Johnson, E. L., Heaver, S. L., Walters, W. A. & Ley, R. E. Microbiome and metabolic disease: revisiting the bacterial phylum Bacteroidetes. J. Mol. Med. 95, 1–8 (2017).