Bacteroides fragilisRANK: SpeciesTAXONOMY: Bacteroidetes/Chlorobi group -> Bacteroidetes -> Bacteroidia -> Bacteroidales -> Bacteroidaceae -> Bacteroides -> Bacteroides fragilis OVERVIEW: Bacteroides fragilis is an obligately anaerobic, Gram-negative, rod-shaped bacterium. It is part of the normal flora of the human colon and is generally commensal, but can cause infection if displaced into the bloodstream or surrounding tissue following surgery, disease, or trauma. Although the B. fragilis group is the most common species found in clinical specimens, it is the least common Bacteroides present in fecal flora, comprising only 0.5% of the bacteria present in stool. Though it is gram negative, it has an altered LPS and does not cause endotoxic shock. Their pathogenicity partly results from their ability to produce capsular polysaccharide, which is protective against phagocytosis and stimulates abscess formation. Identified as constituent of vaginal microbiome. [PMID:23282177] Preliminary in vivo studies with mice suggest that probiotic therapy with B. fragilis may alleviate some of the behavioral and gastrointestinal symptoms associated with Autism Spectrum Disorder. Preliminary in vivo studies indicate that B. fragilis polysaccharide A (PSA) protects mice against experimental colitis induced by Helicobacter hepaticus. Additional in vivo research with mice has shown PSA intermediates in several markers of a healthy mammalian immune system: the levels of CD4 T cells, the balance of T-helper cells, the presence of well-defined follicular structures in the spleen, and in the inflammatory gut response to pathogens. Further studies are needed to establish if these beneficial effects also occur in the human gastrointestinal tract and immune system. It is possible that Bacteroides fragilis and Bacteroides stercosis act as keystone species in individuals consuming high fat diets due to their ability to convert bile into metabolites that are used by other members of the microbial community [PMID: 25054627] Working with lab cultures and mice, Johns Hopkins scientists have found a strain of Bacteroides fragilis which causes colon inflammation, and increases activity of a gene for the enzyme spermine oxidase in the intestine. These results suggest some strains of B. fragilis may increase the colon's exposure to hydrogen peroxide, contributing to DNA damage and the formation of tumors. Further research is needed to establish if these findings are generalizable to other strains of B. fragilis, and whether the effects observed in mice also occur in humans. A cytotoxic T lymphocyte protein 4 (CTLA4) blockade immunotherapy was shown to depend on particular Bacteroides species (Bacteroides thetaiotaomicron and Bacteroides fragilis). Together, these results indicate that the immune response to specific members of the gut microbiota may help set the stage for cancer treatment. [PMID: 26541610] Polysaccharide A (PSA) produced by Bacteroides fragilis protects animals from experimental colitis induced by Helicobacter hepaticus, a commensal bacterium with pathogenic potential. PSA can even cure established experimental colitis in animals by promoting the development of IL-10 producing regulatory T cells. High amounts of Bacteroides fragilis increase the risk of obesity development. [PMID: 18326589 ] This species has been identified as a resident in the human gastrointestinal tract based on the phylogenetic framework of its small subunit ribosomal RNA gene sequences.[PMC 4262072]
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