Bacteroides fragilis

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Bacteroides fragilis

Bacteroides fragilis is a pleomorphic Gram-negative bacillus and an obligate anaerobe of the gut.It is involved in 90% of anaerobic peritoneal infections.Thirty to fifty percent of fecal matter is said to be B. fragilis! Bacteroides species are common in the terminal ileum, and prolific in the colon (10-11 organisms per gram). Bacteroides is the most frequent anaerobic pathogen in man (80% of anaerobic infections). Resistance to some antibiotics may be increasing. Infection can occur anywhere in the body, commonly associated with abscess formation. If documented B. fragilis infection is left untreated, the mortality rate is reported to be about 60%.

**Scientific classification**
Kingdom	Bacteria
Phylum	Bacteroidetes
Class	Bacteroidetes
Order	Bacteroidales
Family	Bacteroidaceae
Genus	Bacteroides
Species	B. fragilis
Binomial	Bacteroides fragilis

[edit] Surface Characteristics

It consists of a polysaccharide capsule that is composed of of two fractions : PSI : β-D-Galp(1-->6)β-D-Galp(1-->6)β-D-Galp(1-->6)Residue1-α-D-Glcp [branched to Residue no. 2](1-->2)α-L-Rhap and PSII : β-D-Galp(1-->4)α-D-Glcp [Branched to β-D-Galp(1-->6)](1-->2)α-L-Rhap.The capsule may confer resistance to host defence mechanisms. Fibrin deposition and abscess formation can be stimulated by purified capsule alone. Surprisingly, the capsular antigens may cause more of a T-cell response than an antibody-mediated response. The capsule comprises 'Polysaccharides A and B' with oppositely charged groups that ionically link the two polysaccharides and seem to be important in abscess formation (Similar in charge, but not structure, to the capsular polysaccharide of S. pneumoniae type 1). Remarkably, vaccination with S. pneumoniae polysaccharide prevents intraperitoneal abscess formation subsequent to B. fragilis instillation, and this protection is not conferred if the charged groups are altered on the capsular vaccine.