Encapsulation and protection against phagocytosis by Bacteroides fragilis.

The capsular polysaccharide of Bacteroides fragilis is most probably an important virulence factor as shown by its ability to protect against phagocytosis in in vitro studies, and by its abscess-promoting ability in experimental in vivo studies. Immunity against the capsular polysaccharide, either in the form of humoral antibodies or as cell-mediated immunity, can facilitate phagocytosis or prevent abscess formation. The capsular polysaccharides in two reference strains examined so far are complex in structure, and seem to be strain specific. The B. fragilis lipopolysaccharide has a relatively low endotoxic activity when compared to lipopolysaccharides from bacteria belonging to Enterobacteriaceae. Structurally the B. fragilis lipopolysaccharide has a relatively low molecular weight, and short polysaccharide chains similar to those found in lipopolysaccharides from rough mutants of Enterobacteriaceae or from capsulated bacteria such as Neisseria meningitidis and Haemophilus influenzae.