Organisms of the genus Bacterioides represent the major group of obligate anaerobes involved in human
infections. Bacteroides usually cause either
bacteremia or localized
abscesses. Of the numerous species of Bacteroides, Bacteroides fragilis is the single most frequent clinical isolate. B. fragilis and Bacteroides melaninogenicus have chemically incomplete
lipopolysaccharides as compared with the
lipopolysaccharides (
endotoxins) of aerobic bacteria, and the
lipopolysaccharides of Bacteroides lack of
biologic potency characteristic of
endotoxin. This inactivity may account for the very infrequent occurrence of
disseminated intravascular coagulation or
purpura that can accompany
sepsis due to these organisms. Furthermore, strains of B. fragilis have an immunologically common capsular
polysaccharide. In an animal model of intraabdominal
sepsis, the encapsulated strains caused
abscesses when given without other organisms. But
abscess formation from unencapsulated strains of Bacteroides generally required the administration of a synergistic aerobe. The
abscesses caused by encapsulated strains were shown to be directly attributable to the capsular
polysaccharide, which is an important
virulence factor of this organism. Antibody against the capsular
antigen is protective against
bacteremia due to B. fragilis, but T cell mediated immunity is protective against experimental intraabdominal
abscesses.