Role of complement in host resistance against members of the Bacteroidaceae

Considerable evidence has been reported in recent years suggesting that complement plays an important role in host resistance against members of the Bacteroidaceae. Most of the investigations in this area have focused on the genus Bacteroides because of its clinical importance. Various species of Bacteroides have been shown to activate the complement system in vitro via the classical and alternative pathways. Complement activation results in the generation of chemotactic factors that mobilize polymorphonuclear leukocytes to sites of infection. Activated complement also facilitates bacteriolysis and opsonophagocytic killing by polymorphonuclear leukocytes and macrophages. Strains possessing dense fibrillar polysaccharide capsules are resistant to both of these defense mechanisms. The putative importance of complement-dependent bacteriolysis and opsonophagocytic killing in resistance against Bacteroides infections in vivo requires confirmation. In addition, the role of complement in synergistic interactions between Bacteroides and facultative bacteria remains to be elucidated.