Bacteroides gingivalis and Bacteroides intermedius recognize different sites on human fibrinogen

Bacteroides (Porphyromonas) gingivalis and Bacteroides (Porphyromonas) intermedius have been implicated in the etiology of human periodontal diseases. These organisms are able to bind and degrade human fibrinogen, and these interactions may play a role in the pathogenesis of periodontal disease. In attempts to map the bacterial binding sites along the fibrinogen molecule, we have found that strains of B. gingivalis and B. intermedius, respectively, recognize spatially distant and distinct sites on the fibrinogen molecule. Isolated reduced and alkylated α-, β-, and γ-fibrinogen chains inhibited binding of ¹²⁵I-fibrinogen to both Bacteroides species in a concentration-dependent manner. Plasmin fragments D and to some extent fragment E, however, produced a concentration-dependent inhibition of ¹²⁵I-fibrinogen binding to B. intermedius strains but did not affect binding of ¹²⁵I-fibrinogen to B. gingivalis strains. Radiolabeled fibrinogen chains and fragments were compared with ¹²⁵I-fibrinogen with respect to specificity and reversibility of binding to bacteria. According to these criteria, γ chain most closely resembled the native fibrinogen molecule in behavior toward B. gingivalis strains and fragments D most closely resembled fibrinogen in behavior toward B. intermedius strains. The ability of anti-human fibrinogen immunoglobulin G (IgG) to inhibit binding of ¹²⁵I-fibrinogen to B. intermedius strains was greatly reduced by absorbing the IgG with fragments D. Absorbing the IgG with fragments D had no effect on the ability of the antibody to inhibit binding of ¹²⁵I-fibrinogen to B. gingivalis strains. A purified staphylococcal fibrinogen-binding protein blocked binding of ¹²⁵I-fibrinogen to B. intermedius strains but not to B. gingivalis strains.