Binding of immunoglobulins and other proteins to bacteroides gingivalis

Bacteroides gingivalis bound IgG labelled with three different enzymes even in well-buffered isotonic systems. This non-immunological binding interferes with enzyme linked immunosorbent assay (ELISA) of salivary antibodies against B.gingivalis.B.melaninogenicus and B.intermedius showed only weak IgG binding while nine other species of oral bacteria did not bind IgG. The broad specificity of the receptor binding IgG to B.gingivalis includes not only IgG and IgA, but also proteins unrelated to immunoglobulins. Like the haemagglutinin of B.gingivalis, the receptor that binds IgG has a specificity for arginine residues with an adjacent or neighbouring aromatic amino acid residue. In contrast to the strong IgG binding of its parent, an avirulent variant B.gingivalis W50/BE1 that is deficient in trypsin-like activity, has a low level of IgG binding. Our results suggest firstly either an inactive or deactivated form of a trypsin- or papain-like enzyme, or a molecule such as the haemagglutinin, binds immunoglobulins and other proteins to the surface of B.gingivalis. Secondly, binding to arginine in proteins is an important ecological factor for B.gingivalis since it is a property shared by its IgG binding receptor, its haemagglutinin and its trypsin- or papain-like enzyme. Finally, the sensitivity of ELISA to non-immunological reactions necessitates establishment of the specificity of immunoassays at levels of sensitivity comparable to those of the assays themselves.