Demonstration of a new type of immunoglobulin G receptor in Streptococcus zooepidemicus strains.

Forty-seven bacterial strains representing four different group C streptococcal species were tested for binding of human and bovine immunoglobulin G (IgG) subclasses. Specific binding sites for IgG were found in all bacterial species studied. The four species included differed, however, in their capacities to interact with various IgG subclasses, indicating the existence of different types of IgG receptors. Streptococcus equisimilis and Streptococcus dysgalactiae were found to carry the same type of IgG receptor, one that is identical to the previously described Fc-binding structure type III. A new type of bacterial IgG receptor was detected in Streptococcus zooepidemicus strains. This receptor exhibits a protein A-like subclass specificity, with binding of human IgG1, IgG2, and IgG4 and of bovine IgG2. However, differences in their capacities to interact with other non-human immunoglobulins indicated that these two immunoglobulin-reactive structures were different. All types of IgG receptors in group C streptococci were found to be heat stable but susceptible to proteolytic enzymes. The inability of human serum albumin or fibrinogen to inhibit the uptake of radiolabeled IgG shows that the IgG receptor is separate from binding sites for these two other proteins on the bacterial cell surface. The existence of similar IgG receptors in closely related streptococcal species suggests that these structures have a common origin.