Identification of distinct Fc-receptor molecules on streptococci and staphylococci

The structural similarity between staphylococcal protein A and streptococcal Fc receptors was examined. Antibody to staphylococcal protein A, proven not to bind to protein A through Fc, was used to determine if the Fc receptors on 4 Fc-receptor positive streptococcal strains were antigenically related to staphylococcal protein A. Anti-protein A antibody bound to Staphylococcus aureus Cowan I, but failed to bind to any of the streptococci. Additionally, when the Cowan strain and 9 other Staphylococcus aureus isolates demonstrating various levels of IgG adsorption capacity were preincubated with antiprotein A antibody, the ability of these bacteria to adsorb IgG was completely inhibited. These results suggest that all the Fc receptors on Staphylococcus aureus are antigenically similar or identical to protein A. Fc receptors on streptococci, while sharing with staphylococcal protein A the capacity to bind to Fc of human IgG, were not antigenically crossreactive with protein A.