Binding of human immunoglobulin G to protein A in encapsulated Staphylococcus aureus

Recent studies of the mechanism of resistance to phagocytosis in encapsulated Staphylococcus aureus have suggested that the capsule is readily penetrated by high-molecular-weight proteins such as antibodies and complement components. S. aureus strains contain a cell wall protein, protein A, that reacts with the Fc portion of immunoglobulins. The binding of immunoglobulin G (IgG) to encapsulated and unencapsulated S. aureus strains has been studied to assess the penetrability of the S. aureus capsule by IgG. Encapsulated S. aureus strains M and Smith diffuse bound large amounts of human IgG which were comparable to amounts bound by the unencapsulated strains Cowan I, M variant, and Smith compact. Trypsin treatment of bacteria reduced their ability to bind IgG. Bound IgG was not removed by extensive washing of bacteria with buffer. A non-protein A-containing, coagulase-negative, encapsulated staphylococcal strain did not bind IgG. These observations suggest that IgG is binding to cell wall protein A in encapsulated S. aureus. No differences in the rates of IgG binding by encapsulated and unencapsulated S. aureus strains were observed. It is concluded that the S. aureus capsule is freely permeable to IgG. This is of importance in considerations of the mechanisms of resistance to phagocytosis and antigen masking in encapsulated microorganisms.