Interactions between human plasma proteins and cell wall components of Staphylococcus aureus

Staphylococcus aureus has surface structures with affinity to human IgG, fibrinogen, and fibronectin. Besides the binding of the Fc-terminal part of IgG from a range of mammalian species, S. aureus protein A binds some IgM, IgA, and IgE molecules. Furthermore, it seems also able to bind immunoglobulins via their Fab-terminal parts. Protein A (Mr 42,000) is the only well-characterized S. aureus cell wall protein, and its structure is known in detail. A considerable number of biological properties of protein A has been demonstrated. Most of these properties seem to be a consequence of the complement activation induced by protein A-IgG complexes. The role of protein A in the phagocytosis of S. aureus is complex. By complement consumption protein A has been found to inhibit the phagocytosis of staphylococci by polymorphonuclear leucocytes. However, it has been demonstrated that protein A-containing staphylococci bind to surface IgG on human alveolar and peritoneal macrophages and thereby promote phagocytosis by these cells. This phenomenon might explain the increased virulence of S. aureus in the presence of human IgG in experimental peritonitis in mice. Fibrinogen binds to a surface structure on S. aureus, designated clumping factor as the binding results in clumping of whole bacteria. Recently, a glycoprotein (Mr of about 400,000) has been isolated from S. aureus. This glycoprotein seems to be the clumping factor. It binds to fibrinogen, inhibits the fibrinogen induced clumping, and seems to be a S. aureus specific, surface component. The isolated component activates human complement in vitro. Also, it induces protection against S. aureus peritonitis in immunized mice. The presence of fibrinogen and an unknown human plasma component increases the virulence of S. aureus in experimental peritonitis in mice, but the role of fibrinogen in human S. aureus infection is unknown. Fibronectin binds to a surface protein on S. aureus, and this binding also results in the clumping of the bacteria. The binding site(s) for fibronectin is different from the binding sites for fibrinogen and IgG. A fibronectin-binding protein (Mr 197,000) has been isolated from S. aureus by affinity chromatography. This protein binds fibronectin and inhibits the fibronectin induced S. aureus clumping. No other biological properties of this protein have yet been demonstrated. The binding of fibronectin to S. aureus opsonize the bacteria for polymorphonuclear leucocytes. The opsonic capacity is, however, low compared to other serum opsonins. It has been suggested that fibronectin plays a role in the attachment of S. aureus, but further studies are needed.(ABSTRACT TRUNCATED AT 400 WORDS)