Fibrinogen-binding proteins from Staphylococcus aureus

Abstract

Staphylococcus aureus produces a large number of proteins that specifically bind to molecules from plasma or from the human extracellular matrix where they are involved in processes associated with the colonization of the host tissues. At least six different fibrinogen-binding proteins have been identified from S. aureus; Clumping factor A and B (ClfA and ClfB), Coagulase, Efb (previously Fib), FbpA , Map and Eap. This work focuses primarily on the function(s) of Efb and Eap.

We have shown that Efb contributes to the virulence of S. aureus in wound infection. An allele replacement mutant of Efb was constructed and compared with the virulence of the wild type in a rat model of wound infection. Sixty-seven % of the animals challenged with the parental strain developed severe clinical signs of wound infection, whereas only 29% of the rats infected with the isogenic mutant showed severe symptoms. The function of Efb in infection is unclear but we have shown in vitro that multiple binding sites are involved in the interaction between Efb and fibrinogen. One binding site is located at the C terminus of Efb and one binding site at the two repeat regions of the N terminus. These N-terminal repeats are homologous to those at the C terminus of coagulase. The divalent binding nature of Efb with Fg leads to the precipitation of the Efb-fibrinogen complex, which can be enhanced by Ca or Zn but not by Mg. At least one of the binding sites for Efb is located at the Aα-chain of Fg (Aα 111 - Aα 197). Binding of Efb to this site may serve to block specific Aα-chain processes. In addition, we have shown that Efb could bind to fibrin as well as to fibrinogen and that the two repeats of Efb could compete with the five repeats of coagulase for the same binding domain on fibrinogen.

We have shown in this study that Eap (extracellular adherence protein) have the ability to enhance the adherence of Staphylococcus aureus to host cells. Eap can bind at least seven plasma proteins, including fibrinogen, prothrombin and fibronectin. Eap is presumably the same, or at least related to a previously described cell surface protein, designated Map for major histocompatibility complex class II analogous protein. In contrast to Map, Eap is mainly extracellular since 70% of the protein was found in the culture supernatant of S. aureus strain Newman. In this study we proposed a novel mechanism for adherence of S. aureus to host components stimulated and mediated by Eap. This is based on the fact that; i) Eap can form olimeric forms ii) Staphylococcus aureus strain Newman adhere to immobilized and soluble Eap, but not to the other staphylococcal proteins tested iii) Eap was able to cause agglutination of Staphylococcus aureus and iv) Attachment of S. aureus to fibroblasts and epithelial cells was significantly enhanced by the presence of Eap.

In conclusion, Staphylococcus aureus produce several fibrinogen-binding proteins that interact with fibrinogen in different ways. Colonization of the host tissues and dissemination of the infection is a multifactorial event in which many extracellular and cellsurface proteins are involved. FgBPs play an important role during infection by: a) A cellsurface FgBP that mediates the direct attachment of S. aureus to damaged heart valves and implanted biomaterial, b) A secreted FgBP with broad binding specificity that promotes colonisation of host tissues and bacterial aggregation, and c) A 15.9 kDa secreted FgBP that influences the severity of a wound infected by S. aureus thereby altering the healing process.