Streptococcus oralis and Streptococcus sanguinis interaction with platelet-fibrin thrombi

Streptococcus oralis and Streptococcus sanguinis are major aetiological agents of infective endocarditis (IE). Direct interaction of S.sanguinis with platelets is thought to be important in the pathogenesis of IE but little is known about the mechanisms of S.oralis-mediated IE. Objectives: To compare the ability of S.oralis and S.sanguinis strains to adhere to platelets and fibrin thrombi in vitro. Methods: Cells (10⁸/ml) from overnight broth cultures of 8 S.sanguinis strains and 6 S.oralis strains were assessed for adhesion to purified fibronectin and fibrinogen, washed human platelets, thrombi formed from plasma, fibronectin-depleted plasma and platelet-rich plasma (PRP). For adhesion to platelets, bacteria were coated onto 96 well microtitre trays, washed platelets (10⁸/ml) added and adherent platelets quantified by acid phosphatase. Thrombi from PRP were formed in 96 well trays coated with type I collagen (300µg/ml). Human plasma was depleted of fibronectin by passage through gelatin-agarose. Clots of normal and depleted plasmas were formed in 96 well trays by addition of thrombin (1h 0.25U, 150mM NaCl, 20mM CaCl₂). Fibrinogen and fibronectin were coated onto ELISA plates at 20µg/ml. For thrombi and purified proteins, adhesion of FITC-labelled bacteria (400µg/ml FITC 1h 37°C) was measured in a fluorescent plate reader. Results: 6/8 S.sanguinis strains adhered strongly to platelets while no S.oralis strains were adhesive. The same S.sanguinis strains adhered strongly to platelet-fibrin thrombi and the other 2 strains adhered moderately. 5/6 S.oralis strains adhered moderately to platelet-fibrin thrombi and just as well as S.sanguinis strains to fibrin-thrombi. Depletion of fibronectin from thrombi did not influence adhesion of either species and adhesion of the two species to purified fibronectin did not differ (p=0.2). In contrast, fibrinogen supported adhesion of S.oralis at higher levels than S.sanguinis (p<0.05). Conclusion: S.sanguinis binds preferentially to platelets in platelet-fibrin thrombi, whereas S.oralis binds to one or more matrix molecules including fibrinogen.