Binding of Fibrinogen Fragment D to Group A Streptococci Causes Strain Dependent Decrease in Cell Surface Hydrophobicity as Measured by the Salt Aggregation Test (SAT) and Cell Clumping in Polyethylene Glycol

Abstract Group A streptococcal reference strains of various M-types as well as three clinical isolates, and clinical isolates of group B, C and G streptococci were compared with respect to their agglutination by fibrinogen, specific binding of monovalent fibrinogen fragment D, and their hydrophobicity measured by salt aggregation in ammonium sulfate dilutions (SAT). Fibrinogen fragment D binding to M positive group A streptococci and clinical isolates of group A reduced salt aggregation of these strains from 0.06 molar to 2 molar in the ammonium sulfate dilution series. Group A, M protein negative and group B, C and G control strains used aggregated only at high ammonium sulfate concentration (2 molar) or not at all. Treatment of these strains with fibrinogen fragments did not influence their weak salt aggregation. By elution of specific retained fragment D from the type 1 M positive strain 40/58II by treatment of cells with 0.1 molar citric acid, 6 molar urea, buffer, pH 3.0, surface hydrophobicity could partially be restored (SAT titer 0.5 molar). Treatment of streptococcal cells with fibronectin or albumin at much higher protein concentrations had a moderate effect on surface hydrophobicity. These results indicate that binding of fibrinogen fragment D to the receptor can block a hydrophobic surface domain of the receptor or induce a sterical hindrance affecting surface exposure of associated surface molecules. A fibrinogen receptor — also responsible for binding of fibrinogen fragment D — is in M positive group A streptococci covalently linked to M protein. Decrease of salt aggregation tendency after mild pepsin treatment to release M protein supported these findings.