Guanidine extraction enhances the binding of human fibrinogen to group-B streptococci

Treatment of group-B streptococci with guanidine chloride significantly enhanced their capacity to bind 125I-labelled fibrinogen. The increase in binding activity was almost proportional in the range of 0.05 M to 6 M guanidine chloride. Repeated extractions with guanidine chloride further increased the capacity of B streptococci to bind the labelled fibrinogen. On the other hand, urea, even at concentrations up to 8 M, did not alter the fibrinogen binding. The enhancement was most pronounced with B streptococci of serotypes III and III-R. Cultivation of these streptococci in Baker and Kasper medium, which is known to stimulate formation of microcapsules, reduced 125I-fibrinogen binding. Subsequent treatment with guanidine increased their binding activity, presumably by facilitating the accessibility of the binding sites.