Inhibition of fibrinogen reaction by polysaccharide of encapsulated Staphylococcus aureus

Encapsulated and nonencapsulated strains of Staphylococcus aureus which lack coagulase or clumping factor (bound coagulase), or both, were examined for the antigen associated with the fibrinogen-cell clumping reaction. Extracts of the cells were tested for the ability to react with fibrinogen or to inhibit fibrinogen precipitation. Antisera prepared against encapsulated (coagulase-positive, clumping factor-negative) variants, as well as against nonencapsulated wild-type (coagulase-positive, clumping factor-positive) S. aureus strains, contained high titers of clumping-inhibiting antibody. When coagulase-negative, clumping factor-negative mutants were the immunizing agents, antisera contained no demonstrable clumping-inhibiting antibody. Phenol extracts of all coagulase-positive strains tested precipitated fibrinogen, regardless of the ability of cells to clump in the presence of fibrinogen. Polysaccharide extracts of encapsulated, clumping factor-negative strains inhibited this fibrinogen-precipitating activity, whereas similar extracts of nonencapsulated staphylococci did not inhibit the fibrinogen reaction. From these results, it appeared that the coagulase-positive, encapsulated staphylococci which do not clump in fibrinogen solution possess clumping factor, but that their capsular polysaccharide inhibits clumping activity. These findings suggested a closer association of clumping factor and coagulase than is now recognized.