ABSTRACT
The ability of Staphylococcus aureus to bind fibrinogen is believed to be important in promoting bacterial adherence to both intravascular catheters and host tissues during infection. We investigated the influence of the global regulator agr on the fibrinogen binding capacity and its relationship to the expression of coagulase (encoded by coa) and clumping factor (encoded by clfA) in strain Newman. Strains were obtained by transducing site-specific mutations of clfA, coa, and agr into strain Newman to obtain single, double, and triple mutants of the respective genes. As expected, the clfA mutant bound less soluble 125I-labeled fibrinogen than the corresponding coa mutant in agr+ strains; however, with agr mutant strains, the upregulation in fibrinogen binding capacity correlated mostly with the increased expression and transcription of coagulase as shown by Western (immunoblot) and Northern (RNA) blot analysis. In particular, the coa agr double mutant resulted in a significant reduction in fibrinogen binding compared with that of the agr mutant. The contribution of clfA to fibrinogen binding in agr-negative strains was less than that of coa (32,740 +/- 1,189 versus 18,141 +/- 334 and 38,919 +/- 1,021 cpm for clfA agr, coa agr, and the single agr mutant, respectively). Thus, coagulase is a major binding protein for soluble fibrinogen in the agr-negative background. In in vitro microtiter and catheter adherence assays with solid-phase fibrinogen, clumping factor, but not coagulase, plays a major role in binding to immobilized fibrinogen. coa transcription was negatively modulated by agr and occurred mainly during the exponential growth phase. In contrast, clfA transcription was agr independent and was strongest during the postexponential phase. Although an agr coa clfA triple mutant bound less soluble fibrinogen than the agr coa double mutant (8,504 +/- 831 versus 18,141 +/- 334 cpm), significant residual fibrinogen binding capacity remained in the triple mutant, thus suggesting an additional fibrinogen binding component. By using direct ligand affinity blotting with 125I-fibrinogen, we could identify coagulase and an additional unidentified 52-kDa protein as a fibrinogen binding component in cell extracts. This band was absent in the extract of the coa clfA double mutant.
