Expression of fibronectin-binding protein FbpA modulates adhesion in Streptococcus gordonii

Fibronectin binding is considered to be an important virulence factor in streptococcal infections. Adhesion of the oral bacterium Streptococcus gordonii to immobilized forms of fibronectin is mediated, in part, by a high molecular mass wall-anchored protein designated CshA. In this study, a second fibronectin-binding protein of S. gordonii is described that has been designated as FbpA (62.7 kDa). This protein, which is encoded by a gene located immediately downstream of the cshA gene, shows 85 and 81% identity to the fibronectin-binding proteins PavA, of Streptococcus pneumoniae, and FBP54, of Streptococcus pyogenes, respectively. Purified recombinant FbpA bound to immobilized human fibronectin in a dose-dependant manner, and isogenic mutants in which the fbpA gene was inactivated were impaired in their binding to fibronectin. This effect was apparent only for cells in the exponential phase of growth, and was associated with reduced surface hydrophobicity and the surface expression of CshA. Cells in the stationary phase of growth were unaffected in their ability to bind to fibronectin. By utilizing gene promoter fusions with cat (encoding chloramphenicol O-acetyltransferase), it was demonstrated that cshA expression was down-regulated during the exponential phase of growth in the fbpA mutant. Expression of fbpA, but not cshA, was sensitive to atmospheric O2 levels, and was found to be up-regulated in the presence of elevated O2 levels. The results suggest that FbpA plays a regulatory role in the modulation of CshA expression and, thus, affects the adhesion of S. gordonii to fibronectin.