Characterization of the fibrinogen-binding surface protein Fbl of Staphylococcus lugdunensis

The fbl gene of Staphylococcus lugdunensis encodes a protein Fbl that is 58 % identical to the clumping factor A (ClfA) of Staphylococcus aureus. The fbl gene was present in eight clinical isolates of S. lugdunensis. When Fbl was expressed on the surface of Lactococcus lactis it promoted adherence to immobilized fibrinogen and cell clumping in a fibrinogen solution. Purified recombinant Fbl region A bound to immobilized fibrinogen in a dose-dependent manner and inhibited the adherence of both Fbl-expressing and ClfA-expressing strains of L. lactis to fibrinogen. Adherence of S. lugdunensis and L. lactis Fbl(+) to immobilized fibrinogen was also inhibited by rabbit anti-Fbl region A antibodies and rabbit anti-ClfA region A antibodies, as well as by human immunoglobulin with a high level of anti-ClfA antibodies. Alignment of the A domains of CflA and Fbl revealed that all of the ClfA residues implicated in binding to the gamma-chain of fibrinogen are conserved in Fbl. Nevertheless Fbl had a tenfold lower affinity for fibrinogen, suggesting that sequence differences that occur elsewhere in the protein, possibly in beta-strand E of domain N2, affect ligand binding.