Endovascular
infections caused by Staphylococcus aureus involve interactions with
fibronectin present as extracellular matrix or surface
ligand on host cells. We examined the expression, structure, and binding activity of the two major S. aureus
fibronectin-
binding proteins (FnBPA, FnBPB) in 10 distinct, methicillin-resistant clinical isolates from patients with either persistent or resolving
bacteremia. The persistent
bacteremia isolates (n = 5) formed significantly stronger bonds with immobilized
fibronectin as determined by dynamic binding measurements performed with atomic force microscopy. Several notable differences were also observed when the results were grouped by clonal complex 5 (CC5) strains (n = 5) versus CC45 strains (n = 5).
Fibronectin-binding receptors on CC5 formed stronger bonds with immobilized
fibronectin (P < 0.001). The fnbA gene was expressed at higher levels in CC45, whereas fnbB was found in only CC5 isolates. The fnbB gene was not sequenced because all CC45 isolates lacked this gene. Instead, comparisons were made for fnbA, which was present in all 10 isolates. Sequencing of fnbA revealed discrete differences within high-affinity,
fibronectin-binding repeats (FnBRs) of FnBPA that included (i) 5-amino-acid polymorphisms in FnBR-9, FnBR-10, and FnBR-11 involving charged or polar side chains, (ii) an extra, 38-amino-acid repeat inserted between FnBR-9 and FnBR-10 exclusively seen in CC45 isolates, and (iii) CC5 isolates had the SVDFEED
epitope in FnBR-11 (a sequence shown to be essential for
fibronectin binding), while this sequence was replaced in all CC45 isolates with GIDFVED (a motif known to favor host cell invasion at the cost of reduced
fibronectin binding). These complementary sequence and binding data suggest that differences in fnbA and fnbB, particularly polymorphisms and duplications in FnBPA, give S. aureus two distinct advantages in human endovascular
infections: (i) FnBPs similar to that of CC5 enhance
ligand binding and foster initiation of disease, and (ii) CC45-like FnBPs promote cell invasion, a key attribute in persistent endovascular
infections.