An isolated, altered fibronectin-binding protein (Fnb) of S. aureus having at least one mutation in an amino acid selected from residues corresponding to Gln103, Gln105, Lys157, Lys503, Lys620, Lys702, Lys762, Gln783 and Gln830 of FnbA of S. aureus strain ATCC49525 is described. Replacement of these reactive residues within the fibronectin-binding protein renders this protein less capable than wild-type Fnb of covalently cross-linking with fibronectin and fibrin. The altered fibronectin-binding protein effectively interferes with adhesion of S. aureus to fibronectin and fibrin, and therefore, an immunogenic composition comprising such altered Fnb exhibits improved immunogenic properties and is safer to use.