Expression of staphylococcal protein Sbi is induced by human IgG

Protein Sbi is an IgG- and β 2 glycoprotein I-binding protein on the surface of Staphylococcus aureus. In most strains, the amount of protein Sbi on the cell surface is very low under normal growth conditions. However, here we show that after growth in the presence of human serum, the amount is significantly increased. In S. aureus strain 8325-4, the observed increase is concentration-dependent and the highest level is found approximately 2 h after serum addition. The active molecule in serum was found to be IgG, which causes an increase of surface-located protein Sbi in S. aureus strain 8325-4, in the clinical isolates tested as well as in protein A-negative mutants. Thus, the results suggest that binding of IgG to protein Sbi upregulates protein Sbi synthesis.