Biochemical characterization of a novel staphylococcal protein A (SpA) interaction with human fibronectin (Fn) and its role in Fn-dependent bacterial adhesion to host cells

Staphylococcus aureus is a harmful bacterium causing serious health conditions like sepsis, endocarditis, and necrotizing pneumonia. The emergence of methicillin- and vancomycin-resistant strains of S. aureus (MRSA and VRSA) poses a significant challenge to human health. The bacterium uses fibronectin (Fn) during the early stages of infection to form clots, facilitating its adhesion to host cells and evasion from the immune system. The most crucial Fn-binding proteins in S. aureus are FnBPA and FnBPB. Staphylococcal protein A (SpA) is another vital molecule for S. aureus pathogenesis, functioning as a superantigen by interacting with immunoglobulins, contributing to the evasion of the host immune system. The study demonstrated that SpA can interact with Fn, and this interaction was investigated using Surface Plasmon Resonance (SPR). Two SpA mutants were used to study the effect of mutations on the interaction. The study found that an anti-SpA antibody could displace Fn from SpA. Carbene footprinting was used to locate the Fn binding site on the SpA C domain. The bacterial adhesion to fibronectin and host cells was tested using three S. aureus strains, and all three strains were significantly impacted by the SpA deletion when adhering to Fn. The adhesion of S. aureus to a HUVEC monolayer was also impacted by SpA deletions in two strains, but not in the third one.