A single amino acid substitution in Fibronectin Binding protein A (FnBPA) governs Staphylococcus aureus virulence via host transglutaminase-mediated fibrin crosslinking.

Staphylococcus aureus is an opportunistic pathogen that can cause different types of infections, ranging from skin lesions to life threatening diseases. Its ability to evade host immunity and establish persistent infections relies on effective adhesion to host tissues, mainly through interactions between bacterial surface proteins and extracellular matrix (ECM) components such as fibrinogen (Fbg). Here we describe a new pathogenic mechanism where S. aureus uses host transglutaminases to covalently anchor itself to Fbg. Specifically, human Factor XIII (FXIII) activated by S. aureus coagulase von Willebrand factor-binding protein (vWbp) crosslinks the bacterial adhesin Fibronectin Binding Protein A (FnBPA) to fibrin(ogen). Beyond vWbp-activated FXIII, also tissue transglutaminase 2 (TG2) is involved in a similar pathway. Previous biochemical studies showed that Gln103 in the N1 subdomain of FnBPA is the main reactive glutamine residue for FXIII-mediated crosslinking. We demonstrated that FnBPA protein mutants lacking Gln103, substituted with an alanine (Q103A) retained non-covalent Fbg binding via the classical Dock, Lock, and Latch mechanism, but failed to form covalent complexes. Furthermore, a mutant bacterial strain expressing FnBPA Q103A showed impaired incorporation into fibrin matrices. Also, in vivo experiments demonstrated that in a murine infection model the same Q103A mutant was less virulent, formed smaller dermonecrotic lesions and had lower bacterial loads than the wild-type strain. Importantly, also TG2, expressed in various tissues and upregulated during inflammation, crosslinks FnBPA to Fbg only in presence of Gln103, highlighting the broader physiological relevance of this mechanism beyond coagulation sites. Together, these results show a new virulence strategy where S. aureus uses vWbp to hijack host transglutaminase activity, stabilizing bacterial-host protein complexes through covalent FnBPA-Fbg interactions.