We have been interested in a family of staphylococcal adhesins forming specific “dock, lock, and latch” (DLL) interactions. In a pioneering paper, we demonstrated that the prototypical DLL adhesin SdrG from S. epidermidis binds its ligand fibrinogen (Fg) with an extremely strong force of ~2 nN, similar to that of a covalent bond. This was the strongest non-covalent bond ever reported in biology, despite an ordinary affinity constant. Inspired by our work, the Gaub group used AFM and all-atom steered molecular dynamics to show that this high mechanostability originates from the formation of an elaborate hydrogen bond network between subdomains of SdrG and the Fg peptide backbone. Following this, I was invited to write a perspective article in Science titled "Force matters in hospital-acquired infections", illustrating the medical importance of this emerging field of ultrastrong forces in pathogen adhesion. We next showed that other adhesins engaged in DLL binding. S. aureus clumping factors ClfA and ClfB, bind their targets with similar ultrastrong forces. We made progress in understanding the "beta zipper" interaction of fibronectin (Fn)-binding adhesins (FnBPs), revealing that they mediate bacterial adhesion to soluble Fn via strong forces (∼1,5 nN), consistent with a tandem β-zipper.11 We also identified forces of ~2 nN between a S. aureus protein A and von Willebrand factor, a plasma glycoprotein mediating the pathogen’s binding to endothelial cell surfaces.So ultrastrong adhesion is not restricted to DLL bond-forming proteins.
Surface-attached bacteria are exposed to mechanical stresses, including hydrodynamic flow and cell-surface contacts.We discovered an intriguing mechanism in which mechanical stress dramatically enhances S. aureus adhesion, providing the pathogen with a means to withstand high shear stress during colonization. ClfA and ClfB adhesins were shown to behave as force-sensitive molecular switches, forming very strong bonds only under high stress. This was explained by a mechanism whereby bacterial adhesion to ligands is enhanced through force-induced conformational changes in the Clf molecules, from a weakly binding folded state to a strongly binding extended state. The newly described stress-enhanced adhesion behaviors point to the formation of unusual "catch-bonds" that reinforce under stress (see below).
Perhaps the most fascinating example of the influence of physical force on bacterial behavior is the formation of catch-bonds that strengthen cellular adhesion under shear stress. Using AFM in the force-clamp spectroscopy mode, we recently provided the first evidence of a catch-bond in a Gram-positive bacterium.14 We showed that the DLL interaction between the staphylococcal adhesin SpsD and Fg is strong and exhibits an unusual catch-slip transition. The bond lifetime first grows with force, but ultimately decreases to behave like a slip bond beyond a critical force that is orders of magnitude higher than for previously investigated complexes. Catch-bonds represent a competitive advantage to help the pathogens tune their adhesion to flow conditions, that is, by binding loosely and spreading under low flow while resisting shear and remaining firmly attached under high flow. Uncovering new catch-bond behaviors could help in the design of strategies as alternatives to antibiotics.
Innovative anti-adhesion compounds represent a promising alternative to antibiotics to fight pathogens, especially nosocomial drug-resistant strains. We have identified inhibitors capable of preventing adhesion, aggregation and biofilm formation. In two studies, we unraveled the molecular interactions by which S. aureus SasG and SdrC mediate cell-cell aggregation leading to biofilm formation. Following that, we discovered that a peptide derived from the neuronal cell adhesion molecule β-neurexin inhibits SdrC-dependent attachment to inert surfaces, cell-cell adhesion, and biofilm formation. The peptide could be used as a platform for designing peptidomimetics with potential to prevent biofilm infections. We also showed that monoclonal antibodies can strongly block the adhesion of the S. aureus collagen-binding protein Cna.
