High Force Catch Bond Mechanism of Bacterial Adhesion in the Human Gut

Bacterial colonization of the human intestine requires firm adhesion of bacteria to insoluble targets under hydrodynamic flow. Here we report the molecular mechanism behind an mechanostable protein complex responsible for resisting high shear forces and adhering bacteria to cellulose fibers in the human gut. Using single-molecule force spectroscopy (SMFS), single-molecule FRET (smFRET), and molecular dynamics (MD) simulations, we resolved two binding modes and three unbinding reaction pathways of a mechanically ultrastable R. champanellensis ( Rc ) Dockerin-Cohesin (Doc-Coh) complex. The complex assembles in two discrete binding modes with significantly different mechanical properties, with one breaking at ~500 pN and the other at ~200 pN at loading rates from 1-100 nN/sec. A neighboring X-module domain allosterically regulates the binding interaction and inhibits one of the low-force pathways at high loading rates, giving rise to a new mechanism of catch bonding that manifests under force ramp protocols. Multi-state Monte Carlo simulations show strong agreement with experimental results, validating the proposed kinetic scheme. These results explain mechanistically how gut microbes regulate cell adhesion strength at high shear stress through intricate molecular mechanisms including dual-binding modes, mechanical allostery and catch bonds.