Catch Bond-mediated Adhesion without a Shear Threshold

Abstract

The FimH protein is the adhesive subunit of Escherichia coli type 1 fimbriae. It mediates shear-dependent bacterial binding to monomannose (1M)-coated surfaces manifested by the existence of a shear threshold for binding, below which bacteria do not adhere. The 1M-specific shear-dependent binding of FimH is consistent with so-called catch bond interactions, whose lifetime is increased by tensile force. We show here that the oligosaccharide-specific interaction of FimH with another of its ligands, trimannose (3M), lacks a shear threshold for binding, since the number of bacteria binding under static conditions is higher than under any flow. However, similar to 1M, the binding strength of surface-interacting bacteria is enhanced by shear. Bacteria transition from rolling into firm stationary surface adhesion as the shear increases. The shear-enhanced bacterial binding on 3M is mediated by catch bond properties of the 1M-binding subsite within the extended oligosaccharidebinding pocket of FimH, since structural mutations in the putative force-responsive region and in the binding site affect 1M-and 3M-specific binding in an identical manner. A shear-dependent conversion of the adhesion mode is also exhibited by P-fimbriated E. coli adhering to digalactose surfaces.Bacterial adhesion, a critical initial step in colonization and biofilm formation, is commonly mediated by carbohydrate-binding lectin-like proteins expressed on the bacterial surface as part of hair-like fimbrial appendages or as nonfimbrial components (1-4). Lectins are a structurally diverse class of receptor proteins that bind monosaccharide or, more commonly, oligosaccharide ligands in a noncovalent and nonenzymatic fashion (5). In general, receptor-ligand interactions (including lectin-saccharide binding) are thought to occur via slip bonds, where the strength of the binding is highest under no tensile force (6, 7). Thus, the surface adhesion of eukaryotic or bacterial cells is expected to be strongest and have the highest level of surface accumulation at the lowest shear stress.However, several studies have demonstrated shear-enhanced bacterial and cell adhesion where surface binding under static or low shear conditions is too weak to mediate adhesion of whole cells but becomes significantly stronger at increased shears, resulting in dramatically higher surface accumulation of cells at elevated shear stresses (i.e. a shear threshold for surface adhesion is seen). This has been shown for von Willebrand-mediated adhesion of platelets (8, 9), for saccharide-specific leukocyte surface rolling mediated by P-and L-selectin (10 -12), and for adhesion of Escherichia coli mediated by binding of the fimbrial lectin FimH to monomannose surfaces (13). It has been suggested that shear thresholds for binding could result by forming not slip bonds but catch bonds whose lifetime is increased by flow-induced tensile forces (14 -17).Type 1 fimbriae are the most common type of adhesive organelles in E. coli and other enterobacteria and mediate mannose-specific adhesio...

Elevated Shear Stress Protects Escherichia coli Cells Adhering to Surfaces via Catch Bonds from Detachment by Soluble Inhibitors

“…In contrast to 3M-RB-coated surfaces, where E. coli slowly rolls along the surface without detaching at shear stress levels below about 10 dynes/cm 2 , E. coli cannot stably adhere to 1M-BSAcoated surfaces below about 0.5 dynes/cm 2 . The cells roll rapidly on FimH-1M bonds, lasting less than 1 s at shear stress levels between roughly 0.5 and 10 dynes/cm 2 , with rolling velocities averaging 5 to 20 m/s compared to averages of 0.1 to 0.4 m/s on 3M-RB at 3.7 dynes/cm 2 (even in spite of the use of a 100-fold lower 3M-RB incubation concentration), consistent with a longer FimH-3M than FimH-1M bond lifetime (23). In 1M-mediated adhesion, bacteria switch to a shearactivated stationary state at shear stress levels above 10 dynes/ cm 2 , similar to what is seen with adhesion to 3M but resulting in bacteria remaining stationary for only about a minute, a time considerably shorter than the 3M-mediated adhesion time (32) (Fig.…”

“…This shear stress-induced enhancement of binding strength correlates with shear stress (and therefore dragging force) rather than shear rate (and transport) (34) and occurs only if the bacteria bind to surfaces coated with mannose but not to surfaces presenting FimH antibodies (33). Furthermore, the shear regime in which maximum adhesion occurs can be tuned by point mutations in the lectin domain of FimH, which gives further structural proof that shear stress-enhanced adhesion is specific to the FimHmannose bond (23,33,34). Finally, while cells are deformable objects, shear stress-enhanced adhesion also occurs for rigid monomannose (1M)-coated beads binding to surfaces coated with purified type 1 fimbriae (8).…”

“…These long-lived bonds mediate firm stationary bacterial adhesion for periods on the order of 1 min on 1M-coated surfaces and for over 30 min on 3M-coated surfaces. The longer lifetime of FimH binding to 3M compared to 1M is likely due to additional FimH-ligand contact points outside the 1M-binding pocket of FimH (23).…”

Mechanism of allosteric propagation across a β‐sheet structure investigated by molecular dynamics simulations

“…Ten percent of protein domains have a β ‐sandwich structure, including the binding domains in catch‐bond forming cadherins,42, 43 and many bacterial adhesins that mediate a shear‐enhanced adhesion that strongly suggests catch bonds 44, 45, 46, 47, 48, 49. It is possible that backbone fluctuations in β ‐sandwich folds are common in catch‐bond forming proteins because the β sandwich would confer the mechanical stability needed to withstand force, while the twisting fluctuations would allow the allostery that provides a mechanism for catch bonds.…”

Microfluidic Studies of Biofilm Formation in Dynamic Environments

“…The untreated glass surface used in those experiments sets this process apart from the surface-specific phenomenon of catch bonds, whereby bacteria, such as Escherichia coli, possess specific adhesins that attach to mannose-coated surfaces in a shear-dependent manner. In these species-specific cases, increasing shear rate strengthens attachment (31). Whereas catch bonds strengthen adhesion after a cell has landed on a surface, shear trapping increases the flux of cells toward the surface.…”