Surface adhesion of bacteria generally occurs in the presence of shear stress, and the lifetime of receptor bonds is expected to be shortened in the presence of external force. However, by using Escherichia coli expressing the lectin-like adhesin FimH and guinea pig erythrocytes in flow chamber experiments, we show that bacterial attachment to target cells switches from loose to firm upon a 10-fold increase in shear stress applied. Steered molecular dynamics simulations of tertiary structure of the FimH receptor binding domain and subsequent site-directed mutagenesis studies indicate that shear-enhancement of the FimH-receptor interactions involves extension of the interdomain linker chain under mechanical force. The ability of FimH to function as a force sensor provides a molecular mechanism for discrimination between surface-exposed and soluble receptor molecules.

Spread of biological species from primary into novel habitats leads to within-species adaptive niche differentiation and is commonly driven by acquisition of point mutations in individual genes that increase fitness in the alternative environment. However, finding footprints of adaptive niche differentiation in specific genes remains a challenge. Here we describe a novel method to analyze the footprint of pathogenicity-adaptive, or pathoadaptive, mutations in the Escherichia coli gene encoding FimH-the major, mannose-sensitive adhesin. Analysis of distribution of mutations across the nodes and branches of the FimH phylogenetic network shows (1) zonal separation of evolutionary primary structural variants of FimH and recently derived ones, (2) dramatic differences in the ratio of synonymous and nonsynonymous changes between nodes from different zones, (3) evidence for replacement hot-spots in the FimH protein, (4) differential zonal distribution of FimH variants from commensal and uropathogenic E. coli, and (5) pathoadaptive functional changes in FimH brought by the mutations. The selective footprint in fimH indicates that the pathoadaptive niche differentiation of E. coli is either in its initial stages or undergoing an evolutionary "source/sink" dynamic.

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...

Escherichia coli NU14, a cystitis isolate used to study the pathogenesis of cystitis and to develop a FimH (type 1 fimbrial adhesin) vaccine, was assessed for extended virulence genotype, phylogenetic background, and FimH sequence and binding phenotype(s). NU14 exhibited the same virulence genotype and was derived from the same (meningitis- and cystitis-associated) subclone of E. coli O18:K1:H7 as the archetypal neonatal bacterial meningitis (NBM) isolate RS218. NU14 also displayed the same Ser62Ala FimH polymorphism as did NBM isolates RS218 and IHE3034-conferring both collagen binding and a distinct monomannose binding capability (which characterizes uropathogenic but not commensal E. coli and dramatically increases adherence to uroepithelial cells). These findings establish that strain NU14 exhibits numerous urovirulence-associated traits and derives from the single most prevalent clonal group in acute cystitis. They provide further evidence of clonal and pathotypic similarities between cystitis and NBM isolates of E. coli O18:K1:H7.

FimH protein is a lectin‐like adhesive subunit of type 1, or mannose‐sensitive, fimbriae that are found on the surface of most Escherichia coli strains. All naturally occurring FimH variants demonstrate a conserved mannotriose‐specific (i.e. multivalent) binding. Here, we demonstrate that replacement of residues 185–279 within the FimH pilin domain with a corresponding segment of the type 1C fimbrial adhesin FocH leads to a loss of the multivalent mannotriose‐specific binding property accompanied by the acquisition of a distinct monomannose‐specific (i.e. monovalent) binding capability. Bacteria expressing the monovalent hybrid adhesins were capable of binding strongly to uroepithelial tissue culture cells and guinea pig erythrocytes. They could not, however, agglutinate yeast or bind human buccal cells – functions readily accomplished by the E. coli‐ expressing mannotriose‐specific FimH variants. Based on the relative potency of inhibiting compounds of different structures, the receptor binding site within monovalent FimH–FocH adhesin has an extended structure with an overall configuration similar to that within the multivalent FimH of natural origin. The monomannose‐only specific phenotype could also be invoked by a single point mutation, E89K, located within the lectin domain of FimH, but distant from the receptor binding site. The structural alterations influence the receptor‐binding valency of the FimH adhesin via distal effects on the combining pocket, obviously by affecting the FimH quaternary structure.