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The first step in the colonization of the human urinary tract by pathogenic Escherichia coli is the mannose-sensitive binding of FimH, the adhesin present at the tip of type 1 pili, to the bladder epithelium. We elucidated crystallographically the interactions of FimH with D-mannose. The unique site binding pocket occupied by D-mannose was probed using site-directed mutagenesis. All but one of the mutants examined had greatly diminished mannose-binding activity and had also lost the ability to bind human bladder cells. The binding activity of the mono-saccharide D-mannose was delineated from this of mannotriose (Man(alpha 1-3)[Man(alpha 1-6)]Man) by generating mutants that abolished D-mannose binding but retained mannotriose binding activity. Our structure/function analysis demonstrated that the binding of the monosaccharide alpha-D-mannose is the primary bladder cell receptor for uropathogenic E. coli and that this event requires a highly conserved FimH binding pocket. The residues in the FimH mannose-binding pocket were sequenced and found to be invariant in over 200 uropathogenic strains of E. coli. Only enterohaemorrhagic E. coli (EHEC) possess a sequence variation within the mannose-binding pocket of FimH, suggesting a naturally occurring mechanism of attenuation in EHEC bacteria that would prevent them from being targeted to the urinary tract.

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Background Escherichia coli strains adhere to the normally sterile human uroepithelium using type 1 pili, that are long, hairy surface organelles exposing a mannose-binding FimH adhesin at the tip. A small percentage of adhered bacteria can successfully invade bladder cells, presumably via pathways mediated by the high-mannosylated uroplakin-Ia and α3β1 integrins found throughout the uroepithelium. Invaded bacteria replicate and mature into dense, biofilm-like inclusions in preparation of fluxing and of infection of neighbouring cells, being the major cause of the troublesome recurrent urinary tract infections.Methodology/Principal FindingsWe demonstrate that α-d-mannose based inhibitors of FimH not only block bacterial adhesion on uroepithelial cells but also antagonize invasion and biofilm formation. Heptyl α-d-mannose prevents binding of type 1-piliated E. coli to the human bladder cell line 5637 and reduces both adhesion and invasion of the UTI89 cystitis isolate instilled in mouse bladder via catheterization. Heptyl α-d-mannose also specifically inhibited biofilm formation at micromolar concentrations. The structural basis of the great inhibitory potential of alkyl and aryl α-d-mannosides was elucidated in the crystal structure of the FimH receptor-binding domain in complex with oligomannose-3. FimH interacts with Manα1,3Manβ1,4GlcNAcβ1,4GlcNAc in an extended binding site. The interactions along the α1,3 glycosidic bond and the first β1,4 linkage to the chitobiose unit are conserved with those of FimH with butyl α-d-mannose. The strong stacking of the central mannose with the aromatic ring of Tyr48 is congruent with the high affinity found for synthetic inhibitors in which this mannose is substituted for by an aromatic group.Conclusions/SignificanceThe potential of ligand-based design of antagonists of urinary tract infections is ruled by the structural mimicry of natural epitopes and extends into blocking of bacterial invasion, intracellular growth and capacity to fluxing and of recurrence of the infection.

FimH, the type 1 pilus adhesin of uropathogenic Escherichia coli (UPEC), contains a receptor-binding domain with an acidic binding pocket specific for mannose. The fim operon, and thus type 1 pilus production, is under transcriptional control via phase variation of an invertible promoter element. FimH is critical during urinary tract infection for mediating colonization and invasion of the bladder epithelium and establishment of intracellular bacterial communities (IBCs). In silico analysis of FimH gene sequences from 279 E. coli strains identified specific amino acids evolving under positive selection outside of its mannose-binding pocket. Mutating two of these residues (A27V/V163A) had no effect on phase variation, pilus assembly, or mannose binding in vitro. However, compared to wild-type, this double mutant strain exhibited a 10,000-fold reduction in mouse bladder colonization 24 h after inoculation and was unable to form IBCs even though it bound normally to mannosylated receptors in the urothelium. In contrast, the single A62S mutation altered phase variation, reducing the proportion of piliated cells, reduced mannose binding 8-fold, and decreased bladder colonization 30-fold in vivo compared to wild-type. A phase-locked ON A62S mutant restored virulence to wild-type levels even though in vitro mannose binding remained impaired. Thus, positive selection analysis of FimH has separated mannose binding from in vivo fitness, suggesting that IBC formation is critical for successful infection of the mammalian bladder, providing support for more general use of in silico positive selection analysis to define the molecular underpinnings of bacterial pathogenesis.type 1 pili ͉ uropathogenic Escherichia coli

SummaryType-1 fimbriae are important virulence factors for the establishment of Escherichia coli urinary tract infections. Bacterial adhesion to the highmannosylated uroplakin Ia glycoprotein receptors of bladder epithelium is mediated by the FimH adhesin. Previous studies have attributed differences in mannose-sensitive adhesion phenotypes between faecal and uropathogenic E. coli to sequence variation in the FimH receptor-binding domain. We find that FimH variants from uropathogenic, faecal and enterohaemorrhagic isolates express the same specificities and affinities for high-mannose structures. The only exceptions are FimHs from O157 strains that carry a mutation (Asn135Lys) in the mannose-binding pocket that abolishes all binding. A high-mannose microarray shows that all substructures are bound by FimH and that the largest oligomannose is not necessarily the best binder. Affinity measurements demonstrate a strong preference towards oligomannosides exposing Mana1-3Man at their non-reducing end. Binding is further enhanced by the b1-4-linkage to GlcNAc, where binding is 100-fold better than that of a-D-mannose. Mana1-3Manb1-4GlcNAc, a major oligosaccharide present in the urine of a-mannosidosis patients, thus constitutes a well-defined FimH epitope. Differences in affinities for high-mannose structures are at least 10-fold larger than differences in numbers of adherent bacteria between faecal and uropathogenic strains. Our results imply that the carbohydrate expression profile of targeted host tissues and of natural inhibitors in urine, such as TammHorsfall protein, are stronger determinants of adhesion than FimH variation.

Uropathogenic Escherichia coli (UPEC) are the major causative agents of urinary tract infections. During infection, UPEC adhere to mannosylated glycoreceptors on the urothelium via the FimH adhesin located at the tip of type 1 pili. Synthetic FimH antiadhesives such as alkyl and phenyl α-D-mannopyranosides are thus ideal candidates for the chemical interception of this crucial step in pathogenesis. The crystal structures of the FimH lectin domain in its ligand-free form and in complexes with eight medium- and high-affinity mannopyranoside inhibitors are presented. The thermodynamic profiles of the FimH-inhibitor interactions indicate that the binding of FimH to α-D-mannopyranose is enthalpy-driven and has a negative entropic change. Addition of a hydrophobic aglycon influences the binding enthalpy and can induce a favorable entropic change. The alleviation of the entropic cost is at least in part explained by increased dynamics in the tyrosine gate (Tyr48 and Tyr137) of the FimH receptor-binding site upon binding of the ligand. Ligands with a phenyl group directly linked to the anomeric oxygen of α-D-mannose introduce the largest dynamics into the Tyr48 side chain, because conjugation with the anomeric oxygen of α-D-mannose forces the aromatic aglycon into a conformation that comes into close contact (≈2.65 Å) with Tyr48. A propargyl group in this position predetermines the orientation of the aglycon and significantly decreases affinity. FimH has the highest affinity for α-D-mannopyranosides substituted with hydrophobic aglycons that are compatible in shape and electrostatic properties to the tyrosine gate, such as heptyl α-D-mannose.

Fullerene hexakis-adducts bearing 12 peripheral mannose moieties have been prepared by grafting sugar derivatives onto the fullerene core and assayed as inhibitors of FimH, a bacterial adhesin, using isothermal titration calorimetry, surface plasmon resonance and hemagglutination assays.