Linker insertion analysis of the FimH adhesin of type 1 fimbriae in an Escherichia coli fimH-null background

Screening of antigenic epitopes related to the adhesion of the avian Escherichia coli Type 1 Fimbrial Agglutinin Domain

BACKGROUND

Avian Escherichia coli (E.coli) type 1 fimbriae adhere to avian tracheal epithelial cells through the FimH protein. However, the adhesion-related antigen is still unknown. The purpose of this study was to analyze the antigenicity of the type 1 fimbrial FimH protein of wild-type avian E. coli, screen antigen epitopes, and prepare monoclonal antibodies (mAbs) that can block the adhesion of avian E. coli.

RESULTS

In this study, the nucleic acid homologies of MG2 (O11), TS12 (O18), and YR5 (O78) with K12 were 97.7%, 99.6%, and 97.7%, respectively, and the amino acid sequence similarity reached 98.7%, 99.3%, and 98.0%, respectively. The epitopes and hydrophilicities of the FimH proteins of these three strains were similar. The more obvious lectin domain epitopes were located at FimH protein positions 111-124 and 154-162. The mAbs 7C2 and 7D8 against these two epitopes were prepared. An adhesion inhibition test showed that 7C2 and 7D8 blocked bacterial adhesion to avian tracheal epithelial cells. The mAb 7C2 against the 111-124 epitope inhibited O78 strain adhesion by 93%, and the mAb 7D8 against the 154-162 epitope inhibited O78 strain adhesion by 49%, indicating that these two epitopes are closely related to the adhesion of type 1 fimbriae. However, only the 111-124 epitope-recognizing mAb 7C2 inhibited bacterial agglutination of erythrocytes, indicating that host cell receptor binding and erythrocyte agglutination are not mediated by the same spatial locations within the FimH protein.

CONCLUSIONS

The results demonstrate that the mAbs 7C2 and 7D8 against FimH protein positions 111-124 and 154-162 could inhibit the adhesion of E.coli to the chicken trachea.

Bacterial Lectin FimH and Its Aggregation Hot-Spots: An Alternative Strategy against Uropathogenic Escherichia coli

Type I fimbriae are the main adhesive organelles of uropathogenic Escherichia coli (UPEC), consisting of four different subunits. Their component with the most important role in establishing bacterial infections is the FimH adhesin located at the fimbrial tip. This two-domain protein mediates adhesion to host epithelial cells through interaction with terminal mannoses on epithelial glycoproteins. Here, we propose that the amyloidogenic potential of FimH can be exploited for the development of therapeutic agents against Urinary Tract Infections (UTIs). Aggregation-prone regions (APRs) were identified via computational methods, and peptide-analogues corresponding to FimH lectin domain APRs were chemically synthesized and studied with the aid of both biophysical experimental techniques and molecular dynamic simulations. Our findings indicate that these peptide-analogues offer a promising set of antimicrobial candidate molecules since they can either interfere with the folding process of FimH or compete for the mannose-binding pocket.

Chaperone-tip adhesin complex is vital for synergistic activation of CFA/I fimbriae biogenesis

Colonization factor CFA/I defines the major adhesive fimbriae of enterotoxigenic Escherichia coli and mediates bacterial attachment to host intestinal epithelial cells. The CFA/I fimbria consists of a tip-localized minor adhesive subunit, CfaE, and thousands of copies of the major subunit CfaB polymerized into an ordered helical rod. Biosynthesis of CFA/I fimbriae requires the assistance of the periplasmic chaperone CfaA and outer membrane usher CfaC. Although the CfaE subunit is proposed to initiate the assembly of CFA/I fimbriae, how it performs this function remains elusive. Here, we report the establishment of an in vitro assay for CFA/I fimbria assembly and show that stabilized CfaA-CfaB and CfaA-CfaE binary complexes together with CfaC are sufficient to drive fimbria formation. The presence of both CfaA-CfaE and CfaC accelerates fimbria formation, while the absence of either component leads to linearized CfaB polymers in vitro. We further report the crystal structure of the stabilized CfaA-CfaE complex, revealing features unique for biogenesis of Class 5 fimbriae.

Next-Generation Glycan Microarray Enabled by DNA-Coded Glycan Library and Next-Generation Sequencing Technology

Interactions of glycans with proteins, cells, and microorganisms play important roles in cell-cell adhesion and host-pathogen interaction. Glycan microarray technology, in which multiple glycan structures are immobilized on a single glass slide and interrogated with glycan-binding proteins (GBPs), has become an indispensable tool in the study of protein-glycan interactions. Despite its great success, the current format of the glycan microarray requires expensive, specialized instrumentation and labor-intensive assay and image processing procedures, which limit automation and possibilities for high-throughput analyses. Furthermore, the current microarray is not suitable for assaying interaction with intact cells due to their large size compared to the two-dimensional microarray surface. To address these limitations, we developed the next-generation glycan microarray (NGGM) based on artificial DNA coding of glycan structures. In this novel approach, a glycan library is presented as a mixture of glycans and glycoconjugates, each of which is coded with a unique oligonucleotide sequence (code). The glycan mixture is interrogated by GBPs followed by the separation of unbound coded glycans. The DNA sequences that identify individual bound glycans are quantitatively sequenced (decoded) by powerful next-generation sequencing (NGS) technology, and copied numbers of the DNA codes represent relative binding specificities of corresponding glycan structures to GBPs. We demonstrate that NGGM generates glycan-GBP binding data that are consistent with that generated in a slide-based glycan microarray. More importantly, the solution phase binding assay is directly applicable to identifying glycan binding to intact cells, which is often challenging using glass slide-based glycan microarrays.

Adhesion of Escherichia coli under flow conditions reveals potential novel effects of FimH mutations

FimH-mediated adhesion of Escherichia coli to bladder epithelium is a prerequisite for urinary tract infections. FimH is also essential for blood-borne bacterial dissemination, but the mechanisms are poorly understood. The purpose of this study was to assess the influence of different FimH mutations on bacterial adhesion using a novel adhesion assay, which models the physiological flow conditions bacteria are exposed to. We introduced 12 different point mutations in the mannose binding pocket of FimH in an E. coli strain expressing type 1 fimbriae only (MSC95-FimH). We compared the bacterial adhesion of each mutant across several commonly used adhesion assays, including agglutination of yeast, adhesion to mono- and tri-mannosylated substrates, and static adhesion to bladder epithelial and endothelial cells. We performed a comparison of these assays to a novel method that we developed to study bacterial adhesion to mammalian cells under flow conditions. We showed that E. coli MSC95-FimH adheres more efficiently to microvascular endothelium than to bladder epithelium, and that only endothelium supports adhesion at physiological shear stress. The results confirmed that mannose binding pocket mutations abrogated adhesion. We demonstrated that FimH residues E50 and T53 are crucial for adhesion under flow conditions. The coating of endothelial cells on biochips and modelling of physiological flow conditions enabled us to identify FimH residues crucial for adhesion. These results provide novel insights into screening methods to determine the effect of FimH mutants and potentially FimH antagonists.

Microevolution in fimH gene of mucosa-associated Escherichia coli strains isolated from pediatric patients with inflammatory bowel disease

Several studies reported increased numbers of mucosa-associated Escherichia coli strains in patients with inflammatory bowel disease (IBD), encompassing Crohn's disease (CD) and ulcerative colitis (UC). The majority of E. coli strains possess type 1 fimbriae, whose tip fibrillum protein, FimH, naturally undergoes amino acid replacements, an important process in the adaptation of commensal E. coli strains to environmental changes, like those observed in IBD and urinary tract infections. In this study, we analyzed mutational patterns in the fimH gene of 52 mucosa-associated E. coli strains isolated from IBD and non-IBD pediatric patients, in order to investigate microevolution of this genetic trait. FimH-positive strains were also phylogenetically typed and tested for their adhesive ability on Caco-2 cells. Specific FimH alleles for each grouping feature were found. Mutations G66S and V27A were related to CD, while mutations A242V, V163A, and T74I were attributed to UC. Otherwise, the G66S, N70S, and S78N mutations were specifically attributed to B2/D phylogroups. The N70S and A119V mutations were related to adhesive E. coli strains. Phylogroup B2, adhesive, and IBD E. coli strains showed a higher site substitution rate (SSR) in the fimH gene, together with a higher number of mutations. The degree of naïve mucosal inflammation was related to specific FimH alleles. Moreover, we could suggest that the V27A mutation is pathoadaptive for the CD intestinal habitat, while we could also suggest that both the N70S and S78N mutations are related to the more virulent E. coli B2 phylogroup. In conclusion, we found some FimH variants that seem to be more involved than others in the evolution of IBD pathogenesis.

Transcriptional response of E. coli upon FimH-mediated fimbrial adhesion

Functionalities which may be genetically programmed into a bacterium are limited by its range of possible activities and its sensory capabilities. Therefore, enhancing the bacterial sensory repertoire is a crucial step for expanded utility in potential biomedical, industrial or environmental applications. Using microarray and qRT-PCR analyses, we have investigated transcription in E. coli (strain CSH50) following FimH-mediated adhesion to biocompatible substrates. Specifically, wild-type FimH-mediated adhesion of E. coli to mannose agarose beads and His-tagged FimH-mediated adhesion to Ni²⁺-NTA beads both led to induction of ahpCF, dps, grxA and marRAB genes among bound cells relative to unbound cells. The strongly-induced genes are known to be regulated by OxyR or SoxS cytoplasmic redox sensors. Some differentially altered genes also overlapped with those implicated in biofilm formation. This study provides an insight into transcriptional events following FimH-mediated adhesion and may provide a platform for elucidation of the signaling circuit necessary for engineering a synthetic attachment response in E. coli.

Reconstitution of pilus assembly reveals a bacterial outer membrane catalyst

Type 1 pili from uropathogenic Escherichia coli are a prototype of adhesive surface organelles assembled and secreted by the conserved chaperone/usher pathway. We reconstituted type 1 pilus biogenesis from purified pilus proteins. The usher FimD acted as a catalyst to accelerate the ordered assembly of protein subunits independently of cellular energy. Its activity was highly dependent on the adhesin subunit FimH, which triggered the conversion of FimD into a high-efficiency assembly catalyst. Furthermore, a simple kinetic model adequately rationalized usher-catalyzed pilus assembly in vivo. Our results contribute to a mechanistic understanding of protein-catalyzed biogenesis of supramolecular protein complexes at the bacterial outer cell membrane.

Recognition of the N-terminal lectin domain of FimH adhesin by the usher FimD is required for type 1 pilus biogenesis

In this work we discover that a specific recognition of the N-terminal lectin domain of FimH adhesin by the usher FimD is essential for the biogenesis of type 1 pili in Escherichia coli. These filamentous organelles are assembled by the chaperone-usher pathway, in which binary complexes between fimbrial subunits and the periplasmic chaperone FimC are recognized by the outer membrane protein FimD (the usher). FimH adhesin initiates fimbriae polymerization and is the first subunit incorporated in the filament. Accordingly, FimD shows higher affinity for the FimC/FimH complex although the structural basis of this specificity is unknown. We have analysed the assembly into fimbria, and the interaction with FimD in vivo, of FimH variants in which the N-terminal lectin domain of FimH was deleted or substituted by different immunoglobulin (Ig) domains, or in which these Ig domains were fused to the N-terminus of full-length FimH. From these data, along with the analysis of a FimH mutant with a single amino acid change (G16D) in the N-terminal lectin domain, we conclude that the lectin domain of FimH is recognized by FimD usher as an essential step for type 1 pilus biogenesis.

The asymptomatic bacteriuria Escherichia coli strain 83972 outcompetes uropathogenic E. coli strains in human urine

Escherichia coli is the most common organism associated with asymptomatic bacteriuria (ABU). In contrast to uropathogenic E. coli (UPEC), which causes symptomatic urinary tract infections (UTI), very little is known about the mechanisms by which these strains colonize the human urinary tract. The prototype ABU E. coli strain 83972 was originally isolated from a girl who had carried it asymptomatically for 3 years. Deliberate colonization of UTI-susceptible individuals with E. coli 83972 has been used successfully as an alternative approach for the treatment of patients who are refractory to conventional therapy. Colonization with strain 83972 appears to prevent infection with UPEC strains in such patients despite the fact that this strain is unable to express the primary adhesins involved in UTI, viz. P and type 1 fimbriae. Here we investigated the growth characteristics of E. coli 83972 in human urine and show that it can outcompete a representative spectrum of UPEC strains for growth in urine. The unique ability of ABU E. coli 83972 to outcompete UPEC in urine was also demonstrated in a murine model of human UTI, confirming the selective advantage over UPEC in vivo. Comparison of global gene expression profiles of E. coli 83972 grown in lab medium and human urine revealed significant differences in expression levels in the two media; significant down-regulation of genes encoding virulence factors such as hemolysin, lipid A, and capsular polysaccharides was observed in cells grown in urine. Clearly, divergent abilities of ABU E. coli and UPEC to exploit human urine as a niche for persistence and survival suggest that these key differences may be exploited for preventative and/or therapeutic approaches.

Molecular characterization of the Escherichia coli asymptomatic bacteriuria strain 83972: the taming of a pathogen

Escherichia coli 83972 is a clinical asymptomatia bacteriuric isolate that is able to colonize the human urinary bladder without inducing an immune response. Here we demonstrate that one of the mechanisms by which this strain has become attenuated is through the mutation of its genes encoding type 1 and P fimbriae.

The Distinct Binding Specificities Exhibited by Enterobacterial Type 1 Fimbriae Are Determined by Their Fimbrial Shafts

Type 1 fimbriae of enterobacteria are heteropolymeric organelles of adhesion composed of FimH, a mannose-binding lectin, and a shaft composed primarily of FimA. We compared the binding activities of recombinant clones expressing type 1 fimbriae from Escherichia coli, Klebsiella pneumoniae, and Salmonella typhimurium for gut and uroepithelial cells and for various soluble mannosylated proteins. Each fimbria was characterized by its capacity to bind particular epithelial cells and to aggregate mannoproteins. However, when each respective FimH subunit was cloned and expressed in the absence of its shaft as a fusion protein with MalE, each FimH bound a wide range of mannose-containing compounds. In addition, we found that expression of FimH on a heterologous fimbrial shaft, e.g. K. pneumoniae FimH on the E. coli fimbrial shaft or vice versa, altered the binding specificity of FimH such that it closely resembled that of the native heterologous type 1 fimbriae. Furthermore, attachment to and invasion of bladder epithelial cells, which were mediated much better by native E. coli type 1 fimbriae compared with native K. pneumoniae type 1 fimbriae, were found to be dependent on the background of the fimbrial shaft (E. coli versus K. pneumoniae) rather than the background of the FimH expressed. Thus, the distinct binding specificities of different enterobacterial type 1 fimbriae cannot be ascribed solely to the primary structure of their respective FimH subunits, but are also modulated by the fimbrial shaft on which each FimH subunit is presented, possibly through conformational constraints imposed on FimH by the fimbrial shaft. The capacity of type 1 fimbrial shafts to modulate the tissue tropism of different enterobacterial species represents a novel function for these highly organized structures.

Inactivation of ompX causes increased interactions of type 1 fimbriated Escherichia coli with abiotic surfaces

During the initial steps of biofilm formation, bacteria have to adapt to a major change in their environment. The adhesion-induced phenotypic changes in a type 1 fimbriated Escherichia coli strain included reductions in the levels of several outer membrane proteins, one of which was identified as OmpX. Here, the phenotypes of mutant strains that differ at the ompX locus were studied with regard to adhesion, cell surface properties, and resistance to stress and antimicrobial compounds. The kinetics of adhesion were measured online by an extended quartz crystal microbalance technique for wild-type and mutant strains with a fimbriated or nonfimbriated background. Deletion of ompX led to significantly increased cell-surface contact in fimbriated strains but to decreased cell-surface contact in a nonfimbriated strain. Phenotypic characterization of the ompX mutant demonstrated that ompX interferes with proper regulation of cell surface structures that play a key role in mediating firm contact of the cell with a surface (i.e., type 1 fimbriae, flagellae, and exopolysaccharides). These phenotypic changes were accompanied by increased tolerance to several antibiotic compounds and sodium dodecyl sulfate. Based on these results, we propose that changes in the composition of outer membrane proteins during fimbria-mediated adhesion may be part of a coordinated adaptive response to the attached mode of growth.

Mapping the binding domain of the F18 fimbrial adhesin

F18 fimbrial Esherichia coli strains are associated with porcine postweaning diarrhea and pig edema disease. Recently, the FedF subunit was identified as the adhesin of the F18 fimbriae. In this study, adhesion domains of FedF were further studied by constructing deletions within the fedF gene and expressing FedF proteins with deletions either together with the other F18 fimbrial subunits or as fusion proteins tagged with maltose binding protein. The region essential for adhesion to porcine intestinal epithelial cells was mapped between amino acid residues 60 and 109 of FedF. To map the binding domain even more closely, all eight charged amino acid residues within this region were independently replaced by alanine. Three of these single point mutants expressing F18 fimbriae exhibited significantly diminished capabilities to adhere to porcine epithelial cells in vitro. In addition, a triple point mutation and a double point mutation completely abolished receptor adhesiveness. The result further confirmed that the region between amino acid residues 60 and 109 is essential for the binding of F18 fimbriae to their receptor. In addition, the adhesion capability of the binding domain was eliminated after treatment with iodoacetamide, suggesting the formation of a disulfide bridge between Cys-63 and Cys-83, whereas Cys-111 and Cys-116 could be deleted without affecting the binding ability of FedF.

Antigen 43-mediated autotransporter display, a versatile bacterial cell surface presentation system

Antigen 43 (Ag43), a self-recognizing outer membrane protein of Escherichia coli, has been converted into an efficient and versatile tool for surface display of foreign protein segments. Ag43 is an autotransporter protein characterized by the feature that all information required for transport to the outer membrane and secretion through the cell envelope is contained within the protein itself. Ag43 consists of two subunits (alpha and beta), where the beta-subunit forms an integral outer membrane translocator to which the alpha-subunit is noncovalently attached. The simplicity of the Ag43 system makes it ideally suited as a surface display scaffold. Here we demonstrate that the Ag43 alpha-module can accommodate and display correctly folded inserts and has the ability to display entire functional protein domains, exemplified by the FimH lectin domain. The presence of heterologous cysteine bridges does not interfere with surface display, and Ag43 chimeras are correctly processed into alpha- and beta-modules, offering optional and easy release of the chimeric alpha-subunits. Furthermore, Ag43 can be displayed in many gram-negative bacteria. This feature is exploited for display of our chimeras in an attenuated Salmonella strain.

Novel Zn(2+)-chelating peptides selected from a fimbria-displayed random peptide library

The display of peptide sequences on the surface of bacteria is a technology that offers exciting applications in biotechnology and medical research. Type 1 fimbriae are surface organelles of Escherichia coli which mediate D-mannose-sensitive binding to different host surfaces by virtue of the FimH adhesin. FimH is a component of the fimbrial organelle that can accommodate and display a diverse range of peptide sequences on the E. coli cell surface. In this study we have constructed a random peptide library in FimH. The library, consisting of approximately 40 million individual clones, was screened for peptide sequences that conferred on recombinant cells the ability to bind Zn(2+). By serial selection, sequences that exhibited various degrees of binding affinity and specificity toward Zn(2+) were enriched. None of the isolated sequences showed similarity to known Zn(2+)-binding proteins, indicating that completely novel Zn(2+)-binding peptide sequences had been isolated. By changing the protein scaffold system, we demonstrated that the Zn(2+)-binding seems to be uniquely mediated by the peptide insert and to be independent of the sequence of the carrier protein. These findings might be applied in the design of biomatrices for bioremediation purposes or in the development of sensors for detection of heavy metals.

Valency conversion in the type 1 fimbrial adhesin of Escherichia coli

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.

Adhesion of type 1-fimbriated Escherichia coli to abiotic surfaces leads to altered composition of outer membrane proteins

Phenotypic differences between planktonic bacteria and those attached to abiotic surfaces exist, but the mechanisms involved in the adhesion response of bacteria are not well understood. By the use of two-dimensional (2D) polyacrylamide gel electrophoresis, we have demonstrated that attachment of Escherichia coli to abiotic surfaces leads to alteration in the composition of outer membrane proteins. A major decrease in the abundance of resolved proteins was observed during adhesion of type 1-fimbriated E. coli strains, which was at least partly caused by proteolysis. Moreover, a study of fimbriated and nonfimbriated mutants revealed that these changes were due mainly to type 1 fimbria-mediated surface contact and that only a few changes occurred in the outer membranes of nonfimbriated mutant strains. Protein synthesis and proteolytic degradation were involved to different extents in adhesion of fimbriated and nonfimbriated cells. While protein synthesis appeared to affect adhesion of only the nonfimbriated strain, proteolytic activity mostly seemed to contribute to adhesion of the fimbriated strain. Using matrix-assisted laser desorption ionization-time of flight mass spectrometry, six of the proteins resolved by 2D analysis were identified as BtuB, EF-Tu, OmpA, OmpX, Slp, and TolC. While the first two proteins were unaffected by adhesion, the levels of the last four were moderately to strongly reduced. Based on the present results, it may be suggested that physical interactions between type 1 fimbriae and the surface are part of a surface-sensing mechanism in which protein turnover may contribute to the observed change in composition of outer membrane proteins. This change alters the surface characteristics of the cell envelope and may thus influence adhesion.

Biofilm formation in a hydrodynamic environment by novel fimh variants and ramifications for virulence

Type 1 fimbriae are surface-located adhesion organelles of Escherichia coli that are directly associated with virulence of the urinary tract. They mediate D-mannose-sensitive binding to different host surfaces by way of the minor fimbrial component FimH. Naturally occurring variants of FimH that bind strongly to terminally exposed monomannose residues have been associated with a pathogenicity-adaptive phenotype that enhances E. coli colonization of extraintestinal locations such as the urinary tract. The FimH adhesin also promotes biofilm formation in a mannose-inhibitable manner on abiotic surfaces under static growth conditions. In this study, we used random mutagenesis combined with a novel selection-enrichment technique to specifically identify mutations in the FimH adhesin that confer on E. coli the ability to form biofilms under hydrodynamic flow (HDF) conditions. We identified three FimH variants from our mutant library that could mediate an HDF biofilm formation phenotype to various degrees. This phenotype was induced by the cumulative effect of multiple changes throughout the receptor-binding region of the protein. Two of the HDF biofilm-forming FimH variants were insensitive to mannose inhibition and represent novel phenotypes not previously identified in naturally occurring isolates. Characterization of our enriched clones revealed some similarities to amino acid alterations that occur in urinary tract infection (UTI) strains. Subsequent screening of a selection of UTI FimH variants demonstrated that they too could promote biofilm formation on abiotic surfaces under HDF conditions. Interestingly, the same correlation was not observed for commensal FimH variants. FimH is a multifaceted protein prone to rapid microevolution. In addition to its previously documented roles in adherence and invasion, we have now demonstrated its function in biofilm formation on abiotic surfaces subjected to HDF conditions. The study indicates that UTI FimH variants possess adaptations that enhance biofilm formation and suggests a novel role for FimH in UTIs associated with medical implants such as catheters.

Chaperone-assisted pilus assembly and bacterial attachment

Bacterial pili assembled by the chaperone-usher pathway can mediate microbial attachment, an early step in the establishment of an infection, by binding specifically to sugars present in host tissues. Recent work has begun to reveal the structural basis both of chaperone function in the biogenesis of these pili and of bacterial attachment.

Inhibitory effects of plant polyphenoloxidase on colonization factors of Streptococcus sobrinus 6715

Exogenously added polyphenoloxidase (EC 1.14.18.1), an enzyme which oxidizes tyrosine residues and is commonly found in many dietary components, abolished the aggregation of Streptococcus sobrinus 6715 by high-molecular-weight dextran. The enzyme decreased glucan-binding lectin and/or glucosyltransferase I activities.

Antigen 43 from Escherichia coli induces inter- and intraspecies cell aggregation and changes in colony morphology of Pseudomonas fluorescens

Antigen 43 (Ag43) is a surface-displayed autotransporter protein of Escherichia coli. By virtue of its self-association characteristics, this protein is able to mediate autoaggregation and flocculation of E. coli cells in static cultures. Additionally, surface display of Ag43 is associated with a distinct frizzy colony morphology in E. coli. Here we show that Ag43 can be expressed in a functional form on the surface of the environmentally important Pseudomonas fluorescens strain SBW25 with ensuing cell aggregation and frizzy colony types. Using green fluorescence protein-tagged cells, we demonstrate that Ag43 can be used as a tool to provide interspecies cell aggregation between E. coli and P. fluorescens. Furthermore, Ag43 expression enhances biofilm formation in P. fluorescens to glass surfaces. The versatility of this protein was also reflected in Ag43 surface display in a variety of other gram-negative bacteria. Display of heterologous Ag43 in selected bacteria might offer opportunities for rational design of multispecies consortia where the concerted action of several bacterial species is required, e.g., waste treatment and degradation of pollutants.

Expression and purification of the mannose recognition domain of the FimH adhesin

Type 1 fimbriae have been shown to be specifically required for Escherichia coli colonisation and pathogenesis of the urinary tract. These structural organelles mediate specific adhesion to alpha-D-mannosides by virtue of the FimH adhesin. FimH is a two-domain protein in which the N-terminal domain contains the receptor-binding site and the C-terminal domain is required for organelle integration. To date, FimH has only been isolated as a complex with the system-specific chaperone FimC. Here we report that a functional form of the FimH receptor-binding domain can be readily isolated and characterised by replacing the C-terminal domain with a histidine tag.

Genetic characterization of Escherichia coli type 1 pilus adhesin mutants and identification of a novel binding phenotype

Five Escherichia coli type 1 pilus mutants that had point mutations in fimH, the gene encoding the type 1 pilus adhesin FimH, were characterized. FimH is a minor component of type 1 pili that is required for the pili to bind and agglutinate guinea pig erythrocytes in a mannose-inhibitable manner. Point mutations were located by DNA sequencing and deletion mapping. All mutations mapped within the signal sequence or in the first 28% of the predicted mature protein. All mutations were missense mutations except for one, a frameshift lesion that was predicted to cause the loss of approximately 60% of the mature FimH protein. Bacterial agglutination tests with polyclonal antiserum raised to a LacZ-FimH fusion protein failed to confirm that parental amounts of FimH cross-reacting material were expressed in four of the five mutants. The remaining mutant, a temperature-sensitive (ts) fimH mutant that agglutinated guinea pig erythrocytes after growth at 31 degrees C but not at 42 degrees C, reacted with antiserum at both temperatures in a manner similar to the parent. Consequently, this mutant was chosen for further study. Temperature shift experiments revealed that new FimH biosynthesis was required for the phenotypic change. Guinea pig erythrocyte and mouse macrophage binding experiments using the ts mutant grown at the restrictive and permissive temperatures revealed that whereas erythrocyte binding was reduced to a level comparable to that of a fimH insertion mutant at the restrictive temperature, mouse peritoneal macrophages were bound with parental efficiency at both the permissive and restrictive temperatures. Also, macrophage binding by the ts mutant was insensitive to mannose inhibition after growth at 42 degrees C but sensitive after growth at 31 degrees C. The ts mutant thus binds macrophages with one receptor specificity at 31 degrees C and another at 42 degrees C.

Fimbriae-assisted bacterial surface display of heterologous peptides

The display of peptide segments on the surface of bacteria offers many new and exciting applications in biotechnology and medical research. Fimbria-assisted display of heterologous sequences is a paradigm for chimeric organelle display on bacteria. Fimbriae are particularly attractive candidates for epitope display for several reasons: (1) they are present in extremely high numbers at the cell surface, (2) they are strong immunogens, (3) they possess inherent adhesive properties, and (4) they can be easily purified. The majority of work dealing with fimbria-assisted peptide display has been focused on the development of recombinant vaccines. A number of different fimbrial types have been used to display immune-relevant sectors of various foreign proteins. Chimeric fimbrial vaccines can be used in the context of purified proteins, however the potential also exists to exploit this technology for the development of live recombinant vaccines. Work has also been performed demonstrating the amenability of fimbriae towards the powerful technology of random peptide display. This review summarises the current state of research in this field.

Functional flexibility of the FimH adhesin: insights from a random mutant library

Type 1 fimbriae are surface organelles of Escherichia coli which mediate D-mannose-sensitive binding to different host surfaces. This binding is conferred by the minor fimbrial component FimH. Naturally occurring variants of the FimH protein have been selected in nature for their ability to recognize specific receptor targets. In particular, variants that bind strongly to terminally exposed monomannose residues have been associated with a pathogenicity-adaptive phenotype that enhances E. coli colonization of extraintestinal locations such as the urinary bladder. In this study we have used random mutagenesis to specifically identify nonselective mutations in the FimH adhesin which modify its binding phenotype. Isogenic E. coli clones expressing FimH variants were tested for their ability to bind yeast cells and model glycoproteins that contain oligosaccharide moieties rich in either terminal monomannose, oligomannose, or nonmannose residues. Both the monomannose- and the oligomannose-binding capacity of type 1 fimbriae could be altered by minor amino acid changes in the FimH protein. The monomannose-binding phenotype was particularly sensitive to changes, with extensive differences in binding being observed in comparison to wild-type FimH levels. Different structural alterations were able to cause similar functional changes in FimH, suggesting a high degree of flexibility to target recognition by this adhesin. Alteration of residue P49 of the mature FimH protein, which occurs within the recently elucidated carbohydrate-binding pocket of FimH, completely abolished its function. Amino acid changes that increased the binding capacity of FimH were located outside receptor-interacting residues, indicating that functional changes relevant to pathogenicity are likely to be due to conformational changes of the adhesin.

Bacterial adhesins: function and structure

Specific adhesion to host tissue cells is an essential virulence factor of most bacterial pathogens. The fundamental processes that determine bacterial attachment to host tissue surfaces are mediated by microbial adhesins. Host specificity and tissue tropism are characteristics exhibited by different bacteria and are determined (at least in part) by the interaction between adhesins and their complementary receptors on host cell surfaces. A detailed picture of how bacteria are able to target to various receptors is emerging. A large number of bacterial adhesins with individual receptor specificities have been identified. Furthermore, recent research has shown that individual adhesins are prone to rapid microevolution that results in changes in the receptor specificity of individual adhesins. Microbial adhesins are often assembled into complex polymeric organelle structures, however non-organelle adhesins linked to the cell surface as monomers or simple oligomers also exist. This review gives an overview of bacterial adhesins and focuses on some general aspects of their biogenesis and role in bacterial colonization of host cell surfaces and as virulence factors.

Antigen 43 and type 1 fimbriae determine colony morphology of Escherichia coli K-12

Colony morphology has been used as an important identification and characterization criterion in bacteriology for many decades. However, the molecular mechanisms underlying the appearance of different colony types have been given little attention. The synthesis of O antigen is defunct in Escherichia coli K-12, and colonies should accordingly only appear to be rough. However, previous reports have noted the presence of different interchangeable colony morphology types. In this study we have addressed the influence of two phase-variable surface structures, antigen 43 and type 1 fimbriae, on colony morphology. Due to differential expression of these structures, four different colony phenotypes could be distinguished. By creating and studying defined mutants of the respective loci, i.e. , flu and fim, we conclude that the presence or absence of the corresponding gene products on the cells correlates with the observed colony morphology forms. Interestingly, the habitat specificity of bacteria under static liquid conditions seems to correlate with the colony phenotypes.

Effect of ionic strength on initial interactions of Escherichia coli with surfaces, studied on-line by a novel quartz crystal microbalance technique

A novel quartz crystal microbalance (QCM) technique was used to study the adhesion of nonfimbriated and fimbriated Escherichia coli mutant strains to hydrophilic and hydrophobic surfaces at different ionic strengths. This technique enabled us to measure both frequency shifts (Deltaf), i.e., the increase in mass on the surface, and dissipation shifts (DeltaD), i.e., the viscoelastic energy losses on the surface. Changes in the parameters measured by the extended QCM technique reflect the dynamic character of the adhesion process. We were able to show clear differences in the viscoelastic behavior of fimbriated and nonfimbriated cells attached to surfaces. The interactions between bacterial cells and quartz crystal surfaces at various ionic strengths followed different trends, depending on the cell surface structures in direct contact with the surface. While Deltaf and DeltaD per attached cell increased for nonfimbriated cells with increasing ionic strengths (particularly on hydrophobic surfaces), the adhesion of the fimbriated strain caused only low-level frequency and dissipation shifts on both kinds of surfaces at all ionic strengths tested. We propose that nonfimbriated cells may get better contact with increasing ionic strengths due to an increased area of contact between the cell and the surface, whereas fimbriated cells seem to have a flexible contact with the surface at all ionic strengths tested. The area of contact between fimbriated cells and the surface does not increase with increasing ionic strengths, but on hydrophobic surfaces each contact point seems to contribute relatively more to the total energy loss. Independent of ionic strength, attached cells undergo time-dependent interactions with the surface leading to increased contact area and viscoelastic losses per cell, which may be due to the establishment of a more intimate contact between the cell and the surface. Hence, the extended QCM technique provides new qualitative information about the direct contact of bacterial cells to surfaces and the adhesion mechanisms involved.

X-ray structure of the FimC-FimH chaperone-adhesin complex from uropathogenic Escherichia coli

Type 1 pili-adhesive fibers expressed in most members of the Enterobacteriaceae family-mediate binding to mannose receptors on host cells through the FimH adhesin. Pilus biogenesis proceeds by way of the chaperone/usher pathway. The x-ray structure of the FimC-FimH chaperone-adhesin complex from uropathogenic Escherichia coli at 2.5 angstrom resolution reveals the basis for carbohydrate recognition and for pilus assembly. The carboxyl-terminal pilin domain of FimH has an immunoglobulin-like fold, except that the seventh strand is missing, leaving part of the hydrophobic core exposed. A donor strand complementation mechanism in which the chaperone donates a strand to complete the pilin domain explains the basis for both chaperone function and pilus biogenesis.

Antigen-43-mediated autoaggregation of Escherichia coli is blocked by fimbriation

Antigen 43 (Ag43), the product of the flu gene, is a surface-displayed autotransporter protein of Escherichia coli. Ag43 is responsible for the autoaggregation and flocculation of static liquid cultures of many E. coli strains. The expression of Ag43 has been reported to be phase variable and controlled by the product of the oxyR gene. Type 1 fimbriae are thin adhesive thread-like surface organelles responsible for bacterial receptor recognition and tissue colonization. Like that of Ag43, the expression of type 1 fimbriae is phase variable. Interestingly, previous results have suggested that the expression of type 1 fimbriae and the expression of Ag43 are mutually exclusive. In the present report, we show, by use of well-defined mutants, that fimbriation abolishes Ag43-mediated autoaggregation but does not affect Ag43 expression. Autoaggregation is shown to require an intercellular Ag43-Ag43 interaction, and the physical presence of fimbriae on the cells seems to abrogate this interaction. The Ag43 or OxyR status does not appear to influence fimbria expression, and our results suggest that the expression of Ag43 and the expression of fimbriae are independent processes.

Bioaccumulation of heavy metals by fimbrial designer adhesins

Naturally occurring adhesins bind to specific molecular targets in a lock-and-key fashion due to the composition of the binding domain of the adhesin. By introduction of random peptide libraries in a suitable surface exposed carrier protein it is possible to create and select designer adhesins with novel binding affinities. Type 1 fimbriae are surface organelles of Escherichia coli which mediate D-mannose sensitive binding to different host surfaces through the FimH adhesin, an integral part of these organelles. We have studied the ability of the FimH adhesin to display random peptide sequences. By serial selection and enrichment procedures specific sequences were identified which conferred the ability on recombinant cells to adhere to various metal oxides (PbO2, CoO, MnO2, Cr2O3). The properties inherent in these sequences permitted the distinct recognition of metals to varying degrees, indicating that this system allow for the isolation of peptide sequences with a variety of binding avidities. These studies demonstrate the potential and versatility of the FimH display system for presenting random peptide sequences. In addition, the possibility exists for the construction of microorganisms for the bioaccumulation of heavy metals from the environment.

Pathogenic adaptation of Escherichia coli by natural variation of the FimH adhesin

Conventional wisdom regarding mechanisms of bacterial pathogenesis holds that pathogens arise by external acquisition of distinct virulence factors, whereas determinants shared by pathogens and commensals are considered to be functionally equivalent and have been ignored as genes that could become adapted specifically for virulence. It is shown here, however, that genetic variation in an originally commensal trait, the FimH lectin of type 1 fimbriae, can change the tropism of Escherichia coli, shifting it toward a urovirulent phenotype. Random point mutations in fimH genes that increase binding of the adhesin to mono-mannose residues, structures abundant in the oligosaccharide moieties of urothelial glycoproteins, confer increased virulence in the mouse urinary tract. These mutant FimH variants, however, are characterized by increased sensitivity to soluble inhibitors bathing the oropharyngeal mucosa, the physiological portal of E. coli. This functional trade-off seems to be detrimental for the intestinal ecology of the urovirulent E. coli. Thus, bacterial virulence can be increased by random functional mutations in a commensal trait that are adaptive for a pathologic environment, even at the cost of reduced physiological fitness in the nonpathologic habitat.

Probing the receptor recognition site of the FimH adhesin by fimbriae-displayed FimH-FocH hybrids

Type 1 fimbriae are surface organelles of Escherichia coli which mediate D-mannose-sensitive binding to different host surfaces. This binding is conferred by the minor fimbrial component FimH. The binding domain of the FimH adhesin has been studied by constructing hybrids of FimH and a homologous protein, FocH, originating from F1C fimbriae. F1C fimbriae do not bind to D-mannosides or confer agglutination of any known types of erythrocytes or yeast. It was previously shown that the FocH protein can be readily substituted by the FimH adhesin, resulting in hybrid fimbriae with the same binding characteristics as type 1 fimbriae. The receptor binding of fimbriae-presented chimeric FimH-FocH hybrids was studied. FimH-FocH fusions encompassing 72% of the N-terminus of FimH fused to the complementary sector of FocH conferred agglutination of erythrocytes and yeast cells at a comparable level to FimH. Surprisingly, it was also found that similar fusions containing between 56 and 66% FimH still conferred binding to yeast cells, D-mannose-BSA and D-mannose-beads but did not give rise to agglutination. The receptor binding capacity of fusions containing 50% or less of the FimH N-terminal region was virtually abolished. The results point to the presence of a D-mannose-receptor-binding core domain in FimH, the affinity of which is modulated by other sectors of the protein to enable binding to extended mannose-containing targets.

Heterobinary adhesins based on the Escherichia coli FimH fimbrial protein

The FimH adhesin of Escherichia coli type 1 fimbriae confers the ability to bind to D-mannosides by virtue of a receptor-binding domain located in its N-terminal region. This protein was engineered into a heterobifunctional adhesin by introducing a secondary binding site in the C-terminal region. The insertion of histidine clusters into this site resulted in coordination of various metal ions by recombinant cells expressing chimeric FimH proteins. In addition, libraries consisting of random peptide sequences inserted into the FimH display system and screened by a "panning" technique were used to identify specific sequences conferring the ability to adhere to Ni2+ and Cu2+. Recombinant cells expressing heterobifunctional FimH adhesins could adhere simultaneously to both metals and saccharides. Finally, combining the metal-binding modifications with alterations in the natural receptor-binding region demonstrated the ability to independently modulate the binding of FimH to two ligands simultaneously.

Authentic display of a cholera toxin epitope by chimeric type 1 fimbriae: effects of insert position and host background

The potential of the major structural protein of type 1 fimbriae as a display system for heterologous sequences was tested. As a reporter-epitope, a heterologous sequence mimicking a neutralizing epitope of the cholera toxin B chain was inserted, in one or two copies, into four different positions in the fimA gene. This was carried out by introduction of new restriction sites by PCR-mediated site-directed mutagenesis of fimA in positions predicted to correspond to optimally surface-located regions of the subunit protein. Subsequently, the synthetic cholera-toxin-encoding DNA segment was inserted. Several of the chosen positions seemed amenable even for large foreign inserts; the chimeric proteins were exposed on the bacterial surface and the cholera toxin epitope was authentically displayed, i.e. it was recognized on bacteria by specific antiserum. Display of chimeric fimbriae was tested with respect to host background in three different Escherichia coli strains, i.e. an isogenic set of K-12 strains, differing in the presence of an indigenous fim gene cluster, as well as a wild-type isolate. Immunization of rabbits with purified chimeric fimbriae resulted in serum which specifically recognized cholera toxin B chain, confirming the utility of the employed strategy.

Type 1 fimbrial expression enhances Escherichia coli virulence for the urinary tract

Type 1 fimbriae are adhesion organelles expressed by many Gram-negative bacteria. They facilitate adherence to mucosal surfaces and inflammatory cells in vitro, but their contribution to virulence has not been defined. This study presents evidence that type 1 fimbriae increase the virulence of Escherichia coli for the urinary tract by promoting bacterial persistence and enhancing the inflammatory response to infection. In a clinical study, we observed that disease severity was greater in children infected with E. coli O1:K1:H7 isolates expressing type 1 fimbriae than in those infected with type 1 negative isolates of the same serotype. The E. coli O1:K1:H7 isolates had the same electrophoretic type, were hemolysin-negative, expressed P fimbriae, and carried the fim DNA sequences. When tested in a mouse urinary tract infection model, the type 1-positive E. coli O1:K1:H7 isolates survived in higher numbers, and induced a greater neutrophil influx into the urine, than O1:K1:H7 type 1-negative isolates. To confirm a role of type 1 fimbriae, a fimH null mutant (CN1016) was constructed from an O1:K1:H7 type 1-positive parent. E. coli CN1016 had reduced survival and inflammatogenicity in the mouse urinary tract infection model. E. coli CN1016 reconstituted with type 1 fimbriae (E. coli CN1018) had restored virulence similar to that of the wild-type parent strain. These results show that type 1 fimbriae in the genetic background of a uropathogenic strain contribute to the pathogenesis of E. coli in the urinary tract.