Fragmentation of Escherichia coli type 1 fimbriae exposes cryptic D-mannose-binding sites

Biochemical characterization of the Escherichia coli surfaceome: a focus on type I fimbriae and flagella

The Escherichia coli surfaceome consists mainly of the large surface organelles expressed by the organism to navigate and interact with the surrounding environment. The current study focuses on type I fimbriae and flagella. These large polymeric surface organelles are composed of hundreds to thousands of subunits, with their large size often preventing them from being studied in their native form. Recent studies are accumulating which demonstrate the glycosylation of surface proteins or virulence factors in pathogens, including E. coli. Using biochemical and glycobiological techniques, including biotin-hydrazide labeling of glycans and chemical and glycosidase treatments, we demonstrate (i) the presence of a well-defined and chemically resistant FimA oligomer in several strains of pathogenic and non-pathogenic E. coli, (ii) the major subunit of type I fimbriae, FimA, in pathogenic and laboratory strains is recognized by concanavalin A, (iii) standard methods to remove N-glycans (PNGase F) or a broad-specificity mannosidase fail to remove the glycan structure, despite the treatments resulting in altered migration in SDS-PAGE, (iv) PNGase F treatment results in a novel 32 kDa band recognized by anti-FliC antiserum. While the exact identity of the glycan(s) and their site of attachment currently elude detection by conventional glycomics/glycoproteomics, the current findings highlight a potential additional layer of complexity of the surface (glyco) proteome of the commensal or adhesive and invasive E. coli strains studied.

Animal Enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) is the most common cause of E. coli diarrhea in farm animals. ETEC are characterized by the ability to produce two types of virulence factors: adhesins that promote binding to specific enterocyte receptors for intestinal colonization and enterotoxins responsible for fluid secretion. The best-characterized adhesins are expressed in the context of fimbriae, such as the F4 (also designated K88), F5 (K99), F6 (987P), F17, and F18 fimbriae. Once established in the animal small intestine, ETEC produce enterotoxin(s) that lead to diarrhea. The enterotoxins belong to two major classes: heat-labile toxins that consist of one active and five binding subunits (LT), and heat-stable toxins that are small polypeptides (STa, STb, and EAST1). This review describes the disease and pathogenesis of animal ETEC, the corresponding virulence genes and protein products of these bacteria, their regulation and targets in animal hosts, as well as mechanisms of action. Furthermore, vaccines, inhibitors, probiotics, and the identification of potential new targets by genomics are presented in the context of animal ETEC.

Adhesins of Enterotoxigenic Escherichia coli Strains That Infect Animals

The first described adhesive antigen of Escherichia coli strains isolated from animals was the K88 antigen, expressed by strains from diarrheic pigs. The K88 antigen was visible by electron microscopy as a surface-exposed filament that was thin and flexible and had hemagglutinating properties. Many different fimbriae have been identified in animal enterotoxigenic E. coli (ETEC) and have been discussed in this article. The role of these fimbriae in the pathogenesis of ETEC has been best studied with K88, K99, 987P, and F41. Each fimbrial type carries at least one adhesive moiety that is specific for a certain host receptor, determining host species, age, and tissue specificities. ETEC are the most frequently diagnosed pathogens among neonatal and post-weaning piglets that die of diarrhea. Immune electron microscopy of animal ETEC fimbriae usually shows that the minor subunits are located at the fimbrial tips and at discrete sites along the fimbrial threads. Since fimbriae most frequently act like lectins by binding to the carbohydrate moieties of glycoproteins or glycolipids, fimbrial receptors have frequently been studied with red blood cells of various animal species. Identification and characterization of the binding moieties of ETEC fimbrial adhesins should be useful for the design of new prophylactic or therapeutic strategies. Some studies describing potential receptor or adhesin analogues that interfere with fimbria-mediated colonization have been described in the article.

Taurolidine antiadhesive properties on interaction with E. coli; its transformation in biological environment and interaction with bacteria cell wall

The taurine amino-acid derivative, taurolidine, bis-(1,1-dioxoperhydro-1,2,4-thiabiazinyl-4)methane, shows broad antibacterial action against gram-positive and gram-negative bacteria, mycobacteria and some clinically relevant fungi. It inhibits, in vitro, the adherence of Escherichia coli and Staphylococcus aureus to human epithelial and fibroblast cells. Taurolidine is unstable in aqueous solution and breaks down into derivatives which are thought to be responsible for the biological activity. To understand the taurolidine antibacterial mechanism of action, we provide the experimental single crystal X-ray diffraction results together with theoretical methods to characterize the hydrolysis/decomposition reactions of taurolidine. The crystal structure features two independent molecules linked through intermolecular H-bonds with one of them somewhat positively charged. Taurolidine in a biological environment exists in equilibrium with taurultam derivatives and this is described theoretically as a 2-step process without an energy barrier: formation of cationic taurolidine followed by a nucleophilic attack of O(hydroxyl) on the exocyclic C(methylene). A concerted mechanism describes the further hydrolysis of the taurolidine derivative methylol-taurultam. The interaction of methylol-taurultam with the diaminopimelic NH(2) group in the E. coli bacteria cell wall (peptidoglycan) has a negative DeltaG value (-38.2 kcal/mol) but a high energy barrier (45.8 kcal/mol) suggesting no reactivity. On the contrary, taurolidine docking into E. coli fimbriae protein, responsible for bacteria adhesion to the bladder epithelium, shows it has higher affinity than mannose (the natural substrate), whereas methylol-taurultam and taurultam are less tightly bound. Since taurolidine is readily available because it is administered in high doses after peritonitis surgery, it may successfully compete with mannose explaining its effectiveness against bacterial infections at laparoscopic lesions.

Carbohydrate receptors of bacterial adhesins: implications and reflections

Bacteria entering a host depend on adhesins to achieve colonization. Adhesins are bacterial surface structures mediating binding to host surficial areas. Most adhesins are composed of one or several proteins. Usually a single bacterial strain is able to express various adhesins. The adhesion type expressed may influence host-, tissue or even cell tropism of Gram-negative and of Gram-positive bacteria. The binding of fimbrial as well as of afimbrial adhesins of Gram-negative bacteria to host carbohydrate structures (=receptors) has been elucidated in great detail. In contrast, in Gram-positives, most well studied adhesins bind to proteinaceous partners. Nevertheless, for both bacterial groups the binding of bacterial adhesins to eukaryotic carbohydrate receptors is essential for establishing colonization or infection. The characterization of this interaction down to the submolecular level provides the basis for strategies to interfere with this early step of infection which should lead to the prevention of subsequent disease. However, this goal will not be achieved easily because bacterial adherence is not a monocausal event but rather mediated by a variety of adhesins.

Intervention with bacterial adhesion by multivalent carbohydrates

Bacterial adhesion is often a prelude to infection. In many cases, this process is governed by protein-carbohydrate interactions. Intervention at this early stage of infection is a conceptually highly attractive alternative to conventional antibiotics that are increasingly prone to resistance. The lack of high-affinity inhibitors of adhesion has proven to be a hurdle for further exploitation of this concept; however, new developments indicate a positive change. Structure-based design at the monovalent level and also evaluation of glycodendrimers and glycopolymers have yielded structures of high affinity. In addition to the development of inhibitors, topics of this review include available structural information of adhesion proteins, carbohydrate specificities of the various pathogens and their adhesion proteins. Other new developments aimed at affecting bacterial adhesion and the use of the adhesins for bacterial detection are also discussed.

Secretory IgA and mucin-mediated biofilm formation by environmental strains of Escherichia coli: role of type 1 pili

Recent studies suggest the importance of secretory IgA (SIgA) and mucin in the mediation of biofilm formation by commensal bacteria within the mammalian gut. Studies using a variety of strains of Escherichia coli have indicated that the interaction between E. coli and SIgA is dependent on the type 1 pilus. In this study, the importance of the pilus in SIgA-mediated biofilm formation by a laboratory strain (MG1655) and environmental (fecal) strains of E. coli was evaluated. Transient expression of the type 1 pilus by the laboratory strain of E. coli failed to facilitate SIgA-mediated biofilm formation, whereas constitutive expression of the type 1 pilus by the laboratory strain was sufficient. In contrast, transient expression of the type 1 pilus was sufficient to facilitate SIgA-mediated biofilm formation by environmental isolates. The "threshold" for mucin-mediated biofilm formation appeared to be lower than that for SIgA-mediated biofilm formation, perhaps reflecting disparate roles of mucin and SIgA in mediating biofilm formation in the gut. These studies also provide the first procedures for the growth of bacterial biofilms on live epithelial cells in vitro, an important development that may facilitate future studies on the effects of bacterial biofilms on epithelial cells.

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.

Immunoglobulin-mediated agglutination of and biofilm formation by Escherichia coli K-12 require the type 1 pilus fiber

The binding of human secretory immunoglobulin A (SIgA), the primary immunoglobulin in the gut, to Escherichia coli is thought to be dependent on type 1 pili. Type 1 pili are filamentous bacterial surface attachment organelles comprised principally of a single protein, the product of the fimA gene. A minor component of the pilus fiber (the product of the fimH gene, termed the adhesin) mediates attachment to a variety of host cell molecules in a mannose inhibitable interaction that has been extensively described. We found that the aggregation of E. coli K-12 by human secretory IgA (SIgA) was dependent on the presence of the pilus fiber, even in the absence of the mannose specific adhesin or in the presence of 25 mM alpha-CH(3)Man. The presence of pilus without adhesin also facilitated SIgA-mediated biofilm formation on polystyrene, although biofilm formation was stronger in the presence of the adhesin. IgM also mediated aggregation and biofilm formation in a manner dependent on pili with or without adhesin. These findings indicate that the pilus fiber, even in the absence of the adhesin, may play a role in biologically important processes. Under conditions in which E. coli was agglutinated by SIgA, the binding of SIgA to E. coli was not increased by the presence of the pili, with or without adhesin. This observation suggests that the pili, with or without adhesin, affect factors such as cell surface rigidity or electrostatic repulsion, which can affect agglutination but which do not necessarily determine the level of bound immunoglobulin.

The Product of the fimI gene is necessary for Escherichia coli type 1 pilus biosynthesis

Site-directed mutagenesis was employed to create lesions in fimI, a gene of uncertain function located in the chromosomal gene cluster (fim) involved in Escherichia coli type 1 pilus biosynthesis. Chromosomal fimI mutations produced a piliation-negative phenotype. Complementation analysis indicated that a fimI'-kan insertion mutation and a fimI frameshift mutation produced polarity-like effects not seen with an in-frame fimI deletion mutation. Minicell analysis associated fimI with a 16.4-kDa noncytoplasmic protein product (FimI). We conclude that FimI has a required role in normal pilus biosynthesis.

The Pix pilus adhesin of the uropathogenic Escherichia coli strain X2194 (O2 : K(-): H6) is related to Pap pili but exhibits a truncated regulatory region

Adhesins provide a major advantage for uropathogenic Escherichia coli in establishing urinary tract infections (UTIs). A novel gene cluster responsible for the expression of a filamentous adhesin of the pyelonephritogenic E. coli strain X2194 has been identified, molecularly cloned, and characterized. The 'pix operon' contains eight open reading frames which exhibit significant sequence homology to corresponding genes in the pap operon encoding P pili, the prevalent E. coli adhesins in non-obstructive acute pyelonephritis in humans. Although a pixB gene corresponding to the PapB regulator was identified, a papI homologue could not be found in the pix operon. Instead, a fragment of the R6 gene of the highly uropathogenic E. coli strain CFT073 was identified upstream of pixB. The R6 gene is located in a pathogenicity island containing several pilus-encoding sequences and shows homology to a transposase of Chelatobacter heintzii. In a pixA-lacZ fusion system it was demonstrated that the expression of Pix pili is regulated at the transcriptional level by the R6 gene sequence. A significantly reduced transcription was observed by deleting this fragment and by lowering the growth temperature from 37 to 26 degrees C. In contrast to other filamentous adhesin systems, Pix pili are mainly expressed in the steady state growth phase and were not repressed by the addition of glucose.

Characterization of Escherichia coli type 1 pilus mutants with altered binding specificities

PCR mutagenesis and a unique enrichment scheme were used to obtain two mutants, each with a single lesion in fimH, the chromosomal gene that encodes the adhesin protein (FimH) of Escherichia coli type 1 pili. These mutants were noteworthy in part because both were altered in the normal range of cell types bound by FimH. One mutation altered an amino acid at a site previously shown to be involved in temperature-dependent binding, and the other altered an amino acid lining the predicted FimH binding pocket.

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.

Snapshots of usher-mediated protein secretion and ordered pilus assembly

Type 1 pilus biogenesis was used as a paradigm to investigate ordered macromolecular assembly at the outer cell membrane. The ability of Gram-negative bacteria to secrete proteins across their outer membrane and to assemble adhesive macromolecular structures on their surface is a defining event in pathogenesis. We elucidated genetic, biochemical, and biophysical requirements for assembly of functional type 1 pili. We discovered that the minor pilus protein FimG plays a critical role in nucleating the formation of the adhesive tip fibrillum. Genetic methods were used to trap pilus subunits during their translocation through the outer membrane usher protein, providing data on the structural interactions that occur between subunit components during type 1 pilus formation. Electron microscopic and biochemical analyses of these stepwise assembly intermediates demonstrated that translocation of pilus subunits occurs linearly through the usher's central channel, with formation of the pilus helix occurring extracellularly. Specialized pilin subunits play unique roles both in this multimerization and in the final ultrastructure of the adhesive pilus.

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.

Molecular basis for the enterocyte tropism exhibited by Salmonella typhimurium type 1 fimbriae

Salmonella typhimurium exhibits a distinct tropism for mouse enterocytes that is linked to their expression of type 1 fimbriae. The distinct binding traits of Salmonella type 1 fimbriae is also reflected in their binding to selected mannosylated proteins and in their ability to promote secondary bacterial aggregation on enterocyte surfaces. The determinant of binding in Salmonella type 1 fimbriae is a 35-kDa structurally distinct fimbrial subunit, FimHS, because inactivation of fimHS abolished binding activity in the resulting mutant without any apparent effect on fimbrial expression. Surprisingly, when expressed in the absence of other fimbrial components and as a translational fusion protein with MalE, FimHS failed to demonstrate any specific binding tropism and bound equally to all cells and mannosylated proteins tested. To determine if the binding specificity of Salmonella type 1 fimbriae was determined by the fimbrial shaft that is intimately associated with FimHS, we replaced the amino-terminal half of FimHS with the corresponding sequence from Escherichia coli FimH (FimHE) that contains the receptor binding domain of FimHE. The resulting hybrid fimbriae bearing FimHES on a Salmonella fimbrial shaft exhibited binding traits that resembled that of Salmonella rather than E. coli fimbriae. Apparently, the quaternary constraints imposed by the fimbrial shaft on the adhesin determine the distinct binding traits of S. typhimurium type 1 fimbriae.

Nucleator function of CsgB for the assembly of adhesive surface organelles in Escherichia coli

Curli are surface organelles in Escherichia coli that assemble outside the bacterium through the precipitation of secreted soluble CsgA monomers, requiring the CsgB nucleator protein. Using immunoelectron microscopy and immunoblotting assays, CsgB is shown to be located on the bacterial surface and also as a minor component of wild-type curli. CsgB lacking its 20 N-terminal residues when fused to maltose-binding protein (MBP) can still trigger polymerization of CsgA monomers in vivo. However, the resulting surface organelles are only formed at one of the two bacterial poles and are morphologically distinct from wild-type curli. These Bfco organelles (CsgB-Free Curli-related Organelles) are highly regular structures reacting with anti-CsgA, but not anti-CsgB antibodies. The CsgB of the active MBP-CsgBII fusion is surface exposed but, unlike the native CsgB in wild-type curli, is not detectable in the Bfco organelles. Overexpression of csgB within a csgA mutant results in the formation of short CsgB polymers on the cell surface. It is suggested that in wild-type bacteria, both CsgA and CsgB are secreted proteins. Interaction between CsgA and CsgB triggers wild-type curli formation, resulting in CsgA-CsgB heteropolymers, while surface-anchored CsgB in MBP-CsgBII triggers morphologically distinct, CsgB-free/CsgA Bfco organelles. In the absence of CsgA, CsgB can self-assemble into polymers.

Localization of a domain in the FimH adhesin of Escherichia coli type 1 fimbriae capable of receptor recognition and use of a domain-specific antibody to confer protection against experimental urinary tract infection

The FimH subunit of type 1-fimbriated Escherichia coli has been implicated as an important determinant of bacterial adherence and colonization of the urinary tract. Here, we sought to localize the functionally important domain(s) within the FimH molecule and to determine if antibodies against this domain would block adherence of type 1-fimbriated E. coli to the bladder mucosa in situ and in vivo in an established mouse model of cystitis. We generated translational fusion proteins of disparate regions of the FimH molecule with an affinity tag MalE, and tested each of the fusion products in vitro for functional activity. The minimum region responsible for binding mouse bladder epithelial cells and a soluble mannoprotein, horseradish peroxidase, was contained within residues 1-100 of the FimH molecule. We validated and extended these findings by demonstrating that antibodies directed at the putative binding region of FimH or at synthetic peptides corresponding to epitopes within the binding domain could specifically block type 1 fimbriae-mediated bacterial adherence to bladder epithelial cells in situ and yeast cells in vitro. Next, we compared the ability of mice passively immunized intraperitoneally with antisera raised against residues 1-25 and 253-264 of FimH or 1-13 of FimA to resist bladder colonization in vivo after intravesicular challenge with type 1-fimbriated E. coli. Only the antibody directed at the putative binding region of FimH (anti- s-FimH1-25) significantly reduced E. coli bladder infections in the experimental mouse model of urinary tract infections. Similar results were obtained when the mice were actively immunized with synthetic peptides corresponding to residues 1-25 and 253-264 of FimH or 1-13 of FimA. The mechanism of protection was attributed, at least in part, to inhibition of bacterial adherence to the bladder surface by s-FimH1-25-specific antibody molecules that had filtered through the kidneys into the urine. The level of FimH antibodies entering the bladder from the circulatory system of the immunized mice was found to be markedly enhanced upon bacterial challenge. The potential broad spectrum activity of the protective FimH antibody was indicated from its serologic cross-reactivity with various urinary tract bacterial isolates bearing type 1 fimbriae. These findings could be relevant in the design of an efficacious and broadly reactive FimH vaccine against urinary tract infections.

Molecular and structural aspects of fimbriae biosynthesis and assembly in Escherichia coli

Fimbriae are long filamentous polymeric protein structures located at the surface of bacterial cells. They enable the bacteria to bind to specific receptor structures and thereby to colonise specific surfaces. Fimbriae consist of so-called major and minor subunits, which form, in a specific order, the fimbrial structure. In this review emphasis is put on the genetic organisation, regulation and especially on the biosynthesis of fimbriae of enterotoxigenic Escherichia coli strains, and more in particular on K88 and related fimbriae, with ample reference to well-studied P and type 1 fimbriae. The biosynthesis of these fimbriae requires two specific and unique proteins, a periplasmic chaperone and an outer membrane located molecular usher ('doorkeeper'). Molecular and structural aspects of the secretion of fimbrial subunits across the cytoplasmic membrane, the interaction of these subunits with periplasmic molecular chaperone, their translocation to the inner site of the outer membrane and their interaction with the usher protein, as well as the (ordered) translocation of the subunits across the outer membrane and their assembly into a growing fimbrial structure will be described. A model for K88 fimbriae is presented.

Ordered translocation of 987P fimbrial subunits through the outer membrane of Escherichia coli

The 987P fimbria of enterotoxigenic Escherichia coli is a heteropolymeric structure which consists essentially of a major FasA subunit and a minor subunit, the FasG adhesin. The latter harbors the binding moiety for receptor molecules on piglet intestinal epithelial cells. In this study, anti-FasF antibody probes were developed and used to demonstrate that the FasF protein represents a new minor fimbrial component. FasF was identified in highly purified fimbriae, and its sequence demonstrated significant levels of similarity with that of FasA. Immune electron microscopy localized both the FasG and FasF proteins at the fimbrial tip as well as at broken ends and at various intervals along the fimbrial length. The presence of these minor proteins in purified 987P fimbriae was corroborated by enzyme-linked immunosorbent assay inhibitions. Finally, the use of nonfimbriated fasG, fasF, and fasA mutants indicated that subunit translocation through the outer membrane follows a specific order, FasG being the first, FasF being the second, and FasA being the third type of exported subunit. Since fimbriae are thought to grow from the base, FasG is proposed to be a tip adhesin and FasF is proposed to be a linker molecule between the adhesin and the fimbrial shaft. Moreover, export of FasG (or FasF) in the absence of FasF (or FasA) indicates that during the process of fimbrial biogenesis in the outer membrane, translocating events precede the initiation of subunit heteropolymerization.

Expression of thioredoxin random peptide libraries on the Escherichia coli cell surface as functional fusions to flagellin: a system designed for exploring protein-protein interactions

We have developed a system for probing protein/protein interactions which makes use of the bacterial flagellum to display random peptide libraries on the surface of E. coli. In developing the system the entire coding sequence of E. coli thioredoxin (trxA) was inserted into a dispensable region of the gene for flagellin (fliC), the major structural component of the E. coli flagellum. The resulting fusion protein (FLITRX) was efficiently exported and assembled into partially functional flagella on the bacterial cell surface. A diverse library of random dodecapeptides were displayed in FLITRX on the exterior of E. coli as conformationally constrained insertions into the thioredoxin active-site loop, a location known to be a highly permissive site for the insertion of exogenous peptide sequences into native thioredoxin. To demonstrate that members of this library could be bound and selected via specific protein/protein interactions to a target protein, a method was devised to enable efficient isolation of those bacteria displaying peptides with affinity to immobilized antibodies. We have unambiguously mapped three different antibody epitopes using this method. Peptides selected as FLITRX active-site fusions retain their binding specificity when made as native thioredoxin active-site loop fusions. This will facilitate future structural characterizations and broaden the general utility of the system for exploring other classes of protein-protein interactions.

FimH adhesin of type 1 pili is assembled into a fibrillar tip structure in the Enterobacteriaceae

Type 1 pili are heteropolymeric mannosebinding fibers produced by all members of the Enterobacteriaceae family. The bulk of the fiber is composed of FimA. Two macromolecular complexes responsible for mediating an interaction with mannose-containing receptors were purified from fimA- Escherichia coli by mannose affinity chromatography and ion-exchange chromatography. One complex contained only the mannose-binding adhesin, FimH, associated with FimG, a minor component of the type 1 pilus. In the other complex the FimG-FimH moiety was loosely associated with a chaperone-minor subunit complex (FimC-FimF), possibly representing an intermediate in tip fibrilla assembly. The FimC chaperone has also been shown to form a preassembly complex with FimH that has been purified and characterized previously. Purified FimC did not bind to the FimG-FimH complex but did recognize FimH dissociated from the FimG-FimH complex. Quick-freeze deep-etch electron microscopy revealed that the FimG-FimH complex had a thin fibrillar architecture. High-resolution electron microscopy of type 1 pili revealed that a 16-nm fibrillar tip structure with an architecture identical to that of the FimG-FimH complex was joined end-to-end to the pilus rod. In a fimH- deletion mutant, the tip fibrillae joined to pilus rods were approximately 3 nm in length. The full-length tip fibrilla was restored by complementation with the fimH gene in trans. The bipartite nature of the type 1 pilus was also demonstrated on pili purified from clinical isolates of members of the Enterobacteriaceae family arguing that it is a conserved feature of the type 1 pilus.

A minor 987P protein different from the structural fimbrial subunit is the adhesin

The 987P fimbriae produced by enterotoxigenic strains of Escherichia coli isolated from piglets mediate bacterial attachment to intestinal epithelial cells. These fimbriae consist essentially of a tight helical arrangement of one structural protein subunit encoded by fasA. Fimbriation and specific adhesion requires the expression of seven additional genes (fasB to fasH). In this study, we investigated whether FasA or another Fas protein, e.g., a potential minor fimbrial component, harbors the binding moiety for the pig 987P receptor glycoproteins. Fas proteins, specifically radiolabeled with an in vivo T7 expression system, were isolated from the periplasm and incubated with receptor-containing brush borders isolated from piglet intestinal epithelial cells. FasG bound best to brush borders, whereas no FasA adhered to them. Additional evidence that FasG, and not FasA, is the 987P adhesin was provided by ligand blotting inhibition assays indicating that FasG alone inhibited fimbrial binding to 987P receptors and that in the absence of FasG, other Fas proteins were not inhibitory. FasG was identified in purified fimbrial preparations with a specific anti-FasG antibody probe. Moreover, FasG was shown to be tightly associated with the fimbrial structure, since it was released only after disassembling fimbriae by heat and sodium dodecyl sulfate treatments. The primary structure of FasG, deduced from the DNA sequence, exhibited 19.1 to 24.4% similarity to FasA and large minor components and/or adhesins of other fimbriae. FasG is the first-described minor fimbrial subunit shown to be essential for both fimbrial biogenesis and specific adhesion.

Type 1 fimbrial shafts of Escherichia coli and Klebsiella pneumoniae influence sugar-binding specificities of their FimH adhesins

The type 1 fimbriae of enterobacteria comprise FimA, which constitutes most of the fimbrial shaft, and a cassette of three minor ancillary subunits including FimH, the mannose-binding moiety. The sugar-binding specificities of Escherichia coli and Klebsiella pneumoniae type 1 fimbriae were examined by determining the relative activities of two aromatic mannosides in inhibiting the yeast aggregation caused by the fimbriated bacteria. 4-Methylumbelliferyl alpha-mannoside (MeUmb alpha Man) was approximately 10-fold more effective than p-nitrophenyl alpha-mannoside (p-NP alpha Man) in inhibiting the yeast aggregation caused by the recombinant expressing native E. coli type 1 fimbriae. In contrast, MeUmb alpha Man was only fourfold more effective than p-NP alpha Man in assays employing the recombinant expressing native K. pneumoniae type 1 fimbriae. In order to elucidate the molecular mechanisms underlying the sugar-binding specificities of type 1 fimbriae in the two species, transcomplementation studies were performed and resulted in the creation of recombinants expressing two types of hybrid fimbriae: one consisting of a cassette of minor subunits of E. coli fimbriae borne on a filamentous shaft of K. pneumoniae FimA subunits and the other consisting of a cassette of K. pneumoniae minor fimbrial subunits borne on a shaft of E. coli FimA subunits. Although the heterologous FimH was incorporated into the fimbrial filaments in amounts comparable to those observed in native fimbriae, the hemagglutination activities of recombinants expressing hybrid fimbriae were significantly lower than those of their counterparts bearing native fimbriae. The sugar-binding specificity of the recombinant expressing hybrid fimbriae consisting of an E. coli shaft bearing K. pneumoniae FimH was different from those of recombinants expressing native K. pneumoniae fimbriae in its affinity for the two aromatic sugars but was remarkably similar to the specificities exhibited by recombinants expressing native E. coli fimbriae. Conversely, the sugar-binding specificity of the recombinant expressing hybrid fimbriae consisting of a K. pneumoniae shaft bearing E. coli FimH was different from that of the recombinant expressing native E. coli fimbriae but was very similar to those of recombinants expressing native K. pneumoniae fimbriae. We conclude that the differences in the sugar-binding specificity between E. coli and K. pneumoniae FimH fimbrial subunits is influenced by the fimbrial shafts which carry the adhesin molecules in a functionally competent form at the distal tips.

Rapid, synchronous, and stable induction of type 1 piliation in Escherichia coli by using a chromosomal lacUV5 promoter

Type 1 pili are filamentous proteinaceous appendages produced by certain members of the family Enterobacteriaceae. In Escherichia coli, the adhesive properties of these pili are due to the binding of at least one minor pilus component to mannose, a sugar common to cell surface molecules of many eukaryotic cells. The study of pilus assembly may be benefited by a rapid way of inducing pilus synthesis de novo. We describe herein the construction and characterization of a strain in which piliation can be rapidly induced by the addition of lactose or its analog isopropyl-beta-D-thiogalactopyranoside. This was accomplished by placing the chromosomal fimA gene (encoding the major structural subunit of pili) under lacUV5 promoter control. Further experiments suggested that transcription of genes downstream of fimA, whose products are required for normal pilus assembly and function, may also be controlled by the lacUV5 promoter. The construction described herein may have a variety of applications apart from aiding the study of pilus assembly since its adhesive properties can be rapidly and easily turned on and off.

Functional heterogeneity of type 1 fimbriae of Escherichia coli

Escherichia coli and other members of the family Enterobacteriaceae express surface fibrillar structures, fimbriae, that promote bacterial adhesion to host receptors. Type 1 fimbriae possess a lectinlike component, FimH, that is commonly thought to cause binding to mannose-containing oligosaccharides of host receptors. Since adhesion of type 1 fimbriated organisms are inhibited by mannose, the reactions are described as mannose sensitive (MS). We have studied the adhesion of the type 1 fimbriated CSH-50 strain of E. coli (which expresses only type 1 fimbriae) to fibronectin (FN). E. coli CSH-50 does not bind detectable amounts of soluble FN but adheres well to immobilized plasma or cellular FN. This adhesion was inhibited by mannose-containing saccharides. By using purified domains of FN, it was found that E. coli CSH-50 adheres primarily to the amino-terminal and gelatin-binding domains, only one of which is glycosylated, in an MS fashion. Binding of the mannose-specific lectin concanavalin A to FN and ovalbumin was eliminated or reduced, respectively, by incubation with periodate or endoglycosidase. Adhesion of E. coli CSH-50 to ovalbumin was reduced by these treatments, but adhesion to FN was unaffected. E. coli CSH-50 also adheres to a synthetic peptide copying a portion of the amino-terminal FN domain (FNsp1) in an MS fashion. Purified CSH-50 fimbriae bound to immobilized FN and FNsp1 in an MS fashion and inhibited adhesion of intact organisms. However, fimbriae purified from HB101 (pPKL4), a recombinant strain harboring the entire type 1 fim gene locus and expressing functional type 1 fimbriae, neither bound to FN or FNsp1 nor inhibited E. coli adhesion to immobilized FN or FNsp1. These novel findings suggest that there are two forms of type 1 MS fimbriae. One form exhibits only the well-known MS lectinlike activity that requires a substratum of mannose-containing glycoproteins. The other form exhibits not only the MS lectinlike activity but also binds to nonglycosylated regions of proteins in an MS manner.

Lesions in two Escherichia coli type 1 pilus genes alter pilus number and length without affecting receptor binding

We describe the characterization of two genes, fimF and fimG (also called pilD), that encode two minor components of type 1 pili in Escherichia coli. Defined, in-frame deletion mutations were generated in vitro in each of these two genes. A double mutation that had deletions identical to both single lesions was also constructed. Examination of minicell transcription and translation products of parental and mutant plasmids revealed that, as predicted from the nucleotide sequence and previous reports, the fimF gene product was a protein of ca. 16 kDa and that the fimG gene product was a protein of ca. 14 kDa. Each of the constructions was introduced, via homologous recombination, into the E. coli chromosome. All three of the resulting mutants produced type 1 pili and exhibited hemagglutination of guinea pig erythrocytes. The latter property was also exhibited by partially purified pili isolated from each of the mutants. Electron microscopic examination revealed that the fimF mutant had markedly reduced numbers of pili per cell, whereas the fimG mutant had very long pili. The double mutant displayed the characteristics of both single mutants. However, pili in the double mutant were even longer than those seen in the fimG mutant, and the numbers of pili were even fewer than those displayed by the fimF mutant. All three mutants could be complemented in trans with a single-copy-number plasmid bearing the appropriate parental gene or genes to give near-normal parental piliation. On the basis of the phenotypes exhibited by the single and double mutants, we believe that the fimF gene product may aid in initiating pilus assembly and that the fimG product may act as an inhibitor of pilus polymerization. In contrast to previous studies, we found that neither gene product was required for type 1 pilus receptor binding.

Glycerol-induced unraveling of the tight helical conformation of Escherichia coli type 1 fimbriae

Glycerol was found to unravel the helical conformation of Escherichia coli type 1 fimbriae without appreciable depolymerization. The linearized fimbrial polymers have a diameter of 2 nm, react strongly with a monoclonal antibody directed at an inaccessible epitope on native fimbriae, and display greater mannose-binding activity and trypsin sensitivity than native fimbriae. Removal of glycerol by dialysis results in spontaneous reassembly of the linear polymers into structures morphologically, antigenically, and functionally indistinguishable from native fimbriae.

Consequences of microbial attachment: directing host cell functions with adhesins

We take the view that adherence is not just a static process of holding hands but rather elicits a response in the targeted cell. From this point of view, adherence is an active process with an outcome. This outcome or fate is predictable only when several parameters of the host cell-adhesin interaction are known: is the adhesin acting alone or in series with other products, is the receptor up- or down-regulated at the time of ligation, which domain of the receptor is bound, and finally, which intracellular response circuits are connected to the receptor in the cell type targeted? Variations in these parameters are the basis for the ability of the adhesins of pathogens to orchestrate outcomes as disparate as simple address recognition versus actin nucleation, cytokine induction, activation of plasmin, derangement of leukocyte migration, or deposition of antibody on host cell membranes. The recognition of the relatedness of some eukaryotic and prokaryotic adhesive domains and the shared use of existing eukaryotic cell-cell interaction systems between host and pathogen suggest that the cellular interactions of interest in eukaryotic cell biology can be revealed by taking clues from the pathogens, which have studied and adapted to them the longest.