Sequencing of the gene encoding the major pilin of pilus colonization factor antigen III (CFA/III) of human enterotoxigenic Escherichia coli and evidence that CFA/III is related to type IV pili

Molecular determinants of surface colonisation in diarrhoeagenic Escherichia coli (DEC): from bacterial adhesion to biofilm formation

Escherichia coli is primarily known as a commensal colonising the gastrointestinal tract of infants very early in life but some strains being responsible for diarrhoea, which can be especially severe in young children. Intestinal pathogenic E. coli include six pathotypes of diarrhoeagenic E. coli (DEC), namely, the (i) enterotoxigenic E. coli, (ii) enteroaggregative E. coli, (iii) enteropathogenic E. coli, (iv) enterohemorragic E. coli, (v) enteroinvasive E. coli and (vi) diffusely adherent E. coli. Prior to human infection, DEC can be found in natural environments, animal reservoirs, food processing environments and contaminated food matrices. From an ecophysiological point of view, DEC thus deal with very different biotopes and biocoenoses all along the food chain. In this context, this review focuses on the wide range of surface molecular determinants acting as surface colonisation factors (SCFs) in DEC. In the first instance, SCFs can be broadly discriminated into (i) extracellular polysaccharides, (ii) extracellular DNA and (iii) surface proteins. Surface proteins constitute the most diverse group of SCFs broadly discriminated into (i) monomeric SCFs, such as autotransporter (AT) adhesins, inverted ATs, heat-resistant agglutinins or some moonlighting proteins, (ii) oligomeric SCFs, namely, the trimeric ATs and (iii) supramolecular SCFs, including flagella and numerous pili, e.g. the injectisome, type 4 pili, curli chaperone-usher pili or conjugative pili. This review also details the gene regulatory network of these numerous SCFs at the various stages as it occurs from pre-transcriptional to post-translocational levels, which remains to be fully elucidated in many cases.

Diarrheagenic Escherichia coli

Most Escherichia coli strains live harmlessly in the intestines and rarely cause disease in healthy individuals. Nonetheless, a number of pathogenic strains can cause diarrhea or extraintestinal diseases both in healthy and immunocompromised individuals. Diarrheal illnesses are a severe public health problem and a major cause of morbidity and mortality in infants and young children, especially in developing countries. E. coli strains that cause diarrhea have evolved by acquiring, through horizontal gene transfer, a particular set of characteristics that have successfully persisted in the host. According to the group of virulence determinants acquired, specific combinations were formed determining the currently known E. coli pathotypes, which are collectively known as diarrheagenic E. coli. In this review, we have gathered information on current definitions, serotypes, lineages, virulence mechanisms, epidemiology, and diagnosis of the major diarrheagenic E. coli pathotypes.

Effects of tcpB Mutations on Biogenesis and Function of the Toxin-Coregulated Pilus, the Type IVb Pilus of Vibrio cholerae

Vibrio cholerae is the etiological agent of the acute intestinal disorder cholera. The toxin-coregulated pilus (TCP), a type IVb pilus, is an essential virulence factor of V. cholerae Recent work has shown that TcpB is a large minor pilin encoded within the tcp operon. TcpB contributes to efficient pilus formation and is essential for all TCP functions. Here, we have initiated a detailed targeted mutagenesis approach to further characterize this salient TCP component. We have identified (thus far) 20 residues of TcpB which affect either the steady-state level of TcpB or alter one or more TCP functions. This study provides a solid framework for further understanding of the complex role of TcpB and will be of use upon determination of the crystal structure of TcpB or related minor pilin orthologs of type IVb pilus systems.

IMPORTANCE

Type IV pili, such as the toxin-coregulated pilus (TCP) in V. cholerae, are bacterial appendages that often act as essential virulence factors. Minor pilins, like TcpB, of these pili systems often play integral roles in pilus assembly and function. In this study, we have generated mutations in tcpB to determine residues of importance for TCP stability and function. Combined with a predicted tertiary structure, characterization of these mutants allows us to better understand critical residues in TcpB and the role they may play in the mechanisms underlying minor pilin functions.

Structural Diversity in the Type IV Pili of Multidrug-resistant Acinetobacter

Acinetobacter baumannii is a Gram-negative coccobacillus found primarily in hospital settings that has recently emerged as a source of hospital-acquired infections. A. baumannii expresses a variety of virulence factors, including type IV pili, bacterial extracellular appendages often essential for attachment to host cells. Here, we report the high resolution structures of the major pilin subunit, PilA, from three Acinetobacter strains, demonstrating that A. baumannii subsets produce morphologically distinct type IV pilin glycoproteins. We examine the consequences of this heterogeneity for protein folding and assembly as well as host-cell adhesion by Acinetobacter Comparisons of genomic and structural data with pilin proteins from other species of soil gammaproteobacteria suggest that these structural differences stem from evolutionary pressure that has resulted in three distinct classes of type IVa pilins, each found in multiple species.

Effects of lng Mutations on LngA Expression, Processing, and CS21 Assembly in Enterotoxigenic Escherichia coli E9034A

Enterotoxigenic Escherichia coli (ETEC) is a major cause of morbidity in children under 5 years of age in low- and middle-income countries and a leading cause of traveler's diarrhea worldwide. The ability of ETEC to colonize the intestinal epithelium is mediated by fimbrial adhesins, such as CS21 (Longus). This adhesin is a type IVb pilus involved in adherence to intestinal cells in vitro and bacterial self-aggregation. Fourteen open reading frames have been proposed to be involved in CS21 assembly, hitherto only the lngA and lngB genes, coding for the major (LngA) and minor (LngB) structural subunit, have been characterized. In this study, we investigated the role of the LngA, LngB, LngC, LngD, LngH, and LngP proteins in the assembly of CS21 in ETEC strain E9034A. The deletion of the lngA, lngB, lngC, lngD, lngH, or lngP genes, abolished CS21 assembly in ETEC strain E9034A and the adherence to HT-29 cells was reduced 90%, compared to wild-type strain. Subcellular localization prediction of CS21 proteins was similar to other well-known type IV pili homologs. We showed that LngP is the prepilin peptidase of LngA, and that ETEC strain E9034A has another peptidase capable of processing LngA, although with less efficiency. Additionally, we present immuno-electron microscopy images to show that the LngB protein could be localized at the tip of CS21. In conclusion, our results demonstrate that the LngA, LngB, LngC, LngD, LngH, and LngP proteins are essential for CS21 assembly, as well as for bacterial aggregation and adherence to HT-29 cells.

Homo-trimeric Structure of the Type IVb Minor Pilin CofB Suggests Mechanism of CFA/III Pilus Assembly in Human Enterotoxigenic Escherichia coli

In gram-negative bacteria, the assembly of type IV pilus (T4P) and the evolutionally related pseudopilus of type II secretion system involves specialized structural proteins called pilins and pseudopilins, respectively, and is dynamically regulated to promote bacterial pathogenesis. Previous studies have suggested that a structural "tip"-like hetero-complex formed through the interaction of at least three minor (pseudo) pilins plays an important role in this process, while some members of the pathogenic type IVb subfamily are known to have only one such minor pilin subunit whose function is still unknown. Here, we determined the crystal structure of the type IVb minor pilin CofB of colonization factor antigen/III from human enterotoxigenic Escherichia coli at 1.88-Å resolution. The crystal structure, in conjunction with physicochemical analysis in solution, reveals a symmetrical homo-trimeric arrangement distinct from the hetero-complexes of minor (pseudo) pilins observed in other T4P and type II secretion systems. Each CofB monomer adopts a unique three-domain architecture, in which the C-terminal β-sheet-rich lectin domain can effectively initiate trimer association of its pilin-like N-terminal domain through extensive hydrophobic interactions followed by domain swapping at the central hinge-like domain. Deletion of cofB produces a phenotype with no detectable pili formation on the cell surface, while molecular modeling indicates that the characteristic homo-trimeric structure of CofB is well situated at the pilus tip of colonization factor antigen/III formed by the major pilin CofA, suggesting a role for the minor pilin in the efficient initiation of T4P assembly.

Fimbriae: Classification and Biochemistry

Proteinaceous, nonflagellar surface appendages constitute a variety of structures, including those known variably as fimbriae or pili. Constructed by distinct assembly pathways resulting in diverse morphologies, fimbriae have been described to mediate functions including adhesion, motility, and DNA transfer. As these structures can represent major diversifying elements among Escherichia and Salmonella isolates, multiple fimbrial classification schemes have been proposed and a number of mechanistic insights into fimbrial assembly and function have been made. Herein we describe the classifications and biochemistry of fimbriae assembled by the chaperone/usher, curli, and type IV pathways.

Crystal Structure of the Minor Pilin CofB, the Initiator of CFA/III Pilus Assembly in Enterotoxigenic Escherichia coli

Type IV pili are extracellular polymers of the major pilin subunit. These subunits are held together in the pilus filament by hydrophobic interactions among their N-terminal α-helices, which also anchor the pilin subunits in the inner membrane prior to pilus assembly. Type IV pilus assembly involves a conserved group of proteins that span the envelope of Gram-negative bacteria. Among these is a set of minor pilins, so named because they share their hydrophobic N-terminal polymerization/membrane anchor segment with the major pilins but are much less abundant. Minor pilins influence pilus assembly and retraction, but their precise functions are not well defined. The Type IV pilus systems of enterotoxigenic Escherichia coli and Vibrio cholerae are among the simplest of Type IV pilus systems and possess only a single minor pilin. Here we show that the enterotoxigenic E. coli minor pilins CofB and LngB are required for assembly of their respective Type IV pili, CFA/III and Longus. Low levels of the minor pilins are optimal for pilus assembly, and CofB can be detected in the pilus fraction. We solved the 2.0 Å crystal structure of N-terminally truncated CofB, revealing a pilin-like protein with an extended C-terminal region composed of two discrete domains connected by flexible linkers. The C-terminal region is required for CofB to initiate pilus assembly. We propose a model for CofB-initiated pilus assembly with implications for understanding filament growth in more complex Type IV pilus systems as well as the related Type II secretion system.

Cloning, expression, purification, crystallization and X-ray crystallographic analysis of CofB, the minor pilin subunit of CFA/III from human enterotoxigenic Escherichia coli

Colonization factor antigen III (CFA/III) is one of the virulence factors of human enterotoxigenic Escherichia coli (ETEC) that forms the long, thin, proteinaceous fibres of type IV pili through assembly of its major and minor subunits CofA and CofB, respectively. The crystal structure of CofA has recently been reported; however, the lack of structural information for CofB, the largest among the known type IV pilin subunits, hampers a comprehensive understanding of CFA/III pili. In this study, constructs of wild-type CofB with an N-terminal truncation and the corresponding SeMet derivative were cloned, expressed, purified and crystallized. The crystals belonged to the rhombohedral space group R32, with unit-cell parameters a = b = 103.97, c = 364.57 Å for the wild-type construct and a = b = 103.47, c = 362.08 Å for the SeMet-derivatized form. Although the diffraction quality of these crystals was initially very poor, dehydration of the crystals substantially improved the resolution limit from ∼ 4.0 to ∼ 2.0 Å. The initial phase was solved by the single-wavelength anomalous dispersion (SAD) method using a dehydrated SeMet CofB crystal, which resulted in an interpretable electron-density map.

Structural and evolutionary analyses show unique stabilization strategies in the type IV pili of Clostridium difficile

Type IV pili are produced by many pathogenic Gram-negative bacteria and are important for processes as diverse as twitching motility, biofilm formation, cellular adhesion, and horizontal gene transfer. However, many Gram-positive species, including Clostridium difficile, also produce type IV pili. Here, we identify the major subunit of the type IV pili of C. difficile, PilA1, and describe multiple 3D structures of PilA1, demonstrating the diversity found in three strains of C. difficile. We also model the incorporation of both PilA1 and a minor pilin, PilJ, into the pilus fiber. Although PilA1 contains no cysteine residues, and therefore cannot form the disulfide bonds found in all Gram-negative type IV pilins, it adopts unique strategies to achieve a typical pilin fold. The structures of PilA1 and PilJ exhibit similarities with the type IVb pilins from Gram-negative bacteria that suggest that the type IV pili of C. difficile are involved in microcolony formation.

Structure of Clostridium difficile PilJ exhibits unprecedented divergence from known type IV pilins

Type IV pili are produced by many pathogenic Gram-negative bacteria and are important for processes as diverse as twitching motility, cellular adhesion, and colonization. Recently, there has been an increased appreciation of the ability of Gram-positive species, including Clostridium difficile, to produce Type IV pili. Here we report the first three-dimensional structure of a Gram-positive Type IV pilin, PilJ, demonstrate its incorporation into Type IV pili, and offer insights into how the Type IV pili of C. difficile may assemble and function. PilJ has several unique structural features, including a dual-pilin fold and the incorporation of a structural zinc ion. We show that PilJ is incorporated into Type IV pili in C. difficile and present a model in which the incorporation of PilJ into pili exposes the C-terminal domain of PilJ to create a novel interaction surface.

Structure and secretion of CofJ, a putative colonization factor of enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) colonize the human gut, causing severe cholera-like diarrhoea. ETEC utilize a diverse array of pili and fimbriae for host colonization, including the Type IVb pilus CFA/III. The CFA/III pilus machinery is encoded on the cof operon, which is similar in gene sequence and synteny to the tcp operon that encodes another Type IVb pilus, the Vibrio cholerae toxin co-regulated pilus (TCP). Both pilus operons possess a syntenic gene encoding a protein of unknown function. In V. cholerae, this protein, TcpF, is a critical colonization factor secreted by the TCP apparatus. Here we show that the corresponding ETEC protein, CofJ, is a soluble protein secreted via the CFA/III apparatus. We present a 2.6 Å resolution crystal structure of CofJ, revealing a large β-sandwich protein that bears no sequence or structural homology to TcpF. CofJ has a cluster of exposed hydrophobic side-chains at one end and structural homology to the pore-forming proteins perfringolysin O and α-haemolysin. CofJ binds to lipid vesicles and epithelial cells, suggesting a role in membrane attachment during ETEC colonization.

Structure of the CFA/III major pilin subunit CofA from human enterotoxigenic Escherichia coli determined at 0.90 Å resolution by sulfur-SAD phasing

CofA, a major pilin subunit of colonization factor antigen III (CFA/III), forms pili that mediate small-intestinal colonization by enterotoxigenic Escherichia coli (ETEC). In this study, the crystal structure of an N-terminally truncated version of CofA was determined by single-wavelength anomalous diffraction (SAD) phasing using five sulfurs in the protein. Given the counterbalance between anomalous signal strength and the undesired X-ray absorption of the solvent, diffraction data were collected at 1.5 Å resolution using synchrotron radiation. These data were sufficient to elucidate the sulfur substructure at 1.38 Å resolution. The low solvent content (29%) of the crystal necessitated that density modification be performed with an additional 0.9 Å resolution data set to reduce the phase error caused by the small sulfur anomalous signal. The CofA structure showed the αβ-fold typical of type IVb pilins and showed high structural homology to that of TcpA for toxin-coregulated pili of Vibrio cholerae, including spatial distribution of key residues critical for pilin self-assembly. A pilus-filament model of CofA was built by computational docking and molecular-dynamics simulation using the previously reported filament model of TcpA as a structural template. This model revealed that the CofA filament surface was highly negatively charged and that a 23-residue-long loop between the α1 and α2 helices filled the gap between the pilin subunits. These characteristics could provide a unique binding epitope for the CFA/III pili of ETEC compared with other type IVb pili.

Structural characterization of CFA/III and Longus type IVb pili from enterotoxigenic Escherichia coli

The type IV pili are helical filaments found on many Gram-negative pathogenic bacteria, with multiple diverse roles in pathogenesis, including microcolony formation, adhesion, and twitching motility. Many pathogenic enterotoxigenic Escherichia coli (ETEC) isolates express one of two type IV pili belonging to the type IVb subclass: CFA/III or Longus. Here we show a direct correlation between CFA/III expression and ETEC aggregation, suggesting that these pili, like the Vibrio cholerae toxin-coregulated pili (TCP), mediate microcolony formation. We report a 1.26-Å resolution crystal structure of CofA, the major pilin subunit from CFA/III. CofA is very similar in structure to V. cholerae TcpA but possesses a 10-amino-acid insertion that replaces part of the α2-helix with an irregular loop containing a 3(10)-helix. Homology modeling suggests a very similar structure for the Longus LngA pilin. A model for the CFA/III pilus filament was generated using the TCP electron microscopy reconstruction as a template. The unique 3(10)-helix insert fits perfectly within the gap between CofA globular domains. This insert, together with differences in surface-exposed residues, produces a filament that is smoother and more negatively charged than TCP. To explore the specificity of the type IV pilus assembly apparatus, CofA was expressed heterologously in V. cholerae by replacing the tcpA gene with that of cofA within the tcp operon. Although CofA was synthesized and processed by V. cholerae, no CFA/III filaments were detected, suggesting that the components of the type IVb pilus assembly system are highly specific to their pilin substrates.

Longus, a type IV pilus of enterotoxigenic Escherichia coli, is involved in adherence to intestinal epithelial cells

Enterotoxigenic Escherichia coli (ETEC) is the leading bacterial cause of diarrhea in the developing world, as well as the most common cause of traveler's diarrhea. The main hallmarks of this type of bacteria are the expression of one or more enterotoxins and fimbriae used for attachment to host intestinal cells. Longus is a pilus produced by ETEC. These bacteria grown in pleuropneumonia-like organism (PPLO) broth at 37 degrees C and in 5% CO(2) produced longus, showing that the assembly and expression of the pili depend on growth conditions and composition of the medium. To explore the role of longus in the adherence to epithelial cells, quantitative and qualitative analyses were done, and similar levels of adherence were observed, with values of 111.44 x 10(4) CFU/ml in HT-29, 101.33 x 10(4) CFU/ml in Caco-2, and 107.11 x 10(4) CFU/ml in T84 cells. In addition, the E9034A Delta lngA strain showed a significant reduction in longus adherence of 32% in HT-29, 22.28% in Caco-2, and 21.68% in T84 cells compared to the wild-type strain. In experiments performed with nonintestinal cells (HeLa and HEp-2 cells), significant differences were not observed in adherence between E9034A and derivative strains. Interestingly, the E9034A and E9034A Delta lngA(pLngA) strains were 30 to 35% more adherent in intestinal cells than in nonintestinal cells. Twitching motility experiments were performed, showing that ETEC strains E9034A and E9034A Delta lngA(pLngA) had the capacity to form spreading zones while ETEC E9034A Delta lngA does not. In addition, our data suggest that longus from ETEC participates in the colonization of human colonic cells.

The Longus type IV pilus of enterotoxigenic Escherichia coli (ETEC) mediates bacterial self-aggregation and protection from antimicrobial agents

Enterotoxigenic Escherichia coli (ETEC) strains are leading causes of childhood diarrhea in developing countries. ETEC pili and non-pili adherence factors designated colonization surface antigens (CSA) are believed to be important in the pathogenesis of diarrhea. Longus, a type IV pilus identified as the CSA(21), is expressed in up to one-third of ETEC strains, and share similarities to the toxin-coregulated pilus of Vibrio cholerae, and the bundle-forming pilus of enteropathogenic E. coli. To identify longus phenotype and possible function, a site-directed mutation of the lngA major subunit gene in the E9034A wild type ETEC strain was constructed. Lack of longus expression from the lngA mutant was demonstrated by immunoblot analysis and electron microscopy using specific anti-LngA antibody. Formation of self-aggregates by ETEC was shown to be dependent on longus expression as the lngA mutant or wild type grown under poor longus expression conditions was unable to express this phenotype. Longus-expressing ETEC were also associated with improved survival when exposed to antibacterial factors including lysozyme and antibiotics. This suggests that longus-mediated bacterial self-aggregates protect bacteria against antimicrobial environmental agents and may promote gut colonization.

pilQ Missense mutations have diverse effects on PilQ multimer formation, piliation, and pilus function in Neisseria gonorrhoeae

Type IV pili are required for virulence in Neisseria gonorrhoeae, as they are involved in adherence to host epithelium, twitching motility, and DNA transformation. The outer membrane secretin PilQ forms a homododecameric ring through which the pilus is proposed to be secreted. pilQ null mutants are nonpiliated, and thus, all pilus-dependent functions are eliminated. Mutagenesis was performed on the middle one-third of pilQ, and mutants with colony morphologies consistent with the colony morphology of nonpiliated or underpiliated bacteria were selected. Nineteen mutants, each with a single amino acid substitution, were isolated and displayed diverse phenotypes in terms of PilQ multimer stability, pilus expression, transformation efficiency, and host cell adherence. The 19 mutants were grouped into five phenotypic classes based on functionality. Four of the five mutant classes fit the current model of pilus functionality, which proposes that a functional pilus assembly apparatus, not necessarily full-length pili, is required for transformation, while high levels of displayed pili are required for adherence. One class, despite having an underpiliated colony morphology, expressed high levels of pili yet adhered poorly, demonstrating that pilus expression is necessary but not sufficient for adherence and indicating that PilQ may be directly involved in host cell adherence. The collection of phenotypes expressed by these mutants suggests that PilQ has an active role in pilus expression and function.

Interaction and localization studies of enteropathogenic Escherichia coli type IV bundle-forming pilus outer membrane components

Typical enteropathogenicEscherichia colistrains express an established virulence factor belonging to the type IV pili family, called the bundle-forming pilus (BFP). BFP are present on the cell surface as bundled filamentous appendages, and are assembled and retracted by proteins encoded by thebfpoperon. These proteins assemble to form a molecular machine. The BFP machine may be conceptually divided into three components: the cytoplasmic membrane (CM) subassembly, which is composed of CM proteins and cytoplasmic nucleotide-binding proteins; the outer membrane (OM) subassembly and the pilus itself. The authors have previously characterized the CM subassembly and the pilus. In this study, a more complete characterization of the OM subassembly was carried out using a combination of biochemical, biophysical and genetic approaches. It is reported that targeting of BfpG to the OM was influenced by the secretin BfpB. BfpG and BfpU interacted with the amino terminus of BfpB. BfpU had a complex cellular distribution pattern and, along with BfpB and BfpG, was part of the OM subassembly.

Type IV pilin structure and assembly: X-ray and EM analyses of Vibrio cholerae toxin-coregulated pilus and Pseudomonas aeruginosa PAK pilin

Pilin assembly into type IV pili is required for virulence by bacterial pathogens that cause diseases such as cholera, pneumonia, gonorrhea, and meningitis. Crystal structures of soluble, N-terminally truncated pilin from Vibrio cholera toxin-coregulated pilus (TCP) and full-length PAK pilin from Pseudomonas aeruginosa reveal a novel TCP fold, yet a shared architecture for the type IV pilins. In each pilin subunit a conserved, extended, N-terminal alpha helix wrapped by beta strands anchors the structurally variable globular head. Inside the assembled pilus, characterized by cryo-electron microscopy and crystallography, the extended hydrophobic alpha helices make multisubunit contacts to provide mechanical strength and flexibility. Outside, distinct interactions of adaptable heads contribute surface variation for specificity of pilus function in antigenicity, motility, adhesion, and colony formation.

Identification of a novel type IV pilus gene cluster required for gastrointestinal colonization of Citrobacter rodentium

Citrobacter rodentium is used as an in vivo model system for clinically significant enteric pathogens such as enterohaemorrhagic Escherichia coli (EHEC) and enteropathogenic E. coli (EPEC). These pathogens all colonize the lumen side of the host gastrointestinal tract via attaching and effacing (A/E) lesion formation. In order to identify genes required for the colonization of A/E-forming pathogens, a library of signature-tagged transposon mutants of C. rodentium was constructed and screened in mice. Of the 576 mutants tested, 14 were attenuated in their ability to colonize the descending colon. Of these, eight mapped to the locus of enterocyte effacement (LEE), which is required for the formation of A/E lesions, underlying the importance of this mechanism for pathogenesis. Another mutant, P5H2, was found to have a transposon insertion in an open reading frame that has strong similarity to type IV pilus nucleotide-binding proteins. The region flanking the transposon insertion was sequenced, identifying a cluster of 12 genes that encode the first described pilus of C. rodentium (named colonization factor Citrobacter, CFC). The proteins encoded by cfc genes have identity to proteins of the type IV COF pilus of enterotoxigenic E. coli (ETEC), the toxin co-regulated pilus of Vibrio cholerae and the bundle-forming pilus of EPEC. A non-polar mutation in cfcI, complementation of this strain with wild-type cfcI and complementation of strain P5H2 with wild-type cfcH confirmed that these genes are required for colonization of the gastrointestinal tract by C. rodentium. Thus, CFC provides a convenient model to study type IV pilus-mediated pathogen-host interactions under physiological conditions in the natural colonic environment.

Virulence typing of Escherichia coli using microarrays

We describe a microarray based broad-range screening technique for Escherichia coli virulence typing. Gene probes were amplified by PCR from a plasmid bank of characterised E. coli virulence genes and were spotted onto a glass slide to form an array of capture probes. Genomic DNA from E. coli strains which were to be tested for the presence of these virulence gene sequences was labelled with fluorescent cyanine dyes by random amplification and then hybridised against the array of probes. The hybridisation, washing and data analysis conditions were optimised for glass slides, and the applicability of the method for identifying the presence of the virulence genes was determined using reference strains and clinical isolates. It was found to be a sensitive screening method for detecting virulence genes, and a powerful tool for determining the pathotype of E. coli. It will be possible to expand and automate this microarray technique to make it suitable for rapid and reliable diagnostic screening of bacterial isolates.

Gene cluster for assembly of pilus colonization factor antigen III of enterotoxigenic Escherichia coli

The assembly of pilus colonization factor antigen III (CFA/III) of enterotoxigenic Escherichia coli (ETEC) requires the processing of CFA/III major pilin (CofA) by a prepilin peptidase (CofP), similar to other type IV pilus formation systems. CofA is produced initially as a 26.5-kDa preform pilin (prepilin) and then processed to a 20.5-kDa mature pilin by CofP which is predicted to be localized in the inner membrane. In the present experiment, we determined the nucleotide sequence of the whole region for CFA/III formation and identified a cluster of 14 genes, including cofA and cofP. Several proteins encoded by cof genes were similar to previously described proteins, such as the toxin-coregulated pili of Vibrio cholerae and the bundle-forming pili of enteropathogenic E. coli. The G+C content of the cof gene cluster was 37%, which was significantly lower than the average for the E. coli genome (50%). The introduction of a recombinant plasmid containing the cof gene cluster into the E. coli K-12 strain conferred CFA/III biogenesis and the ability of adhesion to the human colon carcinoma cell line Caco-2. This is the first report of a complete nucleotide sequence of the type IV pili found in human ETEC, and our results provide a useful model for studying the molecular mechanism of CFA/III biogenesis and the role of CFA/III in ETEC infection.

Identification of enterotoxigenic Escherichia coli harboring longus type IV pilus gene by DNA amplification

DNA amplification of lngA, the structural gene of longus type IV pilus produced by human enterotoxigenic Escherichia coli (ETEC) was achieved by the use of specific oligonucleotide primers designed from the nucleotide sequence of lngA. A 630-bp fragment representing the entire lngA gene was amplified in eight prototype strains previously characterized as longus positive. Five ETEC strains producing colonization factor antigen III (CFA III) (also a type IV pilus) were also positive by PCR, confirming the DNA homology between CFA III and longus. None of the non-ETEC and non-E. coli enteropathogens studied showed the 0.63-kbp amplicon. The procedure thus detected only ETEC strains harboring type IV pili genes with or without other colonization factors. Except for five lngA PCR-positive, probe-positive strains, all lngA PCR-positive strains produced the pilin as demonstrated by immunoblotting. To test the amplification procedure in a clinical setting, a collection of 264 fresh clinical E. coli strains isolated from 88 Mexican children with diarrhea was screened by PCR. Among 82 ETEC isolates found, 30 (36.5%) were lngA PCR-positive. Twenty-seven percent of the children shed ETEC that possessed lngA. In parallel with DNA probes or PCR protocols to detect enterotoxin genes, the lngA PCR method may prove useful for detection of ETEC harboring type IV pilus genes in epidemiological studies.

Evidence for specificity in type 4 pilus biogenesis by enteropathogenic Escherichia coli

Type 4 fimbriae (pili) are surface appendages that are expressed by many species of Gram-negative bacteria. Previous studies have demonstrated that Pseudomonas aeruginosa can express and assemble pilin subunits from several unrelated species, indicating a common mechanism for biogenesis of type 4 pili whereby structural subunits from one system may be interchanged with those of another. In this study, an isogenic mutant of enteropathogenic Escherichia coli (EPEC) was constructed containing the entire tcpA gene from Vibrio cholerae 0395, which encodes the major structural subunit of the toxin-coregulated pilus (TCP), in place of bfpA, which encodes the major structural subunit of the bundle-forming pilus (BFP). Surprisingly, expression of type 4 pilin structures and the associated phenotype of bacterial autoaggregation in culture media were not observed for cells of the EPEC strain containing tcpA nor for those containing an additional mutation in bfpF, which otherwise is associated with a hyperfimbriate phenotype. In addition, cells of a bfpA mutant EPEC strain containing plasmids designed to express either of two different chimeric type 4 pilin subunits containing segments of BfpA and TcpA also failed to form bacterial aggregates and express type 4 pilin structures. Collectively, these results indicate that the type 4 pilin assembly system of EPEC exhibits specificity with regard to pilin subunit recognition and assembly.

The gene encoding the prepilin peptidase involved in biosynthesis of pilus colonization factor antigen III (CFA/III) of human enterotoxigenic Escherichia coli

The assembly of pilus colonization factor antigen III (CFA/III) of human enterotoxigenic Escherichia coli requires the processing of CFA/III major pilin (CofA) by a peptidase, likely another type IV pilus formation system. Western blot analysis of CofA reveals that CofA is produced initially as a 26.5-kDa preform pilin (prepilin) and then processed to 20.5-kDa mature pilin by a prepilin peptidase. This processing is essential for exportation of the CofA from the cytoplasm to the periplasm. In this experiment, the structural gene, cofP, encoding CFA/III prepilin peptidase which cleavages at the Gly-30-Met-31 junction of CofA was identified, and the nucleotide sequence of the gene was determined. CofP consists of 819 bp encoding a 273-amino acid protein with a relative molecular mass of 30,533 Da. CofP is predicted to be localized in the inner membrane based on its hydropathy index. The amino acid sequence of CofP shows a high degree of homology with other prepilin peptidases which play a role in the assembly of type IV pili in several gram-negative bacteria.

Twelve pil genes are required for biogenesis of the R64 thin pilus

The IncI1 plasmid R64 produces two kinds of sex pili: a thin pilus and a thick pilus. The thin pilus, which belongs to the type IV family, is required only for liquid matings. Fourteen genes, pilI to -V, were found in the DNA region responsible for the biogenesis of the R64 thin pilus (S.-R. Kim and T. Komano, J. Bacteriol. 179:3594-3603, 1997). In this study, we introduced frameshift mutations into each of the 14 pil genes to test their requirement for R64 thin pilus biogenesis. From the analyses of extracellular secretion of thin pili and transfer frequency in liquid matings, we found that 12 genes, pilK to -V, are required for the formation of the thin pilus. Complementation experiments excluded the possible polar effects of each mutation on the expression of downstream genes. Two genes, traBC, were previously shown to be required for the expression of the pil genes. In addition, the rci gene is responsible for modulating the structure and function of the R64 thin pilus via the DNA rearrangement of the shufflon. Altogether, 15 genes, traBC, pilK through pilV, and rci, are essential for R64 thin pilus formation and function.

Crystallographic structure reveals phosphorylated pilin from Neisseria: phosphoserine sites modify type IV pilus surface chemistry and fibre morphology

Understanding the structural biology of type IV pili, fibres responsible for the virulent attachment and motility of numerous bacterial pathogens, requires a detailed understanding of the three-dimensional structure and chemistry of the constituent pilin subunit. X-ray crystallographic refinement of Neisseria gonorrhoeae pilin against diffraction data to 2.6 A resolution, coupled with mass spectrometry of peptide fragments, reveals phosphoserine at residue 68. Phosphoserine is exposed on the surface of the modelled type IV pilus at the interface of neighbouring pilin molecules. The site-specific mutation of serine 68 to alanine showed that the loss of the phosphorylation alters the morphology of fibres examined by electron microscopy without a notable effect on adhesion, transformation, piliation or twitching motility. The structural and chemical characterization of protein phosphoserine in type IV pilin subunits is an important indication that this modification, key to numerous regulatory aspects of eukaryotic cell biology, exists in the virulence factor proteins of bacterial pathogens. These O-linked phosphate modifications, unusual in prokaryotes, thus merit study for possible roles in pilus biogenesis and modulation of pilin chemistry for optimal in vivo function.

Mutational analysis of plasmid R64 thin pilus prepilin: the entire prepilin sequence is required for processing by type IV prepilin peptidase

The thin pili of IncI1 plasmid R64, which is required for conjugation in liquid media, belong to the type IV pilus family. They consist of a major subunit, the pilS product, and a minor component, one of the seven pilV products. The pilS product is first synthesized as a 22-kDa prepilin, processed to a 19-kDa mature pilin by the function of the pilU product, and then secreted outside the cell. The mature pilin is assembled to form a thin pilus with the pilV product. To reveal the relationship between the structure and function of the pilS product, 27 missense mutations, three N-terminal deletions, and two C-terminal deletions were constructed by PCR and site-directed mutagenesis. The characteristics of 32 mutant pilS products were analyzed. Four pilS mutant phenotype classes were identified. The products of 10 class I mutants were not processed by prepilin peptidase; the extracellular secretion of the products of two class II mutants was inhibited; from 11 class III mutants, thin pili with reduced activities in liquid mating were formed; from 9 class IV mutants, thin pili with mating activity similar to that of the wild-type pilS gene were formed. The point mutations of the class I mutants were distributed throughout the prepilin sequence, suggesting that processing of the pilS product requires the entire prepilin sequence.

Diarrheagenic Escherichia coli

Escherichia coli is the predominant nonpathogenic facultative flora of the human intestine. Some E. coli strains, however, have developed the ability to cause disease of the gastrointestinal, urinary, or central nervous system in even the most robust human hosts. Diarrheagenic strains of E. coli can be divided into at least six different categories with corresponding distinct pathogenic schemes. Taken together, these organisms probably represent the most common cause of pediatric diarrhea worldwide. Several distinct clinical syndromes accompany infection with diarrheagenic E. coli categories, including traveler's diarrhea (enterotoxigenic E. coli), hemorrhagic colitis and hemolytic-uremic syndrome (enterohemorrhagic E. coli), persistent diarrhea (enteroaggregative E. coli), and watery diarrhea of infants (entero-pathogenic E. coli). This review discusses the current level of understanding of the pathogenesis of the diarrheagenic E. coli strains and describes how their pathogenic schemes underlie the clinical manifestations, diagnostic approach, and epidemiologic investigation of these important pathogens.

Longus pilus of enterotoxigenic Escherichia coli and its relatedness to other type-4 pili--a minireview

Longus is a long pilus produced by human enterotoxigenic Escherichia coli (ETEC) which shares significant structural and biochemical features with class-B type-4 pili. These pili include the toxin-coregulated pilus (TCP) of Vibrio cholerae, the bundle-forming pilus (BFP) of enteropathogenic E. coli and both longus and the colonization factor antigen III (CFA/III) of ETEC. These pili are produced under defined growth conditions indicating that they are under the control of different regulatory elements. While TCP is chromosomally encoded, the remaining pili are encoded on large virulence plasmids. Longus and CFA/III are closely related pili although certain DNA and protein differences also exist between them. This may account for the differences in the regulation, surface presentation, antigenicity, and prevalence of these two pilins among ETEC. Neighboring lngA, a second open reading frame termed lngB was found which encodes a protein with significant homology to proteins which are part of a type-II secretory system such as XcpV, OutC, and PulO of Pseudomonas aeruginosa, Erwinia chrysanthemi, and Klebsiella pneumoniae, respectively. This suggests that lngB may be an accessory gene involved in biogenesis of longus.

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.

A cluster of fourteen genes from enteropathogenic Escherichia coli is sufficient for the biogenesis of a type IV pilus

Enteropathogenic Escherichia coli (EPEC) adhere to epithelial cells in microcolonies, a pattern termed localized adherence (LA). LA is dependent upon the presence of 50-70 MDa plasmids, termed EPEC adherence factor (EAF) plasmids. Expression of an EAF plasmid-encoded type IV fimbria, the bundle-forming pilus (BFP), is associated with the LA phenotype. TnphoA insertions in bfpA, the gene encoding the major structural subunit of the BFP, abolish LA. While bfpA::TnphoA mutants cannot be complemented for LA by plasmids carrying the bfpA gene alone in trans, this work shows that they can be complemented by plasmids carrying the bfpA gene, as well as approximately 10 kb of downstream sequence, suggesting that such mutations have polar effects on downstream genes. The identification and characterization of a cluster of 13 genes immediately downstream of bfpA are described. The introduction into a laboratory Escherichia coli strain of a plasmid containing these 14 bfp gene cluster genes, along with pJPN14, a plasmid containing another fragment derived from the EAF plasmid, confers LA ability and BFP biogenesis. However, when a mutation is introduced into the last gene of the bfp cluster, neither LA nor BFP biogenesis is conferred. This work also provides evidence to show that the fragment cloned in pJPN14 encodes a factor(s) which results in increased levels of the pilin protein. Finally, it is shown that expression of the 14 genes in the bfp cluster from an IPTG-inducible promoter, in the absence of pJPN14, is sufficient to reconstitute BFP biogenesis in a laboratory E. coli strain, but is insufficient for LA. This is the first report demonstrating the reconstitution of a type IV pilus in a laboratory E. coli strain with a defined set of genes. The BFP system should prove to be a useful model for studying the molecular mechanisms of type IV pilus biogenesis.

Structure of the fibre-forming protein pilin at 2.6 A resolution

The crystallographic structure of Neisseria gonorrhoeae pilin, which assembles into the multifunctional pilus adhesion and virulence factor, reveals an alpha-beta roll fold with a striking 85 A alpha-helical spine and an O-linked disaccharide. Key residues stabilize interactions that allow sequence hypervariability, responsible for pilin's celebrated antigenic variation, within disulphide region beta-strands and connections. Pilin surface shape, hydrophobicity and sequence variation constrain pilus assembly to the packing of flat subunit faces against alpha 1 helices. Helical fibre assembly is postulated to form a core of coiled alpha 1 helices banded by beta-sheet, leaving carbohydrate and hypervariable sequence regions exposed to solvent.

Identification of Porphyromonas gingivalis prefimbrilin possessing a long leader peptide: possible involvement of trypsin-like protease in fimbrilin maturation

Fimbriae of Porphyromonas gingivalis have been shown to be important as one of the virulence factors for colonization on mucosal surfaces. The gene (fimA) encoding the fimbrial subunit (fimbrilin) was overexpressed in Escherichia coli by using a bacteriophage T7 promoter-polymerase expression vector system. Analysis of the resulting fimA gene product revealed that the prefimbrilin had a 46 amino acid leader peptide. This extremely long leader peptide was cleaved from the prefimbrilin by treatment with trypsin or P. gingivalis extracts containing trypsin-like protease activity, resulting in production of a mature fimbrilin. We also found that some transposon-induced trypsin-like protease deficient mutants of P. gingivalis exhibited deficiency in fimbriation and that one of the mutants accumulated a fimbrilin precursor possessing a 25 amino acid leader peptide in the cell. The presence of an extremely long leader peptide and the requirement for a leader peptidase with a substrate specificity similar to that of P. gingivalis trypsin-like protease for fimbrilin maturation indicate that P. gingivalis fimbrilin is a novel type that is different from fimbrilins of type I and IV families.

Prevalence and association of the longus pilus structural gene (lngA) with colonization factor antigens, enterotoxin types, and serotypes of enterotoxigenic Escherichia coli

Human enterotoxigenic Escherichia coli (ETEC) produces a plasmid-encoded type IV pilus termed longus (for long pilus). Regardless of the geographic origins of ETEC strains, the longus structural gene lngA was found to have the highest level of association with ETEC producing colonization factor antigen (CFA) CFA/II, followed by ETEC producing CFA/I and CFA/IV. ETEC bearing the less prevalent CFA/III and putative colonization factors and ETEC negative for CFA and putative colonization factor also contained lngA-related sequences. lngA was found in a considerable number of ETEC serotypes and was more often associated with ETEC producing heat-stable enterotoxins than with ETEC producing both heat-labile and heat-stable enterotoxins or heat-labile enterotoxin alone. lngA was found more often in strains isolated from children with diarrhea than in strains from healthy children, suggesting an association with intestinal disease. We conclude that longus is a widely distributed antigenic determinant in ETEC that is highly associated with known plasmid-encoded virulence factors, namely, CFAs and enterotoxins. A longus-specific probe may be a helpful epidemiological tool to assist in the identification of ETEC.

Colonization factors of diarrheagenic E. coli and their intestinal receptors

While Escherichia coli is common as a commensal organism in the distal ileum and colon, the presence of colonization factors (CF) on pathogenic strains of E. coli facilitates attachment of the organism to intestinal receptor molecules in a species- and tissue-specific fashion. After the initial adherence, colonization occurs, and the involvement of additional virulence determinants leads to illness. Enterotoxigenic E. coli (ETEC) is the most extensively studied of the five categories of E. coli that cause diarrheal disease, and has the greatest impact on health worldwide. ETEC can be isolated from domestic animals and humans. The biochemistry, genetics, epidemiology, antigenic characteristics, and cell and receptor binding properties of ETEC have been extensively described. Another major category, enteropathogenic E. coli (EPEC), has virulence mechanisms, primarily effacement and cytoskeletal rearrangement of intestinal brush borders, that are distinct from ETEC. An EPEC CF receptor has been purified and characterized as a sialidated transmembrane glycoprotein complex directly attached to actin, thereby associating CF-binding with host-cell response. Three additional categories of E. coli diarrheal disease, their colonization factors and their host cell receptors, are discussed. It appears that biofilms exist in the intestine in a manner similar to oral bacterial biofilms, and that E. coli is part of these biofilms as both commensals and pathogens.

Use of a new oligonucleotide probe for detection of colonization factor antigen III gene in enterotoxigenic Escherichia coli

An alkaline phosphatase-labeled 30-mer oligonucleotide probe was designed to detect the gene for pilus colonization factor antigen III (CFA/III) of the human type of enterotoxigenic Escherichia coli (ETEC). The CFA/III probe was used to identify CFA/III-producing ETEC among 303 Escherichia coli obtained from subjects with traveler's diarrhea. Six isolates positive for the CFA/III gene were found. This result was confirmed immunologically by using a specific monoclonal antibody developed against CFA/III. These six isolates, isolated from travelers returning from India, Pakistan and China, were all positive for the gene of heat-labile enterotoxin and possessed an identical serotype (025:H-).