The 987P fimbrial gene cluster of enterotoxigenic Escherichia coli is plasmid encoded

Characterization of unstable pEntYN10 from enterotoxigenic Escherichia coli (ETEC) O169:H41

Enterotoxigenic Escherichia coli (ETEC) serotype O169:H41 has been an extremely destructive epidemic ETEC type worldwide. The strain harbors a large unstable plasmid that is regarded as responsible for its virulence, although its etiology has remained unknown. To examine its genetic background specifically on the unstable retention and responsibility in the unique adherence to epithelial cells and enterotoxin production, the complete sequence of a plasmid, pEntYN10, purified from the serotype strain was determined. The length is 145,082 bp; its GC content is 46.15%. It contains 182 CDSs, which include 3 colonization factors (CFs), an enterotoxin, and large number of insertion sequences. The repertory of plasmid stability genes was extraordinarily scant. Uniquely, results showed that 3 CFs, CS6, CS8 (CFA/III)-like, and K88 (F4)-like were encoded redundantly in the plasmid with unique variations among previously known subtypes. These three CFs preserved their respective gene structures similarly to those of other ETEC strains reported previously with unique sequence variations respectively. It is particularly interesting that the K88-like gene cluster of pEntYN10 had 2 paralogous copies of faeG, which encodes the major component of fimbrial structure. It remains to be verified how the unique variations found in the CFs respectively affect the affinity to infected cells, host range, and virulence of the ETEC strain.

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.

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.

Colonization factors of enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) is a major cause of life-threatening diarrheal disease around the world. The major aspects of ETEC virulence are colonization of the small intestine and the secretion of enterotoxins which elicit diarrhea. Intestinal colonization is mediated, in part, by adhesins displayed on the bacterial cell surface. As colonization of the intestine is the critical first step in the establishment of an infection, it represents a potential point of intervention for the prevention of infections. Therefore, colonization factors (CFs) have been important subjects of research in the field of ETEC virulence. Research in this field has revealed that ETEC possesses a large array of serologically distinct CFs that differ in composition, structure, and function. Most ETEC CFs are pili (fimbriae) or related fibrous structures, while other adhesins are simple outer membrane proteins lacking any macromolecular structure. This chapter reviews the genetics, structure, function, and regulation of ETEC CFs and how such studies have contributed to our understanding of ETEC virulence and opened up potential opportunities for the development of preventive and therapeutic interventions.

Pathogenomics of the virulence plasmids of Escherichia coli

Bacterial plasmids are self-replicating, extrachromosomal elements that are key agents of change in microbial populations. They promote the dissemination of a variety of traits, including virulence, enhanced fitness, resistance to antimicrobial agents, and metabolism of rare substances. Escherichia coli, perhaps the most studied of microorganisms, has been found to possess a variety of plasmid types. Included among these are plasmids associated with virulence. Several types of E. coli virulence plasmids exist, including those essential for the virulence of enterotoxigenic E. coli, enteroinvasive E. coli, enteropathogenic E. coli, enterohemorrhagic E. coli, enteroaggregative E. coli, and extraintestinal pathogenic E. coli. Despite their diversity, these plasmids belong to a few plasmid backbones that present themselves in a conserved and syntenic manner. Thanks to some recent research, including sequence analysis of several representative plasmid genomes and molecular pathogenesis studies, the evolution of these virulence plasmids and the implications of their acquisition by E. coli are now better understood and appreciated. Here, work involving each of the E. coli virulence plasmid types is summarized, with the available plasmid genomic sequences for several E. coli pathotypes being compared in an effort to understand the evolution of these plasmid types and define their core and accessory components.

Evolution of the chaperone/usher assembly pathway: fimbrial classification goes Greek

Many Proteobacteria use the chaperone/usher pathway to assemble proteinaceous filaments on the bacterial surface. These filaments can curl into fimbrial or nonfimbrial surface structures (e.g., a capsule or spore coat). This article reviews the phylogeny of operons belonging to the chaperone/usher assembly class to explore the utility of establishing a scheme for subdividing them into clades of phylogenetically related gene clusters. Based on usher amino acid sequence comparisons, our analysis shows that the chaperone/usher assembly class is subdivided into six major phylogenetic clades, which we have termed alpha-, beta-, gamma-, kappa-, pi-, and sigma-fimbriae. Members of each clade share related operon structures and encode fimbrial subunits with similar protein domains. The proposed classification system offers a simple and convenient method for assigning newly discovered chaperone/usher systems to one of the six major phylogenetic groups.

Phase variation of the 987P-like CS18 fimbriae of human enterotoxigenic Escherichia coli is regulated by site-specific recombinases

The gene cluster of the CS18 (PCFO20) fimbriae of human enterotoxigenic Escherichia coli (ETEC) was found to include seven genes (fotA to fotG) that are similar to each of the seven structural and export proteins of the 987P fimbriae. However, no analogous gene to the fasH regulatory gene, which is located at the 3' end of the 987P gene cluster and encodes an AraC-like activator of transcription, could be detected. Surprisingly, two novel genes (fotS and fotT) encoding proteins similar to the site-specific recombinases of the type 1 fimbriae (FimB and FimE) were identified at the 5' end of the fot gene cluster. These genes were shown to be required for the catalysis of a 312 bp-inversion just upstream of fotA. The inversion determines CS18 fimbrial phase variation. FotS participates in inverting the 312 bp-segment in both the ON and OFF orientation, whereas FotT has a bias for the OFF oriented recombination. Similar regulators of fimbriation by phase variation were described in uropathogenic and commensal Enterobacteriaceae. In contrast, only AraC-like transcriptional activators were previously described as regulators of the intestinal colonization factors of human ETEC isolates. Thus, the CS18 and 987P gene clusters encode similar components for fimbrial biogenesis but different types of regulators for fimbriation. The combination of blocks of genes encoding similar structural products but different regulatory proteins underlines how modular DNA rearrangements can evolve by serving pathogen diversification. Acquisition of a phase variation module to regulate fimbrial genes is proposed to be beneficial for the adaptation and transmission of pathogens.

Improved allelic exchange vectors and their use to analyze 987P fimbria gene expression

A series of vectors has been developed to provide improved positive and negative selection for allelic exchange. Based on homologous regions of DNA ranging in size from less than 200 bp to over 1 kb, we have successfully used these new plasmids to introduce or remove markers in chromosomal or plasmid DNA. Wild type fimbria genes were replaced both in Salmonella enteritidis (sefA, agfA and fimC) and Escherichia coli (fasA and fasH). Regulation of 987P fimbriation could be identified after replacement of fasA and fasH with allelic reporter fusions. The expression of fasA but not fasH is dependent upon the osmolarity of the growth medium in an HNS-dependent manner, but unlike some other fimbrial systems expression is not dependent on the exogenous iron concentration.

Identification of major and minor chaperone proteins involved in the export of 987P fimbriae

The 987P fimbriae of Escherichia coli consist mainly of the major subunit, FasA, and two minor subunits, FasF and FasG. In addition to the previously characterized outer membrane or usher protein FasD, the FasB, FasC, and FasE proteins are required for fimbriation. To better understand the roles of these minor proteins, their genes were sequenced and the predicted polypeptides were shown to be most similar to periplasmic chaperone proteins of fimbrial systems. Western blot (immunoblot) analysis and immunoprecipitation of various fas mutants with specific antibody probes identified both the subcellular localizations and associations of these minor components. FasB was shown to be a periplasmic chaperone for the major fimbrial subunit, FasA. A novel periplasmic chaperone, FasC, which stabilizes and specifically interacts with the adhesin, FasG, was identified. FasE, a chaperone-like protein, is also located in the periplasm and is required for optimal export of FasG and possibly other subunits. The use of different chaperone proteins for various 987P subunits is a novel observation for fimbrial biogenesis in bacteria. Whether other fimbrial systems use a similar tactic remains to be discovered.

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.

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.

Identification of plasmid and chromosomal copies of 987P pilus genes in enterotoxigenic Escherichia coli 987

A DNA probe derived from the subunit gene of a cloned 987P determinant was used to characterize the locations of the 987P genes in several Escherichia coli strains. We examined E. coli 987, a nonpiliated mutant of strain 987 (I36) that does not express 987P in vitro, and a variant of I36 that expressed 987P following growth in vivo. We found that plasmid and chromosomal copies of the 987P subunit gene could be differentiated in strain 987 by restriction endonuclease analysis and Southern blot hybridization. The nonpiliated mutant I36 had lost the plasmid copy but retained the chromosomal copy of the 987P gene. A 987P-piliated variant of I36, which did not contain the 987P plasmid, colonized and caused diarrhea in neonatal pigs similarly to wild-type 987. The plasmid that hybridized with the 987P probe was transferred from strain 987 to an E. coli K-12 strain by conjugation. We were unable to demonstrate expression of 987P by these transconjugants. The data suggest that the chromosomal and plasmid copies of the 987P genes may interact to influence 987P expression.

Replicon typing of virulence plasmids of enterotoxigenic Escherichia coli isolates from cattle

Plasmid DNA hybridization with probes for virulence factors used for basic replicons of plasmids was used to identify the virulence plasmids of a collection of enterotoxigenic Escherichia coli isolates from cattle. The virulence probes were derived from the genes coding for the heat-stable enterotoxin STaP and for the F5 (K99) and F41 fimbrial adhesins. The replicon probes were derived from 16 different basic replicons of plasmids (probes repFIA, repFIB, repFIC, repFIIA, repI1, repHI1, repHI2, repL/M, repN, repP, repQ, repT, repU, repW, repX, and repY). The virulence genes coding for the STaP enterotoxin and for the F5 adhesin were located on a single plasmid band in each isolate. The sizes of most of these virulence plasmids were from 65 to 95 MDa. The F41 probe failed to hybridize with any plasmid band. The virulence plasmids had multireplicon types typical of plasmids of the IncF groups. The most common basic replicon association was the triple RepFIA-RepFIB-RepFIC family association.

Swine and cattle enterotoxigenic Escherichia coli-mediated diarrhea. Development of therapies based on inhibition of bacteria-host interactions

Enterotoxigenic Escherichia coli (ETEC) frequently occurs in diarrheal disease afflicting domestic animals. In this paper is summarized the research carried out over the last decade on the two important determinants of virulence that plays a role in the development of the infection, namely the colonizing ability of the small intestine mediated by specific fimbrial adhesins acting as lectins and the production of enterotoxins. Recent progress in knowledge of the phenomenon led to alternative strategies of prevention and cure of enteric infection. Since bacterial recognition of mucosa surface receptors in an initial event in colonization, several approaches based on the competitive inhibition of ETEC adhesion have been developed. This review examines the following approaches: competitive colonization with non pathogenic strains, design of adhesin or toxin vaccines, receptor analog therapy and methods for in vivo suppression of virulence factors.

Colicin V virulence plasmids

ColV plasmids are a heterogeneous group of IncFI plasmids which encode virulence-related properties such as the aerobactin iron uptake system, increased serum survival, and resistance to phagocytosis. These plasmids have been found in invasive strains of Escherichia coli which infect vertebrate hosts including humans and livestock. Colicin V was the first colicin to be identified, in 1925, but not until the field experienced a renewed interest has the mechanism of colicin V activity been explored. As encoded by ColV plasmid pColV-K30, the aerobactin iron uptake system has been extensively investigated, but other ColV-encoded phenotypes remain largely uncharacterized. Restriction enzyme mapping of the 144-kb pColV-K30 and of the 80-kb pColV-B188 has facilitated systematic study, so that questions can be addressed by a molecular and comparative approach regarding the contributions of individual factors and plasmids to the virulence of host E. coli in model systems. The family of large ColV plasmids could be analogous to other families of large virulence plasmids, and insights gained from studying these plasmids should contribute to our understanding of cross-genetic interactions and the role of large plasmids in bacterial pathogenesis.

Specific DNA fragments coding for ST1 and LT1 toxins, and K88 (F4) adhesin in enterotoxigenic Escherichia coli

Ten porcine strains of enterotoxigenic Escherichia coli possessing the K88 (F4) adhesion fimbriae, were selected for study of enterotoxin- and fimbriae-encoding plasmids. Plasmid DNA, separated according to size by gel electrophoresis was transferred to nylon membranes by Southern blotting, and hybridized with enzyme-labelled oligonucleotide probes for ST1 and LT1 enterotoxins, and a 32P-labelled probe for the F4 fimbriae. Plasmids possessing the enterotoxin genes ranged from 50 MDa to 78 MDa in size. The ST1 genes were located on a common 8-MDa EcoR1 restriction endonuclease fragment, while the LT1 genes were located on a 4.5-MDa EcoR1 fragment from the different plasmids. Plasmids with the F4 genes ranged from 50 MDa to 118 MDa in size, but the F4 encoding genes were located on a common 3-MDa HindIII restriction endonuclease fragment. ST1 and LT1 genes were found on the same plasmid in only one strain, LT1 and F4 genes on the same plasmids in 5 strains, while no plasmid contained genes for both ST1 and F4.

Genetic analysis of 987P adhesion and fimbriation of Escherichia coli: the fas genes link both phenotypes

The 987P fimbrial gene cluster has recently been shown to contain eight genes (fasA to fasH) clustered on large plasmids of enterotoxigenic Escherichia coli and adjacent to a Tn1681-like transposon encoding the heat-stable enterotoxin STIa. Different genetic approaches were used to study the relationship between 987P fimbriation and adhesion. TnphoA mutagenesis, complementation assays, and T7 RNA polymerase-promoted gene expression indicated that all of the fas genes were involved in fimbrial expression and adhesion. In contrast to other fimbrial systems, the lack of expression of any single fas gene never resulted in the dissociation of fimbriation and adhesion, indicating that the adhesin is required for fimbrial expression and suggesting that FasA, the fimbrial structural subunit itself, is the adhesin. In addition, fimbrial length was shown to be modulated by the levels of expression of different fas genes.

Enhanced expression of 987P (F6) fimbrial adhesin by cultured enterotoxigenic Escherichia coli

The expression of the 987P fimbrial adhesin by strains of enterotoxigenic Escherichia coli was enhanced and, in some cases, restored by passaging the organisms through Craigie's tubes. In contrast, the fimbrial adhesins K99, K88 and F41 were not optimally expressed in this medium. The results suggest that Craigie's tubes should be used for the optimum expression of 987P.

987P fimbrial gene identification and protein characterization by T7 RNA polymerase-induced transcription and TnphoA mutagenesis

The 987P fimbrial gene cluster has been previously cloned as a 12 kb fragment from prototype strain 987. Gene products encoded by the whole clone were analysed by utilizing an in vivo system based on the induction of transcription by T7 RNA polymerase. The sensitivity of this technique permitted us to identify new proteins involved in 987P fimbriation. In total, eight proteins were detected, their genes (fasA to fasH) were mapped and their orientation of transcription determined. Several of the gene products demonstrated typical properties of exported proteins. Precursor and processed forms could be correlated after inhibiting protein transport with ethanol. The detection of enzymatically active fusion proteins after TnphoA (Tn5IS50L::phoA) mutagenesis supported and complemented these results. One protein encoded by the 12kb fragment was found not to be related to fimbriation but rather the product of the STla gene, identified as a component of a Tn1681-like transposon.

Some infectious causes of diarrhea in young farm animals

Escherichia coli, rotaviruses, and Cryptosporidium parvum are discussed in this review as they relate to enteric disease in calves, lambs, and pigs. These microorganisms are frequently incriminated as causative agents in diarrheas among neonatal food animals, and in some cases different strains or serotypes of the same organism cause diarrhea in humans. E. coli causes diarrhea by mechanisms that include production of heat-labile or heat-stable enterotoxins and synthesis of potent cytotoxins, and some strains cause diarrhea by yet undetermined mechanisms. Rotaviruses and C. parvum induce various degrees of villous atrophy. Rotaviruses infect and replicate within the cytoplasm of enterocytes, whereas C. parvum resides in an intracellular, extracytoplasmic location. E. coli, rotavirus, and C. parvum infections are of concern to producers, veterinarians, and public health officials. These agents are a major cause of economic loss to the producer because of costs associated with therapy, reduced performance, and high morbidity and mortality rates. Moreover, diarrheic animals may harbor, incubate, and act as a source to healthy animals and humans of some of these agents.