Characterization of the adhesin of Escherichia coli F18 fimbriae

ArcA positively regulates the expression of virulence genes and contributes to virulence of porcine Shiga toxin-producing enterotoxigenic Escherichia coli

IMPORTANCE

Enterotoxigenic Escherichia coli (ETEC) cause severe diarrhea in humans and animals, leading to death and huge economic loss worldwide. Thus, elucidation of ETEC's pathogenic mechanisms will provide powerful data for the discovery of drugs serving as prevention or therapeutics against ETEC-caused diarrheal diseases. Here, we report that ArcA plays an essential role in the pathogenicity and virulence regulation in ETEC by positively regulating the expression of several key virulence factors including F18 fimbriae, heat-labile and heat-stable toxins, Shiga toxin 2e, and hemolysin, under microaerobic conditions and in vivo. Moreover, we found that positive regulation of several virulence genes by ArcA requires a global repressor H-NS (histone-like nucleoid structuring), implying that ArcA may exert positive effects by antagonizing H-NS. Collectively, our data established a key role for ArcA in the pathogenicity of porcine ETEC and ETEC strains isolated from human infections. Moreover, our work reveals another layer of regulation in relation to oxygen control of virulence factors in ETEC.

Oral Immunization with Attenuated Salmonella Choleraesuis Expressing the FedF Antigens Protects Mice against the Shiga-Toxin-Producing Escherichia coli Challenge

Edema disease (ED) is a severe and lethal infectious ailment in swine, stemming from Shiga-toxin-producing Escherichia coli (STEC). An efficient, user-friendly, and safe vaccine against ED is urgently required to improve animal welfare and decrease antibiotic consumption. Recombinant attenuated Salmonella vaccines (RASV) administered orally induce both humoral and mucosal immune responses to the immunizing antigen. Their potential for inducing protective immunity against ED is significant through the delivery of STEC antigens. rSC0016 represents an enhanced recombinant attenuated vaccine vector designed for Salmonella enterica serotype Choleraesuis. It combines sopB mutations with a regulated delay system to strike a well-balanced equilibrium between host safety and immunogenicity. We generated recombinant vaccine strains, namely rSC0016 (pS-FedF) and rSC0016 (pS-rStx2eA), and assessed their safety and immunogenicity in vivo. The findings demonstrated that the mouse models immunized with rSC0016 (pS-FedF) and rSC0016 (pS-rStx2eA) generated substantial IgG antibody responses to FedF and rStx2eA, while also provoking robust mucosal and cellular immune responses against both FedF and rStx2eA. The protective impact of rSC0016 (pS-FedF) against Shiga-toxin-producing Escherichia coli surpassed that of rSC0016 (pS-rStx2eA), with percentages of 83.3%. These findings underscore that FedF has greater suitability for vaccine delivery via recombinant attenuated Salmonella vaccines (RASVs). Overall, this study provides a promising candidate vaccine for infection with STEC.

C500 variants conveying complete mucosal immunity against fatal infections of pigs with Salmonella enterica serovar Choleraesuis C78-1 or F18+ Shiga toxin-producing Escherichia coli

Salmonella enterica serovar Choleraesuis (S. Choleraesuis) C500 strain is a live, attenuated vaccine strain that has been used in China for over 40 years to prevent piglet paratyphoid. However, this vaccine is limited by its toxicity and does not offer protection against diseases caused by F18+ Shiga toxin-producing Escherichia coli (STEC), which accounts for substantial economic losses in the swine industry. We recently generated a less toxic derivative of C500 strain with both asd and crp deletion (S. Choleraesuis C520) and assessed its efficacy in mice. In addition, we demonstrate that C520 is also less toxic in pigs and is effective in protecting pigs against S. Choleraesuis when administered orally. To develop a vaccine with a broader range of protection, we prepared a variant of C520 (S. Choleraesuis C522), which expresses rSF, a fusion protein comprised of the fimbriae adhesin domain FedF and the Shiga toxin-producing IIe B domain antigen. For comparison, we also prepared a control vector strain (S. Choleraesuis C521). After oral vaccination of pigs, these strains contributed to persistent colonization of the intestinal mucosa and lymphoid tissues and elicited both cytokine expression and humoral immune responses. Furthermore, oral immunization with C522 elicited both S. Choleraesuis and rSF-specific immunoglobulin G (IgG) and IgA antibodies in the sera and gut mucosa, respectively. To further evaluate the feasibility and efficacy of these strains as mucosal delivery vectors via oral vaccination, we evaluated their protective efficacy against fatal infection with S. Choleraesuis C78-1, as well as the F18+ Shiga toxin-producing Escherichia coli field strain Ee, which elicits acute edema disease. C521 conferred complete protection against fatal infection with C78-1; and C522 conferred complete protection against fatal infection with both C78-1 and Ee. Our results suggest that C520, C521, and C522 are competent to provide complete mucosal immune protection against fatal infection with S. Choleraesuis in swine and that C522 equally qualifies as an oral vaccine vector for protection against F18+ Shiga toxin-producing Escherichia coli.

Evidence-Based Approaches for Determining Effective Target Antigens to Develop Vaccines against Post-Weaning Diarrhea Caused by Enterotoxigenic Escherichia coli in Pigs: A Systematic Review and Network Meta-Analysis

Simple Summary

Owing to the levels of mortality and morbidity associated with enterotoxigenic Escherichia coli (ETEC) inflicted diarrhea, as well as the scope of the global disease burden, several initiatives to develop effective vaccines have been launched. We conducted a meta-analysis to assess the efficacy of commercially available and candidate vaccines against ETEC-associated post-weaning diarrhea in pigs. The effectiveness of vaccines was evaluated using three clinical outcomes: mortality, diarrhea, and average daily weight gain. Subsequently, a simultaneous comparison of vaccines was conducted using a Bayesian network meta-analysis approach to generate evidence-based data on the most effective vaccine based on their target antigens. The results indicated that vaccinated pigs had a significantly lower risk of diarrhea and mortality when compared to non-vaccinated pigs. Furthermore, the findings also showed that a multivalent vaccine that targets both fimbriae and enterotoxins should be prioritized to combat post-weaning diarrhea in pigs.

Abstract

In this study, we conducted a meta-analysis (MA) and systematic review to evaluate the effectiveness of vaccines against post-weaning diarrhea (PWD), caused by enterotoxigenic Escherichia coli (ETEC), in piglets. A Bayesian network meta-analysis (NMA) was also performed to compare the effects of combining different target antigens on vaccine efficacy. Relevant electronic databases were searched using pre-specified search terms, and 17 studies were selected based on three outcomes: diarrhea, mortality, and average daily weight gain (ADWG). In pairwise MA, the vaccinated group showed a significant decrease in diarrhea (OR = 0.124 [0.056, 0.275]) and mortality (OR = 0.273 [0.165, 0.451]), and a significant increase in ADWG (SMD = 0.699 [0.107, 1.290]) compared with those in controls. Furthermore, NMA results showed that all vaccine groups, except for group D (LT enterotoxin), were effective against PWD. Rank probabilities indicated that the F4 + F18 + LT combination was the best regimen for preventing diarrhea (SUCRA score = 0.92) and mortality (SUCRA score = 0.89). NMA also demonstrated that, among the vaccine groups, those inducing simultaneous anti-adhesion and antitoxin immunity had the highest efficacy. Our results provide evidence-based information on the efficacy of vaccines in reducing PWD incidence in pigs and may serve as guidelines for antigen selection for commercial vaccine development in the future.

Dietary inclusion of multispecies probiotics to reduce the severity of post-weaning diarrhea caused by Escherichia coli F18+ in pigs

This study was aimed to determine the efficacy of multispecies probiotics in reducing the severity of post-weaning diarrhea caused by enterotoxigenic Escherichia coli (ETEC) F18⁺ on newly weaned pigs. Thirty-two pigs (16 barrows and 16 gilts, BW = 6.99 ± 0.33 kg) at 21 d of age were individually allotted in a randomized complete block design with 2 × 2 factorial arrangement of treatments. Pigs were selected from sows not infected previously and not vaccinated against ETEC. Pigs were fed experimental diets for 25 d based on 10 d phase 1 and 15 d phase 2. The factors were ETEC challenge (oral inoculation of saline solution or E. coli F18⁺ at 2 × 10⁹ CFU) and probiotics (none or multispecies probiotics 0.15% and 0.10% for phase 1 and 2, respectively). Body weight and feed intake were measured on d 5, 9, 13, 19, and 25. Fecal scores were measured daily. Blood samples were taken on d 19 and 24. On d 25, all pigs were euthanized to obtain samples of digesta, intestinal tissues, and spleen. The tumor necrosis factor alpha (TNFα), malondialdehyde (MDA), peptide YY (PYY), and neuropeptide Y (NPY) were measured in serum and intestinal tissue. Data were analyzed using the MIXED procedure of SAS. The fecal score of pigs was increased (P < 0.05) by ETEC challenge at the post–challenge period. The ETEC challenge decreased (P < 0.05) jejunal villus height and crypt depth, tended to increase (P = 0.056) jejunal TNFα, increased (P < 0.05) ileal crypt depth, and decreased (P < 0.05) serum NPY. The probiotics decreased (P < 0.05) serum TNFα, tended to reduce (P = 0.064) jejunal MDA, tended to increase (P = 0.092) serum PYY, and increased (P < 0.05) jejunal villus height, and especially villus height-to-crypt depth ratio in challenged pigs. Growth performance of pigs were not affected by ETEC challenge, whereas the probiotics increased (P < 0.05) ADG and ADFI and tended to increase (P = 0.069) G:F ratio. In conclusion, ETEC F18⁺ challenge caused diarrhea, intestinal inflammation and morphological damages without affecting the growth performance. The multispecies probiotics enhanced growth performance by reducing intestinal inflammation, oxidative stress, morphological damages.

Engineered Recombinant Escherichia coli Probiotic Strains Integrated with F4 and F18 Fimbriae Cluster Genes in the Chromosome and Their Assessment of Immunogenic Efficacy in Vivo

F4 (K88) and F18 fimbriaed enterotoxigenic Escherichia coli (ETEC) are the predominant causes of porcine postweaning diarrhea (PWD), and vaccines are considered the most effective preventive approach against PWD. Since heterologous DNA integrated into bacterial chromosomes could be effectively expressed with stable inheritance, we chose probiotic EcNc (E. coli Nissle 1917 prototype cured of cryptic plasmids) as a delivery vector to express the heterologous F4 or both F4 and F18 fimbriae and sequentially assessed their immune efficacy of anti-F4 and F18 fimbriae in both murine and piglet models. Employing the CRISPR-cas9 technology, yjcS, pcadA, lacZ, yieN/trkD, maeB, and nth/tppB sites in the chromosome of an EcNc strain were targeted as integration sites to integrate F4 or F18 fimbriae cluster genes under the P_tet promotor to construct two recombinant integration probiotic strains (RIPSs), i.e., nth integration strain (EcNcΔnth/tppB::P_tetF4) and multiple integration strain (EcNc::P_tetF18x4::P_tetF4x2). Expression of F4, both F4 and F18 fimbriae on the surfaces of two RIPSs, was verified with combined methods of agglutination assay, Western blot, and immunofluorescence microscopy. The recombinant strains have improved adherence to porcine intestinal epithelial cell lines. Mice and piglets immunized with the nth integration strain and multiple integration strain through gavage developed anti-F4 and both anti-F4 and anti-F18 IgG immune responses. Moreover, the serum antibodies from the immunized mice and piglets significantly inhibited the adherence of F4⁺ or both F4⁺ and F18⁺ ETEC wild-type strains to porcine intestinal cell lines in vitro, indicating the potential of RIPSs as promising probiotic strains plus vaccine candidates against F4⁺/F18⁺ ETEC infection.

Methodology and application of Escherichia coli F4 and F18 encoding infection models in post-weaning pigs

The enterotoxigenic Escherichia coli (ETEC) expressing F4 and F18 fimbriae are the two main pathogens associated with post-weaning diarrhea (PWD) in piglets. The growing global concern regarding antimicrobial resistance (AMR) has encouraged research into the development of nutritional and feeding strategies as well as vaccination protocols in order to counteract the PWD due to ETEC. A valid approach to researching effective strategies is to implement piglet in vivo challenge models with ETEC infection. Thus, the proper application and standardization of ETEC F4 and F18 challenge models represent an urgent priority. The current review provides an overview regarding the current piglet ETEC F4 and F18 challenge models; it highlights the key points for setting the challenge protocols and the most important indicators which should be included in research studies to verify the effectiveness of the ETEC challenge.

Based on the current review, it is recommended that the setting of the model correctly assesses the choice and preconditioning of pigs, and the timing and dosage of the ETEC inoculation. Furthermore, the evaluation of the ETEC challenge response should include both clinical parameters (such as the occurrence of diarrhea, rectal temperature and bacterial fecal shedding) and biomarkers for the specific expression of ETEC F4/F18 (such as antibody production, specific F4/F18 immunoglobulins (Igs), ETEC F4/F18 fecal enumeration and analysis of the F4/F18 receptors expression in the intestinal brush borders). On the basis of the review, the piglets’ response upon F4 or F18 inoculation differed in terms of the timing and intensity of the diarrhea development, on ETEC fecal shedding and in the piglets’ immunological antibody response. This information was considered to be relevant to correctly define the experimental protocol, the data recording and the sample collections. Appropriate challenge settings and evaluation of the response parameters will allow future research studies to comply with the replacement, reduction and refinement (3R) approach, and to be able to evaluate the efficiency of a given feeding, nutritional or vaccination intervention in order to combat ETEC infection.

Mapping the neutralizing epitopes of F18 fimbrial adhesin subunit FedF of enterotoxigenic Escherichia coli (ETEC)

K88 and F18 fimbrial enterotoxigenic Escherichia coli (ETEC) are the major causes of post-weaning diarrhea (PWD) in pigs. A vaccine that induces broad immunity to prevent K88 and F18 fimbrial ETEC bacterial attachment and colonization in pig small intestines and to neutralize enterotoxin enterotoxicity would be effective for PWD. Structure-based multiepitope-fusion-antigen (MEFA) technology using a backbone immunogen to present neutralizing epitopes of representing virulence factors capacitates development of broadly protective ETEC vaccines. Neutralizing epitopes have been identified from K88 fimbrial adhesin (FaeG) and enterotoxins but not F18 fimbrial adhesin. In this study, we in silico identified immunodominant epitopes from F18ac fimbrial subunit FedF which plays a critical role in F18 fimbrial adherence, genetically fused each epitope to a carrier, examined immunogenicity of each epitope fusion, and determined epitope-derived antibodies neutralizing activities against F18 fimbrial adherence. Data showed that seven immune-dominant epitopes were identified from FedF subunit. Fused to heterologous human ETEC adhesin subunit CfaB, epitope fusions induced anti-F18 antibodies in subcutaneously immunized mice. Moreover, antibodies derived from each fusion significantly blocked adherence of a F18-fimbrial E. coli bacteria to pig intestinal cell line IPEC-J2. While all seven epitopes exhibited neutralizing activity, results from this study identified FedF epitopes #3 (IPSSSGTLTCQAGT) and #7 (QPDATGSWYD) the most effective for antibodies against F18 fimbrial adherence, and suggested their future application in PWD vaccine development.

Host-bacteria interaction and adhesin study for development of therapeutics

Host-pathogen interaction is one of the most important areas of study to understand the adhesion of the pathogen to the host organisms. To adhere on the host cell surface, bacteria assemble the diverse adhesive structures on its surface, which play a foremost role in targeting to the host cell. We have highlighted different bacterial adhesins which are either protein mediated or glycan mediated. The present article listed examples of different bacterial adhesin proteins involved in the interactions with their host, types and subtypes of the fimbriae and non-fimbriae bacterial adhesins. Different bacterial surface adhesin subunits interact with host via different host surface biomolecules. We have also discussed the interactome of some of the pathogens with their host. Therefore, the present study will help researchers to have a detailed understanding of different interacting bacterial adhesins and henceforth, develop new therapies, adhesin specific antibodies and vaccines, which can effectively control pathogenicity of the pathogens.

Mucosal and systemic responses of immunogenic vaccines candidates against enteric Escherichia coli infections in ruminants: A review

Innumerable Escherichia coli of animal origin are identified, which are of economic significance, likewise, cattle, sheep and goats are the carrier of enterohaemorrhagic E. coli, which are less pathogenic, and can spread to people by way of direct contact and through the contamination of foodstuff or portable drinking water, causing serious illness. The immunization of ruminants has been carried out for ages and is largely acknowledged as the most economical and maintainable process of monitoring E. coli infection in ruminants. Yet, only a limited number of E. coli vaccines are obtainable. Mucosal surfaces are the most important ingress for E. coli and thus mucosal immune responses function as the primary means of fortification. Largely contemporary vaccination processes are done by parenteral administration and merely limited number of E. coli vaccines are inoculated via mucosal itinerary, due to its decreased efficacy. Nevertheless, aiming at maximal mucosal partitions to stimulate defensive immunity at both mucosal compartments and systemic site epitomises a prodigious task. Enormous determinations are involved in order to improve on novel mucosal E. coli vaccines candidate by choosing apposite antigens with potent immunogenicity, manipulating novel mucosal itineraries of inoculation and choosing immune-inducing adjuvants. The target of E. coli mucosal vaccines is to stimulate a comprehensive, effective and defensive immunity by specifically counteracting the antibodies at mucosal linings and by the stimulation of cellular immunity. Furthermore, effective E. coli mucosal vaccine would make vaccination measures stress-free and appropriate for large number of inoculation. On account of contemporary advancement in proteomics, metagenomics, metabolomics and transcriptomics research, a comprehensive appraisal of the immeasurable genes and proteins that were divulged by a bacterium is now in easy reach. Moreover, there exist marvellous prospects in this bourgeoning technologies in comprehending the host bacteria affiliation. Accordingly, the flourishing knowledge could massively guarantee to the progression of immunogenic vaccines against E. coli infections in both humans and animals. This review highlight and expounds on the current prominence of mucosal and systemic immunogenic vaccines for the prevention of E. coli infections in ruminants.

Effectiveness of F18+ Fimbrial Antigens Released by a Novel Autolyzed Salmonella Expression System as a Vaccine Candidate against Lethal F18+ STEC Infection

Porcine edema disease (ED) caused by Shiga toxin 2e producing Escherichia coli expressing F18ab⁺ fimbriae (F18ab⁺STEC) frequently occurs in post-weaned piglets, resulting in a significant economic loss in swine industries worldwide. In the present study, we proposed an efficient prevention scheme against ED in which the attenuated Salmonella Typhimurium inactivated by the E-mediated cell lysis to deliver target antigens, FedF and FedA, which function in fimbrial-mediated adhesion and as a major subunit of F18ab+fimbriae, respectively. The co-expression of FedA and FedF protein with outer membrane protein A signal peptide was confirmed in the resultant strains JOL1460 and JOL1464 by immunoblot analysis. Immunization with the candidate strains in mice led to the significant generation of immunoglobulin (Ig) G, specific to both antigens and secretory IgA specific to FedF (P < 0.05). The titers of IgG isotypes, IgG1 and IgG2a, used as markers for T-helpers (Th)-2 and Th-1lymphocytes, respectively, also significantly increased in the immunized group (P < 0.05). The increase in CD3⁺CD4⁺ T lymphocyte subpopulation and in vitro proliferative activity was observed in in vivo stimulated splenocytes, which indicated the immunostimulatory effect of the candidate strains. Moreover, the immunized mice were completely protected from a lethal challenge against wild-type F18⁺STEC whereas 28% of mice died in the non-immunized group. This study demonstrated that the inactivated Salmonella system could efficiently release FedF and FedA and induce robust immune responses specific to the target antigens, which is sufficient to protect the mice from the lethal challenge.

Functional analysis of Escherichia coli Yad fimbriae reveals their potential role in environmental persistence

Initial adhesion of bacterial cells to surfaces or host tissues is a key step in colonisation and biofilm formation processes, and is mediated by cell surface appendages. It was previously demonstrated that Escherichia coli K-12 possesses an arsenal of silenced chaperone-usher fimbriae that were functional when constitutively expressed. Among them, production of prevalent Yad fimbriae induces adhesion to abiotic surfaces. Functional characterisation of Yad fimbriae were undertook, and YadN was identified as the most abundant and potential major pilin, and YadC as the potential tip-protein of Yad fimbriae. It was showed that Yad production participates to binding of E. coli K-12 to human eukaryotic cells (Caco-2) and inhibits macrophage phagocytosis, but also enhances E. coli K-12 binding to xylose, a major component of the plant cell wall, through its tip-lectin YadC. Consistently, it was demonstrated that Yad production provides E. coli with a competitive advantage in colonising corn seed rhizospheres. The latter phenotype is correlated with induction of Yad expression at temperatures below 37°C, and under anaerobic conditions, through a complex regulatory network. Taken together, these results suggest that Yad fimbriae are versatile adhesins that beyond potential capacities to modulate host-pathogen interactions might contribute to E. coli environmental persistence.

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.

Nanobody mediated inhibition of attachment of F18 Fimbriae expressing Escherichia coli

Post-weaning diarrhea and edema disease caused by F18 fimbriated E. coli are important diseases in newly weaned piglets and lead to severe production losses in farming industry. Protective treatments against these infections have thus far limited efficacy. In this study we generated nanobodies directed against the lectin domain of the F18 fimbrial adhesin FedF and showed in an in vitro adherence assay that four unique nanobodies inhibit the attachment of F18 fimbriated E. coli bacteria to piglet enterocytes. Crystallization of the FedF lectin domain with the most potent inhibitory nanobodies revealed their mechanism of action. These either competed with the binding of the blood group antigen receptor on the FedF surface or induced a conformational change in which the CDR3 region of the nanobody displaces the D″-E loop adjacent to the binding site. This D″-E loop was previously shown to be required for the interaction between F18 fimbriated bacteria and blood group antigen receptors in a membrane context. This work demonstrates the feasibility of inhibiting the attachment of fimbriated pathogens by employing nanobodies directed against the adhesin domain.

Structural Sampling of Glycan Interaction Profiles Reveals Mucosal Receptors for Fimbrial Adhesins of Enterotoxigenic Escherichia coli

Fimbriae are long, proteinaceous adhesion organelles expressed on the bacterial envelope, evolutionarily adapted by Escherichia coli strains for the colonization of epithelial linings. Using glycan arrays of the Consortium for Functional Glycomics (CFG), the lectin domains were screened of the fimbrial adhesins F17G and FedF from enterotoxigenic E. coli (ETEC) and of the FimH adhesin from uropathogenic E. coli. This has led to the discovery of a more specific receptor for F17G, GlcNAcβ1,3Gal. No significant differences emerged from the glycan binding profiles of the F17G lectin domains from five different E. coli strains. However, strain-dependent amino acid variations, predominantly towards the positively charged arginine, were indicated by sulfate binding in FedF and F17G crystal structures. For FedF, no significant binders could be observed on the CFG glycan array. Hence, a shotgun array was generated from microvilli scrapings of the distal jejunum of a 3-week old piglet about to be weaned. On this array, the blood group A type 1 hexasaccharide emerged as a receptor for the FedF lectin domain and remarkably also for F18-fimbriated E. coli. F17G was found to selectively recognize glycan species with a terminal GlcNAc, typifying intestinal mucins. In conclusion, F17G and FedF recognize long glycan sequences that could only be identified using the shotgun approach. Interestingly, ETEC strains display a large capacity to adapt their fimbrial adhesins to ecological niches via charge-driven interactions, congruent with binding to thick mucosal surfaces displaying an acidic gradient along the intestinal tract.

Expression of verocytotoxic Escherichia coli antigens in tobacco seeds and evaluation of gut immunity after oral administration in mouse model

Verocytotoxic Escherichia (E.) coli strains are responsible for swine oedema disease, which is an enterotoxaemia that causes economic losses in the pig industry. The production of a vaccine for oral administration in transgenic seeds could be an efficient system to stimulate local immunity. This study was conducted to transform tobacco plants for the seed-specific expression of antigenic proteins from a porcine verocytotoxic E. coli strain. Parameters related to an immunological response and possible adverse effects on the oral administration of obtained tobacco seeds were evaluated in a mouse model. Tobacco was transformed via Agrobacteium tumefaciens with chimeric constructs containing structural parts of the major subunit FedA of the F18 adhesive fimbriae and VT2e B-subunit genes under control of a seed specific GLOB promoter. We showed that the foreign Vt2e-B and F18 genes were stably accumulated in storage tissue by the immunostaining method. In addition, Balb-C mice receiving transgenic tobacco seeds via the oral route showed a significant increase in IgA-positive plasma cell presence in tunica propria when compared to the control group with no observed adverse effects. Our findings encourage future studies focusing on swine for evaluation of the protective effects of transformed tobacco seeds against E. coli infection.

Cloning and Expression of Genes Encoding F107-C and K88-1NT Fimbrial Proteins of Enterotoxigenic Escherichia coli from Piglets

We cloned two genes coding F107-C and K88-1NT fimbrial subunits from strains E. coli C and 1NT isolated from Thua Thien Hue province, Vietnam. The mature peptide of faeG gene from strain E. coli 1NT (called faeG-1NT) is 100 % similarity with faeG gene, while the CDS of fedA gene from strain C (called fedA-C) has a similarity of 97 % with the fedA gene. Expression of the faeG-1NT and fedA-C genes in E. coli BL21 Star™ (DE3) produced proteins of ~31 and 22 kDa, respectively. The effect of IPTG concentration on the K88-1NT and F107-C fimbriae production was investigated. The results showed that 0.5 mM IPTG is suitable for higher expression of K88-1NT subunit, while 0.75 mM IPTG strongly stimulated expression of F107-C subunit. The optimal induction time for expression was also examined. Generally, highest expression of K88-1NT subunit occurred after 6 h of induction, while that of F107-C subunit is after 14 h.

Structural insight in histo-blood group binding by the F18 fimbrial adhesin FedF

F18-positive enterotoxigenic and Shiga toxin-producing Escherichia coli are responsible for post-weaning diarrhoea and oedema disease in pigs and lead to severe production losses in the farming industry. F18 fimbriae attach to the small intestine of young piglets by latching onto glycosphingolipids with A/H blood group determinants on type 1 core. We demonstrate the N-terminal domain of the F18 fimbrial subunit FedF to be responsible for ABH-mediated attachment and present its X-ray structure in ligand-free form and bound to A and B type 1 hexaoses. The FedF lectin domain comprises a 10-stranded immunoglobulin-like β-sandwich. Three linear motives, Q(47) -N(50), H(88) -S(90) and R(117) -T(119), form a shallow glycan binding pocket near the tip of the domain that is selective for type 1 core glycans in extended conformation. In addition to the glycan binding pocket, a polybasic loop on the membrane proximal surface of FedF lectin domain is shown to be required for binding to piglet enterocytes. Although dispensable for ABH glycan recognition, the polybasic surface adds binding affinity in the context of the host cell membrane, a mechanism that is proposed to direct ABH-glycan binding to cell-bound glycosphingolipids and could allow bacteria to avoid clearance by secreted glycoproteins.

Development of vaccine delivery vehicles based on lactic acid bacteria

Live recombinant bacteria represent attractive antigen delivery systems able to induce both mucosal and systemic immune responses against heterologous antigens. The first live recombinant bacterial vectors developed were derived from attenuated pathogenic microorganisms. In addition to the difficulties often encountered in the construction of stable attenuated mutants of pathogenic organisms, attenuated pathogens may retain a residual virulence level that renders them unsuitable for the vaccination of partially immunocompetent individuals such as infants, the elderly or immunocompromised patients. As an alternative to this strategy, non-pathogenic food-grade lactic acid bacteria (LAB) maybe used as live antigen carriers. This article reviews LAB vaccines constructed using antigens other than tetanus toxin fragment C, against bacterial, viral, and parasitic infective agents, for which protection studies have been performed. The antigens utilized for the development of LAB vaccines are briefly described, along with the efficiency of these systems in protection studies. Moreover, the key factors affecting the performance of these systems are highlighted.

Polymerase chain reaction-based method for the typing of F18 fimbriae and distribution of F18 fimbrial subtypes among porcine Shiga toxin-encoding Escherichia coli in Germany

Edema disease is an enterotoxemic disorder of weaned piglets that represents a significant threat to pig husbandry worldwide. The causative Escherichia coli strains are highly adapted to the porcine host and characterized by the production of Shiga toxin type 2e (Stx2e) and adhesive F18 fimbria. The current study assessed the occurrence of F18 fimbrial subtypes in 241 porcine stx2e(+) fedA(+) E. coli strains in Germany, including 116 Shiga toxin-encoding E. coli (STEC) and 125 Shiga toxin E. coli/enterotoxigenic E. coli (STEC/ETEC) isolates. In addition, a novel multiplex polymerase chain reaction (PCR) was developed in order to improve the typing system in terms of costs, time, and discriminative power. Utilizing the novel F18 typing PCR, 93 E. coli strains (38.5%) tested positive for the F18ab fimbrial subtype and 147 strains (61.0%) for the F18ac fimbrial subtype, while 1 strain remained nontypeable. Six strains were classified as F18ac using the F18 typing PCR, but were classified as F18ab using the F18-restriction fragment length polymorphism assay. Nucleotide sequencing of the FedA gene revealed that 5 of these strains encoded F18ac fimbriae, while the FedA of 1 strain did not cluster with F18ab or with F18ac amino acid sequences. The F18 fimbrial subtype was significantly associated with the pathovar of the E. coli strains, as 73.2% of the STEC isolates harbored F18ab genes whereas 93.6% of the STEC/ETEC isolates proved F18ac positive. In conclusion, the novel F18 typing PCR allows a specific identification of the F18 fimbrial subtype. The genetic and phenotypic heterogeneity of F18 fimbriae in porcine E. coli strains should be considered in the development of new vaccines and diagnostic tools.

Protection of piglets against Edema disease by maternal immunization with Stx2e toxoid

Edema disease (ED) in piglets is caused by Shiga toxin Stx2e-producing Escherichia coli. We show that a genetically disarmed Stx2e toxoid is a safe antigen that generates antiserum protecting piglets against the Stx2e toxin. Immunization of suckling piglets with the Stx2e toxoid was safe, had no adverse effects on growth of the piglets, and resulted in effective prevention of edema disease clinical symptoms after challenge with the Stx2e toxin. Our data showed that maternal immunity against the Stx2e toxoid can be transmitted from the vaccinated sows to the piglets via the colostrum. Very high levels of Stx2e-specific serum antibodies persisted in these piglets until 1 month postweaning, bridging the critical period in which the weaned piglets are most susceptible to edema infection. Challenge with Stx2e toxin resulted in clinical signs of edema disease and death of all control piglets from nonimmunized sows, whereas none of the piglets from immunized sows developed clinical signs of ED.

Initial adherence of EPEC, EHEC and VTEC to host cells

Initial adherence to host cells is the first step of the infection of enteropathogenic Escherichia coli (EPEC), enterohaemorrhagic Escherichia coli (EHEC) and verotoxigenic Escherichia coli (VTEC) strains. The importance of this step in the infection resides in the fact that (1) adherence is the first contact between bacteria and intestinal cells without which the other steps cannot occur and (2) adherence is the basis of host specificity for a lot of pathogens. This review describes the initial adhesins of the EPEC, EHEC and VTEC strains. During the last few years, several new adhesins and putative colonisation factors have been described, especially in EHEC strains. Only a few adhesins (BfpA, AF/R1, AF/R2, Ral, F18 adhesins) appear to be host and pathotype specific. The others are found in more than one species and/or pathotype (EPEC, EHEC, VTEC). Initial adherence of EPEC, EHEC and VTEC strains to host cells is probably mediated by multiple mechanisms.

Comparison of Genotypes of Escherichia Coli Strains Carrying F18ab and F18ac Fimbriae from Pigs

Escherichia coli carrying the F18 fimbriae colonize the small intestine of pigs and cause postweaning diarrhea and edema disease. There are 2 closely related antigenic variants of F18, F18ab, and F18ac. While F18ab-positive strains are known to be associated with edema disease, E. coli–carrying F18ac are known to cause diarrhea. One hundred ninety-eight E. coli isolates obtained from cases of diarrhea and edema disease in pigs isolated from feces or intestine were screened for the presence of the fedA gene encoding for F18 fimbriae. To distinguish between F18ab and F18ac, the fedA gene was sequenced in 69 F18-positive isolates/strains. The translated protein sequences of the fedA gene in the 2 variants differ; F18ac protein carries proline at amino acid residue 121, which is substituted or missing in F18ab. The F18ab- and F18ac-positive E. coli strains were compared for the presence of virulence attributes, serotypes of the isolates, and relatedness between the strains. Contrary to earlier reports that E. coli F18ab-positive strains mostly express Shiga toxin and F18ac-positive strains generally express enterotoxins, the current report shows conclusively for the first time that both variant types may carry genes for Shiga toxins and/or enterotoxins. Monoclonal antibodies produced against F18ab or F18ac fimbriae could not distinguish the strains carrying the 2 variants. Therefore, it was concluded that either of the 2 F18 variants, F18ab or F18ac, may be involved in causing postweaning diarrhea or edema disease in pigs.

Recognition of blood group ABH type 1 determinants by the FedF adhesin of F18-fimbriated Escherichia coli

F18-fimbriated Escherichia coli are associated with porcine postweaning diarrhea and edema disease. Adhesion of F18-fimbriated bacteria to the small intestine of susceptible pigs is mediated by the minor fimbrial subunit FedF. However, the target cell receptor for FedF has remained unidentified. Here we report that F18-fimbriated E. coli selectively interact with glycosphingolipids having blood group ABH determinants on type 1 core, and blood group A type 4 heptaglycosylceramide. The minimal binding epitope was identified as the blood group H type 1 determinant (Fucalpha2Galbeta3GlcNAc), while an optimal binding epitope was created by addition of the terminal alpha3-linked galactose or N-acetylgalactosamine of the blood group B type 1 determinant (Galalpha3(Fucalpha2)Galbeta3GlcNAc) and the blood group A type 1 determinant (GalNAcalpha3(Fucalpha2)-Galbeta3GlcNAc). To assess the role of glycosphingolipid recognition by F18-fimbriated E. coli in target tissue adherence, F18-binding glycosphingolipids were isolated from the small intestinal epithelium of blood group O and A pigs and characterized by mass spectrometry and proton NMR. The only glycosphingolipid with F18-binding activity of the blood group O pig was an H type 1 pentaglycosylceramide (Fucalpha2Galbeta3GlcNAc-beta3Galbeta4Glcbeta1Cer). In contrast, the blood group A pig had a number of F18-binding glycosphingolipids, characterized as A type 1 hexaglycosylceramide (GalNAcalpha3(Fucalpha2)Galbeta3GlcNAcbeta3Galbeta4Glcbeta1Cer), A type 4 heptaglycosylceramide (GalNAcalpha3(Fucalpha2)Galbeta3GalNAcbeta3Galalpha4Galbeta4Glcbeta1Cer), A type 1 octaglycosylceramide (GalNAcalpha3(Fucalpha2)Galbeta3GlcNAcbeta3Galbeta3GlcNAcbeta3Galbeta4Glcbeta1Cer), and repetitive A type 1 nonaglycosylceramide (GalNAcalpha3(Fucalpha2)Galbeta3GalNAcalpha3-(Fucalpha2)Galbeta3GlcNAcbeta3Galbeta4Glcbeta1Cer). No blood group antigen-carrying glycosphingolipids were recognized by a mutant E. coli strain with deletion of the FedF adhesin, demonstrating that FedF is the structural element mediating binding of F18-fimbriated bacteria to blood group ABH determinants.

The excretion of F18+ E. coli is reduced after oral immunisation of pigs with a FedF and F4 fimbriae conjugate

Currently, no vaccines are available for edema disease and post-weaning diarrhoea (PWD) in pigs. In the present study, a subunit vaccine containing the F18 fimbrial adhesin FedF was studied. Hereto, recombinant FedF was produced as a fusion protein with maltose-binding protein. Even though the produced MBPFedF was shown to attach in vitro to enterocytes, almost no FedF-specific immune response could be detected after oral administration to piglets. The delivery of FedF to the intestinal mucosa was improved by conjugating the MBPFedF to F4 fimbriae. Indeed, this conjugation induced a systemic and local FedF-specific immune response and led to a reduction in excretion after infection with F18+ E. coli. Although complete protection was not observed, the conjugation between FedF and F4 fimbriae can be considered as a first step towards the development of a combined vaccine against F4+ and F18+ E. coli infections.

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.

Monoclonal antibodies reveal a weak interaction between the F18 fimbrial adhesin FedF and the major subunit FedA

F18+ Escherichia coli can cause post-weaning diarrhoea and oedema disease in pigs. These diseases are responsible for substantial economic losses, but a vaccine is not available. A good knowledge of the characteristic of the fimbriae is useful for the development of a vaccine composed of the fimbrial virulence factor. F18 fimbriae are composed of the major subunit FedA and the minor subunits FedE and the adhesin FedF. In the present study monoclonal antibodies (mAbs) against FedA and FedF were produced. In addition to their diagnostic value, these mAbs revealed a weaker interaction between FedA and FedF compared to the subunit-subunit interactions in other fimbriae, like type 1 and P pili. Further experiments are needed to investigate if this weak interaction could be one of the reasons for the slow colonisation of the small intestinal mucosa by F18+ E. coli.

N-terminal truncation enables crystallization of the receptor-binding domain of the FedF bacterial adhesin

FedF is the two-domain tip adhesin of F18 fimbriae from enterotoxigenic Escherichia coli. Bacterial adherence, mediated by the N-terminal receptor-binding domain of FedF to carbohydrate receptors on intestinal microvilli, causes diarrhoea and oedema disease in newly weaned piglets and induces the secretion of Shiga toxins. A truncate containing only the receptor-binding domain of FedF was found to be further cleaved at its N-terminus. Reconstruction of this N-terminal truncate rendered FedF amenable to crystallization, resulting in crystals with space group P2(1)2(1)2(1) and unit-cell parameters a = 36.20, b = 74.64, c = 99.03 A that diffracted to beyond 2 A resolution. The binding specificity of FedF was screened for on a glycan array, exposing 264 glycoconjugates, to identify specific receptors for cocrystallization with FedF.

Significance of heat-stable and heat-labile enterotoxins in porcine colibacillosis in an additive model for pathogenicity studies

Although heat-stable (ST) and heat-labile (LT) enterotoxins produced by enterotoxigenic Escherichia coli (ETEC) have been documented as important factors associated with diarrheal diseases, investigations assessing the contributions of individual enterotoxins to the pathogenesis of E. coli infection have been limited. To address the individual roles of enterotoxins in the diarrheal disease caused by K88-positive ETEC in young pigs, enterotoxin-positive and -negative isogenic E. coli strains were constructed by using pBR322 to clone and express LT and STb. Four strains, K88+ astA, K88+ astA/pBR322, K88+ astA STb+, and K88+ astA LT+, were constructed and subsequently included in gnotobiotic piglet challenge studies, and their pathogenesis was assessed. The results indicated that all K88+ isogenic strains were able to colonize the small intestines of piglets exhibiting the K88 receptor. However, only LT- and STb-positive strains caused appreciable diarrhea. Piglets inoculated with the K88+ astA LT+ strain became dehydrated within 18 h, while those inoculated with the K88+ astA STb+ strain did not, although diarrhea developed in several piglets. The changes in the blood packed-cell volume and plasma total protein of gnotobiotic piglets inoculated with the LT-positive strains were significantly greater than those of pigs inoculated with the K88 astA/pBR322 strain (P = 0.012, P = 0.002). Immunochemistry image analysis also suggested that LT enhanced bacterial colonization in a gnotobiotic piglet model. This investigation suggested that LT is a major contributor to the virulence of K88+ ETEC and that isogenic constructs are a useful tool for studying the pathogenesis of ETEC infection.

Adhesins of Enterohemorrhagic Escherichia coli

Enterohemorrhagic Escherichia coli (EHEC) was first recognized as a cause of human disease in 1983 and is associated with diarrhea and hemorrhagic colitis, which may be complicated by life-threatening renal and neurological sequelae. EHEC are defined by their ability to produce one or more Shiga-like toxins (Stx), which mediate the systemic complications of EHEC infections, and to induce characteristic attaching and effacing lesions on intestinal epithelia, a phenotype that depends on the locus of enterocyte effacement. Acquisition of Stx-encoding bacteriophages by enteropathogenic E. coli is believed to have contributed to the evolution of EHEC, and consequently some virulence factors are conserved in both pathotypes. A key requirement for E. coli to colonize the intestines and produce disease is the ability to adhere to epithelial cells lining the gastrointestinal tract. Here, we review knowledge of the adhesins produced by EHEC and other Stx-producing E. coli, with emphasis on genetic, structural, and mechanistic aspects and their contribution to pathogenesis.

Escherichia coli in postweaning diarrhea in pigs: an update on bacterial types, pathogenesis, and prevention strategies

Escherichia coli is one of the most important causes of postweaning diarrhea in pigs. This diarrhea is responsible for economic losses due to mortality, morbidity, decreased growth rate, and cost of medication. The E. coli causing postweaning diarrhea mostly carry the F4 (K88) or the F18 adhesin. Recently, an increase in incidence of outbreaks of severe E. coli-associated diarrhea has been observed worldwide. The factors contributing to the increased number of outbreaks of this more severe form of E. coli-associated diarrhea are not yet fully understood. These could include the emergence of more virulent E. coli clones, such as the 0149:LT:STa:STb:EAST1:F4ac, or recent changes in the management of pigs. Development of multiple bacterial resistance to a wide range of commonly used antibiotics and a recent increase in the prevalence and severity of the postweaning syndromes will necessitate the use of alternative measures for their control. New vaccination strategies include the oral immunization of piglets with live avirulent E. coli strains carrying the fimbrial adhesins or oral administration of purified F4 (K88) fimbriae. Other approaches to control this disease include supplementation of the feed with egg yolk antibodies from chickens immunized with F4 or F18 adhesins, breeding of F18- and F4-resistant animals, supplementation with zinc and/ or spray-dried plasma, dietary acidification, phage therapy, or the use of probiotics. To date, not a single strategy has proved to be totally effective and it is probable that the most successful approach on a particular farm will involve a combination of diet modification and other preventive measures.

Receptor binding domain of Escherichia coli F18 fimbrial adhesin FedF can be both efficiently secreted and surface displayed in a functional form in Lactococcus lactis

Adherence of F18 fimbrial Escherichia coli to porcine intestinal epithelial cells is mediated by the adhesin (FedF) of F18 fimbriae. In a previous study, we demonstrated the specificity of the amino acid residues between 60 and 109 as the receptor binding domain of FedF. In this study, different expression, secretion, and anchoring systems for the receptor binding domain of the FedF adhesin in Lactococcus lactis were evaluated. Two partially overlapping receptor binding domains (42 and 62 amino acid residues) were expressed as fusions with L. lactis subsp. cremoris protein PrtP for evaluation of secretion efficiency. To evaluate the cell surface display of these FedF-PrtP fusions, they were further combined with different lengths of PrtP spacers fused with either the L. lactis AcmA anchor or the PrtP cell wall binding domain. An HtrA-defective L. lactis NZ9000 mutant was constructed to determine its effect on the level of secreted or anchored fusion proteins. Recombinant L. lactis clones secreting the receptor binding domain of F18 fimbriae as a fusion with the H domains of L. lactis protein PrtP were first constructed by using two different signal peptides. FedF-PrtP fusions, directed by the signal sequence of L. brevis SlpA, were throughout found to be secreted at significantly higher quantities than corresponding fusions with the signal peptide of L. lactis Usp45. In the surface display systems tested, the L. lactis AcmA anchor performed significantly better, particularly in the L. lactis NZ9000DeltahtrA strain, compared to the L. lactis PrtP anchor region. Of the cell surface display constructs with the AcmA anchor, only those with the longest PrtP spacer regions resulted in efficient binding of recombinant L. lactis cells to porcine intestinal epithelial cells. These results confirmed that it is possible to efficiently produce the receptor binding domain of the F18 adhesin in a functionally active form in L. lactis.

Mapping the binding domain of the F18 fimbrial adhesin

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

Mapping the binding domain of the F18 fimbrial adhesin

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