Transferrin associated with the porcine intestinal mucosa is a receptor specific for K88ab fimbriae of Escherichia coli

A Review of the Mechanisms of Bacterial Colonization of the Mammal Gut

A healthy animal intestine hosts a diverse population of bacteria in a symbiotic relationship. These bacteria utilize nutrients in the host's intestinal environment for growth and reproduction. In return, they assist the host in digesting and metabolizing nutrients, fortifying the intestinal barrier, defending against potential pathogens, and maintaining gut health. Bacterial colonization is a crucial aspect of this interaction between bacteria and the intestine and involves the attachment of bacteria to intestinal mucus or epithelial cells through nonspecific or specific interactions. This process primarily relies on adhesins. The binding of bacterial adhesins to host receptors is a prerequisite for the long-term colonization of bacteria and serves as the foundation for the pathogenicity of pathogenic bacteria. Intervening in the adhesion and colonization of bacteria in animal intestines may offer an effective approach to treating gastrointestinal diseases and preventing pathogenic infections. Therefore, this paper reviews the situation and mechanisms of bacterial colonization, the colonization characteristics of various bacteria, and the factors influencing bacterial colonization. The aim of this study was to serve as a reference for further research on bacteria-gut interactions and improving animal gut health.

Efficacy evaluation of a novel oral silica-based vaccine in inducing mucosal immunity against Mycoplasma hyopneumoniae

Mycoplasma hyopneumoniae, the main etiological agent of Porcine Enzootic Pneumonia, is widely spread in swine production worldwide. Its prevention is of great interest for the productive system, since its colonization in the lung tissue leads to intense production losses. This study aimed to compare the M. hyopneumoniae shedding and acute-phase response in 30 pigs submitted to different vaccination protocols: an experimental oral vaccine using a nanostructured mesoporous silica (SBA-15) as adjuvant (n = 10); an intramuscular commercially available vaccine at 24 days of age (n = 10); and a control group (n = 10) following experimental challenge with M. hyopneumoniae. Laryngeal and nasal swabs were collected weekly and oral fluids were collected at 7, 10, 14, 17, 23, 28, 35, 42, and 49 days post-infection to monitor pathogen excretion by qPCR. Nasal swabs were also used to detect anti-M. hyopneumoniae IgA by ELISA. Blood samples were collected for monitoring acute phase proteins. The antibody response was observed in both immunized groups seven days after vaccination, while the control group became positive for this immunoglobulin at 4 weeks after challenge. Lung lesion score was similar in the immunized groups, and lower than that observed in the control. SBA-15-adjuvanted oral vaccine provided immunological response, decreased shedding of M. hyopneumoniae and led to mucosal protection confirmed by the reduced pulmonary lesions. This study provides useful data for future development of vaccines against M. hyopneumoniae.

Cerebrospinal Fluid Leak Detection with a Carbon Nanotube-Based Field-Effect Transistor Biosensing Platform

Cerebrospinal fluid (CSF) leakage may lead to life-threatening complications if not detected promptly. However, gel electrophoresis, the gold-standard test for confirming CSF leakage by detecting beta2-transferrin (β2-Tf), requires 3-6 h and is labor-intensive. We developed a new β2-Tf detection platform for rapid identification of CSF leakage. The three-step design, which includes two steps of affinity chromatography and a rapid sensing step using a semiconductor-enriched single-walled carbon nanotube field-effect transistor (FET) sensor, circumvented the lack of selectivity that antitransferrin antibody exhibits for transferrin isoforms and markedly shortened the detection time. Furthermore, three different sensing configurations for the FET sensor were investigated for obtaining the optimal β2-Tf sensing results. Finally, body fluid (CSF and serum) tests employing our three-step strategy demonstrated high sensitivity, suggesting its potential to be used as a rapid diagnostic tool for CSF leakage.

Interleukin-17A/F1 Deficiency Reduces Antimicrobial Gene Expression and Contributes to Microbiome Alterations in Intestines of Japanese medaka (Oryzias latipes)

In mammals, interleukin (IL)-17A and F are hallmark inflammatory cytokines that play key roles in protection against infection and intestinal mucosal immunity. In the gastrointestinal tract (GI), the induction of antimicrobial peptide (AMP) production via Paneth cells is a fundamental role of IL-17A and F in maintaining homeostasis of the GI microbiome and health. Although mammalian IL-17A and F homologs (referred to as IL-17A/F1-3) have been identified in several fish species, their function in the intestine is poorly understood. Additionally, the fish intestine lacks Paneth cells, and its GI structure is very different from that of mammals. Therefore, the GI microbiome modulatory mechanism via IL-17A/F genes has not been fully elucidated. In this study, Japanese medaka (Oryzias latipes) were used as a teleost model, and IL-17A/F1-knockout (IL-17A/F1-KO) medaka were established using the CRISPR/Cas9 genome editing technique. Furthermore, two IL-17A/F1-deficient medaka strains were generated, including one strain containing a 7-bp deletion (-7) and another with an 11-bp addition (+11). After establishing F2 homozygous KO medaka, transcriptome analysis (RNA-seq) was conducted to elucidate IL-17A/F1-dependent gene induction in the intestine. Results of RNA-seq and real-time PCR (qPCR) demonstrated down-regulation of immune-related genes, including interleukin-1β (IL-1β), complement 1q subunit C (C1qc), transferrin a (Tfa), and G-type lysozyme (LyzG), in IL-17A/F1-KO medaka. Interestingly, protein and lipid digestive enzyme genes, including phospholipase A2, group IB (pla2g1b), and elastase-1-like (CELA1), were also downregulated in the intestines of IL-17A/F1-KO medaka. Furthermore, to reveal the influence of these downregulated genes on the gut microbiome in IL-17A/F1-KO, 16S rRNA-based metagenomic sequencing analysis was conducted to analyze the microbiome constitution. Under a non-exposed state, the intestinal microbiome of IL-17A/F1-KO medaka differed at the phylum level from wild-type, with significantly higher levels of Verrucomicrobia and Planctomycetes. Additionally, at the operational taxonomic unit (OTU) level of the human and fish pathogens, the Enterobacteriaceae Plesiomonas shigelloides was the dominant species in IL-17A/F1-KO medaka. These findings suggest that IL-17A/F1 is involved in the maintenance of a healthy gut microbiome.

ITGB5 Plays a Key Role in Escherichia coli F4ac-Induced Diarrhea in Piglets

Enterotoxigenic Escherichia coli (ETEC) that expresses F4ac fimbriae is the major pathogenic microorganism responsible for bacterial diarrhea in neonatal piglets. The susceptibility of piglets to ETEC F4ac is determined by a specific receptor on the small intestinal epithelium surface. We performed an iTRAQ-labeled quantitative proteome analysis using a case-control design in which susceptible and resistant full-sib piglets were compared for the protein expression levels. Two thousand two hundred forty-nine proteins were identified, of which 245 were differentially expressed (fold change > 1.5, FDR-adjusted P < 0.05). The differentially expressed proteins fell into four functional classes: (I) cellular adhesion and binding, (II) metabolic process, (III) apoptosis and proliferation, and (IV) immune response. The integrin signaling pathway merited particular interest based on a pathway analysis using statistical overexpression and enrichment tests. Genomic locations of the integrin family genes were determined based on the most recent porcine genome sequence assembly (Sscrofa11.1). Only one gene, ITGB5, which encodes the integrin β5 subunit that assorts with the αv subunit to generate integrin αvβ5, was located within the SSC13q41 region between 13:133161078 and 13:139609422, where strong associations of markers with the ETEC F4ac susceptibility were found in our previous GWAS results. To identify whether integrin αvβ5 is the ETEC F4acR, we established an experimental model for bacterial adhesion using IPEC-J2 cells. Then, the ITGB5 gene was knocked out in IPEC-J2 cell lines using CRISPR/Cas9, resulting in a biallelic deletion cell line (ITGB5 ^-/-). Disruption of ITGB5 significantly reduced ETEC F4ac adhesion to porcine intestinal epithelial cells. In contrast, overexpression of ITGB5 significantly enhanced the adhesion. A GST pull-down assay with purified FaeG and ITGB5 also showed that FaeG binds directly to ITGB5. Together, the results suggested that ITGB5 is a key factor affecting the susceptibility of piglets to ETEC F4ac.

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.

Porcine aminopeptidase N binds to F4+ enterotoxigenic Escherichia coli fimbriae

F4⁺ enterotoxigenic Escherichia coli (ETEC) strains cause diarrheal disease in neonatal and post-weaned piglets. Several different host receptors for F4 fimbriae have been described, with porcine aminopeptidase N (APN) reported most recently. The FaeG subunit is essential for the binding of the three F4 variants to host cells. Here we show in both yeast two-hybrid and pulldown assays that APN binds directly to FaeG, the major subunit of F4 fimbriae, from three serotypes of F4⁺ ETEC. Modulating APN gene expression in IPEC-J2 cells affected ETEC adherence. Antibodies raised against APN or F4 fimbriae both reduced ETEC adherence. Thus, APN mediates the attachment of F4⁺E. coli to intestinal epithelial cells.

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.

Proteomic Analysis of Duodenal Tissue from Escherichia coli F18-Resistant and -Susceptible Weaned Piglets

Diarrhea and edema disease in weaned piglets due to infection by Escherichia coli F18 is a leading cause of economic loss in the pig industry. Resistance to E. coli F18 depends on expression of receptors on intestinal epithelial cells, and individual immunity. This study was conducted in Sutai pig E. coli F18-resistant and -susceptible full sib-pair individuals, identified on the basis of resource populations and verification of adhesion assays. The molecular mechanism underlying E. coli F18 resistance was investigated through analysis of the expression of E. coli F18 receptor associated and innate immunity proteins, using proteomics and bioinformatics techniques. Two-dimensional electrophoresis analysis revealed a total of 20 differentially expressed proteins in E. coli F18-resistant and -susceptible groups (10 upregulated and 10 downregulated). A total of 16 differentially expressed proteins were identified by MALDI TOF/TOF mass spectral analysis. According to gene ontology and pathway analysis, differentially expressed proteins were mainly involved in cell adhesion, immune response and other biologically relevant functions. Network analysis of interactions between differentially expressed proteins indicated a likelihood of their involvement in E. coli F18 infection. The expression levels of several important proteins including actin beta (ACTB), vinculin (VCL), heat stress proteins (HSPs) and transferrin (TF) in E. coli F18-resistant and -susceptible individuals were verified by Western blotting, supporting the identification of ACTB, VCL, HSPs and TF as promising candidate proteins for association with E. coli F18 susceptibility.

Receptor for the F4 fimbriae of enterotoxigenic Escherichia coli (ETEC)

Infection with F4(+) enterotoxigenic Escherichia coli (ETEC) responsible for diarrhea in neonatal and post-weaned piglets leads to great economic losses in the swine industry. These pathogenic bacteria express either of three fimbrial variants F4ab, F4ac, and F4ad, which have long been known for their importance in host infection and initiating protective immune responses. The initial step in infection for the bacterium is to adhere to host enterocytes through fimbriae-mediated recognition of receptors on the host cell surface. A number of receptors for ETEC F4 have now been described and characterized, but their functions are still poorly understood. The current review summarizes the latest research addressing the characteristics of F4 fimbriae receptors and the interactions of F4 fimbriae and their receptors on host cells. These include observations that as follows: (1) FaeG mediates the binding activities of F4 and is an essential component of the F4 fimbriae, (2) the F4 fimbrial receptor gene is located in a region of chromosome 13, (3) the biochemical properties of F4 fimbrial receptors that form the binding site of the bacterium are now recognized, and (4) specific receptors confer susceptibility/resistance to ETEC F4 infection in pigs. Characterizing the host-pathogen interaction will be crucial to understand the pathogenicity of the bacteria, provide insights into receptor activation of the innate immune system, and develop therapeutic strategies to prevent this illness.

F4+ enterotoxigenic Escherichia coli (ETEC) adhesion mediated by the major fimbrial subunit FaeG

The FaeG subunit is the major constituent of F4(+) fimbriae, associated with glycoprotein and/or glycolipid receptor recognition and majorly contributes to the pathogen attachment to the host cells. To investigate the key factor involved in the fimbrial binding of F4(+) Escherichia coli, both the recombinant E. coli SE5000 strains carrying the fae operon gene clusters that express the different types of fimbriae in vitro, named as rF4ab, rF4ac, and rF4ad, respectively, corresponding to the fimbrial types F4ab, F4ac, and F4ad, and the three isogenic in-frame faeG gene deletion mutants were constructed. The adhesion assays and adhesion inhibition assays showed that ΔfaeG mutants had a significant reduction in the binding to porcine brush border as well as the intestinal epithelial cell lines, while the complemented strain ΔfaeG/pfaeG restored the adhesion function. The recombinant bacterial strains rF4ab, rF4ac, and rF4ad have the same binding property as wild-type F4(+) E. coli strains do and improvement in terms of binding to porcine brush border and the intestinal epithelial cells, and the adherence was blocked by the monoclonal antibody anti-F4 fimbriae. These data demonstrate that the fimbrial binding of F4(+) E. coli is directly mediated by the major FaeG subunit.

Structural and functional insight into the carbohydrate receptor binding of F4 fimbriae-producing enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) strains are important causes of intestinal disease in humans and lead to severe production losses in animal farming. A range of fimbrial adhesins in ETEC strains determines host and tissue tropism. ETEC strains expressing F4 fimbriae are associated with neonatal and post-weaning diarrhea in piglets. Three naturally occurring variants of F4 fimbriae (F4ab, F4ac, and F4ad) exist that differ in the primary sequence of their major adhesive subunit FaeG, and each features a related yet distinct receptor binding profile. Here the x-ray structure of FaeGad bound to lactose provides the first structural insight into the receptor specificity and mode of binding by the poly-adhesive F4 fimbriae. A small D'-D″-α1-α2 subdomain grafted on the immunoglobulin-like core of FaeG hosts the carbohydrate binding site. Two short amino acid stretches Phe(150)-Glu(152) and Val(166)-Glu(170) of FaeGad bind the terminal galactose in the lactosyl unit and provide affinity and specificity to the interaction. A hemagglutination-based assay with E. coli expressing mutant F4ad fimbriae confirmed the elucidated co-complex structure. Interestingly, the crucial D'-α1 loop that borders the FaeGad binding site adopts a different conformation in the two other FaeG variants and hints at a heterogeneous binding pocket among the FaeG serotypes.

Refined candidate region for F4ab/ac enterotoxigenic Escherichia coli susceptibility situated proximal to MUC13 in pigs

F4 enterotoxigenic Escherichia coli (F4 ETEC) are an important cause of diarrhea in neonatal and newly-weaned pigs. Based on the predicted differential O-glycosylation patterns of the 2 MUC13 variants (MUC13A and MUC13B) in F4ac ETEC susceptible and F4ac ETEC resistant pigs, the MUC13 gene was recently proposed as the causal gene for F4ac ETEC susceptibility. Because the absence of MUC13 on Western blot from brush border membrane vesicles of F4ab/acR⁺ pigs and the absence of F4ac attachment to immunoprecipitated MUC13 could not support this hypothesis, a new GWAS study was performed using 52 non-adhesive and 68 strong adhesive pigs for F4ab/ac ETEC originating from 5 Belgian farms. A refined candidate region (chr13: 144,810,100–144,993,222) for F4ab/ac ETEC susceptibility was identified with MUC13 adjacent to the distal part of the region. This candidate region lacks annotated genes and contains a sequence gap based on the sequence of the porcine GenomeBuild 10.2. We hypothesize that a porcine orphan gene or trans-acting element present in the identified candidate region has an effect on the glycosylation of F4 binding proteins and therefore determines the F4ab/ac ETEC susceptibility in pigs.

F4-related mutation and expression analysis of the aminopeptidase N gene in pigs

Intestinal infections with F4 enterotoxigenic Escherichia coli (ETEC) are worldwide an important cause of diarrhea in neonatal and recently weaned pigs. Adherence of F4 ETEC to the small intestine by binding to specific receptors is mediated by F4 fimbriae. Porcine aminopeptidase N (ANPEP) was recently identified as a new F4 receptor. In this study, 7 coding mutations and 1 mutation in the 3′ untranslated region (3' UTR)were identified in ANPEP by reverse transcriptase (RT–) PCR and sequencing using 3 F4 receptor-positive (F4R+) and 2 F4 receptor-negative (F4R–) pigs, which were F4 phenotyped based on the MUC4 TaqMan, oral immunization, and the in vitro villous adhesion assay. Three potential differential mutations (g.2615C > T, g.8214A > G, and g.16875C > G) identified by comparative analysis between the 3 F4R+ and 2 F4R– pigs were genotyped in 41 additional F4 phenotyped pigs. However, none of these 3 mutations could be associated with F4 ETEC susceptibility. In addition, the RT-PCR experiments did not reveal any differential expression or alternative splicing in the small intestine of F4R+ and F4R– pigs. In conclusion, we hypothesize that the difference in F4 binding to ANPEP is due to modifications in its carbohydrate moieties.

Adhesive ability means inhibition activities for lactobacillus against pathogens and S-layer protein plays an important role in adhesion

Eighty-five strains of lactobacillus were isolated from the pig intestine and identified by sequencing analysis based on 16S rRNA gene, from which five lactobacillus strains with high adhesive ability were selected. The inhibition ability of the five lactobacillus strains with or without S-layer proteins against adherence of Escherichia coli K88 and Salmonella enteritidis 50335 to Caco-2 was evaluated in vitro with Lactobacillus rhamnosus GG strain (LGG) as a positive control. In addition, tolerance of lactobacilli to heat, acid, bile, Zn(2+) and Cu(2+) were assessed. All five selected strains, Lactobacillus salivarius ZJ614 (JN981856), Lactobacillus reuteri ZJ616 (JN981858), L. reuteri ZJ617 (JN981859), L. reuteri ZJ621 (JN981863) and L. reuteri ZJ623 (JN981865), showed inhibition against the two pathogens, E. coli K88 and S. enteritidis 50335. L. reuteri ZJ621 showed higher inhibition ability than the others to S. enteritidis 50335 (P < 0.05). Sodium dodecyl sulfate-Polyacrylamide gel electrophoresis (SDS-PAGE) analysis indicated that all five strains had abundant bands with molecular weight ranging from 34 to 130 KDa as well as had a common band of approximately 42 KDa. After treatment with 5 M LiCl to remove S-layer protein, the inhibition activities of the lactobacilli against pathogens decreased significantly (P < 0.05). The results showed that higher adhesive ability means higher inhibition activity for lactobacillus against pathogen, in which S-layer proteins plays an important role.

The search for the gene mutations underlying enterotoxigenic Escherichia coli F4ab/ac susceptibility in pigs: a review

Diarrhoea due to enterotoxigenic Escherichia coli with fimbriae F4 (ETEC-F4) is an important problem in neonatal and just weaned piglets and hence for the pig farming industry. There is substantial evidence for a genetic basis for susceptibility to ETEC-F4 since not all piglets suffer from diarrhoea after an ETEC-F4 infection. It is assumed that the wild boar was originally ETEC-F4 resistant and that susceptibility towards ETEC arose after domestication. There are different phenotypes in the pig determined by which of the three existing F4 variants (F4ab, F4ac or F4ad) they are susceptible or resistant for. This suggests that several F4 receptors exist, expressed individually or in combination with each other on the brush border of the piglet’s small intestine. As such, the mucin-type glycoproteins (IMTGP) are described as F4ab/ac receptors, while the intestinal neutral glycospingolipid (IGLad) is proposed as an F4ad receptor. GP74 is a putative F4ab receptor. However, the specific genes that encode for the susceptibility are not yet known. In the past decades, linkage analyses revealed that the loci encoding for the receptor(s) for the two most frequent variants F4ab and F4ac were mapped to the 13^th chromosome of the pig (Sus scrofa 13, SSC13). After fine mapping, the region of interest was mapped between two microsatellite markers, Sw207 and S0075, and interesting candidate genes surfaced. Numerous SNP analyses and a few expression studies on the three MUC-genes (MUC4, MUC13 and MUC20) and the transferrin receptor gene (TFRC) as well as on some other positional candidate genes have been performed in order to find the causative mutation for the ETEC-F4ab/ac receptor(s). However, until today, the exact mutation causing susceptibility to ETEC-F4 remains unknown.

The effect of enterotoxigenic Escherichia coli F4ab,ac on early-weaned piglets: a gene expression study

Diarrhoea in neonatal and early-weaned piglets due to enterotoxigenic Escherichia coli-F4 (ETEC-F4) is an important problem in the pig farming industry. There is substantial evidence for a genetic basis for susceptibility to ETEC-F4 since not all pigs suffer from diarrhoea after an ETEC-F4 infection. A region on SSC13 has been found to be in close linkage to the susceptibility of piglets for ETEC-F4ab,ac. Potential candidate genes on SSC13 have been examined and although some polymorphisms were found to be in linkage disequilibrium with the phenotype, the causative mutation has not yet been found. In this study we are looking at the expression of porcine genes in relation to ETEC-F4ab,ac. With the aid of the Affymetrix GeneChip Porcine Genome Array we were able to find differentially expressed genes between ETEC-F4ab,ac receptor positive (Fab,acR(+)) piglets without diarrhoea and F4ab,acR(+) piglets with diarrhoea or F4ab,acR(-) animals. Since the susceptibility to ETEC-F4ab,ac was described as a Mendelian trait, it is not so surprisingly that only two differentially expressed genes, transferrin receptor (TFRC) and trefoil factor 1 (TFF1), came out of the analysis. Although both genes could pass for functional candidate genes only TFRC also mapped to the region on SSC13 associated with susceptibility for ETEC-F4, which makes TFRC a positional functional candidate gene. Validation by qRT-PCR confirmed the differential expression of TFRC and TFF1. In piglets without diarrhoea, the expression of both genes was higher in F4ab,acR(+) than in F4ab,acR(-) piglets. Similarly, TFRC and TFF1 expression in F4ab,acR(+) piglets without diarrhoea was also higher than in F4ab,acR(+) piglets with diarrhoea. Consequently, although both genes might not play a role as receptor for F4 fimbriae, they could be of great importance during an ETEC-F4 outbreak. An upregulation of TFRC can be a consequence of the piglets ability to raise an effective immune response. An elevation of TFF1, a protein involved in mucin formation, may also affect the piglet's capability to cope with ETEC bacteria, rather than being a receptor for its fimbriae.

Conjugation of transferrin to azide-modified CdSe/ZnS core-shell quantum dots using cyclooctyne click chemistry

Twinkle twinkle quantum dot: Conjugation of biomolecules to azide-modified quantum dots (QDs) through a bifunctional linker, using strain-promoted azide-alkyne cycloaddition with the QD and a squaramide linkage to the biomolecule (see scheme). Transferrin-conjugated QDs were internalized by transferrin-receptor expressing HeLa cells.

Association of B3GNT5 polymorphisms with susceptibility to ETEC F4ab/ac in the white Duroc × Erhualian intercross and 15 outbred pig breeds

The B3GNT5 gene is a candidate for the F4ab/ac receptor conferring susceptibility to enterotoxigenic Escherichia coli (ETEC) F4ab/ac in pigs. In this study, we screened mutations in the complete coding region of the porcine B3GNT5 gene and identified four SNPs in the 3' untranslated regions. We genotyped the four SNPs across a large-scale White Duroc × Chinese Erhualian F2 resource population (total F2 = 755) and 292 purebred piglets representing 15 Chinese and Western breeds. We found that the g.1476G→A locus and haplotypes [A;T;G;T] and [A;G;G;T] had significant association with susceptibility to ETEC F4ac in the resource population. None of the B3GNT5 polymorphisms and haplotypes was associated with susceptibility to ETEC F4ab/ac in outbred piglets. This result, together with other reports, supports the conclusion that B3GNT5 is not the responsible gene encoding the ETEC F4ab/ac receptors.

Characterization of the binding specificity of K88ac and K88ad fimbriae of enterotoxigenic Escherichia coli by constructing K88ac/K88ad chimeric FaeG major subunits

Enterotoxigenic Escherichia coli (ETEC) strains expressing K88 (F4) fimbriae are the major cause of diarrhea in young pigs. Three antigenic variants of K88 fimbriae (K88ab, K88ac, and K88ad) have been identified among porcine ETEC strains. Each K88 fimbrial variant shows a unique pattern in binding to different receptors on porcine enterocytes. Such variant specificity in fimbrial binding is believed to be controlled by the major subunit (FaeG) of the K88 fimbriae, because the genes coding for the only other fimbrial subunit are identical among the three variants. Uniqueness in binding to host receptors may be responsible for differences in the virulence levels of porcine diarrhea disease caused by K88 ETEC strains. To better understand the relationships between the structure of FaeG proteins and fimbrial binding function, and perhaps virulence in disease, we constructed and expressed various K88ac/K88ad faeG gene chimeras and characterized the binding activity of each K88 chimeric fimbria. After verifying biosynthesis of the chimeric fimbriae, we examined their binding specificities in bacterial adherence assays by using porcine brush border vesicles that are specific to either the K88ac or K88ad fimbria. Results showed that each fimbria switched binding specificity to that of the reciprocal type when a peptide comprising amino acids 125 to 163 was exchanged with that of its counterpart. Substitutions of a single amino acid within this region negatively affected the binding capacity of each fimbria. These data indicate that the peptide including amino acids 125 to 163 of the FaeG subunit is essential for K88 variant-specific binding.

Comparison of the contributions of heat-labile enterotoxin and heat-stable enterotoxin b to the virulence of enterotoxigenic Escherichia coli in F4ac receptor-positive young pigs

In swine, the most common and severe enterotoxigenic Escherichia coli (ETEC) infections are caused by strains that express K88 (F4)(+) fimbriae, heat-labile enterotoxin (LT), heat-stable enterotoxin b (STb), and enteroaggregative E. coli heat-stable toxin 1. Previous studies based on a design that involved enterotoxin genes cloned into a nontoxigenic fimbriated strain have suggested that LT but not STb plays an important role in dehydrating diarrheal disease in piglets <1 week old and also enhances bacterial colonization of the intestine. In the present study, we compared these two toxins in terms of importance for piglets >1 week old with a study design that involved construction of isogenic single- and double-deletion mutants and inoculation of 9-day-old F4ac receptor-positive gnotobiotic piglets. Based on the postinoculation percent weight change per h and serum bicarbonate concentrations, the virulence of the STb(-) mutant (Delta estB) did not significantly differ from that of the parent. However, deletion of the LT genes (Delta eltAB) in the STb(-) mutant resulted in a complete abrogation of weight loss, dehydration, and metabolic acidosis in inoculated pigs, and LT complementation restored the virulence of this strain. These results support the hypothesis that LT is a more significant contributor than STb to the virulence of F4(+) ETEC infections in young F4ac receptor-positive pigs less than 2 weeks old. However, in contrast to previous studies with gnotobiotic piglets, there was no evidence that the expression of LT enhanced the ability of the F4(+) ETEC strain to colonize the small intestine.

Characterization of polymorphisms of transferrin receptor and their association with susceptibility to ETEC F4ab/ac in pigs

Diarrhoea caused by enterotoxigenic Escherichia coli (ETEC) expressing F4 (F4ab, F4ac and F4ad) fimbriae is a significant cause of mortality and morbidity in newborn and weaned pigs. The locus controlling susceptibility towards ETEC F4ab/ac has been mapped to SSC13q41, in which TFRC (transferrin receptor) was localized and considered as a positional candidate gene for ETEC F4ab/ac receptor. In this study, we determined susceptibility/resistance to ETEC F4ab/ac in a total of 755 F2 animals from a White Duroc x Erhualian intercross using a microscopic enterocyte adhesion assay. We identified two TFRC polymorphisms (SNPs 591 A>G and 632 A>G) in a single exon after comparative sequencing analysis of 2371-bp amplicons containing the complete coding region of TFRC using RNA of eight full-sib F2 animals with susceptible and resistant phenotypes. The intron sequences flanking the two exon polymorphisms were obtained, revealing an intron polymorphism (SNP 291 C>T). We genotyped the 19 founder animals of the White Duroc x Erhualian intercross for the identified polymorphisms, showing that only the 291 C>T polymorphism is a highly informative marker. We further genotyped all 59 F1 and 755 F2 animals for the 291 C>T polymorphism, and the association of this polymorphism with susceptibility/resistance to ETEC F4ab/ac in these F2 animals was evaluated by the transmission disequilibrium test. The result showed that the 291 C>T polymorphism is not a causal mutation, however, has a significant linkage disequilibrium with the ETEC F4ab/ac, especially F4ac receptor locus.

Chloroplasts assemble the major subunit FaeG of Escherichia coli F4 (K88) fimbriae to strand-swapped dimers

F4 fimbriae encoded by the fae operon are the major colonization factors associated with porcine neonatal and postweaning diarrhoea caused by enterotoxigenic Escherichia coli (ETEC). Via the chaperone/usher pathway, the F4 fimbriae are assembled as long polymers of the major subunit FaeG, which also possesses the adhesive properties of the fimbriae. Intrinsically, the incomplete fold of fimbrial subunits renders them unstable and susceptible to aggregation and/or proteolytic degradation in the absence of a specific periplasmic chaperone. In order to test the possibility of producing FaeG in plants, FaeG expression was studied in transgenic tobacco plants. FaeG was directed to different subcellular compartments by specific targeting signals. Targeting of FaeG to the chloroplast results in much higher yields than FaeG targeting to the endoplasmic reticulum or the apoplast. Two chloroplast-targeted FaeG variants were purified from tobacco plants and crystallized. The crystal structures show that chloroplasts circumvent the absence of the fimbrial assembly machinery by assembling FaeG into strand-swapped dimers. Furthermore, the structures reveal how FaeG combines the structural requirements of a major fimbrial subunit with its adhesive role by grafting an additional domain on its Ig-like core.

Two specific sites for binding of K88ab Escherichia coli fimbriae to porcine intestinal brush border membranes

We have studied the characteristics of the binding of the K88ab Escherichia coli fimbrial antigen to porcine brush border membranes by solid phase binding assay. Binding of biotinylated K88ab to brush border membranes followed a sigmoidal dependence and was saturable, apparent saturation occurring with 0.8 ng of fimbriae (approx. 7 ng of fimbriae per microg of brush border protein) irrespective of incubation temperature in the range of room temperature to 4 degrees C. A Hill plot of log [(fimbriae bound)/(maximal binding-fimbriae bound)] vs. log free fimbriae gave a maximal slope of about 2, indicating the existence of two binding sites. From an analysis of an Scatchard plot, apparent binding constants (1)K(2) and (2)K(2) of 7.1 x 10(8) and 17.1 x 10(8)M(-1) were obtained at room temperature. Nor did temperature have any effect on the rate of binding or on receptor affinity (S(0.5)).

Intestinal mucosal alterations in rats with carbon tetrachloride-induced cirrhosis: changes in glycosylation and luminal bacteria

Spontaneous bacterial peritonitis is a major cause of mortality after liver cirrhosis. Altered permeability of the mucosa and deficiencies in host immune defenses through bacterial translocation from the intestine due to intestinal bacterial overgrowth have been implicated in the development of this complication. Molecular mechanisms underlying the process are not well known. In order to understand mechanisms involved in translocation of bacteria, this study explored the role of oxidative stress in mediating changes in intestinal mucosal glycosylation and luminal bacterial content during cirrhosis. CCl4-induced cirrhosis in rats led to prolonged oxidative stress in the intestine, accompanied by increased sugar content of both intestinal brush border and surfactant layers. This was accompanied by changes in bacterial flora in the gut, which showed increased hydrophobicity and adherence to the mucosa. Inhibition of xanthine oxidase using sodium tungstate or antioxidant supplementation using vitamin E reversed the oxidative stress, changes in brush border membrane sugar content, and bacterial adherence. In conclusion, oxidative stress in the intestine during cirrhosis alters mucosal glycosylation, accompanied by an increased hydrophobicity of luminal bacteria, enabling increased bacterial adherence onto epithelial cells. This might facilitate translocation across the mucosa, resulting in complications such as spontaneous bacterial peritonitis.

Alterations in the intestinal glycocalyx and bacterial flora in response to oral indomethacin

Nonsteroidal anti-inflammatory drugs (NSAIDs), used extensively in clinical medicine, tend to cause adverse effects in the gastrointestinal tract. Earlier work has shown that oral administration of indomethacin produced oxidative damage in the small intestine and attenuation of the glycocalyx layer of the mucosa. The present study assessed, in greater detail, the alterations produced in the glycocalyx of rat small intestinal mucosa in response to indomethacin, with specific reference to surfactant-like particles (SLP) and brush border membranes (BBM). Changes in gut flora in response to the drug were also studied, as it has been shown that luminal bacteria play a role in the pathogenesis of NSAID-induced intestinal damage. The levels of sugars such as sialic acid, fucose, hexose and hexosamine were increased in SLP and decreased in the BBM following indomethacin treatment, with the effects being maximal 24h after the administration of the drug. The composition of lipids in the SLP was also found to be altered. There was a significant increase in the number of bacteria in the luminal contents of the small intestine and caecum in these animals, as compared with controls. The number of bacteria adherent to the intestinal mucosa was also significantly higher in the drug-treated group. In vitro studies revealed that there was an increased tendency for bacteria to adhere to SLP isolated from indomethacin-treated rats. These results suggest that alterations in glycosylation of SLP and BBM in response to indomethacin, along with qualitative and quantitative changes in the luminal bacterial flora, may facilitate translocation of bacteria into the mucosa. These changes may contribute to the enteropathy observed as a result of NSAID treatment.

Inheritance of the F4ab, F4ac and F4ad E. coli receptors in swine and examination of four candidate genes for F4acR

Susceptibility to enterotoxigenic Escherichia coli with fimbriae F4ac is dominantly inherited in the pig. A three-generation pedigree was created to refine the position of F4acR on chromosome 13 comprising 202 pigs: eight parents, 18 F1 and 176 F2 pigs. The 17-point analysis indicates that F4acR lies between Sw207 and S0283. Recombinant offspring specify that the most probable order is Sw207-S0075-F4acR-Sw225-S0283. We observed six phenotypes for the three fimbrial variants F4ab, F4ac and F4ad. The two missing phenotypes F4abR-/F4acR+/F4adR+ and F4abR-/F4acR+/F4adR- indicate that pigs susceptible to F4ac are always susceptible to F4ab. Furthermore, a weak and a strong adhesion of F4ab and F4ad bacteria was observed. The weak receptor F4abR (F4abRw) was present only in pigs devoid of the receptor F4acR (F4abR+/F4acR-). In contrast, in pigs with the phenotype F4abR+/F4acR+, F4ab bacteria adhered to the majority of enterocytes. F4abRw constitutes a frequently observed phenotype whose inheritance is still unclear. Strong adhesion of F4ab and F4ac bacteria is most likely influenced by the same receptor that we name F4bcR. The number of F4ad bacteria that adhered to enterocytes was very variable in the adhesion test. Moreover, expression of F4adR was independent of age. Our segregation analyses indicated a dominant inheritance of F4adR, although the number of susceptible pigs was smaller than expected. We examined four genes as candidates for the F4acR locus: the transferrin receptor gene (TFRC) and three genes members of the glucosyl/galactosyltransferase family (B3GnT5, B3GALT3 and B4GALT4). Comparison of sequences from resistant and homozygous susceptible F4ac pigs did not reveal any causative single nucleotide polymorphism in the four genes. Two silent mutations at the positions 295 (C/T) and 313 (T/C) in B3GALT3 were found. Using the somatic cell hybrid panel, B3GnT5 and B3GALT3 were assigned to the chromosomal region SSC13q23-q41. No mutations were found in the cDNA sequences of these genes associated with the F4acR genotypes.

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.

Altered glycosylation of surfactant and brush border membrane of the small intestine in response to surgical manipulation

BACKGROUND

Surgical stress can lead to bacterial translocation from the intestine into systemic circulation. Adherence of bacteria onto the glycoconjugates of the brush border membrane (BBM) and surfactant coat (SLP) of the mucosal cells is the first step in the translocation of luminal bacteria. Our earlier study showed that surgical manipulation of the intestine results in oxidative stress leading to structural and functional alterations in the mucosa. This study looks at the effect of surgical manipulation on the glycoconjugate alterations of SLP and BBM.

MATERIALS AND METHODS

SLP and BBM were isolated from control and after surgical manipulation and the sugar composition was analyzed. Bacterial adherence using E. coli isolated from cecum was compared after coating microtiter plates with SLP or BBM isolated from control and after surgical manipulation.

RESULTS

An increase in various sugars was seen after surgical manipulation both in SLP and BBM and this increase was maximum at 12 h after surgery. These alterations increased bacterial adherence onto SLP and BBM. Inhibiting superoxide generation by allopurinol treatment prior to surgical manipulation prevented glycosylation alteration and bacterial adherence.

CONCLUSION

Surgical manipulation results in altered glycoconjugates of SLP and BBM which leads to increased bacterial adherence. These alterations are probably brought about by oxygen-free radicals. This is clinically significant because postsurgical complications such as sepsis may be brought about by altered glycosylation.

Role of intestine in postsurgical complications: involvement of free radicals

Surgery at any location in the body leads to surgical stress response and alterations in normal body homeostasis. The intestine is extremely sensitive to surgical stress even at remote locations and the gastrointestinal tract plays an important role in the development of postsurgical complications such as sepsis, the systemic immune response syndrome (SIRS), and multiple organ failure syndrome (MOFS). The generation of free radicals and subsequent biochemical alterations at the cellular and subcellular level in the intestine has been suggested to play an important role in this process. These oxidative stress-induced events in the mucosa might act as an initiator of distant organ damage and also facilitate bacterial adherence onto the epithelium and translocation into the systemic circulation. This review attempts to highlight the important role of intestine and oxygen free radicals in initiating post-surgical complications.

Partial characterization of chorionic gonadotropin-like binding sites from the bacteria Xanthomonas maltophilia

The gram-negative bacterium, Xanthomonas maltophilia, has low- and high-affinity luteinizing hormone/chorionic gonadotropin (LH/CG)-binding sites, similar to the LH/CG receptor found in mammals. Although the low-affinity site binds both LH and human CG (hCG), the high-affinity site is specific for hCG. In the current investigation, these two binding sites were independently isolated from X. maltophilia for further characterization. To isolate functional binding sites, we developed a solubilization method using the detergent zwittergent 3,14 and high glycerol concentrations that allowed for the maintenance of ligand-binding integrity. Gel filtration experiments established molecular weights of 170 and 11.5 kDa for the two binding sites, which were supported by data from photoaffinity labeling and ultracentrifugation experiments. Gel filtration data also suggested the presence of a third binding site of 5.4 kDa. The 170-kDa site had a binding affinity of Kd = 12 x 10(-6) and bound both LH and hCG. The small molecular weight site had an affinity of Kd = 9.4 x 10(-8) and was CG specific. Collectively, these data demonstrate the presence of multiple hormone binding sites in X. maltophilia that differ in molecular size, binding affinity, and ligand specificity.

A transferrin-like protein that does not bind iron is induced by iron deficiency in the alga Dunaliella salina

Iron deficiency induces two major transferrin-like proteins in the plasma membrane (Pm) of the halotolerant alga Dunaliella salina. TTf, a 150-kDa protein, previously identified as a salt-induced triplicated transferrin, having iron-binding characteristics resembling animal transferrins, and a 100-kDa protein designated idi-100 (for iron-deficiency-induced 100 kDa protein). According to the predicted amino acid sequence of idi-100, it is only 30% identical to TTf and differs from it in having two, rather than three, homologous internal repeats and in a lower conservation of canonical iron/bicarbonate binding residues. Both are localized in the outer surface of the membrane; however, TTf can be dissociated from the membrane by treatment with EDTA, whereas release of idi-100 requires detergents. The accumulation of idi-100 under iron deficiency lags behind that of TTf and in contrast to TTf, it is not induced by high salinity, suggesting that induction of idi-100 requires lower Fe threshold levels than that of TTf. In contrast to TTf, idi-100 does not bind Fe; however, there are indications for interactions with bicarbonate ions. These results suggest that despite their common resemblance to transferrins, their similar subcellular localization and their induction by iron deficiency, idi-100 and TTf fulfill different functions.

Evaluation of receptor binding specificity of Escherichia coli K88 (F4) fimbrial adhesin variants using porcine serum transferrin and glycosphingolipids as model receptors

Diarrheal disease caused by enterotoxigenic Escherichia coli expressing the K88 (F4) fimbrial adhesin (K88 ETEC) is a significant source of mortality and morbidity among newborn and weaned piglets. K88 fimbrial adhesins are filamentous surface appendages whose lectin (carbohydrate-binding) activity allows K88 ETEC to attach to specific glycoconjugates (receptors) on porcine intestinal epithelial cells. There are three variants of K88 adhesin (K88ab, K88ac, and K88ad), which possess different, yet related, carbohydrate-binding specificities. We used porcine serum transferrin (pSTf) and purified glycosphingolipids (GSL) to begin to define the minimal recognition sequence for K88 adhesin variants. We found that K88ab adhesin binds with high affinity to pSTf (dissociation constant, 75 microM), while neither K88ac nor K88ad adhesin recognizes pSTf. Degradation of the N-glycan on pSTf by extensive metaperiodate treatment abolished its interaction with the K88ab adhesin, indicating that the K88ab adhesin binds to the single N-glycan found on pSTf. Using exoglycosidase digestion of the pSTf glycan, we demonstrated that K88ab adhesin recognizes N-acetylglucosamine (GlcNAc) residues in the core of the N-glycan on pSTf. All three K88 variants were found to bind preferentially to GSL containing a beta-linked N-acetylhexosamine (HexNAc), either GlcNAc or N-acetylgalactosamine, in the terminal position or, alternatively, in the penultimate position with galactose in the terminal position. Considering the results from pSTf and GSL binding studies together, we propose that the minimal recognition sequence for the K88 adhesin variants contains a beta-linked HexNAc. In addition, the presence of a terminal galactose beta-linked to this HexNAc residue enhances K88 adhesin binding.

Escherichia coli strain RDEC-1 AF/R1 endogenous fimbrial glycoconjugate receptor molecules in rabbit small intestine

Escherichia coli strain RDEC-1 causes a diarrheagenic infection in rabbits with AF/R1 fimbriae, which have been identified as an important colonization factor in RDEC-1 adherence leading to disease. The AF/R1-mediated RDEC-1 adherence model has been used as a model systems for E. coli diarrheal diseases. In this study, RDEC-1 adhered specifically to small intestinal brush borders, with both sialic acid and beta-galactosyl residues apparently involved. The AF/R1-mediated adherence activity of [(14)C]-labeled RDEC-1 was analyzed quantitatively by using 24-well plates coated with purified brush borders and purified microvilli. Two microvillus membrane proteins (130 and 140 kDa) were individually isolated, and chicken antibody raised to each protein inhibited bacterial adherence. These same two proteins, previously shown to be recognized by AF/R1, were individually digested with trypsin, and the amino acid sequences of peptides were determined by reversed-phase capillary liquid chromatography-mass spectrometry tandem mass spectrometry (LC-MS). This LC-MS analysis indicated that these proteins are subunits of the rabbit sucrase-isomaltase protein (SI) complex. Guinea pig serum raised to purified rabbit SI complex inhibited bacterial adherence to microvilli. Additionally, as determined by high-performance thin-layer chromatography and autoradiography, RDEC-1 adhered selectively, via AF/R1 fimbriae, to a glycolipid tentatively identified as galactosylceramide (Gal beta 1-1Cer) in the lipid extract of rabbit small intestinal brush borders. RDEC-1 adherence to Gal beta 1-1Cer was partially inhibited in the presence of galactose. These combined results indicate that the endogenous receptor molecule for AF/R1 fimbriae of RDEC-1 is each individual component of the SI complex, although binding to glycolipid may be responsible for an additional adherence mechanism.

Inhibition of adhesion of Escherichia coli k88ac fimbria to its receptor, intestinal mucin-type glycoproteins, by a monoclonal antibody directed against a variable domain of the fimbria

Strains of enterotoxigenic Escherichia coli that express K88 fimbriae are among the most common causes of diarrhea in young pigs. Adhesion of bacteria to receptors on intestinal epithelial cells, mediated by K88 fimbriae, is the initial step in the establishment of infection. Three antigenic variants of K88 fimbriae exist in nature: K88ab, K88ac, and K88ad. K88ac is the most prevalent and may be the only variant of significance in swine disease. Each K88 fimbrial variant is composed of multiple antigenic determinants. Some of these determinants are shared among the three variants and may be referred to as conserved epitopes, whereas others are unique to a specific variant and may be referred to as variable epitopes. In this study, monoclonal antibodies (MAbs) specific to either variable or conserved epitopes of K88ac fimbriae were produced. The specificity of each MAb was tested by enzyme-linked immunosorbent and immunoblot assays. Fab fragments were prepared from these MAbs and were tested for their ability to block the binding of K88-positive bacteria and purified fimbriae to porcine enterocyte brush border vesicles and purified K88 receptors, respectively. The purified receptors were intestinal mucin-type sialoglycoproteins (IMTGP) isolated from porcine enterocytes (A. K. Erickson, D. R. Baker, B. T. Bosworth, T. A. Casey, D. A. Benfield, and D. H. Francis, Infect. Immun. 62:5404-5410, 1994). Fab fragments prepared from MAbs specific for variable epitopes blocked the binding of bacteria to brush borders and of fimbriae to IMTGP. However, those from MAbs specific for a conserved epitope did not. These observations indicate that the receptor-binding domain of a K88ac fimbria is contained, at least in part, within the antigenically variable epitopes of that fimbria. Epitope mapping for one of the MAbs, which recognizes a linear epitope on K88ac fimbriae, indicated that this MAb binds to the region from amino acid no. 64 to no. 107 on the major subunit of K88ac fimbriae.

K88 adhesins of enterotoxigenic Escherichia coli and their porcine enterocyte receptors

The three antigenic variants of the K88 fimbrial adhesin (K88ab, K88ac, and K88ad) of enterotoxigenic Escherichia coli (ETEC) each exhibit unique specificity with regard to their hemagglutination characteristics. The variants are also unique in the specificity of their binding to the brush borders of enterocytes isolated from pigs with different genetic backgrounds. Diversity in enterocyte binding specificity suggests the existence of several K88 receptors, expressed individually or in various combinations on porcine enterocytes. Three candidate receptors have been identified that may explain the adhesion of K88 fimbrial variants to various porcine enterocytes. These receptors are an intestinal mucin-type sialoglycoprotein (IMTGP), an intestinal transferrin (GP74), and an intestinal neutral glycosphingolipid (IGLad). The IMTGP binds K88ab and K88ac, but not K88ad. The GP74 binds K88ab, but not K88ac or K88ad, and the IGLad binds K88ad, but not K88ab or K88ac. Each of the candidate receptors has been found in brush borders that are adhesive for the fimbriae that bind the respective receptor. They have not been found in brush borders that are not adhesive for those same fimbriae. The presence of IMTGP was highly correlated with susceptibility of neonatal gnotobiotic pigs to ETEC expressing K88ab or K88ac.

The F4 fimbrial antigen of Escherichia coli and its receptors

F4 or K88 fimbriae are long filamentous polymeric surface proteins of enterotoxigenic Escherichia coli (ETEC), consisting of so-called major (FaeG) and minor (FaeF, FaeH, FaeC, and probably FaeI) subunits. Several serotypes of F4 have been described, namely F4ab, F4ac, and F4ad. The F4 fimbriae allow the microorganisms to adhere to F4-specific receptors present on brush borders of villous enterocytes and consequently to colonize the small intestine. Such ETEC infections are responsible for diarrhea and mortality in neonatal and recently weaned pigs. In this review emphasis is put on the morphology, genetic configuration, and biosynthesis of F4 fimbriae. Furthermore, the localization of the different a, b, c, and d epitopes, and the localization of the receptor binding site on the FaeG major subunit of F4 get ample attention. Subsequently, the F4-specific receptors are discussed. When the three variants of F4 (F4ab, F4ac, and F4ad) are considered, six porcine phenotypes can be distinguished with regard to the brush border adhesiveness: phenotype A binds all three variants, phenotype B binds F4ab and F4ac, phenotype C binds F4ab and F4ad, phenotype D binds F4ad, phenotype E binds none of the variants, and phenotype F binds F4ab. The following receptor model is described: receptor bcd is found in phenotype A pigs, receptor bc is found in phenotype A and B pigs, receptor d is found in phenotype C and D pigs, and receptor b is found in phenotype F pigs. Furthermore, the characterization of the different receptors is described in which the bcd receptor is proposed as collection of glycoproteins with molecular masses ranging from 45 to 70 kDa, the bc receptor as two glycoproteins with molecular masses of 210 an 240 kDa, respectively, the b receptor as a glycoprotein of 74 kDa, and the d receptor as a glycosphingolipid with unknown molecular mass. Finally, the importance of F4 fimbriae and their receptors in the study of mucosal immunity in pigs is discussed.

Escherichia coli K88 receptor expression in intestine of disease-susceptible weaned pigs

A challenge trial was carried out in which Escherichia coli O157 K88ac was administered to a litter of weaned pigs and the development of the disease monitored over a five-day experimental period. The eight animals in the trial were assigned to two groups depending on whether they exhibited disease symptoms. Six pigs developed diarrhoea and two appeared unaffected; these were designated as the test (or K88-susceptible) group and the control (or K88-resistant) group, respectively. The animals were euthanised and the intestine was removed and sections processed for brush border membrane vesicle preparation. Microscopic and biochemical assays were undertaken on tissue samples from each animal and a strong correlation was observed between the expression of a glycoprotein receptor complex associated with the brush border membrane and the development of disease symptoms. Further investigation revealed the presence of an analogous glycoprotein complex in the K88-resistant group which did not bind the K88-fimbriae antigen. These results suggest that genetic differences in the glycosyl moieties of the receptor complex provide the basis for disease susceptibility to K88-positive E. coli.

Identification of an intestinal neutral glycosphingolipid as a phenotype-specific receptor for the K88ad fimbrial adhesin of Escherichia coli

In this study, we identified a receptor for the K88ad fimbrial adhesin of Escherichia coli in neutral glycosphingolipid preparations from intestinal epithelial cells of K88ad-adhesive pigs, which was absent in preparations from K88ad-nonadhesive pigs. Neither K88ab nor K88ac adhesin variants bound to this neutral glycosphingolipid. Because this receptor is an intestinal glycosphingolipid that binds K88ad adhesin, it has been designated IGLad. Carbohydrate compositional analysis of a partially purified preparation of IGLad identified galactose, glucose, and N-acetylglucosamine in a ratio of 1.5:1.0:0.5 as the major monosaccharides. Preliminary characterization experiments using lectins showed that IGLad contains the terminal glycanic structure Galbeta1-4GlcNAc. Removal of terminal beta-linked galactose residues from IGLad decreased the recognition of IGLad by the K88ad adhesin, indicating that terminal beta-linked galactose is an essential component of the K88ad adhesin recognition site on IGLad. Studies with purified glycosphingolipid standards demonstrated that K88ad adhesin binds to neolactotetraosylceramide (nLc4Cer) (Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glcbeta1-1Cer) , lactotriosylceramide (GlcNAcbeta1-3Galbeta1-4Glcbeta1-1Cer) and lactotetraosylceramide (Galbeta1-3GlcNAcbeta1-3Galbeta1-4Glcbeta1-1Cer) . Based on these studies, IGLad appears to be nLc4Cer.

Characterization of the carbohydrate moiety of intestinal mucin-type sialoglycoprotein receptors for the K88ac fimbrial adhesin of Escherichia coli

We have previously identified two mucin-type sialoglycoproteins from porcine intestinal epithelial cells with approximate molecular masses of 210 (intestinal mucin-type glycoprotein IMTGP-1) and 240 kDa (IMTGP-2) as receptors for the K88ab and K88ac fimbrial adhesins of Escherichia coli. These receptors are detected in intestinal brush border membrane preparations from pigs with adhesive phenotypes but not from pigs with nonadhesive phenotypes and are postulated to be important determinants of the susceptibility of pigs to K88ab+ and K88ac+ enterotoxigenic E. coli infections. Using exoglycosidase digestion studies, we have now determined that beta-linked galactose is an important component in the recognition of IMTGP-1 and IMTGP-2 by the K88ac adhesin. In addition, we observed a differential distribution of the K88ac adhesin binding activity of IMTGP-1 and IMTGP-2 along the crypt-villus axis, suggesting that receptor activity is dependent on the maturation state of the intestinal epithelial cells. Brush borders from immature intestinal epithelial cells possessed the highest concentrations of IMTGP-1 and IMTGP-2 receptor activity, with a progressive decrease in receptor activity as the cells mature. To characterize the differences in the carbohydrate moieties of IMTGP-1 and IMTGP-2, we developed a procedure for purifying the receptors, using phenol extraction followed by serial lectin affinity chromatography. Carbohydrate compositional analysis of the purified receptors indicated that the carbohydrate moieties of IMTGP-1 and IMTGP-2 consist of both N- and O-glycans containing galactose, glucose, sialic acid, mannose, N-acetylgalactosamine, N-acetylglucosamine, and fucose. The major difference between the two receptors is that IMTGP-2 contains a higher percentage of monosaccharides (mannose and glucose) commonly found in N-glycans.

Multiple receptors on porcine intestinal epithelial cells for the three variants of Escherichia coli K88 fimbrial adhesin

We evaluated intestinal epithelial membrane preparations from five phenotypes of pigs, distinguished by the variant of K88 fimbrial adhesin (K88ab, K88ac, K88ad) which bind to their intestinal epithelial cells (A-all three variants, B-K88ab and K88ac, C-K88ab and K88ad, D-K88ad, and E-none of the variants), for the presence of K88 adhesin receptors. Intestinal brush border membranes were prepared from 20 animals (four from each phenotype). Brush border proteins, that had been separated using SDS-PAGE and transferred to nitrocellulose membranes, were overlaid with biotinylated K88 adhesin, 35S-labelled K88+ Escherichia coli, or biotinylated K88+ E. coli. Biotinylated K88ab and K88ac fimbrial adhesins and labelled E. coli expressing K88ab or K88ac adhesin bound to 210- and 240-kDa receptors in phenotype A and B, but not phenotype C, D, or E animals. In contrast, no phenotype-specific receptors were identified for the K88ad adhesin. Previously, purified K88ab and K88ac fimbriae were shown to block K88ad binding, but purified K88ad fimbriae were unable to block K88ab or K88ac binding in phenotype A animals. These results point to the existence of three K88 adhesin receptors to account for the observed phenotypes: (1) Receptor bcd binds all three variants and is found in phenotype A pigs, (2) Receptor bc (210- and 240-kDa receptors) binds K88ab and K88ac and is found in phenotype A and B pigs, and (3) Receptor d binds K88ad and is found in phenotype C and D pigs.

Distribution of K88 Escherichia coli-adhesive and nonadhesive phenotypes among pigs of four breeds

Enterotoxigenic Escherichia coli expressing K88 fimbrial adhesins often cause diarrhea in young pigs. However, some pigs are inherently resistant to colibacillosis, because they lack receptors on their epithelial cell brush borders to which the fimbriae bind. Phenotypic diversity with respect to the binding of E. coli expressing K88 of the three variant types (K88ab, K88ac, and K88ad) was reported by Bijlsma et al. (1982), and binding specificities for each phenotype were described: A (adhesive to all three variants), B (adhesive to K88ab and K88ac), C (adhesive to K88ab and K88ad), D (adhesive to K88ad) and E (nonadhesive). Because brush border adhesiveness has been correlated with disease susceptibility, swine K88 adhesive phenotypes are of significance in the control of enteric disease. To determine the prevalence of the various K88 adhesive phenotypes in the swine population in the Midwestern United States, we tested epithelial cell brush borders of 24 purebred pigs from each of four breeds (Chester White, Duroc, Hampshire and Yorkshire) for adhesiveness to each of the K88 variants. Four, 4-week-old pigs (the largest and smallest healthy female littermates from two litters) were collected from each of 24 farms. Brush border vesicles from the pigs were tested for ability to bind E. coli expressing each K88 variant. The five brush border adherence patterns described for phenotypes A-E were observed. In addition, brush borders from some pigs only bound K88ab + bacteria. Nearly three quarters of the pigs whose brush borders tested, were found to be phenotype A (43%) or phenotype E (28%). These were the most common phenotypes in each breed, except Hampshire, in which case phenotypes C (17%) and D (25%) were more common than E (8%). There appeared to be no relationship between the phenotype of a pig and its weight relative to its littermate.