A sialoglycoprotein complex linked to the microvillus cytoskeleton acts as a receptor for pilus (AF/R1) mediated adhesion of enteropathogenic Escherichia coli (RDEC-1) in rabbit small intestine

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.

Maternal milk contains antimicrobial factors that protect young rabbits from enteropathogenic Escherichia coli infection

Enteropathogenic Escherichia coli (EPEC) colibacillosis represents a major cause of lethal diarrhea in young children in developing countries. EPEC strains also infect numerous mammal species and represent a major economical problem in rabbit industry. Protection against this pathogen is a challenging goal both in humans and in other mammal species. Despite a good knowledge of the pathogenicity mechanisms of EPEC, the intrinsic and environmental factors that control the expression of EPEC virulence in mammals remain unknown. For instance, the exacerbated sensitivity of young mammals to EPEC infection is still unexplained. Our goal was to investigate if age or other factors, like milk consumption, could be determinants that trigger the disease. We used rabbits as an animal model to study the role of milk in the sensitivity to an EPEC infection. Weaned and suckling rabbits were orally inoculated with EPEC strain E22 (O103:H2:K-) at 28 days of age, and the evolution of the disease was investigated in the two groups. In addition, in order to better characterize the interactions between milk and EPEC, we determined in vitro bacterial growth and the abilities of EPEC cells to adhere to epithelial cells in the presence of milk. Our results demonstrate a protective role of milk in vivo in association with in vitro antibacterial activity. These effects are independent of the presence of specific anti-EPEC antibodies.

Protection against hemorrhagic colitis in an animal model by oral immunization with isogeneic rabbit enteropathogenic Escherichia coli attenuated by truncating intimin

Strains of Shiga toxin (Stx)-producing Escherichia coli, also called enterohemorrhagic E. coli (EHEC), are important food-borne pathogens for humans. Most EHEC strains intimately adhere to the intestinal mucosa in a characteristic attaching and effacing (A/E) pattern, which is mediated by the bacterial adhesin intimin. Subsequent release of Stx1 and/or Stx2 leads to the frequent development of hemorrhagic colitis and, less commonly, to hemolytic-uremic syndrome. The aim of the present study was to develop an attenuated A/E E. coli strain for use as a vaccine against EHEC infection encoding a truncated intimin lacking adhesive capacity, but which would still express somatic antigens, other products of the locus of enterocyte effacement pathogenicity island, and an immunogenic remnant of the intimin molecule. A single-nucleotide deletion was generated in the eae gene in the prototype rabbit A/E E. coli strain RDEC-1 (O15:H-), which resulted in truncation of intimin by 81 C-terminal residues (860 to 939 amino acids) containing a disulfide loop. Inoculation of rabbits with large doses of the truncated intimin mutant (RDEC-1Deltaeae(860-939)) was well tolerated, as observed by the absence of clinical signs of disease or evidence of intestinal A/E lesions. The efficacy of RDEC-1Deltaeae(860-939) as a vaccine was evaluated by orogastric inoculation of rabbits with RDEC-1Deltaeae(860-939) followed by challenge with the virulent strain RDEC-H19A, an Stx1-producing derivative of wild-type RDEC-1 capable of inducing hemorrhagic colitis in rabbits. Following RDEC-H19A challenge, nonimmunized control rabbits exhibited characteristic weight loss with watery to bloody diarrhea and demonstrated intimate bacterial attachment, effacement of microvilli, submucosal edema, mucosal heterophile infiltrates, and Shiga toxin-induced vascular lesions. In contrast, the RDEC-1Deltaeae(860-939)-immunized rabbits showed no clinical signs of disease, maintained normal weight gain, had reduced fecal shedding of challenge organisms, and showed an absence of gross or microscopic lesions in the intestinal mucosa. Serum antibodies specific to intimin were detected among rabbits immunized with RDEC-1Deltaeae(860-939), indicating that truncation of the intimin functional domain not only attenuated bacterial virulence, but also retained at least some of the immunogenicity of native intimin. Although it is not possible to gauge the exact contribution of residual intimin immunity to protection, this attenuation strategy for A/E E. coli strains shows promise for the development of effective vaccines to prevent EHEC infection in humans and animals.

Epithelial M cells: differentiation and function

M cells are distinctive epithelial cells that occur only in the follicle-associated epithelia that overlie organized mucosa-associated lymphoid tissues. They are structurally and functionally specialized for transepithelial transport, delivering foreign antigens and microorganisms to organized lymphoid tissues within the mucosae of the small and large intestines, tonsils and adenoids, and airways. M cell transport is a double-edged sword: Certain pathogens exploit the features of M cells that are intended to promote uptake for the purpose of immunological sampling. Eludication of the molecular architecture of M cell apical surfaces is important for understanding the strategies that pathogens use to exploit this pathway and for utilizing M cell transport for delivery of vaccines to the mucosal immune system. This article reviews the functional and biochemical features that distinguish M cells from other intestinal cell types. In addition it synthesizes the available information on development and differentiation of organized lymphoid tissues and the specialized epithelium associated with these immune inductive sites.

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.

Accessibility of glycolipid and oligosaccharide epitopes on rabbit villus and follicle-associated epithelium

The initial step in many mucosal infections is pathogen attachment to glycoconjugates on the apical surfaces of intestinal epithelial cells. We examined the ability of virus-sized (120-nm) and bacterium-sized (1-microm) particles to adhere to specific glycolipids and protein-linked oligosaccharides on the apical surfaces of rabbit Peyer's patch villus enterocytes, follicle-associated enterocytes, and M cells. Particles coated with the B subunit of cholera toxin, which binds the ubiquitous glycolipid GM1, were unable to adhere to enterocytes or M cells. This confirms that both the filamentous brush border glycocalyx on enterocytes and the thin glycoprotein coat on M cells can function as size-selective barriers. Oligosaccharides containing terminal beta(1,4)-linked galactose were accessible to soluble lectin Ricinus communis type I on all epithelial cells but were not accessible to lectin immobilized on beads. Oligosaccharides containing alpha(2, 3)-linked sialic acid were recognized on all epithelial cells by soluble Maackia amurensis lectin II (Mal II). Mal II coated 120-nm (but not 1-microm) particles adhered to follicle-associated enterocytes and M cells but not to villus enterocytes. The differences in receptor availability observed may explain in part the selective attachment of viruses and bacteria to specific cell types in the intestinal mucosa.

The role of the epidermal growth factor receptor in microbial infections of the gastrointestinal tract

The epidermal growth factor receptor (EGFr) is a transmembrane glycoprotein with an intrinsic tyrosine kinase. Ligand-binding to the EGFr activates cell signaling, phosphorylates protein kinases, and rearranges cytoskeletal proteins - responses that resemble those induced by microbial attachment to cell surfaces, a process known to be mediated by host cell receptors in a number of cases. This article critically reviews the possible role played by the EGFr in microbial colonization, and discusses how modulation of the EGF-EGFr axis may affect infection of the gastrointestinal tract.

Modulation of host cell membrane fluidity: a novel mechanism for preventing bacterial adhesion

Adhesion of bacterial enteropathogens to host mucosal surfaces is a critical primary step in the pathogenesis of diarrheal disease. We investigated the effects of altering the physical properties of eukaryotic cells on bacterial adhesion with the use of a series of three structurally dissimilar membrane fluidizers and several Escherichia coli as test strains. Lipid fluidity of the cell plasma membrane was measured by steady-state fluorescence anisotropy employing the probe 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3, 5-hexatriene. There was a dose-dependent and reversible inhibition of bacterial adhesion with increasing membrane fluidity. Time course experiments indicated that increasing membrane fluidity during the early stages of bacterial adhesion was essential for inhibition of attachment. None of the fluidizers affected the viability of either eukaryotic or prokaryotic cells. These findings demonstrate, for the first time, that changes in plasma membrane physical properties of epithelial cells can prevent microbial adhesion. This also suggests that altering the membrane properties of host cells could form a basis for novel strategies to prevent bacterial adhesion during infection in vivo.

The composition and function of M cell apical membranes: implications for microbial pathogenesis

M cells, an epithelial cell phenotype that occurs only over organized mucosal lymphoid follicles, deliver samples of foreign material by transepithelial transport from the lumen to organized lymphoid tissues within the mucosa of the small and large intestines. The apical membranes of M cells in the intestine are designed to facilitate adherence and uptake of antigens and microorganisms, a prerequisite for immunological sampling. The molecular features of M cell apical surfaces that promote adherence and transport are crucial for understanding the strategies that pathogens use to exploit this pathway.

Effects of orally administered epidermal growth factor on enteropathogenic Escherichia coli infection in rabbits

The increased intestinal absorption induced by epidermal growth factor (EGF) is associated with diffuse lengthening of brush border microvilli. The aim of this study was to examine the in vivo effects of oral administration of EGF during infection with enteropathogenic Escherichia coli. New Zealand White rabbits (4 weeks old) received orogastric EGF daily starting 3 days prior to infection with enteropathogenic E. coli RDEC-1 and were compared with sham-treated infected animals and uninfected controls. Weight gain, food intake, fecal E. coli, and stool consistency were assessed daily. On day 10, segments of jejunum, ileum, proximal, and distal colon were assessed for gram-negative bacterial colonization, disaccharidase activities, and epithelial ultrastructure. Effects of EGF on E. coli RDEC-1 proliferation were studied in vitro. E. coli RDEC-1 caused diarrhea and reduced weight gain. Seven days postinfection, the small and large intestines were colonized with numerous bacteria, brush border microvilli were disrupted, and maltase and sucrase activities were significantly reduced in the jejunum. Daily treatment with EGF prevented the occurrence of diarrhea and reduction of weight gain. These effects were associated with significant inhibition of E. coli colonization in the small and large intestine, improved jejunal maltase and sucrase activities and reduced microvillous injury. EGF did not affect the proliferation of E. coli in vitro. The findings suggest that EGF protects the gastrointestinal tract against colonization by enteropathogenic E. coli.

Fimbrial expression in enteric bacteria: a critical step in intestinal pathogenesis

The ability of species of enteric bacteria to recognize and colonize unique niches along the intestine is mainly based on receptor distribution and interpretation of a combination of environmental signals leading to the expression of specific adherence factors. Such elaborate orchestration of events is critical during the initial steps of pathogenesis.

Colonization factors of diarrheagenic E. coli and their intestinal receptors

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

Effect of enteropathogenic Escherichia coli on adherent properties of Chinese hamster ovary cells

Enteropathogenic Escherichia coli (EPEC) O111:H2, O119:H6, or O142:H6 caused rapid detachment of Chinese hamster ovary (CHO) cell monolayers within 2 to 4 h of cocultivation. CHO cell detachment was not promoted by nonenteropathogenic E. coli (O125:H4, O126:H27, O157:H7, and O26:H11) and could not be attributed to EPEC production of enterohemolysin or Shiga-like toxins. In contrast, EPEC strains did not promote rapid detachment of Lec1, Lec2, or Lec8 CHO cell monolayers. These CHO cell Lec mutants all express abbreviated glycan sequences on membrane glycoproteins and glycolipids. Although EPEC strains failed to alter the adherent properties of Lec2 cells lacking only terminal sialic acid groups, EPEC adherence to the Lec2 mutant was indistinguishable from that observed with wild-type CHO cells. There was also no significant difference in EPEC-induced actin accumulation or invasion of Lec2 cells. In contrast, EPEC localized adherence to Lec1 and Lec8 mutants, lacking sialyllactosamine (Lec1) or sialic acid and galactose (Lec8) sequences, was reduced by 84 and 93%, respectively. Our results suggest that lactosamine sequences [beta Gal(1-4 or 1-3)beta GlcNAc] not containing sialic acid are sufficient for EPEC adherence, actin accumulation, and invasion of CHO cells. Sialic acid groups, however, may be necessary for EPEC-mediated CHO cell detachment.

Characterization of porcine intestinal receptors for the K88ac fimbrial adhesin of Escherichia coli as mucin-type sialoglycoproteins

We have previously identified two K88ac adhesion receptors (210 and 240 kDa) which are present in membrane preparations from adhesive but not nonadhesive porcine intestinal brush border cells; these adhesin receptors are postulated to be important determinants of the susceptibility of pigs to K88ac+ enterotoxigenic Escherichia coli infections (A.K. Erickson, J.A. Willgohs, S.Y. McFarland, D.A. Benfield, and D.F. Francis, Infect. Immun. 60:983-988, 1992). We now describe a procedure for the purification of these two receptors. Receptors were solubilized from adhesive intestinal brush border vesicles using deoxycholate and were purified by gel filtration chromatography on Sepharose CL-4B and then by hydroxyapatite chromatography. Amino acid compositional analyses indicated that the two receptors have similar amino acid compositions. The most distinguishing characteristic of both receptors is a high percentage of threonine and proline residues. Neuraminidase treatment caused the K88ac adhesin receptors to migrate with a slower mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels, indicating that these receptors are sialoglycoproteins. Results from lectin-binding studies indicated that the receptors contain O-linked oligosaccharides composed of galactosyl (beta-1,3)N-acetylgalactosamine, alpha-linked fucose, galactosyl(beta-1,4)N-acetylglucosamine, sialic acid, galactose, and N-acetylgalactosamine. Collectively, these characteristics indicate that the K88ac adhesin receptors are mucin-type sialoglycoproteins.

Adhesion and its role in the virulence of enteropathogenic Escherichia coli

Enteropathogenic Escherichia coli (EPEC) organisms are an important cause of diarrheal disease in young children. The virulence of EPEC is a multifactorial process and involves a number of distinct stages. Initial adherence to intestinal mucosa is mediated by fimbriae which bring about a distinct form of adhesion, localized adhesion. Intimate adhesion of the bacterium to the eukaryotic membrane occurs, resulting in the activation of signal transduction pathways. Microvilli are disrupted and effaced from the apical membrane which then cups around the organism to form pedestal structures, the attaching and effacing lesion. Diarrhea may be produced by alteration of the permeability of the apical membrane and also through a malabsorption mechanism. The pathways involved in the production of the attaching and effacing lesion are described. EPEC organisms were originally thought to belong to a number of distinct serogroups; it is now apparent that many isolates belonging to these serogroups are not pathogenic or belong to other pathogenic groups of E. coli. In addition, isolates falling outside of these serogroups are considered to be true EPEC. The definition of EPEC based on serotyping is inaccurate and should be replaced by methods that specifically detect the virulence properties of EPEC.

Expression of receptors for enterotoxigenic Escherichia coli during enterocytic differentiation of human polarized intestinal epithelial cells in culture

To study the expression of human intestinal receptors for enterotoxigenic Escherichia coli (ETEC), the human polarized intestinal epithelial cell line Caco-2 in culture and several subpopulations of HT-29 cells in culture--parental (mainly undifferentiated) HT-29 cells (HT-29 Std), an enterocytelike subpopulation obtained by selection through glucose deprivation (HT-29 Glc-), and an enterocytelike subpopulation obtained by selection through glucose deprivation which maintains its differentiation characteristics when switched back to standard glucose-containing medium (HT-29 Glc-/+)--were used. Since Caco-2 spontaneously differentiated in culture under standard culture conditions (in the presence of glucose) and HT-29 cells were undifferentiated when cultured under standard conditions (HT-29 Std) and differentiated when grown in a glucose-free medium (HT-29 Glc-), we studied the expression of the receptors for colonization factor antigens (CFA) I, II, and III and the 2230 antigen of ETEC in relation to enterocytic differentiation. We provide evidence that expression of ETEC CFA receptors develops in parallel with other differentiation functions of the cultured cells. The expression of ETEC-specific brush border receptors was studied by indirect immunofluorescence using antibodies raised against purified ETEC CFA. No ETEC receptors were detected in HT-29 Std or short-term-cultured Caco-2 cells. However, among the population of HT-29 Std cells, 2 to 4% of the cells were found to bind ETEC, and these cells expressed positive carcinoembryonic antigen immunoreactivity. This indicated that among the population of undifferentiated HT-29 cells, clusters of differentiated cells were present. ETEC CFA receptors were expressed in the apical and basolateral domains of differentiated HT-29 cells, whereas in differentiated Caco-2 cells only apical expression was observed. Both in HT-29 cells (HT-29 Glc-/+) and in Caco-2 cells cultured under standard conditions, ETEC CFA receptors develop as a function of day in culture. This indicated that the expression of the ETEC CFA receptors was a growth-related event. Indeed, ETEC CFA receptors developed in step with the apical expression of differentiation-associated proteins.

Thin aggregative fimbriae from diarrheagenic Escherichia coli

Four strains of diarrheagenic Escherichia coli originally isolated from distinct geographic regions were found to produce unusual thin aggregative fimbriae requiring depolymerization in formic acid prior to analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Immunoelectron microscopy of native fimbriae and Western blot (immunoblot) analysis of the corresponding 18-kDa fimbrins showed that these E. coli fimbriae were serologically cross-reactive with SEF 17 (Salmonella enteritidis fimbriae with a fimbrin molecular mass of 17 kDa). The E. coli and S. enteritidis fimbrins had similar total amino acid compositions and highly conserved N-terminal amino acid sequences. These results indicate that E. coli and S. enteritidis produce biochemically related, aggregative fimbriae which constitute a new type of intergenerically distributed fimbriae for which we propose the descriptive name GVVPQ fimbriae on the basis of the conserved N-terminal amino acid sequence.