Surface properties of the Vero cytotoxin-producing Escherichia coli O157:H7

Pili Assembled by the Chaperone/Usher Pathway in Escherichia coli and Salmonella

Gram-negative bacteria assemble a variety of surface structures, including the hair-like organelles known as pili or fimbriae. Pili typically function in adhesion and mediate interactions with various surfaces, with other bacteria, and with other types of cells such as host cells. The chaperone/usher (CU) pathway assembles a widespread class of adhesive and virulence-associated pili. Pilus biogenesis by the CU pathway requires a dedicated periplasmic chaperone and integral outer membrane protein termed the usher, which forms a multifunctional assembly and secretion platform. This review addresses the molecular and biochemical aspects of the CU pathway in detail, focusing on the type 1 and P pili expressed by uropathogenic Escherichia coli as model systems. We provide an overview of representative CU pili expressed by E. coli and Salmonella, and conclude with a discussion of potential approaches to develop antivirulence therapeutics that interfere with pilus assembly or function.

Fimbriae: Classification and Biochemistry

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

Role of O-antigen on the Escherichia coli O157:H7 cells hydrophobicity, charge and ability to attach to lettuce

Environmental factors encountered during growing and harvesting may contribute to Escherichia coli O157:H7 contamination of lettuce. Limited nutrients and extended exposure to water may cause E. coli O157:H7 to shed its O antigen. Absence of the O157-polysaccharide antigen could affect the cell's physicochemical properties (hydrophobicity and cell charge) and ultimately influence its attachment to surfaces. The objectives of this study were to evaluate the effect of the E. coli O157:H7 O-antigen on the cell's overall hydrophobicity, charge and ability to attach to cut edge and whole leaf iceberg lettuce surfaces. Three strains of E. coli O157:H7 (86-24 wild type; F-12, mutant lacking the O-antigen and pRFBE, plasmid for O157 gene reintroduced) were examined for their hydrophobicity, overall charge and ability to attach to lettuce. Overall, E. coli O157:H7 attached at higher levels to cut surfaces over whole leaf surfaces (P=0.008) for all strains and treatments. Additionally, the strain lacking the O-antigen (F12)-attached significantly less to lettuce (P=0.015) than the strains expressing the antigen (WT and pRFBE). Cells lacking the O antigen (strain F-12) were also significantly more hydrophobic than strains 86-24 or pRFBE (P≤0.05). Surface charge differed among the strains tested (P≤0.05); however, it did not appear to influence bacterial attachment to lettuce surfaces. The charge was not fully restored in the pRFBE strain (expression of O-antigen reintroduced), therefore, no conclusions can be made pertaining to the effect of charge on attachment in this study. Results indicate that E. coli O157:H7 cells which lack the O-antigen have greater hydrophobicity and attach at lower concentrations than cells expressing the O-antigen, to iceberg lettuce surfaces.

Functional analysis of ycfR and ycfQ in Escherichia coli O157:H7 linked to outbreaks of illness associated with fresh produce

Fresh produce has been associated with multiple outbreaks of illness caused by Escherichia coli O157:H7. The mechanism of E. coli O157:H7 survival through postharvest processing of fresh produce needs to be understood to help develop more effective interventions. In our recent transcriptomic study of strain Sakai, an isolate from the 1996 sprout outbreak in Japan, and strain TW14359, an isolate from the 2006 spinach outbreak in the United States, we showed that ycfR was the most significantly upregulated gene in response to chlorine-based oxidative stress. YcfR is known to be a multiple stress resistance protein and a biofilm regulator in E. coli K-12 strains; however, its role in the pathogenic E. coli O157:H7 has not been clearly defined. In this study, ycfR was replaced with a chloramphenicol resistance cassette oriented in two different directions to construct polar and nonpolar ycfR::cat mutants of Sakai and TW14359. Chlorine resistance and survival on spinach leaf surfaces were assessed in the wild-type strains and the ycfR mutants. Both polar and nonpolar ycfR mutants of Sakai showed significantly less chlorine resistance than their parent strain. In contrast, deletion of ycfR in TW14359 did not change chlorine resistance, indicating that ycfR in these two outbreak-related E. coli O157:H7 strains may function differently. In addition, after a 24-h incubation on spinach leaves in a sublethal concentration of chlorine, the Sakai nonpolar ycfR mutant exhibited lower survival compared to the wild type. The results suggest a role for ycfR in survival of Sakai during chlorine exposure. We also found that the upstream ycfQ, which is annotated as a DNA-binding regulator, acted as a repressor of ycfR. These findings suggest that gene regulation may be a mechanism by which E. coli O157:H7 strain Sakai could survive in the postharvest processing environment.

Histidine-rich glycoprotein: the Swiss Army knife of mammalian plasma

Histidine-rich glycoprotein (HRG), also known as histidine-proline-rich glyco-protein, is an abundant and well-characterized protein of vertebrate plasma. HRG has a multidomain structure that allows the molecule to interact with many ligands, including heparin, phospholipids, plasminogen, fibrinogen, immunoglobulin G, C1q, heme, and Zn2+. The ability of HRG to interact with various ligands simultaneously has suggested that HRG can function as an adaptor molecule and regulate numerous important biologic processes, such as immune complex/necrotic cell/pathogen clearance, cell adhesion, angiogenesis, coagulation, and fibrinolysis. The present review covers the proposed multifunctional roles of HRG with a focus on recent findings that have led to its emergence as a key regulator of immunity and vascular biology. Also included is a discussion of the striking functional similarities between HRG and other important multifunctional proteins found in plasma, such as C-reactive protein, C1q, β2 glycoprotein I, and thrombospondin-1.

Prevalence of Verotoxin-Producing Escherichia coli (VTEC) in a survey of dairy cattle in Najaf, Iraq

BACKGROUND AND OBJECTIVES

Dairy cattle have been implicated as principal reservoir of Verotoxin-Producing Escherichia coli (VTEC), with undercooked ground beef and raw milk being the major vehicles of food borne outbreaks. VTEC has been implicated as an etiological agent of individual cases and outbreaks in developed countries. This study was designed to determine the prevalence of VETEC in diarrheic dairy calves up to 20 days of age in Najaf, Iraq.

MATERIALS AND METHODS

326 fecal samples from diarrheic calves were collected for isolation of Escherichia coli O157:H7 and non-O157 VTEC isolates. Non-sorbitol fermentation, enterohemolysin phenotype, and slide agglutination with antisera were used for screening and detection of these serotypes.

RESULTS

Nineteen (5.8%) non-sorbitol fermenting and 3 (0.9%) enterohemolysin-producing E. coli were obtained. Only 9 were agglutinated with available antisera and none of them belonged to the O157:H7 serotype. Three were found to be verotoxin positive on Vero cell monolayers. These included serotype O111 (2 isolates) and serotype O128 (1 isolate). All three VTEC isolates were resistant to ampicillin and streptomycin. Two exhibited adherence phenotype on HEp-2 cells.

CONCLUSION

E. coli O157:H7 serotype is not prevalent in diarrheic dairy calves, and VTEC is not a frequent cause of diarrhea in calves in Najaf/ Iraq.

Molecular mechanisms of late apoptotic/necrotic cell clearance

Phagocytosis serves as one of the key processes involved in development, maintenance of tissue homeostasis, as well as in eliminating pathogens from an organism. Under normal physiological conditions, dying cells (e.g., apoptotic and necrotic cells) and pathogens (e.g., bacteria and fungi) are rapidly detected and removed by professional phagocytes such as macrophages and dendritic cells (DCs). In most cases, specific receptors and opsonins are used by phagocytes to recognize and bind their target cells, which can trigger the intracellular signalling events required for phagocytosis. Depending on the type of target cell, phagocytes may also release both immunomodulatory molecules and growth factors to orchestrate a subsequent immune response and wound healing process. In recent years, evidence is growing that opsonins and receptors involved in the removal of pathogens can also aid the disposal of dying cells at all stages of cell death, in particular plasma membrane-damaged cells such as late apoptotic and necrotic cells. This review provides an overview of the molecular mechanisms and the immunological outcomes of late apoptotic/necrotic cell removal and highlights the striking similarities between late apoptotic/necrotic cell and pathogen clearance.

The type 4 pili of enterohemorrhagic Escherichia coli O157:H7 are multipurpose structures with pathogenic attributes

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 produces long bundles of polar type 4 pili (T4P) called HCP (for hemorrhagic coli pili) that form physical bridges between bacteria associating with human and animal epithelial cells. Here, we sought to further investigate whether HCP possessed other pathogenicity attributes associated with T4P production. Comparative studies performed with wild-type EHEC EDL933 and an isogenic hcpA mutant revealed that HCP play different roles in the biology of this organism. We found that in addition to promoting bacterial attachment to host cells, HCP mediate (i) invasion of epithelial cells, (ii) hemagglutination of rabbit erythrocytes, (iii) interbacterial connections conducive to biofilm formation, (iv) specific binding to host extracellular matrix proteins laminin and fibronectin but not collagen, and (v) twitching motility. Nonadherent laboratory E. coli strain HB101 complemented with hcpABC genes on plasmid pJX22, which specifies for HCP overproduction in EDL933, became hyperadherent and invasive and produced a thick biofilm, suggesting that the presence of HCP confers HB101(pJX22) new attributes otherwise not exhibited by HB101. Analogous to other bacteria in which T4P are involved in the pathogenesis of several infectious diseases, our data strongly suggest that HCP display multiple functions that may contribute to EHEC colonization of different hosts and to virulence, survival, and transmission of this food-borne pathogen.

Food as a vehicle for transmission of Shiga toxin-producing Escherichia coli

Contaminated food continues to be the principal vehicle for transmission of Escherichia coli O157:H7 and other Shiga toxin-producing E. coli (STEC) to humans. A large number of foods, including those associated with outbreaks (alfalfa sprouts, fresh produce, beef, and unpasteurized juices), have been the focus of intensive research studies in the past few years (2003 to 2006) to assess the prevalence and identify effective intervention and inactivation treatments for these pathogens. Recent analyses of retail foods in the United States revealed E. coli O157:H7 was present in 1.5% of alfalfa sprouts and 0.17% of ground beef but not in some other foods examined. Differences in virulence patterns (presence of both stx1 and stx2 genes versus one stx gene) have been observed among isolates from beef samples obtained at the processing plant compared with retail outlets. Research has continued to examine survival and growth of STEC in foods, with several models being developed to predict the behavior of the pathogen under a wide range of environmental conditions. In an effort to develop effective strategies to minimize contamination, several influential factors are being addressed, including elucidating the underlying mechanism for attachment and penetration of STEC into foods and determining the role of handling practices and processing operations on cross-contamination between foods. Reports of some alternative nonthermal processing treatments (high pressure, pulsed-electric field, ionizing radiation, UV radiation, and ultrasound) indicate potential for inactivating STEC with minimal alteration to sensory and nutrient characteristics. Antimicrobials (e.g., organic acids, oxidizing agents, cetylpyridinium chloride, bacteriocins, acidified sodium chlorite, natural extracts) have varying degrees of efficacy as preservatives or sanitizing agents on produce, meat, and unpasteurized juices. Multiple-hurdle or sequential intervention treatments have the greatest potential to minimize transmission of STEC in foods.

Influence of curli expression by Escherichia coli 0157:H7 on the cell's overall hydrophobicity, charge, and ability to attach to lettuce

Curli fibers are produced by some Escherichia coli cells in response to environmental stimuli. These extracellular proteins enhance the cell's ability to form biofilms on various abiotic surfaces. E. coli 0157:H7 cells readily attach to a variety of fruit and vegetable surfaces. It is not known whether the expression of curli influences the cell's ability to attach to produce surfaces. In this study, the effect of curli expression on the cell's overall hydrophobicity, charge, and ability to attach to cut and whole iceberg lettuce surfaces was examined. All strains, regardless of curli expression, attached preferentially to the cut edges of lettuce (P < 0.05). The curli-producing cells of E. coli 0157:H7 strain E0018 attached in significantly greater numbers to both cut and whole lettuce pieces than did the non-curli-producing E0018 cells (P < 0.05); however, no significant attachment differences were observed between the curli-producing and non-curli-producing cells of E. coli 0157:H7 strains 43894 and 43895. All curli-producing E. coli 0157:H7 strains were significantly more hydrophobic (P < 0.01); however, no association between the cells' hydrophobic characteristics and lettuce attachment was observed. Overall surface charge of the cells did not differ among strains or curli phenotypes. Results indicate that overall hydrophobicity and cell charge in E. coli 0157:H7 strains do not influence attachment to iceberg lettuce surfaces. The presence of curli may not have any influence on attachment of E. coli 0157:H7 cells to produce items. Additional factorsmay influence the attachment of E. coli 0157:H7 to plant surfaces and should be further examined.

Effects of cell surface charge and hydrophobicity on attachment of 16 Salmonella serovars to cantaloupe rind and decontamination with sanitizers

Adherence of bacteria to cantaloupe rind is favored by surface irregularities such as roughness, crevices, and pits, thus reducing the ability of washing or sanitizer treatments to remove or inactivate attached cells. In this study, we compared the surface charge and hydrophobicity of two cantaloupe-related outbreak strains of Salmonella Poona (RM2350 and G-91-1595) to those of 14 additional Salmonella strains using electrostatic and hydrophobic interaction chromatography. The relative abilities of the 16 strains to attach to cantaloupe surfaces and resist removal by washing with water, chlorine (200 ppm), or hydrogen peroxide (2.5%) for 5 min after a storage period of up to 7 days at 5 to 20 degrees C also were determined. Whole cantaloupes were inoculated with each pathogen at 8.36 log CFU/ml, dried for 1 h inside a biosafety cabinet, stored, and then subjected to the washing treatments. Only the positive surface charge of the two cantaloupe-related strains of Salmonella Poona was significantly higher (P < 0.05) than that of the other strains. Initial bacterial attachment to cantaloupe surfaces ranged from 3.68 to 4.56 log CFU/cm2 (highest values for Salmonella Michigan, Newport, Oranienburg, and Mbandaka). The average percentage of the total bacterial population strongly attached to the cantaloupe surface for the Salmonella serovars studied ranged from 0.893 to 0.946 at 5 degrees C and from 0.987 to 0.999 at 25 degrees C. Washing inoculated melons with water did not produce a significant reduction in the concentration of the pathogens (P > 0.05). Chlorine and hydrogen peroxide treatments caused an average 3-log reduction when applied 20 to 40 min postinoculation. However, sanitizer treatments applied 60 min or more postinoculation were less effective (approximately 2.5-log reduction). No significant differences were noted in sanitizer efficacy against the individual strains (P > 0.05). The two cantaloupe-related outbreak Salmonella Poona strains did not significantly differ from the other Salmonella strains tested in negative cell surface charge or hydrophobicity, were not more effective in attaching to whole melon surfaces, and were not more resistant to the various washing treatments when present on rinds.

Conditioned medium from enterohemorrhagic Escherichia coli-infected T84 cells inhibits signal transducer and activator of transcription 1 activation by gamma interferon

Gamma interferon (IFN-gamma) is a cytokine important to host defense which can signal through signal transducer and activator of transcription 1 (Stat1). Enterohemorrhagic Escherichia coli (EHEC) modulates host cell signal transduction to establish infection, and EHEC serotypes O113:H21 and O157:H7 both inhibit IFN-gamma-induced Stat1 tyrosine phosphorylation in vitro. The aim of this study was to delineate both bacterial and host cell factors involved in the inhibition of Stat1 tyrosine phosphorylation. Human T84 colonic epithelial cells were challenged with direct infection, viable EHEC separated from T84 cells by a filter, sodium orthovanadate, isolated flagellin, bacterial culture supernatants, and conditioned medium treated with proteinase K, trypsin, or heat inactivation. Epithelial cells were then stimulated with IFN-gamma and protein extracts were analyzed by immunoblotting. The data showed that IFN-gamma-inducible Stat1 tyrosine phosphorylation was inhibited when EHEC adhered to T84 cells, but not by bacterial culture supernatants or bacteria separated from the epithelial monolayer. Conditioned medium from T84 cells infected with EHEC O157:H7 suppressed Stat1 activation, and this was not reversed by treatment with proteinases or heat inactivation. Use of pharmacological inhibitors showed that time-dependent bacterial, but not epithelial, protein synthesis was involved. Stat1 inhibition was also independent of bacterial flagellin, host proteasome activity, and protein tyrosine phosphatases. Infection led to altered IFN-gamma receptor domain 1 subcellular distribution and decreased expression in cholesterol-enriched membrane microdomains. Thus, suppression of host cell IFN-gamma signaling by production of a contact-dependent, soluble EHEC factor may represent a novel mechanism for this pathogen to evade the host immune system.

Novel effects of the prototype translocating Escherichia coli, strain C25 on intestinal epithelial structure and barrier function

Intestinal bacteria play an etiologic role in triggering and perpetuating chronic inflammatory bowel disorders. However, the precise mechanisms whereby the gut microflora influences intestinal cell function remain undefined. Therefore, the effects of the non-pathogenic prototype translocating Escherichia coli, strain C25 on the barrier properties of human T84 and Madine-Darby canine kidney type 1 epithelial cells were examined. T-84 cells were also infected with commensal E. coil, strains F18 and HB101, and enterohaemorrhagic E. coli, serotype O157:H7. Strains F18 and HB101 had no effect on transepithelial electrical resistance (TER) of T84 monolayers. By contrast, epithelial cells infected with strain C25 displayed a time-dependent decrease in TER, preceded by an altered distribution of the cytoskeletal protein alpha-actinin, comparable to infection with E. coli O157:H7. E. coli C25 infection also led to activation of nuclear factor kappaB (NF-kappaB), interleukin-8 secretion and alterations in localization of claudin-1, but not zona occludens-1 or claudin-4, in T84 cells. There were adherent C25 bacteria on the intact apical surface of infected T84 cells, while mitochondria appeared swollen and vacuolated. These novel findings demonstrate the ability of a translocating commensal bacterium to adhere to and modulate intestinal epithelial barrier function and to induce morphological changes in a manner distinct from the known enteric pathogen, E. coli O157:H7.

Adhesins of Enterohemorrhagic Escherichia coli

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

Physicochemical properties of Shiga toxigenic Escherichia coli

AIMS

To investigate the physicochemical surface properties, such as cellular surface charge, hydrophobicity and electron donor/acceptor potential of a selection of Shiga toxigenic Escherichia coli (STEC) isolates grown in broth and agar culture.

METHODS AND RESULTS

Cellular surface charge was determined using zeta potential measurements. Hydrophobicity of the isolates was determined using bacterial adhesion to hydrocarbons assay, hydrophobic interaction chromatography and contact angle measurements. Microbial adhesion to solvents was used to determine the electron donor/acceptor characteristics. No differences of surface charge measurements were found between broth and agar grown cultures. Isolates belonging to serogroup O157 and serotypes O26:H11 and O111:H- were significantly (P < 0.05) less negatively charged than other STEC serotypes tested. All strains were hydrophilic with most methods and demonstrated a lower hydrophobicity in agar culture compared with broth culture. All strains demonstrated a strong microbial adhesion to chloroform indicating that STEC possess an electron donor and basic character. A relationship between serogroup O157 and other STEC serotypes was apparent using principal-component analysis (PCA).

CONCLUSIONS

Combining the results for physicochemical properties using PCA differentiated between strains belonging to the O157 serogroup and other STEC/non-STEC strains. PCA found similar results for broth and agar grown cultures.

SIGNIFICANCE AND IMPACT OF THE STUDY

Particular serotypes of STEC possess similar physicochemical properties which may play a role in their pathogenicity or potential attachment to various surfaces.

Probiotics reduce enterohemorrhagic Escherichia coli O157:H7- and enteropathogenic E. coli O127:H6-induced changes in polarized T84 epithelial cell monolayers by reducing bacterial adhesion and cytoskeletal rearrangements

The aim of this study was to determine if probiotics reduce epithelial injury following exposure to Escherichia coli O157:H7 and E. coli O127:H6. The pretreatment of intestinal (T84) cells with lactic acid-producing bacteria reduced the pathogen-induced drop in transepithelial electrical resistance. These findings demonstrate that probiotics prevent epithelial injury induced by attaching-effacing bacteria.

Attachment of Escherichia coli O157:H7 grown in tryptic soy broth and nutrient broth to apple and lettuce surfaces as related to cell hydrophobicity, surface charge, and capsule production

This study investigated the effect of growth in tryptic soy broth (TSB) and nutrient broth (NB) on the ability Escherichia coli O157:H7 to attach to lettuce and apple surfaces. In addition, cell surface hydrophobicity, charge and capsule production were determined on cells grown in these media. Cells grown in NB attached less to lettuce and apple surfaces than did those grown in TSB. TSB, but not NB, supported capsule production by E. coli O157:H7. Cells grown in TSB were more hydrophilic than those grown in NB. No difference was found in the electrokinetic properties of cells grown in these media. Electrostatic and hydrophobic interactions and surface proteins did not appear to play an important role in the attachment of E. coli O157:H7 to these surfaces. Of the factors studied, only capsule production was associated with attachment ability.

Critical roles for stx2, eae, and tir in enterohemorrhagic Escherichia coli-induced diarrhea and intestinal inflammation in infant rabbits

Enterohemorrhagic Escherichia coli (EHEC) is a group of food-borne pathogens that can cause diarrhea, colitis, and the hemolytic uremic syndrome (HUS). The importance of several of the proposed EHEC virulence factors lacks experimental verification in animal models. The limitations of current animal models led us to reexamine the infant rabbit model for the study of EHEC pathogenicity. Here, we report that intragastric inoculation of a Shiga toxin 2 (Stx2)-producing E. coli O157:H7 clinical isolate into infant rabbits led to severe diarrhea and intestinal inflammation but no signs of HUS. We constructed a set of isogenic derivatives of this isolate with deletions in several putative virulence genes, including stx(2), eae, tir, and ehxA, to investigate the contribution of individual virulence factors to EHEC pathogenicity. stx(2) increased the severity and duration of EHEC-induced diarrhea. Furthermore, although stx(2) had no role in EHEC intestinal colonization nor was it required for EHEC-induced inflammation, stx(2) altered how the host responded to EHEC infection by promoting heterophilic infiltration of the colonic epithelium and lamina propria. Intragastric inoculation of purified Stx2 also induced inflammation and diarrhea in this model. Diarrhea and intestinal inflammation were also dependent on EHEC colonization, as EHEC derivatives with deletions in eae and tir did not colonize, form attaching and effacing lesions, or develop clinical signs of disease. Our studies indicate that infant rabbits are a useful model for investigation of the intestinal stage of EHEC pathogenesis and suggest that Shiga toxin may play a critical role in causing diarrhea and inflammation in patients infected with EHEC.

Influence of surfactant hydrophobicity on the detachment of Escherichia coli O157:H7 from lettuce

The influence of surfactant hydrophobicity on detachment of Escherichia coli O157:H7 from lettuce was determined. Lettuce pieces inoculated with the pathogen were rinsed with Tween and Span surfactants of different hydrophobicity. Of the Tweens, only Tween 85, the Tween with the lowest hydrophile/lipophile balance (HLB), significantly detached the pathogen from lettuce surface. Span 85 (the surfactant with the lowest HLB studied) exhibited the greatest ability among surfactants tested to detach cells from lettuce. This surfactant removed cells attached to the leaf cuticle but not to the cut edge, and caused no detectable reduction in viability of cells remaining on the lettuce. Treatment with Span 85 did not detach cells when they were allowed to attach in the presence of calcium ions. The combination of NaCl/NaHCO3 (pH 10) and Span 85 did not detach cells possibly due to reduced hydrophobicity of the Span at this pH. This study suggests that surfactants of low HLB disrupt hydrophobic interactions between E. coli O157:H7 and the lettuce surface but cannot cause release of cells adhering to hydrophilic structures such as cut or damaged tissue.

Enterohemorrhagic Escherichia coli O157:H7 disrupts Stat1-mediated gamma interferon signal transduction in epithelial cells

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 is a clinically important bacterial enteropathogen that manipulates a variety of host cell signal transduction cascades to establish infection. However, the effect of EHEC O157:H7 on Jak/Stat signaling is unknown. To define the effect of EHEC infection on epithelial gamma interferon (IFN-gamma)-Stat1 signaling, human T84 and HEp-2 epithelial cells were infected with EHEC O157:H7 and then stimulated with recombinant human IFN-gamma. Cells were also infected with different EHEC strains, heat-killed EHEC, enteropathogenic E. coli (EPEC) O127:H6, and the commensal strain E. coli HB101. Nuclear and whole-cell protein extracts were prepared and were assayed by an electrophoretic mobility shift assay (EMSA) and by Western blotting, respectively. Cells were also processed for immunofluorescence to detect the subcellular localization of Stat1. The EMSA revealed inducible, but not constitutive, Stat1 activation upon IFN-gamma treatment of both cell lines. The EMSA also showed that 6 h of EHEC O157:H7 infection, but not 30 min of EHEC O157:H7 infection, prevented subsequent Stat1 DNA binding induced by IFN-gamma, whereas infection with EPEC did not. Immunoblotting showed that infection with EHEC, but not infection with EPEC, eliminated IFN-gamma-induced Stat1 tyrosine phosphorylation in both dose- and time-dependent fashions and disrupted inducible protein expression of the Stat1-dependent gene interferon regulatory factor 1. Immunofluorescence revealed that EHEC infection did not prevent nuclear accumulation of Stat1 after IFN-gamma treatment. Also, Stat1 tyrosine phosphorylation was suppressed by different EHEC isolates, including intimin-, type III secretion- and plasmid-deficient strains, but not by HB101 and heat-killed EHEC. These findings indicate the novel disruption of host cell signaling caused by EHEC infection but not by EPEC infection.

A murine model of enterohemorrhagic Escherichia coli O157:H7 infection to assess immunopotentiating activity of drugs on mucosal immunity: effect of drugs

An enterohemorrhagic Escherichia coli (EHEC) O157 oral infection murine model was established to examine the potentiating activity of drugs on mucosal immune responses. Groups of ICR mice inoculated intragastrically with 10(11) CFU/kg EHEC O157 showed chronic intestinal infection with the pathogen that persisted over 3 weeks and resulted in the synthesis of relatively high levels of antigen specific fecal IgA antibody. Intraperitoneal administration of 80 NU/kg Neurotropin, an immunopotentiator, augmented the antigen specific mucosal immune responses to EHEC O157. On the other hand, FK506 clearly suppressed the response. To further document the augmenting effect of Neurotropin on mucosal immune responses, mice were immunized intranasally with a mixture of ovalbumin and cholera toxin. Co-administration of 80 NU/kg Neurotropin significantly potentiated the synthesis of fecal IgA and serum IgG antibodies. These results suggest that Neurotropin has potential as a mucosal adjuvant to promote secretory IgA antibody production and that the mice model of oral infection with EHEC O157 is useful for immunopharmacological studies of bacterial infection-defensive mucosal immune responses.

Enterohemorrhagic Escherichia coli O157:H7 disrupts Stat1-mediated gamma interferon signal transduction in epithelial cells

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 is a clinically important bacterial enteropathogen that manipulates a variety of host cell signal transduction cascades to establish infection. However, the effect of EHEC O157:H7 on Jak/Stat signaling is unknown. To define the effect of EHEC infection on epithelial gamma interferon (IFN-gamma)-Stat1 signaling, human T84 and HEp-2 epithelial cells were infected with EHEC O157:H7 and then stimulated with recombinant human IFN-gamma. Cells were also infected with different EHEC strains, heat-killed EHEC, enteropathogenic E. coli (EPEC) O127:H6, and the commensal strain E. coli HB101. Nuclear and whole-cell protein extracts were prepared and were assayed by an electrophoretic mobility shift assay (EMSA) and by Western blotting, respectively. Cells were also processed for immunofluorescence to detect the subcellular localization of Stat1. The EMSA revealed inducible, but not constitutive, Stat1 activation upon IFN-gamma treatment of both cell lines. The EMSA also showed that 6 h of EHEC O157:H7 infection, but not 30 min of EHEC O157:H7 infection, prevented subsequent Stat1 DNA binding induced by IFN-gamma, whereas infection with EPEC did not. Immunoblotting showed that infection with EHEC, but not infection with EPEC, eliminated IFN-gamma-induced Stat1 tyrosine phosphorylation in both dose- and time-dependent fashions and disrupted inducible protein expression of the Stat1-dependent gene interferon regulatory factor 1. Immunofluorescence revealed that EHEC infection did not prevent nuclear accumulation of Stat1 after IFN-gamma treatment. Also, Stat1 tyrosine phosphorylation was suppressed by different EHEC isolates, including intimin-, type III secretion- and plasmid-deficient strains, but not by HB101 and heat-killed EHEC. These findings indicate the novel disruption of host cell signaling caused by EHEC infection but not by EPEC infection.

Identification and characterization of lpfABCC'DE, a fimbrial operon of enterohemorrhagic Escherichia coli O157:H7

The mechanisms underlying the adherence of Escherichia coli O157:H7 and other enterohemorrhagic E. coli (EHEC) strains to intestinal epithelial cells are poorly understood. We have identified a chromosomal region (designated lpfABCC'DE) in EHEC O157:H7 containing six putative open reading frames that was found to be closely related to the long polar (LP) fimbria operon (lpf) of Salmonella enterica serovar Typhimurium, both in gene order and in conservation of the deduced amino acid sequences. We show that lpfABCC'DE is organized as an operon and that its expression is induced during the exponential growth phase. The lpf genes from EHEC strain EDL933 were introduced into a nonfimbriated (Fim(-)) E. coli K-12 strain, and the transformed strain produced fimbriae as visualized by electron microscopy and adhered to tissue culture cells. Anti-LpfA antiserum recognized a ca. 16-kDa LpfA protein when expressed under regulation of the T7 promoter system. The antiserum also cross-reacted with the LP fimbriae in immunogold electron microscopy and Western blot experiments. Isogenic E. coli O157:H7 lpf mutants derived from strains 86-24 and AGT300 showed slight reductions in adherence to tissue culture cells and formed fewer microcolonies compared with their wild-type parent strains. The adherence and microcolony formation phenotypes were restored when the lpf operon was introduced on a plasmid. We propose that LP fimbriae participate in the interaction of E. coli O157:H7 with eukaryotic cells by assisting in microcolony formation.

Relationship of cell surface charge and hydrophobicity to strength of attachment of bacteria to cantaloupe rind

The cantaloupe melon has been associated with outbreaks of Salmonella infections. It is suspected that bacterial surface charge and hydrophobicity may affect bacterial attachment and complicate bacterial detachment from cantaloupe surfaces. The surface charge and hydrophobicity of strains of Salmonella, Escherichia coli (O157:H7 and non-O157:H7), and Listeria monocytogenes were determined by electrostatic and hydrophobic interaction chromatography, respectively. Initial bacterial attachment to cantaloupe surfaces and the ability of bacteria to resist removal by washing with water were compared with surface charge and hydrophobicity. Whole cantaloupes were submerged in inocula containing individual strains or in cocktails containing Salmonella, E. coli, and L. monocytogenes, either as a mixture of strains containing all three genera or as a mixture of strains belonging to a single genus, for 10 min. Inoculated cantaloupes were dried for 1 h in a biosafety cabinet and then stored for up to 7 days at 4 degrees C. Inoculated melons were washed with water, and bacteria still attached to the melon surface, as well as those in the wash water, were enumerated. Initial bacterial attachment was highest for individual strains of E. coli and lowest for L. monocytogenes, but Salmonella exhibited the strongest attachment on days 0, 3, and 7. When mixed-genus cocktails were used, the relative degrees of attachment of the three genera ware altered. The attachment of Salmonella strains was the strongest. but the attachment of E. coli was more extensive than that of L. monocytogenes on days 0, 3, and 7. There was a linear correlation between bacterial cell surface hydrophobicity (r2 = 0.767), negative charge (r2 = 0.738), and positive charge (r2 = 0.724) and the strength of bacterial attachment to cantaloupe surfaces.

Variations in the csgD promoter of Escherichia coli O157:H7 associated with increased virulence in mice and increased invasion of HEp-2 cells

Promoter alterations in the csgD gene of Escherichia coli O157:H7 strains ATCC 43894 and ATCC 43895 are associated with variations in curli expression and the ability to bind Congo red dye. Red variants of each strain were more invasive for cultured HEp-2 cells than were white variants. An ATCC 43895 red variant was more virulent than a white variant in a mouse model. However, there were no differences in Shiga toxin production between red and white variants.

Type 1 fimbriation and its phase switching in diarrheagenic Escherichia coli strains

Type 1 fimbriae can be expressed by most Escherichia coli strains and mediate mannose-sensitive (MS) adherence to mammalian epithelial cells. However, the role of type 1 fimbriae in enteric pathogenesis has been unclear. Expression of type 1 fimbriae in E. coli is phase variable and is associated with the inversion of a short DNA element (fim switch). Forty-six strains of diarrheagenic E. coli were examined for the expression of type 1 fimbriae. Only four of these strains were originally type 1 fimbriated. Seventeen strains, originally nonfimbriated, expressed type 1 fimbriae in association with off-to-on inversion of the fim switch, after serial passages in static culture. The switching frequencies of these strains, from fimbriate to nonfimbriate, were greater than that of the laboratory strain E. coli K-12. None of the 16 strains of serovar O157:H7 or O157:H(-) expressed type 1 fimbriae after serial passages in static culture. The nucleotide sequence analysis of the fim switch region revealed that all of the O157:H7 and O157:H(-) strains had a 16-bp deletion in the invertible element, and the fim switch was locked in the "off" orientation. The results suggest that expression of type 1 fimbriae may be regulated differently in different E. coli pathogens causing enteric infections.

Phosphatidylethanolamine recognition promotes enteropathogenic E. coli and enterohemorrhagic E. coli host cell attachment

Using both solid phase and liposome aggregation assays, we screened a variety of glycolipids and phospholipids and found that EHEC and EPEC bind specifically and in a dose-dependent manner to PE. This binding was consistently observed whether the lipid was immobilized on a thin layer chromatography plate, in a microtitre well or incorporated into a unilamellar vesicle suspended in aqueous solution. There was no evidence of binding to other phospholipids such as phosphatidylcholine (PC) or phosphatidylserine (PS). Bacterial binding to two epithelial cell lines also correlated with the level of outer leaflet PE and was reduced following preincubation with anti-PE. The PE-binding phenotype of EPEC appeared to correlate with the bundle-forming pilus (bfp) genotype of a number of clinical isolates. These results provide evidence of a receptor role for PE in the adhesion of EHEC and EPEC to host cells.

Expression of type I pili is abolished in verotoxin-producing Escherichia coli O157

Verotoxin-producing Escherichia coli (VTEC) were examined for production of type I pili. None of 34 strains of VTEC serogroup O157 examined expressed any pili, whereas 26 strains of 27 VTEC serogroup O26 and seven strains of nine non-VTEC O157 produced type I pili. These VTEC strains were collected from sporadic human cases and cattle in Okinawa in 1997. The genes encoding the major structural component (FimA) and the adhesin (FimH) of type I pili were detected in all 70 strains examined. The inability to express type I pili could be a unique character of VTEC O157 and this trait could be a new candidate to identify the organisms.

Influence of oxygen availability on physiology, verocytotoxin expression and adherence of Escherichia coli O157

A strain of Escherichia coli serotype O157 was grown in steady state chemostat culture under aerobic, oxygen-limited and anaerobic conditions. The growth and metabolic efficiency of oxygen-limited and anaerobic cultures was impaired, with biomass yield and the molar growth yield for glucose, Yglucose, reduced markedly in comparison with aerobic cultures. Steady state cells were typically short rods 2-3 microns long, and were encapsulated by a layer of extracellular material. The majority of cells were non-flagellated and fimbriae were not observed. Chemostat-grown cells were significantly more adhesive for HEp-2 monolayers than cells grown in aerobic batch culture. Furthermore, oxygen-limited and anaerobic cultures were significantly more adhesive for Hep-2 cells when compared with cells grown in aerobic chemostat culture, possibly reflecting increased pathogenicity associated with the induction of novel adhesins. Type 1 pili were not responsible for increased adherence. Verocytotoxins, VT1 and VT2, were expressed constitutively and were not influenced by oxygen availability. This study demonstrates that E. coli O157 is a versatile micro-organism, which responds to environmental conditions likely to be encountered during infection by inducing a phenotype which is more adhesive for human epithelial cells.

Pathogenesis and diagnosis of Shiga toxin-producing Escherichia coli infections

Since their initial recognition 20 years ago, Shiga toxin-producing Escherichia coli (STEC) strains have emerged as an important cause of serious human gastrointestinal disease, which may result in life-threatening complications such as hemolytic-uremic syndrome. Food-borne outbreaks of STEC disease appear to be increasing and, when mass-produced and mass-distributed foods are concerned, can involve large numbers of people. Development of therapeutic and preventative strategies to combat STEC disease requires a thorough understanding of the mechanisms by which STEC organisms colonize the human intestinal tract and cause local and systemic pathological changes. While our knowledge remains incomplete, recent studies have improved our understanding of these processes, particularly the complex interaction between Shiga toxins and host cells, which is central to the pathogenesis of STEC disease. In addition, several putative accessory virulence factors have been identified and partly characterized. The capacity to limit the scale and severity of STEC disease is also dependent upon rapid and sensitive diagnostic procedures for analysis of human samples and suspect vehicles. The increased application of advanced molecular technologies in clinical laboratories has significantly improved our capacity to diagnose STEC infection early in the course of disease and to detect low levels of environmental contamination. This, in turn, has created a potential window of opportunity for future therapeutic intervention.

Signal transduction pathways involved in enterohemorrhagic Escherichia coli-induced alterations in T84 epithelial permeability

Enterohemorrhagic Escherichia coli (EHEC) infection is associated with watery diarrhea and can lead to complications, including hemorrhagic colitis and the hemolytic-uremic syndrome. The mechanisms by which these organisms produce diarrheal disease remain to be elucidated. Changes in T84 epithelial cell electrophysiology were examined following EHEC infection. T84 cell monolayers infected with EHEC O157:H7 displayed a time-dependent decrease in transepithelial resistance. Increases in the transepithelial flux of both [3H]mannitol and 51Cr-EDTA accompanied the EHEC-induced decreases in T84 resistance. Altered barrier function induced by EHEC occurred at the level of the tight junction since immunofluorescent staining of the tight-junction-associated protein ZO-1 was disrupted when examined by confocal microscopy. Decreased resistance induced by EHEC involved a protein kinase C (PKC)-dependent pathway as the highly specific PKC inhibitor, CGP41251, abrogated the EHEC-induced drop in resistance. PKC activity was also increased in T84 cells infected with EHEC. Calmodulin and myosin light chain kinase played a role in EHEC-induced resistance changes as inhibition of these effector molecules partially reversed the effects of EHEC on barrier function. These studies demonstrate that intracellular signal transduction pathways activated following EHEC infection link the increases in T84 epithelial permeability induced by this pathogen.

Divergent signal transduction responses to infection with attaching and effacing Escherichia coli

Shiga toxin-producing Escherichia coli (STEC) O157:H7 is an attaching and effacing pathogen that causes hemorrhagic colitis and the hemolytic-uremic syndrome. Although this organism causes adhesion pedestals, the cellular signals responsible for the formation of these lesions have not been clearly defined. We have shown previously that STEC O157:H7 does not induce detectable tyrosine phosphorylation of host cell proteins upon binding to eukaryotic cells and is not internalized into nonphagocytic epithelial cells. In the present study, tyrosine-phosphorylated proteins were detected under adherent STEC O157:H7 when coincubated with the non-intimately adhering, intimin-deficient, enteropathogenic E. coli (EPEC) strain CVD206. The ability to be internalized into epithelial cells was also conferred on STEC O157:H7 when coincubated with CVD206 ([158 +/- 21] % of control). Neither the ability to rearrange phosphotyrosine proteins nor that to be internalized into epithelial cells was evident following coincubation with another STEC O157:H7 strain or with the nonsignaling espB mutant of EPEC. E. coli JM101(pMH34/pSSS1C), which overproduces surface-localized O157 intimin, also rearranged tyrosine-phosphorylated and cytoskeletal proteins when coincubated with CVD206. In contrast, JM101 (pMH34/pSSS1C) demonstrated rearrangement of cytoskeletal proteins, but not tyrosine-phosphorylated proteins, when coincubated with intimin-deficient STEC (strains CL8KO1 and CL15). These findings indicate that STEC O157:H7 forms adhesion pedestals by mechanisms that are distinct from those in attaching and effacing EPEC. Taken together, these findings point to diverging signal transduction responses to infection with attaching and effacing bacterial enteropathogens.

Translocation of verotoxin-1 across T84 monolayers: mechanism of bacterial toxin penetration of epithelium

Verotoxin-producing Escherichia coli (VTEC) are pathogenic bacteria associated with diarrhea, hemorrhagic colitis, and hemolytic uremic syndrome. Verotoxins (VTs) elaborated by these organisms produce cytopathic effects on a restricted number of cell types, including endothelial cells lining the microvasculature of the bowel and the kidney. Because human intestinal epithelial cells lack the globotriaosylceramide receptor for VT binding, it is unclear how the toxin moves across the intestinal mucosa to the systemic circulation. The aims of this study were to determine the effects of VT-1 on intestinal epithelial cell function and to characterize VT-1 translocation across monolayers of T84 cells, an intestinal epithelial cell line. VT-1 at concentrations up to 1 microgram/ml had no effect on the barrier function of T84 monolayers as assessed by measuring transmonolayer electrical resistance (102 +/- 8% of control monolayers). In contrast, both VT-positive and VT-negative VTEC bacterial strains lowered T84 transmonolayer resistance (45 +/- 7 and 38 +/- 6% of controls, respectively). Comparable amounts of toxin moved across monolayers of T84 cells, exhibiting high-resistance values, as monolayers with VTEC-induced decreases in barrier function, suggesting a transcellular mode of transport. Translocation of VT-1 across T84 monolayers paralleled the movement of a comparably sized protein, horseradish peroxidase. Immunoelectron microscopy confirmed transcellular transport of VT-1, since the toxin was observed within endosomes and associated with specific intracellular targets, including the Golgi network and endoplasmic reticulum. These data present a mode of VT-1 uptake by toxin-insensitive cells and suggest a general mechanism by which bacterial toxins lacking specific intestinal receptors can penetrate the intestinal epithelial barrier.

Invasion of bovine epithelial cells by verocytotoxin-producing Escherichia coli O157:H7

The ability of verocytotoxin-producing Escherichia coli (VTEC) O157:H7 to enter selected human (RPMI-4788 and HeLa) and bovine (MAC-T, mammary secretory; MDBK, kidney) epithelial cell lines was evaluated. All VTEC evaluated efficiently entered RPMI-4788 and MAC-T cell lines. VTEC entered MDBK cells at approximately 4% of MAC-T cells. VTEC were not able to invade HeLa cells. Presence of plasmid had no influence on efficiency of entry, nor did production of shiga-like toxin (SLT I or SLT II). Internalization required microfilaments, but not microtubules. Two types of adherence, localized and diffuse, were exhibited depending on isolate and cell line evaluated. Ability of VTEC to invade bovine mammary epithelial cells may be important in pathogenesis in the bovine, may indicate a route by which raw milk may potentially become contaminated, and may provide a reservoir of bacteria for the contamination of workers, equipment and carcass at time of slaughter.

Heterogeneity in phenotypic and genotypic characteristics among strains of Hafnia alvei

Hafnia alvei is an emerging human pathogen associated with sporadic cases and outbreaks of diarrhea. Bangladeshi isolates of H. alvei possess the Escherichia coli attaching and effacing (eaeA) gene and demonstrate an attaching and effacing phenotype. In the present study we examined 11 Canadian H. alvei isolates and strain 19,982 from Bangladesh to determine if the formation of attaching and effacing lesions is a property shared among multiple isolates. Attaching and effacing lesions were detected by induction of tyrosine kinase protein phosphorylation and cytoskeletal rearrangements in infected tissue culture epithelial cells with immunofluorescence microscopy and by the examination of infected cells with transmission electron microscopy. The presence of the eaeA gene was examined by PCR and colony blot hybridization. Profiles of outer membrane protein extracts, chromosomal macrorestriction fragments, and plasmids were also examined. Accumulation of host phosphotyrosine proteins and rearrangement of the cytoskeletal protein alpha-actinin were both observed in HEp-2 cells infected with H. alvei 19,982. In contrast, none of the other 11 clinical H. alvei isolates demonstrated either of these responses, nor did they form attaching and effacing lesions under electron microscopy. Consistent with the absence of the attaching and effacing phenotype, these clinical isolates did not possess the eaeA gene. The outer membrane protein profiles of all the Canadian isolates were identical but differed from that of H. alvei 19,982. Pulsed-field gel electrophoresis and plasmid profile analyses of the clinical H. alvei isolates differed substantially from those of the Bangladeshi strain. These results indicate that there is heterogeneity among H. alvei strains with respect to signal transduction, attaching and effacing adhesion, outer membrane constituents, and genotype. Epidemiological studies on enteropathogenic H. alvei thus need to go beyond simple species designations and require specific identification of the virulent clones.

Polysaccharide side chains are not required for attaching and effacing adhesion of Escherichia coli O157:H7

Escherichia coli of the serotype O157:H7 is an enterohemorrhagic human pathogen which demonstrates attaching and effacing adhesion to colonocytes in vivo and to epithelial cells grown in tissue culture. Transposon TnphoA mutants of E. coli O157:H7 strain CL-8 were produced. Two of 300 alkaline phosphatase positive mutants, designated JB6 and JB27, did not express O157 side chains as assessed by agglutination with specific polyclonal O157 antiserum, silver staining of lipopolysaccharide extracts separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and Western immunoblots with polyclonal O157-specific antiserum. Both O157-negative mutants and the parent strain demonstrated localized adherence to HEp-2 cells when examined by Giemsa staining and bright-field microscopy. Furthermore, both O157-negative mutants showed enhanced adherence to HEp-2 cells compared with the parent strain when assessed by quantification of adherent bacterial CFUs. The parent strain, CL-8, and both of the mutants produced fluorescent foci when adherent bacteria and HEp-2 cells were stained with fluorescein isothiocyanate-labelled phalloidin. Transmission electron microscopy confirmed attaching and effacing adherence of strain CL-8 and the OO7-negative mutants to HEp-2 cells. These findings indicate that mutants deficient in O157 polysaccharide repeats exhibit adherence to tissue culture cells in vitro and that O157 polysaccharide repeats are not required to produce the attaching and effacing lesion.

Analysis of the EHEC hly operon and its location in the physical map of the large plasmid of enterohaemorrhagic Escherichia coli O157:h7

Almost all clinical enterohaemorrhagic Escherichia coli (EHEC) O157:H7 isolates harbour a large virulence plasmid designated pO157. In this study, pO157 of EHEC O157:H7 reference strain EDL 933 was characterized at the molecular level. A restriction map was constructed by using seven restriction enzymes, with appropriate gel electrophoretic and hybridization methods. The molecular size of pO157 was determined to be 93.6 kb. By sequencing the DNA region extending in the 3'-direction of the previously described EHEC hlyC and hlyA genes, two further genes were discovered and analysed; these were termed EHEC hlyB and EHEC hlyD. The newly discovered genes together with the EHEC hlyC and hlyA genes constitute a typical RTX (Repeats in ToXin) determinant (EHEC hly operon) with the gene order CABD. The map position of the operon was determined by hybridization experiments. Analysis of a DNA fragment carrying the downstream flanking region of the EHEC hly operon revealed an open reading frame which was highly homologous to orf1 of RepFIB, a basic replicon of IncF plasmids. It was located close to the EHEC hly operon.

Signal transduction responses following adhesion of verocytotoxin-producing Escherichia coli

Attaching and effacing adhesion to epithelial cells is a pathognomonic feature of infection by both enteropathogenic Escherichia coli (EPEC) and certain strains of verocytotoxin-producing E. coli (VTEC). EPEC adhesion to tissue culture epithelial cells results in activation of the phosphatidylinositol pathway, with elevated levels of inositol 1,4,5-triphosphate and cytosolic free calcium. In this report, we show that VTEC also activate this signal transduction pathway in infected epithelial cells. Specifically, increased levels of inositol 1,4,5-triphosphate and intracellular free calcium were observed in HEp-2 cells infected with VTEC of serotype O157:H7. VTEC of serotypes O157:H7 and O113:H21 also induced increases in intracellular calcium levels in the human intestinal crypt-like cell line T84, even with minimal or no attaching and effacing activity as monitored by transmission electron microscopy. In contrast to EPEC, VTEC failed to induce tyrosine phosphorylation of epithelial cell proteins in HEp-2 and T84 cells, as determined by indirect immunofluorescence microscopy. These findings suggest that signal transduction responses to VTEC, including elevated levels of inositol triphosphates and intracellular free calcium, are independent of formation of the attaching and effacing lesion. Our findings also show that VTEC pathogenesis may involve signal transduction pathways that are distinct from those induced by EPEC infection.

Catabolite repression of the adhesion of Vero cytotoxin-producing Escherichia coli of serogroups O157 and O111

The virulence traits that mediate Vero cytotoxin-producing E. coli (VTEC) adherence are unclear. Many VTEC strains possess the eaeA gene which is involved in the attaching and effacing effects of enteropathogenic E. coli (EPEC). Most eae-positive VTEC adhered to HEp-2 cells in a localized manner; however some strains did not adhere. Thus we investigated the adhesion of poorly adherent strains, especially those of serogroups O111 and O157. To establish a model, the adherence to HEp-2, INT407 and Caco-2 cells of 12 O157 VTEC and six O111 VTEC isolated from cases of human infection were studied after growth of the bacteria under different conditions. For adhesion tests mannose is usually added during prior broth culture of the bacteria, and during the period of attachment, so that any adhesion due to mannose-sensitive type 1 pili is inhibited. Bacteria cultured in peptone water in the absence of mannose adhered to all three lines; there were localized clusters of bacteria on 1%-82% cells, whether mannose was present during the attachment period or not. Bacteria grown in the presence of D-mannose, or any other sugar that was metabolized, showed little adherence (range 0-9%). alpha-Methyl-glucoside also caused marked inhibition of adhesion. It was concluded that inhibition of adhesion was due to catabolite repression.

Some structures and processes of human epithelial cells involved in uptake of enterohemorrhagic Escherichia coli O157:H7 strains

Several enterohemorrhagic Escherichia coli (EHEC) strains of serotype O157:H7 isolated from patients with hemorrhagic colitis, ischemic colitis, or hemolytic uremic syndrome were all found to be able to invade certain human epithelial cell lines in vitro. Their ability to gain entry into epithelial cells was compared with those of known invasive Shigella flexneri and Salmonella typhi strains and the noninvasive E. coli strain HB101 in invasion assays utilizing gentamicin to kill extracellular bacteria. All EHEC strains under investigation were efficiently internalized into T24 bladder and HCT-8 ileocecal cells. In striking contrast to shigellae, the same EHEC strains were not taken up into human embryonic intestinal INT407 cells or HEp-2 cells any more than the noninvasive E. coli strain HB101. The mechanism(s) of EHEC internalization was characterized by comparing the invasion efficiencies in the absence and presence of a variety of inhibitors acting on structures and processes of prokaryotic or eukaryotic cells. Also, wild-type, plasmid-containing EHEC strains were compared with their plasmid-cured isogenic derivative strains to determine if plasmid genes affect invasion ability. Plasmid-cured EHEC invaded as well as wild-type EHEC, indicating that invasion ability is chromosomally encoded. Inhibition of bacterial protein synthesis by simultaneous addition of bacteria and chloramphenicol to the monolayer blocked EHEC uptake dramatically, suggesting the presence of an invasion protein(s) with a short half-life. Studies utilizing inhibitors which act on eukaryotic cells demonstrated a strong dependence on microfilaments in the process of uptake of all EHEC strains into both T24 and HCT-8 cells. In general, depolymerization of microtubules as well as inhibition of receptor-mediated endocytosis reduced the efficiency of EHEC invasion of T24 cells, whereas interference with endosome acidification reduced EHEC entry into only HCT-8 cells. Taxol-induced stabilization of microtubules did not inhibit internalization into T24 cells or into the HCT-8 cell line. In marked contrast, the ability of S. typhi Ty2 to invade either cell line was inhibited only by depolymerization of microfilaments. In addition to the cell line specificity of EHEC invasion, not all EHEC strains displayed uniform behavior in the presence of inhibitors, suggesting the existence of variant uptake pathways in different strains. Most importantly, previous reports of the inability of EHEC to invade INT407 or HEp-2 cell lines support the currently held belief that EHEC strains are noninvasive.(ABSTRACT TRUNCATED AT 400 WORDS)

Distinct binding properties of eaeA-negative verocytotoxin-producing Escherichia coli of serotype O113:H21

Infection of humans with verotoxin-producing Escherichia coli (VTEC) O113:H21 is associated with clinical features comparable to those associated with infection with attaching and effacing VTEC strains including those of serotype O157:H7. We have shown previously that the adhesion phenotype of VTEC O157:H7 is influenced by the presence of a homolog of the chromosomal eaeA (for E. coli attaching and effacing) gene. In contrast, by colony blot hybridization, VTEC O113:H21 is negative for the eaeA gene. Therefore, the aim of this study was to define the adhesion phenotype of VTEC O113:H21 strain CL-15 to both cultured epithelial cells (HEp-2) and rabbit intestine in vivo. Under transmission electron microscopy, areas of microvillus effacement were observed in regions directly beneath the organism in CL-15-infected cells both in vitro and in vivo. However, F-actin adhesion pedestals on the host plasma membrane were absent. Failure of CL-15 to induce polymerization of actin was confirmed by using staining of F-actin with fluorescein-labeled phalloidin. Under indirect immunofluorescence microscopy, CL-15-infected HEp-2 cells also failed to demonstrate the recruitment of another cytoskeletal element, alpha-actinin, below foci of bacterial adhesion. In contrast, VTEC O157:H7 infection of HEp-2 cells was associated with increased alpha-actinin immunofluorescence. These findings suggest that bacterial factors distinct from those of EaeA are necessary for the adhesion phenotype of VTEC O113:H21.

Signal transduction in human epithelial cells infected with attaching and effacing Escherichia coli in vitro

BACKGROUND/AIMS

Human enteropathogenic Escherichia coli infection of epithelial cells is characterized by attaching and effacing adhesion. To determine if signal transduction responses are involved in this adhesion phenotype, levels of inositol 1,4,5-triphosphate and cytosolic free calcium were measured in tissue culture cells infected with enteropathogenic E. coli strain E2348 (serotype O127:H6).

METHODS

Inositol triphosphate levels were measured by using a commercial binding assay, and intracellular calcium levels were determined by spectrofluorometry.

RESULTS

Elevated levels of both inositol triphosphate (182% +/- 52%; P < 0.05) and intracellular calcium (125% +/- 40%, mean +/- SE; P < 0.05) were seen after infection of HEp-2 cells with strain E2348. In contrast, inositol triphosphate and intracellular calcium levels were not elevated in HEp-2 cells infected with six E. coli strains that did not cause attaching and effacing lesions. Subcellular calcium localization using oxalate precipitation and electron microscopy showed calcium accumulation within the terminal web subjacent to regions of attaching and effacing adhesion. Depleting external calcium did not eliminate formation of attaching and effacing lesions, whereas treatment of HEp-2 cells with an intracellular calcium chelator prevented attaching and effacing lesions.

CONCLUSIONS

Enteropathogenic E. coli infection elevates both inositol triphosphate and intracellular calcium levels in cultured epithelial cells.

Multiple determinants of verotoxin-producing Escherichia coli O157:H7 attachment-effacement

Verotoxin-producing Escherichia coli strains of the serotype O157:H7 belong to a class of gastrointestinal pathogens that adhere to epithelial cells in a characteristic pattern known as attaching and effacing. Recent insight into the nature of E. coli O157:H7 adhesion was provided by the cloning and sequencing of the chromosomal eaeA (for E. coli attaching and effacing) gene homolog (G. Beebakhee, M. Louie, J. De Azavedo, and J. Brunton, FEMS Microbiol. Lett. 91:63-68, 1992, and J. Yu and J. B. Kaper, Mol. Microbiol. 6:411-417, 1992) and isolation of a 60-MDa plasmid referred to as pO157 (I. Toth, M. L. Cohen, H. S. Rumschlag, L. W. Riley, E. H. White, J. H. Carr, W. W. Bond, and I. K. Wachsmuth, Infect. Immun. 58:1223-1231, 1990, and S. Tzipori, H. Karch, K. I. Wachsmuth, R. M. Robins-Browne, A. D. O'Brien, H. Lior, M. L. Cohen, J. Smithers, and M. M. Levine, Infect. Immun. 55:3117-3125, 1987) and an approximately 94-kDa outer membrane protein (94-kDa OMP; P. Sherman, F. Cockerill III, R. Soni, and J. Brunton, Infect. Immun. 59:890-899, 1991). In this study, we examined the gene products of both eaeA and pO157 in relation to the 94-kDa OMP and as candidate effectors for O157:H7 attachment-effacement. Peptide sequencing and immunoassay demonstrated that the C. coli O157:H7 eaeA gene product is distinct from the 94-kDa OMP. Using ultrastructural analyses, we found that both parent and pO157 plasmid-cured O157:H7 strains demonstrated attaching and effacing adhesion to host epithelial cells and reacted equally well to rabbit antiserum raised against the 94-kDa OMP. By both transmission electron microscopy and light microscopy, E. coli HB101 transformed separately with the cloned eaeA gene and the pO157 plasmid did not form attaching and effacing lesions on cultured epithelial cells in vitro and rabbit intestinal tissues in vivo. Since additional determinants may mediate the attaching and effacing phenotype, we examined transposon TnphoA mutants constructed from E. coli O157:H7 strain CL8. Two TnphoA mutants were found deficient in bacterial factors that are necessary for O157:H7 attachment-effacement and likely distinct from the eaeA gene product.

Strains of Escherichia coli O157:H8 from human diarrhoea belong to attaching and effacing class of E coli

AIMS

To determine whether 17 Escherichia coli O157:H8 strains isolated from patients with diarrhoea in the United Kingdom were putative pathogens.

METHODS

The strains had been isolated by the use of O157 antiserum, available for the detection of Vero cytotoxin (VT) producing strains of E coli O157 that are usually of flagellar (H) type 7, but may also be non-motile. The strains were examined for VT production, for their ability to adhere to HEp-2 cells, and for hybridisation with several DNA probes that recognise pathogenic properties of E coli. Their ability to ferment sorbitol and to produce beta-glucuronidase was also investigated, as these tests are used to discriminate VT positive O157 strains.

RESULTS

The O157:H8 strains did not produce VT. All gave localised attachment to HEp-2 cells, associated with a positive fluorescence-actin staining test, and all hybridised with the E coli attaching and effacing (eae) probe. In addition to the difference in VT production, O157:H8 strains could be distinguished from VT positive O157 strains by their beta-glucuronidase activity, their failure to produce enterohaemolysin, and their lack of hybridisation with the CVD419 probe derived from a plasmid in an O157:H7 strain.

CONCLUSIONS

The 0157:H8 strains had in vitro properties characteristic of the class of E coli that causes attaching and effacing lesions in epithelial intestinal cells. They may therefore be considered a putative cause of diarrhoea but their prevalence remains to be established. Several O157:H8 strains failed to ferment sorbitol in agar plates and therefore could be misidentified as VT positive O157 strains. Confirmatory tests for VT production are needed when O157 strains are isolated from faeces.

Demonstration of exopolysaccharide production by enterohemorrhagicEscherichia coli

Enterohemorrhagic Escherichia coli O157:H7 produces visibly slimy colonies when grown on Sorbitol/MacConkey or Maloney's agar plates at room temperature, indicative of exopolysaccharide (EPS) production. Eighteen of 27 (67%) wild-type E. coli O157:H7 isolates produced enough EPS to be visually distinguishable. Of five strains that showed no visible EPS production on these media, four (80%) did produce slimy colonies on media containing higher salt concentrations. Measurements of EPS production by colorimetric determination of uronic acid indicated that EPS production was affected by growth temperature, atmosphere, and medium. Wild-type E. coli O157:H7 strain 932 produced the greatest amounts of EPS when grown anaerobically at 37 degrees C, whereas its plasmid-cured derivative 932P produced large quantities of EPS when grown aerobically at room temperature. Electron micrographs revealed thin, flexible fibers extending from the bacterial cell surface. Cells of strain 932P grown aerobically at room temperature were completely encased in a thick EPS matrix. Chemical analysis of purified EPS revealed that it is very similar or identical to colanic acid. E. coli O157:H7 adheres better to INT 407 cells when grown under conditions that favor high EPS production than when grown under conditions that repress EPS production.

Serotype O157:H7 Escherichia coli from bovine and meat sources

Serotype O157:H7 Escherichia coli strains from several different bovine and meat (beef) sources were studied to determine the diversity of their virulence properties and to compare their plasmid characteristics. Eighteen strains from cattle feces, 2 from water buffalo feces, 3 from beef samples, and 2 from feces of human hemolytic uremic syndrome cases were examined. All of these strains hybridized with the CVD419 DNA probe which identifies serotype O157:H7 and many other serotypes of verocytotoxin-producing E. coli. Of 15 bovine strains that hybridized with two verocytotoxin DNA probes, 8 hybridized with both verocytotoxin 1 (VT1) and VT2 probes, 5 hybridized with only the VT2 probe, and 2 hybridized with only the VT1 probe. This distribution was similar to that reported for O157:H7 E. coli isolated from humans. All three beef isolates hybridized with both VT1 and VT2 probes. All strains that hybridized with the VT probes were positive in the verocytotoxin assay, and all probe-negative strains were negative in the assay. All the strains possessed large plasmids with molecular sizes ranging from 53 to 64 MDa. Fifteen of the 20 cattle and water buffalo strains had one or more additional small plasmids. Restriction patterns resulting from HindIII, SmaI, and BamHI digestions of the large plasmids were used to compare all possible pairs of five different single plasmid-bearing strains from different countries (Egypt, England, and the United States). The restriction patterns of these strains were distinct, and the mean coefficients of similarity for these comparisons ranged from 71 to 91%, indicating a moderate degree of genetic diversity. This diversity and the presence of multiple plasmids in many bovine and human O157:H7 strains reinforce the usefulness of plasmid analysis in future studies. Only four of the 20 bovine strains and 1 of the 3 beef strains possessed the capability for adherence to HEp-2 and Intestine 407 cells in the presence of mannose, indicating that in vitro expression of localized adherence is not a universal property of O157:H7 strains of bovine origin.

Outer membranes are competitive inhibitors of Escherichia coli O157:H7 adherence to epithelial cells

Escherichia coli of serotype O157:H7 are Vero cytotoxin-producing enteric pathogens that have been associated recently with sporadic cases and outbreaks of hemorrhagic colitis and with the hemolytic-uremic syndrome. Adherence of many enteropathogenic bacteria to mucosal surfaces is a critical step in the pathogenesis of diarrheal disease. We showed previously that adherence of E. coli O157:H7 strain CL-56 to epithelial cells in vitro is inhibited by outer membranes. In this study we examined whether outer membranes from a series of E. coli O157:H7 strains mediated competitive inhibition of bacterial binding to epithelial cells grown in tissue culture. We also determined which constituents of the outer membrane mediated inhibition of CL-56 adherence. Binding of six O157:H7 strains to HEp-2 cells was determined by quantitating the number of adherent bacteria in the presence and absence of outer membranes which were extracted from each strain with N-lauroyl sarcosinate (1.7%, wt/vol). After separation of outer membranes by gel electrophoresis, four bands (94, 40, 36, and 30 kDa) were collected by electroelution. Immune sera were raised in rabbits to each of the four eluted bands. Outer membrane extracts from each of the six O157:H7 strains inhibited binding of homologous organisms to the HEp-2 cells. At dilutions which did not cause bacterial agglutination, antiserum raised against the 94-kDa outer membrane protein showed maximal inhibition of bacterial adherence (17.0 +/- 7.3% adherence of control levels). Growth of bacteria in iron-depleted broth did not affect their binding to HEp-2 cells, suggesting that iron-regulated outer membranes were not involved. Fluid accumulation in ileal ligated loops of rabbits in response to E. coli O157:H7 challenge was diminished following both parenteral immunization with outer membranes extracted from the homologous strain and coincubation of organisms with immune serum which contained antibodies to outer membrane extracts. These data indicate that outer membranes are competitive inhibitors of E. coli O157:H7 adherence. Specific constituents of the outer membrane may function as bacterial attachment factors (i.e., adhesins) for E. coli O157:H7 adherence to epithelial cell surfaces.

In vitro surface properties of the newly recognized gastric pathogen Helicobacter pylori

There appears to be a particular association between Helicobacter pylori and the gastric antrum, but the mechanisms by which the organism adheres to and colonizes the gastric mucosa are unclear. Surface hydrophobicity and surface charge mediate the adherence of other bacterial pathogens to mucosal epithelial cell surfaces. Therefore, in this study we characterized both the surface hydrophobicity and the surface charge of 10 H. pylori strains grown in broth culture. Four complementary methods were used to determine hydrophobicity: hydrophobic interaction chromatography, the salt aggregation test, comparison of bacterial adherence to polystyrene with adherence to sulfonated polystyrene, and measurement of contact angle with droplets of water. Three of the methods (salt aggregation test, adherence to polystyrene, and contact angles) indicated that each of the 10 strains expressed a relatively hydrophilic cell surface. In contrast, hydrophobic interaction chromatography determinations with both phenyl- and octyl-Sepharose suggested that the H. pylori strains were relatively hydrophobic. However, tetramethyl urea (0.4 M) did not reduce the binding of H. pylori to phenyl-Sepharose columns. DEAE-cellulose ion-exchange chromatography showed that each of the 10 strains of H. pylori had a surface which, overall, was highly negatively charged. We conclude that H. pylori expresses an overall relatively hydrophilic and negatively charged surface in vitro.

Mouse model for colonization and disease caused by enterohemorrhagic Escherichia coli O157:H7

Enterohemorrhagic Escherichia coli O157:H7 isolates produce Shiga-like toxins and carry a 60-megadalton plasmid which encodes an adhesin for Henle 407 intestinal cells. A streptomycin-treated mouse model was used to compare the intestinal colonizing capacity of E. coli O157:H7 strain 933 with that of its 60-megadalton plasmid-cured derivative, strain 933cu. When fed individually to mice, both 933 and 933cu maintained a stable number of organisms per gram of feces, and the greatest numbers of 933 or 933cu were isolated from cecal and proximal colonic epithelial cells. When 933 and 933cu were simultaneously fed to mice, 933cu was unable to maintain a stable level of colonization in about two-thirds of the mice tested. However, in one-third of the mice, the number of 933cu in feces began to increase rapidly until a stable level of co-colonization with 933 was attained. The isolate from these mice, 933cu-rev, was excreted in high numbers when fed alone to mice and was found on epithelial cells throughout the entire large bowel and distal small intestine. Moreover, 933cu-rev grew in mucus from all segments of the intestine and at higher levels than strain 933 or 933cu. Only mice fed strain 933cu-rev died. Histopathological studies confirmed that mice fed 933cu-rev died from bilateral renal cortical tubular necrosis consistent with toxic insult, perhaps due to Shiga-like toxins. The virulence of 933cu-rev may reflect its ability to grow well in mucus and colonize the small as well as large bowel.