Enteropathogenic Escherichia coli O103 from rabbit elicits actin stress fibers and focal adhesions in HeLa epithelial cells, cytopathic effects that are linked to an analog of the locus of enterocyte effacement

Contribution of pks+ Escherichia coli (E. coli) to Colon Carcinogenesis

Colorectal cancer (CRC) stands as a significant global health concern, ranking second in mortality and third in frequency among cancers worldwide. While only a small fraction of CRC cases can be attributed to inherited genetic mutations, the majority arise sporadically due to somatic mutations. Emerging evidence reveals gut microbiota dysbiosis to be a contributing factor, wherein polyketide synthase-positive Escherichia coli (pks+ E. coli) plays a pivotal role in CRC pathogenesis. pks+ bacteria produce colibactin, a genotoxic protein that causes deleterious effects on DNA within host colonocytes. In this review, we examine the role of the gut microbiota in colon carcinogenesis, elucidating how colibactin-producer bacteria induce DNA damage, promote genomic instability, disrupt the gut epithelial barrier, induce mucosal inflammation, modulate host immune responses, and influence cell cycle dynamics. Collectively, these actions foster a microenvironment conducive to tumor initiation and progression. Understanding the mechanisms underlying pks+ bacteria-mediated CRC development may pave the way for mass screening, early detection of tumors, and therapeutic strategies such as microbiota modulation, bacteria-targeted therapy, checkpoint inhibition of colibactin production and immunomodulatory pathways.

Prevalence of Cyclomodulin-Positive E. coli and Klebsiella spp. Strains in Mexican Patients with Colon Diseases and Antimicrobial Resistance

Colon diseases, such as colorectal cancer (CRC), are multifactor diseases that affect more than one million people per year; recently, the microbiota has been associated with an etiologic factor, specifically bacterial cyclomodulin positivity (CM⁺). Unfortunately, there are no studies from Mexico that detail the presence of bacterial CM⁺ in patients with colon diseases. We therefore performed a comprehensive study to investigate the associations and prevalence of cyclomodulin-positive Diarrheagenic E. coli (DEC), non-DEC, and Klebsiella spp. strains isolated from Mexican subjects with colon diseases. In this work, we analyzed 43 biopsies, 87 different bacteria were isolated, and E. coli was the most frequently noted, followed by Klebsiella spp., and Enterococcus spp. E. coli, non-DEC, and EPEC belonging to phylogroup B2 were the most prevalent. More than 80% of E. coli and Klebsiella were CM⁺. pks, cdt, cnf, and cif were identified. cdt was associated with non-DEC, cif and its combinations with EPEC, as well as cdt and psk with Klebsiella. Lastly, all the CM⁺ bacteria were resistant to at least one antibiotic (34% were MDR, and 48% XDR). In conclusion, the high prevalence of bacterial CM⁺ in colon disease patients suggests that these bacteria play an important role in the genesis of these diseases.

Reversal of Pathogen-Induced Barrier Defects in Intestinal Epithelial Cells by Contra-pathogenicity Agents

BACKGROUND

Environmental enteropathy (EE) is associated with stunting, impairment of responses to oral vaccines, and other adverse health consequences in young children throughout the developing world. EE is characterized by chronic low-grade intestinal inflammation and disrupted epithelial barrier integrity, partly resulting from dysregulation of tight junction proteins, observed in other enteropathies such as celiac disease. During EE, this dysregulation of tight junction expression amplifies translocation of pathogenic bacteria across the intestinal mucosa.

AIMS

The aim was to determine whether enteropathogen-mediated epithelial barrier failure can be ameliorated using contra-pathogenicity therapies.

METHODS

Intestinal epithelial barrier damage was assessed in Caco-2 cells incubated with three important enteropathogens identified in EE patients: Enteropathogenic Escherichia coli (EPEC), Citrobacter rodentium (C. rodentium), and Cryptosporidium parvum (C. parvum). Potential therapeutic molecules were tested to detect effects on transepithelial resistance (TER), bacterial translocation (BT), claudin-4 expression, and regulation of the inflammatory cytokine response.

RESULTS

All three enteropathogens compared to uninfected cells, reduced TER (EPEC; p < 0.0001, C. rodentium; p < 0.0001, C. parvum; p < 0.0007), reduced claudin-4 expression, and permitted BT (EPEC; p < 0.0001, C. rodentium; p < 0.0001, C. parvum; p < 0.0003) through the monolayer. Zinc, colostrum, epidermal growth factor, trefoil factor 3, resistin-like molecule-β, hydrocortisone, and the myosin light chain kinase inhibitor ML7 (Hexahydro-1-[(5-iodo-1-naphthalenyl)sulfonyl]-1H-1,4-diazepine hydrochloride); ML7) improved TER (up to 70%) and decreased BT (as much as 96%). Only zinc demonstrated modest antimicrobial activity.

CONCLUSION

The enteropathogens impaired intestinal-epithelial barrier integrity with dysregulation of claudin-4 and increased bacterial translocation. Enteropathogen-mediated damage was reduced using contra-pathogenicity agents which mitigated the effects of pathogens without direct antimicrobial activity.

Antitumor Effect of Cycle Inhibiting Factor Expression in Colon Cancer via Salmonella VNP20009

BACKGROUND

Colon cancer is one of the major causes of morbidity and mortality worldwide. Cycle inhibiting factors (Cifs) have been shown to deamidate Nedd8, resulting in cell cycle arrest.

OBJECTIVE

To determine the antitumor effect of Cifs on colon cancer by using attenuated Salmonella typhimurium VNP20009.

METHODS

The VNP-SOPE2-cif and VNP-SOPE2-cif-C/A plasmids were transfected into attenuated Salmonella typhimurium VNP20009. The efficiency and specificity of the Cif promoter were validated in colon cancer SW480 cell lines. Western blotting was subsequently performed to evaluate cell cycle regulators, including P21, P27 and Wee1. In vivo, the antitumor effect of VNP20009 was evaluated in a colon cancer xenograft model.

RESULTS

Firstly, VNP-SOPE2-cif and VNP-SOPE2-cif-C/A were selectively expressed both in the bacterial and colon cancer cells. Cif expression in SW480 cells via the VNP tumor-targeted expression system induced the accumulation of Wee1, p21 and p27 expression. Moreover, tumor growth was significantly inhibited in the mice with VNP-SOPE2-cif compared to the mice with VNP with the empty construct.

CONCLUSION

These results suggest that Cif gene delivered by VNP20009 is a promising approach for the treatment of colon cancer.

Enteric pathogens deploy cell cycle inhibiting factors to block the bactericidal activity of Perforin-2

Perforin-2 (MPEG1) is an effector of the innate immune system that limits the proliferation and spread of medically relevant Gram-negative, -positive, and acid fast bacteria. We show here that a cullin-RING E3 ubiquitin ligase (CRL) complex containing cullin-1 and βTrCP monoubiquitylates Perforin-2 in response to pathogen associated molecular patterns such as LPS. Ubiquitylation triggers a rapid redistribution of Perforin-2 and is essential for its bactericidal activity. Enteric pathogens such as Yersinia pseudotuberculosis and enteropathogenic Escherichia coli disarm host cells by injecting cell cycle inhibiting factors (Cifs) into mammalian cells to deamidate the ubiquitin-like protein NEDD8. Because CRL activity is dependent upon NEDD8, Cif blocks ubiquitin dependent trafficking of Perforin-2 and thus, its bactericidal activity. Collectively, these studies further underscore the biological significance of Perforin-2 and elucidate critical molecular events that culminate in Perforin-2-dependent killing of both intracellular and extracellular, cell-adherent bacteria.

DOI:http://dx.doi.org/10.7554/eLife.06505.001

A wide range of bacteria and other microbes can infect animals and cause disease. Throughout evolution, these microbes and their hosts have been fighting never ending arms races in which the microbes deploy ever more elaborate weapons, while the hosts adapt to defend themselves. An animal's first line of defense is provided by its ‘innate’ immune system. This system is activated by the general features of microbial cells; for example, the molecules that make up the walls surrounding most bacteria. Microbes must defeat the innate immune system in order to cause disease, and ultimately to spread from one host to the next.

One component of innate immunity is a protein called Perforin-2 that is present in most, if not all, animal cells. This protein forms pores on bacterial cells, causing them to split open and die. However, it was not clear how Perforin-2 is switched on and what, if anything, bacteria do to counteract it. To address these questions, McCormack et al. infected human and mice cells with bacteria that cause serious diseases of the digestive tract.

The experiments show that when animal cells detect bacteria, or merely a fragment of their cell wall, a specific group of proteins, called the CRL complex, attaches a molecule called ubiquitin to Perforin-2. Ubiquitin works much like the shipping label of a package, enabling the efficient targeting of Perforin-2 to the invading bacteria. McCormack et al. also show that some bacteria use a protein called a cell cycle inhibiting factor (or Cif for short) to inhibit the CRL complex. This blocks the ubiquitin labeling of Perforin-2, which renders it a useless weapon that can no longer be directed towards bacteria.

Mice that are infected with a bacterium called Yersinia pseudotuberculosis become seriously unwell and often die. However, McCormack et al. found that mice infected with mutant Y. pseudotuberculosis that lacked Cif remained healthy. Also, mice that lacked Perforin-2 are highly susceptible to infectious diseases. McCormack et al.'s findings reveal how Perforin-2 is activated during the innate immune response and how some bacteria can defeat this pivotal defense. In the current age of antibiotic resistant bacteria, these studies may spur the development of new drugs that restore or increase the activity of Perforin-2.

DOI:http://dx.doi.org/10.7554/eLife.06505.002

The effect of enterohemorrhagic E. coli infection on the cell mechanics of host cells

Enterohaemorrhagic E. coli (EHEC) is a type of human pathogenic bacteria. The main virulence characteristics of EHEC include the formation of attaching and effacing lesions (A/E lesions) and the production of one or more Shiga-like toxins, which may induce human uremic complications. When EHEC infects host cells, it releases translocated intimin receptor (Tir) and effector proteins inside the host cells, inducing the rearrangement and accumulation of the F-actin cytoskeleton, a phenotype leading to the formation of pedestals in the apical cell surface, and the growth of stress fibers at the base of the cells. To examine the effect of EHEC infection on cell mechanics, we carried out a series of experiments to examine HeLa cells with and without EHEC infection to quantify the changes in (1) focal adhesion area, visualized by anti-vinculin staining; (2) the distribution and orientation of stress fibers; and (3) the intracellular viscoelasticity, via directional video particle tracking microrheology. Our results indicated that in EHEC-infected HeLa cells, the focal adhesion area increased and the actin stress fibers became thicker and more aligned. The cytoskeletal reorganization induced by EHEC infection mediated a dramatic increase in the cytoplasmic elastic shear modulus of the infected cells, and a transition in the viscoelastic behavior of the cells from viscous-like to elastic-like. These changes in mechanobiological characteristics might modulate the attachments between EHEC and the host cell to withstand exfoliation, and between the host cell and the extracellular matrix, and might also alter epithelial integrity.

The cyclomodulin cycle inhibiting factor (CIF) alters cullin neddylation dynamics

The bacterial effector protein cycle inhibiting factor (CIF) converts glutamine 40 of NEDD8 to glutamate (Q40E), causing cytopathic effects and inhibiting cell proliferation. Although these have been attributed to blocking the functions of cullin-RING ubiquitin ligases, how CIF modulates NEDD8-dependent signaling is unclear. Here we use conditional NEDD8-dependent yeast to explore the effects of CIF on cullin neddylation. Although CIF causes cullin deneddylation and the generation of free NEDD8 Q40E, inhibiting the COP9 signalosome (CSN) allows Q40E to form only on NEDD8 attached to cullins. In the presence of the CSN, NEDD8 Q40E is removed from cullins more rapidly than NEDD8, leading to a decrease in steady-state cullin neddylation. As NEDD8 Q40E is competent for cullin conjugation in the absence of functional CSN and with overexpression of the NEDD8 ligase Dcn1, our data are consistent with NEDD8 deamidation causing enhanced deneddylation of cullins by the CSN. This leads to a dramatic change in the extent of activated cullin-RING ubiquitin ligases.

Mutations that impact the enteropathogenic Escherichia coli Cpx envelope stress response attenuate virulence in Galleria mellonella

In this paper, we show that the larvae of the greater wax moth, Galleria mellonella, can be used as a model to study enteropathogenic Escherichia coli (EPEC) virulence. G. mellonella larvae are killed after infection with EPEC type strain E2348/69 but not by an attenuated derivative that expresses diminished levels of the major virulence determinants or by a mutant specifically defective in type III secretion (T3S). Infecting EPEC inhabit the larval hemocoel only briefly and then become localized to melanized capsules, where they remain extracellular. Previously, it was shown that mutations affecting the Cpx envelope stress response lead to diminished expression of the bundle-forming pilus (BFP) and the type III secretion system (T3SS). We demonstrate that mutations that activate the Cpx pathway have a dramatic effect on the ability of the bacterium to establish a lethal infection, and this is correlated with an inability to grow in vivo. Infection with all E. coli strains led to increased expression of the antimicrobial peptides (AMPs) gloverin and cecropin, although strain- and AMP-specific differences were observed, suggesting that the G. mellonella host perceives attenuated strains and Cpx mutants in unique manners. Overall, this study shows that G. mellonella is an economical, alternative infection model for the preliminary study of EPEC host-pathogen interactions, and that induction of the Cpx envelope stress response leads to defects in virulence.

Directed evaluation of enterotoxigenic Escherichia coli autotransporter proteins as putative vaccine candidates

Background

Enterotoxigenic Escherichia coli (ETEC) is a major diarrheal pathogen in developing countries, where it accounts for millions of infections and hundreds of thousands of deaths annually. While vaccine development to prevent diarrheal illness due to ETEC is feasible, extensive effort is needed to identify conserved antigenic targets. Pathogenic Escherichia coli, including ETEC, use the autotransporter (AT) secretion mechanism to export virulence factors. AT proteins are comprised of a highly conserved carboxy terminal outer membrane beta barrel and a surface-exposed amino terminal passenger domain. Recent immunoproteomic studies suggesting that multiple autotransporter passenger domains are recognized during ETEC infection prompted the present studies.

Methodology

Available ETEC genomes were examined to identify AT coding sequences present in pathogenic isolates, but not in the commensal E. coli HS strain. Passenger domains of the corresponding autotransporters were cloned and expressed as recombinant antigens, and the immune response to these proteins was then examined using convalescent sera from patients and experimentally infected mice.

Principal Findings

Potential AT genes shared by ETEC strains, but absent in the E. coli commensal HS strain were identified. Recombinant passenger domains derived from autotransporters, including Ag43 and an AT designated pAT, were recognized by antibodies from mice following intestinal challenge with H10407, and both Ag43 and pAT were identified on the surface of ETEC by flow cytometry. Likewise, convalescent sera from patients with ETEC diarrhea recognized Ag43 and pAT, suggesting that these proteins are expressed during both experimental and naturally occurring ETEC infections and that they are immunogenic. Vaccination of mice with recombinant passenger domains from either pAT or Ag43 afforded protection against intestinal colonization with ETEC.

Conclusions

Passenger domains of conserved autotransporter proteins could contribute to protective immune responses that develop following infection with ETEC, and these antigens consequently represent potential targets to explore in vaccine development.

Diarrheal diseases are responsible for more than 1.5 million deaths annually in developing countries. Enterotoxigenic E. coli (ETEC) are among the most common bacterial causes of diarrhea, accounting for an estimated 300,000–500,000 deaths each year, mostly in young children. There unfortunately is not yet a vaccine that can offer sustained, broad-based protection against ETEC. While most vaccine development effort has focused on plasmid-encoded finger-like ETEC adhesin structures known as colonization factors, additional effort is needed to identify conserved target antigens. Epidemiologic studies suggest that immune responses to uncharacterized, chromosomally encoded antigens could contribute to protection resulting from repeated infections. Earlier studies of immune responses to ETEC infection had identified a class of surface-expressed molecules known as autotransporters (AT). Therefore, available ETEC genome sequences were examined to identify conserved ETEC autotransporters not shared by the commensal E. coli HS strain, followed by studies of the immune response to these antigens, and tests of their utility as vaccine components. Two chromosomally encoded ATs, identified in ETEC, but not in HS, were found to be immunogenic and protective in an animal model, suggesting that conserved AT molecules contribute to protective immune responses that follow natural ETEC infection and offering new potential targets for vaccines.

Cycle inhibiting factors (cifs): cyclomodulins that usurp the ubiquitin-dependent degradation pathway of host cells

Cycle inhibiting factors (Cifs) are type III secreted effectors produced by diverse pathogenic bacteria. Cifs are "cyclomodulins" that inhibit the eukaryotic host cell cycle and also hijack other key cellular processes such as those controlling the actin network and apoptosis. This review summarizes current knowledge on Cif since its first characterization in enteropathogenic Escherichia coli, the identification of several xenologues in distant pathogenic bacteria, to its structure elucidation and the recent deciphering of its mode of action. Cif impairs the host ubiquitin proteasome system through deamidation of ubiquitin or the ubiquitin-like protein NEDD8 that regulates Cullin-Ring-ubiquitin Ligase (CRL) complexes. The hijacking of the ubiquitin-dependent degradation pathway of host cells results in the modulation of various cellular functions such as epithelium renewal, apoptosis and immune response. Cif is therefore a powerful weapon in the continuous arm race that characterizes host-bacteria interactions.

Pathogenic bacteria target NEDD8-conjugated cullins to hijack host-cell signaling pathways

The cycle inhibiting factors (Cif), produced by pathogenic bacteria isolated from vertebrates and invertebrates, belong to a family of molecules called cyclomodulins that interfere with the eukaryotic cell cycle. Cif blocks the cell cycle at both the G₁/S and G₂/M transitions by inducing the stabilization of cyclin-dependent kinase inhibitors p21^waf1 and p27^kip1. Using yeast two-hybrid screens, we identified the ubiquitin-like protein NEDD8 as a target of Cif. Cif co-compartmentalized with NEDD8 in the host cell nucleus and induced accumulation of NEDD8-conjugated cullins. This accumulation occurred early after cell infection and correlated with that of p21 and p27. Co-immunoprecipitation revealed that Cif interacted with cullin-RING ubiquitin ligase complexes (CRLs) through binding with the neddylated forms of cullins 1, 2, 3, 4A and 4B subunits of CRL. Using an in vitro ubiquitylation assay, we demonstrate that Cif directly inhibits the neddylated CUL1-associated ubiquitin ligase activity. Consistent with this inhibition and the interaction of Cif with several neddylated cullins, we further observed that Cif modulates the cellular half-lives of various CRL targets, which might contribute to the pathogenic potential of diverse bacteria.

Among the arsenal of virulence factors used by bacterial pathogens to infect and manipulate their hosts, cyclomodulins are a growing family of bacterial toxins that interfere with the eukaryotic cell-cycle. Cif is one of these cyclomodulins produced by both mammalian and invertebrate pathogenic bacteria. Cif blocks the host cell cycle by inducing the accumulation of two regulators of cell cycle progression: the cyclin-dependent kinase inhibitors p21 and p27. To decipher the mode of action of Cif, we performed yeast two-hybrid screenings. We show that Cif binds to NEDD8 and induce accumulation of neddylated cullins early after infection. Cullins are scaffold components of cullin-RING ubiquitin ligases (CRLs), which ubiquitinate proteins and target them for degradation by the 26S proteasome. We demonstrate that Cif directly inhibits the ubiquitin ligase activity of these CRLs and consequently the targeting of p21 and p27 for ubiquitin-dependent degradation. Targeting at NEDD8 represents a novel strategy for modulation of host cell functions by bacterial pathogens. By inhibiting the most prominent class of ubiquitin-ligases, Cif controls the stability of a cohort of key regulators and impinge on not only cell cycle progression but also on many cellular and biological processes such as immunity, development, transcription, and cell signaling.

Simultaneous inactivation of espB and tir abrogates the strong, but non-protective, inflammatory response induced by EPEC

Enteropathogenic Escherichia coli (EPEC) belong to the attaching and effacing (A/E) family of bacterial pathogens that represent a worldwide health concern. These non-invasive bacteria attach to intestinal enterocytes through a type III secretion system (T3SS), leading to intestinal inflammation and severe diarrhea. To dissect the signals leading to the induction of the inflammatory response and to understand its role in the pathogenesis of infection, we used the rabbit model, which represents a close model of human infections. Rabbits were orally inoculated with either the wild type O103:K-:H2 E22 EPEC strain or with the E22Δtir/espB strain, which bears mutations in two genes involved in the injectisome structure and function. To monitor the development of the inflammatory response, we developed a quantitative real-time RT-PCR (qPCR) assay specific for a panel of rabbit genes. Using combined immunohistochemistry and qPCR, we show here that the inflammatory response triggered by wild type EPEC occurs very early, preceding the bacterial colonization of the epithelium. However, this early response is unable to prevent bacterial attachment on enterocytes. Moreover, our results show that expression of a complete bacterial injectisome is required for the development of inflammation. Finally, infection by the virulent strain, but not by the doubly mutated strain, rapidly induces the development of a specific immune response in the mesenteric lymph nodes, which is not associated with protection. Our findings suggest that the induction of a strong inflammatory response by T3SS dependent components represents a selective advantage for T3SS+ bacteria, thereby facilitating their colonization.

Cif type III effector protein: a smart hijacker of the host cell cycle

During coevolution with their hosts, bacteria have developed functions that allow them to interfere with the mechanisms controlling the proliferation of eukaryotic cells. Cycle inhibiting factor (Cif) is one of these cyclomodulins, the family of bacterial effectors that interfere with the host cell cycle. Acquired early during evolution by bacteria isolated from vertebrates and invertebrates, Cif is an effector protein of type III secretion machineries. Cif blocks the host cell cycle in G1 and G2 by inducing the accumulation of the cyclin-dependent kinase inhibitors p21(waf1/cip1) and p27(kip1). The x-ray crystal structure of Cif reveals it to be a divergent member of a superfamily of enzymes including cysteine proteases and acetyltransferases. This review summarizes and discusses what we know about Cif, from the bacterial gene to the host target.

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 of rabbits against enteropathogenic Escherichia coli (EPEC) using an intimin null mutant

Background

Diarrhea and mortality resulting from infections with enteropathogenic Escherichia coli (EPEC) are of major economic importance in the rabbit meat industry. There is a growing need for an effective vaccine to cope with these problems and to reduce the use of antibiotics. EPEC are characterized by an attaching and effacing virulence mechanism. This is partly mediated by the intimate binding between an adhesin, called intimin, and a translocated receptor (Tir) of prokaryote origin. We constructed an intimin deletion mutant of the rabbit EPEC (REPEC) wild-type strain 97/241.6 (bio-/serogroup 3-/O15) and examined its protective capacity.

Results

After verifying its complete loss of virulence, we used the attenuated strain in vaccination-challenge experiments in which complete protection against a homologous, but virulent, strain was observed. The attenuated strain was able to persist in the intestinal lumen, where it elicited an immune response against EPEC-related virulence proteins, as was shown using an EspB-specific ELISA. Despite the priming of an immune response and the generation of specific antibodies, the intimin mutant was not able to fully protect rabbits against challenges with REPEC strains of other bio-/serogroups.

Conclusion

These data indicate that protection against REPEC infections is at least partly bio-/serogroup dependent and a multivalent vaccine may be needed for protection against the full range of REPEC types. Such a combination vaccine may be developed using intimin null mutants, as the latter were clearly shown to be safe and effective against homologous infections.

Attaching-effacing bacteria in animals

Enteric bacteria with a demonstrable or potential ability to form attaching-effacing lesions, so-called attaching-effacing (AE) bacteria, have been found in the intestinal tracts of a wide variety of warm-blooded animal species, including man. In some host species, for example cattle, pigs, rabbits and human beings, attaching-effacing Escherichia coli (AEEC) have an established role as enteropathogens. In other host species, AE bacteria are of less certain significance. With continuing advances in the detection and typing of AE strains, the importance of these bacteria for many hosts is likely to become clearer. The pathogenic effects of AE bacteria result from adhesion to the intestinal mucosa by a variety of mechanisms, culminating in the formation of the characteristic intimate adhesion of the AE lesion. The ability to induce AE lesions is mediated by the co-ordinated expression of some 40 bacterial genes organized within a so-called pathogenicity island, known as the "Locus for Enterocyte Effacement". It is also believed that the production of bacterial toxins, principally Vero toxins, is a significant virulence factor for some AEEC strains. Recent areas of research into AE bacteria include: the use of Citrobacter rodentium to model human AEEC disease; quorum-sensing mechanisms used by AEEC to modulate virulence gene expression; and the potential role of adhesion in the persistent colonization of the intestine by AE bacteria. This review of AE bacteria covers their molecular biology, their occurrence in various animal species, and the diagnosis, pathology and clinical aspects of animal diseases with which they are associated. Reference is made to human pathogens where appropriate. The focus is mainly on natural colonization and disease, but complementary experimental data are also included.

Cyclomodulins: bacterial effectors that modulate the eukaryotic cell cycle

Microbial pathogens have developed a variety of strategies to manipulate host-cell functions, presumably for their own benefit. We propose the term "cyclomodulins" to describe the growing family of bacterial toxins and effectors that interfere with the eukaryotic cell cycle. Inhibitory cyclomodulins, such as cytolethal distending toxins (CDTs) and the cycle inhibiting factor (Cif), block mitosis and might constitute powerful weapons for immune evasion by inhibiting clonal expansion of lymphocytes. Cell-cycle inhibitors might also impair epithelial-barrier integrity, allowing the entry of pathogenic bacteria into the body or prolonging their local existence by blocking the shedding of epithelia. Conversely, cyclomodulins that promote cellular proliferation, such as the cytotoxic necrotizing factor (CNF), exemplify another subversion mechanism by interfering with pathways of cell differentiation and development. The role of these cyclomodulins in bacterial virulence and carcinogenesis awaits further study and will delineate new perspectives in basic research and therapeutic applications.

Predominance of afr2 and ral fimbrial genes related to those encoding the K88 and CS31A fimbrial adhesins in enteropathogenic Escherichia coli isolates from rabbits with postweaning diarrhea in Central Europe

PCR tests designed in these studies identified three rabbit adhesive factor genes among 43 enteropathogenic E. coli (EPEC) strains: afr1 (2 strains), the F4(K88)/CS31A-related afr2 (10 strains), and ral (15 strains). Several EPEC strains (i.e., O153:H7 and O157:H2) lacked these genes but did adhere to HeLa cells and produced attaching and effacing lesions in rabbits.

Identification of the secretion and translocation domain of the enteropathogenic and enterohemorrhagic Escherichia coli effector Cif, using TEM-1 beta-lactamase as a new fluorescence-based reporter

Enteropathogenic and enterohemorrhagic Escherichia coli (EPEC and EHEC) strains are human and animal pathogens that inject effector proteins into host cells via a type III secretion system (TTSS). Cif is an effector protein which induces host cell cycle arrest and reorganization of the actin cytoskeleton. Cif is encoded by a lambdoid prophage present in most of the EPEC and EHEC strains. In this study, we analyzed the domain that targets Cif to the TTSS by using a new reporter system based on a translational fusion of the effector proteins with mature TEM-1 beta-lactamase. Translocation was detected directly in living host cells by using the fluorescent beta-lactamase substrate CCF2/AM. We show that the first 16 amino acids (aa) of Cif were necessary and sufficient to mediate translocation into the host cells. Similarly, the first 20 aa of the effector proteins Map, EspF, and Tir, which are encoded in the same region as the TTSS, mediated secretion and translocation in a type III-dependent but chaperone-independent manner. A truncated form of Cif lacking its first 20 aa was no longer secreted and translocated, but fusion with the first 20 aa of Tir, Map, or EspF restored both secretion and translocation. In addition, the chimeric proteins were fully able to trigger host cell cycle arrest and stress fiber formation. In conclusion, our results demonstrate that Cif is composed of a C-terminal effector domain and an exchangeable N-terminal translocation signal and that the TEM-1 reporter system is a convenient tool for the study of the translocation of toxins or effector proteins into host cells.

Identification of the secretion and translocation domain of the enteropathogenic and enterohemorrhagic Escherichia coli effector Cif, using TEM-1 beta-lactamase as a new fluorescence-based reporter

Enteropathogenic and enterohemorrhagic Escherichia coli (EPEC and EHEC) strains are human and animal pathogens that inject effector proteins into host cells via a type III secretion system (TTSS). Cif is an effector protein which induces host cell cycle arrest and reorganization of the actin cytoskeleton. Cif is encoded by a lambdoid prophage present in most of the EPEC and EHEC strains. In this study, we analyzed the domain that targets Cif to the TTSS by using a new reporter system based on a translational fusion of the effector proteins with mature TEM-1 beta-lactamase. Translocation was detected directly in living host cells by using the fluorescent beta-lactamase substrate CCF2/AM. We show that the first 16 amino acids (aa) of Cif were necessary and sufficient to mediate translocation into the host cells. Similarly, the first 20 aa of the effector proteins Map, EspF, and Tir, which are encoded in the same region as the TTSS, mediated secretion and translocation in a type III-dependent but chaperone-independent manner. A truncated form of Cif lacking its first 20 aa was no longer secreted and translocated, but fusion with the first 20 aa of Tir, Map, or EspF restored both secretion and translocation. In addition, the chimeric proteins were fully able to trigger host cell cycle arrest and stress fiber formation. In conclusion, our results demonstrate that Cif is composed of a C-terminal effector domain and an exchangeable N-terminal translocation signal and that the TEM-1 reporter system is a convenient tool for the study of the translocation of toxins or effector proteins into host cells.

Enteropathogenic and enterohaemorrhagic Escherichia coli deliver a novel effector called Cif, which blocks cell cycle G2/M transition

Enteropathogenic Escherichia coli (EPEC) and enterohaemorrhagic E. coli (EHEC) are closely related pathogens. Both use a type III secretion system (TTSS) encoded by the 'locus of enterocyte effacement' (LEE) to subvert and attach to epithelial cells through the injection of a repertoire of effector molecules. Here, we report the identification of a new TTSS translocated effector molecule called Cif, which blocks cell cycle G2/M transition and induces the formation of stress fibres through the recruitment of focal adhesions. Cif is not encoded by the LEE but by a lambdoid prophage present in EPEC and EHEC. A cif mutant causes localized effacement of microvilli and intimately attaches to the host cell surface, but is defective in the ability to block mitosis. When expressed in TTSS competent LEE-positive pathogens, Cif is injected into the infected epithelial cells. These cells arrested at the G2/M phase displayed accumulation of inactive phosphorylated Cdk1. In conclusion, Cif is a new member of a growing family of bacterial cyclomodulins that subvert the host eukaryotic cell cycle.

Genetically engineered enteropathogenic Escherichia coli strain elicits a specific immune response and protects against a virulent challenge

Enteropathogenic Escherichia coli (EPEC), a major cause of severe disease with diarrhea in infants, is also involved in weaned rabbit colibacillosis. EPEC O103 is frequent in rabbit-fattening units of Western Europe. It causes high mortality and growth retardation, leading to substantial economic losses. We report here the construction by allelic exchange of an EPEC O103 strain mutated in espB and tir, two essential virulence genes. Upon live oral administration to weaned rabbits, the E22DeltaTir/EspB mutant strain efficiently colonized the intestinal tract without any adverse consequences. The rabbits were challenged with the highly pathogenic parental strain E22. The mutant provided complete protection to rabbits and total resistance to intestinal colonization by E22. In addition, E22DeltaTir/EspB strain induced a specific humoral response against the bacterial adhesin AF/R2. These Abs prevent bacterial attachment to epithelial cells in vitro. These results open the way for the development of an efficient vaccine strategy against rabbit EPEC infections.

Type III secretion-dependent cell cycle block caused in HeLa cells by enteropathogenic Escherichia coli O103

Rabbit enteropathogenic Escherichia coli (EPEC) O103 induces in HeLa cells an irreversible cytopathic effect characterized by the recruitment of focal adhesions, formation of stress fibers, and inhibition of cell proliferation. We have characterized the modalities of the proliferation arrest and investigated its underlying mechanisms. We found that HeLa cells that were exposed to the rabbit EPEC O103 strain E22 progressively accumulated at 4C DNA content and did not enter mitosis. A significant proportion of the cells were able to reinitiate DNA synthesis without division, leading to 8C DNA content. This cell cycle inhibition by E22 was abrogated in mutants lacking EspA, -B, and -D and was restored by transcomplementation. In contrast, intimin and Tir mutants retained the antiproliferative effect. The cell cycle arrest was not a direct consequence of the formation of stress fibers, since their disruption by toxins during exposure to E22 did not reverse the cell cycle inhibition. Likewise, the cell cycle arrest was not dependent on the early tyrosine dephosphorylation events triggered by E22 in the cells. Two key partner effectors controlling entry into mitosis were also investigated: cyclin B1 and the associated cyclin-dependent kinase 1 (Cdk1). Whereas cyclin B1 was not detectably affected in E22-exposed cells, Cdk1 was maintained in a tyrosine-phosphorylated inactive state and lost its affinity for p13(suc1)-agarose beads. This shows that Cdk1 is implicated in the G2/M arrest caused by EPEC strain E22.

Gut feelings: enteropathogenic E. coli (EPEC) interactions with the host

Enteropathogenic Escherichia coli (EPEC) is a gram-negative bacterial pathogen that adheres to human intestinal epithelial cells, resulting in watery, persistent diarrhea. It subverts the host cell cytoskeleton, causing a rearrangement of cytoskeletal components into a characteristic pedestal structure underneath adherent bacteria. In contrast to other intracellular pathogens that affect the actin cytoskeleton from inside the host cytoplasm, EPEC remains extracellular and transmits signals through the host cell plasma membrane via direct injection of virulence factors by a "molecular syringe," the bacterial type III secretion system. One injected factor is Tir, which functions as the plasma membrane receptor for EPEC adherence. Tir directly links extracellular EPEC through the epithelial membrane and firmly anchors it to the host cell actin cytoskeleton, thereby initiating pedestal formation. In addition to stimulating actin nucleation and polymerization in the host cell, EPEC activates several other signaling pathways that lead to tight junction disruption, inhibition of phagocytosis, altered ion secretion, and immune responses. This review summarizes recent developments in our understanding of EPEC pathogenesis and discusses similarities and differences between EPEC pedestals, focal contacts, and Listeria monocytogenes actin tails.

Tir tyrosine phosphorylation and pedestal formation are delayed in enteropathogenic Escherichia coli sepZ::TnphoA mutant 30-5-1(3)

Enteropathogenic Escherichia coli (EPEC) strain 30-5-1(3) has been reported to form attaching and effacing (A/E) lesions without Tir tyrosine phosphorylation. In this study, we show that 30-5-1(3), which has a transposon insertion within the sepZ gene, forms wild-type A/E lesions including Tir tyrosine phosphorylation, but at a slower rate. A/E lesion formation by 30-5-1(3) occurs without detectable secretion of Tir or other EPEC Esp secreted proteins.

Early events and implication of F-actin and annexin I associated structures in the phagocytic uptake of Brucella suis by the J-774A.1 murine cell line and human monocytes

Brucella spp. are facultative, intracellular pathogenic bacteria that cause brucellosis, a zoonosis affecting mammalian species. Brucella entry into myelomonocytic cell lines is highly enhanced by opsonization. Few studies have been undertaken to unravel the first interactions between these bacteria and their host cells. This paper deals with early events following contact of Brucella suis with the J-774A.1 phagocytic cell line and differentiated monocytes. Phagocytic uptake of bacteria was documented under a fluorescence microscope using GFP-expressing B. suis. Unlike entry in the J-774A. 1 cell line, non-opsonized Brucella entered differentiated human monocytes as efficiently as opsonized bacteria. However, following 1 h infections, a mean of only three bacteria were phagocytized and the whole monocyte population was only infected after a 4 h infection. Contact of non-opsonized Brucella with phagocytes did not induce marked structural changes at the cell surface, as revealed by scanning electron microscopy. Contact of Brucella (opsonized or not) elicited transient local recruitment of F-actin, revealed by phalloidin labelling, and of annexin I-associated structures, revealed by immunofluorescence staining. Finally, bacteria appeared to be rapidly internalized in monocytes once they had adhered to the cell surface. A low percentage of infected cells and few adhered and/or internalized bacteria following short-term infections could have resulted either from the fact that there were few sites of entry or the weak bacterial initial interactions with the host-cell membrane or the bacterial receptor.

Presence of the LEE (locus of enterocyte effacement) in pig attaching and effacing Escherichia coli and characterization of eae, espA, espB and espD genes of PEPEC (pig EPEC) strain 1390

In the present study, attaching and effacing Escherichia coli (AEEC) O45 isolates from post-weaning pigs with diarrhoea were examined for the presence of the LEE (locus of enterocyte effacement) using various DNA probes derived from the LEE of human enteropathogenic E. coli (EPEC) strain E2348/69. The LEE fragment was conserved among the eae -positive pig isolates. The attaching and effacing activity of PEPEC (pig EPEC) O45 isolates is highly correlated with the presence of the LEE. Nevertheless, for some PEPEC isolates, the insertion site of the LEE is different or has diverged during evolution. The presence of the LEE fragment in PEPEC isolates provides further evidence that the LEE region is conserved among AEEC of different animal origins. In addition, the nucleotide sequence of the region containing the eae gene and esp genes of a pig AEEC isolate, strain 1390, was determined. Among examined Eae proteins, Eae of strain 1390 showed the highest similarity with Eae belonging to the beta intimin group such as the Eae of rabbit AEEC. Moreover, all pig strains that produced attaching and effacing lesions in piglets and pig ileal explants belonged to the beta intimin group. The deduced amino acid sequences of the EspA, EspB and EspD proteins of strain 1390 showed particularly strong homology to those of AEEC strains presenting a beta intimin allele. Thus, pig AEEC possess the LEE sequences, and for the strain 1390, sequences of the eae and esp regions are related to those of other AEEC, in particular, strains presenting a beta intimin allele, such as the rabbit AEEC.

Role of tir and intimin in the virulence of rabbit enteropathogenic Escherichia coli serotype O103:H2

Attaching and effacing (A/E) rabbit enteropathogenic Escherichia coli (REPEC) strains belonging to serogroup O103 are an important cause of diarrhea in weaned rabbits. Like human EPEC strains, they possess the locus of enterocyte effacement clustering the genes involved in the formation of the A/E lesions. In addition, pathogenic REPEC O103 strains produce an Esp-dependent but Eae (intimin)-independent alteration of the host cell cytoskeleton characterized by the formation of focal adhesion complexes and the reorganization of the actin cytoskeleton into bundles of stress fibers. To investigate the role of intimin and its translocated coreceptor (Tir) in the pathogenicity of REPEC, we have used a newly constructed isogenic tir null mutant together with a previously described eae null mutant. When human HeLa epithelial cells were infected, the tir mutant was still able to induce the formation of stress fibers as previously reported for the eae null mutant. When the rabbit epithelial cell line RK13 was used, REPEC O103 produced a classical fluorescent actin staining (FAS) effect, whereas both the eae and tir mutants were FAS negative. In a rabbit ligated ileal loop model, neither mutant was able to induce A/E lesions. In contrast to the parental strain, which intimately adhered to the enterocytes and destroyed the brush border microvilli, bacteria of both mutants were clustered in the mucus without reaching and damaging the microvilli. The role of intimin and Tir was then analyzed in vivo by oral inoculation of weaned rabbits. Although both mutants were still present in the intestinal flora of the rabbits 3 weeks after oral inoculation, neither mutant strain induced any clinical signs or significant weight loss in the inoculated rabbits whereas the parental strain caused the death of 90% of the inoculated rabbits. Nevertheless, an inflammatory infiltrate was present in the lamina propria of the rabbits infected with both mutants, with an inflammatory response greater for the eae null mutant. In conclusion, we have confirmed the role of intimin in virulence, and we have shown, for the first time, that Tir is also a key factor in vivo for pathogenicity.

In vivo interactions of rabbit enteropathogenic Escherichia coli O103 with its host: an electron microscopic and histopathologic study

A family of human and animal pathogens, including enteropathogenic and enterohemorrhagic Escherichia coli (EPEC and EHEC), trigger formation of 'attaching and effacing' lesions on cultured and intestinal epithelial surfaces. However, our understanding of these events in vivo is incomplete. To further study these interactions in a natural infection model, weaned rabbits were infected with rabbit enteropathogenic E. coli O103 (REPEC O103), followed clinically, and infected tissues were evaluated by electron and light microscopy. Of the 36 rabbits challenged, morbidity and mortality were 65 and 23%, respectively. Twenty-four hours after infection, expression of fimbriae-like organelles was observed on the bacterial surface. Microvilli of ileal Peyer's patches (PP) became disorganized, and intestinal mucus secretion increased which coincided with intraluminal binding of the pathogen in the proximal colon. Forty-eight hours after infection, there was conspicuous lack of fimbriae-like organelle expression, while bacterial adherence preferentially occurred at the domed villi of PP. Seventy-two hours after infection, broad morphological heterogeneity was noted within pedestals beneath attached bacteria, including extended pseudopods. We conclude that REPEC O103 express surface organelles during initial exposure to the host, that the initial target sites of adherence are the domed villi of ileal PP, and that increased mucus secretion occurs during REPEC O103 infection. As well, extended pseudopod formation was demonstrated in vivo.

The long-term cytoskeletal rearrangement induced by rabbit enteropathogenic Escherichia coli is Esp dependent but intimin independent

Attaching and effacing rabbit enteropathogenic Escherichia coli (REPEC) of the O103 serogroup adhere diffusely on HeLa cells and trigger a slow progressive cytopathic effect (CPE) characterized by the recruitment of vinculin and the assembly of actin stress fibres. In contrast to REPEC O103, the reference human EPEC strain E2348/69 is unable to trigger the CPE. In this study, we have shown first that the fimbrial adhesin AF/R2, which mediates the diffuse adhesion of REPEC O103, was not sufficient to induce the CPE capability upon E2348/69. Non-polar mutants of REPEC O103 for espA, espB, espD and eae were then constructed. The four mutants were unable to induce attaching and effacing lesions in the rabbit ileal loop model. The esp mutants were no longer able to induce the CPE, whereas the eae mutant still induced the CPE. Each espA, -B, -D mutant could be fully complemented in trans by the corresponding cloned esp genes from both the parental strain and the CPE-negative E2348/69 strain, indicating that no single esp encodes the information needed to confer the CPE phenotype. In conclusion, the CPE is the first example of an Esp-dependent but Eae (intimin)-independent alteration of the host cell cytoskeleton by certain EPEC strains.

Two enteropathogenic Escherichia coli type III secreted proteins, EspA and EspB, are virulence factors

Enteropathogenic Escherichia coli (EPEC) belongs to a family of related bacterial pathogens, including enterohemorrhagic Escherichia coli (EHEC) O157:H7 and other human and animal diarrheagenic pathogens that form attaching and effacing (A/E) lesions on host epithelial surfaces. Bacterial secreted Esp proteins and a type III secretion system are conserved among these pathogens and trigger host cell signal transduction pathways and cytoskeletal rearrangements, and mediate intimate bacterial adherence to epithelial cell surfaces in vitro. However, their role in pathogenesis is still unclear. To investigate the role of Esp proteins in disease, mutations in espA and espB were constructed in rabbit EPEC serotype O103 and infection characteristics were compared to that of the wild-type strain using histology, scanning and transmission electron microscopy, and confocal laser scanning microscopy in a weaned rabbit infection model. The virulence of EspA and EspB mutant strains was severely attenuated. Additionally, neither mutant strain formed A/E lesions, nor did either one cause cytoskeletal actin rearrangements beneath the attached bacteria in the rabbit intestine. Collectively, this study shows for the first time that the type III secreted proteins EspA and EspB are needed to form A/E lesions in vivo and are indeed virulence factors. It also confirms the role of A/E lesions in disease processes.