Analysis of the function of enteropathogenic Escherichia coli EspB by random mutagenesis

Bioprospecting of inhibitors of EPEC virulence from metabolites of marine actinobacteria from the Arctic Sea

A considerable number of antibacterial agents are derived from bacterial metabolites. Similarly, numerous known compounds that impede bacterial virulence stem from bacterial metabolites. Enteropathogenic Escherichia coli (EPEC) is a notable human pathogen causing intestinal infections, particularly affecting infant mortality in developing regions. These infections are characterized by microvilli effacement and intestinal epithelial lesions linked with aberrant actin polymerization. This study aimed to identify potential antivirulence compounds for EPEC infections among bacterial metabolites harvested from marine actinobacteria (Kocuria sp. and Rhodococcus spp.) from the Arctic Sea by the application of virulence-based screening assays. Moreover, we demonstrate the suitability of these antivirulence assays to screen actinobacteria extract fractions for the bioassay-guided identification of metabolites. We discovered a compound in the fifth fraction of a Kocuria strain that interferes with EPEC-induced actin polymerization without affecting growth. Furthermore, a growth-inhibiting compound was identified in the fifth fraction of a Rhodococcus strain. Our findings include the bioassay-guided identification, HPLC-MS-based dereplication, and isolation of a large phospholipid and a likely antimicrobial peptide, demonstrating the usefulness of this approach in screening for compounds capable of inhibiting EPEC virulence.

The sequence of events of enteropathogenic E. coli's type III secretion system translocon assembly

Many bacterial pathogens employ the type III secretion system (T3SS), a specialized complex that transports effector proteins that manipulate various cellular processes. The T3SS forms a translocon pore within the host-cell membrane consisting of two secreted proteins that transition from a soluble state into a transmembrane complex. Still, the exact sequence of events leading to the formation of a membranous functional pore remains uncertain. Here, we utilized the translocon proteins of enteropathogenic E. coli (EPEC) to investigate the sequence of those steps leading to translocon assembly, including self-oligomerization, hetero-oligomerization, interprotein interaction, and membrane insertion. We found that in EPEC, EspD (SctE) plays a dominant role in pore formation as it assembles into an oligomeric state, regardless of pH, membrane contact, or the presence of EspB (SctB). Subsequently, EspB subunits integrate into EspD homo-oligomers to create EspB-EspD hetero-oligomers that adopt a transmembrane orientation to create a functional pore complex.

Pathophysiology of Enteropathogenic Escherichia coli-induced Diarrhea

Enteropathogenic Escherichia coli (EPEC) are important diarrheal pathogens of infants and young children. Since the availability of molecular diagnosis methods, we now have new insights into the incidence and prevalence of these infections. Recent epidemiological studies indicate that atypical EPEC (aEPEC) are seen more frequently than typical EPEC (tEPEC) worldwide, including in both endemic diarrhea and diarrhea outbreaks. Therefore, it is important to further characterize the pathogenicity of these emerging strains. The virulence mechanisms and pathophysiology of the attaching and effacing lesion (A/E) and the type-three-secretion-system (T3SS) are complex but well-studied. A/E strains use their pool of locus of enterocyte effacement (LEE)-encoded and non-LEE-encoded effector proteins to subvert and modulate cellular and barrier properties of the host. However, the exact mechanisms of diarrhea in EPEC infection are not completely understood. From the clinical perspective, there is a need for fast, easy, and inexpensive diagnostic methods to define optimal treatment and prevention for children in endemic areas. In this article, we present a review of the classification of EPEC, epidemiology, pathogenesis of the disease caused by these bacteria, determinants of virulence, alterations in signaling, determinants of colonization vs. those of disease, and the limited information we have on the pathophysiology of EPEC-induced diarrhea. This article combines peer-reviewed evidence from our own studies and the results of an extensive literature search in the databases PubMed, EMBASE, and Scopus.

Assay for Type III Secretion in Escherichia coli

The type III secretion system (T3SS) is crucial for the virulence of several pathogenic Escherichia coli species as well as for other gram-negative bacterial strains. Therefore, the ability to monitor this system constitutes a valuable tool for assessing the involvement of different proteins in bacterial virulence, for identifying critical domains and specific mutations, and for evaluating the antivirulence activities of various drugs. The major advantage of the T3SS secretion assay for E. coli over assays for other gram-negative pathogens is that it does not necessarily require specific antibodies. Here, we describe how to grow enteropathogenic E. coli (EPEC) and enterohemorrhagic E. coli (EHEC) strains under T3SS-inducing conditions, separate the supernatant fraction from the bacterial pellet, analyze this fraction on sodium dodecyl sulfate (SDS)-polyacrylamide gels, and evaluate the level of T3SS activity. We describe a qualitative analysis using Coomassie staining and a quantitative assay using western blotting.

AtaT Improves the Stability of Pore-Forming Protein EspB by Acetylating Lysine 206 to Enhance Strain Virulence

A novel type II toxin of toxin–antitoxin systems (TAs), Gcn5-related N-acetyltransferase (GNAT) family, was reported recently. GNAT toxins are mainly present in pathogenic species, but studies of their involvement in pathogenicity are rare. This study discovered that the GANT toxin AtaT in enterohemorrhagic Escherichia coli (EHEC) can significantly enhance strain pathogenicity. First, we detected the virulence of ΔataT and ΔataR in cell and animal models. In the absence of ataT, strains showed a lower adhesion number, and host cells presented weaker attaching and effacing lesions, inflammatory response, and pathological injury. Next, we screened the acetylation substrate of AtaT to understand the underlying mechanism. Results showed that E. coli pore-forming protein EspB, which acts as a translocon in type III secretion system (T3SS) in strains, can be acetylated specifically by AtaT. The acetylation of K206 in EspB increases protein stability and maintains the efficiency of effectors translocating into host cells to cause close adhesion and tissue damage.

Transmembrane domains of type III-secreted proteins affect bacterial-host interactions in enteropathogenic E. coli

Many bacterial pathogens utilize a specialized secretion system, termed type III secretion system (T3SS), to translocate effector proteins into host cells and establish bacterial infection. The T3SS is anchored within the bacterial membranes and contains a long needle/filament that extends toward the host-cell and forms, at its distal end, a pore complex within the host membrane. The T3SS pore complex consists of two bacterial proteins, termed SctB and SctE, which have conflicting targeting indications; a signal sequence that targets to secretion to the extracellular environment via the T3SS, and transmembrane domains (TMDs) that target to membrane localization. In this study, we investigate whether the TMD sequences of SctB and SctE have special features that differentiate them from classical TMDs and allow them to escape bacterial membrane integration. For this purpose, we exchanged the SctB and SctE native TMDs for alternative hydrophobic sequences and found that the TMD sequences of SctB and SctE dictate membrane destination (bacterial versus host membrane). Moreover, we examined the role of the SctB TMD sequence in the activity of the full-length protein, post secretion, and found that the TMD does not serve only as a hydrophobic segment, but is also involved in the ability of the protein to translocate itself and other proteins into and across the host cell membrane.

Staying out or Going in? The Interplay between Type 3 and Type 5 Secretion Systems in Adhesion and Invasion of Enterobacterial Pathogens

Enteric pathogens rely on a variety of toxins, adhesins and other virulence factors to cause infections. Some of the best studied pathogens belong to the Enterobacterales order; these include enteropathogenic and enterohemorrhagic Escherichia coli, Shigella spp., and the enteropathogenic Yersiniae. The pathogenesis of these organisms involves two different secretion systems, a type 3 secretion system (T3SS) and type 5 secretion systems (T5SSs). The T3SS forms a syringe-like structure spanning both bacterial membranes and the host cell plasma membrane that translocates toxic effector proteins into the cytoplasm of the host cell. T5SSs are also known as autotransporters, and they export part of their own polypeptide to the bacterial cell surface where it exerts its function, such as adhesion to host cell receptors. During infection with these enteropathogens, the T3SS and T5SS act in concert to bring about rearrangements of the host cell cytoskeleton, either to invade the cell, confer intracellular motility, evade phagocytosis or produce novel structures to shelter the bacteria. Thus, in these bacteria, not only the T3SS effectors but also T5SS proteins could be considered "cytoskeletoxins" that bring about profound alterations in host cell cytoskeletal dynamics and lead to pathogenic outcomes.

Enterococcus faecalis Enhances Expression and Activity of the Enterohemorrhagic Escherichia coli Type III Secretion System

The gut microbiota can significantly impact invading pathogens and the disease they cause; however, many of the mechanisms that dictate commensal-pathogen interactions remain unclear. Enterohemorrhagic Escherichia coli (EHEC) is a potentially lethal human intestinal pathogen that uses microbiota-derived molecules as cues to efficiently regulate virulence factor expression. Here, we investigate the interaction between EHEC and Enterococcus faecalis, a common human gut commensal, and show that E. faecalis affects both expression and activity of the EHEC type III secretion system (T3SS) via two distinct mechanisms. First, in the presence of E. faecalis there is increased transcription of genes encoding the EHEC T3SS. This leads to increased effector translocation and ultimately greater numbers of pedestals formed on host cells. The same effect was observed with several strains of enterococci, suggesting that it is a general characteristic of this group. In a mechanism separate from E. faecalis-induced transcription of the T3SS, we report that an E. faecalis-secreted protease, GelE, cleaves a critical structural component of the EHEC T3SS, EspB. Our data suggest that this cleavage actually increases effector translocation by the T3SS, supporting a model where EspB proteolysis promotes maximum T3SS activity. Finally, we report that treatment of EHEC with E. faecalis-conditioned cell-free medium is insufficient to induce increased T3SS expression, suggesting that this effect relies on cell contact between E. faecalis and EHEC. This work demonstrates a complex interaction between a human commensal and pathogen that impacts both expression and function of a critical virulence factor.IMPORTANCE This work reveals a complex and multifaceted interaction between a human gut commensal, Enterococcus faecalis, and a pathogen, enterohemorrhagic E. coli We demonstrate that E. faecalis enhances expression of the enterohemorrhagic E. coli type III secretion system and that this effect likely depends on cell contact between the commensal and the pathogen. Additionally, the GelE protease secreted by E. faecalis cleaves a critical structural component of the EHEC type III secretion system. In agreement with previous studies, we find that this cleavage actually increases effector protein delivery into host cells by the secretion system. This work demonstrates that commensal bacteria can significantly shape expression and activity of pathogen virulence factors, which may ultimately shape the progression of disease.

Multitalented EspB of enteropathogenic Escherichia coli (EPEC) enters cells autonomously and induces programmed cell death in human monocytic THP-1 cells

Enteropathogenic Escherichia coli (EPEC) subvert host cell signaling pathways by injecting effector proteins via a Type 3 Secretion System (T3SS). The T3SS-dependent EspB protein is a multi-functional effector protein, which contributes to adherence and translocator pore formation and after injection exhibits several intracellular activities. In addition, EspB is also secreted into the environment. Effects of secreted EspB have not been reported thus far. As a surrogate for secreted EspB we employed recombinant EspB (rEspB) derived from the prototype EPEC strain E2348/69 and investigated the interactions of the purified protein with different human epithelial and immune cells including monocytic THP-1 cells, macrophages, dendritic cells, U-937, epithelial T84, Caco-2, and HeLa cells. To assess whether these proteins might exert a cytotoxic effect we monitored the release of lactate dehydrogenase (LDH) as well as propidium iodide (PI) uptake. For comparison, we also investigated several homologs of EspB such as IpaD of Shigella, and SipC, SipD, SseB, and SseD of Salmonella as purified recombinant proteins. Interestingly, cytotoxicity was only observed in THP-1 cells and macrophages, whereas epithelial cells remained unaffected. Cell fractionation and immune fluorescence experiments showed that rEspB enters cells autonomously, which suggests that EspB might qualify as a novel cell-penetrating effector protein (CPE). Using specific organelle tracers and inhibitors of signaling pathways we found that rEspB destroys the mitochondrial membrane potential - an indication of programmed cell death induction in THP-1 cells. Here we show that EspB not only constitutes an essential part of the T3SS-nanomachine and contributes to the arsenal of injected effector proteins but, furthermore, that secreted (recombinant) EspB autonomously enters host cells and selectively induces cell death in immune cells.

Epithelial cells detect functional type III secretion system of enteropathogenic Escherichia coli through a novel NF-κB signaling pathway

Enteropathogenic Escherichia coli (EPEC), a common cause of infant diarrhea, is associated with high risk of mortality in developing countries. The primary niche of infecting EPEC is the apical surface of intestinal epithelial cells. EPEC employs a type three secretion system (TTSS) to inject the host cells with dozens of effector proteins, which facilitate attachment to these cells and successful colonization. Here we show that EPEC elicit strong NF-κB activation in infected host cells. Furthermore, the data indicate that active, pore-forming TTSS per se is necessary and sufficient for this NF-κB activation, regardless of any specific effector or protein translocation. Importantly, upon infection with wild type EPEC this NF-κB activation is antagonized by anti-NF-κB effectors, including NleB, NleC and NleE. Accordingly, this NF-κB activation is evident only in cells infected with EPEC mutants deleted of nleB, nleC, and nleE. The TTSS-dependent NF-κB activation involves a unique pathway, which is independent of TLRs and Nod1/2 and converges with other pathways at the level of TAK1 activation. Taken together, our results imply that epithelial cells have the capacity to sense the EPEC TTSS and activate NF-κB in response. Notably, EPEC antagonizes this capacity by delivering anti-NF-κB effectors into the infected cells.

Inhibition of Adhesion of Enteropathogenic Escherichia coli to HEp-2 Cells by Binding of a Novel Peptide to EspB Protein

Enteropathogenic Escherichia coli (EPEC) is a major cause of infantile diarrhea in developing countries. The translocator EspB is a key virulence factor in the process of the attaching and effacing effect of EPEC and plays a critical role in the pathogenesis of the bacteria. In this study, we aimed to select the peptides binding to EspB protein by phage display library and further investigate whether these peptides can decrease the extent of invasion and virulence of EPEC on host cells by targeting to EspB protein. The expression and purification of EspB protein from E. coli was demonstrated by Western blotting. The Ph.D. 12-mer peptide phage display library was used to screen the candidate peptides binding specifically to EspB protein. Furthermore, the affinity of these candidate peptides bound to EspB was identified by enzyme-linked immunosorbent assay (ELISA). Moreover, we investigated whether these screened peptides could decrease the adherence ratio of EPEC to HEp-2 cells with increasing concentration. Successful purification of EspB protein from pET21b-EspB-transformed E. coli was identified by Western blotting. Then, the candidate peptides including phages 6, 7, 8, and 12 were screened by the Ph.D. 12-mer peptide phage display library and ELISA test demonstrated that their affinity binding to EspB protein was high compared with the control. Functional analysis indicated that synthetic peptide-6 (YFPYSHTSPRQP) significantly decreased the adherence ratio of EPEC to HEp-2 cells with increasing concentration (P < 0.01). Peptide-6 (100 µg/mL) could lead to a 40 % decrease in the adherence ratio of EPEC to HEp-2 cells compared with control (P < 0.01). However, the other three peptides at different concentrations showed only a slight ability to block the adherence of EPEC to host cells. Our data provided a potential strategy to inhibit the adhesion of EPEC to epithelial cells by a candidate peptide targeted toward EspB protein.

Bacterial Control of Pores Induced by the Type III Secretion System: Mind the Gap

Type III secretion systems (T3SSs) are specialized secretion apparatus involved in the virulence of many Gram-negative pathogens, enabling the injection of bacterial type III effectors into host cells. The T3SS-dependent injection of effectors requires the insertion into host cell membranes of a pore-forming "translocon," whose effects on cell responses remain ill-defined. As opposed to pore-forming toxins that damage host cell plasma membranes and induce cell survival mechanisms, T3SS-dependent pore formation is transient, being regulated by cell membrane repair mechanisms or bacterial effectors. Here, we review host cell responses to pore formation induced by T3SSs associated with the loss of plasma membrane integrity and regulation of innate immunity. We will particularly focus on recent advances in mechanisms controlling pore formation and the activity of the T3SS linked to type III effectors or bacterial proteases. The implications of the regulation of the T3SS translocon activity during the infectious process will be discussed.

SepD/SepL-dependent secretion signals of the type III secretion system translocator proteins in enteropathogenic Escherichia coli

The type III protein secretion system (T3SS) encoded by the locus of enterocyte effacement (LEE) is essential for the pathogenesis of attaching/effacing bacterial pathogens, including enteropathogenic Escherichia coli (EPEC), enterohemorrhagic E. coli (EHEC), and Citrobacter rodentium. These pathogens use the T3SS to sequentially secrete three categories of proteins: the T3SS needle and inner rod protein components; the EspA, EspB, and EspD translocators; and many LEE- and non-LEE-encoded effectors. SepD and SepL are essential for translocator secretion, and mutations in either lead to hypersecretion of effectors. However, how SepD and SepL control translocator secretion and secretion hierarchy between translocators and effectors is poorly understood. In this report, we show that the secreted T3SS components, the translocators, and both LEE- and non-LEE-encoded effectors all carry N-terminal type III secretion and translocation signals. These signals all behave like those of the effectors and are sufficient for mediating type III secretion and translocation by wild-type EPEC and hypersecretion by the sepD and sepL mutants. Our results extended previous observations and suggest that the secretion hierarchy of the different substrates is determined by a signal other than the N-terminal secretion signal. We identified a domain located immediately downstream of the N-terminal secretion signal in the translocator EspB that is required for SepD/SepL-dependent secretion. We further demonstrated that this EspB domain confers SepD/SepL- and CesAB-dependent secretion on the secretion signal of effector EspZ. Our results thus suggest that SepD and SepL control and regulate secretion hierarchy between translocators and effectors by recognizing translocator-specific export signals.

IMPORTANCE

Many bacterial pathogens use a syringe-like protein secretion apparatus, termed the type III protein secretion system (T3SS), to secrete and inject numerous proteins directly into the host cells to cause disease. The secreted proteins perform different functions at various stages during infection and are classified into three substrate categories (T3SS components, translocators, and effectors). They all contain secretion signals at their N termini, but how their secretion hierarchy is determined is poorly understood. Here, we show that the N-terminal secretion signals from different substrate categories all behave the same and do not confer substrate specificity. We further characterize the secretion signals of the translocators and identify a translocator-specific signal, demonstrating that substrate-specific secretion signals are required in regulating T3SS substrate hierarchy.

Secretion of the housekeeping protein glyceraldehyde-3-phosphate dehydrogenase by the LEE-encoded type III secretion system in enteropathogenic Escherichia coli

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a multifunctional housekeeping protein secreted by pathogens and involved in adhesion and/or virulence. Previously we reported that enterohemorrhagic (EHEC) and enteropathogenic (EPEC) Escherichia coli secrete GAPDH into the culture medium. This bacterial protein binds human plasminogen and fibrinogen and remains associated with Caco-2 cells upon infection. In these pathogens, GAPDH secretion is not linked to outer membrane vesicles and depends on growth conditions, although the secretion mechanism is still unknown. EPEC is an attaching and effacing pathogen able to secrete and translocate multiple effector proteins into infected cells through a type III secretion system (T3SS). The secretion process is often dependent on a bacterial chaperone. The chaperone CesT displays broad substrate specificity and plays a central role in the recruitment of multiple type III effectors to the T3SS apparatus. Here we provide genetic evidences on GAPDH secretion through T3SS by EPEC grown in DMEM. Secretion of GAPDH is increased in ΔsepD mutants and abolished in mutants defective in the type III ATPase EscN. Complementation with escN gene restores GAPDH secretion. In addition, we prove by means of pull down experiments, overlay immunoblotting and biolayer interferometry a novel interaction between GAPDH and the chaperone CesT. This interaction, which is strong and slow dissociating, may stabilize a population of GAPDH molecules in a secretion competent-state and target them to the type III secretion apparatus. This is the first description of CesT interaction with a housekeeping protein and its export through T3SS.

Enteropathogenic escherichia coli infection in children

PURPOSE OF REVIEW

Enteropathogenic Escherichia coli (EPEC) is an important diarrheal pathogen of young children. As the diagnosis of EPEC is now based mainly on molecular criteria, there has been an important change in its prevalence. The purpose of this study is to review the current epidemiology of EPEC infection and the new insights into its physiopathology.

RECENT FINDINGS

Recent epidemiological studies indicate that atypical EPEC (aEPEC) is more prevalent than typical EPEC (tEPEC) in both developed and developing countries, and that aEPEC is important in both pediatric endemic diarrhea and diarrhea outbreaks. Therefore, it is important to further characterize the pathogenicity of these emerging strains. The virulence mechanisms and physiopathology of the attaching and effacing lesion (A/E) and the type three secretion-system (T3SS) are complex but well studied. A/E strains use their pool of locus of enterocyte effacement (LEE)-encoded and non-LEE-encoded effector proteins to subvert and modulate cellular and barrier properties of the host. However, the exact mechanisms of diarrhea in EPEC infection are not completely understood.

SUMMARY

Remarkable progress has been made to identify virulence determinants required to mediate the pathogenesis of EPEC. However, fast, easy, and inexpensive diagnostic methods are needed in order to define optimal treatment and prevention for children in endemic areas.

The N-terminal amphipathic region of the Escherichia coli type III secretion system protein EspD is required for membrane insertion and function

Enterohemorrhagic Escherichia coli is a causative agent of gastrointestinal and diarrheal diseases. These pathogenic E. coli express a syringe-like protein machine, known as the type III secretion system (T3SS), used for the injection of virulence factors into the cytosol of the host epithelial cell. Breaching the epithelial plasma membrane requires formation of a translocation pore that contains the secreted protein EspD. Here we demonstrate that the N-terminal segment of EspD, encompassing residues 1-171, contains two amphipathic domains spanning residues 24-41 and 66-83, with the latter of these helices being critical for EspD function. Fluorescence and circular dichroism analysis revealed that, in solution, His₆-EspD₁₋₁₇₁ adopts a native disordered structure; however, on binding anionic small unilamellar vesicles composed of phosphatidylserine, His₆-EspD₁₋₁₇₁ undergoes a pH depended conformational change that increases the α-helix content of this protein approximately sevenfold. This change coincides with insertion of the region circumscribing Trp₄₇ into the hydrophobic core of the lipid bilayer. On the HeLa cell plasma membrane, His₆-EspD₁₋₁₇₁ forms a homodimer that is postulated to promote EspD-EspD oligomerization and pore formation. Complementation of ΔespD null mutant bacteria with an espDΔ66-83 gene showed that this protein was secreted but non-functional.

Interactions and predicted host membrane topology of the enteropathogenic Escherichia coli translocator protein EspB

Type 3 secretion systems (T3SSs) are critical for the virulence of numerous deadly Gram-negative pathogens. T3SS translocator proteins are required for effector proteins to traverse the host cell membrane and perturb cell function. Translocator proteins include two hydrophobic proteins, represented in enteropathogenic Escherichia coli (EPEC) by EspB and EspD, which are thought to interact and form a pore in the host membrane. Here we adapted a sequence motif recognized by a host kinase to demonstrate that residues on the carboxyl-terminal side of the EspB transmembrane domain are localized to the host cell cytoplasm. Using functional internal polyhistidine tags, we confirm an interaction between EspD and EspB, and we demonstrate, for the first time, an interaction between EspD and the hydrophilic translocator protein EspA. Using a panel of espB insertion mutations, we describe two regions on either side of a putative transmembrane domain that are required for the binding of EspB to EspD. Finally, we demonstrate that EspB variants incapable of binding EspD fail to adopt the proper host cell membrane topology. These results provide new insights into interactions between translocator proteins critical for virulence.

A bacterial effector targets host DH-PH domain RhoGEFs and antagonizes macrophage phagocytosis

Bacterial pathogens often harbour a type III secretion system (TTSS) that injects effector proteins into eukaryotic cells to manipulate host processes and cause diseases. Identification of host targets of bacterial effectors and revealing their mechanism of actions are crucial for understating bacterial virulence. We show that EspH, a type III effector conserved in enteric bacterial pathogens including enteropathogenic Escherichia coli (EPEC), enterohaemorrhagic E. coli and Citrobacter rodentium, markedly disrupts actin cytoskeleton structure and induces cell rounding up when ectopically expressed or delivered into HeLa cells by the bacterial TTSS. EspH inactivates host Rho GTPase signalling pathway at the level of RhoGEF. EspH directly binds the DH-PH domain in multiple RhoGEFs, which prevents their binding to Rho and thereby inhibits nucleotide exchange-mediated Rho activation. Consistently, infection of mouse macrophages with EPEC harbouring EspH attenuates phagocytosis of the bacteria as well as FcgammaR-mediated phagocytosis. EspH represents the first example of targeting RhoGEFs by bacterial effectors, and our results also reveal an unprecedented mechanism used by enteric pathogens to counteract the host defence system.

Post-transcriptional processing of the LEE4 operon in enterohaemorrhagic Escherichia coli

Enterohaemorrhagic Escherichia coli (EHEC) employs a type III secretion system (T3SS) to export translocator and effector proteins required for mucosal colonization. The T3SS is encoded in a pathogenicity island called the locus of enterocyte effacement (LEE) that is organized in five major operons, LEE1 to LEE5. LEE4 encodes a regulator of secretion (SepL), translocators (EspA, D and B), two chaperones (CesD2 and L0017), a T3SS component (EscF) and an effector protein (EspF). It was originally proposed that the esp transcript is transcribed from a promoter located at the end of sepL but other authors suggested that this transcript is the result of a post-transcriptional processing event. In this study, we established that the espADB mRNA is generated by post-transcriptional processing at the end of the sepL coding sequence. RNase E is the endonuclease involved in the cleavage, but the interaction of this enzyme with other proteins through its C-terminal half is dispensable. A putative transcription termination event in the cesD2 coding region would generate the 3' end of the transcript. Similar to what has been described for other processed transcripts, the cleavage of LEE4 seems a mechanism to differentially regulate SepL and Esp protein production.

Saccharomyces boulardii ameliorates Citrobacter rodentium-induced colitis through actions on bacterial virulence factors

Saccharomyces boulardii has received increasing attention as a probiotic effective in the prevention and treatment of infectious and inflammatory bowel diseases. The aim of this study was to examine the ameliorating effects of S. boulardii on Citrobacter rodentium colitis in vivo and identify potential mechanisms of action. C57BL/6 mice received 2.5 x 10(8) C. rodentium by gavage on day 0, followed by S. boulardii (25 mg; 5 x 10(8) live cells) gavaged twice daily from day 2 to day 9. Animal weights were monitored until death on day 10. Colons were removed and assessed for epithelial barrier function, histology, and myeloperoxidase activity. Bacterial epithelial attachment and type III secreted proteins translocated intimin receptor Tir (the receptor for bacterial intimin) and EspB (a translocation apparatus protein) required for bacterial virulence were assayed. In infected mice, S. boulardii treatment significantly attenuated weight loss, ameliorated crypt hyperplasia (234.7 +/- 7.2 vs. 297.8 +/- 17.6 microm) and histological damage score (0.67 +/- 0.67 vs. 4.75 +/- 0.75), reduced myeloperoxidase activity (2.1 +/- 0.4 vs. 4.7 +/- 0.9 U/mg), and attenuated increased mannitol flux (17.2 +/- 5.0 vs. 31.2 +/- 8.2 nm.cm(-2).h(-1)). The ameliorating effects of S. boulardii were associated with significantly reduced numbers of mucosal adherent C. rodentium, a marked reduction in Tir protein secretion and translocation into mouse colonocytes, and a striking reduction in EspB expression and secretion. We conclude that S. boulardii maintained colonic epithelial barrier integrity and ameliorated inflammatory sequelae associated with C. rodentium infection by attenuating C. rodentium adherence to host epithelial cells through putative actions on the type III secretion system.

Rapid site-directed domain scanning mutagenesis of enteropathogenic Escherichia coli espD

We developed a rapid mutagenesis method based on a modification of the QuikChange(R) system (Stratagene) to systemically replace endogenous gene sequences with a unique similar size sequence tag. The modifications are as follows: 1: the length of the anchoring homologous sequences of both mutagenesis primers were increased to 16 - 22 bp to achieve melting temperatures greater than 80 degrees C. 2: the final concentrations of both primers were increased to 5-10 ng/microl and the final concentration of template to 1-2 ng/mul. 3: the annealing temperature was adjusted when necessary from 52 degrees C to 58 degrees C. We generated 25 sequential mutants in the cloned espD gene (1.2 kb), which encodes an essential component of the type III secretion translocon required for the pathogenesis of enteropathogenic E. coli (EPEC) infection. Each mutation consisted of the replacement of 15 codons (45 bp) with 8 codons representing a 24 bp sequence containing three unique restriction endonuclease sites (KpnI/MfeI/SpeI) starting from the second codon. The insertion of the restriction endonuclease sites provides a convenient method for further insertions of purification and/or epitope tags into permissive domains. This method is rapid, site-directed and allows for the systematic creation of mutants evenly distributed throughout the entire gene of interest.