Enteropathogenic Escherichia coli contains a putative type III secretion system necessary for the export of proteins involved in attaching and effacing lesion formation

The Evasive Enemy: Insights into the Virulence and Epidemiology of the Emerging Attaching and Effacing Pathogen Escherichia albertii

The diarrheic attaching and effacing (A/E) pathogen Escherichia albertii was first isolated from infants in Bangladesh in 1991, although the bacterium was initially classified as Hafnia alvei Subsequent genetic and biochemical interrogation of these isolates raised concerns about their initial taxonomic placement. It was not until 2003 that these isolates were reassigned to the novel taxon Escherichia albertii because they were genetically more closely related to E. coli, although they had diverged sufficiently to warrant a novel species name. Unfortunately, new isolates continue to be mistyped as enteropathogenic E. coli (EPEC) or enterohemorrhagic E. coli (EHEC) owing to shared traits, most notably the ability to form A/E lesions. Consequently, E. albertii remains an underappreciated A/E pathogen, despite multiple reports demonstrating that many provisional EPEC and EHEC isolates incriminated in disease outbreaks are actually E. albertii Metagenomic studies on dozens of E. albertii isolates reveal a genetic architecture that boasts an arsenal of candidate virulence factors to rival that of its better-characterized cousins, EPEC and EHEC. Beyond these computational comparisons, studies addressing the regulation, structure, function, and mechanism of action of its repertoire of virulence factors are lacking. Thus, the paucity of knowledge about the epidemiology, virulence, and antibiotic resistance of E. albertii, coupled with its misclassification and its ability to develop multidrug resistance in a single step, highlights the challenges in combating this emerging pathogen. This review seeks to synthesize our current but incomplete understanding of the biology of E. albertii.

Control freaks-signals and cues governing the regulation of virulence in attaching and effacing pathogens

Enterohaemorrhagic Escherichia coli (EHEC) mediates disease using a type 3 secretion system (T3SS), which is encoded on the locus of enterocyte effacement (LEE) and is tightly controlled by master regulators. This system is further modulated by a number of signals that help to fine-tune virulence, including metabolic, environmental and chemical signals. Since the LEE and its master regulator, Ler, were established, there have been numerous scientific advancements in understanding the regulation and expression of virulence factors in EHEC. This review will discuss the recent advancements in this field since our previous review, with a focus on the transcriptional regulation of the LEE.

Microbiota and Pathogen Proteases Modulate Type III Secretion Activity in Enterohemorrhagic Escherichia coli

The gut microbiota is usually regarded as providing colonization resistance against enteric pathogens. However, some pathogens evolved to thrive with the aid of certain members of the microbiota. Several Gram-negative bacteria employ type three secretion systems (T3SSs), which are molecular syringes that deliver effector proteins to host cells, hijacking host cell function. Here we show that the T3SS of enterohemorrhagic E. coli (EHEC) is cleaved by self and microbiota-derived proteases. Self-cleavage limits effector translocation, while cleavage by the microbiota member Bacteroides thetaiotamicron (Bt) exacerbates effector translocation and lesion formation on epithelial cells.

Enteric pathogens have complex interactions with the gut microbiota. Most of what is known about them has focused on microbiota-derived metabolites or small molecules that serve as nutrients and/or signals to aid in growth or transcriptionally regulate virulence gene expression. A common virulence strategy is to express a type III secretion system (T3SS), which is a molecular syringe deployed by many Gram-negative pathogens to hijack host cell function. Enterohemorrhagic Escherichia coli (EHEC) requires its T3SS to colonize the intestinal tract and cause disease. Here we report that a prominent member of the intestinal microbiota, Bacteroides thetaiotamicron (Bt), secretes proteases that cleave the translocon of the T3SS of EHEC to enhance effector translocation into host cells. This is in contrast from an endogenous protease from EHEC itself (namely, EspP) that cleaves the translocon protein EspB in a different site to limit effector translocation. The EspB protein forms the T3SS pore in mammalian cells, and pore proteins are conserved in the T3SSs from several pathogens. This is the first demonstration of a commensal species directly processing a pathogen’s T3SS, posing a new paradigm for how the microbiota can influence the severity of disease caused by bacterial pathogens. Because T3SSs are employed by many pathogens, this phenomenon has broad implications to commensal-pathogen relationships.

Bacteriophage Transcription Factor Cro Regulates Virulence Gene Expression in Enterohemorrhagic Escherichia coli

Bacteriophage-encoded genetic elements control bacterial biological functions. Enterohemorrhagic Escherichia coli (EHEC) strains harbor lambda-phages encoding the Shiga-toxin (Stx), which is expressed during the phage lytic cycle and associated with exacerbated disease. Phages also reside dormant within bacterial chromosomes through their lysogenic cycle, but how this impacts EHEC virulence remains unknown. We find that during lysogeny the phage transcription factor Cro activates the EHEC type III secretion system (T3SS). EHEC lambdoid phages are lysogenic under anaerobic conditions when Cro binds to and activates the promoters of T3SS genes. Interestingly, the Cro sequence varies among phages carried by different EHEC outbreak strains, and these changes affect Cro-dependent T3SS regulation. Additionally, infecting mice with the related pathogen C. rodentium harboring the bacteriophage cro from EHEC results in greater T3SS gene expression and enhanced virulence. Collectively, these findings reveal the role of phages in impacting EHEC virulence and their potential to affect outbreak strains.

After the Fact(or): Posttranscriptional Gene Regulation in Enterohemorrhagic Escherichia coli O157:H7

To adapt to ever-changing environments, pathogens quickly alter gene expression. This can occur through transcriptional, posttranscriptional, or posttranslational regulation. Historically, transcriptional regulation has been thoroughly studied to understand pathogen niche adaptation, whereas posttranscriptional and posttranslational gene regulation has only relatively recently been appreciated to play a central role in bacterial pathogenesis. Posttranscriptional regulation may involve chaperones, nucleases, and/or noncoding small RNAs (sRNAs) and typically controls gene expression by altering the stability and/or translation of the target mRNA. In this review, we highlight the global importance of posttranscriptional regulation to enterohemorrhagic Escherichia coli (EHEC) gene expression and discuss specific mechanisms of how EHEC regulates expression of virulence factors critical to host colonization and disease progression. The low infectious dose of this intestinal pathogen suggests that EHEC is particularly well adapted to respond to the host environment.

CRISPR Screen Reveals that EHEC's T3SS and Shiga Toxin Rely on Shared Host Factors for Infection

Enterohemorrhagic Escherichia coli (EHEC) has two critical virulence factors—a type III secretion system (T3SS) and Shiga toxins (Stxs)—that are required for the pathogen to colonize the intestine and cause diarrheal disease. Here, we carried out a genome-wide CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats with Cas9) loss-of-function screen to identify host loci that facilitate EHEC infection of intestinal epithelial cells. Many of the guide RNAs identified targeted loci known to be associated with sphingolipid biosynthesis, particularly for production of globotriaosylceramide (Gb3), the Stx receptor. Two loci (TM9SF2 and LAPTM4A) with largely unknown functions were also targeted. Mutations in these loci not only rescued cells from Stx-mediated cell death, but also prevented cytotoxicity associated with the EHEC T3SS. These mutations interfered with early events associated with T3SS and Stx pathogenicity, markedly reducing entry of T3SS effectors into host cells and binding of Stx. The convergence of Stx and T3SS onto overlapping host targets provides guidance for design of new host-directed therapeutic agents to counter EHEC infection.

Enterohemorrhagic Escherichia coli (EHEC) has two critical virulence factors—a type III secretion system (T3SS) and Shiga toxins (Stxs)—that are required for colonizing the intestine and causing diarrheal disease. We screened a genome-wide collection of CRISPR mutants derived from intestinal epithelial cells and identified mutants with enhanced survival following EHEC infection. Many had mutations that disrupted synthesis of a subset of lipids (sphingolipids) that includes the Stx receptor globotriaosylceramide (Gb3) and hence protect against Stx intoxication. Unexpectedly, we found that sphingolipids also mediate early events associated with T3SS pathogenicity. Since antibiotics are contraindicated for the treatment of EHEC, therapeutics targeting sphingolipid biosynthesis are a promising alternative, as they could provide protection against both of the pathogen’s key virulence factors.

The soluble extracellular domain of E-cadherin interferes with EPEC adherence via interaction with the Tir:intimin complex

Enteropathogenic Escherichia coli (EPEC) causes watery diarrhea when colonizing the surface of enterocytes. The translocated intimin receptor (Tir):intimin receptor complex facilitates tight adherence to epithelial cells and formation of actin pedestals beneath EPEC. We found that the host cell adherens junction protein E-cadherin (Ecad) was recruited to EPEC microcolonies. Live-cell and confocal imaging revealed that Ecad recruitment depends on, and occurs after, formation of the Tir:intimin complex. Combinatorial binding experiments using wild-type EPEC, isogenic mutants lacking Tir or intimin, and E. coli expressing intimin showed that the extracellular domain of Ecad binds the bacterial surface in a Tir:intimin-dependent manner. Finally, addition of the soluble extracellular domain of Ecad to the infection medium or depletion of Ecad extracellular domain from the cell surface reduced EPEC adhesion to host cells. Thus, the soluble extracellular domain of Ecad may be used in the design of intervention strategies targeting EPEC adherence to host cells.-Login, F. H., Jensen, H. H., Pedersen, G. A., Amieva, M. R., Nejsum, L. N. The soluble extracellular domain of E-cadherin interferes with EPEC adherence via interaction with the Tir:intimin complex.

Toxins of Locus of Enterocyte Effacement-Negative Shiga Toxin-Producing Escherichia coli

Studies on Shiga toxin-producing Escherichia coli (STEC) typically examine and classify the virulence gene profiles based on genomic analyses. Among the screened strains, a subgroup of STEC which lacks the locus of enterocyte effacement (LEE) has frequently been identified. This raises the question about the level of pathogenicity of such strains. This review focuses on the advantages and disadvantages of the standard screening procedures in virulence profiling and summarizes the current knowledge concerning the function and regulation of toxins encoded by LEE-negative STEC. Although LEE-negative STEC usually come across as food isolates, which rarely cause infections in humans, some serotypes have been implicated in human diseases. In particular, the LEE-negative E. coli O104:H7 German outbreak strain from 2011 and the Australian O113:H21 strain isolated from a HUS patient attracted attention. Moreover, the LEE-negative STEC O113:H21 strain TS18/08 that was isolated from minced meat is remarkable in that it not only encodes multiple toxins, but in fact expresses three different toxins simultaneously. Their characterization contributes to understanding the virulence of the LEE-negative STEC.

Complete Genome Sequence of Escherichia albertii Strain 1551-2, a Potential Extracellular and Intracellular Pathogen

Escherichia albertii has recently been recognized as an emerging human and bird enteric pathogen. Here, we report the complete chromosome sequence of a clinical isolate of E. albertii strain 1551-2, which may provide information about the pathogenic potential of this new species and the mechanisms of evolution of Escherichia species.

Enterohaemorrhagic and other Shiga toxin-producing Escherichia coli (STEC): Where are we now regarding diagnostics and control strategies?

Escherichia coli comprises a highly diverse group of Gram-negative bacteria and is a common member of the intestinal microflora of humans and animals. Generally, such colonization is asymptomatic; however, some E. coli strains have evolved to become pathogenic and thus cause clinical disease in susceptible hosts. One pathotype, the Shiga toxigenic E. coli (STEC) comprising strains expressing a Shiga-like toxin is an important foodborne pathogen. A subset of STEC are the enterohaemorrhagic E. coli (EHEC), which can cause serious human disease, including haemolytic uraemic syndrome (HUS). The diagnosis of EHEC infections and the surveillance of STEC in the food chain and the environment require accurate, cost-effective and timely tests. In this review, we describe and evaluate tests now in routine use, as well as upcoming test technologies for pathogen detection, including loop-mediated isothermal amplification (LAMP) and whole-genome sequencing (WGS). We have considered the need for improved diagnostic tools in current strategies for the control and prevention of these pathogens in humans, the food chain and the environment. We conclude that although significant progress has been made, STEC still remains an important zoonotic issue worldwide. Substantial reductions in the public health burden due to this infection will require a multipronged approach, including ongoing surveillance with high-resolution diagnostic techniques currently being developed and integrated into the routine investigations of public health laboratories. However, additional research requirements may be needed before such high-resolution diagnostic tools can be used to enable the development of appropriate interventions, such as vaccines and decontamination strategies.

Modulation of Host Cell Processes by T3SS Effectors

Two of the enteric Escherichia coli pathotypes-enteropathogenic E. coli (EPEC) and enterohaemorrhagic E. coli (EHEC)-have a conserved type 3 secretion system which is essential for virulence. The T3SS is used to translocate between 25 and 50 bacterial proteins directly into the host cytosol where they manipulate a variety of host cell processes to establish a successful infection. In this chapter, we discuss effectors from EPEC/EHEC in the context of the host proteins and processes that they target-the actin cytoskeleton, small guanosine triphosphatases and innate immune signalling pathways that regulate inflammation and cell death. Many of these translocated proteins have been extensively characterised, which has helped obtain insights into the mechanisms of pathogenesis of these bacteria and also understand the host pathways they target in more detail. With increasing knowledge of the positive and negative regulation of host signalling pathways by different effectors, a future challenge is to investigate how the specific effector repertoire of each strain cooperates over the course of an infection.

Novel insights into the mechanism of SepL-mediated control of effector secretion in enteropathogenic Escherichia coli

Type three secretion systems (T3SSs) are virulence determinants employed by several pathogenic bacteria as molecular syringes to inject effector proteins into host cells. Diarrhea‐producing enteropathogenic Escherichia coli (EPEC) uses a T3SS to colonize the intestinal tract. T3S is a highly coordinated process that ensures hierarchical delivery of three classes of substrates: early (inner rod and needle subunits), middle (translocators), and late (effectors). Translocation of effectors is triggered upon host‐cell contact in response to different environmental cues, such as calcium levels. The T3S substrate specificity switch from middle to late substrates in EPEC is regulated by the SepL and SepD proteins, which interact with each other and form a trimeric complex with the chaperone CesL. In this study, we investigated the link between calcium concentration and secretion regulation by the gatekeeper SepL. We found that calcium depletion promotes late substrate secretion in a translocon‐independent manner. Furthermore, the stability, formation, and subcellular localization of the SepL/SepD/CesL regulatory complex were not affected by the absence of calcium. In addition, we demonstrate that SepL interacts in a calcium‐independent manner with the major export gate component EscV, which in turn interacts with both middle and late secretion substrates, providing a docking site for T3S. These results suggest that EscV serves as a binding platform for both the SepL regulatory protein and secreted substrates during the ordered assembly of the T3SS.

Regulation of the Locus of Enterocyte Effacement in Attaching and Effacing Pathogens

Attaching and effacing (AE) pathogens colonize the gut mucosa using a type three secretion system (T3SS) and a suite of effector proteins. The locus of enterocyte effacement (LEE) is the defining genetic feature of the AE pathogens, encoding the T3SS and the core effector proteins necessary for pathogenesis. Extensive research has revealed a complex regulatory network that senses and responds to a myriad of host- and microbiota-derived signals in the infected gut to control transcription of the LEE. These signals include microbiota-liberated sugars and metabolites in the gut lumen, molecular oxygen at the gut epithelium, and host hormones. Recent research has revealed that AE pathogens also recognize physical signals, such as attachment to the epithelium, and that the act of effector translocation remodels gene expression in infecting bacteria. In this review, we summarize our knowledge to date and present an integrated view of how chemical, geographical, and physical cues regulate the virulence program of AE pathogens during infection.

The Small Regulatory RNA Spot42 Inhibits Indole Biosynthesis to Negatively Regulate the Locus of Enterocyte Effacement of Enteropathogenic Escherichia coli

The locus of enterocyte effacement is necessary for enteropathogenic Escherichia coli (EPEC) to form attaching and effacing (A/E) lesions. A/E lesions are characterized by intimate bacterial adherence to intestinal cells and destruction of microvilli, which leads to diarrhea. Therefore, studies interrogating the regulation of the locus of enterocyte effacement (LEE) are critical for understanding the molecular epidemiology of EPEC infections and developing interventional strategies. Hitherto, most studies have centered on protein-based regulators, whereas the role of small regulatory RNAs remains underappreciated. Previously, we identified the first sRNAs—MgrR, RyhB, and McaS—that regulate the LEE of EPEC. This study was undertaken to identify additional sRNAs that impact the LEE. Our results suggest that the catabolite-responsive sRNA, Spot42, indirectly controls the LEE by inhibiting synthesis of its inducer, indole. Spot42 base-pairs with the tnaCAB mRNA and presumably destabilizes the transcript, thereby preventing expression of the regulatory and structural proteins that are involved in the import and hydrolysis of tryptophan into indole. The absence of intracellular indole leads to reduced transcription of the LEE1-encoded master transcriptional activator Ler, thereby maintaining the LEE in its silenced state and delaying A/E lesion morphogenesis. Our results highlight the importance of riboregulators that synchronize metabolic and virulence pathways in bacterial infection.

The role of EscD in supporting EscC polymerization in the type III secretion system of enteropathogenic Escherichia coli

The type III secretion system (T3SS) is a multi-protein complex that plays a central role in the virulence of many Gram-negative bacterial pathogens. In enteropathogenic Escherichia coli, a prevalent cause of diarrheal diseases, the needle complex base of the T3SS is formed by multi-rings: two concentric inner-membrane rings made by the two oligomerizing proteins (EscD and EscJ), and an outer ring made of a single oligomerizing protein (EscC). Although the oligomerization activity of these proteins is critical for their function and can, therefore, affect the virulence of the pathogen, the mechanisms underlying the oligomerization of these proteins have yet to be identified. In this study, we report that the proteins forming the inner-membrane T3SS rings, EscJ and EscD proteins, are crucial for the oligomerization of EscC. Moreover, we elucidate the oligomerization process of EscD and determine the contribution of individual regions of the protein to its self-oligomerization activity. We show that the oligomerization motif of EscD is located at its N-terminal portion and that its transmembrane domain can self-oligomerize, thus contributing to the self-oligomerization of the full-length EscD.

Type III Secretion in the Melioidosis Pathogen Burkholderia pseudomallei

Burkholderia pseudomallei is a Gram-negative intracellular pathogen and the causative agent of melioidosis, a severe disease of both humans and animals. Melioidosis is an emerging disease which is predicted to be vastly under-reported. Type III Secretion Systems (T3SSs) are critical virulence factors in Gram negative pathogens of plants and animals. The genome of B. pseudomallei encodes three T3SSs. T3SS-1 and -2, of which little is known, are homologous to Hrp2 secretion systems of the plant pathogens Ralstonia and Xanthomonas. T3SS-3 is better characterized and is homologous to the Inv/Mxi-Spa secretion systems of Salmonella spp. and Shigella flexneri, respectively. Upon entry into the host cell, B. pseudomallei requires T3SS-3 for efficient escape from the endosome. T3SS-3 is also required for full virulence in both hamster and murine models of infection. The regulatory cascade which controls T3SS-3 expression and the secretome of T3SS-3 have been described, as well as the effect of mutations of some of the structural proteins. Yet only a few effector proteins have been functionally characterized to date and very little work has been carried out to understand the hierarchy of assembly, secretion and temporal regulation of T3SS-3. This review aims to frame current knowledge of B. pseudomallei T3SSs in the context of other well characterized model T3SSs, particularly those of Salmonella and Shigella.

Comparative genomics and transcriptomics of Escherichia coli isolates carrying virulence factors of both enteropathogenic and enterotoxigenic E. coli

Escherichia coli that are capable of causing human disease are often classified into pathogenic variants (pathovars) based on their virulence gene content. However, disease-associated hybrid E. coli, containing unique combinations of multiple canonical virulence factors have also been described. Such was the case of the E. coli O104:H4 outbreak in 2011, which caused significant morbidity and mortality. Among the pathovars of diarrheagenic E. coli that cause significant human disease are the enteropathogenic E. coli (EPEC) and enterotoxigenic E. coli (ETEC). In the current study we use comparative genomics, transcriptomics, and functional studies to characterize isolates that contain virulence factors of both EPEC and ETEC. Based on phylogenomic analysis, these hybrid isolates are more genomically-related to EPEC, but appear to have acquired ETEC virulence genes. Global transcriptional analysis using RNA sequencing, demonstrated that the EPEC and ETEC virulence genes of these hybrid isolates were differentially-expressed under virulence-inducing laboratory conditions, similar to reference isolates. Immunoblot assays further verified that the virulence gene products were produced and that the T3SS effector EspB of EPEC, and heat-labile toxin of ETEC were secreted. These findings document the existence and virulence potential of an E. coli pathovar hybrid that blurs the distinction between E. coli pathovars.

The GadX regulon affects virulence gene expression and adhesion of porcine enteropathogenic Escherichia coli in vitro

The ability of enteropathogenic Escherichia coli (EPEC) to express virulence factor genes and develop attaching and effacing (AE) lesions is inhibited in acidic environmental conditions. This inhibition is due to the activation of transcription factor GadX, which upregulates expression of glutamic acid decarboxylase (Gad). Gad, in turn, produces γ-aminobutyric acid (GABA), which was recently shown to have a beneficial effect on the jejunal epithelium in vitro due to increased mucin-1 levels.

In the present study, we sought to test whether forced GadX activation/overexpression abolishes virulence associated features of EPEC and provokes increased GABA production. EPEC strains were isolated from diarrheic pigs and submitted to activation of GadX by acidification as well as gadX overexpression via an inducible expression vector plasmid. GABA concentrations in the growth medium, ability for adhesion to porcine intestinal epithelial cells (IPEC-J2) and virulence gene expression were determined.

Growth in acidified media led to increased GABA levels, upregulated gadA/B expression and downregulated mRNA synthesis of the bacterial adhesin intimin. EPEC strains transformed with the gadX gene produced 2.1–3.4-fold higher GABA levels than empty-vector controls and completely lost their ability to adhere to IPEC-J2 cells and to induce actin accumulation.

We conclude that intensified gadX activation can abolish the ability of EPEC to adhere to the intestinal epithelium by reducing the expression of major virulence genes.

Oxygen as a driver of gut dysbiosis

Changes in the composition of gut-associated microbial communities may underlie many inflammatory and allergic diseases. However, the processes that help maintain a stable community structure are poorly understood. Here we review topical work elucidating the nutrient-niche occupied by facultative anaerobic bacteria of the family Enterobacteriaceae, whose predominance within the gut-associated microbial community is a common marker of dysbiosis. A paucity of exogenous respiratory electron acceptors limits growth of Enterobacteriaceae within a balanced gut-associated microbial community. However, recent studies suggest that the availability of oxygen in the large bowel is markedly elevated by changes in host physiology that accompany antibiotic treatment or infection with enteric pathogens, such as Salmonella serovars or attaching and effacing (AE) pathogens. The resulting increase in oxygen availability, alone or in conjunction with other electron acceptors, drives an uncontrolled luminal expansion of Enterobacteriaceae. Insights into the underlying mechanisms provide important clues about factors that control the balance between the host and its resident microbial communities.

A Novel Mechanism for Protein Delivery by the Type 3 Secretion System for Extracellularly Secreted Proteins

The type 3 secretion system (T3SS) is essential for bacterial virulence through delivering effector proteins directly into the host cytosol. Here, we identified an alternative delivery mechanism of virulence factors mediated by the T3SS, which consists of the association of extracellularly secreted proteins from bacteria with the T3SS to gain access to the host cytosol. Both EspC, a protein secreted as an enteropathogenic Escherichia coli (EPEC) autotransporter, and YopH, a protein detected on the surface of Yersinia, require a functional T3SS for host cell internalization; here we provide biophysical and molecular evidence to support the concept of the EspC translocation mechanism, which requires (i) an interaction between EspA and an EspC middle segment, (ii) an EspC translocation motif (21 residues that are shared with the YopH translocation motif), (iii) increases in the association and dissociation rates of EspC mediated by EspA interacting with EspD, and (iv) an interaction of EspC with the EspD/EspB translocon pore. Interestingly, this novel mechanism does not exclude the injection model (i.e., EspF) operating through the T3SS conduit; therefore, T3SS can be functioning as an internal conduit or as an external railway, which can be used to reach the translocator pore, and this mechanism appears to be conserved among different T3SS-dependent pathogens.

The type 3 secretion system is essential for injection of virulence factors, which are delivered directly into the cytosol of the host cells for usurping and subverting host processes. Recent studies have shown that these effectors proteins indeed travel inside an “injectisome” conduit through a single step of translocation by connecting the bacterium and host cell cytoplasms. However, all findings are not compatible with this model. For example, both YopH, a protein detected on the surface of Yersinia, and EspC, an autotransporter protein secreted by enteropathogenic E. coli, require a functional T3SS for host cell translocation. Both proteins have an intermediate extracellular step before their T3SS-dependent translocation. Here, we show an alternative delivery mechanism for these extracellularly secreted virulence factors that are then incorporated into the T3SS to enter the cells; this novel mechanism coexists with but diverges from the canonical injection model that involves the passage of the protein inside the injectisome.

Genetic and Mechanistic Analyses of the Periplasmic Domain of the Enterohemorrhagic Escherichia coli QseC Histidine Sensor Kinase

The histidine sensor kinase (HK) QseC senses autoinducer 3 (AI-3) and the adrenergic hormones epinephrine and norepinephrine. Upon sensing these signals, QseC acts through three response regulators (RRs) to regulate the expression of virulence genes in enterohemorrhagic Escherichia coli (EHEC). The QseB, QseF, and KdpE RRs that are phosphorylated by QseC constitute a tripartite signaling cascade having different and overlapping targets, including flagella and motility, the type three secretion system encoded by the locus of enterocyte effacement (LEE), and Shiga toxin. We modeled the tertiary structure of QseC's periplasmic sensing domain and aligned the sequences from 12 different species to identify the most conserved amino acids. We selected eight amino acids conserved in all of these QseC homologues. The corresponding QseC site-directed mutants were expressed and still able to autophosphorylate; however, four mutants demonstrated an increased basal level of phosphorylation. These mutants have differential flagellar, motility, LEE, and Shiga toxin expression phenotypes. We selected four mutants for more in-depth analyses and found that they differed in their ability to phosphorylate QseB, KdpE, and QseF. This suggests that these mutations in the periplasmic sensing domain affected the region downstream of the QseC signaling cascade and therefore can influence which pathway QseC regulates.IMPORTANCE In the foodborne pathogen EHEC, QseC senses AI-3, epinephrine, and norepinephrine, increases its autophosphorylation, and then transfers its phosphate to three RRs: QseB, QseF, and KdpE. QseB controls expression of flagella and motility, KdpE controls expression of the LEE region, and QseF controls the expression of Shiga toxin. This tripartite signaling pathway must be tightly controlled, given that flagella and the type three secretion system (T3SS) are energetically expensive appendages and Shiga toxin expression leads to bacterial cell lysis. Our data suggest that mutations in the periplasmic sensing loop of QseC differentially affect the expression of the three arms of this signaling cascade. This suggests that these point mutations may change QseC's phosphotransfer preferences for its RRs.

Functional Characterization of EscK (Orf4), a Sorting Platform Component of the Enteropathogenic Escherichia coli Injectisome

The type III secretion system (T3SS) is a supramolecular machine used by many bacterial pathogens to translocate effector proteins directly into the eukaryotic host cell cytoplasm. Enteropathogenic Escherichia coli (EPEC) is an important cause of infantile diarrheal disease in underdeveloped countries. EPEC virulence relies on a T3SS encoded within a chromosomal pathogenicity island known as the locus of enterocyte effacement (LEE). In this work, we pursued the functional characterization of the LEE-encoded protein EscK (previously known as Orf4). We provide evidence indicating that EscK is crucial for efficient T3S and belongs to the SctK (OrgA/YscK/MxiK) protein family, whose members have been implicated in the formation of a sorting platform for secretion of T3S substrates. Bacterial fractionation studies showed that EscK localizes to the inner membrane independently of the presence of any other T3SS component. Combining yeast two-hybrid screening and pulldown assays, we identified an interaction between EscK and the C-ring/sorting platform component EscQ. Site-directed mutagenesis of conserved residues revealed amino acids that are critical for EscK function and for its interaction with EscQ. In addition, we found that T3S substrate overproduction is capable of compensating for the absence of EscK. Overall, our data suggest that EscK is a structural component of the EPEC T3SS sorting platform, playing a central role in the recruitment of T3S substrates for boosting the efficiency of the protein translocation process.

IMPORTANCE

The type III secretion system (T3SS) is an essential virulence determinant for enteropathogenic Escherichia coli (EPEC) colonization of intestinal epithelial cells. Multiple EPEC effector proteins are injected via the T3SS into enterocyte cells, leading to diarrheal disease. The T3SS is encoded within a genomic pathogenicity island termed the locus of enterocyte effacement (LEE). Here we unravel the function of EscK, a previously uncharacterized LEE-encoded protein. We show that EscK is central for T3SS biogenesis and function. EscK forms a protein complex with EscQ, the main component of the cytoplasmic sorting platform, serving as a docking site for T3S substrates. Our results provide a comprehensive functional analysis of an understudied component of T3SSs.

Catabolite and Oxygen Regulation of Enterohemorrhagic Escherichia coli Virulence

The biogeography of the gut is diverse in its longitudinal axis, as well as within specific microenvironments. Differential oxygenation and nutrient composition drive the membership of microbial communities in these habitats. Moreover, enteric pathogens can orchestrate further modifications to gain a competitive advantage toward host colonization. These pathogens are versatile and adept when exploiting the human colon. They expertly navigate complex environmental cues and interkingdom signaling to colonize and infect their hosts. Here we demonstrate how enterohemorrhagic Escherichia coli (EHEC) uses three sugar-sensing transcription factors, Cra, KdpE, and FusR, to exquisitely regulate the expression of virulence factors associated with its type III secretion system (T3SS) when exposed to various oxygen concentrations. We also explored the effect of mucin-derived nonpreferred carbon sources on EHEC growth and expression of virulence genes. Taken together, the results show that EHEC represses the expression of its T3SS when oxygen is absent, mimicking the largely anaerobic lumen, and activates its T3SS when oxygen is available through Cra. In addition, when EHEC senses mucin-derived sugars heavily present in the O-linked and N-linked glycans of the large intestine, virulence gene expression is initiated. Sugars derived from pectin, a complex plant polysaccharide digested in the large intestine, also increased virulence gene expression. Not only does EHEC sense host- and microbiota-derived interkingdom signals, it also uses oxygen availability and mucin-derived sugars liberated by the microbiota to stimulate expression of the T3SS. This precision in gene regulation allows EHEC to be an efficient pathogen with an extremely low infectious dose.

Enteric pathogens have to be crafty when interpreting multiple environmental cues to successfully establish themselves within complex and diverse gut microenvironments. Differences in oxygen tension and nutrient composition determine the biogeography of the gut microbiota and provide unique niches that can be exploited by enteric pathogens. EHEC is an enteric pathogen that colonizes the colon and causes outbreaks of bloody diarrhea and hemolytic-uremic syndrome worldwide. It has a very low infectious dose, which requires it to be an extremely effective pathogen. Hence, here we show that EHEC senses multiple sugar sources and oxygen levels to optimally control the expression of its virulence repertoire. This exquisite regulatory control equips EHEC to sense different intestinal compartments to colonize the host.

Type Three Secretion System in Attaching and Effacing Pathogens

Enteropathogenic Escherichia coli and enterohemorrhagic E. coli are diarrheagenic bacterial human pathogens that cause severe gastroenteritis. These enteric pathotypes, together with the mouse pathogen Citrobacter rodentium, belong to the family of attaching and effacing pathogens that form a distinctive histological lesion in the intestinal epithelium. The virulence of these bacteria depends on a type III secretion system (T3SS), which mediates the translocation of effector proteins from the bacterial cytosol into the infected cells. The core architecture of the T3SS consists of a multi-ring basal body embedded in the bacterial membranes, a periplasmic inner rod, a transmembrane export apparatus in the inner membrane, and cytosolic components including an ATPase complex and the C-ring. In addition, two distinct hollow appendages are assembled on the extracellular face of the basal body creating a channel for protein secretion: an approximately 23 nm needle, and a filament that extends up to 600 nm. This filamentous structure allows these pathogens to get through the host cells mucus barrier. Upon contact with the target cell, a translocation pore is assembled in the host membrane through which the effector proteins are injected. Assembly of the T3SS is strictly regulated to ensure proper timing of substrate secretion. The different type III substrates coexist in the bacterial cytoplasm, and their hierarchical secretion is determined by specialized chaperones in coordination with two molecular switches and the so-called sorting platform. In this review, we present recent advances in the understanding of the T3SS in attaching and effacing pathogens.

Plasmids from Shiga Toxin-Producing Escherichia coli Strains with Rare Enterohemolysin Gene (ehxA) Subtypes Reveal Pathogenicity Potential and Display a Novel Evolutionary Path

Most Shiga toxin-producing Escherichia coli (STEC) strains associated with severe disease, such as hemolytic-uremic syndrome (HUS), carry large enterohemolysin-encoding (ehxA) plasmids, e.g., pO157 and pO103, that contribute to STEC clinical manifestations. Six ehxA subtypes (A through F) exist that phylogenetically cluster into eae-positive (B, C, F), a mix of eae-positive (E) and eae-negative (A), and a third, more distantly related, cluster of eae-negative (D) STEC strains. While subtype B, C, and F plasmids share a number of virulence traits that are distinct from those of subtype A, sequence data have not been available for subtype D and E plasmids. Here, we determined and compared the genetic composition of four subtype D and two subtype E plasmids to establish their evolutionary relatedness among ehxA subtypes and define their potential role in pathogenicity. We found that subtype D strains carry one exceptionally large plasmid (>200 kbp) that carries a variety of virulence genes that are associated with enterotoxigenic and enterohemorrhagic E. coli, which, quite possibly, enables these strains to cause disease despite being food isolates. Our data offer further support for the hypothesis that this subtype D plasmid represents a novel virulence plasmid, sharing very few genetic features with other plasmids; we conclude that these plasmids have evolved from a different evolutionary lineage than the plasmids carrying the other ehxA subtypes. In contrast, the 50-kbp plasmids of subtype E (pO145), although isolated from HUS outbreak strains, carried only few virulence-associated determinants, suggesting that the clinical presentation of subtype E strains is largely a result of chromosomally encoded virulence factors.

IMPORTANCE

Bacterial plasmids are known to be key agents of change in microbial populations, promoting the dissemination of various traits, such as drug resistance and virulence. This study determined the genetic makeup of virulence plasmids from rare enterohemolysin subtype D and E Shiga toxin-producing E. coli strains. We demonstrated that ehxA subtype D plasmids represent a novel E. coli virulence plasmid, and although subtype D plasmids were derived from nonclinical isolates, they encoded a variety of virulence determinants that are associated with pathogenic E. coli In contrast, subtype E plasmids, isolated from strains recovered from severely ill patients, carry only a few virulence determinants. The results of this study reemphasize the plasticity and vast diversity among E. coli plasmids. This work demonstrates that, although E. coli strains of certain serogroups may not be frequently associated with disease, they should not be underestimated in protecting human health and food safety.

Virulence factors enhance Citrobacter rodentium expansion through aerobic respiration

Citrobacter rodentium uses a type III secretion system (T3SS) to induce colonic crypt hyperplasia in mice, thereby gaining an edge during its competition with the gut microbiota through an unknown mechanism. Here, we show that by triggering colonic crypt hyperplasia, the C. rodentium T3SS induced an excessive expansion of undifferentiated Ki67-positive epithelial cells, which increased oxygenation of the mucosal surface and drove an aerobic C. rodentium expansion in the colon. Treatment of mice with the γ-secretase inhibitor dibenzazepine to diminish Notch-driven colonic crypt hyperplasia curtailed the fitness advantage conferred by aerobic respiration during C. rodentium infection. We conclude that C. rodentium uses its T3SS to induce histopathological lesions that generate an intestinal microenvironment in which growth of the pathogen is fueled by aerobic respiration.

Microbiota and pathogen 'pas de deux': setting up and breaking down barriers to intestinal infection

The gut microbiota plays essential roles in human health and disease. In this review, we focus on the role of the intestinal microbiota in promoting resistance to infection by bacterial pathogens as well as how pathogens overcome this barrier. We discuss how the resident microbiota restricts growth and colonization of invading pathogens by limiting availability of nutrients and through generation of a hostile environment. Additionally, we examine how microbiota-derived signaling molecules interfere with bacterial virulence. In turn, we discuss how pathogens exploit non-competitive metabolites to replicate in vivo as well as to precisely control virulence and cause disease. This bacterial two step of creating and overcoming challenges important in preventing and establishing infection highlights the complexities of elucidating interactions between the commensal bacteria and pathogens. Better understanding of microbiota-pathogen interplay will have significant implications for developing novel therapeutics to treat infectious diseases.

The Type Three Secretion System 2-Encoded Regulator EtrB Modulates Enterohemorrhagic Escherichia coli Virulence Gene Expression

Enterohemorrhagic Escherichia coli O157:H7 (EHEC) is a foodborne pathogen that causes bloody diarrhea and hemolytic uremic syndrome throughout the world. A defining feature of EHEC pathogenesis is the formation of attaching and effacing (AE) lesions on colonic epithelial cells. Most of the genes that code for AE lesion formation, including a type three secretion system (T3SS) and effectors, are carried within a chromosomal pathogenicity island called the locus of enterocyte effacement (LEE). In this study, we report that a putative regulator, which is encoded in the cryptic E. coli type three secretion system 2 (ETT2) locus and herein renamed EtrB, plays an important role in EHEC pathogenesis. The etrB gene is expressed as a monocistronic transcript, and EtrB autoregulates expression. We provide evidence that EtrB directly interacts with the ler regulatory region to activate LEE expression and promote AE lesion formation. Additionally, we mapped the EtrB regulatory circuit in EHEC to determine a global role for EtrB. EtrB is regulated by the transcription factor QseA, suggesting that these proteins comprise a regulatory circuit important for EHEC colonization of the gastrointestinal tract.

EspC, an Autotransporter Protein Secreted by Enteropathogenic Escherichia coli, Causes Apoptosis and Necrosis through Caspase and Calpain Activation, Including Direct Procaspase-3 Cleavage

Enteropathogenic Escherichia coli (EPEC) has the ability to antagonize host apoptosis during infection through promotion and inhibition of effectors injected by the type III secretion system (T3SS), but the total number of these effectors and the overall functional relationships between these effectors during infection are poorly understood. EspC produced by EPEC cleaves fodrin, paxillin, and focal adhesion kinase (FAK), which are also cleaved by caspases and calpains during apoptosis. Here we show the role of EspC in cell death induced by EPEC. EspC is involved in EPEC-mediated cell death and induces both apoptosis and necrosis in epithelial cells. EspC induces apoptosis through the mitochondrial apoptotic pathway by provoking (i) a decrease in the expression levels of antiapoptotic protein Bcl-2, (ii) translocation of the proapoptotic protein Bax from cytosol to mitochondria, (iii) cytochrome c release from mitochondria to the cytoplasm, (iv) loss of mitochondrial membrane potential, (v) caspase-9 activation, (vi) cleavage of procaspase-3 and (vii) an increase in caspase-3 activity, (viii) PARP proteolysis, and (ix) nuclear fragmentation and an increase in the sub-G1 population. Interestingly, EspC-induced apoptosis was triggered through a dual mechanism involving both independent and dependent functions of its EspC serine protease motif, the direct cleavage of procaspase-3 being dependent on this motif. This is the first report showing a shortcut for induction of apoptosis by the catalytic activity of an EPEC protein. Furthermore, this atypical intrinsic apoptosis appeared to induce necrosis through the activation of calpain and through the increase of intracellular calcium induced by EspC. Our data indicate that EspC plays a relevant role in cell death induced by EPEC.

IMPORTANCE

EspC, an autotransporter protein with serine protease activity, has cytotoxic effects on epithelial cells during EPEC infection. EspC causes cytotoxicity by cleaving fodrin, a cytoskeletal actin-associated protein, and focal adhesion proteins (i.e., FAK); interestingly, these proteins are also cleaved during apoptosis and necrosis. Here we show that EspC is able to cause cell death, which is characterized by apoptosis: by dissecting the apoptotic pathway and considering that EspC is translocated by an injectisome, we found that EspC induces the mitochondrial apoptotic pathway. Remarkably, EspC activates this pathway by two distinct mechanisms-either by using or not using its serine protease motif. Thus, we show for the first time that this serine protease motif is able to cleave procaspase-3, thereby reaching the terminal stages of caspase cascade activation leading to apoptosis. Furthermore, this overlapped apoptosis appears to potentiate cell death through necrosis, where EspC induces calpain activation and increases intracellular calcium.

Bacterial Adrenergic Sensors Regulate Virulence of Enteric Pathogens in the Gut

Enteric pathogens such as enterohemorrhagic Escherichia coli (EHEC) and Citrobacter rodentium, which is largely used as a surrogate EHEC model for murine infections, are exposed to several host neurotransmitters in the gut. An important chemical exchange within the gut involves the neurotransmitters epinephrine and/or norepinephrine, extensively reported to increase virulence gene expression in EHEC, acting through two bacterial adrenergic sensors: QseC and QseE. However, EHEC is unable to establish itself and cause its hallmark lesions, attaching and effacing (AE) lesions, on murine enterocytes. To address the role of these neurotransmitters during enteric infection, we employed C. rodentium. Both EHEC and C. rodentium harbor the locus of enterocyte effacement (LEE) that is necessary for AE lesion formation. Here we show that expression of the LEE, as well as that of other virulence genes in C. rodentium, is also activated by epinephrine and/or norepinephrine. Both QseC and QseE are required for LEE gene activation in C. rodentium, and the qseC and qseE mutants are attenuated for murine infection. C. rodentium has a decreased ability to colonize dopamine β-hydroxylase knockout (Dbh^−/−) mice, which do not produce epinephrine and norepinephrine. Both adrenergic sensors are required for C. rodentium to sense these neurotransmitters and activate the LEE genes during infection. These data indicate that epinephrine and norepinephrine are sensed by bacterial adrenergic receptors during enteric infection to promote activation of their virulence repertoire. This is the first report of the role of these neurotransmitters during mammalian gastrointestinal (GI) infection by a noninvasive pathogen.

The epinephrine and norepinephrine neurotransmitters play important roles in gut physiology and motility. Of note, epinephrine and norepinephrine play a central role in stress responses in mammals, and stress has profound effects on GI function. Bacterial enteric pathogens exploit these neurotransmitters as signals to coordinate the regulation of their virulence genes. The bacterial QseC and QseE adrenergic sensors are at the center of this regulatory cascade. C. rodentium is a noninvasive murine pathogen with a colonization mechanism similar to that of EHEC, enabling the investigation of host signals in mice. The presence of these neurotransmitters in the gut is necessary for C. rodentium to fully activate its virulence program, in a QseC/QseE-dependent manner, to successfully colonize its murine host. Our study data provide the first example of epinephrine and norepinephrine signaling within the gut to stimulate infection by a bacterial pathogen in a natural animal infection.

Enteric Pathogens Exploit the Microbiota-generated Nutritional Environment of the Gut

Host bacterial associations have a profound impact on health and disease. The human gastrointestinal (GI) tract is inhabited by trillions of commensal bacteria that aid in the digestion of food and vitamin production and play crucial roles in human physiology. Disruption of these relationships and the structure of the bacterial communities that inhabit the gut can contribute to dysbiosis, leading to disease. This fundamental relationship between the host and microbiota relies on chemical signaling and nutrient availability and exchange. GI pathogens compete with the endogenous microbiota for a colonization niche (1, 2). The ability to monitor nutrients and combine this information with the host physiological state is important for the pathogen to precisely program the expression of its virulence repertoire. A major nutrient source is carbon, and although the impact of carbon nutrition on the colonization of the gut by the microbiota has been extensively studied, the extent to which carbon sources affect the regulation of virulence factors by invading pathogens has not been fully defined. The GI pathogen enterohemorrhagic E. coli (EHEC) gages sugar sources as an important cue to regulate expression of its virulence genes. EHEC senses whether it is in a gluconeogenic versus a glycolytic environment, as well as fluctuations of fucose levels to fine tune regulation of its virulence repertoire.

The Locus of Enterocyte Effacement and Associated Virulence Factors of Enterohemorrhagic Escherichia coli

A subset of Shiga toxin-producing Escherichia coli strains, termed enterohemorrhagic E. coli (EHEC), is defined in part by the ability to produce attaching and effacing (A/E) lesions on intestinal epithelia. Such lesions are characterized by intimate bacterial attachment to the apical surface of enterocytes, cytoskeletal rearrangements beneath adherent bacteria, and destruction of proximal microvilli. A/E lesion formation requires the locus of enterocyte effacement (LEE), which encodes a Type III secretion system that injects bacterial proteins into host cells. The translocated proteins, termed effectors, subvert a plethora of cellular pathways to the benefit of the pathogen, for example, by recruiting cytoskeletal proteins, disrupting epithelial barrier integrity, and interfering with the induction of inflammation, phagocytosis, and apoptosis. The LEE and selected effectors play pivotal roles in intestinal persistence and virulence of EHEC, and it is becoming clear that effectors may act in redundant, synergistic, and antagonistic ways during infection. Vaccines that target the function of the Type III secretion system limit colonization of reservoir hosts by EHEC and may thus aid control of zoonotic infections. Here we review the features and functions of the LEE-encoded Type III secretion system and associated effectors of E. coli O157:H7 and other Shiga toxin-producing E. coli strains.

Contributions of EspA Filaments and Curli Fimbriae in Cellular Adherence and Biofilm Formation of Enterohemorrhagic Escherichia coli O157:H7

In Escherichia coli O157:H7 (O157), the filamentous structure of the type III secretion system is produced from the polymerization of the EspA protein. EspA filaments are essential for O157 adherence to epithelial cells. In previous studies, we demonstrated that O157 hha deletion mutants showed increased adherence to HEp-2 cells and produced abundant biofilms. Transcriptional analysis revealed increased expression of espA as well as the csgA gene, which encodes curli fimbriae that are essential for biofilm formation. In the present study, we constructed hha espA, hha csgA, and hha csgA espA deletion mutants to determine the relative importance of EspA and CsgA in O157 adherence to HEp-2 cells and biofilm formation. In vitro adherence assays, conducted at 37°C in a tissue culture medium containing 0.1% glucose, showed that HEp-2 cell adherence required EspA because hha espA and hha csgA espA mutants adhered to HEp-2 cells at higher levels only when complemented with an espA-expressing plasmid. Biofilm assays performed at 28°C in a medium lacking glucose showed dependency of biofilm formation on CsgA; however EspA was not produced under these conditions. Despite production of detectable levels of EspA at 37°C in media supplemented with 0.1% glucose, the biofilm formation occurred independent of EspA. These results indicate dependency of O157 adherence to epithelial cells on EspA filaments, while CsgA promoted biofilm formation under conditions mimicking those found in the environment (low temperature with nutrient limitations) and in the digestive tract of an host animal (higher temperature and low levels of glucose).

The Epinephrine/Norepinephrine/Autoinducer-3 Interkingdom Signaling System in Escherichia coli O157:H7

Epinephrine/norepinephrine/AI-3 signaling is used as an interkingdom chemical signaling system between microbes and their hosts. This system is also exploited by pathogens to regulate virulence traits. In enterohemorrhagic E. coli (EHEC) O157:H7, it is essential for pathogenesis and flagella motility. These three signals activate expression of a pathogenicity island named locus of enterocyte effacement (LEE), Shiga toxin, and the flagella regulon. These signals are sensed by the two-component system QseBC, whereas the bacterial membrane receptor QseC autophosphorylates and phosphorylates the QseB response regulator initiating a complex phosphorelay signaling cascade that activates the expression of a second two-component system, QseEF. The QseEF two-component system is also involved in the expression of the virulence genes, and it senses epinephrine, phosphate, and sulfate. This complex signaling cascade still needs to be completely elucidated.

Commensal 'trail of bread crumbs' provide pathogens with a map to the intestinal landscape

Growth of a microorganism in a host is essential for infection, and bacterial pathogens have evolved to utilize specific metabolites to enhance replication in vivo. Now, emerging data demonstrate that pathogens rely on microbiota-derived metabolites as a form of bacterial-bacterial communication to gain information about location within a host and modify virulence gene expression accordingly. Thus, metabolite-sensing is critical for pathogens to establish infection. Here, we highlight recent examples of how the foodborne pathogen enterohemorrhagic Escherichia coli O157:H7 (EHEC) exploits microbiota-derived metabolites to recognize the host intestinal environment and control gene expression that results in controlled expression of virulence traits.

The Surface Sensor NlpE of Enterohemorrhagic Escherichia coli Contributes to Regulation of the Type III Secretion System and Flagella by the Cpx Response to Adhesion

Although the adhesion of enterohemorrhagic Escherichia coli (EHEC) is central to the EHEC-host interaction during infection, it remains unclear how such adhesion regulates virulence factors. Adhesion to abiotic surfaces by E. coli has been reported to be an outer membrane lipoprotein NlpE-dependent activation cue of the Cpx pathway. Therefore, we investigated the role of NlpE in EHEC on the adhesion-mediated expression of virulence genes. NlpE in EHEC contributed to upregulation of the locus of enterocyte effacement (LEE) genes encoded type III secretion system and to downregulated expression of the flagellin gene by activation of the Cpx pathway during adherence to hydrophobic glass beads and undifferentiated Caco-2 cells. Moreover, LysR homologue A (LrhA) in EHEC was involved in regulating the expression of the LEE genes and flagellin gene in response to adhesion. Gel mobility shift analysis revealed that response regulator CpxR bound to the lrhA promoter region and thereby regulated expressions of the LEE genes and flagellin gene via the transcriptional regulator LrhA in EHEC. Therefore, these results suggest that the sensing of adhesion signals via NlpE is important for regulation of the expression of the type III secretion system and flagella in EHEC during infection.

Tir Is Essential for the Recruitment of Tks5 to Enteropathogenic Escherichia coli Pedestals

Enteropathogenic Escherichia coli (EPEC) is a bacterial pathogen that infects the epithelial lining of the small intestine and causes diarrhea. Upon attachment to the intestinal epithelium, EPEC uses a Type III Secretion System to inject its own high affinity receptor Translocated intimin receptor (Tir) into the host cell. Tir facilitates tight adhesion and recruitment of actin-regulating proteins leading to formation of an actin pedestal beneath the infecting bacterium. The pedestal has several similarities with podosomes, which are basolateral actin-rich extensions found in some migrating animal cells. Formation of podosomes is dependent upon the early podosome-specific scavenger protein Tks5, which is involved in actin recruitment. Although Tks5 is expressed in epithelial cells, and podosomes and EPEC pedestals share many components in their structure and mechanism of formation, the potential role of Tks5 in EPEC infections has not been studied. The aim of this study was to determine the subcellular localization of Tks5 in epithelial cells and to investigate if Tks5 is recruited to the EPEC pedestal. In an epithelial MDCK cell line stably expressing Tks5-EGFP, Tks5 localized to actin bundles. Upon infection, EPEC recruited Tks5-EGFP. Tir, but not Tir phosphorylation was essential for the recruitment. Time-lapse microscopy revealed that Tks5-EGFP was recruited instantly upon EPEC attachment to host cells, simultaneously with actin and N-WASp. EPEC infection of cells expressing a ΔPX-Tks5 deletion version of Tks5 showed that EPEC was able to both infect and form pedestals when the PX domain was deleted from Tks5. Future investigations will clarify the role of Tks5 in EPEC infection and pedestal formation.

Adhesins of Enteropathogenic Escherichia coli

Enteropathogenic Escherichia coli (EPEC) strains induce morphological changes in infected epithelial cells. The resulting attaching and effacing (A/E) lesion is characterized by intimate bacterial adherence to epithelial cells, with microvillus destruction, cytoskeletal rearrangement, and aggregation of host cytoskeletal proteins. This review presents an overview of the adhesion mechanisms used for the colonization of the human gastrointestinal tract by EPEC. The mechanisms underlying EPEC adhesion, prior to and during the formation of the A/E lesion, and the host cytosolic responses to bacterial infection leading to diarrheal disease are discussed.

Pore-forming Activity of the Escherichia coli Type III Secretion System Protein EspD

Enterohemorrhagic Escherichia coli is a causative agent of gastrointestinal and diarrheal diseases. Pathogenesis associated with enterohemorrhagic E. coli involves direct delivery of virulence factors from the bacteria into epithelial cell cytosol via a syringe-like organelle known as the type III secretion system. The type III secretion system protein EspD is a critical factor required for formation of a translocation pore on the host cell membrane. Here, we show that recombinant EspD spontaneously integrates into large unilamellar vesicle (LUV) lipid bilayers; however, pore formation required incorporation of anionic phospholipids such as phosphatidylserine and an acidic pH. Leakage assays performed with fluorescent dextrans confirmed that EspD formed a structure with an inner diameter of ∼2.5 nm. Protease mapping indicated that the two transmembrane helical hairpin of EspD penetrated the lipid layer positioning the N- and C-terminal domains on the extralumenal surface of LUVs. Finally, a combination of glutaraldehyde cross-linking and rate zonal centrifugation suggested that EspD in LUV membranes forms an ∼280-320-kDa oligomeric structure consisting of ∼6-7 subunits.

Core 2 Mucin-Type O-Glycan Is Related to EPEC and EHEC O157:H7 Adherence to Human Colon Carcinoma HT-29 Epithelial Cells

BACKGROUND AND AIM

The roles of host glycosylation in interactions with EPEC and EHEC O157:H7 are largely unclear; this study examined whether O-glycans are involved in EPEC and EHEC O157:H7 adherence to HT-29 cells.

METHODS

Bacterial adherence to the cultured cells was determined using the direct co-staining of adherent bacteria and host cells, the adherent bacteria plating, and/or the direct fluorescent observation of the adherent GFP-labeled bacteria.

RESULTS

A comparison of the adherence of EPEC and EHEC O157:H7 to HT-29-Gal and HT-29 cells indicated that the differentiation of HT-29 cells led to a reduction in the adherence of EPEC and EHEC O157:H7. EPEC and EHEC O157:H7 adhesion decreased after the abrogation of O-glycan biosynthesis mediated by benzyl-α-GalNAc treatment. Core 2 O-glycan-deficient HT-29 cells induced by C2GnT2 knockdown had a significant reduction in EPEC and EHEC O157:H7 adhesion in C2GnT2-sh2/HT-29 cells compared with HT-29 and shRNA-Ctr/HT-29 cells. MUC2 expression in benzyl-α-GalNAc-treated HT-29 cells was significantly reduced but unchanged in C2GnT2-deficient HT-29 cells. EPEC or EHEC O157:H7 infection in C2GnT2-deficient HT-29 cells deteriorated the epithelial barrier function. The occludin expression in the shRNA-Ctr/HT-29 and C2GnT2-sh2/HT-29 cells after infection with EPEC or EHEC O157:H7 was pyknic and discontinuous at the cell surface compared with its continuous distribution of control cells. These data indicate that EPEC and EHEC O157:H7 adherence to HT-29 cells is related to mucin-type core 2 O-glycan.

CONCLUSIONS

This study provides the concepts toward the design of carbohydrate-dependent inhibition of EPEC and EHEC O157:H7 adhesion to human intestinal epithelial cells.

Alteration of the Microbiota and Virulence Gene Expression in E. coli O157:H7 in Pig Ligated Intestine with and without AE Lesions

BACKGROUND

Previously we found that E. coli O157:H7 inoculated into ligated pig intestine formed attaching and effacing (AE) lesions in some pigs but not in others. The present study evaluated changes in the microbial community and in virulence gene expression in E. coli O157:H7 in ligated pig intestine in which the bacteria formed AE lesions or failed to form AE lesions.

METHODOLOGY/PRINCIPAL FINDINGS

The intestinal microbiota was assessed by RNA-based denaturing gradient gel electrophoresis (DGGE) analysis. The DGGE banding patterns showed distinct differences involving two bands which had increased intensity specifically in AE-negative pigs (AE- bands) and several bands which were more abundant in AE-positive pigs. Sequence analysis revealed that the two AE- bands belonged to Veillonella caviae, a species with probiotic properties, and Bacteroides sp. Concurrent with the differences in microbiota, gene expression analysis by quantitative PCR showed that, compared with AE negative pigs, E. coli O157:H7 in AE positive pigs had upregulated genes for putative adhesins, non-LEE encoded nleA and quorum sensing qseF, acid resistance gene ureD, and genes from the locus of enterocyte effacement (LEE).

CONCLUSIONS/SIGNIFICANCE

The present study demonstrated that AE-positive pigs had reduced activities or populations of Veillonella caviae and Bacterioides sp. compared with AE-negative pigs. Further studies are required to understand how the microbiota was changed and the role of these organisms in the control of E. coli O157:H7.

Tir Triggers Expression of CXCL1 in Enterocytes and Neutrophil Recruitment during Citrobacter rodentium Infection

The hallmarks of enteropathogenic Escherichia coli (EPEC) infection are formation of attaching and effacing (A/E) lesions on mucosal surfaces and actin-rich pedestals on cultured cells, both of which are dependent on the type III secretion system effector Tir. Following translocation into cultured cells and clustering by intimin, Tir Y474 is phosphorylated, leading to recruitment of Nck, activation of N-WASP, and actin polymerization via the Arp2/3 complex. A secondary, weak, actin polymerization pathway is triggered via an NPY motif (Y454). Importantly, Y454 and Y474 play no role in A/E lesion formation on mucosal surfaces following infection with the EPEC-like mouse pathogen Citrobacter rodentium. In this study, we investigated the roles of Tir segments located upstream of Y451 and downstream of Y471 in C. rodentium colonization and A/E lesion formation. We also tested the role that Tir residues Y451 and Y471 play in host immune responses to C. rodentium infection. We found that deletion of amino acids 382 to 462 or 478 to 547 had no impact on the ability of Tir to mediate A/E lesion formation, although deletion of amino acids 478 to 547 affected Tir translocation. Examination of enterocytes isolated from infected mice revealed that a C. rodentium strain expressing Tir_Y451A/Y471A recruited significantly fewer neutrophils to the colon and triggered less colonic hyperplasia on day 14 postinfection than the wild-type strain. Consistently, enterocytes isolated from mice infected with C. rodentium expressing Tir_Y451A/Y471A expressed significantly less CXCL1. These result show that Tir-induced actin remodeling plays a direct role in modulation of immune responses to C. rodentium infection.

Quantitative proteomic analysis of Burkholderia pseudomallei Bsa type III secretion system effectors using hypersecreting mutants

Burkholderia pseudomallei is an intracellular pathogen and the causative agent of melioidosis, a severe disease of humans and animals. One of the virulence factors critical for early stages of infection is the Burkholderia secretion apparatus (Bsa) Type 3 Secretion System (T3SS), a molecular syringe that injects bacterial proteins, called effectors, into eukaryotic cells where they subvert cellular functions to the benefit of the bacteria. Although the Bsa T3SS itself is known to be important for invasion, intracellular replication, and virulence, only a few genuine effector proteins have been identified and the complete repertoire of proteins secreted by the system has not yet been fully characterized. We constructed a mutant lacking bsaP, a homolog of the T3SS "gatekeeper" family of proteins that exert control over the timing and magnitude of effector protein secretion. Mutants lacking BsaP, or the T3SS translocon protein BipD, were observed to hypersecrete the known Bsa effector protein BopE, providing evidence of their role in post-translational control of the Bsa T3SS and representing key reagents for the identification of its secreted substrates. Isobaric Tags for Relative and Absolute Quantification (iTRAQ), a gel-free quantitative proteomics technique, was used to compare the secreted protein profiles of the Bsa T3SS hypersecreting mutants of B. pseudomallei with the isogenic parent strain and a bsaZ mutant incapable of effector protein secretion. Our study provides one of the most comprehensive core secretomes of B. pseudomallei described to date and identified 26 putative Bsa-dependent secreted proteins that may be considered candidate effectors. Two of these proteins, BprD and BapA, were validated as novel effector proteins secreted by the Bsa T3SS of B. pseudomallei.

The RpoE Stress Response Pathway Mediates Reduction of the Virulence of Enteropathogenic Escherichia coli by Zinc

Zinc supplements are an effective clinical treatment for infantile diarrheal disease caused by enteric pathogens. Previous studies demonstrated that zinc acts on enteropathogenic Escherichia coli (EPEC) bacteria directly to suppress several virulence-related genes at a concentration that can be achieved by oral delivery of dietary zinc supplements. Our in vitro studies showed that a micromolar concentration of zinc induced the envelope stress response and suppressed virulence in EPEC, providing a possible mechanistic explanation for zinc's therapeutic action. In this report, we investigated the molecular and physiological changes in EPEC induced by zinc. We found that micromolar concentrations of zinc reduced the bacterial growth rate without affecting viability. We observed increased membrane permeability caused by zinc. Zinc upregulated the RpoE-dependent envelope stress response pathway and suppressed EPEC virulence gene expression. RpoE alone was sufficient to inhibit virulence factor expression and to attenuate attaching and effacing lesion formation on human host cells. By mutational analysis we demonstrate that the DNA-binding motif of RpoE is necessary for suppression of the LEE1, but not the LEE4, operon. Predictably, inhibition of the RpoE-mediated envelope stress response in combination with micromolar concentrations of zinc reduced EPEC viability. In conclusion, zinc induces the RpoE and stress response pathways in EPEC, and the alternate sigma factor RpoE downregulates EPEC LEE and non-LEE virulence genes by multiple mechanisms.

Locus of enterocyte effacement: a pathogenicity island involved in the virulence of enteropathogenic and enterohemorragic Escherichia coli subjected to a complex network of gene regulation

The locus of enterocyte effacement (LEE) is a 35.6 kb pathogenicity island inserted in the genome of some bacteria such as enteropathogenic Escherichia coli, enterohemorrhagic E.coli, Citrobacter rodentium, and Escherichia albertii. LEE comprises the genes responsible for causing attaching and effacing lesions, a characteristic lesion that involves intimate adherence of bacteria to enterocytes, a signaling cascade leading to brush border and microvilli destruction, and loss of ions, causing severe diarrhea. It is composed of 41 open reading frames and five major operons encoding a type three system apparatus, secreted proteins, an adhesin, called intimin, and its receptor called translocated intimin receptor (Tir). LEE is subjected to various levels of regulation, including transcriptional and posttranscriptional regulators located both inside and outside of the pathogenicity island. Several molecules were described being related to feedback inhibition, transcriptional activation, and transcriptional repression. These molecules are involved in a complex network of regulation, including mechanisms such as quorum sensing and temporal control of LEE genes transcription and translation. In this mini review we have detailed the complex network that regulates transcription and expression of genes involved in this kind of lesion.