A distinct regulatory sequence is essential for the expression of a subset of nle genes in attaching and effacing Escherichia coli

GrlR, a negative regulator in enteropathogenic E. coli, also represses the expression of LEE virulence genes independently of its interaction with its cognate partner GrlA

Introduction

Enteropathogenic Escherichia coli (EPEC), enterohemorrhagic E. coli (EHEC) and Citrobacter rodentium (CR) belong to a group of pathogens that share the ability to form "attaching and effacing" (A/E) lesions on the intestinal epithelia. A pathogenicity island known as the locus of enterocyte effacement (LEE) contains the genes required for A/E lesion formation. The specific regulation of LEE genes relies on three LEE-encoded regulators: Ler activates the expression of the LEE operons by antagonizing the silencing effect mediated by the global regulator H-NS, GrlA activates ler expression and GrlR represses the expression of the LEE by interacting with GrlA. However, despite the existing knowledge of LEE regulation, the interplay between GrlR and GrlA and their independent roles in gene regulation in A/E pathogens are still not fully understood.

Methods

To further explore the role that GrlR and GrlA in the regulation of the LEE, we used different EPEC regulatory mutants and cat transcriptional fusions, and performed protein secretion and expression assays, western blotting and native polyacrylamide gel electrophoresis.

Results and discussion

We showed that the transcriptional activity of LEE operons increased under LEE-repressing growth conditions in the absence of GrlR. Interestingly, GrlR overexpression exerted a strong repression effect over LEE genes in wild-type EPEC and, unexpectedly, even in the absence of H-NS, suggesting that GrlR plays an alternative repressor role. Moreover, GrlR repressed the expression of LEE promoters in a non-EPEC background. Experiments with single and double mutants showed that GrlR and H-NS negatively regulate the expression of LEE operons at two cooperative yet independent levels. In addition to the notion that GrlR acts as a repressor by inactivating GrlA through protein-protein interactions, here we showed that a DNA-binding defective GrlA mutant that still interacts with GrlR prevented GrlR-mediated repression, suggesting that GrlA has a dual role as a positive regulator by antagonizing GrlR's alternative repressor role. In line with the importance of the GrlR-GrlA complex in modulating LEE gene expression, we showed that GrlR and GrlA are expressed and interact under both inducing and repressing conditions. Further studies will be required to determine whether the GrlR alternative repressor function depends on its interaction with DNA, RNA, or another protein. These findings provide insight into an alternative regulatory pathway that GrlR employs to function as a negative regulator of LEE genes.

NleB2 from enteropathogenic Escherichia coli is a novel arginine-glucose transferase effector

During infection, enteropathogenic Escherichia coli (EPEC) and enterohaemorrhagic E. coli (EHEC) directly manipulate various aspects of host cell function through the translocation of type III secretion system (T3SS) effector proteins directly into the host cell. Many T3SS effector proteins are enzymes that mediate post-translational modifications of host proteins, such as the glycosyltransferase NleB1, which transfers a single N-acetylglucosamine (GlcNAc) to arginine residues, creating an Arg-GlcNAc linkage. NleB1 glycosylates death-domain containing proteins including FADD, TRADD and RIPK1 to block host cell death. The NleB1 paralogue, NleB2, is found in many EPEC and EHEC strains but to date its enzymatic activity has not been described. Using in vitro glycosylation assays combined with mass spectrometry, we found that NleB2 can utilize multiple sugar donors including UDP-glucose, UDP-GlcNAc and UDP-galactose during glycosylation of the death domain protein, RIPK1. Sugar donor competition assays demonstrated that UDP-glucose was the preferred substrate of NleB2 and peptide sequencing identified the glycosylation site within RIPK1 as Arg603, indicating that NleB2 catalyses arginine glucosylation. We also confirmed that NleB2 catalysed arginine-hexose modification of Flag-RIPK1 during infection of HEK293T cells with EPEC E2348/69. Using site-directed mutagenesis and in vitro glycosylation assays, we identified that residue Ser252 in NleB2 contributes to the specificity of this distinct catalytic activity. Substitution of Ser252 in NleB2 to Gly, or substitution of the corresponding Gly255 in NleB1 to Ser switches sugar donor preference between UDP-GlcNAc and UDP-glucose. However, this switch did not affect the ability of the NleB variants to inhibit inflammatory or cell death signalling during HeLa cell transfection or EPEC infection. NleB2 is thus the first identified bacterial Arg-glucose transferase that, similar to the NleB1 Arg-GlcNAc transferase, inhibits host protein function by arginine glycosylation.

Bacterial gut pathogens including enteropathogenic E. coli (EPEC) and enterohaemorrhagic E. coli (EHEC), manipulate host cell function by using a type III secretion system to inject ‘effector’ proteins directly into the host cell cytoplasm. We and others have shown that many of these effectors are novel enzymes, including NleB1, which transfers a single N-acetylglucosamine (GlcNAc) sugar to arginine residues, mediating Arg-GlcNAc glycosylation. Here, we found that a close homologue of NleB1 that is also present in EPEC and EHEC termed NleB2, uses a different sugar during glycosylation. We demonstrated that in contrast to NleB1, the preferred nucleotide-sugar substrate of NleB2 is UDP-glucose and we identified the amino acid residue within NleB2 that dictates this unique catalytic activity. Substitution of this residue in NleB2 and NleB1 switches the sugar donor usage of these enzymes but does not affect their ability to inhibit host cell signalling. Thus, NleB2 is the first identified bacterial arginine-glucose transferase, an activity which has previously only been described in plants and algae.

Differential expression of the virulence gene nleB among Shiga toxin-producing Escherichia coli strains

Shiga toxin-producing Escherichia coli (STEC) is a zoonotic foodborne pathogen associated with hemolytic uremic syndrome (HUS) that vary in their ability to cause disease in humans. STEC represents a serious problem for public health and Argentina is the country with the highest HUS incidence worldwide. Non-LEE effector (nle) genes, present on pathogenicity islands (PAIs), encode translocated substrates of the type III secretion system (T3SS), which could have an important role in STEC virulence. Particularly, nleB is one of the main effector genes proposed as a virulence marker that is involved in the action of T3SS during the STEC infection. NleB inhibits the inflammatory response of the host cell allowing the bacteria to persist in the first stage of the infection. In order to identify the potential risk of STEC strains for public health, the aim of this study was to evaluate and compare basal nleB transcription of 24 STEC strains belonging to 10 serotypes isolated from cattle, food and patients. The results showed differences in nleB transcription among strains. Some non-O157:H7 strains presented transcription levels above the control, an O157:H7 HUS-producing strain. On the other hand, no significant differences were found in basal transcription levels associated with origin or serotype but differences were found between HUS and non-HUS strains. These differences in nleB transcription may be of importance in STEC pathogenesis and could help to differentiate high and low virulence STEC strains.

Molecular subtyping and clonal relatedness of human and cattle verotoxin-producing Escherichia coli O157:H7 isolates

Verotoxin-producing Escherichia coli O157:H7 is the dominant serotype isolated from patients with hemolytic-uremic syndrome (HUS) and, Argentina has the highest rate of HUS in the world. However, not all O157:H7 isolates have the same ability to infect and cause disease in humans. It has been postulated that O157:H7 strains integrate subpopulations related to the origin and virulence. In order to study the population structure and genetic diversity of VTEC O157:H7 from Argentina, a combination of molecular subtyping methods such as multiple loci VNTR analysis (MLVA), single nucleotide polymorphisms (SNP) and phylogroups assignment were used. According to MLVA, high genetic diversity was found among strains isolated from cattle, humans and food. On the other hand, 92% of the isolates presented the allele tir 255 T > A T and 95% were assigned to phylogroup E. We did not find a significant association between the isolates origin and the allele T presence (P > 0,05) postulated as significantly overrepresented in human isolates. Our results show that human and cattle VTEC O157:H7 isolates from Argentina are a homogeneous group and, although it presents high genetic diversity in relation to their MLVA and virulence profiles, it is not possible to distinguish divergent populations. The presence in all the strains of a high number of T3SS effectors genes and the no association of genetic subtypes with strain source, is an alert about the potential risk in public health that VTEC O157:H7 cattle strains possess and, at less, a partial explication about the high incidence of HUS in Argentina.

Type III Secretion Effectors with Arginine N-Glycosyltransferase Activity

Type III secretion systems are used by many Gram-negative bacterial pathogens to inject proteins, known as effectors, into the cytosol of host cells. These virulence factors interfere with a diverse array of host signal transduction pathways and cellular processes. Many effectors have catalytic activities to promote post-translational modifications of host proteins. This review focuses on a family of effectors with glycosyltransferase activity that catalyze addition of N-acetyl-d-glucosamine to specific arginine residues in target proteins, leading to reduced NF-κB pathway activation and impaired host cell death. This family includes NleB from Citrobacter rodentium, NleB1 and NleB2 from enteropathogenic and enterohemorrhagic Escherichia coli, and SseK1, SseK2, and SseK3 from Salmonella enterica. First, we place these effectors in the general framework of the glycosyltransferase superfamily and in the particular context of the role of glycosylation in bacterial pathogenesis. Then, we provide detailed information about currently known members of this family, their role in virulence, and their targets.

Distinct intraspecies virulence mechanisms regulated by a conserved transcription factor

Bacterial pathogens emerge by adapting mechanisms of virulence, differentiating them from their nonpathogenic progenitor. Virulence factors are often encoded on accessory genomic elements not part of the core genome and therefore must be integrated into the regulatory architecture of the cell. Here, we show that a highly conserved transcription factor in Escherichia coli has been relieved of a common purpose and adapted to regulate virulence pleiotropically in 2 distinct genetic backgrounds. This leads to enhanced virulence of both intestinal enterohemorrhagic E. coli and extraintestinal uropathogenic E. coli by exclusive mechanisms. These findings challenge the assumption that conserved transcription factors regulate common pathways maintained within a species and suggest that transcriptional repurposing creates new primary roles on an individual basis.

Tailoring transcriptional regulation to coordinate the expression of virulence factors in tandem with the core genome is a hallmark of bacterial pathogen evolution. Bacteria encode hundreds of transcription factors forming the base-level control of gene regulation. Moreover, highly homologous regulators are assumed to control conserved genes between members within a species that harbor the same genetic targets. We have explored this concept in 2 Escherichia coli pathotypes that employ distinct virulence mechanisms that facilitate specification of a different niche within the host. Strikingly, we found that the transcription factor YhaJ actively regulated unique gene sets between intestinal enterohemorrhagic E. coli (EHEC) and extraintestinal uropathogenic E. coli (UPEC), despite being very highly conserved. In EHEC, YhaJ directly activates expression of type 3 secretion system components and effectors. Alternatively, YhaJ enhances UPEC virulence regulation by binding directly to the phase-variable type 1 fimbria promoter, driving its expression. Additionally, YhaJ was found to override the universal GAD acid tolerance system but exclusively in EHEC, thereby indirectly enhancing type 3 secretion pleiotropically. These results have revealed that within a species, conserved regulators are actively repurposed in a “personalized” manner to benefit particular lifestyles and drive virulence via multiple distinct mechanisms.

H-NS Family Members MvaT and MvaU Regulate the Pseudomonas aeruginosa Type III Secretion System

Pseudomonas aeruginosa is an opportunistic Gram-negative pathogen capable of causing severe disease in immunocompromised individuals. A major P. aeruginosa virulence factor is the type III secretion system (T3SS). The T3SS is used to translocate effector proteins into host cells, causing cytotoxicity. The T3SS is under the transcriptional control of the master regulator ExsA. ExsA is encoded in the exsCEBA operon and autoregulates transcription via the P _exsC promoter. There is also a Vfr-dependent promoter (P _exsA ) located in the intergenic region between exsB and exsA A previous chromatin immunoprecipitation (ChIP)-on-chip experiment identified strong binding signatures for MvaT and MvaU in the intergenic region containing the P _exsA promoter. MvaT and MvaU are DNA-binding histone-like nucleoid-structuring proteins that can repress gene expression. As predicted from the previous ChIP data, purified MvaT specifically bound to the P _exsA promoter region in electrophoretic mobility shift assays. Whereas disruption of mvaT or mvaU by either transposon insertion or clustered regularly interspaced short palindromic repeat interference (CRISPRi) derepressed P _exsA promoter activity and T3SS gene expression, overexpression of MvaT or MvaU inhibited P _exsA promoter activity. Disruption of mvaT, however, did not suppress the Vfr requirement for P _exsA promoter activity. Mutated MvaT/MvaU defective in transcriptional silencing exhibited dominant negative activity, resulting in a significant increase in P _exsA promoter activity. Because no effect of MvaT or MvaU on Vfr expression was detected, we propose a model in which the primary effect of MvaT/MvaU on T3SS gene expression is through direct silencing of the P _exsA promoter.IMPORTANCE Global regulatory systems play a prominent role in controlling the P. aeruginosa T3SS and include the Gac/RsmA, c-di-GMP, and Vfr-cAMP signaling pathways. Many of these pathways appear to directly or indirectly influence exsA transcription or translation. In this study, the histone-like proteins MvaT and MvaU are added to the growing list of global regulators that control the T3SS. MvaT and MvaU bind AT-rich regions in the genome and silence xenogeneic genes, including pathogenicity islands. The T3SS gene cluster has been horizontally transmitted among many Gram-negative pathogens. Control by MvaT/MvaU may reflect a residual effect that has persisted since the initial acquisition of the gene cluster, subsequently imposing a requirement for active regulatory mechanisms to override MvaT/MvaU-mediated silencing.

Host-associated niche metabolism controls enteric infection through fine-tuning the regulation of type 3 secretion

Niche-adaptation of a bacterial pathogen hinges on the ability to recognize the complexity of signals from the environment and integrate that information with the regulation of genes critical for infection. Here we report the transcriptome of the attaching and effacing pathogen Citrobacter rodentium during infection of its natural murine host. Pathogen gene expression in vivo was heavily biased towards the virulence factor repertoire and was found to be co-ordinated uniquely in response to the host. Concordantly, we identified the host-specific induction of a metabolic pathway that overlapped with the regulation of virulence. The essential type 3 secretion system and an associated suite of distinct effectors were found to be modulated co-ordinately through a unique mechanism involving metabolism of microbiota-derived 1,2-propanediol, which dictated the ability to colonize the host effectively. This study provides novel insights into how host-specific metabolic adaptation acts as a cue to fine-tune virulence.

Infection of mice with Citrobacter rodentium is a common model of infection with attaching-and-effacing pathogens. Here, Connolly et al. analyse the transcriptome of C. rodentium during mouse infection, showing host-induced coordinated upregulation of virulence factors and 1,2-propanediol metabolism.

Pathogenicity Islands Distribution in Non-O157 Shiga Toxin-Producing Escherichia coli (STEC)

Shiga toxin-producing Escherichia coli (STEC) are foodborne pathogens associated with outbreaks and hemolytic-uremic syndrome. Cattle and meat foods are the main reservoir and infection source, respectively. Pathogenicity islands (PAIs) play an important role in STEC pathogenicity, and non-locus of the enterocyte effacement(LEE) effector (nle) genes present on them encode translocated substrates of the type III secretion system. A molecular risk assessment based on the evaluation of the nle content has been used to predict which STEC strains pose a risk to humans. The goal was to investigate the distribution of the PAIs OI (O-island)-36 (nleB2, nleC, nleH1-1, nleD), OI-57 (nleG2-3, nleG5-2, nleG6-2), OI-71 (nleA, nleF, nleG, nleG2-1, nleG9, nleH1-2) and OI-122 (ent/espL2, nleB, nleE, Z4321, Z4326, Z4332, Z4333) among 204 clinical, food and animal isolates belonging to 52 non-O157:H7 serotypes. Differences in the frequencies of genetic markers and a wide spectrum of PAI virulence profiles were found. In most LEE-negative strains, only module 1 (Z4321) of OI-122 was present. However, some unusual eae-negative strains were detected, which carried other PAI genes. The cluster analysis, excluding isolates that presented no genes, defined two major groups: eae-negative (determined as seropathotypes (SPTs) D, E or without determination, isolated from cattle or food) and eae-positive (mostly identified as SPTs B, C, or not determined).

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.

Molecular Profiling of Shiga Toxin-Producing Escherichia coli and Enteropathogenic E. coli Strains Isolated from French Coastal Environments

Shiga toxin-producing Escherichia coli (STEC) and enteropathogenic E. coli (EPEC) strains may be responsible for food-borne infections in humans. Twenty-eight STEC and 75 EPEC strains previously isolated from French shellfish-harvesting areas and their watersheds and belonging to 68 distinguishable serotypes were characterized in this study. High-throughput real-time PCR was used to search for the presence of 75 E. coli virulence-associated gene targets, and genes encoding Shiga toxin (stx) and intimin (eae) were subtyped using PCR tests and DNA sequencing, respectively. The results showed a high level of diversity between strains, with 17 unique virulence gene profiles for STEC and 56 for EPEC. Seven STEC and 15 EPEC strains were found to display a large number or a particular combination of genetic markers of virulence and the presence of stx and/or eae variants, suggesting their potential pathogenicity for humans. Among these, an O26:H11 stx1a eae-β1 strain was associated with a large number of virulence-associated genes (n = 47), including genes carried on the locus of enterocyte effacement (LEE) or other pathogenicity islands, such as OI-122, OI-71, OI-43/48, OI-50, OI-57, and the high-pathogenicity island (HPI). One O91:H21 STEC strain containing 4 stx variants (stx1a, stx2a, stx2c, and stx2d) was found to possess genes associated with pathogenicity islands OI-122, OI-43/48, and OI-15. Among EPEC strains harboring a large number of virulence genes (n, 34 to 50), eight belonged to serotype O26:H11, O103:H2, O103:H25, O145:H28, O157:H7, or O153:H2.

IMPORTANCE

The species E. coli includes a wide variety of strains, some of which may be responsible for severe infections. This study, a molecular risk assessment study of E. coli strains isolated from the coastal environment, was conducted to evaluate the potential risk for shellfish consumers. This report describes the characterization of virulence gene profiles and stx/eae polymorphisms of E. coli isolates and clearly highlights the finding that the majority of strains isolated from coastal environment are potentially weakly pathogenic, while some are likely to be more pathogenic.

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.

Understanding the host-adapted state of Citrobacter rodentium by transcriptomic analysis

Citrobacter rodentium (Cr) is a mouse pathogen that mimics many aspects of enteropathogenic Escherichia coli infections including producing attaching and effacing (A/E) lesions. Host-adapted (HA) Cr cells that are shed at the peak of infection have been reported to be hyper-infective. The exact mechanism underlying this phenomenon has remained elusive since the pathogen loses its HA 'status' immediately upon subculturing in laboratory media. We sequenced the entire transcriptome of Cr directly from the feces of infected mice and analyzed the gene expression pattern. We observed that the entire transcriptional machinery as well as several transcriptional regulators to be differentially expressed when compared with the transcriptome of cells grown on laboratory media. Major adhesion and effector genes, tir and eae, were highly expressed in HA along with many genes located on all five loci of enterocyte effacement regions (LEE 1-5). Notable absent among the HA expressed genes were 19 fimbrial operons and non-fimbrial adhesions and several non-LEE encoded effectors. These results demonstrate that host-adapted Cr has a unique transcriptome that is associated with increased host transmission.

A Highly Conserved Bacterial D-Serine Uptake System Links Host Metabolism and Virulence

The ability of any organism to sense and respond to challenges presented in the environment is critically important for promoting or restricting colonization of specific sites. Recent work has demonstrated that the host metabolite D-serine has the ability to markedly influence the outcome of infection by repressing the type III secretion system of enterohaemorrhagic Escherichia coli (EHEC) in a concentration-dependent manner. However, exactly how EHEC monitors environmental D-serine is not understood. In this work, we have identified two highly conserved members of the E. coli core genome, encoding an inner membrane transporter and a transcriptional regulator, which collectively help to "sense" levels of D-serine by regulating its uptake from the environment and in turn influencing global gene expression. Both proteins are required for full expression of the type III secretion system and diversely regulated prophage-encoded effector proteins demonstrating an important infection-relevant adaptation of the core genome. We propose that this system acts as a key safety net, sampling the environment for this metabolite, thereby promoting colonization of EHEC to favorable sites within the host.

Global Regulator of Virulence A (GrvA) Coordinates Expression of Discrete Pathogenic Mechanisms in Enterohemorrhagic Escherichia coli through Interactions with GadW-GadE

Global regulator of virulence A (GrvA) is a ToxR-family transcriptional regulator that activates locus of enterocyte effacement (LEE)-dependent adherence in enterohemorrhagic Escherichia coli (EHEC). LEE activation by GrvA requires the Rcs phosphorelay response regulator RcsB and is sensitive to physiologically relevant concentrations of bicarbonate, a known stimulant of virulence systems in intestinal pathogens. This study determines the genomic scale of GrvA-dependent regulation and uncovers details of the molecular mechanism underlying GrvA-dependent regulation of pathogenic mechanisms in EHEC. In a grvA-null background of EHEC strain TW14359, RNA sequencing analysis revealed the altered expression of over 700 genes, including the downregulation of LEE- and non-LEE-encoded effectors and the upregulation of genes for glutamate-dependent acid resistance (GDAR). Upregulation of GDAR genes corresponded with a marked increase in acid resistance. GrvA-dependent regulation of GDAR and the LEE required gadE, the central activator of GDAR genes and a direct repressor of the LEE. Control of gadE by GrvA was further determined to occur through downregulation of the gadE activator GadW. This interaction of GrvA with GadW-GadE represses the acid resistance phenotype, while it concomitantly activates the LEE-dependent adherence and secretion of immune subversion effectors. The results of this study significantly broaden the scope of GrvA-dependent regulation and its role in EHEC pathogenesis.

IMPORTANCE

Enterohemorrhagic Escherichia coli (EHEC) is an intestinal human pathogen causing acute hemorrhagic colitis and life-threatening hemolytic-uremic syndrome. For successful transmission and gut colonization, EHEC relies on the glutamate-dependent acid resistance (GDAR) system and a type III secretion apparatus, encoded on the LEE pathogenicity island. This study investigates the mechanism whereby the DNA-binding regulator GrvA coordinates activation of the LEE with repression of GDAR. Investigating how these systems are regulated leads to an understanding of pathogenic behavior and novel strategies aimed at disease prevention and control.

Enterohemorrhagic Escherichia coli Virulence Gene Regulation

Coordinated expression of enterohemorrhagic Escherichia coli virulence genes enables the bacterium to cause hemorrhagic colitis and the complication known as hemolytic-uremic syndrome. Horizontally acquired genes and those common to E. coli contribute to the disease process, and increased virulence gene expression is correlated with more severe disease in humans. Researchers have gained considerable knowledge about how the type III secretion system, secreted effectors, adhesin molecules, and the Shiga toxins are regulated by environmental signals and multiple genetic pathways. Also emergent from the data is an understanding of how enterohemorrhagic E. coli regulates response to acid stress, the role of flagellar motility, and how passage through the human host and bovine intestinal tract causes disease and supports carriage in the cattle reservoir, respectively. Particularly exciting areas of discovery include data suggesting how expression of the myriad effectors is coordinately regulated with their cognate type III secretion system and how virulence is correlated with bacterial metabolism and gut physiology.