VirB-mediated positive feedback control of the virulence gene regulatory cascade of Shigella flexneri

Sensing for survival: specialised regulatory mechanisms of Type III secretion systems in Gram-negative pathogens

For centuries, Gram-negative pathogens have infected the human population and been responsible for numerous diseases in animals and plants. Despite advancements in therapeutics, Gram-negative pathogens continue to evolve, with some having developed multi-drug resistant phenotypes. For the successful control of infections caused by these bacteria, we need to widen our understanding of the mechanisms of host-pathogen interactions. Gram-negative pathogens utilise an array of effector proteins to hijack the host system to survive within the host environment. These proteins are secreted into the host system via various secretion systems, including the integral Type III secretion system (T3SS). The T3SS spans two bacterial membranes and one host membrane to deliver effector proteins (virulence factors) into the host cell. This multifaceted process has multiple layers of regulation and various checkpoints. In this review, we highlight the multiple strategies adopted by these pathogens to regulate or maintain virulence via the T3SS, encompassing the regulation of small molecules to sense and communicate with the host system, as well as master regulators, gatekeepers, chaperones, and other effectors that recognise successful host contact. Further, we discuss the regulatory links between the T3SS and other systems, like flagella and metabolic pathways including the tricarboxylic acid (TCA) cycle, anaerobic metabolism, and stringent cell response.

A Tale about Shigella: Evolution, Plasmid, and Virulence

Shigella spp. cause hundreds of millions of intestinal infections each year. They target the mucosa of the human colon and are an important model of intracellular bacterial pathogenesis. Shigella is a pathovar of Escherichia coli that is characterized by the presence of a large invasion plasmid, pINV, which encodes the characteristic type III secretion system and icsA used for cytosol invasion and cell-to-cell spread, respectively. First, we review recent advances in the genetic aspects of Shigella, shedding light on its evolutionary history within the E. coli lineage and its relationship to the acquisition of pINV. We then discuss recent insights into the processes that allow for the maintenance of pINV. Finally, we describe the role of the transcription activators VirF, VirB, and MxiE in the major virulence gene regulatory cascades that control the expression of the type III secretion system and icsA. This provides an opportunity to examine the interplay between these pINV-encoded transcriptional activators and numerous chromosome-encoded factors that modulate their activity. Finally, we discuss novel chromosomal genes icaR, icaT, and yccE that are regulated by MxiE. This review emphasizes the notion that Shigella and E. coli have walked the fine line between commensalism and pathogenesis for much of their history.

Virulence profiling of Shigella flexneri and emergence of serotype 2b as a highly virulent shigellosis causing strain in Pakistan

Bacillary diarrhea caused by Shigella flexneri is mediated by various virulence factors which make it the leading agent of diarrhea in developing countries. Previously, a high prevalence of S. flexneri, associated with diarrhea has been reported in Pakistan but no data is available on their virulence profile. The present study reports for the first time analysis of various virulence factors among S. flexneri serotypes isolated from clinical (diarrheal stool) and non-clinical (retail raw foods and drinking water) sources. A total of 199 S. flexneri (clinical: 155, raw foods: 22, water: 22) belonging to various serotypes were subjected to virulence genes detection and virulence profiling. The most frequent virulence gene was found to be ipaH (100%), followed by sat (98%), ial (71.3%), set1B (65.8%) and set1A (38.7%). A high level of virulence was detected in serotype 2b as compared to other serotypes as 32.3% of all serotype 2b have the entire set of five virulence genes including ipaH (100%), ial (100%), sat (37.7%), set1A (89.3%), and set1B (100%). Seven different virulence gene profiles (V1 - V7) were detected and the most frequently observed to be V1 (ipaH+, ial+, sat+, set1A+, set1B+) followed by V3 (ipaH+, ial+, sat+, set1B+). The predominant virulence gene pattern in serotype 2b isolated from clinical and non-clinical samples were V1 and V3. Furthermore, about 32% strains belonging to serotype 2b contain the complete set of five virulence genes isolated from patients with high disease severity. In conclusion, the current finding revealed for the first times that serotype 2b was the most virulent strains in both clinical and non-clinical samples in Pakistan. In addition, the virulence of serotype 2b was well correlated with high disease severity.

Tchagang C, Tomaro K, Campbell-Valois FX. The T3SS of Shigella: Expression, Structure, Function, and Role in Vacuole Escape

Shigella spp. are one of the leading causes of infectious diarrheal diseases. They are Escherichia coli pathovars that are characterized by the harboring of a large plasmid that encodes most virulence genes, including a type III secretion system (T3SS). The archetypal element of the T3SS is the injectisome, a syringe-like nanomachine composed of approximately 20 proteins, spanning both bacterial membranes and the cell wall, and topped with a needle. Upon contact of the tip of the needle with the plasma membrane, the injectisome secretes its protein substrates into host cells. Some of these substrates act as translocators or effectors whose functions are key to the invasion of the cytosol and the cell-to-cell spread characterizing the lifestyle of Shigella spp. Here, we review the structure, assembly, function, and methods to measure the activity of the injectisome with a focus on Shigella, but complemented with data from other T3SS if required. We also present the regulatory cascade that controls the expression of T3SS genes in Shigella. Finally, we describe the function of translocators and effectors during cell-to-cell spread, particularly during escape from the vacuole, a key element of Shigella’s pathogenesis that has yet to reveal all of its secrets.

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.

Investigating the DNA-Binding Site for VirB, a Key Transcriptional Regulator of Shigella Virulence Genes, Using an In Vivo Binding Tool

The transcriptional anti-silencing and DNA-binding protein, VirB, is essential for the virulence of Shigella species and, yet, sequences required for VirB-DNA binding are poorly understood. While a 7-8 bp VirB-binding site has been proposed, it was derived from studies at a single VirB-dependent promoter, icsB. Our previous in vivo studies at a different VirB-dependent promoter, icsP, found that the proposed VirB-binding site was insufficient for regulation. Instead, the required site was found to be organized as a near-perfect inverted repeat separated by a single nucleotide spacer. Thus, the proposed 7-8 bp VirB-binding site needed to be re-evaluated. Here, we engineer and validate a molecular tool to capture protein-DNA binding interactions in vivo. Our data show that a sequence organized as a near-perfect inverted repeat is required for VirB-DNA binding interactions in vivo at both the icsB and icsP promoters. Furthermore, the previously proposed VirB-binding site and multiple sites found as a result of its description (i.e., sites located at the virB, virF, spa15, and virA promoters) are not sufficient for VirB to bind in vivo using this tool. The implications of these findings are discussed.

Regulatory Hierarchies Controlling Virulence Gene Expression in Shigella flexneri and Vibrio cholerae

Gram-negative enteropathogenic bacteria use a variety of strategies to cause disease in the human host and gene regulation in some form is typically a part of the strategy. This article will compare the toxin-based infection strategy used by the non-invasive pathogen Vibrio cholerae, the etiological agent in human cholera, with the invasive approach used by Shigella flexneri, the cause of bacillary dysentery. Despite the differences in the mechanisms by which the two pathogens cause disease, they use environmentally-responsive regulatory hierarchies to control the expression of genes that have some features, and even some components, in common. The involvement of AraC-like transcription factors, the integration host factor, the Factor for inversion stimulation, small regulatory RNAs, the RNA chaperone Hfq, horizontal gene transfer, variable DNA topology and the need to overcome the pervasive silencing of transcription by H-NS of horizontally acquired genes are all shared features. A comparison of the regulatory hierarchies in these two pathogens illustrates some striking cross-species similarities and differences among mechanisms coordinating virulence gene expression. S. flexneri, with its low infectious dose, appears to use a strategy that is centered on the individual bacterial cell, whereas V. cholerae, with a community-based, quorum-dependent approach and an infectious dose that is several orders of magnitude higher, seems to rely more on the actions of a bacterial collective.

Linalool, citral, eugenol and thymol: control of planktonic and sessile cells of Shigella flexneri

The antimicrobial activity of linalool, citral, eugenol and thymol was determined in growth studies of both planktonic (PC) and biofilm cells (BC) Shigella flexneri. These components were evaluated either in isolation or in combinations using a sequential experimental strategy with Plackett & Burman and central composite rotational designs totaling 47 treatments. The minimum inhibitory concentration for PC was 0.125% (v v⁻¹) for linalool and 0.5% (v v⁻¹) for citral, eugenol and thymol. The biofilm minimum bactericidal concentration was 3 and 1% (v v⁻¹) for linalool and citral, respectively, and 2% (v v⁻¹) for eugenol and thymol. In the mixtures, the minimum concentrations in the efficient assays for PC growth inhibition were 0.0003, 0.0443 and 0.0443% (v v⁻¹), for linalool, citral and thymol, respectively. In the BC, only two assays with concentrations of 0.0558, 0.0558 and 0.319% (v v⁻¹) and 0.035, 0.035 and 0.3999% (v v⁻¹) for linalool, citral and thymol, respectively, inhibited Shigella growth. Synergism was observed among the components, where PC and BC growth inhibition occurred at lower concentrations than those noted individually. The bactericidal effect of the components in microplate was different from the observed in stain steel coupons. Therefore, the obtained model can describe and predict the PC count of S. flexneri in medium with the tested compounds and they could be an alternative for the use in microbiological control in food industry.

MxiN Differentially Regulates Monomeric and Oligomeric Species of the Shigella Type Three Secretion System ATPase Spa47

Shigella rely entirely on the action of a single type three secretion system (T3SS) to support cellular invasion of colonic epithelial cells and to circumvent host immune responses. The ATPase Spa47 resides at the base of the Shigella needle-like type three secretion apparatus (T3SA), supporting protein secretion through the apparatus and providing a likely means for native virulence regulation by Shigella and a much needed target for non-antibiotic therapeutics to treat Shigella infections. Here, we show that MxiN is a differential regulator of Spa47 and that its regulatory impact is determined by the oligomeric state of the Spa47 ATPase, with which it interacts. In vitro and in vivo characterization shows that interaction of MxiN with Spa47 requires the six N-terminal residues of Spa47 that are also necessary for stable Spa47 oligomer formation and activation. This interaction with MxiN negatively influences the activity of Spa47 oligomers while upregulating the ATPase activity of monomeric Spa47. Detailed kinetic analyses of monomeric and oligomeric Spa47 in the presence and absence of MxiN uncover additional mechanistic insights into the regulation of Spa47 by MxiN, suggesting that the MxiN/Spa47 species resulting from interaction with monomeric and oligomeric Spa47 are functionally distinct and that both could be involved in Shigella T3SS regulation. Uncovering regulation of Spa47 by MxiN addresses an important gap in the current understanding of how Shigella controls T3SA activity and provides the first description of differential T3SS ATPase regulation by a native T3SS protein.

Bacterial pathogen gene regulation: a DNA-structure-centred view of a protein-dominated domain

The mechanisms used by bacterial pathogens to regulate the expression of their genes, especially their virulence genes, have been the subject of intense investigation for several decades. Whole genome sequencing projects, together with more targeted studies, have identified hundreds of DNA-binding proteins that contribute to the patterns of gene expression observed during infection as well as providing important insights into the nature of the gene products whose expression is being controlled by these proteins. Themes that have emerged include the importance of horizontal gene transfer to the evolution of pathogens, the need to impose regulatory discipline upon these imported genes and the important roles played by factors normally associated with the organization of genome architecture as regulatory principles in the control of virulence gene expression. Among these architectural elements is the structure of DNA itself, its variable nature at a topological rather than just at a base-sequence level and its ability to play an active (as well as a passive) part in the gene regulation process.

The Galleria mellonella larvae as an in vivo model for evaluation of Shigella virulence

Shigella spp. causing bacterial diarrhea and dysentery are human enteroinvasive bacterial pathogens that are orally transmitted through contaminated food and water and cause bacillary dysentery. Although natural Shigella infections are restricted to humans and primates, several smaller animal models are used to analyze individual steps in pathogenesis. No animal model fully duplicates the human response and sustaining the models requires expensive animals, costly maintenance of animal facilities, veterinary services and approved animal protocols. This study proposes the development of the caterpillar larvae of Galleria mellonella as a simple, inexpensive, informative, and rapid in-vivo model for evaluating virulence and the interaction of Shigella with cells of the insect innate immunity. Virulent Shigella injected through the forelegs causes larvae death. The mortality rates were dependent on the Shigella strain, the infectious dose, and the presence of the virulence plasmid. Wild-type S. flexneri 2a, persisted and replicated within the larvae, resulting in haemocyte cell death, whereas plasmid-cured mutants were rapidly cleared. Histology of the infected larvae in conjunction with fluorescence, immunofluorescence, and transmission electron microscopy indicate that S. flexneri reside within a vacuole of the insect haemocytes that ultrastructurally resembles vacuoles described in studies with mouse and human macrophage cell lines. Some of these bacteria-laden vacuoles had double-membranes characteristic of autophagosomes. These results suggest that G. mellonella larvae can be used as an easy-to-use animal model to understand Shigella pathogenesis that requires none of the time and labor-consuming procedures typical of other systems.

H-NS, Its Family Members and Their Regulation of Virulence Genes in Shigella Species

The histone-like nucleoid structuring protein (H-NS) has played a key role in shaping the evolution of Shigella spp., and provides the backdrop to the regulatory cascade that controls virulence by silencing many genes found on the large virulence plasmid. H-NS and its paralogue StpA are present in all four Shigella spp., but a second H-NS paralogue, Sfh, is found in the Shigella flexneri type strain 2457T, which is routinely used in studies of Shigella pathogenesis. While StpA and Sfh have been proposed to serve as “molecular backups” for H-NS, the apparent redundancy of these proteins is questioned by in vitro studies and work done in Escherichia coli. In this review, we describe the current understanding of the regulatory activities of the H-NS family members, the challenges associated with studying these proteins and their role in the regulation of virulence genes in Shigella.

One Gene and Two Proteins: a Leaderless mRNA Supports the Translation of a Shorter Form of the Shigella VirF Regulator

VirF, an AraC-like activator, is required to trigger a regulatory cascade that initiates the invasive program of Shigella spp., the etiological agents of bacillary dysentery in humans. VirF expression is activated upon entry into the host and depends on many environmental signals. Here, we show that the virF mRNA is translated into two proteins, the major form, VirF₃₀ (30 kDa), and the shorter VirF₂₁ (21 kDa), lacking the N-terminal segment. By site-specific mutagenesis and toeprint analysis, we identified the translation start sites of VirF₃₀ and VirF₂₁ and showed that the two different forms of VirF arise from differential translation. Interestingly, in vitro and in vivo translation experiments showed that VirF₂₁ is also translated from a leaderless mRNA (llmRNA) whose 5′ end is at position +309/+310, only 1 or 2 nucleotides upstream of the ATG84 start codon of VirF₂₁. The llmRNA is transcribed from a gene-internal promoter, which we identified here. Functional analysis revealed that while VirF₃₀ is responsible for activation of the virulence system, VirF₂₁ negatively autoregulates virF expression itself. Since VirF₂₁ modulates the intracellular VirF levels, this suggests that transcription of the llmRNA might occur when the onset of the virulence program is not required. We speculate that environmental cues, like stress conditions, may promote changes in virF mRNA transcription and preferential translation of llmRNA.

Shigella spp. are a major cause of dysentery in humans. In bacteria of this genus, the activation of the invasive program involves a multitude of signals that act on all layers of the gene regulatory hierarchy. By controlling the essential genes for host cell invasion, VirF is the key regulator of the switch from the noninvasive to the invasive phenotype. Here, we show that the Shigella virF gene encodes two proteins of different sizes, VirF₃₀ and VirF₂₁, that are functionally distinct. The major form, VirF₃₀, activates the genes necessary for virulence, whereas the minor VirF₂₁, which shares the C-terminal two-thirds of VirF₃₀, negatively autoregulates virF expression itself. VirF₂₁ is transcribed from a newly identified gene-internal promoter and, moreover, is translated from an unusual leaderless mRNA. The identification of a new player in regulation adds complexity to the regulation of the Shigella invasive process and may help development of new therapies for shigellosis.

Unprecedented Abundance of Protein Tyrosine Phosphorylation Modulates Shigella flexneri Virulence

Evidence is accumulating that protein tyrosine phosphorylation plays a crucial role in the ability of important human bacterial pathogens to cause disease. While most works have concentrated on its role in the regulation of a major bacterial virulence factor, the polysaccharide capsule, recent studies have suggested a much broader role for this post-translational modification. This prompted us to investigate protein tyrosine phosphorylation in the human pathogen Shigella flexneri. We first completed a tyrosine phosphoproteome, identifying 905 unique tyrosine phosphorylation sites on at least 573 proteins (approximately 15% of all proteins). This is the most tyrosine-phosphorylated sites and proteins in a single bacterium identified to date, substantially more than the level seen in eukaryotic cells. Most had not previously been identified and included proteins encoded by the virulence plasmid, which is essential for S. flexneri to invade cells and cause disease. In order to investigate the function of these phosphorylation sites in important virulence factors, phosphomimetic and ablative mutations were constructed in the type 3 secretion system ATPase Spa47 and the master virulence regulator VirB. This revealed that tyrosine residues phosphorylated in our study are critical for Spa47 and VirB activity, and tyrosine phosphorylation likely regulates their functional activity and subsequently the virulence of this major human pathogen. This study suggests that tyrosine phosphorylation plays a critical role in regulating a wide variety of virulence factors in the human pathogen S. flexneri and serves as a base for future studies defining its complete role.

Implications of Spatiotemporal Regulation of Shigella flexneri Type Three Secretion Activity on Effector Functions: Think Globally, Act Locally

Shigella spp. are Gram-negative bacterial pathogens that infect human colonic epithelia and cause bacterial dysentery. These bacteria express multiple copies of a syringe-like protein complex, the Type Three Secretion apparatus (T3SA), which is instrumental in the etiology of the disease. The T3SA triggers the plasma membrane (PM) engulfment of the bacteria by host cells during the initial entry process. It then enables bacteria to escape the resulting phagocytic-like vacuole. Freed bacteria form actin comets to move in the cytoplasm, which provokes bacterial collision with the inner leaflet of the PM. This phenomenon culminates in T3SA-dependent secondary uptake and vacuolar rupture in neighboring cells in a process akin to what is observed during entry and named cell-to-cell spread. The activity of the T3SA of Shigella flexneri was recently demonstrated to display an on/off regulation during the infection. While the T3SA is active when bacteria are in contact with PM-derived compartments, it switches to an inactive state when bacteria are released within the cytosol. These observations indicate that effector proteins transiting through the T3SA are therefore translocated in a highly time and space constrained fashion, likely impacting on their cellular distribution. Herein, we present what is currently known about the composition, the assembly and the regulation of the T3SA activity and discuss the consequences of the on/off regulation of T3SA on Shigella effector properties and functions during the infection. Specific examples that will be developed include the role of effectors IcsB and VirA in the escape from LC3/ATG8-positive vacuoles formed during cell-to-cell spread and of IpaJ protease activity against N-miristoylated proteins. The conservation of a similar regulation of T3SA activity in other pathogens such as Salmonella or Enteropathogenic Escherichia coli will also be briefly discussed.

Riboregulators: Fine-Tuning Virulence in Shigella

Within the past several years, RNA-mediated regulation (ribo-regulation) has become increasingly recognized for its importance in controlling critical bacterial processes. Regulatory RNA molecules, or riboregulators, are perpetually responsive to changes within the micro-environment of a bacterium. Notably, several characterized riboregulators control virulence in pathogenic bacteria, as is the case for each riboregulator characterized to date in Shigella. The timing of virulence gene expression and the ability of the pathogen to adapt to rapidly changing environmental conditions is critical to the establishment and progression of infection by Shigella species; ribo-regulators mediate each of these important processes. This mini review will present the current state of knowledge regarding RNA-mediated regulation in Shigella by detailing the characterization and function of each identified riboregulator in these pathogens.

Elongation factor P and modifying enzyme PoxA are necessary for virulence of Shigella flexneri

Elongation factor P (EF-P) is a universally conserved bacterial translation factor. In many bacteria, EF-P is posttranslationally modified by PoxA, which covalently attaches a β-lysine to a conserved lysine residue of EF-P. Here we show that both EF-P and PoxA are necessary for virulence of the human diarrheal pathogen Shigella flexneri. Loss of either EF-P or PoxA leads to an impaired ability of S. flexneri to invade epithelial cells and form plaques in an epithelial cell monolayer. Proteomic analysis of efp and poxA deletion mutants revealed decreased levels of several virulence effector proteins, including IpaA, -B, and -C and IcsA. Additionally, mRNA levels of virB and virF, which encode master virulence regulators, were decreased in the efp mutant. The reduction in virF transcription was at least partially due to decreased levels of CpxA, which activates virF through the response regulator CpxR. The role of CpxAR in reduced synthesis of VirF and its downstream effectors was indicated by restoration of invasion when a mutation resulting in constitutively activated CpxR was introduced into the efp mutant. Thus, modified EF-P is required for appropriate synthesis of proteins involved in the virulence of this bacterial pathogen.

Phosphate regulated proteins of Xanthomonas citri subsp. citri: a proteomic approach

Xanthomonas citri subsp. citri (X. citri) is the causative agent of the citrus canker, a disease that affects several citrus plants in Brazil and across the world. Although many studies have demonstrated the importance of genes for infection and pathogenesis in this bacterium, there are no data related to phosphate uptake and assimilation pathways. To identify the proteins that are involved in the phosphate response, we performed a proteomic analysis of X. citri extracts after growth in three culture media with different phosphate concentrations. Using mass spectrometry and bioinformatics analysis, we showed that X. citri conserved orthologous genes from Pho regulon in Escherichia coli, including the two-component system PhoR/PhoB, ATP binding cassette (ABC transporter) Pst for phosphate uptake, and the alkaline phosphatase PhoA. Analysis performed under phosphate starvation provided evidence of the relevance of the Pst system for phosphate uptake, as well as both periplasmic binding proteins, PhoX and PstS, which were formed in high abundance. The results from this study are the first evidence of the Pho regulon activation in X. citri and bring new insights for studies related to the bacterial metabolism and physiology. Biological significance Using proteomics and bioinformatics analysis we showed for the first time that the phytopathogenic bacterium X. citri conserves a set of proteins that belong to the Pho regulon, which are induced during phosphate starvation. The most relevant in terms of conservation and up-regulation were the periplasmic-binding proteins PstS and PhoX from the ABC transporter PstSBAC for phosphate, the two-component system composed by PhoR/PhoB and the alkaline phosphatase PhoA.

Virulence factors associated with pediatric shigellosis in Brazilian Amazon

Shigellosis is a global human health problem and the incidence is highest among children. In the present work, main Shigella virulence genes was examined by PCR and compared to symptoms of pediatric shigellosis. Thirty Shigella isolates were identified from an etiologic study at which 1,339 children ranging 0–10 years old were enrolled. S. flexneri was the most frequent species reaching 60.0% of isolates, 22.2% were S. sonnei, and 6.6% were both S. dysenteriae and S. boydii. All Shigella infected children had diarrhea, but not all were accompanied by others symptoms of bacillary dysentery. Among major virulence genes, the PCR typing revealed ipaBCD was present in all isolates, followed by IpaH7.8, set-1A, set-1B, sen/ospD3, virF, and invE. The pathogenic potential of the ShET-1B subunit was observed in relation to dehydration (P < 0.001) and ShET-2 related to the intestinal injury (P = 0.033) evidenced by the presence of bloody diarrhea. Our results show associations among symptoms of shigellosis and virulence genes of clinical isolates of Shigella spp.

Structural insights into VirB-DNA complexes reveal mechanism of transcriptional activation of virulence genes

VirB activates transcription of virulence genes in Shigella flexneri by alleviating heat-stable nucleoid-structuring protein-mediated promoter repression. VirB is unrelated to the conventional transcriptional regulators, but homologous to the plasmid partitioning proteins. We determined the crystal structures of VirB HTH domain bound by the cis-acting site containing the inverted repeat, revealing that the VirB-DNA complex is related to ParB-ParS-like complexes, presenting an example that a ParB-like protein acts exclusively in transcriptional regulation. The HTH domain of VirB docks DNA major groove and provides multiple contacts to backbone and bases, in which the only specific base readout is mediated by R167. VirB only recognizes one half site of the inverted repeats containing the most matches to the consensus for VirB binding. The binding of VirB induces DNA conformational changes and introduces a bend at an invariant A-tract segment in the cis-acting site, suggesting a role of DNA remodeling. VirB exhibits positive cooperativity in DNA binding that is contributed by the C-terminal domain facilitating VirB oligomerization. The isolated HTH domain only confers partial DNA specificity. Additional determinants for sequence specificity may reside in N- or C-terminal domains. Collectively, our findings support and extend a previously proposed model for relieving heat-stable nucleoid-structuring protein-mediated repression by VirB.

Characterization of the ospZ promoter in Shigella flexneri and its regulation by VirB and H-NS

OspZ is an effector protein of the type III secretion system in Shigella spp. that downregulates the human inflammatory response during bacterial infection. The ospZ gene is located on the large virulence plasmid of Shigella. Many genes on this plasmid are transcriptionally repressed by the nucleoid structuring protein H-NS and derepressed by VirB, a DNA-binding protein that displays homology to the plasmid partitioning proteins ParB and SopB. In this study, we characterized the ospZ promoter and investigated its regulation by H-NS and VirB in Shigella flexneri. We show that H-NS represses and VirB partially derepresses the ospZ promoter. H-NS-mediated repression requires sequences located between -731 and -412 relative to the beginning of the ospZ gene. Notably, the VirB-dependent derepression of ospZ requires the same VirB binding sites as are required for the VirB-dependent derepression of the divergent icsP gene. These sites are centered 425 bp upstream of the ospZ gene but over 1 kb upstream of the icsP transcription start site. Although these VirB binding sites lie closer to ospZ than icsP, the VirB-dependent increase in ospZ promoter activity is lower than that observed at the icsP promoter. This indicates that the proximity of VirB binding sites to Shigella promoters does not necessarily correlate with the level of VirB-dependent derepression. These findings have implications for virulence gene regulation in Shigella and other pathogens that control gene expression using mechanisms of transcriptional repression and derepression.