Homology between virF, the transcriptional activator of the Yersinia virulence regulon, and AraC, the Escherichia coli arabinose operon regulator

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.

The phytopathogen Dickeya dadantii 3937 cpxR locus gene participates in the regulation of virulence and the global c-di-GMP network

Bacteria use signal transduction systems to sense and respond to their external environment. The two-component system CpxA/CpxR senses misfolded envelope protein stress and responds by up-regulating envelope protein factors and down-regulating virulence factors in several animal pathogens. Dickeya dadantii is a phytopathogen equipped with a type III secretion system (T3SS) for manipulating the host immune response. We found that deletion of cpxR enhanced the expression of the T3SS marker gene hrpA in a designated T3SS-inducing minimal medium (MM). In the ∆cpxR mutant, multiple T3SS and c-di-GMP regulators were also up-regulated. Subsequent analysis revealed that deletion of the phosphodiesterase gene egcpB in ∆cpxR abolished the enhanced T3SS expression. This suggested that CpxR suppresses EGcpB levels, causing low T3SS expression in MM. Furthermore, we found that the ∆cpxR mutant displayed low c-di-GMP phenotypes in biofilm formation and swimming. Increased production of cellular c-di-GMP by in trans expression of the diguanylate cyclase gene gcpA was negated in the ∆cpxR mutant. Here, we propose that CpxA/CpxR regulates T3SS expression by manipulating the c-di-GMP network, in turn modifying the multiple physiological activities involved in the response to environmental stresses in D. dadantii.

The Regulatory Circuit Underlying Downregulation of a Type III Secretion System in Yersinia enterocolitica by Transcription Factor OmpR

In a previous study, differential proteomic analysis was used to identify membrane proteins of the human enteropathogen Yersinia enterocolitica, whose levels are influenced by OmpR, the transcriptional regulator in the two-component EnvZ/OmpR system. Interestingly, this analysis demonstrated that at 37 °C, OmpR negatively affects the level of over a dozen Ysc-Yop proteins, which constitute a type III secretion system (T3SS) that is essential for the pathogenicity of Y. enterocolitica. Here, we focused our analysis on the role of OmpR in the expression and secretion of Yops (translocators and effectors). Western blotting with anti-Yops antiserum and specific anti-YopD, -YopE and -YopH antibodies, confirmed that the production of Yops is down-regulated by OmpR with the greatest negative effect on YopD. The RT-qPCR analysis demonstrated that, while OmpR had a negligible effect on the activity of regulatory genes virF and yscM1, it highly repressed the expression of yopD. OmpR was found to bind to the promoter of the lcrGVsycD-yopBD operon, suggesting a direct regulatory effect. In addition, we demonstrated that the negative regulatory influence of OmpR on the Ysc-Yop T3SS correlated with its positive role in the expression of flhDC, the master regulator of the flagellar-associated T3SS.

Characterization of the binding motif for the T3SS master regulator LcrF in Yersinia pseudotuberculosis

LcrF is the master regulator that positively regulates the Ysc type III secretion system (T3SS) in Yersinia and shares a high similarity with the DNA-binding domain of the T3SS master regulator ExsA in Pseudomonas aeruginosa. Based on these features, bioinformatics analysis has predicted a putative LcrF-binding site in its target promoters. Here, we experimentally characterized its binding motif. An adenine-rich LcrF-binding region in the lcrG promoter sequence, a typical regulatory target of LcrF, was first confirmed. To obtain detailed information, this binding region was cloned into a synthetized promoter and mutations in this region were further constructed. We demonstrated that the 5'-AAAAA-n5-GnCT-3' sequence is required for LcrF regulation and this motif is strictly located 4-bp upstream of a noncanonical promoter, in which the -35 and -10 elements are separated by a 21-bp spacer. Consistently, the putative binding motif was found in promoters of nine T3SS related operons or genes positively regulated by LcrF. Transcriptome analysis further confirmed that LcrF specifically activates T3SS genes in Yersinia. Collectively, our data suggest that LcrF has evolved to be a specific T3SS activator with a stringent sequence requirement for transcriptional regulation.

Virulence genotyping and antimicrobial resistance profiles of Yersinia enterocolitica isolated from meat and meat products in Egypt

Pathogenic Yersinia enterocolitica (Y. enterocolitica) is one of the food-borne entero-pathogen responsible for yersiniosis in humans. The purpose of this research was to survey the prevalence, virulence-associated genes, and antimicrobial resistance of Y. enterocolitica isolated from meat and meat product samples in Egypt. Forty-one (5.9%) out of 700- samples of chicken meat, beef, ground beef, and sausage were positive Y. enterocolitica with a high prevalence in chicken meat (12%). Five virulence genes (ail, inv, ystA, ystB, and yadA) were characterized among 41 Y. enterocolitica isolates with variable frequencies. Among the strains tested, the ystB gene was detected with a high percentage (78.1%), followed by inv gene (70.7%), ail gene (14.6%), ystA gene (12.2%), and yadA gene (2.4%). A high resistance rate was estimated to amoxicillin-clavulanic acid (100%), followed by cefazolin (95%), ampicillin (65.9%), and doxycycline (51.2%), whilst a high sensitivity rate was observed to gentamicin and ciprofloxacin (97.6% each). Interestingly, the multidrug resistance was specified in the 70.7% of strains and showing 13 resistance patterns. Based on nucleotide sequence analysis of the 16s rRNA gene, the phylogenetic tree showed the genetic relatedness amongst Y. enterocolitica isolates. These findings highlighted the emergence of virulent and multidrug-resistant pathogenic Y. entrocolitica in retailed meat and meat products in Egypt.

Lon Protease- and Temperature-Dependent Activity of a Lysis Cassette Located in the Insecticidal Island of Yersinia enterocolitica

The Yersinia genus comprises pathogens that can adapt to an environmental life cycle stage as well as to mammals. Yersinia enterocolitica strain W22703 exhibits both insecticidal and nematocidal activity conferred by the tripartite toxin complex (Tc) that is encoded on the 19-kb pathogenicity island Tc-PAI _Ye All tc genes follow a strict temperature regulation in that they are silenced at 37°C but activated at lower temperatures. Four highly conserved phage-related genes, located within the Tc-PAI _Ye , were recently demonstrated to encode a biologically functional holin-endolysin gene cassette that lyses its own host W22703 at 37°C. Conditions transcriptionally activating the cassette are not yet known. In contrast to Escherichia coli, the overproduction of holin and endolysin did not result in cell lysis of strain W22703 at 15°C. When the holin-endolysin genes were overexpressed at 15°C in four Y. enterocolitica biovars and in four other Yersinia spp., a heterogenous pattern of phenotypes was observed, ranging from lysis resistance of a biovar 1A strain to the complete growth arrest of a Y. kristensenii strain. To decipher the molecular mechanism underlying this temperature-dependent lysis, we constructed a Lon protease-negative mutant of W22703 in which the overexpression of the lysis cassette leads to cell death at 15°C. Overexpressed endolysin exhibited a high proteolytic susceptibility in strain W22703 but remained stable in the W22703 Δlon strain or in Y. pseudotuberculosis Although artificial overexpression was applied here, the data indicate that Lon protease plays a role in the control of the temperature-dependent lysis in Y. enterocolitica W22703.IMPORTANCE The investigation of the mechanisms that help pathogens survive in the environment is a prerequisite to understanding their evolution and their virulence capacities. In members of the genus Yersinia, many factors involved in virulence, metabolism, motility, or biofilm formation follow a strict temperature-dependent regulation. While the molecular mechanisms underlying the activation of determinants at body temperature have been analyzed in detail, the molecular basis of low-temperature-dependent phenotypes is largely unknown. Here, we demonstrate that a novel phage-related lysis cassette, which is part of the insecticidal and nematocidal pathogenicity island of Y. enterocolitica, does not lyse its own host following overexpression at 15°C and that the Lon protease is involved in this phenotype.

HilD, HilC, and RtsA Form Homodimers and Heterodimers To Regulate Expression of the Salmonella Pathogenicity Island I Type III Secretion System

Salmonella enterica serovar Typhimurium colonizes and invades host intestinal epithelial cells using the type three secretion system (T3SS) encoded on Salmonella pathogenicity island 1 (SPI1). The level of SPI1 T3SS gene expression is controlled by the transcriptional activator HilA, encoded on SPI1. Expression of hilA is positively regulated by three homologous transcriptional regulators, HilD, HilC, and RtsA, belonging to the AraC/XylS family. These regulators also activate the hilD, hilC, and rtsA genes by binding to the same DNA sequences upstream of these promoters, forming a complex feed-forward loop to control SPI1 expression. Despite the apparent redundancy in function, HilD has a unique role in SPI1 regulation because the majority of external regulatory inputs act exclusively through HilD. To better understand SPI1 regulation, the nature of interaction between HilD, HilC, and RtsA has been characterized using biochemical and genetic techniques. Our results showed that HilD, HilC, and RtsA can form heterodimers as well as homodimers in solution. Comparison with other AraC family members identified a putative α-helix in the N-terminal domain, which acts as the dimerization domain. Alanine substitution in this region results in reduced dimerization of HilD and HilC and also affects their ability to activate hilA expression. The dimer interactions of HilD, HilC, and RtsA add another layer of complexity to the SPI1 regulatory circuit, providing a more comprehensive understanding of SPI1 T3SS regulation and Salmonella pathogenesis.IMPORTANCE The SPI1 type three secretion system is a key virulence factor required for Salmonella to both cause gastroenteritis and initiate serious systemic disease. The system responds to numerous environmental signals in the intestine, integrating this information via a complex regulatory network. Here, we show that the primary regulatory proteins in the network function as both homodimers and heterodimers, providing information regarding both regulation of virulence in this important pathogen and general signal integration to control gene expression.

Maintaining Integrity Under Stress: Envelope Stress Response Regulation of Pathogenesis in Gram-Negative Bacteria

The Gram-negative bacterial envelope is an essential interface between the intracellular and harsh extracellular environment. Envelope stress responses (ESRs) are crucial to the maintenance of this barrier and function to detect and respond to perturbations in the envelope, caused by environmental stresses. Pathogenic bacteria are exposed to an array of challenging and stressful conditions during their lifecycle and, in particular, during infection of a host. As such, maintenance of envelope homeostasis is essential to their ability to successfully cause infection. This review will discuss our current understanding of the σ^E- and Cpx-regulated ESRs, with a specific focus on their role in the virulence of a number of model pathogens.

Enteropathogens: Tuning Their Gene Expression for Hassle-Free Survival

Enteropathogenic bacteria have been the cause of the majority of foodborne illnesses. Much of the research has been focused on elucidating the mechanisms by which these pathogens evade the host immune system. One of the ways in which they achieve the successful establishment of a niche in the gut microenvironment and survive is by a chain of elegantly regulated gene expression patterns. Studies have shown that this process is very elaborate and is also regulated by several factors. Pathogens like, enteropathogenic Escherichia coli (EPEC), Salmonella Typhimurium, Shigellaflexneri, Yersinia sp. have been seen to employ various regulated gene expression strategies. These include toxin-antitoxin systems, quorum sensing systems, expression controlled by nucleoid-associated proteins (NAPs), several regulons and operons specific to these pathogens. In the following review, we have tried to discuss the common gene regulatory systems of enteropathogenic bacteria as well as pathogen-specific regulatory mechanisms.

RNA Thermometers in Bacterial Pathogens

Temperature variation is one of the multiple parameters a microbial pathogen encounters when it invades a warm-blooded host. To survive and thrive at host body temperature, human pathogens have developed various strategies to sense and respond to their ambient temperature. An instantaneous response is mounted by RNA thermometers (RNATs), which are integral sensory structures in mRNAs that modulate translation efficiency. At low temperatures outside the host, the folded RNA blocks access of the ribosome to the translation initiation region. The temperature shift upon entering the host destabilizes the RNA structure and thus permits ribosome binding. This reversible zipper-like mechanism of RNATs is ideally suited to fine-tune virulence gene expression when the pathogen enters or exits the body of its host. This review summarizes our present knowledge on virulence-related RNATs and discusses recent developments in the field.

Yersinia enterocolitica CsrA regulates expression of the Ysa and Ysc type 3 secretion system in unique ways

This study investigated how carbon storage regulator A (CsrA) affects expression of the Ysa and Ysc type 3 secretion (T3S) system genetic regulatory cascades that control Ysps (Yersinia secreted proteins) and Yops (Yersinia outer proteins) export, respectively. Given that most often CsrA acts as a mediator of mRNA stability, an activity that can be monitored using lacZ transcriptional fusions, we employed a collection of reporter strains to assess Ysa and Ysc gene expression. To this end, bacteria were cultivated to induce either the Ysa or the Ysc T3S system. Comparison of csrA mutants to the wild-type strain revealed that, in response to the respective inducing conditions, genes spanning the Ysa and Ysc gene cascades displayed increased expressions. Then, the possibility that CsrA affects secretion of Ysps and Yops was tested and the profiles of secreted proteins by wild-type and csrA mutant strains were compared by proteomic analysis. Ysps were over-secreted and Yops were under-secreted, for the csrA mutant. These results support the hypothesis that CsrA affects both the Ysa and Ysc T3S systems in Yersinia enterocolitica. They further support the conclusion that CsrA plays an important role in controlling adaptation of this pathogenic bacterium during its lifecycle shift between a terrestrial and parasitic existence.

Environmental Regulation of Yersinia Pathophysiology

Hallmarks of Yersinia pathogenesis include the ability to form biofilms on surfaces, the ability to establish close contact with eukaryotic target cells and the ability to hijack eukaryotic cell signaling and take over control of strategic cellular processes. Many of these virulence traits are already well-described. However, of equal importance is knowledge of both confined and global regulatory networks that collaborate together to dictate spatial and temporal control of virulence gene expression. This review has the purpose to incorporate historical observations with new discoveries to provide molecular insight into how some of these regulatory mechanisms respond rapidly to environmental flux to govern tight control of virulence gene expression by pathogenic Yersinia.

Transcriptional profiling of Bordetella pertussis reveals requirement of RNA chaperone Hfq for Type III secretion system functionality

Bordetella pertussis, the causative agent of human whooping cough (pertussis) produces a complex array of virulence factors in order to establish efficient infection in the host. The RNA chaperone Hfq and small regulatory RNAs are key players in posttranscriptional regulation in bacteria and have been shown to play an essential role in virulence of a broad spectrum of bacterial pathogens. This study represents the first attempt to characterize the Hfq regulon of the human pathogen B. pertussis under laboratory conditions as well as upon passage in the host and indicates that loss of Hfq has a profound effect on gene expression in B. pertussis. Comparative transcriptional profiling revealed that Hfq is required for expression of several virulence factors in B. pertussis cells including the Type III secretion system (T3SS). In striking contrast to the wt strain, T3SS did not become operational in the hfq mutant passaged either through mice or macrophages thereby proving that Hfq is required for the functionality of the B. pertussis T3SS. Likewise, expression of virulence factors vag8 and tcfA encoding autotransporter and tracheal colonization factor, respectively, was strongly reduced in the hfq mutant. Importantly, for the first time we demonstrate that B. pertussis T3SS can be activated upon contact with macrophage cells in vitro.

Yersinia Type III Secretion System Master Regulator LcrF

Many Gram-negative pathogens express a type III secretion (T3SS) system to enable growth and survival within a host. The three human-pathogenic Yersinia species, Y. pestis, Y. pseudotuberculosis, and Y. enterocolitica, encode the Ysc T3SS, whose expression is controlled by an AraC-like master regulator called LcrF. In this review, we discuss LcrF structure and function as well as the environmental cues and pathways known to regulate LcrF expression. Similarities and differences in binding motifs and modes of action between LcrF and the Pseudomonas aeruginosa homolog ExsA are summarized. In addition, we present a new bioinformatics analysis that identifies putative LcrF binding sites within Yersinia target gene promoters.

Regulatory principles governing Salmonella and Yersinia virulence

Enteric pathogens such as Salmonella and Yersinia evolved numerous strategies to survive and proliferate in different environmental reservoirs and mammalian hosts. Deciphering common and pathogen-specific principles for how these bacteria adjust and coordinate spatiotemporal expression of virulence determinants, stress adaptation, and metabolic functions is fundamental to understand microbial pathogenesis. In order to manage sudden environmental changes, attacks by the host immune systems and microbial competition, the pathogens employ a plethora of transcriptional and post-transcriptional control elements, including transcription factors, sensory and regulatory RNAs, RNAses, and proteases, to fine-tune and control complex gene regulatory networks. Many of the contributing global regulators and the molecular mechanisms of regulation are frequently conserved between Yersinia and Salmonella. However, the interplay, arrangement, and composition of the control elements vary between these closely related enteric pathogens, which generate phenotypic differences leading to distinct pathogenic properties. In this overview we present common and different regulatory networks used by Salmonella and Yersinia to coordinate the expression of crucial motility, cell adhesion and invasion determinants, immune defense strategies, and metabolic adaptation processes. We highlight evolutionary changes of the gene regulatory circuits that result in different properties of the regulatory elements and how this influences the overall outcome of the infection process.

Transcriptomic Analysis of Yersinia enterocolitica Biovar 1B Infecting Murine Macrophages Reveals New Mechanisms of Extracellular and Intracellular Survival

Yersinia enterocolitica is typically considered an extracellular pathogen; however, during the course of an infection, a significant number of bacteria are stably maintained within host cell vacuoles. Little is known about this population and the role it plays during an infection. To address this question and to elucidate the spatially and temporally dynamic gene expression patterns of Y. enterocolitica biovar 1B through the course of an in vitro infection, transcriptome sequencing and differential gene expression analysis of bacteria infecting murine macrophage cells were performed under four distinct conditions. Bacteria were first grown in a nutrient-rich medium at 26 °C to establish a baseline of gene expression that is unrelated to infection. The transcriptomes of these bacteria were then compared to bacteria grown in a conditioned cell culture medium at 37 °C to identify genes that were differentially expressed in response to the increased temperature and medium but not in response to host cells. Infections were then performed, and the transcriptomes of bacteria found on the extracellular surface and intracellular compartments were analyzed individually. The upregulated genes revealed potential roles for a variety of systems in promoting intracellular virulence, including the Ysa type III secretion system, the Yts2 type II secretion system, and the Tad pilus. It was further determined that mutants of each of these systems had decreased virulence while infecting macrophages. Overall, these results reveal the complete set of genes expressed by Y. enterocolitica in response to infection and provide the groundwork for future virulence studies.

RcsB positively regulates the Yersinia Ysc-Yop type III secretion system by activating expression of the master transcriptional regulator LcrF

The Rcs phosphorelay is a complex signaling pathway used by the family Enterobacteriaceae to sense, respond and adapt to environmental changes during free-living or host-associated lifestyles. In this study, we show that the Rcs phosphorelay pathway positively regulates the virulence plasmid encoded Ysc-Yop type III secretion system (T3SS) in the enteropathogen Yesinia pseudotuberculosis. Both the overexpression of the wild-type Rcs regulator RcsB or the constitutive active RscB(D56E) variant triggered more abundant Ysc-Yop synthesis and secretion, whereas the non-phosphorylatable mutant RcsB(D56Q) negated this. Congruently, enhanced Yops expression and secretion occurred in an in cis rscB(D56E) mutant but not in an isogenic rscB(D56Q) mutant. Screening for regulatory targets of RcsB identified the virG-lcrF operon that encodes for LcrF, the Ysc-Yop T3SS master regulator. Protein-DNA binding assays confirmed that RcsB directly bound to this operon promoter, which subsequently caused stimulated lcrF transcription. Moreover, active RcsB enhanced the ability of bacteria to deliver Yop effectors into immune cells during cell contact, and this promoted an increase in bacterial viability. Taken together, our study demonstrates the role of the Rcs system in regulating the Ysc-Yop T3SS in Yersinia and reports on RcsB being the first transcriptional activator known to directly control lcrF transcription.

IscR is essential for yersinia pseudotuberculosis type III secretion and virulence

Type III secretion systems (T3SS) are essential for virulence in dozens of pathogens, but are not required for growth outside the host. Therefore, the T3SS of many bacterial species are under tight regulatory control. To increase our understanding of the molecular mechanisms behind T3SS regulation, we performed a transposon screen to identify genes important for T3SS function in the food-borne pathogen Yersinia pseudotuberculosis. We identified two unique transposon insertions in YPTB2860, a gene that displays 79% identity with the E. coliiron-sulfur cluster regulator, IscR. A Y. pseudotuberculosis iscR in-frame deletion mutant (ΔiscR) was deficient in secretion of Ysc T3SS effector proteins and in targeting macrophages through the T3SS. To determine the mechanism behind IscR control of the Ysc T3SS, we carried out transcriptome and bioinformatic analysis to identify Y. pseudotuberculosis genes regulated by IscR. We discovered a putative IscR binding motif upstream of the Y. pseudotuberculosis yscW-lcrF operon. As LcrF controls transcription of a number of critical T3SS genes in Yersinia, we hypothesized that Yersinia IscR may control the Ysc T3SS through LcrF. Indeed, purified IscR bound to the identified yscW-lcrF promoter motif and mRNA levels of lcrF and 24 other T3SS genes were reduced in Y. pseudotuberculosis in the absence of IscR. Importantly, mice orally infected with the Y. pseudotuberculosis ΔiscR mutant displayed decreased bacterial burden in Peyer's patches, mesenteric lymph nodes, spleens, and livers, indicating an essential role for IscR in Y. pseudotuberculosis virulence. This study presents the first characterization of Yersinia IscR and provides evidence that IscR is critical for virulence and type III secretion through direct regulation of the T3SS master regulator, LcrF.

Bacterial pathogens use regulators that sense environmental cues to enhance their fitness. Here, we identify a transcriptional regulator in the human gut pathogen, Yersinia pseudotuberculosis, which controls a specialized secretion system essential for bacterial growth in mammalian tissues. This regulator was shown in other bacterial species to alter its activity in response to changes in iron concentration and oxidative stress, but has never been studied in Yersinia. Importantly, Y. pseudotuberculosis experiences large changes in iron bioavailability upon transit from the gut to deeper tissues and iron is a critical component in Yersinia virulence, as individuals with iron overload disorders have enhanced susceptibility to systemic Yersinia infections. Our work places this iron-modulated transcriptional regulator within the regulatory network that controls virulence gene expression in Y. pseudotuberculosis, identifying it as a potential new target for antimicrobial agents.

LcrQ blocks the role of LcrF in regulating the Ysc-Yop type III secretion genes in Yersinia pseudotuberculosis

Pathogenic Yersinia species employ the Ysc-Yop type III secretion system (T3SS) encoded by a highly conserved pYV virulence plasmid to export the virulence effectors into host cells. The Ysc-Yop T3SS is tightly regulated by multiple contributing proteins that function at different levels. However, systematic transcriptional regulation analysis of Ysc-Yop T3SS is lacking and the detailed mechanism under this regulation process is still elusive. Aimed at systematically characterizing transcriptional regulations of all T3SS genes in Y. pseudotuberculosis, we amplified 97 non-coding fragments from the pYV plasmid and analyzed transcriptional responses of the T3SS genes under different growth conditions. Transcriptions of T3SS genes were induced at 37°C and genes encoding T3SS effectors were highly induced by further depletion of Ca2+. The temperature induced gene transcription process is mediated by modules encoded on the chromosome, while the Ca2+ depletion-induced process is controlled by the positive regulatory protein LcrF as well as the negative regulatory protein LcrQ. In this process, LcrQ shares the same targets with LcrF and the effect of LcrQ is dependent on the presence of LcrF. Furthermore, over-expression of LcrF showed the same phenotype as that of the lcrQ mutant strain and intracellular amount balance of LcrQ and LcrF is important in T3SS regulation. When the expression level of LcrF exceeds LcrQ, expression of the Ysc-Yop T3SS genes is activated and vice versa. Together, these data support a model in which LcrQ blocks the activation role of LcrF in regulating the transcription of T3SS genes in Yersinia.

YmoA negatively controls the expression of insecticidal genes in Yersinia enterocolitica

Yersinia enterocolitica is toxic towards invertebrates due to the presence of the toxin complex (tc) genes that are activated by the thermolabile regulator TcaR2. In the search for further regulatory factors involved in insecticidal gene expression, the modulator of yersinial virulence, YmoA, was identified to silence all tc genes of the Y. enterocolitica strain W22703 (biovar 2, serovar O:9). Using promoter fusions with the luciferase reporter, we found that the deletion of ymoA results in elevated transcription of tcaR1, tcaR2, tcaA, tcaB, tcaC, tccC1 and tccC2 at both 15 °C and 37 °C. Complementation by episomal ymoA significantly reduced tc gene expression, thus validating the inhibitory activity of YmoA on the production of insecticidal proteins. YmoA contributes to the binding properties of H-NS to the tc promoters by forming a complex with this nucleoid-associated protein, and this complex not only binds to the upstream regions of all tc genes, but also to intragenic sites of tcaA and tcaB that play an important role in controlling the expression of both genes. At low temperature, the intracellular amount of thermostable YmoA is not reduced, but the repressor is less functional. These data point to H-NS/YmoA as an antagonist of the inducer TcaR2.

Thermosensing to adjust bacterial virulence in a fluctuating environment

The lifecycle of most microbial pathogens can be divided into two states: existence outside and inside their hosts. The sudden temperature upshift experienced upon entry from environmental or vector reservoirs into a warm-blooded host is one of the most crucial signals informing the pathogens to adjust virulence gene expression and their host-stress survival program. This article reviews the plethora of sophisticated strategies that bacteria have evolved to sense temperature, and outlines the molecular signal transduction mechanisms used to modulate synthesis of crucial virulence determinants. The molecular details of thermal control through conformational changes of DNA, RNA and proteins are summarized, complex and diverse thermosensing principles are introduced and their potential as drug targets or synthetic tools are discussed.

RfaL is required for Yersinia pestis type III secretion and virulence

Yersinia pestis, the causative agent of plague, uses a type III secretion system (T3SS) to inject cytotoxic Yop proteins directly into the cytosol of mammalian host cells. The T3SS can also be activated in vitro at 37°C in the absence of calcium. The chromosomal gene rfaL (waaL) was recently identified as a virulence factor required for proper function of the T3SS. RfaL functions as a ligase that adds the terminal N-acetylglucosamine to the lipooligosaccharide core of Y. pestis. We previously showed that deletion of rfaL prevents secretion of Yops in vitro. Here we show that the divalent cations calcium, strontium, and magnesium can partially or fully rescue Yop secretion in vitro, indicating that the secretion phenotype of the rfaL mutant may be due to structural changes in the outer membrane and the corresponding feedback inhibition on the T3SS. In support of this, we found that the defect can be overcome by deleting the regulatory gene lcrQ. Consistent with a defective T3SS, the rfaL mutant is less virulent than the wild type. We show here that the virulence defect of the mutant correlates with a decrease in both T3SS gene expression and ability to inject innate immune cells, combined with an increased sensitivity to cationic antimicrobial peptides.

Post-transcriptional regulation of gene expression in Yersinia species

Proper regulation of gene expression is required by bacterial pathogens to respond to continually changing environmental conditions and the host response during the infectious process. While transcriptional regulation is perhaps the most well understood form of controlling gene expression, recent studies have demonstrated the importance of post-transcriptional mechanisms of gene regulation that allow for more refined management of the bacterial response to host conditions. Yersinia species of bacteria are known to use various forms of post-transcriptional regulation for control of many virulence-associated genes. These include regulation by cis- and trans-acting small non-coding RNAs, RNA-binding proteins, RNases, and thermoswitches. The effects of these and other regulatory mechanisms on Yersinia physiology can be profound and have been shown to influence type III secretion, motility, biofilm formation, host cell invasion, intracellular survival and replication, and more. In this review, we discuss these and other post-transcriptional mechanisms and their influence on virulence gene regulation, with a particular emphasis on how these processes influence the virulence of Yersinia in the host.

Protein export according to schedule: architecture, assembly, and regulation of type III secretion systems from plant- and animal-pathogenic bacteria

Flagellar and translocation-associated type III secretion (T3S) systems are present in most gram-negative plant- and animal-pathogenic bacteria and are often essential for bacterial motility or pathogenicity. The architectures of the complex membrane-spanning secretion apparatuses of both systems are similar, but they are associated with different extracellular appendages, including the flagellar hook and filament or the needle/pilus structures of translocation-associated T3S systems. The needle/pilus is connected to a bacterial translocon that is inserted into the host plasma membrane and mediates the transkingdom transport of bacterial effector proteins into eukaryotic cells. During the last 3 to 5 years, significant progress has been made in the characterization of membrane-associated core components and extracellular structures of T3S systems. Furthermore, transcriptional and posttranscriptional regulators that control T3S gene expression and substrate specificity have been described. Given the architecture of the T3S system, it is assumed that extracellular components of the secretion apparatus are secreted prior to effector proteins, suggesting that there is a hierarchy in T3S. The aim of this review is to summarize our current knowledge of T3S system components and associated control proteins from both plant- and animal-pathogenic bacteria.

Elevated CpxR~P levels repress the Ysc-Yop type III secretion system of Yersinia pseudotuberculosis

One way that Gram-negative bacteria respond to extracytoplasmic stress is through the CpxA-CpxR system. An activated CpxA sensor kinase phosphorylates the CpxR response regulator to instigate positive auto-amplification of Cpx pathway activation, as well as synthesis of various bacterial survival factors. In the absence of CpxA, human enteropathogenic Yersinia pseudotuberculosis accumulates high CpxR~P levels aided by the action of low molecular weight phosphodonors such as acetyl~P. Critically, these bacteria are also defective for plasmid-encoded Ysc-Yop-dependent type III synthesis and secretion, an essential determinant of virulence. Herein, we investigated whether elevated CpxR~P levels account for lost Ysc-Yop function. Decisively, reducing CpxR∼P in Yersinia defective for CpxA phosphatase activity - through incorporating second-site suppressor mutations in ackA-pta or cpxR - dramatically restored Ysc-Yop T3S function. Moreover, the repressive effect of accumulated CpxR∼P is a direct consequence of binding to the promoter regions of the T3S genes. Thus, Cpx pathway activation has two consequences in Yersinia; one, to maintain quality control in the bacterial envelope, and the second, to restrict ysc-yop gene expression to those occasions where it will have maximal effect.

Concerted actions of a thermo-labile regulator and a unique intergenic RNA thermosensor control Yersinia virulence

Expression of all Yersinia pathogenicity factors encoded on the virulence plasmid, including the yop effector and the ysc type III secretion genes, is controlled by the transcriptional activator LcrF in response to temperature. Here, we show that a protein- and RNA-dependent hierarchy of thermosensors induce LcrF synthesis at body temperature. Thermally regulated transcription of lcrF is modest and mediated by the thermo-sensitive modulator YmoA, which represses transcription from a single promoter located far upstream of the yscW-lcrF operon at moderate temperatures. The transcriptional response is complemented by a second layer of temperature-control induced by a unique cis-acting RNA element located within the intergenic region of the yscW-lcrF transcript. Structure probing demonstrated that this region forms a secondary structure composed of two stemloops at 25°C. The second hairpin sequesters the lcrF ribosomal binding site by a stretch of four uracils. Opening of this structure was favored at 37°C and permitted ribosome binding at host body temperature. Our study further provides experimental evidence for the biological relevance of an RNA thermometer in an animal model. Following oral infections in mice, we found that two different Y. pseudotuberculosis patient isolates expressing a stabilized thermometer variant were strongly reduced in their ability to disseminate into the Peyer's patches, liver and spleen and have fully lost their lethality. Intriguingly, Yersinia strains with a destabilized version of the thermosensor were attenuated or exhibited a similar, but not a higher mortality. This illustrates that the RNA thermometer is the decisive control element providing just the appropriate amounts of LcrF protein for optimal infection efficiency.

Yersinia enterocolitica bacteremia and enterocolitis in a previously healthy 20-month-old girl

Yersinia enterocolitica is a gram-negative bacillus that can cause illness ranging from a self-limiting enterocolitis to life-threatening bacteremia. Y. enterocolitica biotype 1B, serotype O:8 (1B/O:8), is the most pathogenic of the Yersinia species because of the presence of the high-pathogenicity island and the Yersinia virulence plasmid (pYV). Here, we report a pediatric case of Y. enterocolitica 1B/O:8 bacteremia and enterocolitis. A 20-month-old girl was admitted to hospital with fever,pharyngitis, and abdominal pain on day 2. Blood culture on admission was positive for Y. enterocolitica 1B/O:8. Stool culture on day 5 after cefotaxime treatment was also positive for Y. enterocolitica 1B/O:8, but only after cold enrichment at 4°C for 3 weeks. PCR assays identified the pYV only in stool specimens, indicating that strains from routine blood culture at 37°C lacked the pYV. The present case showed the usefulness of stool culture with cold enrichment and agglutination test for the diagnosis of Y. enterocolitica infection. We would therefore like to emphasize the importance of collection and preservation of stool specimens for the identification of pYV. To our knowledge, this is the first reported pediatric case of Y. enterocolitica 1B/O:8 bacteremia.

Oligomeric coiled-coil adhesin YadA is a double-edged sword

Yersinia adhesin A (YadA) is an essential virulence factor for the food-borne pathogens Yersinia enterocolitica and Yersinia pseudotuberculosis. Surprisingly, it is a pseudogene in Yersinia pestis. Even more intriguing, the introduction of a functional yadA gene in Y. pestis EV76 was shown to correlate with a decrease in virulence in a mouse model. Here, we report that wild type (wt) Y. enterocolitica E40, as well as YadA-deprived E40 induced the synthesis of neutrophil extracellular traps (NETs) upon contact with neutrophils, but only YadA-expressing Y. enterocolitica adhered to NETs and were killed. As binding seemed to be a prerequisite for killing, we searched for YadA-binding substrates and detected the presence of collagen within NETs. E40 bacteria expressing V98D,N99A mutant YadA with a severely reduced ability to bind collagen were found to be more resistant to killing, suggesting that collagen binding contributes significantly to sensitivity to NETs. Wt Y. pestis EV76 were resistant to killing by NETs, while recombinant EV76 expressing YadA from either Y. pseudotuberculosis or Y. enterocolitica were sensitive to killing by NETs, outlining the importance of YadA for susceptibility to NET-dependent killing. Recombinant EV76 endowed with YadA from Y. enterocolitica were also less virulent for the mouse than wt EV76, as shown before. In addition, EV76 carrying wt YadA were less virulent for the mouse than EV76 expressing YadA(V₉₈D,N₉₉A). The observation that YadA makes Yersinia sensitive to NETs provides an explanation as for why evolution selected for the inactivation of yadA in the flea-borne Y. pestis and clarifies an old enigma. Since YadA imposes the same cost to the food-borne Yersinia but was nevertheless conserved by evolution, this observation also illustrates the duality of some virulence functions.

Cell membrane is impaired, accompanied by enhanced type III secretion system expression in Yersinia pestis deficient in RovA regulator

BACKGROUND

In the enteropathogenic Yersinia species, RovA regulates the expression of invasin, which is important for enteropathogenic pathogenesis but is inactivated in Yersinia pestis. Investigation of the RovA regulon in Y. pestis at 26 °C has revealed that RovA is a global regulator that contributes to virulence in part by the direct regulation of psaEFABC. However, the regulatory roles of RovA in Y. pestis at 37 °C, which allows most virulence factors in mammalian hosts to be expressed, are still poorly understood.

METHODOLOGY/PRINCIPAL FINDINGS

The transcriptional profile of an in-frame rovA mutant of Y. pestis biovar Microtus strain 201 was analyzed under type III secretion system (T3SS) induction conditions using microarray techniques, and it was revealed that many cell-envelope and transport/binding proteins were differentially expressed in the ΔrovA mutant. Most noticeably, many of the T3SS genes, including operons encoding the translocon, needle and Yop (Yersinia outer protein) effectors, were significantly up-regulated. Analysis of Yop proteins confirmed that YopE and YopJ were also expressed in greater amounts in the mutant. However, electrophoresis mobility shift assay results demonstrated that the His-RovA protein could not bind to the promoter sequences of the T3SS genes, suggesting that an indirect regulatory mechanism is involved. Transmission electron microscopy analysis indicated that there are small loose electron dense particle-like structures that surround the outer membrane of the mutant cells. The bacterial membrane permeability to CFSE (carboxyfluorescein diacetate succinimidyl ester) was significantly decreased in the ΔrovA mutant compared to the wild-type strain. Taken together, these results revealed the improper construction and dysfunction of the membrane in the ΔrovA mutant.

CONCLUSIONS/SIGNIFICANCE

We demonstrated that the RovA regulator plays critical roles in the construction and functioning of the bacterial membrane, which sheds considerable light on the regulatory functions of RovA in antibiotic resistance and environmental adaptation. The expression of T3SS was upregulated in the ΔrovA mutant through an indirect regulatory mechanism, which is possibly related to the altered membrane construction in the mutant.

Control of the type 3 secretion system in Vibrio harveyi by quorum sensing through repression of ExsA

The type 3 secretion system (T3SS) genes of Vibrio harveyi are activated at low cell density and repressed at high cell density by quorum sensing (QS). Repression requires LuxR, the master transcriptional regulator of QS-controlled genes. Here, we determine the mechanism underlying the LuxR repression of the T3SS system. Using a fluorescence-based cell sorting approach, we isolated V. harveyi mutants that are unable to express T3SS genes at low cell density and identified two mutations in the V. harveyi exsBA operon. While LuxR directly represses the expression of exsBA, complementation and epistasis analyses reveal that it is the repression of exsA expression, but not exsB expression, that is responsible for the QS-mediated repression of T3SS genes at high cell density. The present work further defines the genes in the V. harveyi QS regulon and elucidates a mechanism demonstrating how multiple regulators can be linked in series to direct the expression of QS target genes specifically at low or high cell density.

Brucella melitensis VjbR and C12-HSL regulons: contributions of the N-dodecanoyl homoserine lactone signaling molecule and LuxR homologue VjbR to gene expression

Background

Quorum sensing is a communication system that regulates gene expression in response to population density and often regulates virulence determinants. Deletion of the luxR homologue vjbR highly attenuates intracellular survival of Brucella melitensis and has been interpreted to be an indication of a role for QS in Brucella infection. Confirmation for such a role was suggested, but not confirmed, by the demonstrated in vitro synthesis of an auto-inducer (AI) by Brucella cultures. In an effort to further delineate the role of VjbR to virulence and survival, gene expression under the control of VjbR and AI was characterized using microarray analysis.

Results

Analyses of wildtype B. melitensis and isogenic ΔvjbR transciptomes, grown in the presence and absence of exogenous N-dodecanoyl homoserine lactone (C₁₂-HSL), revealed a temporal pattern of gene regulation with variances detected at exponential and stationary growth phases. Comparison of VjbR and C₁₂-HSL transcriptomes indicated the shared regulation of 127 genes with all but 3 genes inversely regulated, suggesting that C₁₂-HSL functions via VjbR in this case to reverse gene expression at these loci. Additional analysis using a ΔvjbR mutant revealed that AHL also altered gene expression in the absence of VjbR, up-regulating expression of 48 genes and a luxR homologue blxR 93-fold at stationary growth phase. Gene expression alterations include previously un-described adhesins, proteases, antibiotic and toxin resistance genes, stress survival aids, transporters, membrane biogenesis genes, amino acid metabolism and transport, transcriptional regulators, energy production genes, and the previously reported fliF and virB operons.

Conclusions

VjbR and C₁₂-HSL regulate expression of a large and diverse number of genes. Many genes identified as virulence factors in other bacterial pathogens were found to be differently expressed, suggesting an important contribution to intracellular survival of Brucella. From these data, we conclude that VjbR and C₁₂-HSL contribute to virulence and survival by regulating expression of virulence mechanisms and thus controlling the ability of the bacteria to survive within the host cell. A likely scenario occurs via QS, however, operation of such a mechanism remains to be demonstrated.

Novel drug delivery systems: desired feat for tuberculosis

Tuberculosis has claimed its victims throughout much of known human history and is currently the most devastating human bacterial disease. The ability to infect human population on a global scale, combined with the widespread emergence of multi-drug resistant strains, has led to the placement of Mycobacterium tuberculosis on the National Institute of Allergy and Infectious Diseases (NIAID) list of Biodefence and Emerging Infectious Disease Threats Agents. The resurgence of interest in tuberculosis (TB) has stemmed because of increased evidences from developed countries. Contrary to expectations, no country has reached the phase of elimination and in no subsection of society TB has been completely eliminated. A deeper understanding of the process will assist in the identification of the host and mycobacterial efforts involved and provide targets for therapeutic strategies against tuberculosis. The article presents a view on pathogenesis of tuberculosis and its diverse manifestations, host defense evasion, mechanisms of microbial persistence, emergence of Multiple Drug Resistance and Extensive Drug Resistance, conventional therapy used and the possible novel systems which are under extensive investigation as drug carriers for improving the cytosolic concentration of the anti-tubercular agents.

Growth of calcium-blind mutants of Yersinia pestis at 37 degrees C in permissive Ca2+-deficient environments

Cells of wild-type Yersinia pestis exhibit a low-calcium response (LCR) defined as bacteriostasis with expression of a pCD-encoded type III secretion system (T3SS) during cultivation at 37 degrees C without added Ca(2+) versus vegetative growth with downregulation of the T3SS with Ca(2+) (>or=2.5 mM). Bacteriostasis is known to reflect cumulative toxicity of Na(+), l-glutamic acid and culture pH; control of these variables enables full-scale growth ('rescue') in the absence of Ca(2+). Several T3SS regulatory proteins modulate the LCR, because their absence promotes a Ca(2+)-blind phenotype in which growth at 37 degrees C ceases and the T3SS is constitutive even with added Ca(2+). This study analysed the connection between the LCR and Ca(2+) by determining the response of selected Ca(2+)-blind mutants grown in Ca(2+)-deficient rescue media containing Na(+) plus l-glutamate (pH 5.5), where the T3SS is not expressed, l-glutamate alone (pH 6.5), where l-aspartate is fully catabolized, and Na(+) alone (pH 9.0), where the electrogenic sodium pump NADH : ubiquinone oxidoreductase becomes activated. All three conditions supported essentially full-scale Ca(2+)-independent growth at 37 degrees C of wild-type Y. pestis as well as lcrG and yopN mutants (possessing a complete but dysregulated T3SS), indicating that bacteriostasis reflects a Na(+)-dependent lesion in bioenergetics. In contrast, mutants lacking the negative regulator YopD or the YopD chaperone (LcrH) failed to grow in any rescue medium and are therefore truly temperature-sensitive. The Ca(2+)-blind yopD phenotype was fully suppressed in a Ca(2+)-independent background lacking the injectisome-associated inner-membrane component YscV but not peripheral YscK, suggesting that the core translocon energizes YopD.

Detection of Yersinia enterocolitica in alfalfa, mung bean, cilantro, and mamey sapote (Pouteria sapota) food matrices using DNA microarray chip hybridization

Four different food matrices (alfalfa, cilantro, mamey sapote, and mung bean) were contaminated with three different dilutions 10(6), 10(4), and 10(3) cfu/g of Yersinia enterocolitica. DNA was isolated from each food mix and used in chromosomal amplifications. The amplified DNA was used as templates in single PCR reactions of the four genes (virF, ail, yst, and blaA) followed by mixing the four reactions for one PCR primer extension reaction. The presence and the limit of detection of four genes in four food matrices were established by microarray hybridization. Data revealed the diversity of signal intensities. Neither the microarray chip hybridization nor the single PCR amplification could detect virF's presence located on a plasmid. Ail was detected in 10(3) cfu/g, whereas blaA and yst were detected from 10(5) to 10(6) cfu/g in all food matrices. Therefore, the ail gene could be the gene of choice in identifying Y. enterocolitica in alfalfa, cilantro, mamey, and mung bean. Other genes--blaA, yst, virF--exhibited wide variability in hybridization signals, highlighting the need of a better DNA purification step prior to DNA microarray hybridization.

Gene expression profiling of Yersinia pestis with deletion of lcrG, a known negative regulator for Yop secretion of type III secretion system

Yersinia pestis injects a set of virulent proteins into the cytosol of eukaryotic cells by a type III secretion system (T3SS). LcrG is a known negative regulator for secretion of Yersinia outer-membrane proteins (Yops) by blocking the secretion apparatus (Ysc) from the inner membrane. To further understand the effect of lcrG deletion on Y. pestis T3SS regulation, transcriptional profiles from the DeltalcrG mutant and wild-type Y. pestis strains were compared. The results showed that although the DeltalcrG mutant was markedly attenuated (600-fold increase of LD(50) in s.c. challenged BALB/c mice), transcriptions of almost all the type III genes were upregulated significantly in the DeltalcrG mutant. The immunoblotting analysis of YopM and LcrV demonstrated that their expressions were also increased in the DeltalcrG mutant in comparison to the wild-type strain. We speculate that, in addition to the negative regulation of the Yop secretion, LcrG could possibly play a negative regulatory role in the transcription of T3SS genes through indirect mechanisms. Furthermore, this report also revealed significant transcriptional changes in the genes encoding cell-envelope-related proteins and a virulence-related transcription factor RovA in the DeltalcrG mutant.

Effect of flagellar mutations on Yersinia enterocolitica biofilm formation

Yersinia enterocolitica biovar 1B is one of a number of strains pathogenic to humans in the genus Yersinia. It has three different type III secretion systems, Ysc, Ysa, and the flagella. In this study, the effect of flagella on biofilm formation was evaluated. In a panel of 31 mutant Y. enterocolitica strains, we observed that mutations that abolish the structure or rotation of the flagella greatly reduce biofilm formation when the bacteria are grown under static conditions. These results were further evaluated by assessing biofilm formation under continuous culture using a flow cell chamber. The results confirmed the important contribution of flagella to the initiation of biofilm production but indicated that there are differences in the progression of biofilm development between static growth and flow conditions. Our results suggest that flagella play a critical role in biofilm formation in Y. enterocolitica.

Investigation of virulence genes in clinical isolates of Yersinia enterocolitica

In this study, we aimed to investigate the distribution of virulence genes in clinical isolates of pathogenic Yersinia enterocolitica. Two thousand six hundred stool samples were collected from 2600 patients with diarrhea, and were tested using the culture method and real-time PCR. Then, all isolates of pathogenic Y. enterocolitica cultured from the culture method were examined for virulence genes (inv, ail, ystA, ystB, ystC, yadA, virF) by PCR and for the presence of plasmid by four phenotypic tests. As a result, 160 pathogenic strains were successfully detected by the culture method, including bio/serotype 1A/unknown (4), 1B/unknown (8), 2/O:9 (39), 2/unknown (7), 3/O:3 (22), 3/unknown (6), 4/O:3 (55), 4/unknown (10) and 5/unknown (9). The positive rate of virulence genes tested in 160 isolates was inv (100%), ail (94%), ystA (93%), ystB (7.5%), ystC (5%), yadA (89%) and virF (82%) while the phenotypic test included autoagglutination (87%), binding of crystal violet (89%), calcium-dependent growth (74%) and Congo red absorption (78%), respectively. Finally, we found that not all pathogenic Y. enterocolitica necessarily carry all traditional virulence genes in both chromosomes and plasmids to cause illness. Perhaps, some of them, lacking some traditional virulence genes, contain other unknown virulence markers that interact with each other and play an important role in the diverse pathogenesis of pathogenic Y. enterocolitica.

Pleiotropic phenotypes of a Yersinia enterocolitica flhD mutant include reduced lethality in a chicken embryo model

Background

The Yersinia enterocolitica flagellar master regulator FlhD/FlhC affects the expression levels of non-flagellar genes, including 21 genes that are involved in central metabolism. The sigma factor of the flagellar system, FliA, has a negative effect on the expression levels of seven plasmid-encoded virulence genes in addition to its positive effect on the expression levels of eight of the flagellar operons. This study investigates the phenotypes of flhD and fliA mutants that result from the complex gene regulation.

Results

Phenotypes relating to central metabolism were investigated with Phenotype MicroArrays. Compared to the wild-type strain, isogenic flhD and fliA mutants exhibited increased growth on purines and reduced growth on N-acetyl-D-glucosamine and D-mannose, when used as a sole carbon source. Both mutants grew more poorly on pyrimidines and L-histidine as sole nitrogen source. Several intermediates of the tricarboxylic acid and the urea cycle, as well as several dipeptides, provided differential growth conditions for the two mutants. Gene expression was determined for selected genes and correlated with the observed phenotypes. Phenotypes relating to virulence were determined with the chicken embryo lethality assay. The assay that was previously established for Escherichia coli strains was modified for Y. enterocolitica. The flhD mutant caused reduced chicken embryo lethality when compared to wild-type bacteria. In contrast, the fliA mutant caused wild-type lethality. This indicates that the virulence phenotype of the flhD mutant might be due to genes that are regulated by FlhD/FlhC but not FliA, such as those that encode the flagellar type III secretion system.

Conclusion

Phenotypes of flhD and fliA mutants are related to central metabolism and virulence and correlate with gene regulation.

Expression of the Yersinia enterocolitica pYV-encoded type III secretion system is modulated by lipopolysaccharide O-antigen status

We show that the expression of a Yersinia enterocolitica O:8 pYV-encoded type III secretion system was altered in a rough mutant (YeO8-R) due to elevated levels of FlhDC. H-NS might underlie flhDC upregulation in YeO8-R, and the data suggest a relationship between the absence of O antigen and the expression of H-NS.

Global gene regulation in Yersinia enterocolitica: effect of FliA on the expression levels of flagellar and plasmid-encoded virulence genes

This study describes the involvement of the sigma factor of the flagellar system, FliA, in global gene regulation of Yersinia enterocolitica. In addition to exhibiting a positive effect upon the expression levels of eight class III flagellar operons, FliA also exhibited a negative effect upon the expression levels of four virulence operons that are located on the pYV virulence plasmid. These are yadA, virC, yopQ, and the insertion element ISYen1. While the positive effect on class III flagellar operons by FliA is most likely direct, the negative effect on the virulence operons appears to require the known transcriptional activator of these genes, VirF. This was determined using microarray analysis, quantitative PCR and a search for putative binding sites for FliA. In addition to the FliA regulation of flagellar and plasmid-encoded virulence genes, we studied temperature regulation of these genes. While wild-type cells exhibited increased expression levels of flagellar genes and decreased expression levels of plasmid-encoded virulence genes at 25 degrees C (as compared to 37 degrees C), temperature dependence of gene expression was much reduced in the fliA mutants. We conclude that FliA contributes to the inverse temperature regulation of flagellar and plasmid-encoded virulence genes. We present a network of transcriptional regulation around FlhD/FlhC and FliA.

Molecular identification of Yersinia enterocolitica isolated from pasteurized whole milk using DNA microarray chip hybridization

A DNA microarray chip of four virulence genes and 16S ribosomal DNA gene conserved region among all Gram negative species, including Yersinia, as a positive control was developed and evaluated using 22 Yersinia enterocolitica isolates. Eight different oligonucleotide probes (oligoprobes) with an average size of 22 bp, complementary to the unique sequences of each gene, were designed and immobilized on the surface of chemically modified slides. Multiplex PCR was used to simultaneously amplify DNA target regions of all five genes, and single stranded DNA (ssDNA) samples for microarray analysis were prepared by using a primer extension of amplicons in the presence of one primer of all genes. The presence of genes in Y. enterocolitica was established by hybridization of the fluorescently labeled ssDNA representing different samples of the microarray gene-specific oligoprobes and confirmed by PCR. Results of the study showed specificity of genotyping Y. enterocolitica using multiple microarray-based assays. Final validation of the chip's ability to identify Y. enterocolitica genes from adulterated pasteurized whole milk was confirmed and successful. The limit of chip detection of virulence genes in pasteurized whole milk was found to be 1000 CFU per hybridization.

Improved system for construction and analysis of single-copy beta-galactosidase operon fusions in Yersinia enterocolitica

We report a significantly improved system for studying single-copy lacZ operon fusions in Yersinia enterocolitica: a simple procedure for the stable integration of lacZ operon fusions into the ara locus and a strain with a deletion mutation that abolishes the low level of endogenous beta-galactosidase activity.

Systematic targeted mutagenesis of Brucella melitensis 16M reveals a major role for GntR regulators in the control of virulence

In order to identify transcriptional regulators involved in virulence gene control in Brucella melitensis, we generated a collection of 88 mutants in the AraC, ArsR, Crp, DeoR, GntR, IclR, LysR, MerR, RpiR, and TetR families of regulators. This collection was named LiMuR (library of mutants for regulators). We developed a method to test several mutants simultaneously in one animal in order to identify those unable to survive. This method, called the plasmid-tagged mutagenesis method, was used to test the residual virulence of mutants after 1 week in a mouse model of infection. Ten attenuated mutants, of which six and three belong to the GntR and LysR families, respectively, were identified and individually confirmed to replicate at lower rates in mice. Among these 10 mutants, only gntR10 and arsR6 are attenuated in cellular models. The LiMuR also allows simple screenings to identify regulators of a particular gene or operon. As a first example, we analyzed the expression of the virB operon in the LiMuR mutants. We carried out Western blottings of whole-cell extracts to analyze the production of VirB proteins using polyclonal antisera against VirB proteins. Four mutants produced small amounts of VirB proteins, and one mutant overexpressed VirB proteins compared to the wild-type strain. In these five mutants, reporter analysis using the virB promoter fused to lacZ showed that three mutants control virB at the transcriptional level. The LiMuR is a resource that will provide straightforward identification of regulators involved in the control of genes of interest.

Distinct mechanisms of integrin binding by Yersinia pseudotuberculosis adhesins determine the phagocytic response of host macrophages

The enteropathogenic yersiniae express two outer membrane adhesins, invasin and YadA, that contribute to pathogenesis. While invasin binds directly to beta1 integrin receptors with high affinity, YadA binds indirectly through extracellular matrix (ECM) components. In this study, Yersinia pseudotuberculosis inv and yadA mutants were used to investigate how these distinct binding mechanisms compare and potentially compete in activating signalling pathways and promoting bacterial uptake by host macrophages. The efficiency of adhesin-mediated phagocytic responses was found to be dependent on the relative expression of invasin and YadA on the bacterial surface as well as the expression of ECM proteins in the extracellular milieu. Under conditions of low concentrations of ECM, invasin was found to be the dominant adhesin, promoting high levels of phagocytosis coincident with robust and sustained activation of the protein tyrosine kinases Fak and Pyk2, phosphorylation of the adaptor molecule Cas and activation of the small GTPase Rac1. In the presence of higher concentrations of ECM, YadA became the dominant functional adhesin through its ability to engage integrin receptors via an ECM bridge. We propose a model whereby invasin promotes robust and prolonged activation of phagocytic signalling cascades by inducing a 'high-affinity' integrin conformation as well as integrin clustering. We postulate that YadA-ECM promotes phagocytosis through a more transient activation of signalling cascades that arises from integrin clustering in the context of a cross-linked fibrillar ECM network.