Activation of the RcsC/YojN/RcsB phosphorelay system attenuates Salmonella virulence

The TviA auxiliary protein renders the Salmonella enterica serotype Typhi RcsB regulon responsive to changes in osmolarity

In response to osmolarity, Salmonella enterica serotype Typhi (S. Typhi) regulates genes required for Vi capsular antigen expression oppositely to those required for motility and invasion. Previous studies suggest that osmoregulation of motility, invasion and capsule expression is mediated through the RcsC/RcsD/RcsB phosphorelay system. Here we performed gene expression profiling and functional studies to determine the role of TviA, an auxiliary protein of the RcsB response regulator, in controlling virulence gene expression in S. Typhi. TviA repressed expression of genes encoding flagella and the invasion-associated type III secretion system (T3SS-1) through repression of the flagellar regulators flhDC and fliZ, resulting in reduced invasion, reduced motility and reduced expression of FliC. Both RcsB and TviA repressed expression of flhDC, but only TviA altered flhDC expression in response to osmolarity. Introduction of tviA into S. enterica serotype Typhimurium rendered flhDC transcription sensitive to changes in osmolarity. These data suggest that the auxiliary TviA protein integrates a new regulatory input into the RcsB regulon of S. Typhi, thereby altering expression of genes encoding flagella, the Vi antigen and T3SS-1 in response to osmolarity.

Envelope Stress Responses

The gram-negative bacterial envelope is a complex extracytoplasmic compartment responsible for numerous cellular processes. Among its most important functions is its service as the protective layer separating the cytoplasmic space from the ever-changing external environment. To adapt to the diverse conditions encountered both in the environment and within the mammalian host, Escherichia coli and Salmonella species have evolved six independent envelope stress response systems . This review reviews the sE response, the CpxAR and BaeSR two-component systems (TCS) , the phage shock protein response, and the Rcs phosphorelay system. These five signal transduction pathways represent the most studied of the six known stress responses. The signal for adhesion to abiotic surfaces enters the pathway through the novel outer membrane lipoprotein NlpE, and activation on entry into the exponential phase of growth occurs independently of CpxA . Adhesion could disrupt NlpE causing unfolding of its unstable N-terminal domain, leading to activation of the Cpx response. The most recent class of genes added to the Cpx regulon includes those involved in copper homeostasis. Two separate microarray experiments revealed that exposure of E. coli cells to high levels of external copper leads to upregulation of several Cpx regulon members. The BaeSR TCS has also been shown to mediate drug resistance in Salmonella. Similar to E. coli, the Bae pathway of Salmonella enterica mediates resistance to oxacillin, novobiocin, deoxycholate, β-lactams, and indole.

Understanding the regulation of Group B Streptococcal virulence factors

Bacterial infections remain a significant threat to the health of newborns and adults. Group B Streptococci (GBS) are Gram-positive bacteria that are common asymptomatic colonizers of healthy adults. However, this opportunistic organism can also subvert suboptimal host defenses to cause severe invasive disease and tissue damage. The increasing emergence of antibiotic-resistant GBS raises more concerns for sustained measures in treatment of the disease. A number of factors that are important for virulence of GBS have been identified. This review summarizes the functions of some well-characterized virulence factors, with an emphasis on how GBS regulates their expression. Regulatory and signaling molecules are attractive drug targets in the treatment of bacterial infections. Consequently, understanding signaling responses of GBS is essential for elucidation of pathogenesis of GBS infection and for the identification of novel therapeutic agents.

The Rcs phosphorelay system is essential for pathogenicity in Erwinia amylovora

The Rcs phosphorelay system is a modified two-component signal transduction system found exclusively in Enterobacteriaceae. In this study, we characterized the roles of the Rcs system in Erwinia amylovora, a highly virulent and necrogenic enterobacterium causing fire blight disease on rosaceous plants. Our results showed that rcsB, rcsC, rcsD and rcsBD mutants were non-pathogenic on immature pear fruit. The bacterial growth of these mutants was also greatly reduced compared with that of the wild-type strain in immature pear fruit. In an in vitro amylovoran assay, rcsB and rcsD mutants were deficient in amylovoran production, whereas the rcsC mutant exhibited higher amylovoran production than that of the wild-type. Consistent with amylovoran production, expression of the amylovoran biosynthetic gene amsG, using green fluorescent protein as a reporter, was not detectable in rcsB, rcsD and rcsBD mutants both in vitro and in vivo. The expression of amsG in vitro was higher in the rcsC mutant than in the wild-type, whereas its expression in vivo was higher in the wild-type than in the rcsC mutant. In addition, rcs mutants were more susceptible to polymyxin B treatment than the wild-type, suggesting that the Rcs system conferred some level of resistance to polymyxin B. Furthermore, rcs mutants showed irregular and slightly reduced motility on swarming plates. Together, these results indicate that the Rcs system plays a major role in virulence and survival of E. amylovora in immature pear fruit.

Regulation of igaA and the Rcs system by the MviA response regulator in Salmonella enterica

IgaA is a membrane protein that prevents overactivation of the Rcs regulatory system in enteric bacteria. Here we provide evidence that igaA is the first gene in a sigma(70)-dependent operon of Salmonella enterica serovar Typhimurium that also includes yrfG, yrfH, and yrfI. We also show that the Lon protease and the MviA response regulator participate in regulation of the igaA operon. Our results indicate that MviA regulates igaA transcription in an RpoS-dependent manner, but the results also suggest that MviA may regulate RcsB activation in an RpoS- and IgaA-independent manner.

Threonine phosphorylation prevents promoter DNA binding of the Group B Streptococcus response regulator CovR

All living organisms communicate with the external environment for their survival and existence. In prokaryotes, communication is achieved by two-component systems (TCS) comprising histidine kinases and response regulators. In eukaryotes, signalling is accomplished by serine/threonine and tyrosine kinases. Although TCS and serine/threonine kinases coexist in prokaryotes, direct cross-talk between these families was first described in Group B Streptococcus (GBS). A serine/threonine kinase (Stk1) and a TCS (CovR/CovS) co-regulate toxin expression in GBS. Typically, promoter binding of regulators like CovR is controlled by phosphorylation of the conserved active site aspartate (D53). In this study, we show that Stk1 phosphorylates CovR at threonine 65. The functional consequence of threonine phosphorylation of CovR in GBS was evaluated using phosphomimetic and silencing substitutions. GBS encoding the phosphomimetic T65E allele are deficient for CovR regulation unlike strains encoding the non-phosphorylated T65A allele. Further, compared with wild-type or T65A CovR, the T65E CovR is unable to bind promoter DNA and is decreased for phosphorylation at D53, similar to Stk1-phosphorylated CovR. Collectively, we provide evidence for a novel mechanism of response regulator control that enables GBS (and possibly other prokaryotes) to fine-tune gene expression for environmental adaptation.

Genome expression analyses revealing the modulation of the Salmonella Rcs regulon by the attenuator IgaA

Intracellular growth attenuator A (IgaA) was identified as a Salmonella enterica regulator limiting bacterial growth inside fibroblasts. Genetic evidence further linked IgaA to repression of the RcsCDB regulatory system, which responds to envelope stress. How IgaA attenuates this system is unknown. Here, we present genome expression profiling data of S. enterica serovar Typhimurium igaA mutants grown at high osmolarity and displaying exacerbated Rcs responses. Transcriptome data revealed that IgaA attenuates gene expression changes requiring phosphorylated RcsB (RcsB~P) activity. Some RcsB-regulated genes, yciGFE and STM1862 (pagO)-STM1863-STM1864, were equally expressed in wild-type and igaA strains, suggesting a maximal expression at low levels of RcsB ~P. Other genes, such as metB, ypeC, ygaC, glnK, glnP, napA, glpA, and nirB, were shown for the first time and by independent methods to be regulated by the RcsCDB system. Interestingly, IgaA-deficient strains with reduced RcsC or RcsD levels exhibited different Rcs responses and distinct virulence properties. spv virulence genes were differentially expressed in most of the analyzed strains. spvA expression required RcsB and IgaA but, unexpectedly, was also impaired upon stimulation of the RcsC-->RcsD-->RcsB phosphorelay. Overproduction of either RcsB(+) or a nonphosphorylatable RcsB(D56Q) variant in strains displaying low spvA expression unveiled that both dephosphorylated RcsB and RcsB~P are required for optimal spvA expression. Taken together, our data support a model with IgaA attenuating the RcsCDB system by favoring the switch of RcsB~P to the dephosphorylated state. This role of IgaA in constantly fine-tuning the RcsB~P/RcsB ratio may ensure the proper expression of important virulence factors, such as the Spv proteins.

Instability of the Salmonella RcsCDB signalling system in the absence of the attenuator IgaA

IgaA is a Salmonella enterica membrane protein that attenuates the response of the RcsCDB signalling system to envelope stress. This protein is essential unless the RcsCDB system is inactivated, suggesting that IgaA may constantly adjust the magnitude of the response. Such a functional link is also supported by the concurrence of the igaA and rcsD-rcsB-rcsC loci in genomes of enteric bacteria and the selection of spontaneous mutations in the RcsCDB system following IgaA deprivation. However, the exact nature of the spontaneous mutations rendering IgaA dispensable remains undefined. In this work, we examined how the transduction of an igaA null allele affects the status of the RcsCDB system. Loss of RcsCDB response was registered in approximately 90 % of the IgaA-defective clones, which failed to produce the capsule material positively regulated by this system. About half of these non-mucoid clones suppressed the loss of IgaA with large deletions encompassing variable regions of the rcsD-rcsB-rcsC locus. Unexpectedly, mucoid transductants were also reproducibly obtained and indicated the capacity of S. enterica to retain a functional RcsCDB system in the absence of IgaA. Decreased levels of either RcsC or RcsD were shown in 'mucoid' clones lacking IgaA and displaying low responsiveness to stimuli. Taken together, these data demonstrate that the stability and responsiveness of the RcsCDB system relies on its attenuator IgaA. The type of suppressions found also support a model with IgaA controlling the level of signal flowing through RcsC and RcsD.

The Salmonella PmrAB regulon: lipopolysaccharide modifications, antimicrobial peptide resistance and more

Microbes are able to sense and respond to their environment primarily through the use of two-component regulatory systems. Many of these systems activate virulence-factor expression and are regulated by host-derived signals, having evolved to control gene expression at the key time and place for optimal establishment and maintenance of infection. Salmonella spp. are enteric pathogens that are able to survive both within host macrophages during systemic spread and killing by innate immune factors at intestinal mucosal surfaces. This review focuses on a key mechanism of pathogenesis that involves the PmrA-PmrB two-component system, which is activated in vivo by direct or indirect means and regulates genes that modify lipopolysaccharide, aiding survival in host (and non-host) environments.

The Response regulator HrpY of Dickeya dadantii 3937 regulates virulence genes not linked to the hrp cluster

HrpX/Y is a putative two-component system (TCS) encoded within the type III secretion system (T3SS) gene cluster of Dickeya dadantii. A linear regulatory cascade initiated by HrpX/Y that leads to activation of the downstream T3SS genes via HrpS and HrpL was described previously. Therefore, in D. dadantii, HrpX/Y plays an important role in regulation of genes involved in bacteria-plant interactions and bacterial aggregation via the T3SS. HrpX/Y is the only TCS shared among the plant-pathogenic enterobacteria that is not also present in animal-associated enterobacteria. To date, the genes known to be regulated by HrpY are restricted to the hrp and hrc genes and no signal has been identified that triggers HrpY-dependent gene expression. We demonstrated that HrpY interacts with the hrpS promoter in vitro. We then used a transposon-based system to isolate previously unidentified HrpY-dependent genes, including genes previously shown to affect virulence, including kdgM and acsC. HrpY is a dual regulator, positively regulating at least 10 genes in addition to those in the hrp gene cluster and negatively regulating at least 5 genes. The regulatory effect on one gene depended on the culture medium used. Of the 16 HrpY-regulated genes identified in this screen, 14 are not present in Pectobacterium atrosepticum, the nearest relative of D. dadantii with a sequenced genome. None of the newly identified HrpY-regulated genes were required for bacterial aggregation; thus, neither acyl-homoserine lactone-mediated quorum sensing nor the Rcs signal transduction system which regulates colanic acid, a molecule that plays an important role in biofilm formation in other enterobacteria, are required for D. dadantii aggregation.

Comparative analysis of the Photorhabdus luminescens and the Yersinia enterocolitica genomes: uncovering candidate genes involved in insect pathogenicity

Background

Photorhabdus luminescens and Yersinia enterocolitica are both enteric bacteria which are associated with insects. P. luminescens lives in symbiosis with soil nematodes and is highly pathogenic towards insects but not to humans. In contrast, Y. enterocolitica is widely found in the environment and mainly known to cause gastroenteritis in men, but has only recently been shown to be also toxic for insects. It is expected that both pathogens share an overlap of genetic determinants that play a role within the insect host.

Results

A selective genome comparison was applied. Proteins belonging to the class of two-component regulatory systems, quorum sensing, universal stress proteins, and c-di-GMP signalling have been analysed. The interorganismic synopsis of selected regulatory systems uncovered common and distinct signalling mechanisms of both pathogens used for perception of signals within the insect host. Particularly, a new class of LuxR-like regulators was identified, which might be involved in detecting insect-specific molecules. In addition, the genetic overlap unravelled a two-component system that is unique for the genera Photorhabdus and Yersinia and is therefore suggested to play a major role in the pathogen-insect relationship. Our analysis also highlights factors of both pathogens that are expressed at low temperatures as encountered in insects in contrast to higher (body) temperature, providing evidence that temperature is a yet under-investigated environmental signal for bacterial adaptation to various hosts. Common degradative metabolic pathways are described that might be used to explore nutrients within the insect gut or hemolymph, thus enabling the proliferation of P. luminescens and Y. enterocolitica in their invertebrate hosts. A strikingly higher number of genes encoding insecticidal toxins and other virulence factors in P. luminescens compared to Y. enterocolitica correlates with the higher virulence of P. luminescens towards insects, and suggests a putative broader insect host spectrum of this pathogen.

Conclusion

A set of factors shared by the two pathogens was identified including those that are involved in the host infection process, in persistence within the insect, or in host exploitation. Some of them might have been selected during the association with insects and then adapted to pathogenesis in mammalian hosts.

The Salmonella SPI1 type three secretion system responds to periplasmic disulfide bond status via the flagellar apparatus and the RcsCDB system

Upon contact with intestinal epithelial cells, Salmonella enterica serovar Typhimurium injects a set of effector proteins into the host cell cytoplasm via the Salmonella pathogenicity island 1 (SPI1) type III secretion system (T3SS) to induce inflammatory diarrhea and bacterial uptake. The master SPI1 regulatory gene hilA is controlled directly by three AraC-like regulators: HilD, HilC, and RtsA. Previous work suggested a role for DsbA, a periplasmic disulfide bond oxidase, in SPI1 T3SS function. RtsA directly activates dsbA, and deletion of dsbA leads to loss of SPI1-dependent secretion. We have studied the dsbA phenotypes by monitoring expression of SPI1 regulatory, structural, and effector genes. Here we present evidence that loss of DsbA independently affects SPI1 regulation and SPI1 function. The dsbA-mediated feedback inhibition of SPI1 transcription is not due to defects in the SPI1 T3SS apparatus. Rather, the transcriptional response is dependent on both the flagellar protein FliZ and the RcsCDB system, which also affects fliZ transcription. Thus, the status of disulfide bonds in the periplasm affects expression of the SPI1 system indirectly via the flagellar apparatus. RcsCDB can also affect SPI1 independently of FliZ. All regulation is through HilD, consistent with our current model for SPI1 regulation.

The RcsCDB signaling system and swarming motility in Salmonella enterica serovar typhimurium: dual regulation of flagellar and SPI-2 virulence genes

The Rcs phosphorelay is a multicomponent signaling system that positively regulates colanic acid synthesis and negatively regulates motility and virulence. We have exploited a spontaneously isolated mutant, IgaA(T191P), that is nearly maximally activated for the Rcs system to identify a vast set of genes that respond to the stimulation, and we report new regulatory properties of this signaling system in Salmonella enterica serovar Typhimurium. Microarray data show that the Rcs system normally functions as a positive regulator of SPI-2 and other genes important for the growth of Salmonella in macrophages, although when highly activated the system completely represses the SPI-1/SPI-2 virulence, flagellar, and fimbrial biogenesis pathways. The auxiliary protein RcsA, which works with RcsB to positively regulate colanic acid and other target genes, not only stimulates but also antagonizes the positive regulation of many genes in the igaA mutant. We show that RcsB represses motility through the RcsB box in the promoter region of the master operon flhDC and that RcsA is not required for this regulation. Curiously, RcsB selectively stimulates expression of the flagellar type 3 secretion genes fliPQR; an RcsAB box located downstream of fliR influences this regulation. We show that excess colanic acid impairs swimming and inhibits swarming motility, consistent with the inverse regulation of the two pathways by the Rcs system.

Phenotypic analysis of Yersinia pseudotuberculosis 32777 response regulator mutants: new insights into two-component system regulon plasticity in bacteria

Two-component regulatory systems (2CSs) typically comprise a sensor kinase and a response regulator that, in concert, monitor the concentration of particular extracellular factors and mediate the transcription of specific genes accordingly. As such, 2CSs play an important role in the regulation of bacterial pathogenesis. On the basis of genome-wide in silico analysis, the Gram-negative enteropathogenic bacterium Yersinia pseudotuberculosis is thought to encode 24 complete 2CSs. In the present work, we mutated the corresponding 2CS response regulator-encoding genes in Y. pseudotuberculosis strain 32777 and assessed the in vitro resistance of each mutant to the various types of stress encountered by Yersinia cells in the digestive tract. Eight of the generated regulatory mutants (phoP, ompR, pmrA, ntrC-, arcA-, rstA-, rcsB-, and yfhA-like mutants) showed significant changes in tolerance towards at least one type of stress, when compared with the wild-type strain. Of these eight, four (ompR, phoP, rstA-, and yfhA-like mutants) were found to be less virulent than the wild type in the BALB/c mouse model. Although some mutant phenotypes were consistent with those (when known) of the corresponding, putative ortholog mutants in other pathogenic species, several response regulators behaved differently in Y. pseudotuberculosis; these included the PmrA, PhoP, and ArcA-like response regulators, which were found to control bile salt resistance in a manner different from that observed in Salmonella. Hence, in addition to genome evolution, transcriptional network remodeling may be a major cause of phenotypic adaptation (and thus species divergence) in Y. pseudotuberculosis.

The Rcs phosphorelay modulates the expression of plant cell wall degrading enzymes and virulence inPectobacterium carotovorumssp.carotovorum

Production of plant cell wall degrading enzymes, the major virulence factors of soft-rot Pectobacterium species, is controlled by many regulatory factors. Pectobacterium carotovorum ssp. carotovorum SCC3193 encodes an Rcs phosphorelay system that involves two sensor kinases, RcsC(Pcc) and RcsD(Pcc), and a response regulator RcsB(Pcc) as key components of this system, and an additional small lipoprotein RcsF(Pcc). This study indicates that inactivation of rcsC(Pcc), rcsD(Pcc) and rcsB(Pcc) enhances production of virulence factors with the highest effect detected for rcsB(Pcc). Interestingly, mutation of rcsF(Pcc) has no effect on virulence factors synthesis. These results suggest that in SCC3193 a parallel phosphorylation mechanism may activate the RcsB(Pcc) response regulator, which acts as a repressor suppressing the plant cell wall degrading enzyme production. Enhanced production of virulence factors in Rcs mutants is more pronounced when bacteria are growing in the absence of plant signal components.

Rcs and PhoPQ regulatory overlap in the control of Salmonella enterica virulence

Genetic screens based on the use of MudJ-generated lac fusions permitted the identification of novel genes regulated by the Rcs signal transduction system in Salmonella enterica serovar Typhimurium. Besides genes that are also found in the Escherichia coli genome, our screens identified Salmonella-specific genes regulated by RcsB, including bapA, siiE, srfA, and srfB. Here we show that the srfABC operon is negatively regulated by RcsB and by PhoP. In vivo studies using mutants with constitutive activation of the Rcs and/or PhoPQ system suggested that there is an overlap between these regulatory systems in the control of Salmonella virulence.

The glutamate-dependent acid resistance system in Escherichia coli: essential and dual role of the His-Asp phosphorelay RcsCDB/AF

The RcsCDB signal transduction system is an atypical His-Asp phosphorelay. Notably, the response regulator RcsB can be activated either by phosphorylation through the RcsCD pathway or by an accessory cofactor RcsA. Although conserved in Enterobacteriaceae, the role of this system in adaptation to environmental stress conditions is largely unknown. This study reveals that the response regulator RcsB is essential to glutamate-dependent acid resistance, a condition pertinent to the lifestyle of Escherichia coli. The requirement for RcsB is independent of its activation by either the RcsCD or the RcsA pathway. The basal activity of RcsB appears to be necessary and sufficient for acid resistance. The sensitivity of the rcsB strain to low pH is correlated to a strong reduction of the expression of the glutamate decarboxylase genes, gadA and gadB, during the stationary phase of growth. This effect on gadA/B expression is not mediated by the general stress sigma factor RpoS, but does require a functional gadE allele and the previously identified GadE box. Therefore activation of gadAB expression and acid resistance absolutely requires both GadE and RcsB. In contrast, an increase in RcsB activity through the activation of the RcsCD phosphorelay or the RcsA pathway or through overproduction of the protein leads to general repression of the expression of the gad genes and a corresponding reduction in acid resistance.

A positive feedback loop promotes transcription surge that jump-starts Salmonella virulence circuit

The PhoP/PhoQ two-component system is a master regulator of Salmonella pathogenicity. Here we report that induction of the PhoP/PhoQ system results in an initial surge of PhoP phosphorylation; the occupancy of target promoters by the PhoP protein; and the transcription of PhoP-activated genes, which then subsides to reach new steady-state levels. This surge in PhoP activity is due to PhoP positively activating its own transcription, because a strain constitutively expressing the PhoP protein attained steady-state levels of activation asymptotically, without the surge. The strain constitutively expressing the PhoP protein was attenuated for virulence in mice, demonstrating that the surge conferred by PhoP's positive feedback loop is necessary to jump-start Salmonella's virulence program.

The Rcs phosphorelay system is specific to enteric pathogens/commensals and activates ydeI, a gene important for persistent Salmonella infection of mice

Bacteria utilize phosphorelay systems to respond to environmental or intracellular stimuli. Salmonella enterica encodes a four-step phosphorelay system that involves two sensor kinase proteins, RcsC and RcsD, and a response regulator, RcsB. The physiological stimulus for Rcs phosphorelay activation is unknown; however, Rcs-regulated genes can be induced in vitro by osmotic shock, low temperature and antimicrobial peptide exposure. In this report we investigate the role of the Rcs pathway using phylogenetic analysis and experimental techniques. Phylogenetic analysis determined that full-length RcsC- and RcsD-like proteins are generally restricted to Enterobacteriaceae species that have an enteric pathogenic or commensal relationship with the host. Experimental data show that RcsD and RcsB, in addition to RcsC, are important for systemic infection in mice and polymyxin B resistance in vitro. To identify Rcs-regulated genes that confer these phenotypes, we took advantage of our observation that RcsA, a transcription factor and binding partner of RcsB, is not required for polymyxin B resistance or survival in mice. S. enterica serovar Typhimurium oligonucleotide microarrays were used to identify 18 loci that are activated by RcsC, RcsD and RcsB but not RcsA. Five of the 18 loci encode genes that contribute to polymyxin B resistance. One of these genes, ydeI, was shown by quantitative real-time PCR to be regulated by the Rcs pathway independently of RcsA. Additionally, the stationary-phase sigma factor, RpoS (sigmaS), regulates ydeI transcription. In vivo infections show that ydeI mutants are out-competed by wild type 10- to 100-fold after oral inoculation, but are only modestly attenuated after intraperitoneal inoculation. These data indicate that ydeI is an Rcs-activated gene that plays an important role in persistent infection of mice, possibly by increasing bacterial resistance to antimicrobial peptides.

Sensing by bacterial regulatory systems in host and non-host environments

Free-living organisms have the ability to gauge their surroundings and modify their gene expression patterns in ways that help them cope with new environments. Here we discuss the physiological significance of recent reports describing the ability of the Salmonella typhimurium PhoP/PhoQ two-component system to recognize and respond to host-derived antimicrobial peptides.

The role of the Rcs phosphorelay in Enterobacteriaceae

The Rcs phosphorelay is composed of the sensor kinase, RcsC, the HPt-domain protein RcsD and the response regulator, RcsB. In this review we discuss the role of the Rcs phosphorelay in the Enterobacteriaceae, highlighting the observation that the Rcs phosphorelay appears to play a key role in the temporal regulation of biofilm formation and pathogenicity.

Regulation of bacterial virulence by two-component systems

In bacteria, two-component systems (TCS) are widely used signal transduction devices which are engaged in a multitude of gene regulatory systems that respond to changing growth conditions. Many pathogenic bacteria encounter different microenvironments during their infectious cycle and their ability to efficiently adapt to different niches inside and outside of their host organisms is frequently mediated by TCSs, which can, therefore, be considered as an essential prerequisite for their pathogenicity. Although significant progress has been made in the elucidation of basic principles of the signal transduction process itself, in many pathogens the contribution of TCS to bacterial virulence is insufficiently recognized.

New concepts in Salmonella virulence: the importance of reducing the intracellular growth rate in the host

The literature refers to Salmonella enterica as an intracellular bacterial pathogen that proliferates within vacuoles of mammalian cells. However, recent in vivo studies have revealed that the vast majority of infected cells contain very few intracellular bacteria (three to four organisms). Salmonella intracellular growth is also limited in cultured dendritic cells and fibroblasts, two cell types abundant in tissues located underneath the intestinal epithelium. Recently, a Salmonella factor previously known for its role as a negative regulator of intracellular growth has been shown to tightly repress certain pathogen functions upon host colonization and to be critical for virulence. The connection between virulence and the negative control of intracellular growth is further sustained by the fact that some attenuated mutants overgrow in non-phagocytic cells located in the intestinal lamina propria. These findings are changing our classical view of Salmonella as a fast growing intracellular pathogen and suggest that this pathogen may trigger responses directed to reduce the growth rate within the infected cell. These responses could play a critical role in modulating the delicate balance between disease and persistence.

Environmental regulation and virulence attributes of the Ysa type III secretion system of Yersinia enterocolitica biovar 1B

Pathogenic biovars of Yersinia enterocolitica maintain the well-studied plasmid-encoded Ysc type III secretion (TTS) system, which has a definitive role in virulence. Y. enterocolitica biovar 1B additionally has a distinct chromosomal locus, the Yersinia secretion apparatus pathogenicity island (YSA PI) that encodes the Ysa TTS system. The signals to which the Ysa TTS system responds and its role in virulence remain obscure. This exploratory study was conducted to define environmental cues that promote the expression of Ysa TTS genes and to define how the Ysa TTS system influences bacterium-host interactions. Using a genetic approach, a collection of Y. enterocolitica Ysa TTS mutants was generated by mutagenesis with a transposon carrying promoterless lacZYA. This approach identified genes both within and outside of the YSA PI that contribute to Ysa TTS. Expression of these genes was regulated in response to growth phase, temperature, NaCl, and pH. Additional genetic analysis demonstrated that two regulatory genes encoding components of the YsrR-YsrS (ysrS) and RcsC-YojN-RcsB (rcsB) phosphorelay systems affect the expression of YSA PI genes and each other. The collection of Ysa TTS-defective transposon mutants, along with other strains carrying defined mutations that block Ysa and Ysc TTS, was examined for changes in virulence properties by using the BALB/c mouse model of infection. This analysis revealed that the Ysa TTS system impacts the ability of Y. enterocolitica to colonize gastrointestinal tissues. These results reveal facets of how Y. enterocolitica controls the function of the Ysa TTS system and uncovers a role for the Ysa TTS during the gastrointestinal phase of infection.

The origin and evolution of human pathogens

What are the genetic origins of human pathogens? An international group of scientists discussed this topic at a workshop that took place in late October 2004 in Baeza (Spain). Focusing primarily on bacterial pathogens, they examined the role that pathogenicity islands and bacteriophages play on determining the virulence properties that distinguish closely related members of a given species, such as host range and tissue specificity. They also discussed an instance in which closely related bacterial species differ in the production of a cell surface modification mediating resistance to an antibiotic as a result of the disparate regulation of homologous genes. In certain pathogens, genes normally carrying out housekeeping functions may adopt new functions, whereas in other organisms, genes that respond to stresses associated with non-host environments are silenced during infection to prevent the expression of products that interfere with the normal colonization process. The adaptive behaviour of certain pathogens relies on gene variation at certain loci that by virtue of containing polymeric repeats in regulatory or coding regions, can generate variants that may or may not express products that modify the cell surface of the organism. The meeting also addressed the properties of ORFan genes, which have no homologues in the sequence databases, as well as the creation of genes de novo by duplication and divergence.

Dual regulatory pathways integrating the RcsC-RcsD-RcsB signalling system control enterohaemorrhagic Escherichia coli pathogenicity

Bacterial pathogenesis is strictly regulated in response to changes in environmental conditions. A His-Asp phosphorelay system consisting of a sensor kinase and response regulator is used by Gram-positive and Gram-negative bacteria to control gene expression in response to environmental stimuli. We screened His-Asp phosphorelay systems for their effect on virulence expression in enterohaemorrhagic Escherichia coli (EHEC), and found rcsD or rcsB overexpression enhanced locus for enterocyte effacement (LEE) gene transcription and adherence to Caco-2 cells through transcriptional activation of the ler regulatory gene. An EHEC-specific regulator GrvA, encoded by ECs1274, was required for ler transcription activation by RcsB. Furthermore, GrvA activated ler transcription in E. coli K12. Stimulation of the RcsDCB regulatory system by RcsF overexpression slightly increased EspB expression in the wild type but not the ECs1274 mutant. However, EspB expression in an rcsB deletion mutant increased compared with wild type, suggesting that RcsB negatively regulates LEE gene expression and that active RcsB protein is present under normal growth conditions. Deletion of pchA, which encodes a positive regulator for ler, abolished the effect of the rcsB deletion, suggesting that pchA mediated the negative RcsB effect. pchA transcript levels decreased when RcsB expression increased. Thus, LEE gene transcription may be regulated by RcsB through two oppositely regulated O157-specific regulators, PchA and GrvA.

The Salmonella membrane protein IgaA modulates the activity of the RcsC-YojN-RcsB and PhoP-PhoQ regulons

The Salmonella enterica serovar Typhimurium membrane protein IgaA and the PhoP-PhoQ two-component system are used by this pathogen to attenuate the intracellular growth rate within fibroblasts. IgaA has also recently been shown to contribute to virulence by exerting tight repression of the RcsC-YojN-RcsB phosphorelay in host tissues. Here we show that loss of repression of the RcsC-YojN-RcsB system, linked to an R188H mutation in the IgaA protein (igaA1 allele), is accompanied by altered expression of PhoP-PhoQ-activated (pag) genes. The changes in gene expression were different depending on the specific pag gene analyzed. Thus, transcription of ugd, which is required for lipopolysaccharide modification and colanic acid capsule synthesis, was enhanced in the igaA1 mutant. RcsB and its coregulator RcsA promoted this alteration in a PhoP-PmrA-independent manner. Unlike ugd, activation of the RcsC-YojN-RcsB phosphorelay negatively affected the expression of all other pag genes tested. In this case, RcsB alone was responsible for this effect. We also found that PhoP, but not PmrA, negatively modulates the expression of gmm, a gene required for colanic acid synthesis that is regulated positively by RcsC-YojN-RcsB. Finally, it was observed that the fine regulation of pag genes exerted by RcsB requires the RpoS protein and that an active RcsB, but not RcsA, diminishes expression of the phoP gene. These data support the hypothesis that in Salmonella there is an intimate regulatory circuit between the PhoP-PhoQ and RcsC-YojN-RcsB phosphorelays, which is revealed only when the RcsC-YojN-RcsB signaling route is derepressed. Consistent with the phenotypes observed in fibroblast cells, IgaA is predicted to favor expression of the entire PhoP-PhoQ regulon based on its repression of the RcsC-YojN-RcsB phosphorelay.