The Vibrio cholerae VarS/VarA two-component system controls the expression of virulence proteins through ToxT regulation

The activity of the quorum sensing regulator HapR is modulated by the bacterial extracellular vesicle (BEV)-associated protein ObfA of Vibrio cholerae

Vibrio cholerae, a facultative human pathogen and causative agent of the severe diarrheal disease cholera, transits between the human intestinal tract and aquatic reservoirs. Like other bacterial species, V. cholerae continuously releases bacterial extracellular vesicles (BEVs) from its surface, which have been recently characterised for their role during in vivo colonisation. However, between epidemic outbreaks, V. cholerae persists in the biofilm mode for extended periods in aquatic reservoirs, which enhances environmental fitness and host transition. In this study, we investigated the effect of V. cholerae BEVs on biofilm formation, a critical feature for ex vivo survival. In contrast to BEVs from planktonic cultures, our results show that physiological concentrations of BEVs from dynamic biofilm cultures facilitate V. cholerae biofilm formation, which could be linked to a proteinaceous factor. Comparative proteomic analyses of planktonic- and biofilm-derived BEVs identified a previously uncharacterised outer membrane protein as an abundant component of dynamic biofilm-derived BEVs, which was found to be responsible for the BEV-dependent enhancement of biofilm production. Consequently, this protein was named outer membrane-associated biofilm facilitating protein A (ObfA). Comprehensive molecular studies unravelled ObfA as a negative modulator of HapR activity. HapR is a key transcriptional regulator of the V. cholerae quorum sensing (QS) cascade acting as a potent repressor of biofilm formation and virulence. Consistently, obfA mutants not only exhibited reduced biofilm production but also reduced colonisation fitness. Surprisingly, our results demonstrate that ObfA does not affect HapR through the canonical QS system but via the Csr-cascade altering the expression of the small regulatory RNAs CsrC and CsrD. In summary, this study elucidates a novel intraspecies BEV-based communication in V. cholerae that influences biofilm formation and colonisation fitness via a new regulatory pathway involving HapR, Csr-cascade and the BEV-associated protein ObfA.

Ultrasonic cavitation induced Vibrio parahaemolyticus entering an apoptosis-like death process through SOS response

As an effective non-thermal sterilization method, ultrasound remains at the level of passive bacterial death despite the initial understanding of its sterilization mechanism. Here, we present the perspective that bacteria can choose to actively enter an apoptosis-like death state in response to external ultrasonic stress. In this study, Vibrio parahaemolyticus exhibited apoptotic markers such as phosphatidylserine ectropion and activated caspases when subjected to ultrasound stress. Additionally, the accumulation of reactive oxygen species (ROS) and enhanced calcium signaling were observed. Further transcriptomic analysis was conducted to investigate the regulatory mechanism of the SOS response in Vibrio parahaemolyticus during an apoptosis-like state. The results showed that the genes encoding the citrate cycle were down-regulated in Vibrio parahaemolyticus cells adapted to ultrasonic stress, leading to an apoptosis-like state and a decrease in production capacity and ability to catabolize carbon dioxide. Furthermore, the level of oxidized glutathione increased, suggesting that the bacteria were engaged in various anti-oxidative stress responses, ultimately leading to apoptosis. Moreover, the ultrasound field activated the regulatory factor CsrA, which facilitates stress survival as cells transition from rapid growth to an apoptotic state through a stringent response and catabolic inhibition system. Parallel reaction monitoring (PRM) revealed that the expression of certain key SOS proteins in Vibrio parahaemolyticus was up-regulated following ultrasound treatment, resulting in a gradual adaptation of the cells to external stress and ultimately leading to active cell death. In conclusion, the biological lethal effect of ultrasound treatment is not solely a mechanical cell necrosis process as traditionally viewed, but also a programmed cell death process regulated by cellular adaptation. This enriched the biological effect pathway of ultrasound sterilization.

Vibrio cholerae RND efflux systems: mediators of stress responses, colonization and pathogenesis

Resistance Nodulation Division (RND) efflux systems are ubiquitous transporters in gram-negative bacteria that provide protection against antimicrobial agents and thereby enhance survival in virtually all environments these prokaryotes inhabit. Vibrio cholerae is a dual lifestyle enteric pathogen that spends much of its existence in aquatic environments. An unwitting encounter with a human host can lead to V. cholerae intestinal colonization by strains that encode cholera toxin and toxin co-regulated pilus virulence factors leading to potentially fatal cholera diarrhea and dissemination in the environment. Adaptive response mechanisms to host factors encountered by these pathogens are therefore critical both to engage survival mechanisms such as RND-mediated transporters and to induce timely expression of virulence factors. Sensing of cues encountered in the host may therefore activate more than protective responses such as efflux systems, but also be coordinated to initiate expression of virulence factors. This review summarizes recent advances that contribute towards the understanding of RND efflux physiological functions and how the transport systems interface with the regulation of virulence factor production in V. cholerae.

Mechanisms Underlying Vibrio cholerae Biofilm Formation and Dispersion

Biofilms are a widely observed growth mode in which microbial communities are spatially structured and embedded in a polymeric extracellular matrix. Here, we focus on the model bacterium Vibrio cholerae and summarize the current understanding of biofilm formation, including initial attachment, matrix components, community dynamics, social interactions, molecular regulation, and dispersal. The regulatory network that orchestrates the decision to form and disperse from biofilms coordinates various environmental inputs. These cues are integrated by several transcription factors, regulatory RNAs, and second-messenger molecules, including bis-(3'-5')-cyclic dimeric guanosine monophosphate (c-di-GMP). Through complex mechanisms, V. cholerae weighs the energetic cost of forming biofilms against the benefits of protection and social interaction that biofilms provide. Expected final online publication date for the Annual Review of Microbiology, Volume 76 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

On or Off: Life-Changing Decisions Made by Vibrio cholerae Under Stress

Vibrio cholerae, the causative agent of the infectious disease, cholera, is commonly found in brackish waters and infects human hosts via the fecal-oral route. V. cholerae is a master of stress resistance as V. cholerae's dynamic lifestyle across different physical environments constantly exposes it to diverse stressful circumstances. Specifically, V. cholerae has dedicated genetic regulatory networks to sense different environmental cues and respond to these signals. With frequent outbreaks costing a tremendous amount of lives and increased global water temperatures providing more suitable aquatic habitats for V. cholerae, cholera pandemics remain a probable catastrophic threat to humanity. Understanding how V. cholerae copes with different environmental stresses broadens our repertoire of measures against infectious diseases and expands our general knowledge of prokaryotic stress responses. In this review, we summarize the regulatory mechanisms of how V. cholerae fights against stresses in vivo and in vitro.

Vibrio Pathogenicity Island-1: The Master Determinant of Cholera Pathogenesis

Cholera is an acute secretory diarrhoeal disease caused by the bacterium Vibrio cholerae. The key determinants of cholera pathogenicity, cholera toxin (CT), and toxin co-regulated pilus (TCP) are part of the genome of two horizontally acquired Mobile Genetic Elements (MGEs), CTXΦ, and Vibrio pathogenicity island 1 (VPI-1), respectively. Besides, V. cholerae genome harbors several others MGEs that provide antimicrobial resistance, metabolic functions, and other fitness traits. VPI-1, one of the most well characterized genomic island (GI), deserved a special attention, because (i) it encodes many of the virulence factors that facilitate development of cholera (ii) it is essential for the acquisition of CTXΦ and production of CT, and (iii) it is crucial for colonization of V. cholerae in the host intestine. Nevertheless, VPI-1 is ubiquitously present in all the epidemic V. cholerae strains. Therefore, to understand the role of MGEs in the evolution of cholera pathogen from a natural aquatic habitat, it is important to understand the VPI-1 encoded functions, their acquisition and possible mode of dissemination. In this review, we have therefore discussed our present understanding of the different functions of VPI-1 those are associated with virulence, important for toxin production and essential for the disease development.

Small RNAs as Fundamental Players in the Transference of Information During Bacterial Infectious Diseases

Communication shapes life on Earth. Transference of information has played a paramount role on the evolution of all living or extinct organisms since the appearance of life. Success or failure in this process will determine the prevalence or disappearance of a certain set of genes, the basis of Darwinian paradigm. Among different molecules used for transmission or reception of information, RNA plays a key role. For instance, the early precursors of life were information molecules based in primitive RNA forms. A growing field of research has focused on the contribution of small non-coding RNA forms due to its role on infectious diseases. These are short RNA species that carry out regulatory tasks in cis or trans. Small RNAs have shown their relevance in fine tuning the expression and activity of important regulators of essential genes for bacteria. Regulation of targets occurs through a plethora of mechanisms, including mRNA stabilization/destabilization, driving target mRNAs to degradation, or direct binding to regulatory proteins. Different studies have been conducted during the interplay of pathogenic bacteria with several hosts, including humans, animals, or plants. The sRNAs help the invader to quickly adapt to the change in environmental conditions when it enters in the host, or passes to a free state. The adaptation is achieved by direct targeting of the pathogen genes, or subversion of the host immune system. Pathogens trigger also an immune response in the host, which has been shown as well to be regulated by a wide range of sRNAs. This review focuses on the most recent host-pathogen interaction studies during bacterial infectious diseases, providing the perspective of the pathogen.

Small RNA coaR contributes to intestinal colonization in Vibrio cholerae via the two-component system EnvZ/OmpR

Vibrio cholerae is a waterborne bacterium responsible for worldwide outbreaks of acute and fatal cholera. Recently, small regulatory RNAs (sRNAs) have become increasingly recognized as important regulators of virulence gene expression in response to environmental signals. In this study, we determined that two-component system EnvZ/OmpR was required for intestinal colonization in V. cholerae O1 EI Tor strain E12382. Analysis of the characteristics of OmpR revealed a potential binding site in the intergenic region between vc1470 and vc1471, and qRT-PCR showed that expression of the intergenic region increased 5.3-fold in the small intestine compared to LB medium. Race and northern blot assays were performed and demonstrated a new sRNA, coaR (cholerae osmolarity and acidity related regulatory RNA). A ΔcoaR mutant showed a deficient colonization ability in small intestine with CI of 0.15. We identified a target of coaR, tcpI, a negative regulator of the major pilin subunit of TcpA. The ΔtcpI mutant has an increased colonization with CI of 3.16. The expression of coaR increased 2.8-fold and 3.3-fold under relative acidic and hypertonic condition. In summary, coaR was induced under the condition of high osmolarity and acid stress via EnvZ/OmpR and explained that tcpI relieves pH-mediated repression of toxin co-regulated pilus synthesis.

Shedding Light on Specificity: Population Genomic Structure of a Symbiosis Between a Coral Reef Fish and Luminous Bacterium

All organisms depend on symbiotic associations with bacteria for their success, yet how these interspecific interactions influence the population structure, ecology, and evolution of microbial symbionts is not well understood. Additionally, patterns of genetic variation in interacting species can reveal ecological traits that are important to gene flow and co-evolution. In this study, we define patterns of spatial and temporal genetic variation of a coral reef fish, Siphamia tubifer, and its luminous bacterial symbiont, Photobacterium mandapamensis in the Okinawa Islands, Japan. Using restriction site-associated sequencing (RAD-Seq) methods, we show that populations of the facultative light organ symbiont of S. tubifer exhibit genetic structure at fine spatial scales of tens of kilometers despite the absence of physical barriers to dispersal and in contrast to populations of the host fish. These results suggest that the host’s behavioral ecology and environmental interactions between host and symbiont help to structure symbiont populations in the region, consequently fostering the specificity of the association between host generations. Our approach also revealed several symbiont genes that were divergent between host populations, including hfq and a homolog of varS, both of which play a role in host association in Vibrio cholerae. Overall, this study highlights the important role that a host animal can play in structuring the distribution of its bacterial symbiont, particularly in highly connected marine environments, thereby promoting specificity of the symbiosis between host generations.

Vibrio cholerae CsrA Directly Regulates varA To Increase Expression of the Three Nonredundant Csr Small RNAs

CsrA, an RNA-binding global regulator, is an essential protein in Vibrio choleraeV. cholerae CsrA is regulated by three small RNAs (sRNAs), namely, CsrB, CsrC, and CsrD, which act to sequester and antagonize the activity of CsrA. Although the sRNAs were considered to be largely redundant, we found that they differ in expression, half-life, and the ability to regulate CsrA. Further, we identified a feedback loop in the Csr system in which CsrA increases the synthesis of these antagonistic sRNAs. Because the Csr sRNAs are positively regulated by VarA, we determined the effects of CsrA on VarA levels. The level of VarA was reduced in a csrA mutant, and we found that CsrA directly bound to varA mRNA in an electrophoretic mobility shift assay in vitro and in an CsrA-RNA immunoprecipitation assay in vivo Thus, varA mRNA is an in vivo-verified direct target of CsrA in V. cholerae, and this is the first demonstration of CsrA directly binding to a varA/uvrY/gacA homolog. Additionally, we demonstrated that a varA translational fusion was less active in a csrA mutant than in wild-type V. cholerae, suggesting that CsrA enhances varA translation. We propose that this autoregulatory feedback loop, in which CsrA increases the production of the nonredundant Csr sRNAs by regulating the amount of VarA, provides a mechanism for fine-tuning the availability of CsrA and, thus, of its downstream targets.IMPORTANCEVibrio cholerae is a major human pathogen, causing epidemics and pandemics of cholera. V. cholerae persists in the aquatic environment, providing a constant source for human infection. Success in transitioning from the environment to the human host and back requires the bacterium to rapidly respond and to adjust its gene expression and metabolism to these two very different habitats. Our findings show that CsrA, an RNA-binding regulatory protein, plays a central role in regulating these transitions. CsrA activity is controlled by the antagonistic sRNAs CsrB, CsrC, and CsrD, and these sRNAs respond to changes in the availability of nutrients. CsrA autoregulates its own activity by controlling these sRNAs via their primary regulator VarA. Thus, the change in CsrA availability in response to nutrient availability allows V. cholerae to alter gene expression in response to environmental cues.

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

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

Molecular dynamics simulation (MDS) analysis of Vibrio cholerae ToxT virulence factor complexed with docked potential inhibitors

The ToxT transcription factor mediates the transcription of the two major virulence factors in Vibrio cholerae. It has a DNA binding domain made of α4, α5, α6, α7, α8, α9 and α10 helices that is responsible for the transcription of virulence genes. Therefore, it is of interest to screen ToxT against the ZINC ligand database containing data for a million compounds. The QSAR model identified 40 top hits for ToxT. Two target protein complexes with ligands Lig N1 and Lig N2 with high score were selected for molecular dynamics simulation. Simulation data shows that ligands are stable in the DNA binding domain of ToxT. Moreover, Lig N1 and Lig N2 passed pharmacological as well as ADME filters along with g-mmpbsa analysis with binding affinity of -199.831 kJ/mol for Lig N1 and - 286.951 kJ/mol for Lig N2. Moreover, no Lipinski and PhysChem violations were identified. It is further observed that these compounds were not inhibitors of P-glycoprotein, CYP450 and renal organic cation transporters. The LD50 of 2.5804 mol/kg for Lig N1 and 2.7788 mol/kg for Lig N2 was noted with acceptable toxicity index.

Expression of Collagenase is Regulated by the VarS/VarA Two-Component Regulatory System in Vibrio alginolyticus

Vibrio alginolyticus is an opportunistic pathogen in both humans and marine animals. Collagenase encoded by colA is considered to be one of the virulence factors. Expression of colA is regulated by multiple environmental factors, e.g., temperature, growth phase, and substrate. To elucidate the mechanism of regulation of colA expression, transposon mutagenesis was performed. VarS, a sensor histidine kinase of the two-component regulatory system, was demonstrated to regulate the expression of colA. VarA, a cognate response regulator of VarS, was also identified and shown to be involved in the regulation of colA expression. In vitro phosphorylation assays showed that phosphorylated VarS acted as a phosphoryl group donor to VarA. A site-directed mutagenesis study showed that the His300, Asp718 and His874 residues in VarS were essential for the phosphorylation of VarS, and the Asp54 residue in VarA was likely to receive the phosphoryl group from VarS. The results demonstrate that the VarS/VarA two-component regulatory system regulates the expression of collagenase in V. alginolyticus.

The Two-Component Signal Transduction System VxrAB Positively Regulates Vibrio cholerae Biofilm Formation

Two-component signal transduction systems (TCSs), typically composed of a sensor histidine kinase (HK) and a response regulator (RR), are the primary mechanism by which pathogenic bacteria sense and respond to extracellular signals. The pathogenic bacterium Vibrio cholerae is no exception and harbors 52 RR genes. Using in-frame deletion mutants of each RR gene, we performed a systematic analysis of their role in V. cholerae biofilm formation. We determined that 7 RRs impacted the expression of an essential biofilm gene and found that the recently characterized RR, VxrB, regulates the expression of key structural and regulatory biofilm genes in V. choleraevxrB is part of a 5-gene operon, which contains the cognate HK vxrA and three genes of unknown function. Strains carrying ΔvxrA and ΔvxrB mutations are deficient in biofilm formation, while the ΔvxrC mutation enhances biofilm formation. The overexpression of VxrB led to a decrease in motility. We also observed a small but reproducible effect of the absence of VxrB on the levels of cyclic di-GMP (c-di-GMP). Our work reveals a new function for the Vxr TCS as a regulator of biofilm formation and suggests that this regulation may act through key biofilm regulators and the modulation of cellular c-di-GMP levels.IMPORTANCE Biofilms play an important role in the Vibrio cholerae life cycle, providing protection from environmental stresses and contributing to the transmission of V. cholerae to the human host. V. cholerae can utilize two-component systems (TCS), composed of a histidine kinase (HK) and a response regulator (RR), to regulate biofilm formation in response to external cues. We performed a systematic analysis of V. cholerae RRs and identified a new regulator of biofilm formation, VxrB. We demonstrated that the VxrAB TCS is essential for robust biofilm formation and that this system may regulate biofilm formation via its regulation of key biofilm regulators and cyclic di-GMP levels. This research furthers our understanding of the role that TCSs play in the regulation of V. cholerae biofilm formation.

rstB Regulates Expression of the Photobacterium damselae subsp. damselae Major Virulence Factors Damselysin, Phobalysin P and Phobalysin C

The marine pathogenic bacterium Photobacterium damselae subsp. damselae causes septicemia in marine animals and in humans. The pPHDD1 plasmid-encoded hemolysins damselysin (Dly) and phobalysin P (PhlyP), and the chromosome-encoded hemolysin phobalysin C (PhlyC) constitute its main virulence factors. However, the mechanisms by which expression of these three hemolysins is regulated remain unknown. Here we report the isolation of a mini-Tn10 transposon mutant which showed a strong impairment in its hemolytic activity. The transposon disrupted a putative sensor histidine kinase gene vda_000600 (rstB), which together with vda_000601 (rstA) is predicted to encode a putative two-component regulatory system. This system showed to be homologous to the Vibrio cholerae CarSR/VprAB and Escherichia coli RstAB systems. Reconstruction of the mutant by allelic exchange of rstB showed equal impairment in hemolysis, and complementation with a plasmid expressing rstAB restored hemolysis to wild-type levels. Remarkably, we demonstrated by promoter expression analyses that the reduced hemolysis in the rstB mutant was accompanied by a strong decrease in transcription activities of the three hemolysin genes dly (damselysin), hlyA_pl (phobalysin P) and hlyA_ch (phobalysin C). Thus, RstB, encoded in the small chromosome, regulates plasmid and chromosomal virulence genes. We also found that reduced expression of the three virulence genes correlated with a strong decrease in virulence in a sea bass model, demonstrating that RstB constitutes a master regulator of the three P. damselae subsp. damselae hemolysins and plays critical roles in the pathogenicity of this bacterium. This study represents the first evidence of a direct role of a RstAB-like system in the regulation of bacterial toxins.

Integration of AI-2 Based Cell-Cell Signaling with Metabolic Cues in Escherichia coli

The quorum sensing molecule Autoinducer-2 (AI-2) is generated as a byproduct of activated methyl cycle by the action of LuxS in Escherichia coli. AI-2 is synthesized, released and later internalized in a cell-density dependent manner. Here, by mutational analysis of the genes, uvrY and csrA, we describe a regulatory circuit of accumulation and uptake of AI-2. We constructed a single-copy chromosomal luxS-lacZ fusion in a luxS⁺ merodiploid strain and evaluated its relative expression in uvrY and csrA mutants. At the entry of stationary phase, the expression of the fusion and AI-2 accumulation was positively regulated by uvrY and negatively regulated by csrA respectively. A deletion of csrA altered message stability of the luxS transcript and CsrA protein exhibited weak binding to 5’ luxS regulatory region. DNA protein interaction and chromatin immunoprecipitation analysis confirmed direct interaction of UvrY with the luxS promoter. Additionally, reduced expression of the fusion in hfq deletion mutant suggested involvement of small RNA interactions in luxS regulation. In contrast, the expression of lsrA operon involved in AI-2 uptake, is negatively regulated by uvrY and positively by csrA in a cell-density dependent manner. The dual role of csrA in AI-2 synthesis and uptake suggested a regulatory crosstalk of cell signaling with carbon regulation in Escherichia coli. We found that the cAMP-CRP mediated catabolite repression of luxS expression was uvrY dependent. This study suggests that luxS expression is complex and regulated at the level of transcription and translation. The multifactorial regulation supports the notion that cell-cell communication requires interaction and integration of multiple metabolic signals.

Vibrio cholerae CsrA Regulates ToxR Levels in Response to Amino Acids and Is Essential for Virulence

ToxR is a major virulence gene regulator in Vibrio cholerae. Although constitutively expressed under many laboratory conditions, our previous work demonstrated that the level of ToxR increases significantly when cells are grown in the presence of the 4 amino acids asparagine, arginine, glutamate, and serine (NRES). We show here that the increase in ToxR production in response to NRES requires the Var/Csr global regulatory circuit. The VarS/VarA two-component system controls the amount of active CsrA, a small RNA-binding protein involved in the regulation of a wide range of cellular processes. Our data show that a varA mutant, which is expected to overproduce active CsrA, had elevated levels of ToxR in the absence of the NRES stimulus. Conversely, specific amino acid substitutions in CsrA were associated with defects in ToxR production in response to NRES. These data indicate that CsrA is a positive regulator of ToxR levels. Unlike previously described effects of CsrA on virulence gene regulation, the effects of CsrA on ToxR were not mediated through quorum sensing and HapR. CsrA is likely essential in V. cholerae, since a complete deletion of csrA was not possible; however, point mutations in CsrA were tolerated well. The CsrA Arg6His mutant had wild-type growth in vitro but was severely attenuated in the infant mouse model of V. cholerae infection, showing that CsrA is critical for pathogenesis. This study has broad implications for our understanding of how V. cholerae integrates its response to environmental cues with the regulation of important virulence genes.

IMPORTANCE

In order to colonize the human host, Vibrio cholerae must sense and respond to environmental signals to ensure appropriate expression of genes required for pathogenesis. Uncovering how V. cholerae senses its environment and activates its virulence gene repertoire is critical for our understanding of how V. cholerae transitions from its natural aquatic habitat to the human host. Here we demonstrate a previously unknown link between the global regulator CsrA and the major V. cholerae virulence gene regulator ToxR. The role of CsrA in the cell is to receive input from the environment and coordinate an appropriate cellular response. By linking environmental sensing to the ToxR regulon, CsrA effectively acts as a switch that controls pathogenesis in response to specific signals. We demonstrate that CsrA is critical for virulence in the infant mouse model of V. cholerae infection, consistent with its role as an in vivo regulator of virulence gene expression.

Vibrio cholerae Response Regulator VxrB Controls Colonization and Regulates the Type VI Secretion System

Two-component signal transduction systems (TCS) are used by bacteria to sense and respond to their environment. TCS are typically composed of a sensor histidine kinase (HK) and a response regulator (RR). The Vibrio cholerae genome encodes 52 RR, but the role of these RRs in V. cholerae pathogenesis is largely unknown. To identify RRs that control V. cholerae colonization, in-frame deletions of each RR were generated and the resulting mutants analyzed using an infant mouse intestine colonization assay. We found that 12 of the 52 RR were involved in intestinal colonization. Mutants lacking one previously uncharacterized RR, VCA0566 (renamed VxrB), displayed a significant colonization defect. Further experiments showed that VxrB phosphorylation state on the predicted conserved aspartate contributes to intestine colonization. The VxrB regulon was determined using whole genome expression analysis. It consists of several genes, including those genes that create the type VI secretion system (T6SS). We determined that VxrB is required for T6SS expression using several in vitro assays and bacterial killing assays, and furthermore that the T6SS is required for intestinal colonization. vxrB is encoded in a four gene operon and the other vxr operon members also modulate intestinal colonization. Lastly, though ΔvxrB exhibited a defect in single-strain intestinal colonization, the ΔvxrB strain did not show any in vitro growth defect. Overall, our work revealed that a small set of RRs is required for intestinal colonization and one of these regulators, VxrB affects colonization at least in part through its regulation of T6SS genes.

Pathogenic bacteria experience varying conditions during infection of human hosts and often use two-component signal transduction systems (TCSs) to monitor their environment. TCS consists of a histidine kinase (HK), which senses environmental signals, and a corresponding response regulator (RR), which mediates a cellular response. The genome of the human pathogen V. cholerae contains a multitude of genes encoding HKs and RRs proteins. In the present study, we systematically analyzed the role of each V. cholerae RR for its role in pathogenesis. We identified a previously uncharacterized RR, VxrB, as a new virulence factor. We demonstrated that VxrB controls expression of the type VI secretion system (T6SS), a virulence nanomachine that directly translocates effectors into bacterial or host cells, thereby facilitating colonization by competing with sister cells and intestinal microbiota. This study represents the first systematic analysis of the role of all RRs in V. cholerae pathogenesis and provides a foundation for understanding the signal transduction pathways controlling V. cholerae pathogenesis.

Role of AcsR in expression of the acetyl-CoA synthetase gene in Vibrio vulnificus

BACKGROUND

VarS/VarA is one of the global factors regulating diverse aspects of the metabolism and virulence of bacteria including pathogenic Vibrio spp. An experiment to identify the VarS/VarA-regulon in V. vulnificus revealed that a putative LuxR-type transcriptional regulator was down-regulated in ΔvarA mutant. To investigate the roles of this regulatory cascade, the target gene regulated by a LuxR-regulator was identified and its expression was characterized.

RESULTS

Transcriptomic analysis of the mutant deficient in this LuxR-type regulator showed that the acsA gene encoding acetyl-CoA synthetase was down-regulated. Thus, this regulator was named AcsR for "regulator of acetyl-CoA synthetase". A putative histidine kinase gene, acsS, was located five ORFs downstream of the acsR gene. Expression of an acsA::luxAB transcriptional fusion was decreased in both ΔacsR and ΔacsS mutants. Similar to a ΔacsA mutant, strains carrying deletions either in acsR or acsS grew slowly than wild type in a minimal medium with acetate as a sole carbon source. Growth defect of the ΔacsR strain in acetate-minimal medium was restored by complementation. To investigate if AcsR directly regulates acsA expression, in vitro-gel shift assays were performed using the recombinant AcsR and the regulatory region of the acsA gene, showing that AcsR specifically bound the upstream region of the acsA ORF.

CONCLUSION

This study indicates that the VarS/VarA system plays a role in V. vulnificus metabolism via regulating AcsR, which in turn controls acetate metabolism by activating the transcription of the acetyl-CoA synthetase gene.

The VieB auxiliary protein negatively regulates the VieSA signal transduction system in Vibrio cholerae

BACKGROUND

Vibrio cholerae is a facultative pathogen that lives in the aquatic environment and the human host. The ability of V. cholerae to monitor environmental changes as it transitions between these diverse environments is vital to its pathogenic lifestyle. One way V. cholerae senses changing external stimuli is through the three-component signal transduction system, VieSAB, which is encoded by the vieSAB operon. The VieSAB system plays a role in the inverse regulation of biofilm and virulence genes by controlling the concentration of the secondary messenger, cyclic-di-GMP. While the sensor kinase, VieS, and the response regulator, VieA, behave similar to typical two-component phosphorelay systems, the role of the auxiliary protein, VieB, is unclear.

RESULTS

Here we show that VieB binds to VieS and inhibits its autophosphorylation and phosphotransfer activity thus preventing phosphorylation of VieA. Additionally, we show that phosphorylation of the highly conserved Asp residue in the receiver domain of VieB regulates the inhibitory activity of VieB.

CONCLUSION

Taken together, these data point to an inhibitory role of VieB on the VieSA phosphorelay, allowing for additional control over the signal output. Insight into the function and regulatory mechanism of the VieSAB system improves our understanding of how V. cholerae controls gene expression as it transitions between the aquatic environment and human host.

Gene fitness landscapes of Vibrio cholerae at important stages of its life cycle

Vibrio cholerae has evolved to adeptly transition between the human small intestine and aquatic environments, leading to water-borne spread and transmission of the lethal diarrheal disease cholera. Using a host model that mimics the pathology of human cholera, we applied high density transposon mutagenesis combined with massively parallel sequencing (Tn-seq) to determine the fitness contribution of >90% of all non-essential genes of V. cholerae both during host infection and dissemination. Targeted mutagenesis and validation of 35 genes confirmed our results for the selective conditions with a total false positive rate of 4%. We identified 165 genes never before implicated for roles in dissemination that reside within pathways controlling many metabolic, catabolic and protective processes, from which a central role for glycogen metabolism was revealed. We additionally identified 76 new pathogenicity factors and 414 putatively essential genes for V. cholerae growth. Our results provide a comprehensive framework for understanding the biology of V. cholerae as it colonizes the small intestine, elicits profuse secretory diarrhea, and disseminates into the aquatic environment.

Fitness factors in vibrios: a mini-review

Vibrios are Gram-negative curved bacilli that occur naturally in marine, estuarine, and freshwater systems. Some species include human and animal pathogens, and some vibrios are necessary for natural systems, including the carbon cycle and osmoregulation. Countless in vivo and in vitro studies have examined the interactions between vibrios and their environment, including molecules, cells, whole animals, and abiotic substrates. Many studies have characterized virulence factors, attachment factors, regulatory factors, and antimicrobial resistance factors, and most of these factors impact the organism's fitness regardless of its external environment. This review aims to identify common attributes among factors that increase fitness in various environments, regardless of whether the environment is an oyster, a rabbit, a flask of immortalized mammalian cells, or a planktonic chitin particle. This review aims to summarize findings published thus far to encapsulate some of the basic similarities among the many vibrio fitness factors and how they frame our understanding of vibrio ecology. Factors representing these similarities include hemolysins, capsular polysaccharides, flagella, proteases, attachment factors, type III secretion systems, chitin binding proteins, iron acquisition systems, and colonization factors.

Neglected but amazingly diverse type IVb pili

This review provides an overview of current knowledge concerning type IVb pili in Gram-negative bacteria. The number of these pili identified is steadily increasing with genome sequencing and mining studies, but studies of these pili are somewhat uneven, because their expression is tightly regulated and the signals or regulators controlling expression need to be identified. However, as illustrated here, they have a number of interesting functional, assembly-related and regulatory features.