Researching bacterial pathogenesis

 

 

 

 

 

Enhanced Interaction of Vibrio cholerae Virulence Regulators TcpP and ToxR under Oxygen-Limiting Conditions

Fan F., Z. Liu, N. Jabeen, L.D. Birdwell, J. Zhu and Kan B.

Infect Immun. 2014 Feb. 3.

Abstract

Vibrio cholerae is the causative agent of the diarrheal disease cholera. The ability of V. cholerae to colonize and cause disease requires the intricately regulated expression of a number of virulence factors during infection. One of the signals sensed by V. cholerae is oxygen-limiting conditions in the gut. It has been shown that the virulence activator AphB plays a key role to sense low oxygen concentration and induce the transcription of another key virulence activator, TcpP. In this study, we applied a bacterial two-hybrid system to further examine the oxygen effect on different virulence regulators. We found that anoxic conditions enhanced interaction between TcpP and ToxR, identified as the first positive regulator of V. cholerae virulence genes. We further demonstrated that the TcpP-ToxR interaction was dependent on the primary periplasmic protein disulfide formation enzyme DsbA and cysteine residues in the periplasmic domains of both ToxR and TcpP. Furthermore, we showed that in V. cholerae, an interaction between TcpP-ToxR is important for virulence gene induction. Under anaerobic growth conditions, we detected ToxR-TcpP heterodimers, which were abolished in the presence of the reducing agent DTT. Our results suggest that V. cholerae may sense intestinal anoxic signals by multiple components to activate virulence.

 

A novel protein protects bacterial iron-dependent metabolism from nitric oxide.

Stern, A.M., B. Liu, L.R. Bakken, J.P. Shapleigh, and Zhu J.

J. Bacteriol. 195:4702-4708, 2013

Abstract

Reactive nitrogen species (RNS), in particular nitric oxide (NO), are toxic to bacteria, and bacteria have mechanisms to allow growth despite this stress. Understanding how bacteria interact with NO is essential to understanding bacterial physiology in many habitats, including pathogenesis; however, many targets of NO and enzymes involved in NO resistance remain uncharacterized. We performed for the first time a metabolomic screen on NO-treated and -untreated bacteria to define broadly the effects of NO on bacterial physiology, as well as to identify the function of NnrS, a previously uncharacterized enzyme involved in defense against NO. We found many known and novel targets of NO. We also found that iron-sulfur cluster enzymes were preferentially inhibited in a strain lacking NnrS due to the formation of iron-NO complexes. We then demonstrated that NnrS is particularly important for resistance to nitrosative stress under anaerobic conditions. Our data thus reveal the breadth of the toxic effects of NO on metabolism and identify the function of an important enzyme in alleviating this stress.

 

Quorum sensing regulatory cascades control Vibrio fluvialis pathogenesis.

Wang, Y., H. Wang, W. Liang, A. J. Hay, Z. Zhong, B. Kan, and Zhu J.

J. Bacteriol. 195:3583-3589, 2013

Abstract

Quorum sensing (QS) is a process by which individual bacteria are able to communicate with one another, thereby enabling the population as a whole to coordinate gene regulation and subsequent phenotypic outcomes. Communication is accomplished through production and detection of small molecules in the extracellular milieu. In many bacteria, particularly Vibrio species, multiple QS systems result in multiple signals, as well as cross talk between systems. In this study, we identify two QS systems in the halophilic enteric pathogen Vibrio fluvialis: one acyl-homoserine lactone (AHL) based and one CAI-1/AI-2 based. We show that a LuxI homolog, VfqI, primarily produces 3-oxo-C10-HSL, which is sensed by a LuxR homolog, VfqR. VfqR-AHL is required to activate vfqI expression and autorepress vfqR expression. In addition, we have shown that similar to that in V. cholerae and V. harveyi, V. fluvialis produces CAI-1 and AI-2 signal molecules to activate the expression of a V. cholerae HapR homolog through LuxO. Although VfqR-AHL does not regulate hapR expression, HapR can repress vfqR transcription. Furthermore, we found that QS in V. fluvialis positively regulates production of two potential virulence factors, an extracellular protease and hemolysin. QS also affects cytotoxic activity against epithelial tissue cultures. These data suggest that V. fluvialis integrates QS regulatory pathways to play important physiological roles in pathogenesis.

 

Applying reversible mutations of nodulation and nitrogen-fixation genes to study social cheating in Rhizobium etli-legume interaction.

Ling, J., H. Zheng, D.S. Katzianer, H. Wang, Z. Zhong, and Zhu, J.

PLoS One.8:e70138, 2013

Abstract

Mutualisms are common in nature, though these symbioses can be quite permeable to cheaters in situations where one individual parasitizes the other by discontinuing cooperation yet still exploits the benefits of the partnership. In the Rhizobium-legume system, there are two separate contexts, namely nodulation and nitrogen fixation processes, by which resident Rhizobium individuals can benefit by cheating. Here, we constructed reversible and irreversible mutations in key nodulation and nitrogen-fixation pathways of Rhizobium etli and compared their interaction with plant hosts Phaseolus vulgaris to that of wild type. We show that R. etli reversible mutants deficient in nodulation factor production are capable of intra-specific cheating, wherein mutants exploit other Rhizobium individuals capable of producing these factors. Similarly, we show that R. etli mutants are also capable of cheating inter-specifically, colonizing the host legume yet contributing nothing to the partnership in terms of nitrogen fixation. Our findings indicate that cheating is possible in both of these frameworks, seemingly without damaging the stability of the mutualism itself. These results may potentially help explain observations suggesting that legume plants are commonly infected by multiple bacterial lineages during the nodulation process.

 

Bile-salt induced intermolecular disulfide bond formation activates Vibrio cholerae virulence.

Yang, M., Z. Liu, C. Hughes, A.M. Stern, H. Wang, Z. Zhong, B. Kan, W. Fenical and Zhu, J.

Proc. Natl. Acad. Sci U.S.A. 110:2348-2353, 2013.

Abstract

To be successful pathogens, bacteria must often restrict the expression of virulence genes to host environments. This requires a physical or chemical marker of the host environment as well as a cognate bacterial system for sensing the presence of a host to appropriately time the activation of virulence. However, there have been remarkably few such signal-sensor pairs identified, and the molecular mechanisms for host-sensing are virtually unknown. By directly applying a reporter strain of Vibrio cholerae, the causative agent of cholera, to a thin layer chromatography (TLC) plate containing mouse intestinal extracts, we found two host signals that activate virulence gene transcription. One of these was revealed to be the bile salt taurocholate. We then show that a set of bile salts cause dimerization of the transmembrane transcription factor TcpP by inducing intermolecular disulfide bonds between cysteine (C)-207 residues in its periplasmic domain. Various genetic and biochemical analyses led us to propose a model in which the other cysteine in the periplasmic domain, C218, forms an inhibitory intramolecular disulfide bond with C207 that must be isomerized to form the active C207-C207 intermolecular bond. We then found bile salt-dependent effects of these cysteine mutations on survival in vivo, correlating to our in vitro model. Our results are a demonstration of a mechanism for direct activation of the V. cholerae virulence cascade by a host signal molecule. They further provide a paradigm for recognition of the host environment in pathogenic bacteria through periplasmic cysteine oxidation.

 

LysR-family activator regulated major facilitator superfamily transporters are involved in Vibrio cholerae antimicrobial compound resistance and intestinal colonization.

Chen, S., H. Wang, D.S. Katzianer, Z. Zhong, and Zhu, J.

Int. J. Antimicrob. Agents 41:188-192, 2013.

Abstract

Expression of a multidrug resistance transporter renders bacterial cells resistant to a variety of drugs. The major facilitator superfamily (MFS) comprises the largest group of bacterial multidrug transporters. There are over 20 MFS efflux pumps annotated on the genome of Vibrio cholerae, but little is known about their functions and regulation. In this study, five MFS efflux pumps were characterised, each of which is associated with a divergently transcribed putative LysR-type transcriptional regulator (MfsR). It was found that each of these MFS structural genes is regulated by the corresponding MfsR regulator. Deletion of these five mfs genes results in increased susceptibility to tetracycline and crude bile as well as a colonisation defect in an infant mouse colonisation model. Moreover, tetracycline and unknown intestinal signals could serve as co-inducers for the MfsR regulators. These data suggest that MFS efflux pumps are important both for antimicrobial resistance and V. cholerae pathogenesis.

 

Catalases promote resistance of oxidative stress in Vibrio cholerae.

Wang, H., S. Chen, J. Zhang, F.P. Rothenbacher, T. Jiang, B. Kan, Z. Zhong, and Zhu, J.

PLoS One.7:e53383, 2012

Abstract

Oxidative stress is a major challenge faced by bacteria. Many bacteria control oxidative stress resistance pathways through the transcriptional regulator OxyR. The human pathogen Vibrio cholerae is a Gram-negative bacterium that is the causative agent of cholera. V. cholerae lives in both aquatic environments and human small intestines, two environments in which it encounters reactive oxygen species (ROS). To study how V. cholerae responds to oxidative stress, we constructed an in-frame oxyR deletion mutant. We found that this mutant was not only sensitive to H(2)O(2), but also displayed a growth defect when diluted in rich medium. Further study showed that two catalases, KatG and KatB, either when expressed in living cells, present in culture supernatants, or added as purified recombinant proteins, could rescue the oxyR growth defect. Furthermore, although it could colonize infant mouse intestines similar to that of wildtype, the oxyR mutant was defective in zebrafish intestinal colonization. Alternatively, co-infection with wildtype, but not katG-katB deletion mutants, greatly enhanced oxyR mutant colonization. Our study suggests that OxyR in V. cholerae is critical for antioxidant defense and that the organism is capable of scavenging environmental ROS to facilitate population growth.

 

Succession in the Gut Microbiome following Antibiotic and Antibody Therapies for Clostridium difficile.

Peterfreund, G.L., L.E. Vandivier, R. Sinha, A.J. Marozsan, W.C. Olson, J. Zhu, and Bushman F.D.

PLoS One.7:e46966, 2012

Abstract

Antibiotic disruption of the intestinal microbiota may cause susceptibility to pathogens that is resolved by progressive bacterial outgrowth and colonization. Succession is central to ecological theory but not widely documented in studies of the vertebrate microbiome. Here, we study succession in the hamster gut after treatment with antibiotics and exposure to Clostridium difficile. C. difficile infection is typically lethal in hamsters, but protection can be conferred with neutralizing antibodies against the A and B toxins. We compare treatment with neutralizing monoclonal antibodies (mAb) to treatment with vancomycin, which prolongs the lives of animals but ultimately fails to protect them from death. We carried out longitudinal deep sequencing analysis and found distinctive waves of succession associated with each form of treatment. Clindamycin sensitization prior to infection was associated with the temporary suppression of the previously dominant Bacteroidales and the fungus Saccinobaculus in favor of Proteobacteria. In mAb-treated animals, C. difficile proliferated before joining Proteobacteria in giving way to re-expanding Bacteroidales and the fungus Wickerhamomyces. However, the Bacteroidales lineages returning by day 7 were different from those that were present initially, and they persisted for the duration of the experiment. Animals treated with vancomycin showed a different set of late-stage lineages that were dominated by Proteobacteria as well as increased disparity between the tissue-associated and luminal cecal communities. The control animals showed no change in their gut microbiota. These data thus suggest different patterns of ecological succession following antibiotic treatment and C. difficile infection.

 

The NorR regulon is critical for Vibrio cholerae resistance to nitric oxide and sustained colonization of the intestines.

Stern AM, Hay AJ, Liu Z, Desland FA, Zhang J, Zhong Z, Zhu J.

MBio. 2012 Apr 17;3(2):e00013-12. doi: 10.1128/mBio.00013-12. Print 2012.

Abstract

Vibrio cholerae, the cause of an often fatal infectious diarrhea, remains a large global public health threat. Little is known about the challenges V. cholerae encounters during colonization of the intestines, which genes are important for overcoming these challenges, and how these genes are regulated. In this study, we examined the V. cholerae response to nitric oxide (NO), an antibacterial molecule derived during infection from various sources, including host inducible NO synthase (iNOS). We demonstrate that the regulatory protein NorR regulates the expression of NO detoxification genes hmpA and nnrS, and that all three are critical for resisting low levels of NO stress under microaerobic conditions in vitro. We also show that prxA, a gene previously thought to be important for NO detoxification, plays no role in NO resistance under microaerobic conditions and is upregulated by H(2)O(2), not NO. Furthermore, in an adult mouse model of prolonged colonization, hmpA and norR were important for the resistance of both iNOS- and non-iNOS-derived stresses. Our data demonstrate that NO detoxification systems play a critical role in the survival of V. cholerae under microaerobic conditions resembling those of an infectious setting and during colonization of the intestines over time periods similar to that of an actual V. cholerae infection. IMPORTANCE: Little is known about what environmental stresses Vibrio cholerae, the etiologic agent of cholera, encounters during infection, and even less is known about how V. cholerae senses and counters these stresses. Most prior studies of V. cholerae infection relied on the 24-h infant mouse model, which does not allow the analysis of survival over time periods comparable to that of an actual V. cholerae infection. In this study, we used a sustained mouse colonization model to identify nitric oxide resistance as a function critical for the survival of V. cholerae in the intestines and further identified the genes responsible for sensing and detoxifying this stress.

 

The VarS/VarA two-component system modulates the activity of the Vibrio cholerae quorum-sensing transcriptional regulator HapR.

Tsou AM, Liu Z, Cai T, Zhu J.

Microbiology. 2011 Jun; 157(Pt 6):1620-8. Epub 2011 Mar 10.

Abstract

The human pathogen Vibrio cholerae uses quorum sensing to regulate the expression of a number of phenotypes, including virulence factor production, in response to changes in cell density. It produces small molecules called autoinducers that increase in concentration as cell density increases, and these autoinducers bind to membrane sensors once they reach a certain threshold. This binding leads to signalling through a downstream phosphorelay pathway to alter the expression of the transcriptional regulator HapR. Previously, it was shown that the VarS/VarA two-component system acts on a component of the phosphorelay pathway upstream of HapR to regulate HapR expression levels. Here, we show that in addition to this mechanism of regulation, VarS and VarA also indirectly modulate HapR protein activity. This modulation is mediated by the small RNA CsrB but is independent of the known quorum-sensing system that links the autoinducers to HapR. Thus, the VarS/VarA two-component system intersects with the quorum-sensing network at two levels. In both cases, the effect of VarS and VarA on quorum sensing is dependent on the Csr small RNAs, which regulate carbon metabolism, suggesting that V. cholerae may integrate nutrient status and cell density sensory inputs to tailor its gene expression profile more precisely to surrounding conditions.

Vibrio cholerae anaerobic induction of virulence gene expression is controlled by thiol-based switches of virulence regulator AphB

Liu Z, Yang M, Peterfreund GL, Tsou AM, Selamoglu N, Daldal F, Zhong Z, Kan B, Zhu J.

Proc Natl Acad Sci U.S.A. 2011 Jan 11;108(2):810-5. Epub 2010 Dec 27.

Abstract

Bacterial pathogens have evolved sophisticated signal transduction systems to coordinately control the expression of virulence determinants. For example, the human pathogen Vibrio cholerae is able to respond to host environmental signals by activating transcriptional regulatory cascades. The host signals that stimulate V. cholerae virulence gene expression, however, are still poorly understood. Previous proteomic studies indicated that the ambient oxygen concentration plays a role in V. cholerae virulence gene expression. In this study, we found that under oxygen-limiting conditions, an environment similar to the intestines, V. cholerae virulence genes are highly expressed. We show that anaerobiosis enhances dimerization and activity of AphB, a transcriptional activator that is required for the expression of the key virulence regulator TcpP, which leads to the activation of virulence factor production. We further show that one of the three cysteine residues in AphB, C(235), is critical for oxygen responsiveness, as the AphB(C235S) mutant can activate virulence genes under aerobic conditions in vivo and can bind to tcpP promoters in the absence of reducing agents in vitro. Mass spectrometry analysis suggests that under aerobic conditions, AphB is modified at the C(235) residue. This modification is reversible between oxygen-rich aquatic environments and oxygen-limited human hosts, suggesting that V. cholerae may use a thiol-based switch mechanism to sense intestinal signals and activate virulence.

Virulence regulator AphB enhances toxR transcription in Vibrio cholerae.

Xu X, Stern AM, Liu Z, Kan B, Zhu J.

BMC Microbiol. 2010 Jan 6;10:3.

Abstract

BACKGROUND:

Vibrio cholerae is the causative agent of cholera. Extensive studies reveal that complicated regulatory cascades regulate expression of virulence genes, the products of which are required for V. cholerae to colonize and cause disease. In this study, we investigated the expression of the key virulence regulator ToxR under different conditions.

RESULTS:

We found that compared to that of wild type grown to stationary phase, the toxR expression was lower in an aphB mutant strain. AphB has been previously shown to be a key virulence regulator that is required to activate the expression of tcpP. When expressed constitutively, AphB is able to activate the toxR promoter. Furthermore, gel shift analysis indicates that AphB binds toxR promoter region directly. We also characterize the effect of AphB on the levels of the outer membrane porins OmpT and OmpU, which are known to be regulated by ToxR.

CONCLUSIONS:

Our data indicate that V. cholerae possesses an additional regulatory loop that use AphB to activate the expression of two virulence regulators, ToxR and TcpP, which together control the expression of the master virulence regulator ToxT.

Metabolism of bile salts in mice influences spore germination in Clostridium difficile.

Giel JL, Sorg JA, Sonenshein AL, Zhu J.

PLoS One. 2010 Jan 15;5(1):e8740.

Abstract

Clostridium difficile, a spore-forming bacterium, causes antibiotic-associated diarrhea. In order to produce toxins and cause disease, C. difficile spores must germinate and grow out as vegetative cells in the host. Although a few compounds capable of germinating C. difficile spores in vitro have been identified, the in vivo signal(s) to which the spores respond were not previously known. Examination of intestinal and cecal extracts from untreated and antibiotic-treated mice revealed that extracts from the antibiotic-treated mice can stimulate colony formation from spores to greater levels. Treatment of these extracts with cholestyramine, a bile salt binding resin, severely decreased the ability of the extracts to stimulate colony formation from spores. This result, along with the facts that the germination factor is small, heat-stable, and water-soluble, support the idea that bile salts stimulate germination of C. difficile spores in vivo. All extracts able to stimulate high level of colony formation from spores had a higher proportion of primary to secondary bile salts than extracts that could not. In addition, cecal flora from antibiotic-treated mice was less able to modify the germinant taurocholate relative to flora from untreated mice, indicating that the population of bile salt modifying bacteria differed between the two groups. Taken together, these data suggest that an in vivo-produced compound, likely bile salts, stimulates colony formation from C. difficile spores and that levels of this compound are influenced by the commensal gastrointestinal flora.

 

The virulence transcriptional activator AphA enhances biofilm formation by Vibrio cholerae by activating expression of the biofilm regulator VpsT.

Yang M, Frey EM, Liu Z, Bishar R, Zhu J.

Infect Immun. 2010 Feb;78(2):697-703. Epub 2009 Nov 23.

Abstract

Vibrio cholerae is the agent of the severe diarrheal disease cholera, and it perpetuates in aquatic reservoirs when not in the host. Within the host's intestines, the bacteria execute a complex regulatory pathway culminating with the production of virulence factors that allow colonization and cause disease. The ability of V. cholerae to form biofilms is thought to aid its persistence in the aquatic environment and passage through the gastric acid barrier of the stomach. The transcriptional activators VpsR and VpsT are part of the biofilm formation-regulatory network. In this study, we screened a V. cholerae genomic library in Escherichia coli cells containing a P(vpsT)-luxCDBAE transcriptional fusion reporter and found that a plasmid clone containing the aphA gene activates the expression of vpsT in E. coli. AphA is a master virulence regulator in V. cholerae that is required to activate the expression of tcpP, whose gene products in turn activate all virulence genes including those responsible for the synthesis of the toxin-coregulated pilus (TCP) and cholera toxin through the activation of toxT. AphA has a direct effect on the vpsT promoter, as gel shift experiments demonstrated that AphA binds to the vpsT promoter region. Furthermore, V. cholerae aphA mutants exhibit significantly lower levels of vpsT expression as well as reduced biofilm formation. AphA thus links the expression of virulence and biofilm synthesis genes.

Quorum sensing negatively regulates hemolysin transcriptionally and posttranslationally in Vibrio cholerae.

Tsou AM, Zhu J.

Infect Immun. 2010 Jan; 78(1):461-7. Epub 2009 Oct 26.

Abstract

Recent work has shown that in addition to cholera toxin (CT) and the toxin-coregulated pilus (TCP), other cytotoxic proteins in Vibrio cholerae also cause disease symptoms, and this is particularly evident in strains lacking CT. One such protein is the hemolysin encoded by hlyA. Here we show that, like CT and TCP, HlyA is repressed by the quorum-sensing-regulated transcription factor HapR. This repression occurs on two levels: one at the transcriptional level that is independent of the metalloprotease HapA and one at the posttranslational level that is mediated by HapA. The transcriptional regulation is significantly more apparent on solid media than in liquid cultures. This is the first time that hemolysis has been shown to be directly regulated by quorum sensing in V. cholerae, and it is interesting that, like other virulence factors, HlyA is also repressed by HapR, which is expressed late in infection.

Coordinated regulation of virulence by quorem sensing and motility pathways during the initial stages of Vibrio cholerae infection.

Tsou AM, Frey EM, Hsiao A, Liu Z, Zhu J.

Commun Integr Biol. 2008;1(1):42-4

Abstract

Pathogenic bacteria, such as Vibrio cholerae, must be capable of adapting to diverse living conditions, especially when transitioning from life in environmental reservoirs to life in a host. The abilities to sense arrival at a site suitable for colonization or infection and to respond with appropriate alterations in gene expression are crucial for a pathogen's success. Recently, we have shown that V. cholerae is able to recognize that it has reached its colonization site in the small intestine by sensing breakage of its flagellum as it penetrates the mucosal layer overlaying the intestinal epithelium. Flagellar loss results in the release of the anti-sigma factor FlgM and subsequent activation of the alternative sigma-factor FliA. FliA represses the quorum sensing-controlled transcriptional regulator, HapR, allowing increased expression of virulence factors such as Cholera Toxin (CT) and the Toxin Coregulated Pilus (TCP). In this way, the de-repression of virulence factor expression coincides with the arrival of bacteria at the site of infection at the intestinal mucosa. Our work reveals an interesting interplay between motility and quorum sensing signaling pathways to precisely time virulence gene expression during colonization.

Regulatory targets of quorem sensing in Vibrio cholerae: evidence for two distinct HapR-binding motifs.

Tsou AM, Cai T, Liu Z, Zhu J, Kulkarni RV.

Nucleic Acids Res. 2009 May;37(8):2747-56. Epub 2009 Mar 10.

Abstract

The quorum-sensing pathway in Vibrio cholerae controls the expression of the master regulator HapR, which in turn regulates several important processes such as virulence factor production and biofilm formation. While HapR is known to control several important phenotypes, there are only a few target genes known to be transcriptionally regulated by HapR. In this work, we combine bioinformatic analysis with experimental validation to discover a set of novel direct targets of HapR. Our results provide evidence for two distinct binding motifs for HapR-regulated genes in V. cholerae. The first binding motif is similar to the motifs recently discovered for orthologs of HapR in V. harveyi and V. vulnificus. However, our results demonstrate that this binding motif can be of variable length in V. cholerae. The second binding motif shares common elements with the first motif, but is of fixed length and lacks dyad symmetry at the ends. The contributions of different bases to HapR binding for this second motif were demonstrated using systematic mutagenesis experiments. The current analysis presents an approach for systematically expanding our knowledge of the quorum-sensing regulon in V. cholerae and other related bacteria.

Genetic tools to study gene expression during bacterial pathogen infection.

Hsiao A, Zhu J.

Adv Appl Microbiol. 2009;67:297-314

Abstract

The study of bacterial pathogenesis is in many ways the study of the regulatory mechanisms at work in the microbe during infection. The astonishing flexibility and adaptability of the bacterial cell has enabled many pathogenic species to freely transition between dramatically different environmental conditions. The transcriptional changes that underlie this ability can determine the success of the pathogen in the host. Many techniques have been devised to examine the transcriptional repertoire of bacteria in vivo during infection. Here, we review a class of technologies known as in vivo expression technology (IVET), which use promoter-trapping with a variety of different reporter constructs to allow researchers to probe the transcriptional changes taking place in bacteria under various environmental conditions. Using IVET techniques, researchers have been able to catalogue a wide variety of virulence factors in the host for several important human pathogens, as well as examining the timing of virulence gene regulation. Most recently, IVET techniques have also been used to identify transcriptional repression events in vivo, such as the suppression of anti-colonization factors deleterious to infection. As the array of IVET reporters and promoter-trapping strategies grow, researchers are increasingly able to illuminate the myriad transcriptional activities that allow bacteria to survive and cause disease in the host.

Direct regulation by the Vibrio cholerae regulator ToxT to modulate colonization and anticolonization pilus expression.

Hsiao A, Xu X, Kan B, Kulkarni RV, Zhu J.

Infect Immun. 2009 Apr;77(4):1383-8. Epub 2009 Jan 21.

Abstract

The pathogen Vibrio cholerae uses a large number of coordinated transcriptional regulatory events to transition from its environmental reservoir to the host and establish itself at its preferred colonization site at the host intestinal mucosa. The key regulator in this process is the AraC/XylS family transcription factor, ToxT, which plays critical roles in pathogenesis, including the regulation of two type IV pili, the anticolonization factor mannose-sensitive hemagglutinin and the toxin-coregulated pilus. Previously, it was thought ToxT required dimerization in order to effect transcriptional regulation at its cognate promoters. Here, we present evidence that ToxT directly represses transcription of the msh operon by binding to three promoters within this operon and that dimerization may not be required for transcriptional repression of target promoters by ToxT, suggesting that this regulator uses different mechanisms to modulate the transcriptional repertoire of V. cholerae.

Mucosal penetration primes Vibrio cholerae for host colonization by repressing quorum sensing

Liu Z, Miyashiro T, Tsou A, Hsiao A, Goulian M, Zhu J.

Proc Natl Acad Sci U.S.A. 2008 July 15;105(28):9769-74. Epub 2008 Jul 7.

Abstract

To successfully infect a host and cause the diarrheal disease cholera, Vibrio cholerae must penetrate the intestinal mucosal layer and express virulence genes. Previous studies have demonstrated that the transcriptional regulator HapR, which is part of the quorum sensing network in V. cholerae, represses the expression of virulence genes. Here, we show that hapR expression is also modulated by the regulatory network that governs flagellar assembly. Specifically, FliA, which is the alternative sigma-factor (sigma(28)) that activates late-class flagellin genes in V. cholerae, represses hapR expression. In addition, we show that mucin penetration by V. cholerae is sufficient to break flagella and so cause the secretion of FlgM, the anti-sigma factor that inhibits FliA activity. During initial colonization of host intestinal tissue, hapR expression is repressed because of low cell density. However, full repression of hapR expression does not occur in fliA mutants, which results in attenuated colonization. Our results suggest that V. cholerae uses flagellar machinery to sense particular intestinal signals before colonization and enhance the expression of virulence genes by modulating the output of quorum sensing signaling.

Post-transcriptional cross-talk between pro- and anti-colonization pili biosynthesis systems in Vibrio cholerae.

 

Hsiao A, Toscano K, Zhu J.

Mol Microbiol. 2008 Feb;67(4):849-60. Epub 2007 Dec 19.

Abstract

The pathogen Vibrio cholerae modulates the expression of many genes in order to transition from its environmental reservoir to its niche in the human host. Among these are genes encoding two related Type IV pili, the mannose-sensitive haemagglutinin (MSHA) pilus, which aids V. cholerae persistence in aquatic environments but causes clearance of bacteria by host immune defences, and the toxin co-regulated pilus (TCP) required for colonization. These antagonistic effects are resolved transcriptionally by the regulator ToxT, which represses msh genes while activating tcp genes during infection. We show that these two pili systems are also intertwined post-transcriptionally through the ToxT-regulated pre-pilin peptidase TcpJ. We found that the major MSHA pilin, MshA, was degraded in V. cholerae in a TcpJ-dependent fashion. In a heterologous Escherichia coli system, TcpJ can recognize both MshA and its cognate substrate, the TCP subunit TcpA, but that processing by TcpJ causes the degradation of MshA. Through site-directed mutagenesis and chimeric pilin analysis, we show that this process targets a combination of MshA N-terminal motifs and depends on the proteolytic activity of TcpJ. Moreover, overexpression of tcpJ partially restored the ability of bacteria unable to transcriptionally downregulate msh genes to colonize infant mice. These findings describe co-ordinated proteolysis as a regulatory mechanism in V. cholerae and illustrate this organism's adaptability in the face of dramatic environmental changes.

 

 

 

 

 

 

 

 

 

                   

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