Catabolism of mucus components influences motility of Vibrio cholerae in the presence of environmental reservoirs.

Vibrio cholerae O1, the etiological agent of cholera, is a natural inhabitant of aquatic ecosystems. Motility is a critical element for the colonization of both the human host and its environmental reservoirs. In this study, we investigated the molecular mechanisms underlying the chemotactic response of V. cholerae in the presence of some of its environmental reservoirs. We found that, from the several oligosaccharides found in mucin, two specifically triggered motility of V. cholerae O1: N-acetylneuraminic acid (Neu5Ac) and N-acetylglucosamine (GlcNAc). We determined that the compounds need to be internally catabolized in order to trigger motility of V. cholerae. Interestingly, the catabolism of Neu5Ac and GlcNAc converges and the production of one molecule common to both pathways, glucosamine-6-phosphate (GlcN-6P), is essential to induce motility in the presence of both compounds. Mutants unable to produce GlcN-6P show greatly reduced motility towards mucin. Furthermore, we determined that the production of GlcN-6P is necessary to induce motility of V. cholerae in the presence of some of its environmental reservoirs such as crustaceans or cyanobacteria, revealing a molecular link between the two distinct modes of the complex life cycle of V. cholerae. Finally, cross-species comparisons revealed varied chemotactic responses towards mucin, GlcNAc, and Neu5Ac for environmental (non-pathogenic) strains of V. cholerae, clinical and environmental isolates of the human pathogens Vibrio vulnificus and Vibrio parahaemolyticus, and fish and squid isolates of the symbiotic bacterium Vibrio fischeri. The data presented here suggest nuance in convergent strategies across species of the same bacterial family for motility towards suitable substrates for colonization.

Identification of a Small Molecule Activator for AphB, a LysR-Type Virulence Transcriptional Regulator in Vibrio cholerae.

AphB is a LysR-type transcriptional regulator (LTTR) that cooperates with a second transcriptional activator, AphA, at the tcpPH promoter to initiate expression of the virulence cascade in Vibrio cholerae. Because it is not yet known whether AphB responds to a natural ligand in V. cholerae that influences its ability to activate transcription, we used a computational approach to identify small molecules that influence its activity. In silico docking was used to identify potential ligands for AphB, and saturation transfer difference nuclear magnetic resonance was subsequently employed to access the validity of promising targets. We identified a small molecule, BP-15, that specifically binds the C-terminal regulatory domain of AphB and increases its activity. Interestingly, molecular docking predicts that BP-15 does not bind in the putative primary effector-binding pocket located at the interface of RD-I and RD-II as in other LTTRs, but rather at the dimerization interface. The information gained in this study helps us to further understand the mechanism by which transcriptional activation by AphB is regulated by suggesting that AphB has a secondary ligand binding site, as observed in other LTTRs. This study also lays the groundwork for the future design of inhibitory molecules to block the V. cholerae virulence cascade, thereby preventing the devastating symptoms of cholera infection.

Bile salts and alkaline pH reciprocally modulate the interaction between the periplasmic domains of Vibrio cholerae ToxR and ToxS.

ToxR is a transmembrane transcription factor that is essential for virulence gene expression and human colonization by Vibrio cholerae. ToxR requires its operon partner ToxS, a periplasmic integral membrane protein, for full activity. These two proteins are thought to interact through their respective periplasmic domains, ToxRp and ToxSp. In addition, ToxR is thought to be responsive to various environmental cues, such as bile salts and alkaline pH, but how these factors influence ToxR is not yet understood. Using NMR and reciprocal pull down assays, we present the first direct evidence that ToxR and ToxS physically interact. Furthermore, using NMR and DSF, it was shown that the bile salts cholate and chenodeoxycholate interact with purified ToxRp and destabilize it. Surprisingly, bile salt destabilization of ToxRp enhanced the interaction between ToxRp and ToxSp. In contrast, alkaline pH, which is one of the factors that leads to ToxR proteolysis, decreased the interaction between ToxRp and ToxSp. Taken together, these data suggest a model whereby bile salts or other detergents destabilize ToxR, increasing its interaction with ToxS to promote full ToxR activity. Subsequently, as V. cholerae alkalinizes its environment in late stationary phase, the interaction between the two proteins decreases, allowing ToxR proteolysis to proceed.

Production of putative enhanced oral cholera vaccine strains that express toxin-coregulated pilus.

The use of whole cell killed (WCK) oral cholera vaccines is an important strategy for cholera prevention in endemic areas. To overcome current vaccine limitations, we engineered strains of V. cholerae to be non-toxigenic and to express the protective protein colonization factor, toxin-coregulated pilus (TCP), under scale-up conditions potentially amenable to vaccine production. Two V. cholerae clinical strains were selected and their cholera toxin genes deleted. The tcp operon was placed under control of a rhamnose-inducible promoter. Production and stability of TCP were assessed under various conditions. The strains lack detectable cholera toxin production. The addition of 0.1% rhamnose to the growth medium induced robust production of TCP and TcpA antigen. The strains produced intact TCP in larger growth volumes (1 L), and pili appeared stable during heat-killing or acid treatment of the bacterial cultures. To date, no WCK cholera vaccines have included TCP. We have constructed putative strains of V. cholerae for use in a vaccine that produce high levels of stable TCP antigen, which has not previously been achieved.

A new class of inhibitors of the AraC family virulence regulator Vibrio cholerae ToxT.

Vibrio cholerae is responsible for the diarrheal disease cholera that infects millions of people worldwide. While vaccines protecting against cholera exist, and oral rehydration therapy is an effective treatment method, the disease will remain a global health threat until long-term solutions such as improved sanitation and access to clean water become widely available. Because of this, there is a pressing need for potent therapeutics that can either mitigate cholera symptoms, or act prophylactically to prevent the virulent effects of a cholera infection. Here we report the design, synthesis, and characterization of a set of compounds that bind and inhibit ToxT, the transcription factor that directly regulates the two primary V. cholerae virulence factors. Using the folded structure of the monounsaturated fatty acid observed in the X-ray structure of ToxT as a template, we designed ten novel compounds that inhibit the virulence cascade to a greater degree than any known inhibitor. Our findings provide a structural and functional basis for the development of viable antivirulence therapeutics that combat cholera and, potentially, other forms of bacterial pathogenic disease.

Architecture of the Vibrio cholerae toxin-coregulated pilus machine revealed by electron cryotomography.

Type IV pili (T4P) are filamentous appendages found on many Bacteria and Archaea. They are helical fibres of pilin proteins assembled by a multi-component macromolecular machine we call the basal body. Based on pilin features, T4P are classified into type IVa pili (T4aP) and type IVb pili (T4bP)1,2. T4aP are more widespread and are involved in cell motility3, DNA transfer4, host predation5 and electron transfer6. T4bP are less prevalent and are mainly found in enteropathogenic bacteria, where they play key roles in host colonization7. Following similar work on T4aP machines8,9, here we use electron cryotomography10 to reveal the three-dimensional in situ structure of a T4bP machine in its piliated and non-piliated states. The specific machine we analyse is the Vibrio cholerae toxin-coregulated pilus machine (TCPM). Although only about half of the components of the TCPM show sequence homology to components of the previously analysed Myxococcus xanthus T4aP machine (T4aPM), we find that their structures are nevertheless remarkably similar. Based on homologies with components of the M. xanthus T4aPM and additional reconstructions of TCPM mutants in which the non-homologous proteins are individually deleted, we propose locations for all eight TCPM components within the complex. Non-homologous proteins in the T4aPM and TCPM are found to form similar structures, suggesting new hypotheses for their functions and evolutionary histories.

The Fatty Acid Regulator FadR Influences the Expression of the Virulence Cascade in the El Tor Biotype of Vibrio cholerae by Modulating the Levels of ToxT via Two Different Mechanisms.

FadR is a master regulator of fatty acid (FA) metabolism that coordinates the pathways of FA degradation and biosynthesis in enteric bacteria. We show here that a ΔfadR mutation in the El Tor biotype of Vibrio cholerae prevents the expression of the virulence cascade by influencing both the transcription and the posttranslational regulation of the master virulence regulator ToxT. FadR is a transcriptional regulator that represses the expression of genes involved in FA degradation, activates the expression of genes involved in unsaturated FA (UFA) biosynthesis, and also activates the expression of two operons involved in saturated FA (SFA) biosynthesis. Since FadR does not bind directly to the toxT promoter, we determined whether the regulation of any of its target genes indirectly influenced ToxT. This was accomplished by individually inserting a double point mutation into the FadR-binding site in the promoter of each target gene, thereby preventing their activation or repression. Although preventing FadR-mediated activation of fabA, which encodes the enzyme that carries out the first step in UFA biosynthesis, did not significantly influence either the transcription or the translation of ToxT, it reduced its levels and prevented virulence gene expression. In the mutant strain unable to carry out FadR-mediated activation of fabA, expressing fabA ectopically restored the levels of ToxT and virulence gene expression. Taken together, the results presented here indicate that V. cholerae FadR influences the virulence cascade in the El Tor biotype by modulating the levels of ToxT via two different mechanisms.IMPORTANCE Fatty acids (FAs) play important roles in membrane lipid homeostasis and energy metabolism in all organisms. In Vibrio cholerae, the causative agent of the acute intestinal disease cholera, they also influence virulence by binding into an N-terminal pocket of the master virulence regulator, ToxT, and modulating its activity. FadR is a transcription factor that coordinately controls the pathways of FA degradation and biosynthesis in enteric bacteria. This study identifies a new link between FA metabolism and virulence in the El Tor biotype by showing that FadR influences both the transcription and posttranslational regulation of the master virulence regulator ToxT by two distinct mechanisms.

Origins of pandemic Vibrio cholerae from environmental gene pools.

Some microorganisms can transition from an environmental lifestyle to a pathogenic one1-3. This ecological switch typically occurs through the acquisition of horizontally acquired virulence genes4,5. However, the genomic features that must be present in a population before the acquisition of virulence genes and emergence of pathogenic clones remain unknown. We hypothesized that virulence adaptive polymorphisms (VAPs) circulate in environmental populations and are required for this transition. We developed a comparative genomic framework for identifying VAPs, using Vibrio cholerae as a model. We then characterized several environmental VAP alleles to show that while some of them reduced the ability of clinical strains to colonize a mammalian host, other alleles conferred efficient host colonization. These results show that VAPs are present in environmental bacterial populations before the emergence of virulent clones. We propose a scenario in which VAPs circulate in the environment and become selected and enriched under certain ecological conditions, and finally a genomic background containing several VAPs acquires virulence factors that allow for its emergence as a pathogenic clone.

The Vibrio cholerae Minor Pilin TcpB Initiates Assembly and Retraction of the Toxin-Coregulated Pilus.

Type IV pilus (T4P) systems are complex molecular machines that polymerize major pilin proteins into thin filaments displayed on bacterial surfaces. Pilus functions require rapid extension and depolymerization of the pilus, powered by the assembly and retraction ATPases, respectively. A set of low abundance minor pilins influences pilus dynamics by unknown mechanisms. The Vibrio cholerae toxin-coregulated pilus (TCP) is among the simplest of the T4P systems, having a single minor pilin TcpB and lacking a retraction ATPase. Here we show that TcpB, like its homolog CofB, initiates pilus assembly. TcpB co-localizes with the pili but at extremely low levels, equivalent to one subunit per pilus. We used a micropillars assay to demonstrate that TCP are retractile despite the absence of a retraction ATPase, and that retraction relies on TcpB, as a V. cholerae tcpB Glu5Val mutant is fully piliated but does not induce micropillars movements. This mutant is impaired in TCP-mediated autoagglutination and TcpF secretion, consistent with retraction being required for these functions. We propose that TcpB initiates pilus retraction by incorporating into the growing pilus in a Glu5-dependent manner, which stalls assembly and triggers processive disassembly. These results provide a framework for understanding filament dynamics in more complex T4P systems and the closely related Type II secretion system.

Effects of tcpB Mutations on Biogenesis and Function of the Toxin-Coregulated Pilus, the Type IVb Pilus of Vibrio cholerae.

UNLABELLED: Vibrio cholerae is the etiological agent of the acute intestinal disorder cholera. The toxin-coregulated pilus (TCP), a type IVb pilus, is an essential virulence factor of V. cholerae Recent work has shown that TcpB is a large minor pilin encoded within the tcp operon. TcpB contributes to efficient pilus formation and is essential for all TCP functions. Here, we have initiated a detailed targeted mutagenesis approach to further characterize this salient TCP component. We have identified (thus far) 20 residues of TcpB which affect either the steady-state level of TcpB or alter one or more TCP functions. This study provides a solid framework for further understanding of the complex role of TcpB and will be of use upon determination of the crystal structure of TcpB or related minor pilin orthologs of type IVb pilus systems. IMPORTANCE: Type IV pili, such as the toxin-coregulated pilus (TCP) in V. cholerae, are bacterial appendages that often act as essential virulence factors. Minor pilins, like TcpB, of these pili systems often play integral roles in pilus assembly and function. In this study, we have generated mutations in tcpB to determine residues of importance for TCP stability and function. Combined with a predicted tertiary structure, characterization of these mutants allows us to better understand critical residues in TcpB and the role they may play in the mechanisms underlying minor pilin functions.

Role of ToxS in the proteolytic cascade of virulence regulator ToxR in Vibrio cholerae.

Two of the primary virulence regulators of Vibrio cholerae, ToxR and TcpP, function together with cognate effector proteins. ToxR undergoes regulated intramembrane proteolysis (RIP) during late stationary phase in response to nutrient limitation at alkaline pH; however, the specific function of its cognate ToxS remains unresolved. In this work, we found that ToxR rapidly becomes undetectable in a ΔtoxS mutant when cultures are exposed to either starvation conditions or after alkaline pH shock individually. A ΔtoxS mutant enters into a dormant state associated with the proteolysis of ToxR at a faster rate than wild-type, closely resembling a ΔtoxR mutant. Using a mutant with a periplasmic substitution in ToxS, we found that the proteases DegS and DegP function additively with VesC and a novel protease, TapA, to degrade ToxR in the mutant. Overall, the results shown here reveal a role for ToxS in the stabilization of ToxR by protecting the virulence regulator from premature proteolysis.

Intestinal Colonization Dynamics of Vibrio cholerae.

To cause the diarrheal disease cholera, Vibrio cholerae must effectively colonize the small intestine. In order to do so, the bacterium needs to successfully travel through the stomach and withstand the presence of agents such as bile and antimicrobial peptides in the intestinal lumen and mucus. The bacterial cells penetrate the viscous mucus layer covering the epithelium and attach and proliferate on its surface. In this review, we discuss recent developments and known aspects of the early stages of V. cholerae intestinal colonization and highlight areas that remain to be fully understood. We propose mechanisms and postulate a model that covers some of the steps that are required in order for the bacterium to efficiently colonize the human host. A deeper understanding of the colonization dynamics of V. cholerae and other intestinal pathogens will provide us with a variety of novel targets and strategies to avoid the diseases caused by these organisms.

Proteolysis of virulence regulator ToxR is associated with entry of Vibrio cholerae into a dormant state.

Vibrio cholerae O1 is a natural inhabitant of aquatic environments and causes the diarrheal disease, cholera. Two of its primary virulence regulators, TcpP and ToxR, are localized in the inner membrane. TcpP is encoded on the Vibrio Pathogenicity Island (VPI), a horizontally acquired mobile genetic element, and functions primarily in virulence gene regulation. TcpP has been shown to undergo regulated intramembrane proteolysis (RIP) in response to environmental conditions that are unfavorable for virulence gene expression. ToxR is encoded in the ancestral genome and is present in non-pathogenic strains of V. cholerae, indicating it has roles outside of the human host. In this study, we show that ToxR undergoes RIP in V. cholerae in response to nutrient limitation at alkaline pH, a condition that occurs during the stationary phase of growth. This process involves the site-2 protease RseP (YaeL), and is dependent upon the RpoE-mediated periplasmic stress response, as deletion mutants for the genes encoding these two proteins cannot proteolyze ToxR under nutrient limitation at alkaline pH. We determined that the loss of ToxR, genetically or by proteolysis, is associated with entry of V. cholerae into a dormant state in which the bacterium is normally found in the aquatic environment called viable but nonculturable (VBNC). Strains that can proteolyze ToxR, or do not encode it, lose culturability, experience a change in morphology associated with cells in VBNC, yet remain viable under nutrient limitation at alkaline pH. On the other hand, mutant strains that cannot proteolyze ToxR remain culturable and maintain the morphology of cells in an active state of growth. Overall, our findings provide a link between the proteolysis of a virulence regulator and the entry of a pathogen into an environmentally persistent state.

The 40-residue insertion in Vibrio cholerae FadR facilitates binding of an additional fatty acyl-CoA ligand.

FadR is a master regulator of fatty acid metabolism and influences virulence in certain members of Vibrionaceae. Among FadR homologues of the GntR family, the Vibrionaceae protein is unusual in that it contains a C-terminal 40-residue insertion. Here we report the structure of Vibrio cholerae FadR (VcFadR) alone, bound to DNA, and in the presence of a ligand, oleoyl-CoA. Whereas Escherichia coli FadR (EcFadR) contains only one acyl-CoA-binding site in each monomer, crystallographic and calorimetric data indicate that VcFadR has two. One of the binding sites resembles that of EcFadR, whereas the other, comprised residues from the insertion, has not previously been observed. Upon ligand binding, VcFadR undergoes a dramatic conformational change that would more fully disrupt DNA binding than EcFadR. These findings suggest that the ability to bind and respond to an additional ligand allows FadR from Vibrionaceae to function as a more efficient regulator.

Secretion of TcpF by the Vibrio cholerae toxin-coregulated pilus biogenesis apparatus requires an N-terminal determinant.

Type IV pili are important for microcolony formation, biofilm formation, twitching motility, and attachment. We and others have shown that type IV pili are important for protein secretion across the outer membrane, similar to type II secretion systems. This study explored the relationship between protein secretion and pilus formation in Vibrio cholerae. The toxin-coregulated pilus (TCP), a type IV pilus required for V. cholerae pathogenesis, is necessary for the secretion of the colonization factor TcpF (T. J. Kirn, N. Bose, and R. K. Taylor, Mol. Microbiol. 49:81-92, 2003). This phenomenon is not unique to V. cholerae; secreted virulence factors that are dependent on the presence of components of the type IV pilus biogenesis apparatus for secretion have been reported with Dichelobacter nodosus (R. M. Kennan, O. P. Dhungyel, R. J. Whittington, J. R. Egerton, and J. I. Rood, J. Bacteriol. 183:4451-4458, 2001) and Francisella tularensis (A. J. Hager et al., Mol. Microbiol. 62:227-237, 2006). Using site-directed mutagenesis, we demonstrated that the secretion of TcpF is dependent on the presence of selected amino acid R groups at position five. We were unable to find other secretion determinants, suggesting that Y5 is the major secretion determinant within TcpF. We also report that proteins secreted in a type IV pilus biogenesis apparatus-dependent manner have a YXS motif within the first 15 amino acids following the Sec cleavage site. The YXS motif is not present in proteins secreted by type II secretion systems, indicating that this is unique to type IV pilus-mediated secretion. Moreover, we show that TcpF interacts with the pilin TcpA, suggesting that these proteins are secreted by the type IV pilus biogenesis system. These data provide a starting point for understanding how type IV pili can mediate secretion of virulence factors important for bacterial pathogenesis.

Cholera: Environmental Reservoirs and Impact on Disease Transmission.

Vibrio cholerae is widely known to be the etiological agent of the life-threatening diarrheal disease cholera. Cholera remains a major scourge in many developing countries, infecting hundreds of thousands every year. Remarkably, V. cholerae is a natural inhabitant of brackish riverine, estuarine, and coastal waters, and only a subset of strains are known to be pathogenic to humans. Recent studies have begun to uncover a very complex network of relationships between V. cholerae and other sea dwellers, and the mechanisms associated with the occurrence of seasonal epidemics in regions where cholera is endemic are beginning to be elucidated. Many of the factors required for the organism's survival and persistence in its natural environment have been revealed, as well as the ubiquitous presence of horizontal gene transfer in the emergence of pathogenic strains of V. cholerae. In this article, we will focus on the environmental stage of pathogenic V. cholerae and the interactions of the microorganism with other inhabitants of aquatic environments. We will discuss the impact that its environmental reservoirs have on disease transmission and the distinction between reservoirs of V. cholerae and the vectors that establish cholera as a zoonosis.

Cholera: Environmental Reservoirs and Impact on Disease Transmission.

Vibrio cholerae is widely known to be the etiological agent of the life-threatening diarrheal disease cholera. Cholera remains a major scourge in many developing countries, infecting hundreds of thousands every year. Remarkably, V. cholerae is a natural inhabitant of brackish riverine, estuarine, and coastal waters, and only a subset of strains are known to be pathogenic to humans. Recent studies have begun to uncover a very complex network of relationships between V. cholerae and other sea dwellers, and the mechanisms associated with the occurrence of seasonal epidemics in regions where cholera is endemic are beginning to be elucidated. Many of the factors required for the organism's survival and persistence in its natural environment have been revealed, as well as the ubiquitous presence of horizontal gene transfer in the emergence of pathogenic strains of V. cholerae. In this article, we will focus on the environmental stage of pathogenic V. cholerae and the interactions of the microorganism with other inhabitants of aquatic environments. We will discuss the impact that its environmental reservoirs have on disease transmission and the distinction between reservoirs of V. cholerae and the vectors that establish cholera as a zoonosis.

Characterization of BreR interaction with the bile response promoters breAB and breR in Vibrio cholerae.

The Vibrio cholerae BreR protein is a transcriptional repressor of the breAB efflux system operon, which encodes proteins involved in bile resistance. In a previous study (F. A. Cerda-Maira, C. S. Ringelberg, and R. K. Taylor, J. Bacteriol. 190:7441-7452, 2008), we used gel mobility shift assays to determine that BreR binds at two independent binding sites at the breAB promoter and a single site at its own promoter. Here it is shown, by DNase I footprinting and site-directed mutagenesis, that BreR is able to bind at a distal and a proximal site in the breAB promoter. However, only one of these sites, the proximal 29-bp site, is necessary for BreR-mediated transcriptional repression of breAB expression. In addition, it was determined that BreR represses its own expression by recognizing a 28-bp site at the breR promoter. These sites comprise regions of dyad symmetry within which residues critical for BreR function could be identified. The BreR consensus sequence AANGTANAC-N(6)-GTNTACNTT overlaps the -35 region at both promoters, implying that the repression of gene expression is achieved by interfering with RNA polymerase binding at these promoters.

Growth and maintenance of Escherichia coli laboratory strains.

Escherichia coli is a Gram-negative bacterium, commonly used in both teaching and research laboratories. This unit includes protocols for the growth and maintenance of E. coli in any teaching- or research-associated laboratory.

The crystal structure of AphB, a virulence gene activator from Vibrio cholerae, reveals residues that influence its response to oxygen and pH.

Expression of the two critical virulence factors of Vibrio cholerae, toxin-coregulated pilus and cholera toxin, is initiated at the tcpPH promoter by the regulators AphA and AphB. AphA is a winged helix DNA-binding protein that enhances the ability of AphB, a LysR-type transcriptional regulator, to activate tcpPH expression. We present here the 2.2 Å X-ray crystal structure of full-length AphB. As reported for other LysR-type proteins, AphB is a tetramer with two distinct subunit conformations. Unlike other family members, AphB must undergo a significant conformational change in order to bind to DNA. We have found five independent mutations in the putative ligand-binding pocket region that allow AphB to constitutively activate tcpPH expression at the non-permissive pH of 8.5 and in the presence of oxygen. These findings indicate that AphB is responsive to intracellular pH as well as to anaerobiosis and that residues in the ligand-binding pocket of the protein influence its ability to respond to both of these signals. We have solved the structure of one of the constitutive mutants, and observe conformational changes that would allow DNA binding. Taken together, these results describe a pathway of conformational changes allowing communication between the ligand and DNA binding regions of AphB.