Chemotaxis in Vibrio cholerae

Regulatory mechanisms of two-component systems in Vibrio cholerae: Enhancing pathogenicity and environmental adaptation

Cholera, which is caused by the bacterium Vibrio cholerae, is a highly dangerous disease characterized by severe symptoms such as watery diarrhea, dehydration, and even death. V. cholerae can both colonize the host intestine and survive in environmental reservoirs. Two-component systems (TCSs) are essential regulatory mechanisms that allow bacteria to adapt to changing environments. This review focuses on the regulatory mechanisms of TCS-mediated gene expression in V. cholerae. We first summarize the composition and classification of TCSs in V. cholerae N16961. We then discuss the roles of TCSs in facilitating adaptation to diverse environmental stimuli and increasing pathogenicity. Furthermore, we analyze the distribution of TCSs in pandemic and nonpandemic-V. cholerae strains, demonstrating their indispensable role in promoting virulence and facilitating the widespread dissemination of pandemic strains. Elucidation of these mechanisms is crucial for devising new strategies to combat cholera and prevent future outbreaks, ultimately contributing to improved public health outcomes.

Coordinated regulation of chemotaxis and resistance to copper by CsoR in Pseudomonas putida

Copper is an essential enzyme cofactor in bacteria, but excess copper is highly toxic. Bacteria can cope with copper stress by increasing copper resistance and initiating chemorepellent response. However, it remains unclear how bacteria coordinate chemotaxis and resistance to copper. By screening proteins that interacted with the chemotaxis kinase CheA, we identified a copper-binding repressor CsoR that interacted with CheA in Pseudomonas putida. CsoR interacted with the HPT (P1), Dimer (P3), and HATPase_c (P4) domains of CheA and inhibited CheA autophosphorylation, resulting in decreased chemotaxis. The copper-binding of CsoR weakened its interaction with CheA, which relieved the inhibition of chemotaxis by CsoR. In addition, CsoR bound to the promoter of copper-resistance genes to inhibit gene expression, and copper-binding released CsoR from the promoter, leading to increased gene expression and copper resistance. P. putida cells exhibited a chemorepellent response to copper in a CheA-dependent manner, and CsoR inhibited the chemorepellent response to copper. Besides, the CheA-CsoR interaction also existed in proteins from several other bacterial species. Our results revealed a mechanism by which bacteria coordinately regulated chemotaxis and resistance to copper by CsoR.

Non-O1/Non-O139 Vibrio cholerae-An Underestimated Foodborne Pathogen? An Overview of Its Virulence Genes and Regulatory Systems Involved in Pathogenesis

In recent years, the number of foodborne infections with non-O1 and non-O139 Vibrio cholerae (NOVC) has increased worldwide. These have ranged from sporadic infection cases to localized outbreaks. The majority of case reports describe self-limiting gastroenteritis. However, severe gastroenteritis and even cholera-like symptoms have also been described. All reported diarrheal cases can be traced back to the consumption of contaminated seafood. As climate change alters the habitats and distribution patterns of aquatic bacteria, there is a possibility that the number of infections and outbreaks caused by Vibrio spp. will further increase, especially in countries where raw or undercooked seafood is consumed or clean drinking water is lacking. Against this background, this review article focuses on a possible infection pathway and how NOVC can survive in the human host after oral ingestion, colonize intestinal epithelial cells, express virulence factors causing diarrhea, and is excreted by the human host to return to the environment.

PlzD modifies Vibrio vulnificus foraging behavior and virulence in response to elevated c-di-GMP

IMPORTANCE

Many free-swimming bacteria propel themselves through liquid using rotary flagella, and mounting evidence suggests that the inhibition of flagellar rotation initiates biofilm formation, a sessile lifestyle that is a nearly universal surface colonization paradigm in bacteria. In general, motility and biofilm formation are inversely regulated by the intracellular second messenger bis-(3´-5´)-cyclic dimeric guanosine monophosphate (c-di-GMP). Here, we identify a protein, PlzD, bearing a conserved c-di-GMP binding PilZ domain that localizes to the flagellar pole in a c-di-GMP-dependent manner and alters the foraging behavior, biofilm, and virulence characteristics of the opportunistic human pathogen, Vibrio vulnificus. Our data suggest that PlzD interacts with components of the flagellar stator to decrease bacterial swimming speed and changes in swimming direction, and these activities are enhanced when cellular c-di-GMP levels are elevated. These results reveal a physical link between a second messenger (c-di-GMP) and an effector (PlzD) that promotes transition from a motile to a sessile state in V. vulnificus.

D-amino acids signal a stress-dependent run-away response in Vibrio cholerae

To explore favourable niches while avoiding threats, many bacteria use a chemotaxis navigation system. Despite decades of studies on chemotaxis, most signals and sensory proteins are still unknown. Many bacterial species release D-amino acids to the environment; however, their function remains largely unrecognized. Here we reveal that D-arginine and D-lysine are chemotactic repellent signals for the cholera pathogen Vibrio cholerae. These D-amino acids are sensed by a single chemoreceptor MCPDRK co-transcribed with the racemase enzyme that synthesizes them under the control of the stress-response sigma factor RpoS. Structural characterization of this chemoreceptor bound to either D-arginine or D-lysine allowed us to pinpoint the residues defining its specificity. Interestingly, the specificity for these D-amino acids appears to be restricted to those MCPDRK orthologues transcriptionally linked to the racemase. Our results suggest that D-amino acids can shape the biodiversity and structure of complex microbial communities under adverse conditions.

The Aer2 chemoreceptor from Vibrio vulnificus is a tri-PAS-heme oxygen sensor

The marine pathogen Vibrio vulnificus senses and responds to environmental stimuli via two chemosensory systems and 42-53 chemoreceptors. Here, we present an analysis of the V. vulnificus Aer2 chemoreceptor, VvAer2, which is the first V. vulnificus chemoreceptor to be characterized. VvAer2 is related to the Aer2 receptors of other gammaproteobacteria, but uncharacteristically contains three PAS domains (PAS1-3), rather than one or two. Using an E. coli chemotaxis hijack assay, we determined that VvAer2, like other Aer2 receptors, senses and responds to O₂ . All three VvAer2 PAS domains bound pentacoordinate b-type heme and exhibited similar O₂ affinities. PAS2 and PAS3 both stabilized O₂ via conserved Iβ-Trp residues, but PAS1, which was easily oxidized in vitro, was unaffected by Iβ-Trp replacement. Our results support a model in which PAS1 is largely dispensable for O₂ -mediated signaling, whereas PAS2 modulates PAS3 signaling, and PAS3 signals to the downstream domains. Each PAS domain appeared to be positionally optimized, because PAS swapping caused altered signaling properties, and neither PAS1 nor PAS2 could replace PAS3. Our findings strengthen previous conclusions that Aer2 receptors are O₂ sensors, but with distinct N-terminal domain arrangements that facilitate, modulate and tune responses based on environmental signals.

PAS Domain-Containing Chemoreceptors Influence the Signal Sensing and Intestinal Colonization of Vibrio cholerae

Bacterial chemotaxis is the phenomenon in which bacteria migrate toward a more favorable niche in response to chemical cues in the environment. The methyl-accepting chemotaxis proteins (MCPs) are the principal sensory receptors of the bacterial chemotaxis system. Aerotaxis is a special form of chemotaxis in which oxygen serves as the signaling molecule; the process is dependent on the aerotaxis receptors (Aer) containing the Per-Arnt-Sim (PAS) domain. Over 40 MCPs are annotated on the genome of Vibrio cholerae; however, little is known about their functions. We investigated six MCPs containing the PAS domain in V. cholerae El Tor C6706, namely aer2, aer3, aer4, aer5, aer6, and aer7. Deletion analyses of each aer homolog gene indicated that these Aer receptors are involved in aerotaxis, chemotaxis, biofilm formation, and intestinal colonization. Swarming motility assay indicated that the aer2 gene was responsible for sensing the oxygen gradient independent of the other five homologs. When bile salts and mucin were used as chemoattractants, each Aer receptor influenced the chemotaxis differently. Biofilm formation was enhanced by overexpression of the aer6 and aer7 genes. Moreover, deletion of the aer2 gene resulted in better bacterial colonization of the mutant in adult mice; however, virulence gene expression was unaffected. These data suggest distinct roles for different Aer homologs in V. cholerae physiology.

Impact of Gene Repression on Biofilm Formation of Vibrio cholerae

Vibrio cholerae, the etiological agent of cholera, is a facultative intestinal pathogen which can also survive in aquatic ecosystems in the form of biofilms, surface-associated microbial aggregates embedded in an extracellular matrix, which protects them from predators and hostile environmental factors. Biofilm-derived bacteria and biofilm aggregates are considered a likely source for cholera infections, underscoring the importance of V. cholerae biofilm research not just to better understand bacterial ecology, but also cholera pathogenesis in the human host. While several studies focused on factors induced during biofilm formation, genes repressed during this persistence stage have been fairly neglected. In order to complement these previous studies, we used a single cell-based transcriptional reporter system named TetR-controlled recombination-based in-biofilm expression technology (TRIBET) and identified 192 genes to be specifically repressed by V. cholerae during biofilm formation. Predicted functions of in-biofilm repressed (ibr) genes range from metabolism, regulation, surface association, transmembrane transport as well as motility and chemotaxis. Constitutive (over)-expression of these genes affected static and dynamic biofilm formation of V. cholerae at different stages. Notably, timed expression of one candidate in mature biofilms induced their rapid dispersal. Thus, genes repressed during biofilm formation are not only dispensable for this persistence stage, but their presence can interfere with ordered biofilm development. This work thus contributes new insights into gene silencing during biofilm formation of V. cholerae.

The global regulators ArcA and CytR collaboratively modulate Vibrio cholerae motility

Background

Vibrio cholerae, a Gram-negative bacterium, is highly motile owing to the presence of a single polar flagellum. The global anaerobiosis response regulator, ArcA regulates the expression of virulence factors and enhance biofilm formation in V. cholerae. However, the function of ArcA for the motility of V. cholerae is yet to be elucidated. CytR, which represses nucleoside uptake and catabolism, is known to play a chief role in V. cholerae pathogenesis and flagellar synthesis but the mechanism that CytR influences motility is unclear.

Results

In this study, we found that the ΔarcA mutant strain exhibited higher motility than the WT strain due to ArcA directly repressed flrA expression. We further discovered that CytR directly enhanced fliK expression, which explained why the ΔcytR mutant strain was retarded in motility. On the other hand, cytR was a direct ArcA target and cytR expression was directly repressed by ArcA. As expected, cytR expression was down-regulated.

Conclusions

Overall, ArcA plays a critical role in V. cholerae motility by regulating flrA expression directly and fliK indirectly in the manner of cytR.

Bioactive small molecules produced by the human gut microbiome modulate Vibrio cholerae sessile and planktonic lifestyles

Humans live in symbiosis with a diverse community of microorganisms, which has evolved to carry out many specific tasks that benefit the host, including protection against invading pathogens. Within the chemical diversity of the gastrointestinal tract, small molecules likely constitute chemical cues for the communication between the microbiota and pathogens. Therefore, we sought to investigate if molecules produced by the human gut microbiota show biological activity against the human pathogen Vibrio cholerae. To probe the effects of the gut metabolome on V. cholerae, we investigated its response to small-molecule extracts from human feces, from a complex bacterial community cultivated in vitro, and from culture supernatants of Enterocloster citroniae, Bacteroides thetaiotaomicron, and Bacteroides vulgatus. Using RNA sequencing, we determined the impact of the human gut metabolome on V. cholerae global gene expression. Among the genes downregulated in the presence of the fecal extract, the most overrepresented functional category was cell motility, which accounted for 39% of repressed genes. Repression of V. cholerae motility by the fecal extract was confirmed phenotypically, and E. citroniae extracts reproduced this phenotype. A complex in vitro microbial community led to increased motility, as did extracts from B. vulgatus, a species present in this community. Accordingly, mucin penetration was also repressed by fecal and E. citroniae extracts, suggesting that the phenotypes observed may have implications for host colonization. Together with previous studies, this work shows that small molecules from the gut metabolome may have a widespread, significant impact on microbe-microbe interactions established in the gut environment.

First insights into a pyruvate sensing and uptake system in Vibrio campbellii and its importance for virulence

Pyruvate is a key metabolite in living cells and has been shown to play a crucial role in the virulence of several bacterial pathogens. The bioluminescent Vibrio campbellii, a severe infectious burden for marine aquaculture, excretes extraordinarily large amounts of pyruvate during growth and rapidly retrieves it by an as-yet unknown mechanism. We have now identified the responsible pyruvate transporter, here named BtsU, and our results show that it is the only pyruvate transporter in V. campbellii. Expression of btsU is tightly regulated by the membrane-integrated LytS-type histidine kinase BtsS, a sensor for extracellular pyruvate, and the LytTR-type response regulator BtsR. Cells lacking either the pyruvate transporter or sensing system show no chemotactic response towards pyruvate, indicating that intracellular pyruvate is required to activate the chemotaxis system. Moreover, pyruvate sensing and uptake were found to be important for the resuscitation of V. campbellii from the viable but nonculturable (VBNC) state and the bacterium's virulence against brine shrimp larvae. IMPORTANCE Bacterial infections are a serious threat to marine aquaculture, one of the fastest growing food sectors on earth. Therefore, it is extremely important to learn more about the pathogens responsible, one of which is Vibrio campbellii. This study sheds light on the importance of pyruvate sensing and uptake for V. campbellii, and reveals that the bacterium possesses only one pyruvate transporter, which is activated by a pyruvate-responsive histidine kinase/response regulator system. Without the ability to sense or take up pyruvate, the virulence of V. campbellii towards gnotobiotic brine shrimp larvae is strongly reduced.

The polar flagellar transcriptional regulatory network in Vibrio campbellii deviates from canonical Vibrio species

Swimming motility is a critical virulence factor in pathogenesis for numerous Vibrio species. Vibrio campbellii DS40M4 is a wild isolate that has been recently established as a highly tractable model strain for bacterial genetics studies. We sought to exploit the tractability and relevance of this strain for characterization of flagellar gene regulation in V. campbellii. Using comparative genomics, we identified homologs of V. campbellii flagellar and chemotaxis genes conserved in other members of the Vibrionaceae and determined the transcriptional profile of these loci using differential RNA-seq. We systematically deleted all 63 predicted flagellar and chemotaxis genes in V. campbellii and examined their effects on motility and flagellum production. We specifically focused on the core regulators of the flagellar hierarchy established in other vibrios: RpoN (σ⁵⁴), FlrA, FlrC, and FliA. Our results show that V. campbellii transcription of flagellar and chemotaxis genes is governed by a multi-tiered regulatory hierarchy similar to other motile Vibrio species. However, there are several critical differences in V. campbellii: (i) the σ⁵⁴-dependent regulator FlrA is dispensable for motility, (ii) the flgA, fliEFGHIJ, flrA, and flrBC operons do not require σ⁵⁴ for expression, and (iii) FlrA and FlrC co-regulate class II genes. Our model proposes that the V. campbellii flagellar transcriptional hierarchy has three classes of genes, in contrast to the four-class hierarchy in Vibrio cholerae. Our genetic and phenotypic dissection of the V. campbellii flagellar regulatory network highlights the differences that have evolved in flagellar regulation across the Vibrionaceae. Importance Vibrio campbellii is a Gram-negative bacterium that is free-living and ubiquitous in marine environments and is an important global pathogen of fish and shellfish. Disruption of the flagellar motor significantly decreases host mortality of V. campbellii, suggesting that motility is a key factor in pathogenesis. Using this model organism, we identified >60 genes that encode proteins with predicted structural, mechanical, or regulatory roles in function of the single polar flagellum in V. campbellii. We systematically tested strains containing single deletions of each gene to determine the impact on motility and flagellum production. Our studies have uncovered differences in the regulatory network and function of several genes in V. campbellii as compared to established systems in Vibrio cholerae and Vibrio parahaemolyticus.

Clostridioides difficile Single Cell Swimming Strategy: A Novel Motility Pattern Regulated by Viscoelastic Properties of the Environment

Flagellar motility is important for the pathogenesis of many intestinal pathogens, allowing bacteria to move to their preferred ecological niche. Clostridioides difficile is currently the major cause for bacterial health care-associated intestinal infections in the western world. Most clinical strains produce peritrichous flagella and are motile in soft-agar. However, little knowledge exists on the C. difficile swimming behaviour and its regulation at the level of individual cells. We report here on the swimming strategy of C. difficile at the single cell level and its dependency on environmental parameters. A comprehensive analysis of motility parameters from several thousand bacteria was achieved with the aid of a recently developed bacterial tracking programme. C. difficile motility was found to be strongly dependent on the matrix elasticity of the medium. Long run phases of all four motile C. difficile clades were only observed in the presence of high molecular weight molecules such as polyvinylpyrrolidone (PVP) and mucin, which suggests an adaptation of the motility apparatus to the mucin-rich intestinal environment. Increasing mucin or PVP concentrations lead to longer and straighter runs with increased travelled distance per run and fewer turnarounds that result in a higher net displacement of the bacteria. The observed C. difficile swimming pattern under these conditions is characterised by bidirectional, alternating back and forth run phases, interrupted by a short stop without an apparent reorientation or tumbling phase. This motility type was not described before for peritrichous bacteria and is more similar to some previously described polar monotrichous bacteria.

Vibrio cholerae's mysterious Seventh Pandemic island (VSP-II) encodes novel Zur-regulated zinc starvation genes involved in chemotaxis and cell congregation

Vibrio cholerae is the causative agent of cholera, a notorious diarrheal disease that is typically transmitted via contaminated drinking water. The current pandemic agent, the El Tor biotype, has undergone several genetic changes that include horizontal acquisition of two genomic islands (VSP-I and VSP-II). VSP presence strongly correlates with pandemicity; however, the contribution of these islands to V. cholerae's life cycle, particularly the 26-kb VSP-II, remains poorly understood. VSP-II-encoded genes are not expressed under standard laboratory conditions, suggesting that their induction requires an unknown signal from the host or environment. One signal that bacteria encounter under both host and environmental conditions is metal limitation. While studying V. cholerae's zinc-starvation response in vitro, we noticed that a mutant constitutively expressing zinc starvation genes (Δzur) congregates at the bottom of a culture tube when grown in a nutrient-poor medium. Using transposon mutagenesis, we found that flagellar motility, chemotaxis, and VSP-II encoded genes were required for congregation. The VSP-II genes encode an AraC-like transcriptional activator (VerA) and a methyl-accepting chemotaxis protein (AerB). Using RNA-seq and lacZ transcriptional reporters, we show that VerA is a novel Zur target and an activator of the nearby AerB chemoreceptor. AerB interfaces with the chemotaxis system to drive oxygen-dependent congregation and energy taxis. Importantly, this work suggests a functional link between VSP-II, zinc-starved environments, and energy taxis, yielding insights into the role of VSP-II in a metal-limited host or aquatic reservoir.

RNA-mediated control of cell shape modulates antibiotic resistance in Vibrio cholerae

Vibrio cholerae, the cause of cholera disease, exhibits a characteristic curved rod morphology, which promotes infectivity and motility in dense hydrogels. Periplasmic protein CrvA determines cell curvature in V. cholerae, yet the regulatory factors controlling CrvA are unknown. Here, we discover the VadR small RNA (sRNA) as a post-transcriptional inhibitor of the crvA mRNA. Mutation of vadR increases cell curvature, whereas overexpression has the inverse effect. We show that vadR transcription is activated by the VxrAB two-component system and triggered by cell-wall-targeting antibiotics. V. cholerae cells failing to repress crvA by VadR display decreased survival upon challenge with penicillin G indicating that cell shape maintenance by the sRNA is critical for antibiotic resistance. VadR also blocks the expression of various key biofilm genes and thereby inhibits biofilm formation in V. cholerae. Thus, VadR is an important regulator for synchronizing peptidoglycan integrity, cell shape, and biofilm formation in V. cholerae.

Vibrio cholerae Type VI Activity Alters Motility Behavior in Mucin

Motility is required for many bacterial pathogens to reach and colonize target sites. Vibrio cholerae traverses a thick mucus barrier coating the small intestine to reach the underlying epithelium. We screened a transposon library in motility medium containing mucin to identify factors that influence mucus transit. Lesions in structural genes of the type VI secretion system (T6SS) were among those recovered. Two-dimensional (2D) and 3D single-cell tracking was used to compare the motility behaviors of wild-type cells and a mutant that collectively lacked three essential T6SS structural genes (T6SS^-). In the absence of mucin, wild-type and T6SS^- cells exhibited similar speeds and run-reverse-flick (RRF) swimming patterns, in which forward-moving cells briefly backtrack before stochastically reorienting (flicking) in a new direction upon resuming forward movement. We show that mucin induced T6SS expression and activity in wild-type bacteria but significantly decreased their swimming speed and flicking, yielding curvilinear or near-surface circular traces for many cells. Conversely, mucin slowed T6SS^- cells to a lesser extent, and many continued to flick and produce RRF-like traces. ΔcheY3 cells, which exclusively swim in the forward direction and thus cannot flick, also produced curvilinear traces with or without mucin present and, on occasion, near-surface circular traces in the presence of mucin. The dependence of flicking on swimming speed suggested that mucin-induced T6SS activity further decreased V. cholerae motility and thereby reduced flicking probability during reverse-to-forward transitions. We propose that this encourages cells to continue on their current trajectory rather than reorienting, which may benefit those tracking toward the epithelial surface.IMPORTANCE V. cholerae deploys an arsenal of virulence factors as it attempts to traverse a protective mucus layer and reach the epithelial surface of the distal small intestine. The T6SS used to cull bacterial competition during infection is induced by mucus. We show that this activity may serve an additional purpose by further decreasing motility in the presence of mucin, thereby reducing the probability of speed-dependent, near-perpendicular directional changes. We posit that this encourages cells to maintain course rather than change direction, which may aid those attempting to reach and colonize the epithelial surface.

Exploring Computational and Biophysical Tools to Study the Presence of G-Quadruplex Structures: A Promising Therapeutic Solution for Drug-Resistant Vibrio cholerae

Vibrio cholerae, a gram-negative bacterium that causes cholera, has already caused seven major pandemics across the world and infects roughly 1.3–4 million people every year. Cholera treatment primarily involves oral rehydration therapy supplemented with antibiotics. But recently, multidrug-resistant strains of V. cholerae have emerged. High genomic plasticity further enhances the pathogenesis of this human pathogen. Guanines in DNA or RNA assemble to form G-quadruplex (GQ) structures which have begun to be seen as potential drug targeting sites for different pathogenic bacteria and viruses. In this perspective, we carried out a genome-wide hunt in V. cholerae using a bio-informatics approach and observed ∼85 G-quadruplex forming motifs (VC-PGQs) in chromosome I and ∼45 putative G-quadruplexs (PGQs) in chromosome II. Ten putative G-quadruplex forming motifs (VC-PGQs) were selected on the basis of conservation throughout the genus and functional analysis displayed their location in the essential genes encoding bacterial proteins, for example, methyl-accepting chemotaxis protein, orotate phosphoribosyl transferase protein, amidase proteins, etc. The predicted VC-PGQs were validated using different bio-physical techniques, including Nuclear Magnetic Resonance spectroscopy, Circular Dichroism spectroscopy, and electrophoretic mobility shift assay, which demonstrated the formation of highly stable GQ structures in the bacteria. The interaction of these VC-PGQs with the known specific GQ ligand, TMPyP4, was analyzed using ITC and molecular dynamics studies that displayed the stabilization of the VC-PGQs by the GQ ligands and thus represents a potential therapeutic strategy against this enteric pathogen by inhibiting the PGQ harboring gene expression, thereby inhibiting the bacterial growth and virulence. In summary, this study reveals the presence of conserved GQ forming motifs in the V. cholerae genome that has the potential to be used to treat the multi-drug resistance problem of the notorious enteric pathogen.

Motility of Vibrio spp.: regulation and controlling strategies

Flagellar motility in bacteria is a highly regulated and complex cellular process that requires high energy investment for movement and host colonization. Motility plays an important role in the lifestyle of Vibrio spp. in the aquatic environment and during host colonization. Flagellar motility in vibrios is associated with several cellular processes, such as movement, colonization, adhesion, biofilm formation, and virulence. The transcription of all flagella-related genes occurs hierarchically and is regulated positively or negatively by several transcription factors and regulatory proteins. The flagellar regulatory hierarchy is well studied in Vibrio cholerae and Vibrio parahaemolyticus. Here, we compared the regulatory cascade and molecules involved in the flagellar motility of V. cholerae and V. parahaemolyticus in detail. The evolutionary relatedness of the master regulator of the polar and lateral flagella in different Vibrio species is also discussed. Although they can form symbiotic associations of some Vibrio species with humans and aquatic organisms can be harmed by several species of Vibrio as a result of surface contact, characterized by flagellar movement. Thus, targeting flagellar motility in pathogenic Vibrio species is considered a promising approach to control Vibrio infections. This approach, along with the strategies for controlling flagellar motility in different species of Vibrio using naturally derived and chemically synthesized compounds, is discussed in this review. KEY POINTS: • Vibrio species are ubiquitous and distributed across the aquatic environments. • The flagellar motility is responsible for the chemotactic movement and initial colonization to the host. • The transition from the motile into the biofilm stage is one of the crucial events in the infection. • Several signaling pathways are involved in the motility and formation of biofilm. • Attenuation of motility by naturally derived or chemically synthesized compounds could be a potential treatment for preventing Vibrio biofilm-associated infections.

Transcriptional profiling of Vibrio cholerae O1 following exposure to human anti- lipopolysaccharide monoclonal antibodies

Following an episode of cholera, a rapidly dehydrating, watery diarrhea caused by the Gram-negative bacterium, Vibrio cholerae O1, humans mount a robust anti-lipopolysaccharide (LPS) antibody response that is associated with immunity to subsequent re-infection. In neonatal mouse and rabbit models of cholera, passively administered anti-LPS polyclonal and monoclonal (MAb) antibodies reduce V. cholerae colonization of the intestinal epithelia by inhibiting bacterial motility and promoting vibrio agglutination. Here we demonstrate that human anti-LPS IgG MAbs also arrest V. cholerae motility and induce bacterial paralysis. A subset of those MAbs also triggered V. cholerae to secrete an extracellular matrix (ECM). To identify changes in gene expression that accompany antibody exposure and that may account for motility arrest and ECM production, we subjected V. cholerae O1 El Tor to RNA-seq analysis after treatment with ZAC-3 IgG, a high affinity MAb directed against the core/lipid A region of LPS. We identified > 160 genes whose expression was altered following ZAC-3 IgG treatment, although canonical outer membrane stress regulons were not among them. ompS (VCA1028), a porin associated with virulence and indirectly regulated by ToxT, and norR (VCA0182), a σ54-dependent transcription factor involved in late stages of infection, were two upregulated genes worth noting.

Phenotypic and Genotypic Characterization of Veterinary Vibrio cincinnatiensis Isolates

Vibrio cincinnatiensis is a halophilic species which has been found in marine and estuarine environments worldwide. The species is considered a rare pathogen for which the significance for humans is unclear. In this study, nine veterinary isolates were investigated that were obtained from domestic animals in Germany. The isolates were mostly recovered from abortion material of pigs, cattle, and horse (amnion or fetuses). One isolate was from a goose. A human clinical strain from a case of enteritis in Germany described in the literature was also included in the study. Whole-genome sequencing (WGS) of all isolates and MALDI-TOF MS (matrix-assisted-laser-desorption/ionization time-of-flight mass spectrometry) were performed to verify the species assignment. All strains were investigated for phenotypic traits including antimicrobial resistance (AMR), biochemical properties, and two virulence-associated phenotypes (hemolytic activity and resistance to human serum). WGS data and MS spectra confirmed that all veterinary isolates are closely related to the type strain V. cincinnatiensis NCTC12012. An exception was the human isolate from Germany which is related to the other isolates but could belong to another species. The isolates were similar in most biochemical phenotypes. Only one strain showed a very weak hemolytic activity against sheep erythrocytes, and serum resistance was intermediate in two strains. AMR phenotypes were more variable between the isolates. Resistances were observed against ß-lactams ampicillin and cefoxitin and against tetracycline and the sulfonamide antibiotics trimethoprim and sulfamethoxazole. Some acquired AMR genes were identified by bioinformatics analyses. WGS and MALDI-TOF MS data reveal a close relationship of the veterinary isolates and the type strain V. cincinnatiensis NCTC12012, which is a clinical human isolate. As the veterinary isolates of this study were mostly recovered from abortion material (amnions and fetuses), a zoonotic potential of the veterinary isolates seems possible.

Structural basis of the binding affinity of chemoreceptors Mlp24p and Mlp37p for various amino acids

Environmental sensing is crucial for bacterial survival and pathogenicity. Bacteria sense environmental chemicals using chemoreceptor proteins, such as Methyl-accepting Chemotaxis Proteins (MCPs). Vibrio cholerae, the etiological agent of cholera, has at least 44 chemoreceptor proteins homologous to MCP-Like Proteins (MLPs). Mlp24 and Mlp37 are dCACHE type chemoreceptors that senses various amino acids. Mlp24 is important for cholera toxin production, whereas Mlp37 is related to biofilm formation. The periplasmic ligand binding regions of Mlp24 and Mlp37 (Mlp24p and Mlp37p, respectively) share similar amino acid sequences, tertiary and quaternary structures, and a common mechanism for the ligand amino acid backbone recognition. However, Mlp37p recognizes various l-amino acids and taurine with similar affinity whereas Mlp24p shows different binding affinities for various l-amino acids and does not bind taurine. Here we solved the crystal structure of Mlp37p in complex with l-arginine and compared it with previously determined structures of Mlp37p, Mlp24p and their ligand complexes. We found that Mlp37p changes the conformation of the loop that forms the upper wall of the ligand binding pocket according to size and shape of the ligand, and thereby show similar affinity for various ligands.

Expanding dynamics of the virulence-related gene variations in the toxigenic Vibrio cholerae serogroup O1

Background

Toxigenic Vibrio cholerae serogroup O1 is the causative pathogen in the sixth and seventh cholera pandemics. Cholera toxin is the major virulent factor but other virulence and virulence-related factors play certain roles in the pathogenesis and survival in the host. Along with the evolution of the epidemic strains, the virulence-related genes also experience variation, gain and loss, and lead to genetic divergence in different strains.

Results

In this study, we analyzed the virulence-related gene profiles in the toxigenic serogroup O1 strains isolated from 1923 to 2015, the genomes of which were publicly available. The virulence-related genes of the V. cholerae O1 strains were annotated based on the Virulence Factors Database (VFDB). An average of 230.1 virulence-related genes per strain were identified; significant differences in the average numbers were found between the classical and El Tor biotypes, and increasing trends in the number of virulence-related genes along with the isolation years were observed in the El Tor biotype strains. A total of 176 homologs of virulence-related genes were found from these strains, of which 25 belonged to the core genes, suggesting their conservative and necessary roles in V. cholerae pathogenesis. We described the diversities of the homologs by defining gene sequence type, and illustrated its association with gene duplication; we found that gene duplication clearly increased the complexity of the gene sequence types in the core virulence-related genes. In addition, we provided virulence-related gene profiles whose genetic characteristic depend on the isolation years from the view of gene gain and loss, variation, gene duplication and gene sequence type number.

Conclusions

Our study reveals the comprehensive variation dynamics of the virulence-related genes in toxigenic V. cholerae serogroup O1 during epidemics. The increasing trend for the virulence-related genes may suggest the evolutional advantage of strains by gaining virulence-related genes with diverse functional categories.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5725-y) contains supplementary material, which is available to authorized users.

Vibrio Flagellar Synthesis

Vibrio spp. are highly motile Gram-negative bacteria, ubiquitously found in aquatic environments. Some Vibrios are responsible for disease and morbidity of marine invertebrates and humans, while others are studied for their symbiotic interactions. Vibrio spp. are motile due to synthesis of flagella that rotate and propel the bacteria. Many Vibrio spp. synthesize monotrichous polar flagella (e.g., V. cholerae, V. alginolyticus); however, some synthesize peritrichous or lophotrichous flagella. Flagellar-mediated motility is intimately connected to biological and cellular processes such as chemotaxis, biofilm formation, colonization, and virulence of Vibrio spp. This review focuses on the polar flagellum and its regulation in regard to Vibrio virulence and environmental persistence.

The effect of bacterial chemotaxis on host infection and pathogenicity

Chemotaxis enables microorganisms to move according to chemical gradients. Although this process requires substantial cellular energy, it also affords key physiological benefits, including enhanced access to growth substrates. Another important implication of chemotaxis is that it also plays an important role in infection and disease, as chemotaxis signalling pathways are broadly distributed across a variety of pathogenic bacteria. Furthermore, current research indicates that chemotaxis is essential for the initial stages of infection in different human, animal and plant pathogens. This review focuses on recent findings that have identified specific bacterial chemoreceptors and corresponding chemoeffectors associated with pathogenicity. Pathogenicity-related chemoeffectors are either host and niche-specific signals or intermediates of the host general metabolism. Plant pathogens were found to contain an elevated number of chemotaxis signalling genes and functional studies demonstrate that these genes are critical for their ability to enter the host. The expanding body of knowledge of the mechanisms underlying chemotaxis in pathogens provides a foundation for the development of new therapeutic strategies capable of blocking infection and preventing disease by interfering with chemotactic signalling pathways.

Multiple intraintestinal signals coordinate the regulation of Vibrio cholerae virulence determinants

Vibrio cholerae is a Gram-negative motile bacterium capable of causing fatal pandemic disease in humans via oral ingestion of contaminated water or food. Within the human intestine, the motile vibrios must evade the innate host defense mechanisms, penetrate the mucus layer covering the small intestine, adhere to and multiply on the surface of the microvilli and cause disease via the action of cholera toxin. The explosive diarrhea associated with V. cholerae intestinal colonization leads to dissemination of the vibrios back into the environment to complete this phase of the life cycle. The host phase of the vibrio life cycle is made possible via the concerted action of a signaling cascade that controls the synthesis of V. cholerae colonization determinants. These virulence proteins are coordinately synthesized in response to specific host signals that are still largely undefined. A more complete understanding of the molecular events involved in the V. cholerae recognition of intraintestinal signals and the subsequent transcriptional response will provide important information regarding how pathogenic bacteria establish infection and provide novel methods for treating and/or preventing bacterial infections such as Asiatic cholera. This review will summarize what is currently known in regard to host intraintestinal signals that inform the complex ToxR regulatory cascade in order to coordinate in a spatial and temporal fashion virulence protein synthesis within the human small intestine.

Influence of Chemotaxis and Swimming Patterns on the Virulence of the Coral Pathogen Vibrio coralliilyticus

Chemotaxis, the directed movement toward or away from a chemical signal, can be essential to bacterial pathogens for locating hosts or avoiding hostile environments. The coral pathogen Vibrio coralliilyticus chemotaxes toward coral mucus; however, chemotaxis has not been experimentally demonstrated to be important for virulence. To further examine this, in-frame mutations were constructed in genes predicted to be important for V. coralliilyticus chemotaxis. Most Vibrio genomes contain multiple homologs of various chemotaxis-related genes, and two paralogs of each for cheB, cheR, and cheA were identified. Based on single mutant analyses, the paralogs cheB₂, cheR₂, and cheA₁ were essential for chemotaxis in laboratory assays. As predicted, the ΔcheA₁ and ΔcheR₂ strains had a smooth-swimming pattern, while the ΔcheB₂ strain displayed a zigzag pattern when observed under light microscopy. However, these mutants, unlike the parent strain, were unable to chemotax toward the known attractants coral mucus, dimethylsulfoniopropionate, and N-acetyl-d-glucosamine. The ΔcheB₂ strain and an aflagellate ΔfliG₁ strain were avirulent to coral, while the ΔcheA₁ and ΔcheR₂ strains were hypervirulent (90 to 100% infection within 14 h on average) compared to the wild-type strain (66% infection within 36 h on average). Additionally, the ΔcheA₁ and ΔcheR₂ strains appeared to better colonize coral fragments than the wild-type strain. These results suggest that although chemotaxis may be involved with infection (the ΔcheB₂ strain was avirulent), a smooth-swimming phenotype is important for bacterial colonization and infection. This study provides valuable insight into understanding V. coralliilyticus pathogenesis and how this pathogen may be transmitted between hosts.IMPORTANCE Corals are responsible for creating the immense structures that are essential to reef ecosystems; unfortunately, pathogens like the bacterium Vibrio coralliilyticus can cause fatal infections of reef-building coral species. However, compared to related human pathogens, the mechanisms by which V. coralliilyticus initiates infections and locates new coral hosts are poorly understood. This study investigated the effects of chemotaxis, the directional swimming in response to chemical signals, and bacterial swimming patterns on infection of the coral Montipora capitata Infection experiments with different mutant strains suggested that a smooth-swimming pattern resulted in hypervirulence. These results demonstrate that the role of chemotaxis in coral infection may not be as straightforward as previously hypothesized and provide valuable insight into V. coralliilyticus pathogenesis.

Vibrio cholerae accessory colonisation factor AcfC: a chemotactic protein with a role in hyperinfectivity

Vibrio cholerae O1 El Tor is an aquatic Gram-negative bacterium responsible for the current seventh pandemic of the diarrheal disease, cholera. A previous whole-genome analysis on V. cholerae O1 El Tor strains from the 2010 epidemic in Pakistan showed that all strains contained the V. cholerae pathogenicity island-1 and the accessory colonisation gene acfC (VC_0841). Here we show that acfC possess an open reading frame of 770 bp encoding a protein with a predicted size of 28 kDa, which shares high amino acid similarity with two adhesion proteins found in other enteropathogens, including Paa in serotype O45 porcine enteropathogenic Escherichia coli and PEB3 in Campylobacter jejuni. Using a defined acfC deletion mutant, we studied the specific role of AcfC in V. cholerae O1 El Tor environmental survival, colonisation and virulence in two infection model systems (Galleria mellonella and infant rabbits). Our results indicate that AcfC might be a periplasmic sulfate-binding protein that affects chemotaxis towards mucin and bacterial infectivity in the infant rabbit model of cholera. Overall, our findings suggest that AcfC contributes to the chemotactic response of WT V. cholerae and plays an important role in defining the overall distribution of the organism within the intestine.

THE AER2 RECEPTOR FROM VIBRIO CHOLERAE IS A DUAL PAS-HEME OXYGEN SENSOR

The diarrheal pathogen Vibrio cholerae navigates complex environments using three chemosensory systems and 44-45 chemoreceptors. Chemosensory cluster II modulates chemotaxis, whereas clusters I and III have unknown functions. Ligands have been identified for only five V. cholerae chemoreceptors. Here we report that the cluster III receptor, VcAer2, binds and responds to O₂ . VcAer2 is an ortholog of Pseudomonas aeruginosa Aer2 (PaAer2), but differs in that VcAer2 has two, rather than one, N-terminal PAS domain. We have determined that both PAS1 and PAS2 form homodimers and bind penta-coordinate b-type heme via an Eη-His residue. Heme binding to PAS1 required the entire PAS core, but receptor function also required the N-terminal cap. PAS2 functioned as an O₂ -sensor [K_d(O2) , 19 μM], utilizing the same Iβ Trp (W276) as PaAer2 to stabilize O₂ . The crystal structure of PAS2-W276L was similar to that of PaAer2-PAS, but resided in an active conformation mimicking the ligand-bound state, consistent with its signal-on phenotype. PAS1 also bound O₂ [K_d(O2), 12 μM], although O₂ binding was stabilized by either a Trp or Tyr residue. Moreover, PAS1 appeared to function as a signal modulator, regulating O₂ -mediated signaling from PAS2, and resulting in activation of the cluster III chemosensory pathway. This article is protected by copyright. All rights reserved.

Identification of chemotaxis operon cheYZA and cheA gene expression under stressful conditions in Piscirickettsia salmonis

Piscirickettsia salmonis is the etiological agent of piscirickettsiosis, which, as the main systemic disease in the Chilean salmon industry, causes significant economic losses. This bacterium can produce biofilm as a persistence and survival strategy in adverse conditions. In other bacteria, cheA is a key gene for modulating the onset of bacterial chemotaxis, as well as having a secondary role in biofilm production. Notwithstanding this association, the potential relationships between biofilm formation and genes involved in P. salmonis chemotaxis are poorly understood. This study aimed to determine P. salmonis cheA gene expression when grown in different culture media known to induce biofilm production. Piscirickettsia salmonis AUSTRAL-005 produced moderate/high biofilm levels after 144 h of incubation in the AUSTRAL-SRS and marine broths. In contrast, LF-89 biofilm production was weak/nonexistent in the aforementioned broths. Both assessed P. salmonis strains contained the cheYZA operon. Additionally, AUSTRAL-005 cheA transcripts increased in both culture media. In conclusion, these results suggest potential relationships between biofilm formation and genes related to chemotaxis in the fish pathogen P. salmonis.

Chemotactic Behaviors of Vibrio cholerae Cells

Vibrio cholerae, the causative agent of cholera, swims in aqueous environments with a single polar flagellum. In a spatial gradient of a chemical, the bacterium can migrate in "favorable" directions, a property that is termed chemotaxis. The chemotaxis of V. cholerae is not only critical for survival in various environments and but also is implicated in pathogenicity. In this chapter, we describe how to characterize the chemotactic behaviors of V. cholerae: these methods include swarm assay, temporal stimulation assay, capillary assay, and receptor methylation assay.

Glucose induces delocalization of a flagellar biosynthesis protein from the flagellated pole

To survive in a continuously changing environment, bacteria sense concentration gradients of attractants or repellents, and purposefully migrate until a more favourable habitat is encountered. While glucose is known as the most effective attractant, the flagellar biosynthesis and hence chemotactic motility has been known to be repressed by glucose in some bacteria. To date, the only known regulatory mechanism of the repression of flagellar synthesis by glucose is via downregulation of the cAMP level, as shown in a few members of the family Enterobacteriaceae. Here we show that, in Vibrio vulnificus, the glucose-mediated inhibition of flagellar motility operates by a completely different mechanism. In the presence of glucose, EIIA(Glc) is dephosphorylated and inhibits the polar localization of FapA (flagellar assembly protein A) by sequestering it from the flagellated pole. A loss or delocalization of FapA results in a complete failure of the flagellar biosynthesis and motility. However, when glucose is depleted, EIIA(Glc) is phosphorylated and releases FapA such that free FapA can be localized back to the pole and trigger flagellation. Together, these data provide new insight into a bacterial strategy to reach and stay in the glucose-rich area.

Identification of a Vibrio cholerae chemoreceptor that senses taurine and amino acids as attractants

Vibrio cholerae, the etiological agent of cholera, was found to be attracted by taurine (2-aminoethanesulfonic acid), a major constituent of human bile. Mlp37, the closest homolog of the previously identified amino acid chemoreceptor Mlp24, was found to mediate taxis to taurine as well as L-serine, L-alanine, L-arginine, and other amino acids. Methylation of Mlp37 was enhanced upon the addition of taurine and amino acids. Isothermal titration calorimetry demonstrated that a purified periplasmic fragment of Mlp37 binds directly to taurine, L-serine, L-alanine and L-arginine. Crystal structures of the periplamic domain of Mlp37 revealed that L-serine and taurine bind to the membrane-distal PAS domain in essentially in the same way. The structural information was supported by characterising the in vivo properties of alanine-substituted mutant forms of Mlp37. The fact that the ligand-binding domain of the L-serine complex had a small opening, which would accommodate a larger R group, accounts for the broad ligand specificity of Mlp37 and allowed us to visualise ligand binding to Mlp37 with fluorescently labelled L-serine. Taken together, we conclude that Mlp37 serves as the major chemoreceptor for taurine and various amino acids.

The Vibrio cholerae ToxR Regulon Encodes Host-Specific Chemotaxis Proteins that Function in Intestinal Colonization

Virulence gene regulation in Vibrio cholerae is under the control of the ToxR-ToxT regulatory cascade. Chemotaxis and net motility have been shown to influence the infectivity of Vibrio cholerae. V. cholerae toxR mutants do not synthesize proteins required for chemotaxis towards mucus. The inability of the toxR mutant strain to recognize and swim towards mucus is due to their failure to synthesize AcfB, a methyl-accepting chemotaxis protein. AcfB has previously been shown to be involved in intestinal colonization using the infant mouse model of cholera infection. Wild type V. cholerae recognizes galactose-6-sulfate in the capillary tube assay whereas V. cholerae acfB mutants fail to migrate into the capillary tubes. Vibrio strains carrying a mutation in tcpI, a ToxR regulated gene found within the Vibrio Pathogenicity Island (VPI), which encodes a methyl accepting chemotaxis protein are fully chemotactic towards mucus and galactose-6-sulfate.

RpoS and quorum sensing control expression and polar localization of Vibrio cholerae chemotaxis cluster III proteins in vitro and in vivo

The diarrheal pathogen Vibrio cholerae contains three gene clusters that encode chemotaxis-related proteins, but only cluster II appears to be required for chemotaxis. Here, we present the first characterization of V. cholerae's 'cluster III' chemotaxis system. We found that cluster III proteins assemble into foci at bacterial poles, like those formed by cluster II proteins, but the two systems assemble independently and do not colocalize. Cluster III proteins are expressed in vitro during stationary phase and in conjunction with growth arrest linked to carbon starvation. This expression, as well as expression in vivo in suckling rabbits, is dependent upon RpoS. V. cholerae's CAI-1 quorum sensing (QS) system is also required for cluster III expression in stationary phase and modulates its expression in vivo, but is not required for cluster III expression in response to carbon starvation. Surprisingly, even though the CAI-1 and AI-2 QS systems are thought to feed into the same signaling pathway, the AI-2 system inhibited cluster III gene expression, revealing that the outputs of the two QS systems are not always the same. The distinctions between genetic determinants of cluster III expression in vitro and in vivo highlight the distinctive nature of the in vivo environment.

RNA-seq analysis identifies new genes regulated by the histone-like nucleoid structuring protein (H-NS) affecting Vibrio cholerae virulence, stress response and chemotaxis

The histone-like nucleoid structuring protein (H-NS) functions as a transcriptional silencer by binding to AT-rich sequences at bacterial promoters. However, H-NS repression can be counteracted by other transcription factors in response to environmental changes. The identification of potential toxic factors, the expression of which is prevented by H-NS could facilitate the discovery of new regulatory proteins that may contribute to the emergence of new pathogenic variants by anti-silencing. Vibrio cholerae hns mutants of the El Tor biotype exhibit altered virulence, motility and environmental stress response phenotypes compared to wild type. We used an RNA-seq analysis approach to determine the basis of the above hns phenotypes and identify new targets of H-NS transcriptional silencing. H-NS affected the expression of 18% of all predicted genes in a growth phase-dependent manner. Loss of H-NS resulted in diminished expression of numerous genes encoding methyl-accepting chemotaxis proteins as well as chemotaxis toward the attractants glycine and serine. Deletion of hns also induced an endogenous envelope stress response resulting in elevated expression of rpoE encoding the extracytoplamic sigma factor E (σ^E). The RNA-seq analysis identified new genes directly repressed by H-NS that can affect virulence and biofilm development in the El Tor biotype cholera bacterium. We show that H-NS and the quorum sensing regulator HapR silence the transcription of the vieSAB three-component regulatory system in El Tor biotype V. cholerae. We also demonstrate that H-NS directly represses the transcription of hlyA (hemolysin), rtxCA (the repeat in toxin or RTX), rtxBDE (RTX transport) and the biosynthesis of indole. Of these genes, H-NS occupancy at the hlyA promoter was diminished by overexpression of the transcription activator HlyU. We discuss the role of H-NS transcriptional silencing in phenotypic differences exhibited by V. cholerae biotypes.

Comparative proteomic analysis of Clostridium difficile isolates of varying virulence

The soluble proteome of three Clostridium difficile strains of varying pathogenic potential, designated B-1, Tra 5/5 and 027 SM, were compared using differential in-gel electrophoresis in which the proteins of each strain were labelled with CyDyes. This enabled visual inspection of the 2D profiles of strains and identification of differentially expressed proteins using image analysis software. Unlabelled protein reference maps of the predominant proteins were then generated for each strain using 2D gel electrophoresis followed by protein sequencing of each spot using a Reflectron matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometer. Increased coverage of the proteome was achieved using 1D gel electrophoresis in a bottom-up approach using LC-MS/MS of 1 cm gel slices. A total of 888 different proteins were detected by comparative analysis of isolates grown in parallel for 64 h on blood agar plates. Of these, only 38 % were shared between all isolates. One hundred and ten proteins were identified as showing ≥2-fold difference in expression between strains. Differential expression was shown in a number of potential virulence and colonization factors. Toxin B was detected in the more virulent strains B-1 and 027 SM, but not in the lower virulent strain Tra 5/5, despite all strains possessing an intact pathogenicity locus. The S-layer protein (Cwp2) was identified in strains 027 SM and Tra 5/5 but not strain B-1, and differences in the post-translational modification of SlpA were noted for strain B-1. The variant S-layer profile of strain B-1 was confirmed by genomic comparison, which showed a 58 kb insertion in the S-layer operon of strain B-1. Differential post-translation modification events were also noted in flagellar proteins, thought to be due to differential glycosylation. This study highlights genomic and proteomic variation of different Clostridium difficile strains and suggests a number of factors may play a role in mediating the varying virulence of these different strains.

Structure, gene regulation and environmental response of flagella in Vibrio

Vibrio species are Gram-negative, rod-shaped bacteria that live in aqueous environments. Several species, such as V. harveyi, V. alginotyticus, and V. splendidus, are associated with diseases in fish or shellfish. In addition, a few species, such as V. cholerae and V. parahaemolyticus, are risky for humans due to infections from eating raw shellfish infected with these bacteria or from exposure of wounds to the marine environment. Bacterial flagella are not essential to live in a culture medium. However, most Vibrio species are motile and have rotating flagella which allow them to move into favorable environments or to escape from unfavorable environments. This review summarizes recent studies about the flagellar structure, function, and regulation of Vibrio species, especially focused on the Na⁺-driven polar flagella that are principally responsible for motility and sensing the surrounding environment, and discusses the relationship between flagella and pathogenicity.

A bacterial pathogen uses dimethylsulfoniopropionate as a cue to target heat-stressed corals

Diseases are an emerging threat to ocean ecosystems. Coral reefs, in particular, are experiencing a worldwide decline because of disease and bleaching, which have been exacerbated by rising seawater temperatures. Yet, the ecological mechanisms behind most coral diseases remain unidentified. Here, we demonstrate that a coral pathogen, Vibrio coralliilyticus, uses chemotaxis and chemokinesis to target the mucus of its coral host, Pocillopora damicornis. A primary driver of this response is the host metabolite dimethylsulfoniopropionate (DMSP), a key element in the global sulfur cycle and a potent foraging cue throughout the marine food web. Coral mucus is rich in DMSP, and we found that DMSP alone elicits chemotactic responses of comparable intensity to whole mucus. Furthermore, in heat-stressed coral fragments, DMSP concentrations increased fivefold and the pathogen's chemotactic response was correspondingly enhanced. Intriguingly, despite being a rich source of carbon and sulfur, DMSP is not metabolized by the pathogen, suggesting that it is used purely as an infochemical for host location. These results reveal a new role for DMSP in coral disease, demonstrate the importance of chemical signaling and swimming behavior in the recruitment of pathogens to corals and highlight the impact of increased seawater temperatures on disease pathways.

Conformational barrier of CheY3 and inability of CheY4 to bind FliM control the flagellar motor action in Vibrio cholerae

Vibrio cholerae contains multiple copies of chemotaxis response regulator (VcCheY1-VcCheY4) whose functions are elusive yet. Although previous studies suggested that only VcCheY3 directly switches the flagellar rotation, the involvement of VcCheY4 in chemotaxis could not be ruled out. None of these studies, however, focused on the structure, mechanism of activation or molecular basis of FliM binding of the VcCheYs. From the crystal structures of Ca(2+) and Mg(2+) bound VcCheY3 we proposed the presence of a conformational barrier composed of the hydrophobic packing of W61, M88 and V106 and a unique hydrogen bond between T90 and Q97 in VcCheY3. Lesser fluorescence quenching and higher Km value of VcCheY3, compared to its mutants VcCheY3-Q97A and VcCheY3-Q97A/E100A supported our proposition. Furthermore, aforesaid biochemical data, in conjunction with the structure of VcCheY3-Q97A, indicated that the coupling of T90 and Q97 restricts the movement of T90 toward the active site reducing the stabilization of the bound phosphate and effectively promoting autodephosphorylation of VcCheY3. The structure of BeF3(-) activated VcCheY3 insisted us to argue that elevated temperature and/or adequacy of phosphate pool might break the barrier of the free-state VcCheY3 and the conformational changes, required for FliM binding, occur upon phosphorylation. Structure of VcCheY4 has been solved in the free and sulfated states. VcCheY4(sulf), containing a bound sulfate at the active site, appears to be more compact and stable with a longer α4 helix, shorter β4α4 loop and hydrogen bond between T82 and the sulfate compared to VcCheY4(free). While pull down assay of VcCheYs with VcFliMNM showed that only activated VcCheY3 can interact with VcFliMNM and VcCheY4 cannot, a knowledge based docking explained the molecular mechanism of the interactions between VcCheY3 and VcFliM and identified the limitations of VcCheY4 to interact with VcFliM even in its phosphorylated state.

Chemoreceptor gene loss and acquisition via horizontal gene transfer in Escherichia coli

Chemotaxis allows bacteria to more efficiently colonize optimal microhabitats within their larger environment. Chemotaxis in Escherichia coli is the best-studied model system, and a large number of E. coli strains have been sequenced. The Escherichia/Shigella genus encompasses a great variety of commensal and pathogenic strains, but the role of chemotaxis in their association with the host remains poorly understood. Here we show that the core chemotaxis genes are lost in many, but not all, nonmotile strains but are well preserved in all motile strains. The genes encoding the Tar, Tsr, and Aer chemoreceptors, which mediate chemotaxis to a broad spectrum of chemical and physical cues, are also nearly uniformly conserved in motile strains. In contrast, the clade of extraintestinal pathogenic E. coli strains apparently underwent an ancestral loss of Trg and Tap chemoreceptors, which sense sugars, dipeptides, and pyrimidines. The broad range of time estimated for the loss of these genes (1 to 3 million years ago) corresponds to the appearance of the genus Homo.

Diverse populations of lake water bacteria exhibit chemotaxis towards inorganic nutrients

Chemotaxis allows microorganisms to rapidly respond to different environmental stimuli; however, understanding of this process is limited by conventional assays, which typically focus on the response of single axenic cultures to given compounds. In this study, we used a modified capillary assay coupled with flow cytometry and 16S rRNA gene amplicon pyrosequencing to enumerate and identify populations within a lake water microbial community that exhibited chemotaxis towards ammonium, nitrate and phosphate. All compounds elicited chemotactic responses from populations within the lake water, with members of Sphingobacteriales exhibiting the strongest responses to nitrate and phosphate, and representatives of the Variovorax, Actinobacteria ACK-M1 and Methylophilaceae exhibiting the strongest responses to ammonium. Our results suggest that chemotaxis towards inorganic substrates may influence the rates of biogeochemical processes.

Studies of dynamic protein-protein interactions in bacteria using Renilla luciferase complementation are undermined by nonspecific enzyme inhibition

The luciferase protein fragment complementation assay is a powerful tool for studying protein-protein interactions. Two inactive fragments of luciferase are genetically fused to interacting proteins, and when these two proteins interact, the luciferase fragments can reversibly associate and reconstitute enzyme activity. Though this technology has been used extensively in live eukaryotic cells, split luciferase complementation has not yet been applied to studies of dynamic protein-protein interactions in live bacteria. As proof of concept and to develop a new tool for studies of bacterial chemotaxis, fragments of Renilla luciferase (Rluc) were fused to the chemotaxis-associated response regulator CheY3 and its phosphatase CheZ in the enteric pathogen Vibrio cholerae. Luciferase activity was dependent on the presence of both CheY3 and CheZ fusion proteins, demonstrating the specificity of the assay. Furthermore, enzyme activity was markedly reduced in V. cholerae chemotaxis mutants, suggesting that this approach can measure defects in chemotactic signaling. However, attempts to measure changes in dynamic CheY3-CheZ interactions in response to various chemoeffectors were undermined by nonspecific inhibition of the full-length luciferase. These observations reveal an unexpected limitation of split Rluc complementation that may have implications for existing data and highlight the need for great caution when evaluating small molecule effects on dynamic protein-protein interactions using the split luciferase technology.

Chemotaxis of Marinobacter adhaerens and its impact on attachment to the diatom Thalassiosira weissflogii

Alga-bacterium interactions are crucial for aggregate formation and carbon cycling in aquatic systems. To understand the initiation of these interactions, we investigated bacterial chemotaxis within a bilateral model system. Marinobacter adhaerens HP15 has been demonstrated to attach to the diatom Thalassiosira weissflogii and induce transparent exopolymeric particle and aggregate formation. M. adhaerens possesses one polar flagellum and is highly motile. Bacterial cells were attracted to diatom cells, as demonstrated by addition of diatom cell homogenate or diatom culture supernatant to soft agar, suggesting that chemotaxis might be important for the interaction of M. adhaerens with diatoms. Three distinct chemotaxis-associated gene clusters were identified in the genome sequence of M. adhaerens, with the clusters showing significant sequence similarities to those of Pseudomonas aeruginosa PAO1. Mutations in the genes cheA, cheB, chpA, and chpB, which encode histidine kinases and methylesterases and which are putatively involved in either flagellum-associated chemotaxis or pilus-mediated twitching motility, were generated and mutants with the mutations were phenotypically analyzed. ΔcheA and ΔcheB mutants were found to be swimming deficient, and all four mutants were impaired in biofilm formation on abiotic surfaces. Comparison of the HP15 wild type and its chemotaxis mutants in cocultures with the diatom revealed that the fraction of bacteria attaching to the diatom decreased significantly for mutants in comparison to that for the wild type. Our results highlight the importance of M. adhaerens chemotaxis in initiation of its interaction with the diatom. In-depth knowledge of these basic processes in interspecies interactions is pivotal to obtain a systematic understanding of organic matter flux and nutrient cycling in marine ecosystems.

Mlp24 (McpX) of Vibrio cholerae implicated in pathogenicity functions as a chemoreceptor for multiple amino acids

The chemotaxis of Vibrio cholerae, the causative agent of cholera, has been implicated in pathogenicity. The bacterium has more than 40 genes for methyl-accepting chemotaxis protein (MCP)-like proteins (MLPs). In this study, we found that glycine and at least 18 L-amino acids, including serine, arginine, asparagine, and proline, serve as attractants to the classical biotype strain O395N1. Based on the sequence comparison with Vibrio parahaemolyticus, we speculated that at least 17 MLPs of V. cholerae may mediate chemotactic responses. Among them, Mlp24 (previously named McpX) is required for the production of cholera toxin upon mouse infection. mlp24 deletion strains of both classical and El Tor biotypes showed defects in taxis toward several amino acids, which were complemented by the expression of Mlp24. These amino acids enhanced methylation of Mlp24. Serine, arginine, asparagine, and proline were shown to bind directly to the periplasmic fragment of Mlp24. The structural information of its closest homolog, Mlp37, predicts that Mlp24 has two potential ligand-binding pockets per subunit, the membrane distal of which was suggested, by mutational analyses, to be involved in sensing of amino acids. These results suggest that Mlp24 is a chemoreceptor for multiple amino acids, including serine, arginine, and asparagine, which were previously shown to stimulate the expression of several virulence factors, implying that taxis toward a set of amino acids plays critical roles in pathogenicity of V. cholerae.

Bacterial swimming, swarming and chemotactic response to heavy metal presence: which could be the influence on wastewater biotreatment efficiency?

Fixed-bed reactors are usually designed for wastewater biotreatments, where the biofilm establishment and maintenance play the most important roles. Biofilm development strictly relies on different types of bacterial motility: swimming, swarming, and chemotaxis, which can be altered by the microenvironment conditions. The aim of this work is to do an integrated study on the effects of Cu(II), Cd(II), Zn(II) and Cr(VI) on swimming, swarming and chemotaxis of Pseudomonas veronii 2E, Delftia acidovorans AR and Ralstonia taiwanensis M2 to improve biofilm development and maintenance for metal loaded wastewater biotreatment in fixed-bed bioreactors. Swimming, swarming and chemotactic response evaluation experiments were carried out at different metal concentrations. P. veronii 2E motility was not affected by metal presence, being this strain optimal for fixed-bed reactors. D. acidovorans AR swarming was inhibited by Cd and Zn. Although R. taiwanensis M2 showed high resistance to Cu, Cd, Cr and Zn, motility was definitively altered, so further studies on R. taiwanensis M2 resistance mechanisms would be particularly interesting.

Proteomics analyses of the opportunistic pathogen Burkholderia vietnamiensis using protein fractionations and mass spectrometry

The main objectives of this work were to obtain a more extensive coverage of the Burkholderia vietnamiensis proteome than previously reported and to identify virulence factors using tandem mass spectrometry. The proteome of B. vietnamiensis was precipitated into four fractions to as extracellular, intracellular, cell surface and cell wall proteins. Two different approaches were used to analyze the proteins. The first was a gel-based method where 1D SDS-PAGE was used for separation of the proteins prior to reverse phase liquid chromatography tandem mass spectrometry (LC-MS/MS). The second method used MudPIT analysis (Multi dimensional Protein Identification Technique), where proteins are digested and separated using cation exchange and reversed phase separations before the MS/MS analysis (LC/LC-MS/MS). Overall, gel-based LC-MS/MS analysis resulted in more protein identifications than the MudPIT analysis. Combination of the results lead to identification of more than 1200 proteins, approximately 16% of the proteins coded from the annotated genome of Burkholderia species. Several virulence factors were detected including flagellin, porin, peroxiredoxin and zinc proteases.

Overexpression, purification, crystallization and preliminary X-ray analysis of CheY4 from Vibrio cholerae O395

Chemotaxis and motility greatly influence the infectivity of Vibrio cholerae, although the role of chemotaxis genes in V. cholerae pathogenesis is poorly understood. In contrast to the single copy of CheY found in Escherichia coli and Salmonella typhimurium, four CheYs (CheY1-CheY4) are present in V. cholerae. While insertional disruption of the cheY4 gene results in decreased motility, insertional duplication of this gene increases motility and causes enhanced expression of the two major virulence genes. Additionally, cheY3/cheY4 influences the activation of the transcription factor NF-κB, which triggers the generation of acute inflammatory responses. V. cholerae CheY4 was cloned, overexpressed and purified by Ni-NTA affinity chromatography followed by gel filtration. Crystals of CheY4 grown in space group C2 diffracted to 1.67 Å resolution, with unit-cell parameters a = 94.4, b = 31.9, c = 32.6 Å, β = 96.5°, whereas crystals grown in space group P3(2)21 diffracted to 1.9 Å resolution, with unit-cell parameters a = b = 56.104, c = 72.283 Å, γ = 120°.

A model for Vibrio cholerae colonization of the human intestine

Vibrio cholerae is a strict human pathogen that causes the disease cholera. It is an old-world pathogen that has re-emerged as a new threat since the early 1990s. V. cholerae colonizes the upper, small intestine where it produces a toxin that leads to watery diarrhea, characterizing the disease (Kahn et al., 1988). The dynamics of colonization by the bacteria of the intestines are largely unknown. Although a large initial infectious dose is required for infection, data suggests that only a smaller sub-population colonizes a portion of the small bowel leading to disease. There are many barriers to colonization in the intestines including peristalsis, fluid wash-out, viscosity of the mucus layer, and pH. We are interested in identifying the mechanisms that allow this sub-population of bacteria to survive and colonize the intestines when faced with these barriers. To elaborate the dynamics of V. cholerae infection, we have developed a mathematical model based on a convection-diffusion-reaction-swimming equation capturing bacterial dynamics coupled with Stokes equations governing fluid velocity where we developed a novel non-local boundary condition. Our results indicate that both host and bacterial factors contribute to bacterial density in the gut. Host factors include intestinal diffusion and convection rates while bacterial factors include adherence, motility and growth rates. This model can ultimately be used to test therapeutic strategies against V. cholerae.

A family of ParA-like ATPases promotes cell pole maturation by facilitating polar localization of chemotaxis proteins

Stochastic processes are thought to mediate localization of membrane-associated chemotaxis signaling clusters in peritrichous bacteria. Here, we identified a new family of ParA-like ATPases (designated ParC [for partitioning chemotaxis]) encoded within chemotaxis operons of many polar-flagellated γ-proteobacteria that actively promote polar localization of chemotaxis proteins. In Vibrio cholerae, a single ParC focus is found at the flagellated old pole in newborn cells, and later bipolar ParC foci develop as the cell matures. The cell cycle-dependent redistribution of ParC occurs by its release from the old pole and subsequent relocalization at the new pole, consistent with a "diffusion and capture" model for ParC dynamics. Chemotaxis proteins encoded in the same cluster as ParC have a similar unipolar-to-bipolar transition; however, they reach the new pole after the arrival of ParC. Cells lacking ParC exhibit aberrantly localized foci of chemotaxis proteins, reduced chemotaxis, and altered motility, which likely accounts for their enhanced colonization of the proximal small intestine in an animal model of cholera. Collectively, our findings indicate that ParC promotes the efficiency of chemotactic signaling processes. In particular, ParC-facilitated development of a functional chemotaxis apparatus at the new pole readies this site for its development into a functional old pole after cell division.