The Vibrio cholerae acfB colonization determinant encodes an inner membrane protein that is related to a family of signal-transducing proteins

Antibacterial carbonic anhydrase inhibitors targeting Vibrio cholerae enzymes

INTRODUCTION

Cholera is a bacterial diarrheal disease caused by pathogen bacteria Vibrio cholerae, which produces the cholera toxin (CT). In addition to improving water sanitation, oral cholera vaccines have been developed to control infection. Besides, rehydration and antibiotic therapy are complementary treatment strategies for cholera. ToxT regulatory protein activates transcription of CT gene, which is enhanced by bicarbonate (HCO3-).

AREAS COVERED

This review delves into the genomic blueprint of V. cholerae, which encodes for α-, β-, and γ- carbonic anhydrases (CAs). We explore how the CAs contribute to the pathogenicity of V. cholerae and discuss the potential of CA inhibitors in mitigating the disease's impact.

EXPERT OPINION

CA inhibitors can reduce the virulence of bacteria and control cholera. Here, we reviewed all reported CA inhibitors, noting that α-CA from V. cholerae (VchCAα) was the most effective inhibited enzyme compared to the β- and γ-CA families (VchCAβ and VchCAγ). Among the CA inhibitors, acyl selenobenzenesulfonamidenamides and simple/heteroaromatic sulfonamides were the best VchCA inhibitors in the nM range. It was noted that some antibacterial compounds show good inhibitory effects on all three bacterial CAs. CA inhibitors belonging to other classes may be synthesized and tested on VchCAs to harness cholera.

Adaptation to bile and anaerobicity limits Vibrio cholerae phage adsorption

IMPORTANCE

Vibrio cholerae is the bacterial pathogen responsible for cholera, a diarrheal disease that impacts people in areas without access to potable water. In regions that lack such infrastructure, cholera represents a large proportion of disease outbreaks. Bacteriophages (phages, viruses that infect bacteria) have recently been examined as potential therapeutic and prophylactic agents to treat and prevent bacterial disease outbreaks like cholera due to their specificity and stability. This work examines the interaction between V. cholerae and vibriophages in consideration for a cholera prophylaxis regimen (M. Yen, L. S. Cairns, and A. Camilli, Nat Commun 8:14187, 2017, https://doi.org/10.1038/ncomms14187) in the context of stimuli found in the intestinal environment. We discover that common signals in the intestinal environment induce cell surface modifications in V. cholerae that also restrict some phages from binding and initiating infection. These findings could impact considerations for the design of phage-based treatments, as phage infection appears to be limited by bacterial adaptations to the intestinal environment.

Sargasso Sea Vibrio bacteria: Underexplored potential pathovars in a perturbed habitat

We fully sequenced the genomes of 16 Vibrio cultivars isolated from eel larvae, plastic marine debris (PMD), the pelagic brown macroalga Sargassum, and seawater samples collected from the Caribbean and Sargasso Seas of the North Atlantic Ocean. Annotation and mapping of these 16 bacterial genome sequences to a PMD-derived Vibrio metagenome-assembled genome created for this study showcased vertebrate pathogen genes closely-related to cholera and non-cholera pathovars. Phenotype testing of cultivars confirmed rapid biofilm formation, hemolytic, and lipophospholytic activities, consistent with pathogenic potential. Our study illustrates that open ocean vibrios represent a heretofore undescribed group of microbes, some representing potential new species, possessing an amalgam of pathogenic and low nutrient acquisition genes, reflecting their pelagic habitat and the substrates and hosts they colonize.

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.

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

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

The chemosensory systems of Vibrio cholerae

Vibrio cholerae, the causative agent of the acute diarrheal disease cholera, is able to thrive in diverse habitats such as natural water bodies and inside human hosts. To ensure their survival, these bacteria rely on chemosensory pathways to sense and respond to changing environmental conditions. These pathways constitute a highly sophisticated cellular control system in Bacteria and Archaea. Reflecting the complex life cycle of V. cholerae, this organism has three different chemosensory pathways that together contain over 50 proteins expressed under different environmental conditions. Only one of them is known to control motility, while the function of the other two remains to be discovered. Here, we provide an overview of the chemosensory systems in V. cholerae and the advances toward understanding their structure and function.

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.

Genomic epidemiology of Vibrio cholerae reveals the regional and global spread of two epidemic non-toxigenic lineages

Non-toxigenic Vibrio cholerae isolates have been found associated with diarrheal disease globally, however, the global picture of non-toxigenic infections is largely unknown. Among non-toxigenic V. cholerae, ctxAB negative, tcpA positive (CNTP) isolates have the highest risk of disease. From 2001 to 2012, 71 infectious diarrhea cases were reported in Hangzhou, China, caused by CNTP serogroup O1 isolates. We sequenced 119 V. cholerae genomes isolated from patients, carriers and the environment in Hangzhou between 2001 and 2012, and compared them with 850 publicly available global isolates. We found that CNTP isolates from Hangzhou belonged to two distinctive lineages, named L3b and L9. Both lineages caused disease over a long time period with usually mild or moderate clinical symptoms. Within Hangzhou, the spread route of the L3b lineage was apparently from rural to urban areas, with aquatic food products being the most likely medium. Both lineages had been previously reported as causing local endemic disease in Latin America, but here we show that global spread of them has occurred, with the most likely origin of L3b lineage being in Central Asia. The L3b lineage has spread to China on at least three occasions. Other spread events, including from China to Thailand and to Latin America were also observed. We fill the missing links in the global spread of the two non-toxigenic serogroup O1 V. cholerae lineages that can cause human infection. The results are important for the design of future disease control strategies: surveillance of V. cholerae should not be limited to ctxAB positive strains.

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.

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.

AcfA is an essential regulator for pathogenesis of fish pathogen Vibrio alginolyticus

V. alginolyticus is an important opportunistic pathogen which causes vibriosis in aquatic animals. AcfA, as an accessory colonization factor, is hypothesized to be involved in the pathogenesis of infection. In this study, a mutant strain with an in-frame deletion removed nucleotides 86 to 561 of the acfA gene was constructed to reveal the role of AcfA in the physiology and virulence from V. alginolyticus. An acfA mutant showed a similar growth level, an obvious decrease in swarming motility and the activity of ECPase, a higher LD₅₀ value by intraperitoneal injection of grouper fish compared to that of the wild-type. Furthermore, the deletion of acfA could enhance the level of biofilm formation and suppress the polar flagellum forming. The comparative proteomic analysis demonstrated the deletion mutation of acfA could up-regulate the expression of 4 proteins of p4alcd, deoD, phb and DctP, and down-regulate the expression of 8 proteins of Clp, hpV36980, ABCtp, pepD, arA, aggp, fla and ompA compared to that of the wild-type. The analysis of RT-qPCR showed the mRNA levels of DctP and deoD were significantly induced, and the mRNA levels of pepD, arA, fla and ompA were significantly reduced in acfA mutant compared with the wild-type. The results suggest that acfA may contribute to the overall success in the pathogenesis of V. alginolyticus by regulating the expression of some relevant genes.

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.

Isolation and Characterization of NDM-Positive Escherichia coli from Municipal Wastewater in Jeddah, Saudi Arabia

The emergence of resistance to last-resort antibiotics is a public health concern of global scale. Besides direct person-to-person propagation, environmental pathways might contribute to the dissemination of antibiotic-resistant bacteria and antibiotic resistance genes (ARGs). Here, we describe the incidence of bla_NDM-1, a gene conferring resistance to carbapenems, in the wastewater of the city of Jeddah, Saudi Arabia, over a 1-year period. bla_NDM-1 was detected at concentrations ranging from 10⁴ to 10⁵ copies/m³ of untreated wastewater during the entire monitoring period. These results indicate the ubiquity and high incidence of bla_NDM-1 in the local wastewater. To track the bacteria carrying bla_NDM-1, we isolated Escherichia coli PI7, a strain of sequence type 101 (ST101), from wastewater around the Hajj event in October 2013. Genome sequencing of this strain revealed an extensive repertoire of ARGs as well as virulence and invasive traits. These traits were further confirmed by antibiotic resistance profiling and in vitro cell internalization in HeLa cell cultures. Given that this strain remains viable even after a certain duration in the sewerage, and that Jeddah lacks a robust sanitary infrastructure to fully capture all generated sewage, the presence of this bacterium in the untreated wastewater represents a potential hazard to the local public health. To the best of our knowledge, this is the first report of a bla_NDM-1-positive E. coli strain isolated from a nonnosocomial environment in Saudi Arabia and may set a priority concern for the need to establish improved surveillance for carbapenem-resistant E. coli in the country and nearby regions.

Molecular and Genomic Characterization of Vibrio mimicus Isolated from a Frozen Shrimp Processing Facility in Mexico

Vibrio mimicus is a gram-negative bacterium responsible for diseases in humans. Three strains of V. mimicus identified as V. mimicus 87, V. mimicus 92 and V. mimicus 93 were isolated from a shrimp processing facility in Guaymas, Sonora, Mexico. The strains were analyzed using several molecular techniques and according to the cluster analysis they were different, their similarities ranged between 51.3% and 71.6%. ERIC-PCR and RAPD (vmh390R) were the most discriminatory molecular techniques for the differentiation of these strains. The complete genomes of two strains (V. mimicus 87, renamed as CAIM 1882, and V. mimicus 92, renamed as CAIM 1883) were sequenced. The sizes of the genomes were 3.9 Mb in both strains, with 2.8 Mb in ChI and 1.1 Mb in ChII. A 12.7% difference was found in the proteome content (BLAST matrix). Several virulence genes were detected (e.g. capsular polysaccharide, an accessory colonization factor and genes involved in quorum-sensing) which were classified in 16 categories. Variations in the gene content between these genomes were observed, mainly in proteins and virulence genes (e.g., hemagglutinin, mobile elements and membrane proteins). According to these results, both strains were different, even when they came from the same source, giving an insight of the diversity of V. mimicus. The identification of various virulence genes, including a not previously reported V. mimicus gene (acfD) in ChI in all sequenced strains, supports the pathogenic potential of this species. Further analysis will help to fully understand their potential virulence, environmental impact and evolution.

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.

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.

The complexity of ToxT-dependent transcription in Vibrio cholerae

Vibrio cholerae is the causative agent of the disease cholera, characterized by profuse watery diarrhoea. Two of the main virulence factors associated with the disease are cholera toxin (CT) and toxin-coregulated pilus (TCP). Expression of CT and TCP is regulated via a complex cascade of factors that respond to environmental signals, but ultimately ToxT is the direct transcriptional activator of the genes encoding CT and TCP. Recent studies have begun to unveil the mechanisms behind ToxT-dependent transcription. We review current knowledge of transcriptional activation by ToxT and the environmental stimuli that allow ToxT to regulate virulence gene expression, resulting in cholera pathogenesis.

Identification of Vibrio cholerae type III secretion system effector proteins

AM-19226 is a pathogenic O39 serogroup Vibrio cholerae strain that lacks the typical virulence factors for colonization (toxin-coregulated pilus [TCP]) and toxin production (cholera toxin [CT]) and instead encodes a type III secretion system (T3SS). The mechanism of pathogenesis is unknown, and few effector proteins have been identified. We therefore undertook a survey of the open reading frames (ORFs) within the ∼49.7-kb T3SS genomic island to identify potential effector proteins. We identified 15 ORFs for their ability to inhibit growth when expressed in yeast and then used a β-lactamase (TEM1) fusion reporter system to demonstrate that 11 proteins were bona fide effectors translocated into HeLa cells in vitro in a T3SS-dependent manner. One effector, which we named VopX (A33_1663), is conserved only in V. cholerae and Vibrio parahaemolyticus T3SS-positive strains and has not been previously studied. A vopX deletion reduces the ability of strain AM-19226 to colonize in vivo, and the bile-induced expression of a vopX-lacZ transcriptional fusion in vitro is regulated by the T3SS-encoded transcriptional regulators VttR(A) and VttR(B). An RLM1 yeast deletion strain rescued the growth inhibition induced by VopX expression, suggesting that VopX interacts with components of the cell wall integrity mitogen-activated protein kinase (MAPK) pathway. The collective results show that the V. cholerae T3SS encodes multiple effector proteins, one of which likely has novel activities that contribute to disease via interference with eukaryotic signaling pathways.

Energy taxis drives Campylobacter jejuni toward the most favorable conditions for growth

Campylobacter jejuni is a serious food-borne bacterial pathogen in the developed world. Poultry is a major reservoir, and C. jejuni appears highly adapted to the gastrointestinal tract of birds. Several factors are important for chicken colonization and virulence, including a taxis mechanism for environmental navigation. To explore the mechanism of chemotaxis in C. jejuni, we constructed mutants with deletions of five putative mcp (methyl-accepting chemotaxis protein) genes (tlp1, tlp2, tlp3, docB, and docC). Surprisingly, the deletions did not affect the chemotactic behavior of the mutants compared to that of the parental strain. However, the tlp1, tlp3, docB, and docC mutant strains displayed a 10-fold decrease in the ability to invade human epithelial and chicken embryo cells, hence demonstrating that the corresponding proteins affect the host interaction. l-Asparagine, formate, d-lactate, and chicken mucus were identified as new attractants of C. jejuni, and we observed that chemical substances promoting tactic attraction are all known to support the growth of this organism. The attractants could be categorized as carbon sources and electron donors and acceptors, and we furthermore observed a correlation between an attractant's potency and its efficiency as an energy source. The tactic attraction was inhibited by the respiratory inhibitors HQNO (2-n-heptyl-4-hydroxyquinoline N-oxide) and sodium azide, which significantly reduce energy production by oxidative phosphorylation. These findings strongly indicate that energy taxis is the primary force in environmental navigation by C. jejuni and that this mechanism drives the organism toward the optimal chemical conditions for energy generation and colonization.

Flexibility of Vibrio cholerae ToxT in transcription activation of genes having altered promoter spacing

Cholera, a severe diarrheal disease, is caused by ingestion of the gram-negative bacterium Vibrio cholerae. Expression of V. cholerae virulence factors is highly regulated at the transcriptional and posttranscriptional levels by a complex network of proteins and small noncoding RNAs. The direct activator of transcription of most V. cholerae virulence genes is the ToxT protein. ToxT binds to a 13-bp sequence, the toxbox, located upstream of genes in its regulon. However, the organization of toxboxes relative to each other and to the core promoter elements at different genes varies dramatically. At different ToxT-activated genes a single toxbox may be necessary and sufficient for full activation, or pairs of toxboxes organized as either inverted or direct repeats may be required for full activation. Although all toxboxes are located at positions consistent with a class I promoter architecture, the locations of toxboxes relative to the transcription start site also vary from gene to gene. To further assess the ability of ToxT to activate transcription from different configurations relative to the core promoter elements, we constructed promoter-lacZ fusions having altered spacing both between toxbox pairs and between the promoter-proximal toxbox and the -35 box at five different ToxT-activated promoters. Our results suggest that that ToxT has remarkable flexibility in its positioning as a transcription activator and that different interactions between ToxT and RNA polymerase occur during transcription activation of promoters having different toxbox configurations.

Regulation of virulence in Vibrio cholerae: the ToxR regulon

Vibrio cholerae is a gram-negative bacterium that is the causative agent of cholera. This disease consists of enormous fluid loss through stools, which can be fatal. Cholera epidemics appear in explosive outbreaks that have occurred repeatedly throughout history. The virulence factors toxin coregulated pilus (TCP) and cholera toxin (CT) are essential for colonization of the host and enterotoxicity, respectively. These virulence factors are under the control of ToxT, an AraC/XylS family protein that activates transcription of the genes encoding TCP and CT. ToxT is under the control of a virulence regulatory cascade known as the ToxR regulon, which responds to environmental stimuli to ensure maximal virulence-factor induction within the human intestine. An understanding of this intricate signaling pathway is essential for the development of methods to treat and prevent this devastating disease.

Only one of the five CheY homologs in Vibrio cholerae directly switches flagellar rotation

Vibrio cholerae has three sets of chemotaxis (Che) proteins, including three histidine kinases (CheA) and four response regulators (CheY) that are encoded by three che gene clusters. We deleted the cheY genes individually or in combination and found that only the cheY3 deletion impaired chemotaxis, reinforcing the previous conclusion that che cluster II is involved in chemotaxis. However, this does not exclude the involvement of the other clusters in chemotaxis. In other bacteria, phospho-CheY binds directly to the flagellar motor to modulate its rotation, and CheY overexpression, even without CheA, causes extremely biased swimming behavior. We reasoned that a V. cholerae CheY homolog, if it directly controls flagellar rotation, should also induce extreme swimming behavior when overproduced. This was the case for CheY3 (che cluster II). However, no other CheY homolog, including the putative CheY (CheY0) protein encoded outside the che clusters, affected swimming, demonstrating that these CheY homologs cannot act directly on the flagellar motor. CheY4 very slightly enhanced the spreading of an Escherichia coli cheZ mutant in semisolid agar, raising the possibility that it can affect chemotaxis by removing a phosphoryl group from CheY3. We also found that V. cholerae CheY3 and E. coli CheY are only partially exchangeable. Mutagenic analyses suggested that this may come from coevolution of the interacting pair of proteins, CheY and the motor protein FliM. Taken together, it is likely that the principal roles of che clusters I and III as well as cheY0 are to control functions other than chemotaxis.

Going against the grain: chemotaxis and infection in Vibrio cholerae

Chemotaxis is the process by which motile cells move in a biased manner both towards favourable and away from unfavourable environments. The requirement of this process for infection has been examined in several bacterial pathogens, including Vibrio cholerae. The single polar flagellum of Vibrio species is powered by a sodium-motive force across the inner membrane, and can rotate to produce speeds of up to 60 cell-body lengths (approximately 60microm) per second. Investigating the role of the chemotactic control of rapid flagellar motility during V. cholerae infection has revealed some unexpected and intriguing results.

Campylobacter jejuni gene expression in the chick cecum: evidence for adaptation to a low-oxygen environment

Transcriptional profiling of Campylobacter jejuni during colonization of the chick cecum identified 59 genes that were differentially expressed in vivo compared with the genes in vitro. The data suggest that C. jejuni regulates electron transport and central metabolic pathways to alter its physiological state during establishment in the chick cecum.

Chemotaxis in Vibrio cholerae

The ability of motile bacteria to swim toward or away from specific environmental stimuli, such as nutrients, oxygen, or light provides cells with a survival advantage, especially under nutrient-limiting conditions. This behavior, called chemotaxis, is mediated by the bacteria changing direction by briefly reversing the direction of rotation of the flagellar motors. A sophisticated signal transduction system, consisting of signal transducer proteins, a histidine kinase, a response regulator, a coupling protein, and enzymes that mediate sensory adaptation, relates the input signal to the flagellar motor. Chemotaxis has been extensively studied in bacteria such as Escherichia coli and Salmonella enterica serovar Typhimurium, and depends on the activity of single copies of proteins in a linear pathway. However, growing evidence suggests that chemotaxis in other bacteria is more complex with many bacterial species having multiple paralogues of the various chemotaxis genes found in E. coli and, in most cases, the detailed functions of these potentially redundant genes have not been elucidated. Although the completed genome of Vibrio cholerae, the causative agent of cholera, predicted a multitude of genes with homology to known chemotaxis-related genes, little is known about their relative contribution to chemotaxis or other cellular functions. Furthermore, the role of chemotaxis during the environmental or infectious phases of this organism is not yet fully understood. This review will focus on the complex relationship between chemotaxis and virulence in V. cholerae.

Examination of diverse toxin-coregulated pilus-positive Vibrio cholerae strains fails to demonstrate evidence for Vibrio pathogenicity island phage

The major virulence factors of toxigenic Vibrio cholerae are cholera toxin, which is encoded by a lysogenic filamentous bacteriophage (CTXPhi), and toxin-coregulated pilus (TCP), an essential colonization factor that is also the receptor for CTXPhi. The genes involved in the biosynthesis of TCP reside in a pathogenicity island, which has been reported to correspond to the genome of another filamentous phage (designated VPIPhi) and to encode functions necessary for the production of infectious VPIPhi particles. We examined 46 V. cholerae strains having diverse origins and carrying different genetic variants of the TCP island for the production of the VPIPhi and CTXPhi in different culture conditions, including induction of prophages with mitomycin C and UV irradiation. Although 9 of 10 V. cholerae O139 strains and 12 of 15 toxigenic El Tor strains tested produced extracellular CTXPhi, none of the 46 TCP-positive strains produced detectable VPIPhi in repeated assays, which detected as few as 10 particles of a control CTX phage per ml. These results contradict the previous report regarding VPIPhi-mediated horizontal transfer of the TCP genes and suggest that the TCP island is unable to support the production of phage particles. Further studies are necessary to understand the mechanism of horizontal transfer of the TCP island.

Pathogenic potential of environmental Vibrio cholerae strains carrying genetic variants of the toxin-coregulated pilus pathogenicity island

The major virulence factors of toxigenic Vibrio cholerae are cholera toxin (CT), which is encoded by a lysogenic bacteriophage (CTXPhi), and toxin-coregulated pilus (TCP), an essential colonization factor which is also the receptor for CTXPhi. The genes for the biosynthesis of TCP are part of a larger genetic element known as the TCP pathogenicity island. To assess their pathogenic potential, we analyzed environmental strains of V. cholerae carrying genetic variants of the TCP pathogenicity island for colonization of infant mice, susceptibility to CTXPhi, and diarrheagenicity in adult rabbits. Analysis of 14 environmental strains, including 3 strains carrying a new allele of the tcpA gene, 9 strains carrying a new allele of the toxT gene, and 2 strains carrying conventional tcpA and toxT genes, showed that all strains colonized infant mice with various efficiencies in competition with a control El Tor biotype strain of V. cholerae O1. Five of the 14 strains were susceptible to CTXPhi, and these transductants produced CT and caused diarrhea in adult rabbits. These results suggested that the new alleles of the tcpA and toxT genes found in environmental strains of V. cholerae encode biologically active gene products. Detection of functional homologs of the TCP island genes in environmental strains may have implications for understanding the origin and evolution of virulence genes of V. cholerae.

Two predicted chemoreceptors of Helicobacter pylori promote stomach infection

Helicobacter pylori must be motile or display chemotaxis to be able to fully infect mammals, but it is not known how this chemotaxis is directed. We disrupted two genes encoding predicted chemoreceptors, tlpA and tlpC. H. pylori mutants lacking either of these genes are fully motile and chemotactic in vitro and are as able as the wild type to infect mice when they are the sole infecting strains. In contrast, when mice are coinfected with the H. pylori SS1 tlpA or tlpC mutant and the wild type, we find more wild type than mutant after 2 weeks of colonization. Neither strain has an in vitro growth defect. These results suggest that the tlpA- and tlpC-encoded proteins assist colonization of the stomach environment.

Sodium ion cycle in bacterial pathogens: evidence from cross-genome comparisons

Analysis of the bacterial genome sequences shows that many human and animal pathogens encode primary membrane Na+ pumps, Na+-transporting dicarboxylate decarboxylases or Na+ translocating NADH:ubiquinone oxidoreductase, and a number of Na+ -dependent permeases. This indicates that these bacteria can utilize Na+ as a coupling ion instead of or in addition to the H+ cycle. This capability to use a Na+ cycle might be an important virulence factor for such pathogens as Vibrio cholerae, Neisseria meningitidis, Salmonella enterica serovar Typhi, and Yersinia pestis. In Treponema pallidum, Chlamydia trachomatis, and Chlamydia pneumoniae, the Na+ gradient may well be the only energy source for secondary transport. A survey of preliminary genome sequences of Porphyromonas gingivalis, Actinobacillus actinomycetemcomitans, and Treponema denticola indicates that these oral pathogens also rely on the Na+ cycle for at least part of their energy metabolism. The possible roles of the Na+ cycling in the energy metabolism and pathogenicity of these organisms are reviewed. The recent discovery of an effective natural antibiotic, korormicin, targeted against the Na+ -translocating NADH:ubiquinone oxidoreductase, suggests a potential use of Na+ pumps as drug targets and/or vaccine candidates. The antimicrobial potential of other inhibitors of the Na+ cycle, such as monensin, Li+ and Ag+ ions, and amiloride derivatives, is discussed.

Role of sodium bioenergetics in Vibrio cholerae

The ability of the bacterium to use sodium in bioenergetic processes appears to play a key role in both the environmental and pathogenic phases of Vibrio cholerae. Aquatic environments, including fresh, brackish, and coastal waters, are an important factor in the transmission of cholera and an autochthonous source. The organism is considered to be halophilic and has a strict requirement for Na(+) for growth. Furthermore, expression of motility and virulence factors of V. cholerae is intimately linked to sodium bioenergetics and to each other. Several lines of evidence indicated that the activity of the flagellum of V. cholerae might have an impact on virulence gene regulation. As the V. cholerae flagellum is sodium-driven and the Na(+)-NQR enzyme is known to create a sodium motive force across the bacterial membrane, it was recently suggested that the increased toxT expression observed in a nqr-negative strain is mediated by affecting flagella activity. It was suggested that the V. cholerae flagellum might respond to changes in membrane potential and the resulting changes in flagellar rotation might serve as a signal for virulence gene expression. However, we recently demonstrated that although the flagellum of V. cholerae is not required for the effects of ionophores on virulence gene expression, changes in the sodium chemical potential are sensed and thus alternative mechanisms, perhaps involving the TcpP/H proteins, for the detection of these conditions must exist. Analyzing the underlying mechanisms by which bacteria respond to changes in the environment, such as their ability to monitor the level of membrane potential, will probably reveal complex interplays between basic physiological processes and virulence factor expression in a variety of pathogenic species.

Analysis of the role of flagellar activity in virulence gene expression in Vibrio cholerae

Expression of virulence factors and motility of VIBRIO: cholerae are intimately linked by an as yet uncharacterized mechanism. Several lines of evidence indicate that the activity of the flagellum of V. cholerae might have an impact on virulence gene regulation, as alterations of the motility phenotype, either by mutation or by inhibitory drugs, result in varied levels of virulence factor production. The Na(+)-driven polar flagella of some VIBRIO: species are proposed to act as mechanosensors, sensing media viscosity. It has been suggested that the V. cholerae flagellum might act as a 'voltmeter', responding to changes in membrane potential, or might sense some environmental conditions that lead to the repression of virulence factors in V. cholerae. To test these hypotheses, beta-galactosidase levels of several types of non-motile mutant derivatives of a V. cholerae toxT::lacZ reporter strain were analysed following changes in media viscosity, membrane potential and other environmental conditions. Like the parental strain, the non-motile strain showed increased toxT::lacZ expression in high-viscosity media, suggesting that the sensing of media viscosity does not occur via the flagella. Other molecules that might be able to detect changes in media viscosity could include mechanosensitive (MS) ion channels found in the bacterial membrane. However, a V. cholerae derivative strain mutated in two putative MS channels still showed increased toxT::lacZ expression in high-viscosity media, indicating that these putative ion channels of V. cholerae are not involved in the viscosity effect and suggesting an as yet uncharacterized mechanism for sensing of media viscosity. The flagellum does not appear to act as a voltmeter, as beta-galactosidase activities of the non-flagellate derivative strain were found to be similar to those of the parental strain after artificially changing the sodium membrane bioenergetics. Similarly, several environmental conditions known to reduce toxin expression were equally effective in reducing toxT::lacZ expression in the motile or non-motile strains. In conclusion, the flagellum of V. cholerae does not act as a mechanosensor, voltmeter or signal transducer for environmental conditions. Thus, alternative mechanisms for the detection of these conditions must exist that likely do not involve the ToxR molecule, as the sensing of all of the tested parameters occurred when the TcpP/H proteins alone activated the toxT::lacZ reporter gene.

Bacterial virulence gene regulation: an evolutionary perspective

Coevolution between bacteria and their plant or animal hosts determines characteristics of the interaction, the bacterial virulence genes involved, and the regulatory systems controlling expression of virulence genes. The long-standing association between Salmonellae and their animal hosts has resulted in the acquisition by Salmonella subspecies of a variety of virulence genes and the evolution of complex regulatory networks. The particular repertoire of virulence genes acquired by different Salmonella enterica subspecies and the regulatory systems that control them dictate subspecies-specific infection characteristics. Although the association between Vibrio cholerae and humans appears to be more recent, to reflect a simpler pathogenic strategy, and to involve fewer virulence genes than that of Salmonellae, complex virulence-regulatory networks have nonetheless evolved. In contrast, there is no evidence for acquisition of virulence genes by horizontal gene transfer in bordetellae, and their virulence regulon is less complex in overall structure than those of salmonellae and Vibrio cholerae. In Bordetellae, subspecies-specific differences in pathogenic strategy appear to result from differential gene expression within and across Bordetella subspecies.

Vibrio cholerae intestinal population dynamics in the suckling mouse model of infection

The suckling mouse has been used as a model to identify Vibrio cholerae intestinal colonization factors for over two decades, yet little is known about the location of recoverable organisms along the gastrointestinal (GI) tract following intragastric inoculation. In the present study, we determined the population dynamics of wild-type and avirulent mutant derivatives of both classical and El Tor biotype strains throughout the entire suckling mouse GI tract at various times after intragastric inoculation. Wild-type strains preferentially colonized the middle small bowel with a sharp demarcation between more proximal segments which had manyfold-fewer recoverable cells. Surprisingly, large and stable populations of viable cells were also recovered from the cecum and large bowel. Strains lacking toxin-coregulated pili (TCP(-)) were cleared from the small bowel; however, an El Tor TCP(-) strain colonized the cecum and large bowel almost as well as the wild-type strain. Strains lacking lipopolysaccharide O antigen (OA(-)) were efficiently cleared from the small bowel at early times but then showed net growth for the remainder of the infections. Moreover, large populations of the OA(-) strains were maintained in the large bowel. These results show that for the El Tor biotype neither TCP nor OA is required for colonization of the suckling mouse large bowel. Finally, similar percent recoveries of wild-type, TCP(-), and OA(-) strains from the small bowel at an early time after infection suggest that TCP and OA are not required for strains of either biotype to resist bactericidal mechanisms in the suckling mouse GI tract.

Molecular characterization of a new ribotype of Vibrio cholerae O139 Bengal associated with an outbreak of cholera in Bangladesh

Vibrio cholerae O139 Bengal initially appeared in the southern coastal region of Bangladesh and spread northward, causing explosive epidemics during 1992 and 1993. The resurgence of V. cholerae O139 during 1995 after its transient displacement by a new clone of El Tor vibrios demonstrated rapid changes in the epidemiology of cholera in Bangladesh. A recent outbreak of cholera in two north-central districts of Bangladesh caused by V. cholerae O139 led us to analyze strains collected from the outbreak and compare them with V. cholerae O139 strains isolated from other regions of Bangladesh and neighboring India to investigate their origins. Analysis of restriction fragment length polymorphisms in genes for conserved rRNA (ribotype) revealed that the recently isolated V. cholerae O139 strains belonged to a new ribotype which was distinct from previously described ribotypes of toxigenic V. cholerae O139. All strains carried the genes for toxin-coregulated pili (tcpA and tcpI) and accessory colonization factor (acfB), the regulatory gene toxR, and multiple copies of the lysogenic phage genome encoding cholera toxin (CTXPhi) and belonged to a previously described ctxA genotype. Comparative analysis of the rfb gene cluster by PCR revealed the absence of a large region of the O1-specific rfb operon downstream of the rfaD gene and the presence of an O139-specific genomic region in all O139 strains. Southern hybridization analysis of the O139-specific genomic region also produced identical restriction patterns in strains belonging to the new ribotype and those of previously described ribotypes. These results suggested that the new ribotype of Bengal vibrios possibly originated from an existing strain of V. cholerae O139 by genetic changes in the rRNA operons. In contrast to previously isolated O139 strains which mostly had resistance to trimethoprim, sulfamethoxazole, and streptomycin encoded by a transposon (SXT element), 68.6% of the toxigenic strains analyzed in the present study, including all strains belonging to the new ribotype, were susceptible to these antibiotics. Molecular analysis of the SXT element revealed possible deletion of a 3.6-kb region of the SXT element in strains which were susceptible to the antibiotics. Thus, V. cholerae O139 strains in Bangladesh are also undergoing considerable reassortments in genetic elements encoding antimicrobial resistance.

Effects of changes in membrane sodium flux on virulence gene expression in Vibrio cholerae

The expression of several virulence factors of Vibrio cholerae is coordinately regulated by the ToxT molecule and the membrane proteins TcpP/H and ToxR/S, which are required for toxT transcription. To identify proteins that negatively affect toxT transcription, we screened transposon mutants of V. cholerae carrying a chromosomally integrated toxT::lacZ reporter construct for darker blue colonies on media containing 5-bromo-4-chlor-3-indolyl beta-D galactoside (X-gal). Two mutants had transposon insertions in a region homologous to the nqr gene cluster of Vibrio alginolyticus, encoding a sodium-translocating NADH-ubiquinone oxidoreductase (NQR). In V. alginolyticus, NQR is a respiration-linked Na+ extrusion pump generating a sodium motive force that can be used for solute import, ATP synthesis, and flagella rotation. Inhibition of NQR enzyme function in V. cholerae by the specific inhibitor 2-n-heptyl-4-hydroxyquinoline N-oxide (HQNO) resulted in elevated toxT::lacZ activity. Increased toxT::lacZ expression in an nqr mutant strain compared with the parental strain was observed when the TcpP/H molecules alone were strongly expressed, suggesting that the negative effect of the NQR complex on toxT transcription is mediated through TcpP/H. However, the ability of the TcpP/H proteins to activate the toxT::lacZ reporter construct was greatly diminished in the presence of high NaCl concentrations in the growth medium. The flagellar motor of V. cholerae appears to be driven by a sodium motive force, and modulation of flagella rotation by inhibitory drugs, high media viscosity, or specific mutations resulted in increases of toxT::lacZ expression. Thus, the regulation of the main virulence factors of V. cholerae appears to be modulated by endogenous and exogenous sodium levels in a complex way.

Analysis of clinical and environmental strains of nontoxigenic Vibrio cholerae for susceptibility to CTXPhi: molecular basis for origination of new strains with epidemic potential

Toxigenic Vibrio cholerae strains are lysogens of CTXPhi, a filamentous phage which encodes cholera toxin. The receptor for CTXPhi for invading V. cholerae cells is the toxin-coregulated pilus (TCP), the genes for which reside in a larger genetic element, the TCP pathogenicity island. We analyzed 146 CTX-negative strains of V. cholerae O1 or non-O1 isolated from patients or surface waters in five different countries for the presence of the TCP pathogenicity island, the regulatory gene toxR, and the CTXPhi attachment sequence attRS, as well as for susceptibility of the strains to CTXPhi, to investigate the molecular basis for the emergence of new clones of toxigenic V. cholerae. DNA probe or PCR assays for tcpA, tcpI, acfB, toxR, and attRS revealed that 6.85% of the strains, all of which belonged to the O1 serogroup, carried the TCP pathogenicity island, toxR, and multiple copies of attRS, whereas the remaining 93.15% of the strains were negative for TCP but positive for either one or both or neither of toxR and attRS. An analysis of the strains for susceptibility to CTXPhi, using a genetically marked derivative of the phage CTX-KmPhi, showed that all TCP-positive CTX-negative strains and 1 of 136 TCP-negative strains were infected by the phage either in vitro or in the intestines of infant mice. The phage genome integrated into the chromosome of infected V. cholerae O1 cells forming stable lysogens. Comparative analysis of rRNA gene restriction patterns revealed that the lysogens derived from nontoxigenic progenitors were either closely related to or distinctly different from previously described clones of toxigenic V. cholerae. To our knowledge, this is the first demonstration of lysogenic conversion of naturally occurring nontoxigenic V. cholerae strains by CTXPhi. The results of this study further indicated that strains belonging to the O1 serogroup of V. cholerae are more likely to possess the TCP pathogenicity island and hence to be infected by CTXPhi, leading to the origination of potential new epidemic clones.

Epidemiology, genetics, and ecology of toxigenic Vibrio cholerae

Cholera caused by toxigenic Vibrio cholerae is a major public health problem confronting developing countries, where outbreaks occur in a regular seasonal pattern and are particularly associated with poverty and poor sanitation. The disease is characterized by a devastating watery diarrhea which leads to rapid dehydration, and death occurs in 50 to 70% of untreated patients. Cholera is a waterborne disease, and the importance of water ecology is suggested by the close association of V. cholerae with surface water and the population interacting with the water. Cholera toxin (CT), which is responsible for the profuse diarrhea, is encoded by a lysogenic bacteriophage designated CTXPhi. Although the mechanism by which CT causes diarrhea is known, it is not clear why V. cholerae should infect and elaborate the lethal toxin in the host. Molecular epidemiological surveillance has revealed clonal diversity among toxigenic V. cholerae strains and a continual emergence of new epidemic clones. In view of lysogenic conversion by CTXPhi as a possible mechanism of origination of new toxigenic clones of V. cholerae, it appears that the continual emergence of new toxigenic strains and their selective enrichment during cholera outbreaks constitute an essential component of the natural ecosystem for the evolution of epidemic V. cholerae strains and genetic elements that mediate the transfer of virulence genes. The ecosystem comprising V. cholerae, CTXPhi, the aquatic environment, and the mammalian host offers an understanding of the complex relationship between pathogenesis and the natural selection of a pathogen.

Comparison of sample sequences of the Salmonella typhi genome to the sequence of the complete Escherichia coli K-12 genome

Raw sequence data representing the majority of a bacterial genome can be obtained at a tiny fraction of the cost of a completed sequence. To demonstrate the utility of such a resource, 870 single-stranded M13 clones were sequenced from a shotgun library of the Salmonella typhi Ty2 genome. The sequence reads averaged over 400 bases and sampled the genome with an average spacing of once every 5,000 bases. A total of 339,243 bases of unique sequence was generated (approximately 7% representation). The sample of 870 sequences was compared to the complete Escherichia coli K-12 genome and to the rest of the GenBank database, which can also be considered a collection of sampled sequences. Despite the incomplete S. typhi data set, interesting categories could easily be discerned. Sixteen percent of the sequences determined from S. typhi had close homologs among known Salmonella sequences (P < 1e-40 in BlastX or BlastN), reflecting the proportion of these genomes that have been sequenced previously; 277 sequences (32%) had no apparent orthologs in the complete E. coli K-12 genome (P > 1e-20), of which 155 sequences (18%) had no close similarities to any sequence in the database (P > 1e-5). Eight of the 277 sequences had similarities to genes in other strains of E. coli or plasmids, and six sequences showed evidence of novel phage lysogens or sequence remnants of phage integrations, including a member of the lambda family (P < 1e-15). Twenty-three sample sequences had a significantly closer similarity a sequence in the database from organisms other than the E. coli/Salmonella clade (which includes Shigella and Citrobacter). These sequences are new candidate lateral transfer events to the S. typhi lineage or deletions on the E. coli K-12 lineage. Eleven putative junctions of insertion/deletion events greater than 100 bp were observed in the sample, indicating that well over 150 such events may distinguish S. typhi from E. coli K-12. The need for automatic methods to more effectively exploit sample sequences is discussed.

The lipopolysaccharide biosynthesis locus of Campylobacter jejuni 81116

Most Campylobacter jejuni strains express lipo-oligosaccharides. Some strains also express lipopolysaccharides (LPS), with O-antigen-like carbohydrate repeats. C. jejuni 81116 expresses an LPS containing both lipo-oligosaccharides and O-antigen-like repeats, but nothing is known about the structure or sugar composition of these LPS species. A cosmid library of the genome of C. jejuni 81116 was constructed and probed with Campylobacter hyoilei genes involved in LPS synthesis. Five cosmids hybridized with the probe and two of these expressed C. jejuni 81116 LPS in Escherichia coli. By subcloning, a 16 kb DNA region was identified which contains the genetic information required to express C. jejuni LPS. DNA sequence analysis revealed 11 ORFs homologous to genes involved in LPS synthesis of other bacteria. They consisted of three homologues of sugar biosynthesis genes, two homologues of transport genes and six homologues of sugar transferases.

Analysis of ToxR-dependent transcription activation of ompU, the gene encoding a major envelope protein in Vibrio cholerae

The membrane proteins ToxR and ToxS regulate a variety of genes associated with the virulence of Vibrio cholerae, the agent of human cholera. One of the ToxRS-regulated genes is the ompU gene, which encodes a porin that may also act as an adhesin. To begin to understand the mechanism of ompU transcription activation by ToxRS, we performed genetic and biochemical studies on the ompU promoter. Deletions with a 5' end-point at or downstream of -128, relative to the start site for transcription, did not direct expression of a lacZ reporter gene in wild-type V. cholerae, although the -128 promoter fragment did direct ToxRS-dependent reporter gene activity under conditions of ToxR overexpression in E. coli. Consistent with the activation data is that membranes containing ToxR and ToxS caused a gel electrophoretic mobility shift when mixed at low concentrations with deletion fragments whose end-point is at -211, but not with -128 or -68 fragments. ToxRS membranes did shift the -128 fragment when added at higher concentrations. DNase I footprinting analysis of ompU promoter DNA complexed with ToxRS membranes demonstrated protection of three sites: an upstream site ranging from -238 to -139, and two downstream sites ranging from -116 to -58 and -53 to -24. Within the DNA protected from DNase I digestion by ToxRS membranes, there are no elements bearing similarity to those identified previously within the promoters of two other ToxR-dependent genes, ctxA and toxT. We suggest a model for transcription activation that involves sequential ToxR-binding events to distinct regions in the ompU promoter.

Rhizobium leguminosarum contains a group of genes that appear to code for methyl-accepting chemotaxis proteins

Methyl-accepting chemotaxis proteins (MCPs) play important roles in the chemotactic response of many bacteria. Oligonucleotide primers designed to amplify the conserved signalling domain of MCPs by PCR were used to identify potential MCP-encoding genes in Rhizobium leguminosarum. Using a PCR-derived probe created from these primers a genomic library of R. leguminosarum VF39SM was screened; at least five putative MCP-encoding genes (termed mcpB to mcpF) were identified and isolated from the library. One of these putative genes (mcpC) is located on one of the indigenous plasmids of VF39SM. Fifteen different cosmids showing homology to an mcpD probe were also isolated from a genomic library. The complete DNA sequences of mcpB, mcpC and mcpD were obtained. All three genes code for proteins with characteristics typical of MCPs. However, the protein encoded by mcpB has a relatively large periplasmic domain compared to that in other MCPs. Partial DNA sequences of mcpE and mcpF had strong similarity to sequences from the methylation domains of known MCPs. Mutants defective in mcpB, mcpC, mcpD or mcpE were created using insertional mutagenesis strategies. Mutation of mcpB resulted in impairment of chemotaxis to a wide range of carbon sources on swarm plates; phenotypes for the other three mutants have yet to be elucidated. The mcpB, mcpC and mcpD mutants were tested for loss of nodulation competitiveness. When co-inoculated with the wild-type, the mcpB and mcpC mutants formed fewer nodules than the wild-type, whereas the mcpD mutant was just as competitive as the wild-type. The results overall suggest that R. leguminosarum possesses mcp-like genes, and that at least some of these play a role in early steps in the plant-microbe interaction.

Alterations in Vibrio cholerae motility phenotypes correlate with changes in virulence factor expression

Motility is thought to contribute to the virulence of Vibrio cholerae, but the role it plays in pathogenesis is not completely understood. To investigate the influence of motility on virulence gene expression and intestinal colonization, we have isolated mutants with altered swarming abilities in soft agar medium. Both spontaneous hyperswarmer (exhibiting faster swarm rates) and spontaneous or transposon-induced nonmotile mutants of strain 0395 were obtained. Surprisingly, we found that two of three classes of hyperswarmer mutants were defective in autoagglutination, a phenotype associated with expression of toxin-coregulated pili (TCP), an essential ToxR-regulated colonization factor of V. cholerae. In contrast, nonmotile mutants exhibited autoagglutination under growth conditions that normally repress this phenotype. Further characterization of mutant strains revealed differences in the expression of other virulence determinants. Class I hyperswarmer mutants were defective in production of TCP, cholera toxin, and a cell-associated hemolysin but showed increased levels of protease and fucose-sensitive hemagglutinin. All nonmotile mutants examined, including those with insertions in a sequence homologous to motB, exhibited increased expression of TCP pilin, cholera toxin, and cell-associated hemolysin but dramatically decreased levels of fucose-sensitive hemagglutinin and HEp-2 adhesins. In general, nonmotile mutants displayed few or no defects in intestinal colonization, while class I hypermotile mutants were highly defective in colonization. These results suggest that the motility phenotype of V. cholerae is tightly coupled to the expression of multiple ToxR-regulated and non-ToxR-regulated virulence determinants.

Comparison of the promoter proximal regions of the toxin-co-regulated tcp gene cluster in classical and El Tor strains of Vibrio cholerae O1

A physical map has been constructed of the 5-kb XbaI fragment encoding the promoter proximal of region the tcp gene cluster encoding the toxin-coregulated pilus (TCP) of Vibrio cholerae. This fragment contains the major regulatory regions for TCP. Comparison of the nucleotide (nt) sequences from strains of the classical and El Tor biotypes demonstrates that the regions are essentially identical, with several notable exceptions. The intergenic regions, between tcpI and tcpP, and between tcpH and tcpA, show significant sequence divergence which may account for the biotype-related differences in TCP, since this is the location of the major promoter sequences. The C-terminal coding regions of the major pilin subunit, TcpA, also differ. Southern hybridization analyses suggest that the tcpA nt sequence is conserved within a biotype, and Western blot analysis suggests that the two forms of TcpA are antigenically different, but related. Besides tcpA, tcpB, tcpH and tcpI, the genes encoding two additional proteins, TcpP and TcpQ, but not previously defined, were also identified. TcpH and TcpI have been previously suggested to be regulatory proteins but homology data imply that TcpI is a methyl-accepting chemotaxis protein (MCP), as recently reported [Harkey et al., Infect. Immun. 62 (1994) 2669-2678], and TcpH is predicted to be a periplasmic or exported protein. TcpP is thought to be a trans-cytoplasmic membrane (CM) protein which may have a regulatory role.