Whole genome PCR scanning reveals the syntenic genome structure of toxigenic Vibrio cholerae strains in the O1/O139 population.

Vibrio cholerae is commonly found in estuarine water systems. Toxigenic O1 and O139 V. cholerae strains have caused cholera epidemics and pandemics, whereas the nontoxigenic strains within these serogroups only occasionally lead to disease. To understand the differences in the genome and clonality between the toxigenic and nontoxigenic strains of V. cholerae serogroups O1 and O139, we employed a whole genome PCR scanning (WGPScanning) method, an rrn operon-mediated fragment rearrangement analysis and comparative genomic hybridization (CGH) to analyze the genome structure of different strains. WGPScanning in conjunction with CGH revealed that the genomic contents of the toxigenic strains were conservative, except for a few indels located mainly in mobile elements. Minor nucleotide variation in orthologous genes appeared to be the major difference between the toxigenic strains. rrn operon-mediated rearrangements were infrequent in El Tor toxigenic strains tested using I-CeuI digested pulsed-field gel electrophoresis (PFGE) analysis and PCR analysis based on flanking sequence of rrn operons. Using these methods, we found that the genomic structures of toxigenic El Tor and O139 strains were syntenic. The nontoxigenic strains exhibited more extensive sequence variations, but toxin coregulated pilus positive (TCP+) strains had a similar structure. TCP+ nontoxigenic strains could be subdivided into multiple lineages according to the TCP type, suggesting the existence of complex intermediates in the evolution of toxigenic strains. The data indicate that toxigenic O1 El Tor and O139 strains were derived from a single lineage of intermediates from complex clones in the environment. The nontoxigenic strains with non-El Tor type TCP may yet evolve into new epidemic clones after attaining toxigenic attributes.

Proteins involved in difference of sorbitol fermentation rates of the toxigenic and nontoxigenic Vibrio cholerae El Tor strains revealed by comparative proteome analysis.

BACKGROUND: The nontoxigenic V. cholerae El Tor strains ferment sorbitol faster than the toxigenic strains, hence fast-fermenting and slow-fermenting strains are defined by sorbitol fermentation test. This test has been used for more than 40 years in cholera surveillance and strain analysis in China. Understanding of the mechanisms of sorbitol metabolism of the toxigenic and nontoxigenic strains may help to explore the genome and metabolism divergence in these strains. Here we used comparative proteomic analysis to find the proteins which may be involved in such metabolic difference. RESULTS: We found the production of formate and lactic acid in the sorbitol fermentation medium of the nontoxigenic strain was earlier than of the toxigenic strain. We compared the protein expression profiles of the toxigenic strain N16961 and nontoxigenic strain JS32 cultured in sorbitol fermentation medium, by using fructose fermentation medium as the control. Seventy-three differential protein spots were found and further identified by MALDI-MS. The difference of product of fructose-specific IIA/FPR component gene and mannitol-1-P dehydrogenase, may be involved in the difference of sorbitol transportation and dehydrogenation in the sorbitol fast- and slow-fermenting strains. The difference of the relative transcription levels of pyruvate formate-lyase to pyruvate dehydrogenase between the toxigenic and nontoxigenic strains may be also responsible for the time and ability difference of formate production between these strains. CONCLUSION: Multiple factors involved in different metabolism steps may affect the sorbitol fermentation in the toxigenic and nontoxigenic strains of V. cholerae El Tor.

[Molecular characterization of Vibrio cholerae phage-type 6b epidemic isolates from 1998 to 2001 in Sichuan province].

OBJECTIVE: To investigate the molecular characteristics of phage-type 6b isolates emerging in 1998-2001 cholera epidemics in Sichuan province. METHODS: Isolates were analyzed by phage-typing, pulsed field gel electrophoresis (PFGE) and ompW gene sequencing. RESULTS: All phage-type 1b and 6b isolates in Sichuan province from 1998 to 2001 were toxigenic. A total of 24 patterns were identified after PFGE analysis, and one predominant pattern consisted of 13 isolates. Several 1b and 6b isolates from Sichuan and isolates of the 1b from other provinces showed the same PFGE pattern. Mutation in ompW gene was found in 6b isolates. CONCLUSION: V.cholerae O1 6b isolates in Sichuan province from 1998 to 2001 have special genetic markers, and might genetically correlate with contemporaneous 1b isolates.

Pleiotropic effects of the twin-arginine translocation system on biofilm formation, colonization, and virulence in Vibrio cholerae.

BACKGROUND: The Twin-arginine translocation (Tat) system serves to translocate folded proteins, including periplasmic enzymes that bind redox cofactors in bacteria. The Tat system is also a determinant of virulence in some pathogenic bacteria, related to pleiotropic effects including growth, motility, and the secretion of some virulent factors. The contribution of the Tat pathway to Vibrio cholerae has not been explored. Here we investigated the functionality of the Tat system in V. cholerae, the etiologic agent of cholera. RESULTS: In V. cholerae, the tatABC genes function in the translocation of TMAO reductase. Deletion of the tatABC genes led to a significant decrease in biofilm formation, the ability to attach to HT-29 cells, and the ability to colonize suckling mouse intestines. In addition, we observed a reduction in the output of cholera toxin, which may be due to the decreased transcription level of the toxin gene in tatABC mutants, suggesting an indirect effect of the mutation on toxin production. No obvious differences in flagellum biosynthesis and motility were found between the tatABC mutant and the parental strain, showing a variable effect of Tat in different bacteria. CONCLUSION: The Tat system contributes to the survival of V. cholerae in the environment and in vivo, and it may be associated with its virulence.

Molecular characterization of a novel adult diarrhoea rotavirus strain J19 isolated in China and its significance for the evolution and origin of group B rotaviruses.

The complete genome of a novel adult diarrhoea rotavirus strain J19 was cloned and sequenced using an improved single-primer sequence-independent method. The complete genome is 17,961 bp and is AU-rich (66.49 %). Northern blot analysis and genomic sequence analysis indicated that segments 1-11 encode 11 viral proteins, respectively. Protein alignments with the corresponding proteins of J19 with B219, and groups A, B and C rotaviruses, produced higher per cent sequence identities to B219. Among groups A, B and C rotaviruses, 10 proteins from group B rotaviruses exhibited slightly higher amino acid sequence identity to the J19 proteins, but proteins of J19 showed low amino acid sequence identity with groups A and C rotaviruses. Construction of unrooted phylogenetic trees using a set of known proteins and representatives of three known rotavirus groups revealed that six structural proteins were positioned close to B219 and the basal nodes of groups A, B and C lineages, although with a preferred association with group B lineages. Phylogenetic analysis of the five non-structural proteins showed a similar trend. The results of the serological analysis, protein sequence analysis and phylogenetic analysis suggested that J19 would be a novel rotavirus strain with great significance to the evolution and origin of group B rotaviruses.

Genetic diversity of toxigenic and nontoxigenic Vibrio cholerae serogroups O1 and O139 revealed by array-based comparative genomic hybridization.

Toxigenic serogroups O1 and O139 of Vibrio cholerae may cause cholera epidemics or pandemics. Nontoxigenic strains within these serogroups also exist in the environment, and also some may cause sporadic cases of disease. Herein, we investigate the genomic diversity among toxigenic and nontoxigenic O1 and O139 strains by comparative genomic microarray hybridization with the genome of El Tor strain N16961 as a base. Conservation of the toxigenic O1 El Tor and O139 strains is found as previously reported, whereas accumulation of genome changes was documented in toxigenic El Tor strains isolated within the 40 years of the seventh pandemic. High phylogenetic diversity in nontoxigenic O1 and O139 strains is observed, and most of the genes absent from nontoxigenic strains are clustered together in the N16961 genome. By comparing these toxigenic and nontoxigenic strains, we observed that the small chromosome of V. cholerae is quite conservative and stable, outside of the superintegron region. In contrast to the general stability of the genome, the superintegron demonstrates pronounced divergence among toxigenic and nontoxigenic strains. Additionally, sequence variation in virulence-related genes is found in nontoxigenic El Tor strains, and we speculate that these intermediate strains may have pathogenic potential should they acquire CTX prophage alleles and other gene clusters. This genome-wide comparison of toxigenic and nontoxigenic V. cholerae strains may promote understanding of clonal differentiation of V. cholerae and contribute to an understanding of the origins and clonal selection of epidemic strains.

A Vibrio cholerae serogroup O1 vaccine candidate against CTX ET Phi infection.

Cholera is a severe diarrheal disease that may spread rapidly. Vaccination is considered a valid measure against it. We developed a new vaccine candidate, IEM109, against Vibrio cholerae. To generate this candidate, a chromosomal fragment containing the TLC element, attB of the CTX Phi integration site, and RTX cluster responsible for the cytotoxic activity for mammalian cells was deleted through homologous recombination from the previously described El Tor biotype, IEM101. The protective genes ctxB and rstR, which establish resistance to CTX Phi infections, were inserted into that same location on the chromosome of IEM109 to enhance the safety and genetic stability of the vaccine candidate and to prevent horizontal gene transfer. In in vivo tests, cell cultures showed that the cytotoxic effect of IEM109 on Hep-2 was negative. Furthermore, the infection rate of El Tor biotype CTX Phi to that of IEM109 in the rabbit intestine is 3000-fold lower than that of IEM101. Intraintestinal vaccination of rabbits with a single dose of IEM109 elicits high titers of anti-CTB IgG and vibriocidal antibodies. When challenged with 0.5-2 microg CT and 10(5) to 10(8)CFU of four wild toxigenic strains of different biotypes and serogroups, IEM109 conferred full protection. Thus, IEM109 is a stable vaccine candidate that evokes not only antitoxic and vibriocidal immunities, but also resistance to the El Tor biotype CTX Phi infection.

Resistance of the cholera vaccine candidate IEM108 against CTXPhi infection.

The cholera toxin (CT) genes ctxAB are carried on a lysogenic phage of Vibrio cholerae, CTXPhi, which can transfer ctxAB between toxigenic and nontoxigenic strains of bacteria. This transfer may pose a problem when live oral cholera vaccine is given to people in epidemic areas, because the toxin genes can be reacquired by the vaccine strains. To address this problem, we have constructed a live vaccine candidate, IEM108, which carries an El Tor-derived rstR gene. This gene encodes a repressor and can render bacterial resistance to CTXPhi infection. In this study, we evaluated the resistance of IEM108 against CTXPhi infection by using a CTXPhi marked for chloramphenicol (CAF) resistance and an in vivo model. We found that the cloned rstR gene rendered IEM108 immune to infection with the marked CTXPhi. In addition, the infection rate of IEM108 was even lower than that of the native CTXPhi-positive strain. These results suggest that the vaccine candidate IEM108 is resistant to infection by CTXPhi.

[Sequence analysis of housekeeping genes recA, dnaE, and mdh in different serogroups or biotypes of Vibrio cholerae isolated from China].

OBJECTIVE: To analyze sequences of the housekeeping genes including recA, dnaE, and mdh in different serogroups or different biotypes of Vibrio cholerae (VC) strains isolated from China. METHODS AND RESULTS: recA, dnaE, and mdh genes of Vibrio cholerae were obtained by PCR, sequenced and analyzed. Forty-four variable bases were identified in the 500 bases of recA gene of 18 VC strains, and the mutation of only 3 variable bases could result in changes of 2 amino acids. In the 600 bases of dnaE genes of 18 strains, 32 variable bases were found and only 1 contributed to an amino acid change. In the 367 bases of mdh genes of 18 strains, only 1 variable base was identified whose mutation involved an amino acid convertion. Toxic EI Tor biotype (EVC) strains and toxic O139 serogroup strains were closely related. Non-toxic strains of different serogroups or types were lowly related. Non-toxic and toxic strains of different serogroups or types were lowly related. CONCLUSION: Toxic EVC and toxic O139 serogroup strains isolated from different areas of China may evolve from a common original strain, and toxic O139 VC strains may come from toxic EVC strains.

[Study on infection of different strains of Vibro cholerae O1 by El Tor CTXPhi].

To study the horizontal transfer efficiencies of filamentous bacteriophage CTXPhi in different V. cholera O1 strains and the phage immunities of these strains. The infectious El Tor CTXPhi particles genetic marked by chloramphenicol resistance gene were used to infect four different V. cholerae O1 strains in vivo and in vitro. Selected the infected clones based on its character of chloramphenicol resistance and identified and judged the exist form of CTXPhi genome through Southern bolt and other hybridization methods. Calculated the infection rates of different strains and compared each other. Then we analyzed the mechanism of infection and phage immunity. The infection rate of classic strain 1119 with the genetic marked CTX(ET)Phi in vivo is much higher than that in vitro. In vitro experiment, the rate of 1119 is higher than other three El Tor strains. And in El Tor strains, the infection frequency of IEM101 that had no rstR gene is 100 to 1000 times higher than other two strains containing rstR. Classical biotype strain is more susceptible to CTX(ET)Phi particles than El Tor strains. Expression of TCP and the phage immunity mediated by rstR gene affect the horizontal transfection of CTXPhi in V. cholerae strains.

[Sequence analysis of the mutated genes in CTXEVC Phi and nct-CTXnew Phi genomes in Vibrio cholerae JS94484 strain].

OBJECTIVE: To analyze the sequences of the mutated genes in CTX(EVC)Phi and nct-CTX(new)Phi genomes in Vibrio cholerae JS94484 strain. METHODS: The mutated genes in CTX(EVC)Phi and nct-CTX(new)Phi genome were obtained by PCR, sequenced and analyzed. RESULTS: ig1, rstR, ig2, and ctxAB genes in CTX(EVC)Phi genome of V. cholerae strain JS94484 were highly homologous with those of standard EVC strain N16961, while ig1, rstR and ig2 genes in nct-CTX(new)Phi genome of the strain JS94484 shared low homology with those of the other 3 biotypes of V. cholerae. Considerable difference was detected in the last 60 bp of zot genes between CTX(EVC)Phi and nct-CTX(new)Phi genomes, indicating possible difference in the amino sequences of the Zot proteins encoded by these two genes. The sequence of toxin-coregulated pilus A subunit gene (tcpA) of the strain JS94484 was identical with that of strain N16961. CONCLUSION: ig1, rstR and ig2 genes of nct-CTX(new)Phigenome are of a novel type, and their functions await further investigation.

Construction and characterization of a thyA mutant derived from cholera vaccine candidate IEM101.

A naturally cholera toxin gene negative Vibrio cholerae (O1, El Tor, Ogawa) strain, named IEM101, was isolated in China. The human volunteer tests showed that this strain was safe, able to colonize the intestinal mucosa, and able to induce a strong immune response. Also other studies indicated that it was an efficient live vector to deliver heterologous antigens. In this article, a thymidylate synthase gene (thyA)-defined mutant was constructed using homologous recombination. Except for the morphological changes in minimal medium and slightly reduced colonization capacity, mutant strain IEM101-T maintained most of the desirable features as the wild-type strain IEM101 in terms of growth rate and immunogenicity. However, the mutant was more biosafe than its parent strain. In conclusion, IEM101-T may be a promising strain to develop live vaccine candidate of cholera or an attractive vaccine vector to deliver heterologous antigens in vivo.

[Functional analysis of promoters of Vibrio cholerea typing phage VP1 with reporter system].

Phage VP1 infects and lyses Vibrio cholerae. The VP1 genome is a circular double-strand DNA and its size is 32176 base pairs. Analysis of the sequence of the VP1 genome revealed the presence of 15 putative promoter sequence. The activities of these putative promoters in V. cholerae were assayed by transformation of reporter gene plasmid and phage infection together. Promoter regions were ligated into pRS1274/BamH I/EcoR I. Then transformed into E. coli JM109 and all of clone display blue. The recombinant plasmids were transformed into V. cholerae 7743 deltaZ by electroporation, then bacteriophage VP1 infect transformant. The time-course expressing lacZ gene and detecting change of beta-galactosidase enzyme activity in V. cholerae transformants at latent period, indicated P17 probably is a early promoter; P2 and P3 and P9 etc are medium-term promoters; P18 is a late promoter.

[Sequencing and analysis of Vibrio cholerae typing phage VP3 genome].

DNA sequence and the genome of phage VP3 (a typing phage of V. cholera) were analyzed. A random library of VP3 DNA was constructed by shot-gun library method. The VP3 genome sequence was assembled with contigs sequences, the gaps between different contigs were filled with sequencing data from primer walking. ORFs were predicted; Phylogeny of DNA polymerase sequences was analyzed to determine the class of VP3; The activity of putative promoter genes were analyzed using lacZ report system. VP3 genome is a 39504bp of circular double-stranded DNA. Twenty-seven out of forty-nine putative ORFs were annotated; twenty gene products were homologous with T7-like phages, including DNAP, DNA replicative protein, capsid, tail tubular, tail fiber protein, and DNA packaged protein. The activity of the putative promoter regions was confirmed through cloning those regions to LacZ-fuse plasmid pRS1274 and analysis of the expression of beta galactosidase. The complete genomic sequence of VP3 and phylogenetic tree analysis suggests VP3 is a member of T7 phage family.

Construction and evaluation of a safe, live, oral Vibrio cholerae vaccine candidate, IEM108.

IEM101, a Vibrio cholerae O1 El Tor Ogawa strain naturally deficient in CTXPhi, was previously selected as a live cholera vaccine candidate. To make a better and safer vaccine that can induce protective immunity against both the bacteria and cholera toxin (CT), a new vaccine candidate, IEM108, was constructed by introducing a ctxB gene and an El Tor-derived rstR gene into IEM101. The ctxB gene codes for the protective antigen CTB subunit, and the rstR gene mediates phage immunity. The stable expression of the two genes was managed by a chromosome-plasmid lethal balanced system based on the housekeeping gene thyA. Immunization studies indicate that IEM108 generates good immune responses against both the bacteria and CT. After a single-dose intraintestinal vaccination with 10(9) CFU of IEM108, both anti-CTB immunoglobulin G and vibriocidal antibodies were detected in the immunized-rabbit sera. However, only vibriocidal antibodies are detected in rabbits immunized with IEM101. In addition, IEM108 but not IEM101 conferred full protection against the challenges of four wild-type toxigenic strains of V. cholerae O1 and 4 micro g of CT protein in a rabbit model. By introducing the rstR gene, the frequency of conjugative transfer of a recombinant El Tor-derived RS2 suicidal plasmid to IEM108 was decreased 100-fold compared to that for IEM101. This indicated that the El Tor-derived rstR cloned in IEM108 was fully functional and could effectively inhibit the El Tor-derived CTXPhi from infecting IEM108. Our results demonstrate that IEM108 is an efficient and safe live oral cholera vaccine candidate that induces antibacterial and antitoxic immunity and CTXPhi phage immunity.

Molecular epidemiology of Vibrio cholerae O139 in China: polymorphism of ribotypes and CTX elements.

Vibrio cholerae O139, the second etiological serogroup of cholera, triggered the first outbreak of O139 cholera in China in 1993. To analyze the clone polymorphism of O139 isolates in China, 117 strains of V. cholerae O139, isolated from different areas in China between 1993 and 1999, were selected to characterize the phylogenetic relationships by molecular techniques. Analysis of restriction fragment length polymorphism in the conserved 16S rRNA gene revealed seven different ribotypes within the 117 strains. Among these strains, there were eight that lacked the cholera toxin gene (ctxAB), zot, and the repetitive sequence (RS); these eight strains belonged to three individual ribotypes. Our results suggested that V. cholerae O139 strains in China had clone diversity in phylogeny. The results of our hybridization patterns for CTX genetic elements (ctxAB, zot, and RS) showed that CTXPhi genomes in most V. cholerae O139 strains had two or more copies and had extensive restriction patterns even for the strains which belong to the same ribotype. For 22 (20.1%) strains, the copies of ctxAB were different from those of zot, suggesting that a ctxAB-negative CTXPhi genome may exist in O139 strains. This ctxAB-negative CTXPhi genome may coexist with the intact CTXPhi genome in a strain. In addition, the dendrogram for I-CeuI-generated pulsed-field gel electrophoresis patterns showed that V. cholerae serogroup O139 has a closer relationship with one strain of serogroup O22 than with the strains of serogroup O1. The results of this study showed the clonal diversity and the distribution of O139 strains in China, suggesting multiple origins of the O139 cholera epidemic or sporadic events.

[Screening for the peptides blocking cholera phage VP1 transfection to host cell by phage random amino acid peptide library].

Use Vibrio cholerae El Tor Typing Phage VP1 as ligand to screen phage-display random 7 amino acid peptide library, ELISA and inactivation experiment were used to identify positive clone. The ratio of output to input was increased after three rounds of screening. Pseudo-positive was decreased stepwise. It indicated the efficient enrichment. After three rounds of screening, 312 out of 360 phage clones were positive in ELISA, 1 clone shows blocking VP1 adsorbs to Vibrio cholerae by inactivation experiment. Sequencing result indicate that amino acid sequences are p245: LeuGlnGlnLysHisLeuLeu; p40, p55: GlnLeuIleMetIleArgHis; p69, p274 IleThrProArgAsnArgSer. Getting some peptides can mimick the VP1 receptor epitopes, one peptide can block VP1 transfection to host cell, it was a clue to study receptor's structure and phage infectious mechanism to host cell.

[A study on genetic polymorphism of rRNA gene pattern of Vibrio cholerae O139 in China].

OBJECTIVE: To investigated the genetic polymorphism of the isolated strains using ribotyping method. METHODS: One hundred twenty-two strains of V. cholerae O139 isolated from different areas of China from 1993 to 1999 were selected and characterized with ribotyping, including 16s rDNA and 23s rDNA probes. RESULTS: One hundred twenty-two strains were differentiated into 10 different ribotypes (RT1-RT10) on the basis of rRNA gene probes hybridization (with Bgl I digestion), which consisted of 7 - 9 bands between 12 and 1.5 kb in size. RT1 and RT3, as two predominant ribotypes, comprised most number of the strains which spread to the extensive range. Nine strains, which are negative to ctxAB, zot and RS individually, belong to 4 special ribotypes. The dendrogram revealing genetic relationship among different clones of V. cholerae O139 showed that the clones belonging to RT1 and RT2 had genetic similarity on high degree, although they were isolated from different regions. The two predominant ribotypes (RT1 and RT3) were distant in genetic relationship. CONCLUSION: Results showed the clonal diversity and the wide area distribution of V. cholerae O139 strains in China, suggesting the multiple origins of O139 epidemics.

[Analysis of gene cluster of Tat-dependent protein export system of Vibrio cholerae and its function].

The Tat (Twin-arginine translocatin) system is a recently defined protein export pathway that serves to translocate folded proteins. The substrates of the Tat pathway contain specific amino-terminal signal peptides that exhibit a conserved amino acid consensus motif-S/T-R-R-x-F-L-X. Here is the report of knocked out the tatA, tatB and tatC genes of the V. cholerae by suicide plasmid homologous recombination technology. Mutant strains showed obvious changes of growth characteristics. The transport of a typical Tat pathway substrate, trimethylamine N-oxide reductase (molybdoenzyme), was completely blocked. The physiochemical reactions of the parent and mutant strains were also analyzed. Four physiochemical reactions using D-galactose, L-asparagine, glyl-L-aspartic acid and D-L-alpha-glycerol phosphate as substrates were negative in mutant strains, which might be affected by the inactivation of the Tat-dependent system.