Transformation of Vibrio cholerae by plasmid DNA

A Strategy for the Rapid Development of a Safe Vibrio cholerae Candidate Vaccine Strain

Approximately 1/6 of humanity is at high risk of experiencing cholera epidemics. The development of effective and safe vaccines against Vibrio cholerae, the primary cause of cholera, is part of the public health measures to prevent cholera epidemics. Natural nontoxigenic V. cholerae isolates represent a source of new genetically improved and relatively safe vaccine strains. However, the genomic engineering of wild-type V. cholerae strains is difficult, and these strains are genetically unstable due to their high homologous recombination activity. We comprehensively characterized two V. cholerae isolates using genome sequencing, bioinformatic analysis, and microscopic, physiological, and biochemical tests. Genetic constructs were Gibson assembled and electrotransformed into V. cholerae. Bacterial colonies were assessed using standard microbiological and immunological techniques. As a result, we created a synthetic chromoprotein-expressing reporter operon. This operon was used to improve the V. cholerae genome engineering approach and monitor the stability of the genetic constructs. Finally, we created a stable candidate V. cholerae vaccine strain bearing a recA deletion and expressing the β-subunit of cholera toxin. Thus, we developed a strategy for the rapid creation of genetically stable and relatively safe candidate vaccine strains. This strategy can be applied not only to V. cholerae but also to other important human bacterial pathogens.

Involvement of in vivo induced icmF gene of Vibrio cholerae in motility, adherence to epithelial cells, and conjugation frequency

Previously, using global transcription profile approach icmF gene of Vibrio cholerae was identified as an in vivo induced gene. In the present study, the icmF gene of V. cholerae O395 was cloned, sequenced, and used to construct an icmF insertion mutant. This IcmF is homologous to Legionella pneumophila IcmF, belonging to the icm cassette responsible for macrophage killing and intracellular survival of the organism. The icmF insertion mutant exhibited reduced motility and increased adherence to human intestinal epithelial cells. The presence of ATP-GTP-binding site suggests further a possible role of IcmF as a signaling molecule. Triparental-mating assay, with the mutant as a recipient, showed higher conjugation frequency than wild type. We propose that the increased adherence to epithelial cell line and increased conjugation frequency of the mutant result from some sort of cell surface reorganization.

First evidence for a restriction-modification system in Leptospira sp

The LE1 leptophage exhibited a host range restricted to the saprophytic Leptospira biflexa [Saint Girons et al., Res. Microbiol. 141 (1990) 1131-1133] and mainly to the Patoc 1 strain (hereafter called PFRA) kept in the Paris, France collection. Results of titration of LE1 lysates indicated the presence of a host-controlled modification and restriction system within PUSA (Patoc 1 strain maintained in the Morgantown, WV, USA collection) that was absent in PFRA. Because genomic DNA of PITAL (Patoc 1 strain maintained in Trieste, Italy) appeared smeared in pulsed field gel electrophoresis (PFGE), this strain is likely to contain nucleases that are activated upon DNA isolation. Moreover, comparative NotI digestions of PUSA and PFRA DNAs, as visualized by PFGE, indicated that PUSA belonged to a different serovar than PFRA. Finally, 16S ribosomal sequence analysis indicated that PUSA belonged to the saprophytic Leptospira meyeri species, while PITAL and PFRA appertained to L. biflexa. The evolutionary significance and the importance of the restriction and modification enzymes or non-specific nucleases within strains for genetic experiments are discussed.

Use of RNA arbitrarily primed-PCR fingerprinting to identify Vibrio cholerae genes differentially expressed in the host following infection

Evidence suggests that a repertoire of Vibrio cholerae genes are differentially expressed in vivo, and regulation of virulence factors in vivo may follow a different pathway. Our work was aimed at characterization of in vivo-grown bacteria and identification of genes that are differentially expressed following infection by RNA arbitrarily primed (RAP)-PCR fingerprinting. The ligated rabbit ileal loop model was used. The motility of in vivo-grown bacteria increased by 350% over that of in vitro-grown bacteria. Also, the in vivo-grown cells were more resistant to killing by human serum. By using the RAP-PCR strategy, five differentially expressed transcripts were identified. Two in vitro-induced transcripts encoded polypeptides for the leucine tRNA synthatase and the 50S ribosomal protein, and the three in vivo-induced transcripts encoded the SucA and MurE proteins and a polypeptide of unknown function. MurE is a protein involved in the peptidoglycan biosynthetic pathway. The lytic profiles of in vivo- and in vitro-grown cells suspended in distilled water were compared; the former was found to be slightly less sensitive to lysis. Ultrathin sections of both cells observed under the transmission electron microscope revealed that in contrast to the usual wavy discontinuous membrane structure of the in vitro-grown cells, in vivo-grown cells had a more rigid, clearly visible double-layered structure. The V. cholerae murE gene was cloned and sequenced. The sequence contained an open reading frame of 1,488 nucleotides with its own ribosome-binding site. A plasmid containing the murE gene of V. cholerae was transformed into V. cholerae 569B, and a transformed strain, 569BME, containing the plasmid was obtained. Ultrathin sections of 569BME viewed under a transmission electron microscope revealed a slightly more rigid cell wall than that of wild-type 569B. When V. cholerae 569B and 569BME cells were injected separately into ligated rabbit ileal loops, the transformed cells had a preference for growth in the ileal loops versus laboratory conditions.

Role of DnaK in in vitro and in vivo expression of virulence factors of Vibrio cholerae

The dnaK gene of Vibrio cholerae was cloned, sequenced, and used to construct a dnaK insertion mutant which was then used to examine the role of DnaK in expression of the major virulence factors of this important human pathogen. The central regulator of several virulence genes of V. cholerae is ToxR, a transmembrane DNA binding protein. The V. cholerae dnaK mutant grown in standard laboratory medium exhibited phenotypes characteristic of cells deficient in ToxR activity. Using Northern blot analysis and toxR transcriptional fusions, we demonstrated a reduction in expression of the toxR gene in the dnaK mutant strain together with a concomitant increase in expression of a htpG-like heat shock gene that is located immediately upstream and is divergently transcribed from toxR. This may be due to increased heat shock induction in the dnaK mutant. In vivo, however, although expression from heat shock promoters in the dnaK mutant was similar to that observed in vitro, expression of both toxR and htpG was comparable to that by the parental strain. In both strains, in vivo expression of toxR was significantly higher than that observed in vitro, but no reciprocal decrease in htpG expression was observed. These results suggest that the modulation of toxR expression in vivo may be different from that observed in vitro.

Ordered cloned DNA map of the genome of Vibrio cholerae 569B and localization of genetic markers

By using a low-resolution macrorestriction map as the foundation (R. Majumder et al., J. Bacteriol. 176:1105-1112, 1996), an ordered cloned DNA map of the 3.2-Mb chromosome of the hypertoxinogenic strain 569B of Vibrio cholerae has been constructed. A cosmid library the size of about 4,000 clones containing more than 120 Mb of V. cholerae genomic DNA (40-genome equivalent) was generated. By combining landmark analysis and chromosome walking, the cosmid clones were assembled into 13 contigs covering about 90% of the V. cholerae genome. A total of 92 cosmid clones were assigned to the genome and to regions defined by NotI, SfiI, and CeuI macrorestriction maps. Twenty-seven cloned genes, 9 rrn operons, and 10 copies of a repetitive DNA sequence (IS1004) have been positioned on the ordered cloned DNA map.

Physical map of the genome of Vibrio cholerae 569B and localization of genetic markers

A combined physical and genetic map of the genome of the classical O1 hypertoxinogenic strain 569B of Vibrio cholerae has been constructed. The enzymes NotI, SfiI and CeuI generated DNA fragments of suitable size distribution that could be resolved by pulsed-field gel electrophoresis. The digests produced 37, 22, and 7 fragments, respectively. The CeuI maps of the genomes of strains 569B and O395, constructed by partial restriction digestion, were identical, and the data are consistent with the concept of circular chromosomes. The genome size of each of the strains was estimated to be about 3.2 Mb. The NotI and SfiI digestion profiles of the genomic DNAs of strains 569B and O395 exhibited distinct restriction fragment length polymorphism. The linkages between the 37 NotI fragments of the genome of strain 569B were determined by combining three approaches: isolation of linking clones, analysis of partial digestion fragments, and identification of NotI fragments in isolated CeuI and SfiI fragments. To align linked fragments precisely, NotI-digested genomic DNA was end labeled and separated in the same gel with the NotI-digested DNA to be probed with linking clones. This also allowed the identification of smaller restriction fragments that are not visible in ethidium bromide-stained gels. The presence of repetitive DNA sequences in the V. cholerae 569B genome has been demonstrated. Twenty cloned homologous and heterologous genes and seven rrn operons have been positioned on the physical map. The two copies of the Ctx genetic element in the genome of strain 569B are located about 1,000 kb apart.

Heat shock response and heat shock protein antigens of Vibrio cholerae

Sixteen heat shock proteins (Hsps) have been identified in the hypertoxinogenic strain 569B of Vibrio cholerae which are synthesized in response to small and large elevations of temperature. The induction of the Hsps is necessary for the cells to survive the deleterious effects of heat. There is no difference in the pattern of induction of the Hsps in V. cholerae strains varying in levels of toxinogenicity. One of the major low-molecular-mass Hsps, a 16-kDa protein, is preferentially degraded following shift down of temperature. This protein is induced at a much lower level at high temperatures in cells maintained in the laboratory for a prolonged period. The only Hsp located in the outer membrane of V. cholerae cells is a 23-kDa protein. Western immunoblot analysis with human immune sera collected from convalescent cholera patients revealed that this protein is markedly immunogenic. The human immune serum also reacted with the 69- and 16-kDa major Hsps and the 88-, 66-, and 46-kDa Hsps but not with the 61-kDa major Hsp identified as the groEL gene product. All major Hsps reacted with rabbit anti-V. cholerae sera. Ethanol stress leads to the induction of four of the major Hsps and three additional proteins.

The DNA adenine methyltransferase-encoding gene (dam) of Vibrio cholerae

The DNA adenine methyltransferase (MTase)-encoding gene (dam) of Vibrio cholerae, an organism belonging to the family Vibrionaceae, has been cloned and the complete nucleotide (nt) sequence determined. V. cholerae dam encodes a 21.5-kDa protein and is directly involved in methyl-directed DNA mismatch repair. It can substitute for the Escherichia coli enzyme and can suppress the phenotypic traits associated with E. coli dam mutants. Overproduction of V. cholerae Dam MTase does not result in hypermutability in either V. cholerae or E. coli cells. Overproduction of V. cholerae Dam in a pUC plasmid, however, fails to suppress the 2-aminopurine (2-AP)-sensitive phenotype of E. coli dam mutants. Homology between the nt and deduced amino acid (aa) sequences of the E. coli and V. cholerae dam genes is only 30-35%.

Recombinant derivative of a naturally occurring non-toxinogenic Vibrio cholerae 01 expressing the B subunit of cholera toxin: a potential oral vaccine strain

A clinical isolate of Vibrio cholerae 01 was identified which did not possess the heat-labile (CT), the heat-stable (ST) or the zonula occludens (Zot) toxin genes. Rabbit ileal loop assays showed that no other CT-like toxin was produced by this strain. The partly deleted cholera toxin gene which carries the intact gene for the B subunit was cloned and the recombinant plasmid, pURD110, was introduced into this non-toxinogenic natural human isolate. The transformed cells (strain URD2) secreted the B subunit gene product which competed with the holotoxin secreted by the hypertoxinogenic strain 569B of V. cholerae for the GM1 ganglioside binding sites in vivo. This strain can colonize the rabbit intestine as detected by the removable intestinal tie adult rabbit diarrhoea (RITARD) model. This construct has an advantage over other live oral attenuated V. cholerae strains used as vaccines in that the latter strains were made non-toxinogenic by only deleting part of the gene coding for the A subunit of cholera toxin while the strain described here is naturally non-toxinogenic.

A 14-kilodalton inner membrane protein of Vibrio cholerae biotype e1 tor confers resistance to group IV choleraphage infection to classical vibrios

Choleraphage phi 149 differentiates the two biotypes, classical and el tor, of Vibrio cholerae. This phage cannot replicate in V. cholerae biotype el tor cells because the concatemeric DNA intermediates produced are unstable and cannot be chased to mature phage DNA. A V. cholerae biotype el tor gene coding for a 14,000-Da inner membrane protein which destabilizes the concatemeric DNA intermediates by hindering their binding to the cell membrane has been identified. Presumably, a 22,000-Da V. cholerae biotype el tor protein might also have a role in conferring phage phi 149 resistance to cells belonging to the biotype el tor. A nucleotide sequence homologous to the 1.2-kb V. cholerae biotype el tor DNA coding for both the 14,000- and 22,000-Da proteins is present in all strains of classical vibrios but is not transcribed. The nucleotide sequence of the gene coding for the 14,000-Da protein has been determined.