Molecular cloning and characterization of the genetic determinants that express the complete Shigella serotype D (Shigella sonnei) lipopolysaccharide in heterologous live attenuated vaccine strains

A snapshot of diversity: Intraclonal variation of Escherichia coli clones as commensals and pathogens

Whole-genome sequencing has enabled detailed studies on bacterial evolution during infection, but there is limited knowledge on intraclonal variation. In this study, we sought to provide a snapshot of the intraclonal diversity of Escherichia coli as both commensal in the faecal environment and pathogen during urinary tract infection, respectively. This was performed by whole-genome sequencing and analyses of single nucleotide polymorphisms (SNPs) and gene-content variation in ten isolates belonging to the same clone and isolated from rectal swabs or urine samples. We identified only one clone in eight of the nine urines sampled (89 %). In both the commensal and pathogenic state, the within-host diversity was limited with intraclonal SNP diversity of 0-2 non-synonymous SNPs for each clone. The genetic diversity showed variation in gene content in a range of 2-15 genes in total for all clones, including genes positioned on plasmids, and in the K- and O-antigen cluster. The observed SNP- and gene variation shows that sampling of one colony would be enough for surveillance, outbreak investigations and clonal evolution. However, for studies of adaptation during or between colonization and infection, this variation is relevant to consider.

Specificity of monoclonal antibodies against Escherichia coli K-12 lipopolysaccharide

Three monoclonal antibodies against the lipopolysaccharide (LPS) of Escherichia coli strain K-12 were obtained after immunization of BALB/c mice with heat-killed bacteria and were serologically characterized by hemagglutination, enzyme-immunoassay, Western, dot, and colony blot using as antigens bacteria, LPS, de-O-acylated LPS, dephosphorylated LPS, the core oligosaccharide and synthetic partial structures representing a di-, tri-and tetrasaccharide of the K-12 outer core region. In addition, deacylated LPS and synthetic oligosaccharides were covalently linked to bovine serum albumin resulting in artificial glycoconjugate antigens. In all assays used, the antibodies were specific for K-12 LPS. These antibodies detect E. coli K-12 LPS in its isolated form as well as on the surface of bacteria and are, thus, useful for all working with this microorganism.

Plague Vaccines: Status and Future

Three major plague pandemics caused by the gram-negative bacterium Yersinia pestis have killed nearly 200 million people in human history. Due to its extreme virulence and the ease of its transmission, Y. pestis has been used purposefully for biowarfare in the past. Currently, plague epidemics are still breaking out sporadically in most of parts of the world, including the United States. Approximately 2000 cases of plague are reported each year to the World Health Organization. However, the potential use of the bacteria in modern times as an agent of bioterrorism and the emergence of a Y. pestis strain resistant to eight antibiotics bring out severe public health concerns. Therefore, prophylactic vaccination against this disease holds the brightest prospect for its long-term prevention. Here, we summarize the progress of the current vaccine development for counteracting plague.

Pushing the Bacterial Envelope

Over the past three decades, a powerful array of techniques has been developed for expressing heterologous proteins and saccharides on the surface of bacteria. Surface-engineered bacteria, in turn, have proven useful in a variety of settings, including high-throughput screening, biofuel production, and vaccinology. In this chapter, we provide a comprehensive review of methods for displaying polypeptides and sugars on the bacterial cell surface, and discuss the many innovative applications these methods have found to date. While already an important biotechnological tool, we believe bacterial surface display may be further improved through integration with emerging methodology in other fields, such as protein engineering and synthetic chemistry. Ultimately, we envision bacterial display becoming a multidisciplinary platform with the potential to transform basic and applied research in bacteriology, biotechnology, and biomedicine.

Live attenuated oral cholera vaccines

Live, orally administered, attenuated vaccine strains of Vibrio cholerae have many theoretical advantages over killed vaccines. A single oral inoculation could result in intestinal colonization and rapid immune responses, obviating the need for repetitive dosing. Live V. cholerae organisms can also respond to the intestinal environment and immunological exposure to in vivo expressed bacterial products, which could result in improved immunological protection against wild-type V. cholerae infection. The concern remains that live oral cholera vaccines may be less effective among partially immune individuals in cholera endemic areas as pre-existing antibodies can inhibit live organisms and decrease colonization of the gut. A number of live oral cholera vaccines have been developed to protect against cholera caused by the classical and El Tor serotypes of V. cholerae O1, including CVD 103-HgR, Peru-15 and V. cholerae 638. A number of live oral cholera vaccines have also been similarly developed to protect against cholera caused by V. cholerae O139, including CVD 112 and Bengal-15. Live, orally administered, attenuated cholera vaccines are in various stages of development and evaluation.

Considerations for development of whole cell bacterial vaccines to prevent diarrheal diseases in children in developing countries

Enteric pathogens constitute a major pediatric threat in the developing world through their impact on morbidity and mortality, physical and cognitive development and cause and effect relationship with malnutrition. Although many bacterial pathogens can cause diarrheal diseases, a group of less than 10 including Shigella spp., enterotoxigenic Escherichia coli (ETEC), Vibrio cholerae, and possibly, Campylobacter jejuni account for a significant percentage of these diseases in developing countries. Rotavirus is also a major cause of diarrheal diseases. Vaccines against these agents offer a potentially effective control measure against these diseases, but safe, practical, and effective vaccines for many of these agents have yet to be realized. Many vaccine development approaches are under investigation, but the one that is currently most advanced and that has been most widely applied to enteric pathogens is the use of orally administered live or killed whole pathogen preparations. If inactivated, these vaccines will probably be administered as multiple doses with approximately 10(10) to 10(11) total particles per dose, but they are relatively safe for oral administration. Further, they may not require a buffer for delivery and can be stored in liquid formulations. Fewer doses may be required for some live attenuated pathogen vaccines, but a buffer will most likely be required for oral delivery and the product must be stored in a dried formulation. Also, safety becomes more of a concern with live pathogens depending on the degree of attenuation, host immunocompetence, and the total number and kinds of attenuated pathogens which may be present in a combined agent vaccine. Both live and killed whole pathogen vaccines can be immunogenic and have the possibility to serve as vectors for other antigens. Although many organisms and serotypes are clinically important, by exploiting antigenic cross reactivity and using some pathogen components as vectors for cloned antigens of other pathogens, it could be possible to induce immunity against major enteric pathogens/serotypes with <10 whole pathogen components in a multi-agent vaccine. Safe and effective mucosal adjuvants may in the future be useful in whole pathogen vaccines, but they do not seem to be essential for immunization. Further, dietary supplements such as zinc, mixed routes of delivery and new regimens are under study which may in the future enhance further the effectiveness of the whole pathogen vaccines which now seem realizable in the near term. For this to happen, however, a coordinated and committed effort is necessary now to address the immunologic, regulatory, manufacturing, testing and implementation issues which will be involved in the realization of this important product to benefit children's health worldwide.

Molecular cloning and characterization of genes for Shigella sonnei form I O polysaccharide: proposed biosynthetic pathway and stable expression in a live salmonella vaccine vector

The gene region for biosynthesis of Shigella sonnei form I O polysaccharide (O-Ps) and flanking sequences, totaling >18 kb, was characterized by deletion analysis to define a minimal construct for development of Salmonella-based live vaccine vector strains. Lipopolysaccharide (LPS) expression and DNA sequence studies of plasmid deletion derivatives indicated form I O-Ps expression from a 12.3-kb region containing a putative promoter and 10 contiguous open reading frames (ORFs), one of which is the transposase of IS630. A detailed biosynthetic pathway, consistent with the predicted functions of eight of the nine essential ORFs and the form I O-Ps structure, is proposed. Further sequencing identified partial IS elements (i.e., IS91 and IS630) and wzz upstream of the form I coding region and a fragment of aqpZ and additional full or partial IS elements (i.e., IS629, IS91, and IS911) downstream of this region. The stability of plasmid-based form I O-Ps expression was greater from low-copy vectors than from high-copy vectors and was enhanced by deletion of the downstream IS91 from plasmid inserts. Both core-linked (i.e., LPS) and non-core-linked (i.e., capsule-like) surface expression of form I O-Ps were detected by Western blotting and silver staining of polyacrylamide gel electrophoresis-separated Shigella and Escherichia coli extracts. However, salmonellae, which have a core that is chemically dissimilar to that of shigellae, expressed only non-core-linked surface-associated form I O-Ps. Finally, attenuated Salmonella enterica serovar Typhi live vaccine vector candidates, containing minimal-sized form I operon constructs, elicited immune protection in mice against virulent S. sonnei challenge, thereby supporting the promise of live, oral vaccines for the prevention of shigellosis.

CpsK of Streptococcus agalactiae exhibits alpha2,3-sialyltransferase activity in Haemophilus ducreyi

Streptococcus agalactiae (GBS) is a major cause of serious newborn bacterial infections. Crucial to GBS evasion of host immunity is the production of a capsular polysaccharide (CPS) decorated with sialic acid, which inactivates the alternative complement pathway. The CPS operons of serotypes Ia and III GBS have been described, but the CPS sialyltransferase gene was not identified. We identified cpsK, an open reading frame in the CPS operon of most serotypes, which was homologous to the lipooligosaccharide (LOS) sialyltransferase gene, lst, of Haemophilus ducreyi. To determine if cpsK might encode a sialyltransferase, we complemented a H. ducreyi lst mutant with cpsK. CpsK was expressed in H. ducreyi and LOS was isolated and analysed for sialic acid content by SDS-PAGE and high-performance liquid chromatography (HPLC). Sialo-LOS was seen in the wild-type, cpsK- or lst-complemented mutant strains, but not in the mutant without cpsK. Addition of Neu5Ac to the LOS was confirmed by mass spectroscopy. Lectin binding studies detected terminal Neu5Ac(alpha 2-->3)Gal(beta 1- on LOS produced by the wild-type, cpsK or lst-complemented mutant strain LOS, compared with the mutant alone. Our data characterize the first sialyltransferase gene from a Gram- positive bacterium and provide compelling evidence that its product catalyses the alpha2,3 addition of Neu5Ac to H. ducreyi LOS and therefore the terminal side-chain of GBS CPS. Phylogenetic studies further indicated that lst and cpsK are related but distinct from sialyltransferases of most other bacteria and, along with their similar codon usage bias and G + C content, suggests acquisition by lateral transfer from an ancestral low G + C organism.

Comparison of mucosal and systemic humoral immune responses after transcutaneous and oral immunization strategies

In order to compare the ability of transcutaneous and oral immunization strategies to induce mucosal and systemic immune responses, we inoculated mice transcutaneously with cholera toxin (CT) or the non-toxic B subunit of cholera toxin (CtxB), or orally with Peru2(pETR1), an attenuated vaccine strain of Vibrio cholerae expressing CtxB. In addition, we also evaluated dual immunization regimens (oral inoculation with transcutaneous boosting, and transcutaneous immunization with oral boosting) in an attempt to optimize induction of both mucosal and systemic immune responses. We found that transcutaneous immunization with purified CtxB or CT induces much more prominent systemic IgG anti-CtxB responses than does oral inoculation with a vaccine vector strain of V. cholerae expressing CtxB. In comparison, anti-CtxB IgA in serum, stool and bile were comparable in mice either transcutaneously or orally immunized. Overall, the most prominent systemic and mucosal anti-CtxB responses occurred in mice that were orally primed with Peru2(pETR1) and transcutaneously boosted with CT. Our results suggest that combination oral and transcutaneous immunization strategies may most prominently induce both mucosal and systemic humoral responses.

O-antigen structural variation: mechanisms and possible roles in animal/plant-microbe interactions

Current data from bacterial pathogens of animals and from bacterial symbionts of plants support some of the more general proposed functions for lipopolysaccharides (LPS) and underline the importance of LPS structural versatility and adaptability. Most of the structural heterogeneity of LPS molecules is found in the O-antigen polysaccharide. In this review, the role and mechanisms of this striking flexibility in molecular structure of the O-antigen in bacterial pathogens and symbionts are illustrated by some recent findings. The variation in O-antigen that gives rise to an enormous structural diversity of O-antigens lies in the sugar composition and the linkages between monosaccharides. The chemical composition and structure of the O-antigen is strain-specific (interstrain LPS heterogeneity) but can also vary within one bacterial strain (intrastrain LPS heterogeneity). Both LPS heterogeneities can be achieved through variations at different levels. First of all, O-polysaccharides can be modified non-stoichiometrically with sugar moieties, such as glucosyl and fucosyl residues. The addition of non-carbohydrate substituents, i.e. acetyl or methyl groups, to the O-antigen can also occur with regularity, but in most cases these modifications are again non-stoichiometric. Understanding LPS structural variation in bacterial pathogens is important because several studies have indicated that the composition or size of the O-antigen might be a reliable indicator of virulence potential and that these important features often differ within the same bacterial strain. In general, O-antigen modifications seem to play an important role at several (at least two) stages of the infection process, including the colonization (adherence) step and the ability to bypass or overcome host defense mechanisms. There are many reports of modifications of O-antigen in bacterial pathogens, resulting either from altered gene expression, from lysogenic conversion or from lateral gene transfer followed by recombination. In most cases, the mechanisms underlying these changes have not been resolved. However, in recent studies some progress in understanding has been made. Changes in O-antigen structure mediated by lateral gene transfer, O-antigen conversion and phase variation, including fucosylation, glucosylation, acetylation and changes in O-antigen size, will be discussed. In addition to the observed LPS heterogeneity in bacterial pathogens, the structure of LPS is also altered in bacterial symbionts in response to signals from the plant during symbiosis. It appears to be part of a molecular communication between bacterium and host plant. Experiments ex planta suggest that the bacterium in the rhizosphere prepares its LPS for its roles in symbiosis by refining the LPS structure in response to seed and root compounds and the lower pH at the root surface. Moreover, modifications in LPS induced by conditions associated with infection are another indication that specific structures are important. Also during the differentiation from bacterium to bacteroid, the LPS of Rhizobium undergoes changes in the composition of the O-antigen, presumably in response to the change of environment. Recent findings suggest that, during symbiotic bacteroid development, reduced oxygen tension induces structural modifications in LPS that cause a switch from predominantly hydrophilic to predominantly hydrophobic molecular forms. However, the genetic mechanisms by which the LPS epitope changes are regulated remain unclear. Finally, the possible roles of O-antigen variations in symbiosis will be discussed.

Comparison of O-antigen gene clusters of Escherichia coli (Shigella) sonnei and Plesiomonas shigelloides O17: sonnei gained its current plasmid-borne O-antigen genes from P. shigelloides in a recent event

Escherichia coli Sonnei has an O antigen identical to that of Plesiomonas shigelloides O17, and its O-antigen gene cluster is located on a plasmid. By sequencing the chromosomal O-antigen gene cluster of P. shigelloides O17 and comparing it with that of Sonnei, we showed that Sonnei gained its O-antigen genes recently.

Genetic variation of dTDP-L-rhamnose pathway genes in Salmonella enterica

The genetic variation in the dTDP-L-rhamnose pathway gene set (rmlB, rmlD, rmlA, rmlC) in Salmonella enterica was examined after sequencing the four genes from 11 rml-containing gene clusters encoding seven O antigens, and a 903 bp rmlB segment from another 23 strains representing the seven subspecies. There was considerable sequence variation and strong polarity in the nature and level of variation among rml genes. The 5' end of the rml gene set, including rmlB, rmlD and most of rmlA, is in general subspecies specific. In contrast, the 3' end, including part of rmlA and all of rmlC, is O antigen specific. The G+C content of the 3' end is lower than that of the 5' end. The variation in the 3' end of the gene set is much greater than that of the 5' end. It is apparent that the rml gene set of S. enterica includes genes with two different evolutionary histories. In addition, there has been extensive recombination in the gene set, probably related to O antigen transfer between subspecies. These findings provide evidence for the lateral transfer of O antigen genes between species and among subspecies of S. enterica. The results have also shown that conserved genes at the end of an O antigen gene cluster play a major role in mediating exchange of the central serogroup-specific regions.

The complete DNA sequence of the O antigen gene region of Plesiomonas shigelloides serotype O17 which is identical to Shigella sonnei form I antigen

We cloned and determined the sequence of a DNA region of approximately 15-kb containing the cluster of genes required for O17 antigen expression in the Escherichia coli K-12 strain from the chromosome of Plesiomonas shigelloides serotype O17:H2 strain. The sequencing analysis revealed that the minimum essential region of the P. shigelloides O17 antigen gene cluster had a size of approximately 11.5-kb and contained 9 contiguous open reading frames (ORFs), which were almost identical to the corresponding ORFs of Shigella sonnei form I antigen gene region, except for IS630 sequence, at the DNA as well as amino acid levels. The putative function of most of the ORFs could be determined on the basis of amino acid sequence similarities and characteristics. In addition, the G+C content of the P. shigelloides O17 antigen genes was lower than that of the chromosomal DNA of P. shigelloides and S. sonnei, suggesting that both P. shigelloides O17 and S. sonnei form I antigen genes had been derived from the same origin with a low G+C content.

Identification of Shigella serotypes by restriction of amplified O-antigen gene cluster

Due to the scarcity of distinctive biochemical reactions for differentiation of Shigella-Escherichia coli, antigenic analysis has long been used for identification and typing of Shigella isolates. Nevertheless, several intra- and interspecific cross-reactions have been reported to disturb serotyping assays. Shigella serotyping is also occasionally affected by the transition from the smooth (S) form to the rough (R) form. Thus, there is a need for the development of novel robust and discriminating methods for Shigella identification and typing. Characteristically, all genes specifically involved in O-antigen synthesis are clustered in E. coli, Shigella, and Salmonella. Published oligonucleotide sequences complementary to JUMPstart and gene gnd, the conserved flanking sequences upstream and downstream of O-antigen gene clusters, were used to amplify the O-antigen gene cluster of representative strains of each Shigella serotype. A unique, amplified fragment was generally observed for each serotype (size ranging from 6 kbp to 17 kbp). Clearly identifiable and reproducible patterns were obtained for each serotype after MboII digestion of the products, except for S. boydii 12 which showed two distinct patterns, and S. flexneri serotypes 1 to 5 and X and Y which showed a single pattern. A database was built with the Taxotron package allowing automated identification of clinical Shigella isolates to all known serotypes.

Development of primers to O-antigen biosynthesis genes for specific detection of Escherichia coli O157 by PCR

The chemical composition of each O-antigen subunit in gram-negative bacteria is a reflection of the unique DNA sequences within each rfb operon. By characterizing DNA sequences contained with each rfb operon, a diagnostic serotype-specific probe to Escherichia coli O serotypes that are commonly associated with bacterial infections can be generated. Recently, from an E. coli O157:H7 cosmid library, O-antigen-positive cosmids were identified with O157-specific antisera. By using the cosmid DNAs as probes, several DNA fragments which were unique to E. coli O157 serotypes were identified by Southern analysis. Several of these DNA fragments were subcloned from O157-antigen-positive cosmids and served as DNA probes in Southern analysis. One DNA fragment within plasmid pDS306 which was specific for E. coli O157 serotypes was identified by Southern analysis. The DNA sequence for this plasmid revealed homology to two rfb genes, the first of which encodes a GDP-mannose dehydratase. These rfb genes were similar to O-antigen biosynthesis genes in Vibrio cholerae and Yersinia enterocolitica serotype O:8. An oligonucleotide primer pair was designed to amplify a 420-bp DNA fragment from E. coli O157 serotypes. The PCR test was specific for E. coli O157 serotypes. PCR detected as few as 10 cells with the O157-specific rfb oligonucleotide primers. Coupled with current enrichment protocols, O157 serotyping by PCR will provide a rapid, specific, and sensitive method for identifying E. coli O157.

Genetic analysis of Shigella sonnei form I antigen: identification of a novel IS630 as an essential element for the form I antigen expression

The form I coding region of Shigella sonnei was cloned and shown to have an operon-like rfb organization. It was found that the 11.0 kb HindIII-XbaI fragment of pHH201 encoding the form I antigen contains 10 contiguous open reading frames (ORF), ORF1 to ORF10. Deletions from either end of pHH201, within ORF1 or ORF10, eliminated form I expression. ORF1 and ORF2 share significant nucleic and amino acids homologies to two ORF's of the Salmonella typhi Vi antigen genes. ORF5 in pHH201 is identical to IS630. pHH2064, derived from pHH201, lacks the IS630 element and can stably express the form I antigen inE. coli HB101. However, pHH2064 is structurally unstable in a S. sonnei form II host. This indicates that the presence of the IS630 gene within the S. sonnei rfb operon may be necessary for the stability of form I expression in S. sonnei. This finding is substantiated by the observation that all virulent S. sonnei isolates examined in this study retained the IS630 element within their rfb operon.

Escherichia coli clone Sonnei (Shigella sonnei) had a chromosomal O-antigen gene cluster prior to gaining its current plasmid-borne O-antigen genes

O antigen is part of the lipopolysaccharide present in the outer membrane of gram-negative bacteria. The surface-exposed O antigen is subject to selection by the host immune system, which may account for the maintenance of many different O-antigen forms. Characteristically, all genes specific to O-antigen synthesis are clustered in a region close to the his and gnd genes on the chromosome of Escherichia coli and related species. Shigella sonnei, essentially a clone of E. coli (E. coli clone Sonnei), is an important human pathogen and is unusual in that its O-antigen gene cluster is located on a plasmid. Our results suggest that it once had a normal chromosomal O-antigen gene cluster which has been largely deleted. We suggest that the O antigen encoded by the plasmid-borne genes offered a selective advantage in adapting to a new environment and that the chromosomal O-antigen genes were eventually inactivated. We also identified, by PCR and sequencing, a potential ancestor of E. coli Sonnei among the 166 known E. coli serotype strains.

Construction and characterization of genetically-marked bivalent anti-Shigella dysenteriae 1 and anti-Shigella flexneri Y live vaccine candidates

Bivalent vaccine candidates were developed against Shigella dysenteriae 1 and Shigella flexneri, which are among the most frequent causative agents of shigellosis in developing countries. The rfp and rfb gene clusters, which code for S. dysenteriae serotype 1 O-antigen biosynthesis, were inserted into an arsenite resistance minitransposon and randomly integrated into the attenuated S. flexneri aroD serotype Y strain SFL124. Nine recombinant clones that efficiently expressed both homologous and heterologous O-antigens were obtained. Southern blot analysis showed that in one clone the S. dysenteriae 1 genes had integrated into the chromosome, whereas in all the others they had integrated into the virulence plasmid. All recombinant clones exhibited normal growth characteristics, were able to invade and survive within eukaryotic cells to the same extent as the parental strain, and expressed efficiently the recombinant lipopolysaccharide within invaded cells. Immunization of mice with two of the recombinant clones resulted in the production of antibodies specific for both homologous and heterologous O-antigens. The recombinant clones constitute promising vaccine candidates which can readily be distinguished from endemic shigellae by their non-antibiotic resistance marker.

Construction and characterization of a live attenuated vaccine candidate against Shigella dysenteriae type 1

Vaccine candidates against Shigella dysenteriae type 1, which is associated with the most severe cases of bacillary dysentery, were constructed. The rfp and rfb gene clusters, which code for S. dysenteriae 1 O antigen biosynthesis, were randomly integrated into either the chromosome or the virulence plasmid of the rough attenuated Shigella flexneri aroD strain SFL124-27 with a minitransposon carrying an arsenite resistance selection marker. The recombinant clones efficiently expressed the recombinant O antigen, exhibited a normal growth pattern, were able to invade and survive within eukaryotic cells to the same extent as the parental strain, and expressed the recombinant antigen within invaded cells. A clone was selected as the vaccine candidate, which was demonstrated to be immunogenic and safe in animal models, leading to 47% full protection and 53% partial protection against challenge with the wild-type strain.

Versatile cosmid vectors for facilitated analysis and subcloning of the insert

A series of cosmid vectors, termed pSSVI215, pSSVI216-1, pSSVI216-2, pSSVI217, and pSSVI218, were constructed in order to facilitate the downstream processing of large inserts. Each vector has dual cos sites as well as a kanamycin resistance (KmR) gene flanked by recognition sites for the very rare cutter I-SceI meganuclease as well as symmetrical NotI and SwaI sites (SCEKAN cassette). Several unique cloning sites, including BamHI, are present on one side of the cassette between the I-SceI and NotI/SwaI sites. The various cosmids differ from each other by one or more of the following features: origin of replication (ori), size, host range, and conjugal transfer capability. Inserts combined with the SCEKAN cassette can be isolated on a NotI or SwaI fragment from any of these vectors and easily subcloned into the vector of choice by selecting for the adjacent KmR gene which can later be removed by I-SceI restriction and self-ligation. In addition, the SCEKAN cassette can be conveniently excised from plasmid pSSVI214 such that any plasmid can easily be fitted with the present system. The subcloning strategy afforded by the new vectors was successfully applied to an approximately 37-kb fragment from the V. cholerae O139 genome carrying the rfb locus which encodes the O-serotype specificity of this organism.

Further molecular characterization and stability of the live oral attenuated cholera vaccine strain CVD103-HgR

Vibrio cholerae CVD103-HgR, the first live attenuated vaccine licensed for human use produced by recombinant DNA technology, was genetically compared to its parent strains 569B and CVD103. The genetic stability for both lyophilized vaccine in final container form and for viable organisms shed from vaccinees was determined. Results obtained lead us to conclude: (i) the genetic composition of the examined genes in CVD103-HgR is identical to that of the parent strains except for the alterations induced; (ii) the level of mercury resistance depends on the orientation of the mer operon within hlyA, with the highest level being observed for the orientation found in CVD103-HgR; (iii) no DNA sequences from plasmids used in construction remain in the genome; (iv) the strain is genetically stable; and (v) both CVD103-HgR and its parent strains contain defective lysogenic prophages. We have further confirmed that a certain amount of restriction fragment length polymorphism (RFLP) exists around the chromosomal ctx locus within V. cholerae strains of the classical biotype (detectable on chromosomal DNA restricted by either HindIII or EcoRI, but not PstI).

Development of Shigella sonnei live oral vaccines based on defined rfbInaba deletion mutants of Vibrio cholerae expressing the Shigella serotype D O polysaccharide

Previous experimentation has highlighted a number of difficulties in the development of carrier-based bivalent vaccines (J.-F. Viret and D. Favre, Biologicals 22:361-372, 1994) In an attempt to obviate these carrier strains. Toward this aim, a series of defined rfbInaba deletion (delta rfbInaba) mutants of the cholera vaccine strain V. cholerae CVD103-HgR (O1 Inaba serotype) and derivative bearing the chromosomally integrated locus encoding the S. sonnei O-PS were constructed and characterized. The various mutations disrupt genes thought to be involved in either the synthesis of perosamine, the synthesis of 3-deoxy-L-glycero tetronic acid, or the O-PS transport functions together with synthesis of the perosamine synthetase. Some deletions were obtained only in strains expressing the heterologous lipopolysaccharide (LPS). Viable delta rfbInaba deletions in CVD103-HgR profoundly altered some of its phenotypic properties. The same deletions present in CVD103-HgR derivatives expressing the heterologous LPS affected their phenotypes only to a lesser extent. Only in strains in which perosamine synthesis was specifically abolished could high amounts of core-bound S. sonnei O-PS be synthesized. Two such strains (CH21, which expresses both the R1 core and the S. sonnei O-PS, and CH22, which expresses only the latter antigenic determinant) were further analyzed and were found to be indistinguishable from CVD103-HgR with regard to lack of enterotoxin activity, choleragenoid production, mercury resistance, pilin production, and, for CH22, motility. Mice immunized with CH22 produced high titers of S. sonnei O-PS-specific antibodies.

Lateral transfer of rfb genes: a mobilizable ColE1-type plasmid carries the rfbO:54 (O:54 antigen biosynthesis) gene cluster from Salmonella enterica serovar Borreze

Plasmid pWQ799 is a 6.9-kb plasmid isolated from Salmonella enterica serovar Borreze. Our previous studies have shown that the plasmid contains a functional biosynthetic gene cluster for the expression of the O:54 lipopolysaccharide O-antigen of this serovar. The minimal replicon functions of pWQ799 have been defined, and a comparison with nucleotide and protein databases revealed this replicon to be virtually identical to ColE1. This is the first report of involvement of ColE1-related plasmids in O-antigen expression. The replicon of pWQ799 is predicted to encode two RNA molecules, typical of other ColE1-type plasmids. RNAII, the putative replication primer from pWQ799, shares regions of homology with RNAII from ColE1. RNA1 is an antisense regulator of DNA replication in ColE1-related plasmids. The coding region for RNAI from pWQ799 shares no homology with any other known RNAI sequence but is predicted to adopt a secondary structure characteristic of RNAI molecules. pWQ799 may therefore represent a new incompatibility group within this family. pWQ799 also possesses cer, rom, and mob determinants, and these differ minimally from those of ColE1. The plasmid is mobilizable in the presence of either the broad-host-range helper plasmid pRK2013 or the IncI1 plasmid R64drd86. Mobilization and transfer of pWQ799 to other organisms provides the first defined mechanism for lateral transfer of O-antigen biosynthesis genes in S. enterica and explains both the distribution of related plasmids and coexpression of the O:54 factor with other O-factors in different Salmonella serovars. The base composition of the pWQ799 replicon sequences gives an average percent G+C value typical of Salmonella spp. In contrast, the percent G+C value is dramatically lower with rfb0:54, consistent with the possibility that the cluster was acquired from an organism with much lower G+C composition.