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

Glucose- but not rice-based oral rehydration therapy enhances the production of virulence determinants in the human pathogen Vibrio cholerae

Despite major attempts to prevent cholera transmission, millions of people worldwide still must address this devastating disease. Cholera research has so far mainly focused on the causative agent, the bacterium Vibrio cholerae, or on disease treatment, but rarely were results from both fields interconnected. Indeed, the treatment of this severe diarrheal disease is mostly accomplished by oral rehydration therapy (ORT), whereby water and electrolytes are replenished. Commonly distributed oral rehydration salts also contain glucose. Here, we analyzed the effects of glucose and alternative carbon sources on the production of virulence determinants in the causative agent of cholera, the bacterium Vibrio cholerae during in vitro experimentation. We demonstrate that virulence gene expression and the production of cholera toxin are enhanced in the presence of glucose or similarly transported sugars in a ToxR-, TcpP- and ToxT-dependent manner. The virulence genes were significantly less expressed if alternative non-PTS carbon sources, including rice-based starch, were utilized. Notably, even though glucose-based ORT is commonly used, field studies indicated that rice-based ORT performs better. We therefore used a spatially explicit epidemiological model to demonstrate that the better performing rice-based ORT could have a significant impact on epidemic progression based on the recent outbreak of cholera in Haiti. Our results strongly support a change of carbon source for the treatment of cholera, especially in epidemic settings.

Cholera research has so far mainly focused on the causative agent, the bacterium Vibrio cholerae, or on disease treatment, but rarely were results from both fields interconnected. Indeed, the treatment of this severe diarrheal disease is mostly accomplished by oral rehydration therapy (ORT). ORT aims at rehydrating patients through the provision of water and oral rehydration salts; the latter being composed of electrolytes as well as glucose as a carbon source. Although glucose-based ORS is commonly used to treat diarrheal diseases and is recommended by the WHO, field studies on cholera indicated that rice-based ORT performs better than glucose-based ORT. Here, we investigated the impact that glucose, starch, or other carbon sources exert on V. cholerae. We demonstrated that glucose leads to an increased expression of the major virulence genes in the pathogen and, accordingly, to an enhanced production of cholera toxin during in vitro experimentation. Because the cholera toxin is primarily responsible for the severe symptoms that are associated with the disease, our study highlights the negative effects of glucose-based ORT. Next, we used a spatially explicit epidemiological model to demonstrate that the better performing rice-based ORS could have a significant impact on epidemic progression based on the recent outbreak of cholera in Haiti.

Small-molecule inhibitors of toxT expression in Vibrio cholerae

Vibrio cholerae, a Gram-negative bacterium, infects humans and causes cholera, a severe disease characterized by vomiting and diarrhea. These symptoms are primarily caused by cholera toxin (CT), whose production by V. cholerae is tightly regulated by the virulence cascade. In this study, we designed and carried out a high-throughput chemical genetic screen to identify inhibitors of the virulence cascade. We identified three compounds, which we named toxtazin A and toxtazin B and B', representing two novel classes of toxT transcription inhibitors. All three compounds reduce production of both CT and the toxin-coregulated pilus (TCP), an important colonization factor. We present evidence that toxtazin A works at the level of the toxT promoter and that toxtazins B and B' work at the level of the tcpP promoter. Treatment with toxtazin B results in a 100-fold reduction in colonization in an infant mouse model of infection, though toxtazin A did not reduce colonization at the concentrations tested. These results add to the growing body of literature indicating that small-molecule inhibitors of virulence genes could be developed to treat infections, as alternatives to antibiotics become increasingly needed.

IMPORTANCE

V. cholerae caused more than 580,000 infections worldwide in 2011 alone (WHO, Wkly. Epidemiol. Rec. 87:289-304, 2012). Cholera is treated with an oral rehydration therapy consisting of water, glucose, and electrolytes. However, as V. cholerae is transmitted via contaminated water, treatment can be difficult for communities whose water source is contaminated. In this study, we address the need for new therapeutic approaches by targeting the production of the main virulence factor, cholera toxin (CT). The high-throughput screen presented here led to the identification of two novel classes of inhibitors of the virulence cascade in V. cholerae, toxtazin A and toxtazins B and B'. We demonstrate that (i) small-molecule inhibitors of virulence gene production can be identified in a high-throughput screen, (ii) targeting virulence gene production is an effective therapeutic strategy, and (iii) small-molecule inhibitors can uncover unknown layers of gene regulation, even in well-studied regulatory cascades.

Vibrio cholerae VexH encodes a multiple drug efflux pump that contributes to the production of cholera toxin and the toxin co-regulated pilus

The resistance-nodulation-division (RND) efflux systems are ubiquitous transporters that function in antimicrobial resistance. Recent studies showed that RND systems were required for virulence factor production in Vibrio cholerae. The V. cholerae genome encodes six RND efflux systems. Three of the RND systems (VexB, VexD, and VexK) were previously shown to be redundant for in vitro resistance to bile acids and detergents. A mutant lacking the VexB, VexD, and VexK RND pumps produced wild-type levels of cholera toxin (CT) and the toxin co-regulated pilus (TCP) and was moderately attenuated for intestinal colonization. In contrast, a RND negative mutant produced significantly reduced amounts of CT and TCP and displayed a severe colonization defect. This suggested that one or more of the three uncharacterized RND efflux systems (i.e. VexF, VexH, and VexM) were required for pathogenesis. In this study, a genetic approach was used to generate a panel of V. cholerae RND efflux pump mutants in order to determine the function of VexH in antimicrobial resistance, virulence factor production, and intestinal colonization. VexH contributed to in vitro antimicrobial resistance and exhibited a broad substrate specificity that was redundant with the VexB, VexD, and VexK RND efflux pumps. These four efflux pumps were responsible for in vitro antimicrobial resistance and were required for virulence factor production and intestinal colonization. Mutation of the VexF and/or VexM efflux pumps did not affect in vitro antimicrobial resistance, but did negatively affect CT and TCP production. Collectively, our results demonstrate that the V. cholerae RND efflux pumps have redundant functions in antimicrobial resistance and virulence factor production. This suggests that the RND efflux systems contribute to V. cholerae pathogenesis by providing the bacterium with protection against antimicrobial compounds that are present in the host and by contributing to the regulated expression of virulence factors.

A bistable switch and anatomical site control Vibrio cholerae virulence gene expression in the intestine

A fundamental, but unanswered question in host-pathogen interactions is the timing, localization and population distribution of virulence gene expression during infection. Here, microarray and in situ single cell expression methods were used to study Vibrio cholerae growth and virulence gene expression during infection of the rabbit ligated ileal loop model of cholera. Genes encoding the toxin-coregulated pilus (TCP) and cholera toxin (CT) were powerfully expressed early in the infectious process in bacteria adjacent to epithelial surfaces. Increased growth was found to co-localize with virulence gene expression. Significant heterogeneity in the expression of tcpA, the repeating subunit of TCP, was observed late in the infectious process. The expression of tcpA, studied in single cells in a homogeneous medium, demonstrated unimodal induction of tcpA after addition of bicarbonate, a chemical inducer of virulence gene expression. Striking bifurcation of the population occurred during entry into stationary phase: one subpopulation continued to express tcpA, whereas the expression declined in the other subpopulation. ctxA, encoding the A subunit of CT, and toxT, encoding the proximal master regulator of virulence gene expression also exhibited the bifurcation phenotype. The bifurcation phenotype was found to be reversible, epigenetic and to persist after removal of bicarbonate, features consistent with bistable switches. The bistable switch requires the positive-feedback circuit controlling ToxT expression and formation of the CRP-cAMP complex during entry into stationary phase. Key features of this bistable switch also were demonstrated in vivo, where striking heterogeneity in tcpA expression was observed in luminal fluid in later stages of the infection. When this fluid was diluted into artificial seawater, bacterial aggregates continued to express tcpA for prolonged periods of time. The bistable control of virulence gene expression points to a mechanism that could generate a subpopulation of V. cholerae that continues to produce TCP and CT in the rice water stools of cholera patients.

Most pathogenic microorganisms infect in a stepwise manner: colonization of host surfaces is followed by invasion and injury of host tissues and, late in the infectious process, dissemination to other hosts occurs. During its residence in the host, the pathogen produces essential virulence determinants and often replicates rapidly, leading to a vast expansion of its biomass. Although this scenario is well established also for Vibrio cholerae, the cause of a potentially fatal diarrheal illness, it has not previously been possible to identify precisely when or where virulence determinants are produced in the intestine. We addressed this question by investigating the expression of virulence genes by individual V. cholerae during infection of the small intestine. Virulence genes were found to be powerfully expressed early in the infectious process by bacteria in close proximity to epithelial surfaces. Increased replication rates were also localized to epithelial surfaces. During later stages of the infection, the population of V. cholerae bifurcates into two fractions: one subpopulation continues to express virulence genes, whereas these genes are silenced in the other subpopulation. The genetic program controlling the continued production of virulence genes may mediate the persistence of a hyper-infectious subpopulation of bacteria in the stools of cholera patients.

Evolutionary genetic analysis of the emergence of epidemic Vibrio cholerae isolates on the basis of comparative nucleotide sequence analysis and multilocus virulence gene profiles

Vibrio cholerae, the causative agent of cholera, is a natural inhabitant of the aquatic ecosystem. We examined a unique collection of V. cholerae clinical and environmental isolates of widespread geographic distribution recovered over a 60-year period to determine their evolutionary genetic relationships based on analysis of two housekeeping genes, malate dehydrogenase (mdh) and a chaperonin (groEL). In addition, the phylogenetic distribution of 12 regions associated with virulence was determined. Comparative sequence analysis of mdh revealed that all V. cholerae O1 and O139 serogroup isolates belonged to the same clonal lineage. Single-strand conformational polymorphism (SSCP) analysis of these O1 and O139 strains at groEL confirmed the presence of an epidemic clonal complex. Of the 12 virulence regions examined, only three regions, Vibrio seventh pandemic island 1 (VSP-I), VSP-II, and RS1, were absent from all classical V. cholerae isolates. Most V. cholerae El Tor biotype and O139 serogroup isolates examined encoded all 12 virulence regions assayed. Outside of V. cholerae O1/O139 serogroup isolates, only one strain, VO7, contained VSP-I. Two V. cholerae El Tor isolates, GP155 and 2164-78, lacked both VSP-I and VSP-II, and one El Tor isolate, GP43, lacked VSP-II. Five non-O1/non-O139 serogroup isolates had an mdh sequence identical to that of the epidemic O1 and O139 strains. These isolates, similar to classical strains, lack both VSP-I and VSP-II. Four of the 12 virulence regions examined were found to be present in all isolates: hlyA, pilE, MSHA and RTX. Among non-O1/non-O139 isolates, however, the occurrence of the additional eight regions was considerably lower. The evolutionary relationships and multilocus virulence gene profiles of V. cholerae natural isolates indicate that consecutive pandemic strains arose from a common O1 serogroup progenitor through the successive acquisition of new virulence regions.

Expression of cholera toxin under non-AKI conditions in Vibrio cholerae El Tor induced by increasing the exposed surface of cultures

The regulatory systems controlling expression of the ctxAB genes encoding cholera toxin (CT) in the classical and El Tor biotypes of pathogenic Vibrio cholerae have been characterized and found to be almost identical. Notwithstanding this, special in vitro conditions, called AKI conditions, are required for El Tor bacteria to produce CT. The AKI conditions involve biphasic cultures. In phase 1 the organism is grown in a still tube for 4 h. In phase 2 the medium is poured into a flask to continue growth with shaking. Virtually no expression of CT occurs if this protocol is not followed. Here we demonstrated that CT expression takes place in single-phase still cultures if the volume-to-surface-area ratio is decreased, both under air and under an inert atmosphere. The expression of key genes involved in the regulation of CT production was analyzed, and we found that the expression pattern closely resembles the in vivo expression pattern.

Identification of a novel type IV pilus gene cluster required for gastrointestinal colonization of Citrobacter rodentium

Citrobacter rodentium is used as an in vivo model system for clinically significant enteric pathogens such as enterohaemorrhagic Escherichia coli (EHEC) and enteropathogenic E. coli (EPEC). These pathogens all colonize the lumen side of the host gastrointestinal tract via attaching and effacing (A/E) lesion formation. In order to identify genes required for the colonization of A/E-forming pathogens, a library of signature-tagged transposon mutants of C. rodentium was constructed and screened in mice. Of the 576 mutants tested, 14 were attenuated in their ability to colonize the descending colon. Of these, eight mapped to the locus of enterocyte effacement (LEE), which is required for the formation of A/E lesions, underlying the importance of this mechanism for pathogenesis. Another mutant, P5H2, was found to have a transposon insertion in an open reading frame that has strong similarity to type IV pilus nucleotide-binding proteins. The region flanking the transposon insertion was sequenced, identifying a cluster of 12 genes that encode the first described pilus of C. rodentium (named colonization factor Citrobacter, CFC). The proteins encoded by cfc genes have identity to proteins of the type IV COF pilus of enterotoxigenic E. coli (ETEC), the toxin co-regulated pilus of Vibrio cholerae and the bundle-forming pilus of EPEC. A non-polar mutation in cfcI, complementation of this strain with wild-type cfcI and complementation of strain P5H2 with wild-type cfcH confirmed that these genes are required for colonization of the gastrointestinal tract by C. rodentium. Thus, CFC provides a convenient model to study type IV pilus-mediated pathogen-host interactions under physiological conditions in the natural colonic environment.

Vibrio pathogenicity island and cholera toxin genetic element-associated virulence genes and their expression in non-O1 non-O139 strains of Vibrio cholerae

A non-O1 non-O139 Vibrio cholerae strain, 10259, belonging to the serogroup O53 was shown to harbor genes related to the vibrio pathogenicity island (VPI) and a cholera toxin (CT) genetic element called CTX. While the nucleotide sequence of the strain 10259 tcpA gene differed significantly (26 and 28%) from those of O1 classical and El Tor biotype strains, respectively, partial sequence analysis data of certain other VPI-associated genes (aldA, tagA, tcpP/H, toxT, acfB/C, and int) and intergenic regions (tcpF to toxT and tcpH to tcpA) of the strain showed only minor variations (0.4 to 4.8%) from corresponding sequences in O1 strains. Strain 10259 also contained CTX element-associated toxin genes with sequences almost identical to those of O1 strains. Growth of the organism in Luria broth (LB) under ToxR inducing conditions (30 degrees C and pH 6.5) led to transcriptional activation of tcpP/H, toxR, toxT, and tcpA genes, but not of ctxA, as determined by reverse transcription-PCR (RT-PCR). Subsequent analysis revealed that strain 10259 possessed only two copies (instead of three or more copies found in epidemic-causing O1 or O139 strains) of the heptanucleotide (TTTTGAT) repeats in the intergenic region upstream of ctxAB. Therefore, a strain 10259 mutant was generated by replacement of this region with a homologous region (1.4 kb) derived from a V. cholerae O1 classical biotype strain (O395) that contained seven such repeats. The resultant recombinant strain (10259R) was found to be capable of coordinately regulated expression of toxT, ctxA, and tcpA when grown under the ToxR inducing conditions. Serological studies also demonstrated that the recombinant strain produced TcpA and a significantly ( approximately 1,000-fold) higher level of CT in vitro compared to that of the parent strain. Virulence gene expression in two other non-O1 non-O139 strains (serogroup O37) containing VPI and the CTX element was studied by RT-PCR and serological assay. One strain (S7, which was involved in an epidemic in Sudan in 1968) showed coordinately regulated expression of virulence genes leading to the production of both CT and TcpA in LB medium. However, the other strain, V2, produced RT-PCR-detectable transcripts of toxT, ctxA, or tcpA genes in the early phase (6 h), but not in the late phase (16 h) of growth in LB medium. These results are consistent with the low levels of production of CT and TcpA by the strain that were serologically detectable. The significance of these results is discussed in relation to the role of virulence genes and their expression to the pathogenic potential of V. cholerae strains belonging to non-O1 serogroups.

Evolutionary and functional analyses of variants of the toxin-coregulated pilus protein TcpA from toxigenic Vibrio cholerae non-O1/non-O139 serogroup isolates

The toxin-coregulated pilus (TCP) is a critical determinant of the pathogenicity of Vibrio cholerae. This bundle-forming pilus is an essential intestinal colonization factor and also serves as a receptor for CTXphi, the filamentous phage that encodes cholera toxin (CT). TCP is a polymer of repeating subunits of the major pilin protein TcpA and tcpA is found within the Vibrio pathogenicity island (VPI). In this study genetic variation at the tcpA locus in toxigenic isolates of V. cholerae was investigated and three novel TcpA sequences from V. cholerae strains V46, V52 and V54, belonging to serogroups O141, O37 and O8, respectively, were identified. These novel tcpA alleles grouped into three distinct clonal lineages. The polymorphisms in TcpA were predominantly located in the carboxyl region of TcpA in surface-exposed regions of TCP fibres. Comparison of the genetic diversity among V. cholerae isolates at the tcpA locus with that of aldA, another locus within the VPI, and mdh, a chromosomal locus, revealed that tcpA sequences are far more diverse than these other loci. Most likely, this diversity is a reflection of diversifying selection in adaptation to the host immune response or to CTXphi susceptibility. An assessment of the functional properties of the variant tcpA sequences in the non-O1 V. cholerae strains was carried out by analysing whether these strains could be infected by CTXphi and colonize the suckling mouse. Similar to El Tor strains of V. cholerae O1, in vitro CTXphi infection of these strains required the exogenous expression of toxT, suggesting that in these strains ToxT regulates TCP expression and that these TcpA variants can serve as CTXphi receptors. All the V. cholerae non-O1 serogroup isolates tested were capable of colonizing the suckling mouse small intestine, suggesting that the different TcpA variants could function as colonization factors.

Detection of virulence associated genes in clinical strains of vibrio mimicus

A total of 42 clinical strains of Vibrio mimicus were examined for the presence of virulence associated genes toxR, toxS, toxT, tcpP, ctx and tcpA by PCR assay. Almost all strains were shown to have the toxR gene, while the toxS gene was found in 27 strains. On the other hand, five strains possessed both toxT and tcpP genes, but others had neither. Only two strains were positive for amplification of the ctx gene, whereas no PCR product with tcpA primers was detected. The results indicate the incomplete copies of virulence cascade in V mimicus strains. The pathogenesis and epidemic potential of this species is also discussed.

Comparison of Vibrio cholerae pathogenicity islands in sixth and seventh pandemic strains

Epidemic Vibrio cholerae strains possess a large cluster of essential virulence genes on the chromosome called the Vibrio pathogenicity island (VPI). The VPI contains the tcp gene cluster encoding the type IV pilus toxin-coregulated pilus colonization factor which can act as the cholera toxin bacteriophage (CTXPhi) receptor. The VPI also contains genes that regulate virulence factor expression. We have fully sequenced and compared the VPI of the seventh-pandemic (El Tor biotype) strain N16961 and the sixth-pandemic (classical biotype) strain 395 and found that the N16961 VPI is 41,272 bp and encodes 29 predicted proteins, whereas the 395 VPI is 41,290 bp. In addition to various nucleotide and amino acid polymorphisms, there were several proteins whose predicted size differed greatly between the strains as a result of frameshift mutations. We hypothesize that these VPI sequence differences provide preliminary evidence to help explain the differences in virulence factor expression between epidemic strains (i.e., the biotypes) of V. cholerae.

The Vibrio cholerae ToxR/TcpP/ToxT virulence cascade: distinct roles for two membrane-localized transcriptional activators on a single promoter

ToxR is required in Vibrio cholerae for transcriptional activation of the toxT gene, the protein product of which activates numerous genes involved in virulence. Although ToxR cannot activate the toxT promoter in Escherichia coli, the products of the tcpPH operon are shown here to activate the toxT promoter, and co-expression with ToxRS enhances activation. An identical pattern was seen in a DeltatcpPDeltatoxR strain of V. cholerae when TcpPH or ToxRS was expressed from plasmids. Although overexpression of the TcpP/H proteins in V. cholerae partially complemented both a DeltatoxR strain and a DeltatcpPDeltatoxR double mutant for toxin production and toxT-lacZ activation, the presence of ToxR greatly increased their expression. Analysis of a toxT-lacZ promoter deletion series demonstrated that TcpP was able to interact functionally with the toxT promoter downstream of the ToxR binding site. This was confirmed using electrophoretic mobility shift assays of this toxT promoter deletion series and DNase I footprinting analysis, which showed that TcpP interacts with the promoter region from -51 to -32, whereas ToxR protected a region from -100 to -69. In addition, membranes containing endogenous levels of ToxR bound more readily to the toxT promoter than did membranes containing only TcpP. Characterization of a number of tcpP substitution mutants revealed one derivative (TcpP-H93L) that, when overexpressed, was markedly defective for toxT activation, cholera toxin and TcpA (toxin co-regulated pilus) production and DNA binding; however, toxT activation by TcpP-H93L was restored in the presence of ToxR, suggesting that ToxR can provide the promoter recognition function for toxT activation. Two additional mutant derivatives, TcpP-W68L and TcpP-R86A, failed to activate toxT or direct toxin and TcpA production in the presence or absence of ToxR. Both TcpP-W68L and TcpP-R86A, like TcpP-H93L, were defective for DNA binding. Finally, a ToxR mutant derivative, ToxR-G80S, served to separate the different roles of ToxR on different promoters. Although ToxR-G80S was inefficient at activating the ompU promoter in V. cholerae (ompU encodes an outer membrane porin regulated by ToxR), it was fully capable of activating the toxT promoter. These data suggest that ToxR is not a direct activator in the toxT expression system but, instead, enhances the activity of TcpP, perhaps by recruiting it to the toxT promoter under conditions in which expression levels of TcpP are too low for it to activate toxT efficiently on its own.

Differential activation of the tcpPH promoter by AphB determines biotype specificity of virulence gene expression in Vibrio cholerae

Vibrio cholerae strains of the classical biotype express the genes encoding cholera toxin (CT) and toxin-coregulated pilus (TCP) under a variety of environmental conditions in vitro, whereas El Tor biotype strains express these genes only under specialized culture conditions. We show here that a single base-pair difference at positions -65 and -66 of the classical and El Tor tcpPH promoters, respectively, is responsible for the differential regulation of virulence gene expression in these two disease-causing biotypes. Analysis of tcpP-lacZ fusions in both V. cholerae and Escherichia coli indicated that transcriptional activation of the El Tor tcpPH promoter by the LysR regulator AphB was significantly reduced relative to that of the classical promoter. Reciprocal exchange of the tcpPH promoter between the two biotypes in V. cholerae showed that the ability to activate the transcription of tcpPH is not dependent on the biotype of the strain per se but on the tcpPH promoter itself. Classical and El Tor tcpP-lacZ promoter chimeras in E. coli localized the region responsible for the differential activation of tcpPH by AphB to within 75 bp of the transcriptional start site. Individual base-pair changes within this region showed that the presence of either an A or a G at position -65 or -66 conferred the classical or El Tor, respectively, pattern of tcpPH activation by AphB. Reciprocal exchange of these base pairs between biotypes in V. cholerae switched the biotype-specific pattern of expression of tcpPH as well as the production of CT and TCP in response to environmental stimuli.

Classical and El Tor biotypes of Vibrio cholerae differ in timing of transcription of tcpPH during growth in inducing conditions

Two protein pairs in Vibrio cholerae, ToxRS and TcpPH, are necessary for transcription from the toxT promoter and subsequent expression of cholera virulence genes. We have previously shown that transcription of tcpPH in classical strains of V. cholerae is activated at mid-log-phase growth in ToxR-inducing conditions, while transcription of tcpPH in El Tor strains is not. In this study, we showed that while transcription of tcpPH differs at mid-log-phase growth in ToxR-inducing conditions between the biotypes, transcription is equivalently high during growth in AKI conditions. We used tcpPH::gusA transcriptional fusions to quantitate expression of tcpPH in each biotype throughout growth in ToxR-inducing conditions and showed that although transcription of tcpPH is reduced at mid-log-phase growth in an El Tor strain, transcription is turned on later in growth to levels in excess of those in the classical strain (although cholera toxin is not produced). This suggests that the difference in expression of cholera virulence factors in response to ToxR-inducing conditions between the El Tor and classical biotypes of V. cholerae may be related to the timing of transcription of tcpPH rather than the absolute levels of transcription.

Evidence for specificity in type 4 pilus biogenesis by enteropathogenic Escherichia coli

Type 4 fimbriae (pili) are surface appendages that are expressed by many species of Gram-negative bacteria. Previous studies have demonstrated that Pseudomonas aeruginosa can express and assemble pilin subunits from several unrelated species, indicating a common mechanism for biogenesis of type 4 pili whereby structural subunits from one system may be interchanged with those of another. In this study, an isogenic mutant of enteropathogenic Escherichia coli (EPEC) was constructed containing the entire tcpA gene from Vibrio cholerae 0395, which encodes the major structural subunit of the toxin-coregulated pilus (TCP), in place of bfpA, which encodes the major structural subunit of the bundle-forming pilus (BFP). Surprisingly, expression of type 4 pilin structures and the associated phenotype of bacterial autoaggregation in culture media were not observed for cells of the EPEC strain containing tcpA nor for those containing an additional mutation in bfpF, which otherwise is associated with a hyperfimbriate phenotype. In addition, cells of a bfpA mutant EPEC strain containing plasmids designed to express either of two different chimeric type 4 pilin subunits containing segments of BfpA and TcpA also failed to form bacterial aggregates and express type 4 pilin structures. Collectively, these results indicate that the type 4 pilin assembly system of EPEC exhibits specificity with regard to pilin subunit recognition and assembly.

Infectious CTXPhi and the vibrio pathogenicity island prophage in Vibrio mimicus: evidence for recent horizontal transfer between V. mimicus and V. cholerae

Vibrio mimicus differs from Vibrio cholerae in a number of genotypic and phenotypic traits but like V. cholerae can give rise to diarrheal disease. We examined clinical isolates of V. mimicus for the presence of CTXPhi, the lysogenic filamentous bacteriophage that carries the cholera toxin genes in epidemic V. cholerae strains. Four V. mimicus isolates were found to contain complete copies of CTXPhi. Southern blot analyses revealed that V. mimicus strain PT5 contains two CTX prophages integrated at different sites within the V. mimicus genome whereas V. mimicus strains PT48, 523-80, and 9583 each contain tandemly arranged copies of CTXPhi. We detected the replicative form of CTXPhi, pCTX, in all four of these V. mimicus isolates. The CTX prophage in strain PT5 was found to produce infectious CTXPhi particles. The nucleotide sequences of CTXPhi genes orfU and zot from V. mimicus strain PT5 and V. cholerae strain N16961 were identical, indicating contemporary horizontal transfer of CTXPhi between these two species. The receptor for CTXPhi, the toxin-coregulated pilus, which is encoded by another lysogenic filamentous bacteriophage, VPIPhi, was also present in the CTXPhi-positive V. mimicus isolates. The nucleotide sequences of VPIPhi genes aldA and toxT from V. mimicus strain PT5 and V. cholerae N16961 were identical, suggesting recent horizontal transfer of this phage between V. mimicus and V. cholerae. In V. mimicus, the vibrio pathogenicity island prophage was integrated in the same chromosomal attachment site as in V. cholerae. These results suggest that V. mimicus may be a significant reservoir for both CTXPhi and VPIPhi and may play an important role in the emergence of new toxigenic V. cholerae isolates.

Differential transcription of the tcpPH operon confers biotype-specific control of the Vibrio cholerae ToxR virulence regulon

Epidemic strains of Vibrio cholerae O1 are divided into two biotypes, classical and El Tor. In both biotypes, regulation of virulence gene expression depends on a cascade in which ToxR activates expression of ToxT, and ToxT activates expression of cholera toxin and other virulence genes. In the classical biotype, maximal expression of this ToxR regulon in vitro occurs at 30 degrees C at pH 6.5 (ToxR-inducing conditions), whereas in the El Tor biotype, production of these virulence genes only occurs under very limited conditions and not in response to temperature and pH; this difference between biotypes is mediated at the level of toxT transcription. In the classical biotype, two other proteins, TcpP and TcpH, are needed for maximal toxT transcription. Transcription of tcpPH in the classical biotype is regulated by pH and temperature independently of ToxR or ToxT, suggesting that TcpP and TcpH couple environmental signals to transcription of toxT. In this study, we show a near absence of tcpPH message in the El Tor biotype under ToxR-inducing conditions of temperature and pH. However, once expressed, El Tor TcpP and TcpH appear to be as effective as classical TcpP and TcpH in activating toxT transcription. These results suggest that differences in regulation of virulence gene expression between the biotypes of V. cholerae primarily result from differences in expression of tcpPH message in response to environmental signals. We present an updated model for control of the ToxR virulence regulon in V. cholerae.

Pathogenic and vaccine significance of toxin-coregulated pili of Vibrio cholerae E1 Tor

Vibrio cholerae O1 strains are classified into one of two biotypes, classical and E1 Tor, the latter being primarily responsible for cholera cases worldwide since 1961. Recent studies in our laboratory have focused upon the pathogenic and vaccine significance of the toxin-coregulated pili (TCP) produced by strains of E1 Tor biotype. Mutants in which the tcpA gene (encoding the pilin subunit protein) has been inactivated are dramatically attenuated in the infant mouse cholera model, showing markedly reduced colonisation potential in mixed-infection competition experiments. Significantly, in the vaccine context, antibodies to TCP are sufficient to prevent experimental infection, although our data suggest that this protective effect might be limited to strains of homologous biotype. Since we have shown that tcpA sequences are conserved within a biotype but differ between biotypes, this latter observation suggests that the biotype-restricted pilin epitopes might have greater vaccine significance. Similar studies indicate that TCP also play a critical role in colonisation by strains of the recently-recognised O139 serogroup, which is thought to have evolved from an O1 E1 Tor strain. In contrast to the effect of introducing mutations in the tcpA gene, strains carrying inactivated mshA genes (encoding the subunit of the mannose-sensitive haemagglutinin pilus) show unaltered in vivo behaviour. Consistent with this finding is our inability to demonstrate any protective effect associated with antibodies to MSHA. Ongoing approaches to vaccine development are variously aimed at improving the immunogenicity of the current inactivated whole-cell vaccine, or assessing the field efficacy of a promising live attenuated strain. The possible implications of our findings are discussed in relation to both of these options.

Transient transcriptional activation of the Vibrio cholerae El Tor virulence regulator toxT in response to culture conditions

Vibrio cholerae El Tor require special in vitro culture conditions, consisting of an initial static growth period followed by shift to shaking (AKI conditions), for expression of cholera toxin (CT) and toxin coregulated pili (TCP). ToxT, a regulator whose initial transcription depends on the ToxR regulator, positively modulates expression of CT and TCP. To help understand control of CT and TCP in El Tor vibrios, we monitored ctxAB and ToxR-dependent toxT transcription by time course primer extension assays. AKI conditions stimulated CT synthesis with an absence of ctxAB transcription during static growth followed by induction upon shaking. ToxR-dependent toxT transcription was induced at the end of the static growth period but was transient, stopping shortly after shaking was initiated but, interestingly, also if the static phase was prolonged. Immunoblot assays showed that ToxR protein levels were not coincidentally transient, implying a protein on/off switch mechanism for ToxR. Despite the transient activation by ToxR, transcription of ctxAB was maintained during shaking. This finding suggested continued toxT expression, possibly through relay transcription from another promoter. The 12.6-kb distant upstream tcpA promoter responsible for expression of the TCP operon has been proposed to provide an alternate toxT message by readthrough transcription. Activation of the tcpA promoter is supported by increased expression of TcpA protein during the shaking phase of the culture. Readthrough transcription of toxT from tcpA would be compatible with reverse transcription-PCR evidence for a toxT mRNA at times when ToxR-dependent transcription was no longer detectable by primer extension.

Analysis of an autoregulatory loop controlling ToxT, cholera toxin, and toxin-coregulated pilus production in Vibrio cholerae

Coordinate expression of many virulence genes in the human pathogen Vibrio cholerae is controlled by the ToxR, TcpP, and ToxT proteins. These proteins function in a regulatory cascade in which ToxR and TcpP, two inner membrane proteins, are required to activate toxT and ToxT is the direct activator of virulence gene expression. ToxT-activated genes include those whose products are required for the biogenesis of cholera toxin (CTX) and the toxin-coregulated pilus, the major subunit of which is TcpA. This work examined control of toxT transcription. We tested a model whereby activation of toxT by ToxR and TcpP is required to prime an autoregulatory loop in which ToxT-dependent transcription of the tcpA promoter reads through a proposed terminator between the tcpF and toxT genes to result in continued ToxT production. Primer extension analysis of RNA from wild-type classical strain O395 showed that there are two products encoding toxT, one of which is longer than the other by 105 bp. Deletion of the toxT promoter (toxTDeltapro) resulted in the abolishment of toxT transcription, as predicted. Deletion of the tcpA promoter (tcpADeltapro) had no effect on subsequent detection of the smaller toxT primer extension product, but the larger toxT product was not detected, indicating that this product may be the result of transcription from the tcpA promoter and not of initiation directly upstream of toxT. Neither mutant strain produced detectable TcpA, but the CTX levels of the strains were different. The toxTDeltapro strain produced little detectable CTX, while the tcpADeltapro strain produced CTX levels intermediate between those of the wild-type and toxTDeltapro strains. Dependence of toxT transcription on TcpP and TcpH was confirmed by analyzing RNAs from strains carrying deletions in the genes encoding these regulators. The tcpP defect resulted in undetectable toxT transcription, whereas the tcpH mutation led to a diminishing of toxT RNA but not complete abolishment. Taken together, these results suggest that toxT transcription is dependent on two different promoters; one is directly upstream and is activated in part by TcpP and TcpH, and the other is much further upstream and is activated by ToxT.

Toxin-co-regulated pilus cluster in non-O1, non-toxigenic Vibrio cholerae: evidence of a third allele of pilin gene

Polymerase chain reaction has been used to detect the presence of the virulence associated gene, tcpA and part of the promoter distal region of the toxin-co-regulated pilus cluster in non-O1, non-toxigenic, Vibrio cholerae. The amplified regions were characterised by restriction fragment length polymorphism and heteroduplex motility assay. We describe the nucleotide sequence of the tcpA gene fragment from non-toxigenic vibrios from clinical and environmental sources. The present study shows that there are at least three types of the tcpA gene among V. cholerae and the primers specific for the classical tcpA gene, amplify all biotypes. A sequence similarity in other regions of the toxin-co-regulated pilus cluster is suggested. The evidences for the presence of this cluster among non-toxigenic vibrios is, to our knowledge, reported for the first time. The use of restriction fragment length polymorphism for typing the tcpA and studying the alleles distribution is proposed.

The tcp gene cluster of Vibrio cholerae

The toxin co-regulated pilus (TCP) has been identified as a critical colonization factor in both animal models and humans for Vibrio cholerae O1. The major pilin subunit, TcpA (and also TcpB), is similar to type-4 pilins but TCP probably more appropriately belongs to a sub-class which includes the bundle-forming pilus of enteropathogenic Escherichia coli. The genes for TCP biosynthesis and assembly are clustered with the exception of housekeeping functions such as TcpG (=DsbA, a periplasmic disulfide bond epimerase). The nt sequences from El Tor and classical strains show only minor differences corresponding to the major regulatory regions and in TcpA itself. These differences are thought to account for the alternate conditions required for expression of TCP by the two biotypes and the antigenic variation and lack of cross-protection. Aside from the TcpA only a few of the proteins have had their roles in TCP biogenesis defined. Regulation of TCP is controlled by the ToxR regulon via ToxT with a possible involvement of TcpP and the cAMP-CRP system. Experiments using the infant mouse cholera model have now shown that TCP is a colonization factor and protective antigen for both classical and El Tor O1 strains and in the O139 Bengal serotype and that the mannose-sensitive haemagglutinin pilus does not appear to play a comparable role.

Cyclic AMP and its receptor protein negatively regulate the coordinate expression of cholera toxin and toxin-coregulated pilus in Vibrio cholerae

Insertion mutations in two Vibrio cholerae genes, cya and crp, which encode adenylate cyclase and the cyclic AMP (cAMP) receptor protein (CRP), respectively, derepressed the expression of a chromosomal cholera toxin (CT) promoter-lacZ fusion at the nonpermissive temperature of 37 degrees C. In the classical biotype strain O395, the crp mutation increased the production of both CT and toxin-coregulated pilus (TCP) in vitro under a variety of growth conditions not normally permissive for their expression. The most dramatic increase in CT and TCP was observed with the crp mutant in Luria-Bertani (LB) medium pH 8.5, at 30 degrees C. El Tor biotype strains differ from classical strains in that they do not produce CT or TCP when grown in LB media. Incorporation of the crp mutation into El Tor strain C6706 permitted production of these proteins in LB medium pH 6.5, at 30 degrees C. In the infant mouse cholera model, the crp mutation decreased colonization in both biotypes at least 100-fold relative to the wild-type strains. The data presented here suggest a model whereby cAMP-CRP negatively regulates the expression of CT and TCP in both classical and El Tor biotypes under certain environmental conditions and also influences pathogenesis by regulating other processes necessary for optimal growth in vivo.

Toxin-coregulated pilus, but not mannose-sensitive hemagglutinin, is required for colonization by Vibrio cholerae O1 El Tor biotype and O139 strains

The relative contributions of toxin-coregulated pilus (TCP) and cell-associated mannose-sensitive hemagglutinin (MSHA) to the colonization ability of Vibrio cholerae O1 El Tor biotype strains and O139 Bengal strains was determined by using isogenic parental and in-frame deletion mutant pairs in the infant mouse cholera model. Both the El Tor and O139 tcpA mutant strains showed a dramatic defect in colonization as indicated by their competitive indices, whereas deletion of mshA had a negligible effect on colonization in either background.

Regulation of tcp genes in classical and El Tor strains of Vibrio cholerae O1

Expression of genes encoding the toxin-co-regulated pilus (TCP) varies between the two biotypes of Vibrio cholerae O1. Sequence analysis of the tcp locus from the classical and El Tor strains has revealed differences in the intergenic regions between tcpI and tcpP, and tcpH and tcpA, which may be involved in regulation. To investigate this possibility, transcription of tcpA, and the predicted upstream promoters for tcpI and tcpP, has been analysed in the classical and El Tor strains using promoter-cat (chloramphenicol acetyltransferase) fusions. Together with primer extension analyses, these studies indicate that the tcpA and tcpP promoters are toxR-dependent and suggest that TcpP may be involved in activation of both the tcpI and tcpP promoters. We conclude that differences in the level of tcpA expression in a classical and an El Tor strain are likely to be due to the effect of sequence variation on the ability of control factors to act on these regulatory regions.