In vivo adherence and colonization of Vibrio cholerae strains that differ in hemagglutinating activity and motility

Recent Vibrio cholerae O1 Epidemic Strains Are Unable To Replicate CTXΦ Prophage Genome

Cholera, an acute diarrheal disease, is caused by pathogenic strains of Vibrio cholerae generated by the lysogenization of the filamentous cholera toxin phage CTXΦ. Although CTXΦ phage in the classical biotype are usually integrated solitarily or with a truncated copy, those in El Tor biotypes are generally found in tandem and/or with related genetic elements. Due to this structural difference in the CTXΦ prophage array, the prophage in the classical biotype strains does not yield extrachromosomal CTXΦ DNA and does not produce virions, whereas the El Tor biotype strains can replicate the CTXΦ genome and secrete infectious CTXΦ phage particles. However, information on the CTXΦ prophage array structure of pathogenic V. cholerae is limited. Therefore, we investigated the complete genomic sequences of five clinical V. cholerae isolates obtained in Kolkata (India) during 2007 to 2011. The analysis revealed that recent isolates possessed an altered CTXΦ prophage array of the prototype El Tor strain. These strains were defective in replicating the CTXΦ genome. All recent isolates possessed identical rstA and intergenic sequence 1 (Ig-1) sequences and comparable rstA expression in the prototype El Tor strain, suggesting that the altered CTXΦ array was responsible for the defective replication of the prophage. Therefore, CTXΦ structures available in the database and literatures can be classified as replicative and nonreplicative. Furthermore, V. cholerae epidemic strains became capable of producing CTXΦ phage particles since the 1970s. However, V. cholerae epidemic strains again lost the capacity for CTXΦ production around the year 2010, suggesting that a significant change in the dissemination pattern of the current cholera pandemic occurred.

IMPORTANCE Cholera is an acute diarrheal disease caused by pathogenic strains of V. cholerae generated by lysogenization of the filamentous cholera toxin phage CTXΦ. The analysis revealed that recent isolates possessed altered CTXΦ prophage array of prototype El Tor strain and were defective in replicating the CTXΦ genome. Classification of CTXΦ structures in isolated years suggested that V. cholerae epidemic strains became capable of producing CTXΦ phage particles since the 1970s. However, V. cholerae epidemic strains again lost the capacity for CTXΦ production around the year 2010, suggesting that a critical change had occurred in the dissemination pattern of the current cholera pandemic.

Origins of the current seventh cholera pandemic

Vibrio cholerae has caused seven cholera pandemics since 1817, imposing terror on much of the world, but bacterial strains are currently only available for the sixth and seventh pandemics. The El Tor biotype seventh pandemic began in 1961 in Indonesia, but did not originate directly from the classical biotype sixth-pandemic strain. Previous studies focused mainly on the spread of the seventh pandemic after 1970. Here, we analyze in unprecedented detail the origin, evolution, and transition to pandemicity of the seventh-pandemic strain. We used high-resolution comparative genomic analysis of strains collected from 1930 to 1964, covering the evolution from the first available El Tor biotype strain to the start of the seventh pandemic. We define six stages leading to the pandemic strain and reveal all key events. The seventh pandemic originated from a nonpathogenic strain in the Middle East, first observed in 1897. It subsequently underwent explosive diversification, including the spawning of the pandemic lineage. This rapid diversification suggests that, when first observed, the strain had only recently arrived in the Middle East, possibly from the Asian homeland of cholera. The lineage migrated to Makassar, Indonesia, where it gained the important virulence-associated elements Vibrio seventh pandemic island I (VSP-I), VSP-II, and El Tor type cholera toxin prophage by 1954, and it then became pandemic in 1961 after only 12 additional mutations. Our data indicate that specific niches in the Middle East and Makassar were important in generating the pandemic strain by providing gene sources and the driving forces for genetic events.

RpoS controls the Vibrio cholerae mucosal escape response

Vibrio cholerae causes a severe diarrhoeal disease by secreting a toxin during colonization of the epithelium in the small intestine. Whereas the initial steps of the infectious process have been intensively studied, the last phases have received little attention. Confocal microscopy of V. cholerae O1-infected rabbit ileal loops captured a distinctive stage in the infectious process: 12 h post-inoculation, bacteria detach from the epithelial surface and move into the fluid-filled lumen. Designated the “mucosal escape response,” this phenomenon requires RpoS, the stationary phase alternative sigma factor. Quantitative in vivo localization assays corroborated the rpoS phenotype and showed that it also requires HapR. Expression profiling of bacteria isolated from ileal loop fluid and mucus demonstrated a significant RpoS-dependent upregulation of many chemotaxis and motility genes coincident with the emigration of bacteria from the epithelial surface. In stationary phase cultures, RpoS was also required for upregulation of chemotaxis and motility genes, for production of flagella, and for movement of bacteria across low nutrient swarm plates. The hapR mutant produced near-normal numbers of flagellated cells, but was significantly less motile than the wild-type parent. During in vitro growth under virulence-inducing conditions, the rpoS mutant produced 10- to 100-fold more cholera toxin than the wild-type parent. Although the rpoS mutant caused only a small over-expression of the genes encoding cholera toxin in the ileal loop, it resulted in a 30% increase in fluid accumulation compared to the wild-type. Together, these results show that the mucosal escape response is orchestrated by an RpoS-dependent genetic program that activates chemotaxis and motility functions. This may furthermore coincide with reduced virulence gene expression, thus preparing the organism for the next stage in its life cycle.

Vibrio cholerae, a pathogenic microbe, causes a severe diarrhoeal disease mainly in Third World countries. Although the pathogenicity of this organism has been intensively studied for more than a century, most research has focused on the initial stages of the infection, especially colonization of the intestine and virulence gene expression. However, the last stages of the infectious process have received very little attention. In the present manuscript, the authors use the rabbit ileal loop model of cholera to show how this organism, late in the infection, detaches from the epithelial surface and migrates into the luminal fluid, a process the authors termed the “mucosal escape response.” This study identifies, for the first time, how the alternative starvation sigma factor RpoS regulates this process. Features of this genetic program include the dramatic induction of genes involved in motility and chemotaxis functions. This study furthermore identifies RpoS as an important regulator of virulence gene expression and shows that the mucosal escape response may coincide with diminished virulence gene expression. This work is essential for understanding a key and under-appreciated step in the life cycle of this important human pathogen: its exit from the intestine and how this serves to prepare it for transmission into environmental reservoirs or to new human hosts.

Contribution of hemagglutinin/protease and motility to the pathogenesis of El Tor biotype cholera

Vibrio cholerae is a highly motile organism that secretes a Zn-dependent metalloprotease, hemagglutinin/protease (HapA). HapA has been shown to have mucinase activity and contribute to the reactogenicity of live vaccine candidates, but its role in cholera pathogenesis is not yet clear. The contribution of motility to pathogenesis is not fully understood, since conflicting results have been obtained with different strains, mutants, and animal models. The objective of this work was to determine the contribution of HapA and motility to the pathogenesis of El Tor biotype cholera. To this end we constructed isogenic motility (motY) and mucinase (hapA) single and double mutants of an El Tor biotype V. cholerae strain. Mutants were characterized for the expression of major virulence factors in vitro and in vivo. The motility mutant showed a remarkable increase in cholera toxin (CT), toxin coregulated pilus major subunit (TcpA), and HapA production in vitro. Increased TcpA and CT production could be explained by increased transcription of tcpA, ctxA, and toxT. No effect was detected on the transcription of hapA in the motility mutant. The sodium ionophore monensin diminished production of HapA in the parent but not in the motility mutant. Phenamil, a specific inhibitor of the flagellar motor, diminished CT production in the wild-type and motY strains. The hapA mutant showed increased binding to mucin. In contrast, the motY mutation diminished adherence to biotic and abiotic surfaces including mucin. Lack of HapA did not affect colonization in the suckling mouse model. The motility and mucinase defects did not prevent induction of ctxA and tcpA in the mouse intestine as measured by recombinase-based in vivo expression technology. Analysis of mutants in the rabbit ileal loop model showed that both V. cholerae motility and HapA were necessary for full expression of enterotoxicity.

Genetics of stress adaptation and virulence in toxigenic Vibrio cholerae

Vibrio cholerae, a Gram-negative bacterium belonging to the gamma-subdivision of the family Proteobacteriaceae is the etiologic agent of cholera, a devastating diarrheal disease which occurs frequently as epidemics. Any bacterial species encountering a broad spectrum of environments during the course of its life cycle is likely to develop complex regulatory systems and stress adaptation mechanisms to best survive in each environment encountered. Toxigenic V. cholerae, which has evolved from environmental nonpathogenic V. cholerae by acquisition of virulence genes, represents a paradigm for this process in that this organism naturally exists in an aquatic environment but infects human beings and cause cholera. The V. cholerae genome, which is comprised of two independent circular mega-replicons, carries the genetic determinants for the bacterium to survive both in an aquatic environment as well as in the human intestinal environment. Pathogenesis of V. cholerae involves coordinated expression of different sets of virulence associated genes, and the synergistic action of their gene products. Although the acquisition of major virulence genes and association between V. cholerae and its human host appears to be recent, and reflects a simple pathogenic strategy, the establishment of a productive infection involves the expression of many more genes that are crucial for survival and adaptation of the bacterium in the host, as well as for its onward transmission and epidemic spread. While a few of the virulence gene clusters involved directly with cholera pathogenesis have been characterized, the potential exists for identification of yet new genes which may influence the stress adaptation, pathogenesis, and epidemiological characteristics of V. cholerae. Coevolution of bacteria and mobile genetic elements (plasmids, transposons, pathogenicity islands, and phages) can determine environmental survival and pathogenic interactions between bacteria and their hosts. Besides horizontal gene transfer mediated by genetic elements and phages, the evolution of pathogenic V. cholerae involves a combination of selection mechanisms both in the host and in the environment. The occurrence of periodic epidemics of cholera in endemic areas appear to enhance this process.

The role of motility as a virulence factor in bacteria

Many bacteria that cause diseases of humans, animals and plants use flagella to move. This review summarises recent studies that have analysed the role of motility and chemotaxis in the host-parasite relationship of pathogenic bacteria. These studies have shown that for many pathogens, motility is essential in some phases of their life cycle and that virulence and motility are often intimately linked by complex regulatory networks. Possibilities to exploit bacterial motility as a specific therapeutic antibacterial target to cure or prevent disease are discussed.

Genetic characterization of a new type IV-A pilus gene cluster found in both classical and El Tor biotypes of Vibrio cholerae

The Vibrio cholerae genome contains a 5.4-kb pil gene cluster that resembles the Aeromonas hydrophila tap gene cluster and other type IV-A pilus assembly operons. The region consists of five complete open reading frames designated pilABCD and yacE, based on the nomenclature of related genes from Pseudomonas aeruginosa and Escherichia coli K-12. This cluster is present in both classical and El Tor biotypes, and the pilA and pilD genes are 100% conserved. The pilA gene encodes a putative type IV pilus subunit. However, deletion of pilA had no effect on either colonization of infant mice or adherence to HEp-2 cells, demonstrating that pilA does not encode the primary subunit of a pilus essential for these processes. The pilD gene product is similar to other type IV prepilin peptidases, proteins that process type IV signal sequences. Mutational analysis of the pilD gene showed that pilD is essential for secretion of cholera toxin and hemagglutinin-protease, mannose-sensitive hemagglutination (MSHA), production of toxin-coregulated pili, and colonization of infant mice. Defects in these functions are likely due to the lack of processing of N termini of four Eps secretion proteins, four proteins of the MSHA cluster, and TcpB, all of which contain type IV-A leader sequences. Some pilD mutants also showed reduced adherence to HEp-2 cells, but this defect could not be complemented in trans, indicating that the defect may not be directly due to a loss of pilD. Taken together, these data demonstrate the effectiveness of the V. cholerae genome project for rapid identification and characterization of potential virulence factors.

Distinct roles of an alternative sigma factor during both free-swimming and colonizing phases of the Vibrio cholerae pathogenic cycle

Vibrio cholerae, the bacterium that causes cholera, has a pathogenic cycle consisting of a free-swimming phase outside its host, and a sessile virulent phase when colonizing the human small intestine. We have cloned the V. cholerae homologue of the rpoN gene (encoding sigma54) and determined its role in the cholera pathogenic cycle by constructing an rpoN null mutant. The V. cholerae rpoN mutant is non-motile; examination of this mutant by electron microscopy revealed that it lacks a flagellum. In addition to flagellar synthesis, sigma54 is involved in glutamine synthetase expression. Moreover, the rpoN mutant is defective for colonization in an infant mouse model of cholera. We present evidence that the colonization defect is distinct from the non-motile and Gln phenotypes of the rpoN mutant, implicating multiple and distinct roles of sigma54 during the V. cholerae pathogenic cycle. RNA polymerase containing sigma54 (sigma54-holoenzyme) has an absolute requirement for an activator protein to initiate transcription. We have identified three regulatory genes, flrABC (flagellar regulatory proteins ABC) that are additionally required for flagellar synthesis. The flrA and flrC gene products are sigma54-activators and form a flagellar transcription cascade. flrA and flrC mutants are also defective for colonization; this phenotype is probably independent of non-motility. An flrC constitutive mutation (M114-->I) was isolated that is independent of its cognate kinase FlrB. Expression of the constitutive FlrCM114-->I from the cholera toxin promoter resulted in a change in cell morphology, implicating involvement of FlrC in cell division. Thus, sigma54 holoenzyme, FlrA and FlrC transcribe genes for flagellar synthesis and possibly cell division during the free-swimming phase of the V. cholerae life cycle, and some as yet unidentified gene(s) that aid colonization within the host.

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

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

The measurement of swimming velocity of Vibrio cholerae and Pseudomonas aeruginosa using the video tracking methods

The swimming velocities of two monotrichous flagellated bacteria were measured by a computer-assisted video tracking method. Tracing the moving path of the individual bacterium revealed that the bacterial cell did not swim continuously in a straight direction, but frequently changed swimming direction and velocity. The average swimming velocities calculated from the 3-sec path were 75.4 +/- 9.4 microns/sec in four strains of Vibrio cholerae and 51.3 +/- 8.4 microns/sec in five strains of Pseudomonas aeruginosa. These results suggest that V. cholerae swim faster than P. aeruginosa at 30 C in nutrient broth. This method is useful for a detailed analysis of bacterial movement and moving patterns in different environmental conditions.

Cholera

Despite more than a century of study, cholera still presents challenges and surprises to us. Throughout most of the 20th century, cholera was caused by Vibrio cholerae of the O1 serogroup and the disease was largely confined to Asia and Africa. However, the last decade of the 20th century has witnessed two major developments in the history of this disease. In 1991, a massive outbreak of cholera started in South America, the one continent previously untouched by cholera in this century. In 1992, an apparently new pandemic caused by a previously unknown serogroup of V. cholerae (O139) began in India and Bangladesh. The O139 epidemic has been occurring in populations assumed to be largely immune to V. cholerae O1 and has rapidly spread to many countries including the United States. In this review, we discuss all aspects of cholera, including the clinical microbiology, epidemiology, pathogenesis, and clinical features of the disease. Special attention will be paid to the extraordinary advances that have been made in recent years in unravelling the molecular pathogenesis of this infection and in the development of new generations of vaccines to prevent it.

Identification of a 33 kDa antigen associated with an adhesive and colonizing strain of Vibrio cholerae El Tor and its role in protection

Proteins from the cell-free lysates of the wild-type strain KB207 of Vibrio cholerae El Tor and the isogenic non-adhesive mutant CD11 were analysed by native and denaturing polyacrylamide gel electrophoresis. A protein of 33 kDa present in KB207 was absent from CD11. Antiserum to the surface antigens of KB207 was absorbed with CD11. Antibodies remaining in the serum after absorption reacted to KB207 but not to CD11 as judged by slide agglutination, double gel diffusion and dot blot ELISA. Antibodies in the absorbed serum inhibited adherence of KB207 to rabbit intestinal mucosa and colonization in an infant mice model. The 33 kDa protein was isolated from KB207 by immunoaffinity chromatography. Antibodies present in the absorbed serum were used as ligand. The 33 kDa antigen was immunogenic and conferred protection in the rabbit ileal loop model. Combined administration of 33 kDa protein and B-subunit of cholera toxin offered full protection.

Roles of motility and flagellar structure in pathogenicity of Vibrio cholerae: analysis of motility mutants in three animal models

Wild-type Vibrio cholerae of both El Tor and classical biotypes (strains N16961 and 395, respectively) and nonmotile mutant derivatives with and without flagellar structures were characterized in three different animal models: (i) the rabbit ileal loop, (ii) the removable intestinal tie adult rabbit diarrhea (RITARD) model, and (iii) the suckling mouse model. Both the wild-type strains and nonmotile mutants were toxinogenic in the rabbit ileal loop and the suckling mouse models. However, all of the nonmotile mutants produced significantly less fluid accumulation than did the wild-type parental strains. The two nonmotile mutants of strain N16961 did not adhere to rabbit ileal mucosa, but both nonmotile mutants derived from strain 395 exhibited adherence. In the RITARD model, the motile El Tor strains were more virulent than both the flagellate and aflagellate nonmotile mutants (all infected rabbits died within 18 h), while the nonmotile mutants, when fatalities occurred, required 78 to 105 h to produce a fatal outcome. Likewise, the motile classical parent 395 produced a fatal outcome within ca. 25 h, while nonmotile mutants required 69 to 96 h. The nonmotile flagellate strain KR31 was not significantly more virulent than the nonmotile aflagellate strain KR26. Of the two classical nonmotile mutants, KR1, which produces a coreless sheathlike structure, was clearly more virulent (5 of 10 rabbits died within 96 h), while KR3 (nonmotile, aflagellate) did not produce fatalities in any of the 10 rabbits tested. Similarly, no significant difference in diarrheagenicity or colonizing ability was detected between the two nonmotile mutants derived from the El Tor strain, but the classical nonmotile mutant with the coreless sheath caused significantly greater diarrhea and colonized for a longer time than did the isogenic nonmotile aflagellate strain, KR3. No significant differences between the nonmotile mutants were detected in competition studies done with suckling mice. Analysis of the wild-type and mutant strains in these three animal models clearly demonstrated a role for motility in V. cholerae pathogenicity, while analysis of only the nonmotile mutants derived from the classical parent suggested a role for flagellar structures.

Binding of bacteria to carbohydrates immobilized on beads to demonstrate the presence of cell-associated hemagglutinins in Vibrio cholerae

We describe a phase contrast microscopy method for direct observation of classical and El Tor vibrios to agarose beads containing covalently attached L-fucose or D-mannose. Binding of the vibrios to L-fucose beads was found to correlate with fucose-sensitive agglutination of human O erythrocytes, while binding of bacteria to beads with D-mannose was consistent with mannose-sensitive agglutination of chicken erythrocytes. Furthermore, vibrios expressing both fucose and mannose-sensitive hemagglutinins adhered equally to L-fucose and D-mannose-containing beads. Because this procedure is neither subject to biological variations in different populations of erythrocytes nor affected by other factors known to interfere with hemagglutination tests, it offers a suitable, more robust and specific alternative to detect functional adhesins in Vibrio cholerae and other bacteria.

Pili of a Vibrio parahaemolyticus strain as a possible colonization factor

Pili of Vibrio parahaemolyticus were purified from a Kanagawa phenomenon-positive strain (Ha7) that belongs to serogroup O2:K3 and is adhesive to rabbit intestine. The organisms treated with the Fab fraction of antipilus antibody failed to adhere to the intestine. Purified pili had the ability to adhere to the intestine, but the pretreatment of the intestine with purified pili did not allow adherence of the organisms to the intestine. These results suggest that pili of this V. parahaemolyticus strain play an important role in colonization.

Hemagglutination and intestinal adherence properties of clinical and environmental isolates of non-O1 Vibrio cholerae

Hemagglutination and intestinal adherence properties of non-O1 Vibrio cholerae were studied in vitro. No definite correlation between the cell-associated hemagglutinin titers and the intestinal adhesion indices was noted. Sugar- and glycoprotein-mediated inhibition data also indicated differences between the hemagglutination and adherence processes in respect to the receptor structures. Intestinal adherence of most V. cholerae strains could be inhibited to various extents by N-acetyl D-glucosamine. This observation provides a likely explanation for the ecological behavior of these organisms, which are known to associate themselves with chitinous (chitin:homopolymer of N-acetyl D-glucosamine) surfaces of zooplankton. The absence of any significant difference between the intestinal adherence indices of clinical and environmental isolates suggests that intestinal adhesion may be an essential but not sufficient prerequisite for colonization by and subsequent expression of pathogenicity of these microorganisms.

Pili of Vibrio cholerae O1 biotype E1 Tor: a comparative study on adhesive and non-adhesive strains

Pili were found on the cell surface of non-adhesive Vibrio cholerae O1 Biotype E1 Tor as well as the adhesive strain. Purified pili of the adhesive and non-adhesive strains were morphologically, electrophoretically, and immunologically, indistinguishable from each other. The molecular weights of both pilin (subunit protein of the pilus) were about 16,000 daltons as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. These 16 kDa pili are different from the pilus colonization factor, which is a 20.5 kDa protein, reported by Taylor et al. The 16 kDa pili of Vibrio cholerae O1 Biotype E1 Tor have hemagglutinating activity, but may have no role in colonization, because non-adhesive strains also have such pili.