Diverse CTX phages among toxigenic Vibrio cholerae O1 and O139 strains isolated between 1994 and 2002 in an area where cholera is endemic in Bangladesh

Altered Molecular Attributes and Antimicrobial Resistance Patterns of Vibrio cholerae O1 El Tor Strains Isolated from the Cholera Endemic Regions of India

AIMS

The present study aimed to document the comparative analysis of differential hyper-virulent features of Vibrio cholerae O1 strains isolated during 2018 from cholera endemic regions in Gujarat and Maharashtra (Western India) and West Bengal (Eastern India).

METHODS AND RESULTS

A total of 87 V. cholerae O1 clinical strains from Western India and 48 from Eastern India were analyzed for a number of biotypic and genotypic features followed by antimicrobial resistance (AMR) profile. A novel PCR was designed to detect a large fragment deletion in the Vibrio seventh pandemic island II (VSP-II) genomic region, which is a significant genetic feature of the V. cholerae strains that has caused Yemen cholera outbreak. All the strains from Western India were belong to the Ogawa serotype, polymyxin B-sensitive, hemolytic, had a deletion in VSP-II (VSP-IIC) region and carried Haitian genetic alleles of ctxB, tcpA and rtxA. Conversely, 14.6% (7/48) of the strains from Eastern India belonged to the Inaba serotype, polymyxin B-resistant, non-hemolytic, harbored VSP-II other than VSP-IIC type, classical ctxB, Haitian tcpA and El Tor rtxA alleles. Resistance to tetracycline and chloramphenicol has been observed in strains from both the regions.

CONCLUSIONS

This study showed hyper-virulent, polymyxin B-sensitive epidemic causing strains in India along with the strains with polymyxin B-resistant and non-hemolytic traits that may spread and cause serious disease outcome in future.

SIGNIFICANCE AND IMPACT OF THE STUDY

The outcomes of this study can help to improve the understanding of the hyper-pathogenic property of recently circulating pandemic V. cholerae strains in India. A special attention is also needed on the monitoring of AMR surveillance because V. cholerae strains are losing susceptibility to many antibiotics used as a second line of defense in the treatment of cholera.

Genomic diversities of ctxB, tcpA and rstR alleles of Vibrio cholerae O139 strains isolated from Odisha, India

The genome of Vibrio cholerae O139 strains has undergone cryptic changes since its first emergence in 1992 in South India. This study aimed to determine the presence of genotypic changes marked in ctxB, tcpA and rstR genes located within the CTX prophages among the strains of V. cholerae O139 isolated from 1999 to 2017 in Odisha. Antibiotic susceptibility test was conducted on 59 V. cholerae O139 strains. A conventional PCR assay was done for ctxB gene typing followed by sequencing along with identification of rstR and tcpA gene. Pulsed-field gel electrophoresis (PFGE) was carried out to reveal clonal variations among the V. cholerae O139 strains. Among V. cholerae O139 isolates more than 60% showed resistance to ampicillin, co-trimoxazole, furazolidone, streptomycin, neomycin and nalidixic acid. The ctxB sequencing and rstR allele-specific PCR assay revealed the presence of three genotypes 1, 3 and 4 with at least one copy of CTX Calc φ in addition to CTX ET and CTX Cl prophages in V. cholerae O139 isolates. PFGE analysis revealed 13 pulsotypes with two clades having 60% similarity among V. cholerae O139 strains. The circulating V. cholerae O139 strains in Odisha showed variation in genotypes with multiple clonal expansions over the years.

Molecular characterization of Vibrio cholerae O1 El Tor strains in Malaysia revealed genetically diverse variant lineages

Vibrio cholerae O1 El Tor variants have been the major causative agents of cholera worldwide since their emergence in the 2000s. Cholera remains endemic in some regions in Malaysia. Therefore, we aimed to investigate the genetic characteristics of the V. cholerae O1 El Tor strains associated with outbreaks and sporadic cases to elucidate the molecular evolution among the strains circulating in this region. A total of 45 V. cholerae O1 El Tor strains isolated between 1991 and 2011 were examined. All strains were subjected to phenotypic characterization, and molecular characterization including detection of virulence genes and CTX prophage (CTXΦ) by Polymerase Chain Reaction (PCR) and genotyping by Pulsed-Field Gel Electrophoresis (PFGE). All strains were phenotypically confirmed as El Tor biotype and were mostly Ogawa serotype (96%). Antimicrobial susceptibility testing showed that the outbreak strains isolated in 1991 (Sabah) and 2009 (Terengganu) were all multidrug-resistant while the sporadic strains were resistant to erythromycin and furazolidone only. All strains (n = 45) were resistant to erythromycin. The virulence genes ctxA, ctxB, ompW, rfb, rtxC, tcpA, tcpI, rstR, zot and hlyA were present in all strains. The outbreak strains isolated in 1991 harboured El Tor cholera toxin gene (ctxB3) while sporadic strains from 2004 to 2011 harboured classical ctxB1. Four distinctive CTXΦ arrays were identified among the El Tor variants, one of which co-occurred with El Tor strains during the 2009 outbreak in Terengganu. PFGE analysis revealed that a genetically diverse El Tor variants population persisted in Sabah. The co-existence of multiple El Tor variants together with the prototypic El Tor strains suggested a multiclonal emergence of V. cholerae O1 El Tor variants in this region.

Genetic Diversity of ctxB Gene Among Classical O1 and El Tor Strains of Vibrio cholerae using High-Resolution Melting Curve Analysis

Background & Objective:

Vibrio cholerae is a natural inhabitant of the environment and causes severe diarrhea ailments (cholera) that affects thousands of people each year worldwide. The most important virulence factors of this pathogen are cholera toxin (cholera toxin CT) and Type IV pili (toxin co-regulated pili TCP), which are encoded within the genome of the filamentous bacteriophage CTXφ. In the present study, according to researchers’ report on genotypic variations of cholera toxin, we evaluated the sequence of ctxB subunit obtained from 100 strains of patients infected with cholera in Iran.

Methods:

The evaluation of genotype variations of cholera toxin was made by high-resolution melting curve analysis illustrating a single nucleotide change. Then, ctxB gene sequencing was performed. Through this analysis and the sequencing process, two standard samples were studied.

Results:

Using serologic tests, all the strains analyzed in this study were identified to be in O1 serotype. However, there have been differences in sequences of ctxB as some were similar to V. cholerae O1 biovar El Tor str. N16961 while others were similar to the genotype of V. cholerae ATCC 14035. We did not observe any particular pattern within the process of mutation.

Conclusion:

The analysis of the new samples of ctxB showed that they were potentially different. It seems that these complicated species were affected by a new genetic exchange of El Tor and classic genotypes.

Cholera toxin phage: structural and functional diversity between Vibrio cholerae biotypes

Cholera is a severe form of watery diarrhea caused by Vibrio cholerae toxigenic strains. Typically, the toxigenic variants of V. cholerae harbor a bacteriophage, cholera toxin phage, integrated in their genome. The ctxAB genes from the phage genome encode the cholera toxin, which is responsible for the major clinical symptoms of the disease. Although ctxAB genes are crucial to V. cholerae strains for cholera manifestation, the genetic structure of cholera toxin phage, DNA sequence of its genes, spatial organization in the host genome and its satellite phage content are not homogenous between V. cholerae biotypes-classical and El Tor. Differences in cholera toxin phage and its genes play a significant role in the identification of V. cholerae biotypes and in the understanding of their pathogenic and epidemic potentials. Here, we present an account of the variations of cholera toxin phage and its genes in V. cholerae biotypes as well as their usefulness in the identification of classical and El Tor strains.

Laboratory Culturing Techniques and Maintenance of Vibrio cholerae

Cholera is a severe diarrheal disease caused by the consumption of food or water contaminated with the aquatic gram-negative bacterium Vibrio cholerae. Infected hosts will experience vomiting and severe watery diarrhea and if not treated properly will ultimately succumb to death by dehydration. Due to the global prevalence and severity of cholera, V. cholerae has been extensively studied in both environmental and laboratory settings. Herein, we describe proper V. cholerae maintenance, in addition to classical and El Tor biotype culturing in a laboratory setting.

Genotypic and Phenotypic Assays to Distinguish Vibrio cholerae Biotype

Vibrio cholerae is a motile gram-negative bacterium found in brackish water and the etiological agent of the fecal-oral disease cholera. Classical and El Tor are two main biotypes that make up the V. cholerae O1 serogroup, which each display unique genotypic and phenotypic characteristics that allow for reliable biotype characterization. While treatment for cholera is much the same despite the causative strain's biotype, such classification can be imperative for laboratory experiments and may have broader impacts in the biomedical field. In the early 2000s, clinical isolates were identified that contained genotypic and phenotypic traits from both biotypes. The newly identified hybrids, termed El Tor variants, have caused clinical and environmental isolate biotype identification to be more complicated than previous single-assay identification. Herein, we describe a series of PCR-based genetic screens (tcpA and ctxB) and phenotypic assays (polymyxin B resistance, citrate metabolism, proteolytic activity, hemolytic activity, motility, and Voges-Proskauer). Together, these assays are used for reliable biotype characterization of V. cholerae clinical (and environmental) isolates.

Design and Construction of Vibrio cholerae Strains That Harbor Various CTX Prophage Arrays

Toxigenic Vibrio cholerae strains arise upon infection and integration of the lysogenic cholera toxin phage, the CTX phage, into bacterial chromosomes. The V. cholerae serogroup O1 strains identified to date can be broadly categorized into three main groups: the classical biotype strains, which harbor CTX-cla; the prototype El Tor strains (Wave 1 strains), which harbor CTX-1; and the atypical El Tor strains, which harbor CTX-2 (Wave 2 strains) or CTX-3~6 (Wave 3 strains). The efficiencies of replication and transmission of CTX phages are similar, suggesting the possibility of existence of more diverse bacterial strains harboring various CTX phages and their arrays in nature. In this study, a set of V. cholerae strains was constructed by the chromosomal integration of CTX phages into strains that already harbored CTX phages or those that did not harbor any CTX phage or RS1 element. Strains containing repeats of the same kind of CTX phage, strains containing the same kind of CTX phage in each chromosome, strains containing alternative CTX phages in one chromosome, or containing different CTX phages in each chromosome have been constructed. Thus, strains with any CTX array can be designed and constructed. Moreover, the strains described in this study contained the toxT-139F allele, which enhances the expression of TcpA and cholera toxin. These characteristics are considered to be important for cholera vaccine development. Once their capacity to provoke immunity in human against V. cholerae infection is evaluated, some of the generated strains could be developed further to yield cholera vaccine strains.

The role of CTX and RS1 satellite phages genomic arrangement in Vibrio cholera toxin production in two recent cholera outbreaks (2012 and 2013) in IR Iran

The objective of the present study was to investigate the genomic arrangement of CTX/RS1 prophages in 30 Vibrio cholerae strains obtained from 2 consecutive years of cholera outbreak and to compare the role of different CTX/RS1 arrangements in cholera toxin expression among the El Tor strains. Profile A with TLC-RS1-CTX-RTX arrangement was observed in 46.7% of the isolates with RS1 phage locating adjacent to TLC element. About 50% of the isolates showed Profile B with TLC-CTX-RS1-RTX arrangement and one single isolate (3.3%) revealed TLC-CTX-RS1-RS1-RTX arrangement (Profile C). No RS1 element was detected to be adjacent to TLC element in B and C profiles. No truncated CTX phage genome was detected among the isolates of 2 years. Different CTX-RS1 arrangement profiles (A, B, and C) with different RS1 copy numbers and locations uniformly showed low level of cholera toxin production in El Tor strains with no significant difference, revealing that different RS1 copy numbers and locations have no effect on cholera toxin production level (p-value >0.05). However, increased cholera toxin expression was observed for control V. cholerae classical biotype strain. In conclusion, variations in RS1 prophage did not affect CT expression level in related El Tor V. cholerae strains. CTX genotyping establishes a more valuable database for epidemiologic, pathogenesis, and source tracking purposes.

Retrospective genomic analysis of Vibrio cholerae O1 El Tor strains from different places in India reveals the presence of ctxB-7 allele found in Haitian isolates

A total of 45 strains of Vibrio cholerae O1 isolated from 10 different places in India where they were associated with cases of cholera between the years 2007 and 2008 were examined by molecular methods. With the help of phenotypic and genotypic tests the strains were confirmed to be O1 El Tor biotype strains with classical ctxB gene. Polymerase chain reaction (PCR) analysis by double - mismatch amplification mutation assay PCR showed 16 of these strains carried the ctxB-7 allele reported in Haitian strains. Sequencing of the ctxB gene in all the 45 strains revealed that in 16 strains the histidine at the 20th amino acid position had been replaced by asparagine and this single nucleotide polymorphism did not affect cholera toxin production as revealed by beads enzyme-linked immunosorbent assay. This study shows that the new ctxB gene sequence was circulating in different places in India. Seven representatives of these 45 strains analysed by pulsed - field gel electrophoresis showed four distinct Not I digested profiles showing that multiple clones were causing cholera in 2007 and 2008.

Laboratory Techniques Used to Maintain and Differentiate Biotypes of Vibrio cholerae Clinical and Environmental Isolates

The aquatic Gram-negative bacterium Vibrio cholerae is the etiological agent of the infectious gastrointestinal disease cholera. Due to the global prevalence and severity of this disease, V. cholerae has been extensively studied in both environmental and laboratory settings, requiring proper maintenance and culturing techniques. Classical and El Tor are two main biotypes that compose the V. cholerae O1 serogroup, each displaying unique genotypic and phenotypic characteristics that provide reliable mechanisms for biotype characterization, and require distinct virulence inducing culturing conditions. Regardless of the biotype of the causative strain for any given infection or outbreak, the standard treatment for the disease involves rehydration therapy supplemented with a regimen of antibiotics. However, biotype classification may be necessary for laboratory studies and may have broader impacts in the biomedical field. In the early 2000's clinical isolates were identified which exhibit genotypic and phenotypic traits from both classical and El Tor biotypes. The newly identified hybrids, termed El Tor variants, have caused clinical and environmental isolate biotype identification to become more complex than previous traditional single assay identification protocols. In addition to describing V. cholerae general maintenance and culturing techniques, this manuscript describes a series of gene specific (ctxB and tcpA) PCR-based genetic screens and phenotypic assays (polymyxin B resistance, citrate metabolism, proteolytic activity, hemolytic activity, motility, and glucose metabolism via Voges-Proskauer assay) collectively used to characterize and/or distinguish between classical and El Tor biotypes. Together, these assays provide an efficient systematic approach to be used as an alternative, or in addition, to costly, labor-intensive experiments in the characterization of V. cholerae clinical (and environmental) isolates.

Single Nucleotide Polymorphisms in Regulator-Encoding Genes Have an Additive Effect on Virulence Gene Expression in a Vibrio cholerae Clinical Isolate

Cholera, an infectious disease of the small intestine caused by the aquatic bacterium Vibrio cholerae, often results in vomiting and acute watery diarrhea. If left untreated or if the response is too slow, the symptoms can quickly lead to extreme dehydration and ultimately death of the patient. Recent anecdotal evidence of cholera patients suffering from increasingly severe symptoms and of disease progression at a much higher rate than previously observed has emerged. As recent cholera outbreaks caused by increasingly virulent strains have resulted in higher mortality rates, the need to investigate the mechanism(s) allowing this observed increased virulence is apparent. The significance of our research is in identifying the mechanism for increased virulence capabilities, which will allow the development of a model that will greatly enhance our understanding of cholera disease and V. cholerae pathogenesis, leading to broader biomedical impacts, as cholera serves as a model for other enteric diarrheal diseases.

Vibrio cholerae is the etiological agent of the infectious disease cholera, which is characterized by vomiting and severe watery diarrhea. Recently, V. cholerae clinical isolates have demonstrated increased virulence capabilities, causing more severe symptoms with a much higher rate of disease progression than previously observed. We have identified single nucleotide polymorphisms (SNPs) in four virulence-regulatory genes (hapR, hns, luxO, and vieA) of a hypervirulent V. cholerae clinical isolate, MQ1795. Herein, all SNPs and SNP combinations of interest were introduced into the prototypical El Tor reference strain N16961, and the effects on the production of numerous virulence-related factors, including cholera toxin (CT), the toxin-coregulated pilus (TCP), and ToxT, were analyzed. Our data show that triple-SNP (hapR hns luxO and hns luxO vieA) and quadruple-SNP combinations produced the greatest increases in CT, TCP, and ToxT production. The hns and hns luxO SNP combinations were sufficient for increased TCP and ToxT production. Notably, the hns luxO vieA triple-SNP combination strain produced TCP and ToxT levels similar to those of MQ1795. Certain SNP combinations (hapR and hapR vieA) had the opposite effect on CT, TCP, and ToxT expression. Interestingly, the hns vieA double-SNP combination strain increased TCP production while decreasing CT production. Our findings suggest that SNPs identified in the four regulatory genes, in various combinations, are associated with increased virulence capabilities observed in V. cholerae clinical isolates. These studies provide insight into the evolution of highly virulent strains.

IMPORTANCE Cholera, an infectious disease of the small intestine caused by the aquatic bacterium Vibrio cholerae, often results in vomiting and acute watery diarrhea. If left untreated or if the response is too slow, the symptoms can quickly lead to extreme dehydration and ultimately death of the patient. Recent anecdotal evidence of cholera patients suffering from increasingly severe symptoms and of disease progression at a much higher rate than previously observed has emerged. As recent cholera outbreaks caused by increasingly virulent strains have resulted in higher mortality rates, the need to investigate the mechanism(s) allowing this observed increased virulence is apparent. The significance of our research is in identifying the mechanism for increased virulence capabilities, which will allow the development of a model that will greatly enhance our understanding of cholera disease and V. cholerae pathogenesis, leading to broader biomedical impacts, as cholera serves as a model for other enteric diarrheal diseases.

The Hybrid Pre-CTXΦ-RS1 Prophage Genome and Its Regulatory Function in Environmental Vibrio cholerae O1 Strains

The cholera toxin genes of Vibrio cholerae are encoded by CTXΦ, a lysogenic bacteriophage. Infection with this phage plays a determinant role in toxigenicity conversion and the emergence of new clones of pathogenic V. cholerae. Multiple phage alleles, defined by sequence types of the repressor gene rstR, have been found, showing the divergence of phage genomes. Pre-CTXΦ, which is characterized by the absence of toxin genes, is predicted to be the precursor of CTXΦ. We have found a new pre-CTXΦ prophage genome (named pre-CTXZJΦ for its novel rstR allele) in nontoxigenic V. cholerae O1 isolates that were obtained during surveillance of the estuary water of the Zhujiang River. A novel hybrid genome of the helper phage RS1 was identified in an environmental strain carrying pre-CTXZJΦ in this study. The chromosomal integration and genomic arrangement of pre-CTXZJΦ and RS1 were determined. The RS2 of pre-CTXZJΦ was shown to have a function in replication, but it seemed to have lost its ability to integrate. The RstR of pre-CTXZJΦ exerted the highest repression of its own rstA promoter compared to other RstRs, suggesting rstR-specific phage superinfection immunity and potential coinfection with other pre-CTXΦ/CTXΦ alleles. The environmental strain carrying pre-CTXZJΦ could still be infected by CTXETΦ, the most common phage allele in the strains of the seventh cholera pandemic, suggesting that this nontoxigenic clone could potentially undergo toxigenicity conversion by CTXΦ infection and become a new toxigenic clone despite already containing the pre-CTXΦ prophage.

Hybrid Vibrio cholerae El Tor lacking SXT identified as the cause of a cholera outbreak in the Philippines

Cholera continues to be a global threat, with high rates of morbidity and mortality. In 2011, a cholera outbreak occurred in Palawan, Philippines, affecting more than 500 people, and 20 individuals died. Vibrio cholerae O1 was confirmed as the etiological agent. Source attribution is critical in cholera outbreaks for proper management of the disease, as well as to control spread. In this study, three V. cholerae O1 isolates from a Philippines cholera outbreak were sequenced and their genomes analyzed to determine phylogenetic relatedness to V. cholerae O1 isolates from recent outbreaks of cholera elsewhere. The Philippines V. cholerae O1 isolates were determined to be V. cholerae O1 hybrid El Tor belonging to the seventh-pandemic clade. They clustered tightly, forming a monophyletic clade closely related to V. cholerae O1 hybrid El Tor from Asia and Africa. The isolates possess a unique multilocus variable-number tandem repeat analysis (MLVA) genotype (12-7-9-18-25 and 12-7-10-14-21) and lack SXT. In addition, they possess a novel 15-kb genomic island (GI-119) containing a predicted type I restriction-modification system. The CTXΦ-RS1 array of the Philippines isolates was similar to that of V. cholerae O1 MG116926, a hybrid El Tor strain isolated in Bangladesh in 1991. Overall, the data indicate that the Philippines V. cholerae O1 isolates are unique, differing from recent V. cholerae O1 isolates from Asia, Africa, and Haiti. Furthermore, the results of this study support the hypothesis that the Philippines isolates of V. cholerae O1 are indigenous and exist locally in the aquatic ecosystem of the Philippines.

Genetic characterization and phylogenomics analysis of outbreak strains have proven to be critical for probing clonal relatedness to strains isolated in different geographical regions and over time. Recently, extensive genetic analyses of V. cholerae O1 strains isolated in different countries have been done. However, genome sequences of V. cholerae O1 isolates from the Philippines have not been available for epidemiological investigation. In this study, molecular typing and phylogenetic analysis of Vibrio cholerae isolated from both clinical and environmental samples in 2011 confirmed unique genetic features of the Philippines isolates, which are helpful to understand the global epidemiology of cholera.

Phenotypic and genotypic characteristics of Vibrio cholerae O1 isolated from the Sierra Leone cholera outbreak in 2012

BACKGROUND

This study describes phenotypic, genotypic and antibiotic susceptibility patterns of the strains isolated from the 2012 Sierra Leone cholera outbreak. Rectal swabs were collected from patients and cultured for Vibrio cholerae O1.

METHODS

The isolates were subjected to multiplex PCR, mismatch amplification mutation assay (MAMA) PCR, pulsed field gel electrophoresis (PFGE), and antibiotic sensitivity tests using disk diffusion and minimum inhibitory concentration (MIC) E-test following standard procedures.

RESULTS

Out of 17 rectal swabs tested, 15 yielded V. cholerae O1 biotype El Tor, serotype Ogawa. All the strains belonged to 'altered' variants as MAMA PCR result showed the presence of classical cholera toxin B. PFGE result revealed four pulse types. Using antibiotic disk diffusion, all the isolates were resistant to erythromycin, chloramphenicol, furazolidone, and trimethoprim/sulfamethoxazole (SXT) except SL1 which was sensitive to chloramphenicol and SXT. All the isolates were sensitive to nalidixic acid, tetracycline, doxycycline, azithromycin, and ciprofloxacin except SL2 which was resistant to nalidixic acid. However, variable sensitivity patterns were observed for kanamycin. The ranges of MIC were 0.125-0.50 mg/l, 0.003-0.023 mg/l and 0.38-0.75 mg/l for azithromycin, ciprofloxacin and tetracycline, respectively.

CONCLUSIONS

This study demonstrates that altered variants of V. cholerae O1 of four clonal types were responsible for the 2012 outbreak of cholera in Sierra Leone.

Genomic and phenotypic characterization of Vibrio cholerae non-O1 isolates from a US Gulf Coast cholera outbreak

Between November 2010, and May 2011, eleven cases of cholera, unrelated to a concurrent outbreak on the island of Hispaniola, were recorded, and the causative agent, Vibrio cholerae serogroup O75, was traced to oysters harvested from Apalachicola Bay, Florida. From the 11 diagnosed cases, eight isolates of V. cholerae were isolated and their genomes were sequenced. Genomic analysis demonstrated the presence of a suite of mobile elements previously shown to be involved in the disease process of cholera (ctxAB, VPI-1 and -2, and a VSP-II like variant) and a phylogenomic analysis showed the isolates to be sister taxa to toxigenic V. cholerae V51 serogroup O141, a clinical strain isolated 23 years earlier. Toxigenic V. cholerae O75 has been repeatedly isolated from clinical cases in the southeastern United States and toxigenic V. cholerae O141 isolates have been isolated globally from clinical cases over several decades. Comparative genomics, phenotypic analyses, and a Caenorhabditis elegans model of infection for the isolates were conducted. This analysis coupled with isolation data of V. cholerae O75 and O141 suggests these strains may represent an underappreciated clade of cholera-causing strains responsible for significant disease burden globally.

The role of Vibrio cholerae genotyping in Africa

Toxigenic Vibrio cholerae, the causative agent of the disease cholera, is prevalent in the African continent from the 1970s when the seventh pandemic spread from Asia to Africa. In the past decade, cholera has caused devastating outbreaks in much of Africa, illustrated by the recent cholera epidemics in Zimbabwe and regions of central Africa. Given the extent of cholera in Africa, a robust and efficient surveillance system should be in place to prevent and control the disease in this continent. Such a surveillance system would be greatly bolstered by use of molecular typing techniques to identify genetic subtypes. In this review, we highlight the role that modern molecular typing techniques can play in tracking and aborting the spread of cholera.

Detection, isolation, and identification of Vibrio cholerae from the environment

Recent molecular advances in microbiology have greatly improved the detection of bacterial pathogens in the environment. These improvements and a downward trend in the cost of molecular detection methods have contributed to increased frequency of detection of pathogenic microorganisms where traditional culture-based detection methods have failed. Culture methods also have been greatly improved, and the confluence of the two suites of methods provides a powerful tool for detection, isolation, and characterization of pathogens. While molecular detection provides data on the presence and type of pathogens, culturing methods allow a researcher to preserve the organism of interest for "-omics" studies, such as genomic, metabolomic, secretomic, and transcriptomic analysis, which are rapidly becoming more affordable. This has yielded a clearer understanding of the ecology and epidemiology of microorganisms that cause disease. In this unit, we present commonly accepted methods for isolation, detection, and characterization of V. cholerae, providing more extensive knowledge of the ecology and epidemiology of this organism. This unit has been fully revised and updated from the earlier version with the latest knowledge and additional information not previously included.

A rapid and reliable species-specific identification of clinical and environmental isolates of Vibrio cholerae using a three-test procedure and recA polymerase chain reaction

PURPOSE

Vibrio cholerae, the cause of cholera, is one of the leading causes of morbidity and mortality in many developing countries. Most laboratories initially rely on biochemical tests for a presumptive identification of these strains, followed by a polymerase chain reaction (PCR)-based method to confirm their identification. The aim of this study is to establish a rapid and reliable identification scheme for V. cholerae using a minimal, but highly specific number of biochemical tests and a PCR assay.

MATERIALS AND METHODS

We developed a species-specific PCR to identify V. cholerae, using a housekeeping gene recA, and used that to evaluate the sensitivity and specificity of 12 biochemical tests commonly used for screening and / or presumptive identification of V. cholerae in the clinical and environmental samples.

RESULTS

Here we introduced a combination of three biochemical tests, namely, sucrose fermentation, oxidase test, and growth in trypton broth containing 0% NaCl, as also the PCR of the recA gene, for rapid identification of V. cholerae isolates, with 100% sensitivity and specificity. The established method accurately identified a collection of 47 V. cholerae strains isolated from the clinical cases (n = 26) and surface waters (n = 21), while none of the 32 control strains belonging to different species were positive in this assay.

CONCLUSION

The triple-test procedure introduced here is a simple and useful assay which can be adopted in cholera surveillance programs for efficient monitoring of V. cholerae in surface water and fecal samples.

Molecular diversity of CTX prophage in Vibrio cholerae

AIMS

The objective of this study was to investigate the molecular diversity of CTX genetic element within toxigenic Vibrio cholerae genomes and to determine the genetic diversity of V. cholerae population collected in a 6-year period (2004-2009) in Iran.

METHODS AND RESULTS

The results of mismatch amplification mutation assay (MAMA)-PCR and sequencing showed cytosine nucleotide in positions 203 and 115 in all 50 El Tor V. cholerae strains, which is the same as classical ctxB sequence. One strain yielded amplicons with both El Tor and classical biotype primers in MAMA-PCR indicative of presence of two copies of CTX phages with different genotypes (rstR(ET) ctxB(class) and rstR(ET) ctxB(ET)) integrated within the genome of this isolate, which suggested the integration of two different CTX phages at different occasions or point mutation in one copy of CTX. Sequencing and PCR analysis indicated the presence of hybrid CTX genotype (rstR(ET) ctx(class)) in 70.6% of the isolates; however, only El Tor RS1 phage has been integrated in flanking to the CTX phages with different genotypes.

CONCLUSIONS

Enterobacterial repetitive intergenic consensus-PCR (ERIC-PCR) and ribosomal gene spacer-PCR (RS-PCR) showed a relatively homogenous population in different years. Our findings indicate that sequence analysis of RS and ctxB regions has more discriminative power than restriction-based methods.

SIGNIFICANCE AND IMPACT OF THE STUDY

Investigating the molecular diversity of CTX prophage among V. cholerae strains helps to establish a new valuable database of genetic information about isolates, which is of great importance for epidemiologic studies in Iran and other countries encountering cholera epidemics.

Environmental and epidemiological surveillance of Vibrio cholerae in a cholera-endemic region in India with freshwater environs

AIM

To conduct epidemiological and ecological surveillance of cholera in freshwater environments.

METHODS AND RESULTS

A freshwater region of India was surveyed between April 2007 and December 2008. Vibrio cholerae was isolated from 59·5% of water and plankton samples (n = 357) and 35·5% of stool samples (n = 290). Isolation from water was dependent on air (r = 0·44) and water temperatures (r = 0·49) (P < 0·01) but was independent of rainfall (r = 0·15), chlorophyll a (r = 0·18), salinity (r = 0·2) or pH (r = 0·2) (P > 0·05). Isolation from plankton was dependent on temperature of air (r = 0·45), water temperature (r = 0·44), chlorophyll a concentration (r = 0·42), pH (r = 0·23) and salinity (r = 0·39) (P < 0·01). Cholera cases correlated with rainfall (r = 0·82, P < 0·01) and chlorophyll a concentration (r = 0·42, P < 0·05), but not with air temperature (r = 0·3, P = 0·37). Vibrio cholerae O1 possessed ctxB, ctxA, rstR and tcpA (ElTor), toxR, toxT, rtxA, rtxC, mshA and hylA. Among non-O1-non-O139, the distribution of virulence-associated and regulatory protein genes was heterogeneous with - 0·7, 2·2, 94·77, 97·76, 99·25, 100 and 100% isolates being positive for tcpA, toxT, rtxA, rtxC, hylA, toxR and mshA, respectively. Two-thirds of non-O1-non-O139 isolates exhibited antibiotic resistance to various antibiotics that did not correlate with geographical site or time of origin for the isolates. RAPD and AFLP showed V. cholerae to be a diverse bacterium. AFLP demonstrated separate lineages for non-O1-non-O139 and O1 isolates.

CONCLUSION

Environmental parameters played a significant role in the emergence and spread of cholera and the abundance of V. cholerae. But based on virulence gene profiling and genetic fingerprinting, the possibility of origin of toxigenic isolates from nontoxigenic environmental isolates seems unlikely in freshwater environs of India.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study explains the ecology, epidemiology and seasonality of cholera in freshwater environs.

Vibrio cholerae infection, novel drug targets and phage therapy

Vibrio cholerae is the causative agent of the diarrheal disease cholera. Although antibiotic therapy shortens the duration of diarrhea, excessive use has contributed to the emergence of antibiotic resistance in V. cholerae. Mobile genetic elements have been shown to be largely responsible for the shift of drug resistance genes in bacteria, including some V. cholerae strains. Quorum sensing communication systems are used for interaction among bacteria and for sensing environmental signals. Sequence analysis of the ctxB gene of toxigenic V. cholerae strains demonstrated its presence in multiple cholera toxin genotypes. Moreover, bacteriophage that lyse the bacterium have been reported to modulate epidemics by decreasing the required infectious dose of the bacterium. In this article, we will briefly discuss the disease, its clinical manifestation, antimicrobial resistance and the novel approaches to locate drug targets to treat cholera.

Characterization of Vibrio cholerae O1 El Tor biotype variant clinical isolates from Bangladesh and Haiti, including a molecular genetic analysis of virulence genes

Vibrio cholerae serogroup O1, the causative agent of the diarrheal disease cholera, is divided into two biotypes: classical and El Tor. Both biotypes produce the major virulence factors toxin-coregulated pilus (TCP) and cholera toxin (CT). Although possessing genotypic and phenotypic differences, El Tor biotype strains displaying classical biotype traits have been reported and subsequently were dubbed El Tor variants. Of particular interest are reports of El Tor variants that produce various levels of CT, including levels typical of classical biotype strains. Here, we report the characterization of 10 clinical isolates from the International Centre for Diarrhoeal Disease Research, Bangladesh, and a representative strain from the 2010 Haiti cholera outbreak. We observed that all 11 strains produced increased CT (2- to 10-fold) compared to that of wild-type El Tor strains under in vitro inducing conditions, but they possessed various TcpA and ToxT expression profiles. Particularly, El Tor variant MQ1795, which produced the highest level of CT and very high levels of TcpA and ToxT, demonstrated hypervirulence compared to the virulence of El Tor wild-type strains in the infant mouse cholera model. Additional genotypic and phenotypic tests were conducted to characterize the variants, including an assessment of biotype-distinguishing characteristics. Notably, the sequencing of ctxB in some El Tor variants revealed two copies of classical ctxB, one per chromosome, contrary to previous reports that located ctxAB only on the large chromosome of El Tor biotype strains.

Genetic Screens and Biochemical Assays to Characterize Vibrio cholerae O1 Biotypes: Classical and El Tor

Vibrio cholerae serogroup O1 has two biotypes, classical and El Tor, the latter of which has displaced the prior and has been the causative agent for the ongoing seventh pandemic. However, reports since 2001 have identified clinical isolates of El Tor that have classical O1 biotype genetic and phenotypic characteristics. These El Tor variants have been emerging in clinical settings with increased frequency, including the 2010 cholera outbreak in Haiti. The emergence of El Tor variants warrants the proper and timely identification of clinical (or environmental) isolates' biotype. This unit describes some quick and simple genetic screens and phenotypic assays (biochemical characterization), to be performed simultaneously, commonly used to distinguish biotype and initiate characterization of any clinical (or environmental) isolates of Vibrio cholerae O1.

Genetic characterization of Vibrio cholerae O1 strains isolated in Zambia during 1996-2004 possessing the unique VSP-II region of El Tor variant

New variants of Vibrio cholerae O1 have appeared in different time-frames in various endemic regions, especially in Asia and Africa. Sixty-nine strains of V. cholerae O1 isolated in Zambia between 1996 and 2004 were investigated by various genotypic techniques to determine the lineage of virulence signatures and clonality. All strains were positive for Vibrio seventh pandemic Islands (VSP)-I and VSP-II and repeat toxin (RTX) gene clusters attesting their El Tor lineage. Interestingly, strains isolated in recent times (2003-2004) were identified as an altered variant (El Tor biotype that harbours El Tor type rstR but produce classical ctxB) that replaced completely the progenitor El Tor strains prevalent in 1996-1997. Recent altered variant strains differed from prototype El Tor strains isolated earlier in that these strains lacked two ORFs, VC0493 and VC0498, in the VSP-II region. PFGE analysis revealed two major clonal lineages in the strains; cluster A represented the strains isolated before 2003 and cluster B the altered strains isolated in 2003-2004. Cluster A was closely related to prototype El Tor reference strain isolated in Bangladesh in 1971. Cluster B was found to be matched with Bangladeshi altered strains but was different from the hybrid strains isolated from Mozambique and Bangladesh. This report provides important information on the genesis of altered strains of V. cholerae O1 isolated in Zambia and emphasizes the need for further studies to follow the trends of evolutionary changes.

Molecular characterization reveals involvement of altered El Tor biotype Vibrio cholerae O1 strains in cholera outbreak at Hyderabad, India

Thirty-four Vibrio cholerae isolates collected from a cholera outbreak in Hyderabad, South India were found to belong to serogroup Ol biotype El Tor serotype Ogawa. The genotype of all the isolates was confirmed by PCR assays. All the isolates were found PCR positive for ctxAB, ompW, rflOl, rtxC, and tcpA genes. All the isolates but one harboured rstR ( El Tor ) allele. However, one isolate carried both rstR ( EL Tor ) as well as rstR ( Classical ) alleles. Cholera toxin (ctxB) genotyping of the isolates confirmed the presence of altered cholera toxin B of classical biotype in all the isolates. All the isolates except VCH35 harboured an RS1-CTX prophage array on the large chromosome. The isolate VCH35 contained a tandem repeat of classical CTX prophage on the small chromosome. The clonal relationship among the V. cholerae isolates as carried out by enterobacterial repetitive intergenic consensus sequences PCR, BOX PCR and randomly amplified polymorphic DNA, uniformly showed a genetic relationship among the outbreak isolates. The results of this study suggest that altered El Tor biotype V. cholerae with the classical cholera toxin gene are involved in cholera outbreaks in India.

Phenotypic, genotypic, and antibiotic sensitivity patterns of strains isolated from the cholera epidemic in Zimbabwe

This paper details the phenotypic, genotypic, and antibiotic sensitivity patterns of 88 Vibrio cholerae strains from Zimbabwe. Of the 88 strains, 83 were classified as "altered El Tor" and 5 as "hybrid El Tor" strains. All of the strains were susceptible to tetracycline, doxycycline, ciprofloxacin, and azithromycin by disc diffusion, but susceptibility to tetracycline and azithromycin diminished when observed using the MIC method.

Binding efficiencies of carbohydrate ligands with different genotypes of cholera toxin B: molecular modeling, dynamics and docking simulation studies

Vibrio cholerae produces cholera toxin (CT) that consists of two subunits, A and B, and is encoded by a filamentous phage CTXΦ. The A subunit carries enzymatic activity that ribosylates ADP, whereas the B subunit binds to monosialoganglioside (GM1) receptor in epithelial cells. Molecular analysis of toxigenic V. cholerae strains indicated the presence of multiple ctxB genotypes. In this study, we employed a comparative modeling approach to define the structural features of all known variants of ctxB found in O139 serogroup V. cholerae. Modeling, molecular dynamics and docking simulations studies suggested subtle variations in the binding ability of ctxB variants to carbohydrate ligands of GM1 (galactose, sialic acid and N-acetyl galactosamine). These findings throw light on the molecular efficiencies of pathogenic isolates of V. cholerae harboring natural variants of ctxB in causing the disease, thus suggesting the need to consider ctxB variations when designing vaccines against cholera.

Effect of storage and sodium chloride on excision of CTXPhi or pre-CTXPhi and CTXPhi from Vibrio cholerae O139 strains

We examined the effect of storage and sodium chloride on excision of CTXPhi or pre-CTXPhi and CTXPhi from Vibrio cholerae O139 strains. We found that one strain of V. cholerae O139 VO146P showed loss of the complete phage array, and other strain VO170P showed partial loss of the phage array giving rise to altered strains designated as VO146N and VO170N. Results of PCR and RFLP analysis revealed that both strains (VO146P and VO170P) possessed a single copy of pre-CTX(ET)Phi and two copies of CTXPhi comprising CTX(Class)Phi and CTX(Calc)Phi arranged in tandem, and integrated in the large chromosome. The presence of classical ctxB was detected in CTX(Calc)Phi of both V. cholerae O139 strains. Nucleotide sequencing of three housekeeping genes showed no difference between parent and altered strains of V. cholerae O139.

Sequence analysis of Vibrio cholerae orfU and zot from pre-CTXΦ and CTXΦ reveals multiple origin of pre-CTXΦ and CTXΦ

A multiplex PCR was developed to detect pre-CTXΦ and CTXΦ in Vibrio cholerae. A total of 115 V. cholerae were tested, of which 42 V. cholerae O1 and 18 V. cholerae O139 contained CTXΦ. Six V. cholerae O139 contained only pre-CTXΦ and three V. cholerae O1 and 23 V. cholerae O139 contained both pre-CTXΦ and CTXΦ. None of the V. cholerae non-O1 and non-O139 that were tested had pre-CTXΦ or CTXΦ. Results of Restriction Fragment Length Polymorphism (RFLP) analysis revealed the V. cholerae isolates possessed single or multiple copies of pre-CTXΦ and CTXΦ, always proceeded by a tandemly arranged RS1 element. Comparative nucleotide sequence analyses of the core region genes, orfU and zot, of 15 V. cholerae showed pre-CTX(ET) Φ and CTX(ET) Φ lineage with V. cholerae El Tor and pre-CTX(Class) Φ, pre-CTX(Calc) Φ, and CTX(Calc) Φ with classical V. cholerae O1 and O139. Two distinct types of ctxB were detected in V. cholerae O139. Multi-locus Sequence Typing (MLST) of seven V. cholerae housekeeping genes indicated clonal origin, irrespective of the presence of pre-CTXΦ and/or CTXΦ.

Molecular characterisation of Vibrio cholerae O1 strains carrying an SXT/R391-like element from cholera outbreaks in Kenya: 1994-2007

BACKGROUND

Over the last decade, cholera outbreaks in parts of Kenya have become common. Although a number of recent studies describe the epidemiology of cholera in Kenya, there is paucity of information concerning the diversity and occurrence of mobile genetic elements in Vibrio cholerae strains implicated in these outbreaks. A total of 65 Vibrio cholerae O1 El Tor serotype Inaba isolated between 1994 and 2007 from various outbreaks in Kenya were investigated for mobile genetic elements including integrons, transposons, the integrating conjugative elements (ICEs), conjugative plasmids and for their genotypic relatedness.

RESULTS

All the strains were haemolytic on 5% sheep blood and positive for the Vibrio cholerae El Tor-specific haemolysin toxin gene (hylA) by PCR. They all contained strB, sulII, floR and the dfrA1 genes encoding resistance to streptomycin, sulfamethoxazole, chloramphenicol and trimethoprim respectively. These genes, together with an ICE belonging to the SXT/R391 family were transferable to the rifampicin-resistant E. coli C600 en bloc. All the strains were negative for integron class 1, 2 and 3 and for transposase gene of transposon Tn7 but were positive for integron class 4 and the trpM gene of transposon Tn21. No plasmids were isolated from any of the 65 strains. All the strains were also positive for all V. cholera El Tor pathogenic genes except the NAG- specific heat-stable toxin (st) gene. None of the strains were positive for virulence genes associated with the V. cholerae classical biotype. All the strains were positive for El Tor-specific CTXphi bacteriophage rstrR repressor gene (CTXETPhi) but negative for the Classical, Calcutta, and the Environmental repressor types. Pulse Field Gel Electrophoresis (PFGE) showed that regardless of the year of isolation, all the strains bearing the SXT element were clonally related.

CONCLUSIONS

This study demonstrates that the V. cholerae O1 strains carrying an SXT/R391-like element implicated in recent cholera outbreaks in Kenya has not changed significantly between 1994 and 2007 and are clonally related.

Classical RS1 and environmental RS1 elements in Vibrio cholerae O1 El Tor strains harbouring a tandem repeat of CTX prophage: revisiting Mozambique in 2005

Currently, Vibrio cholerae O1 serogroup biotype El Tor strains producing classical type cholera toxin (altered strains or El Tor variants) are prevalent in Asia and in Mozambique. Mozambican strains collected in 2004 contained a tandem repeat of CTX prophage on the small chromosome and each CTX prophage harboured the classical rstR and classical ctxB. We found that the majority of the strains collected in 2005 in Mozambique contained extra elements on the large chromosome in addition to the tandem repeat of CTX prophage on the small chromosome. New type RS1 elements RS1(cla) and RS1(env), and a CTX(env) with rstR(env) and the classical ctxB were identified on the large chromosome of the Mozambican isolates collected in 2005.

Evolution of new variants of Vibrio cholerae O1

Vibrio cholerae typically contains a prophage that carries the genes encoding the cholera toxin, which is responsible for the major clinical symptoms of the disease. In recent years, new pathogenic variants of V. cholerae have emerged and spread throughout many Asian and African countries. These variants display a mixture of phenotypic and genotypic traits from the two main biotypes (known as 'classical' and 'El Tor'), suggesting that they are genetic hybrids. Classical and El Tor biotypes have been the most epidemiologically successful cholera strains during the past century, and it is believed that the new variants (which we call here 'atypical El Tor') are likely to develop successfully in a manner similar to these biotypes. Here, we describe recent advances in our understanding of the epidemiology and evolution of the atypical El Tor strains.

Genetic analysis of CTX prophages with special reference to ctxB and rstR alleles of Vibrio cholerae O139 strains isolated from Kolkata over a decade

Chronological analysis of 125 Vibrio cholerae O139 strains isolated during 1993-2005 in Kolkata revealed the prevalence of two new genotypes of cholera toxin (CT) and novel combinations of ctxB and rstR alleles resulting in variant CTX prophages. One of the new genotypes of ctxB, which first appeared in 1996 with the re-emerged V. cholerae O139 strains that had CTX Calcutta phage, was designated as genotype 4. In 1998, another new genotype, designated as genotype 5, was detected that prevailed mostly in CTX phages with El Tor rstR. The prototype El Tor CTX phage with genotype 3 gradually disappeared in O139, and since 2002 the predominant CTX prophages in O139 are Calcutta phages with genotype 4 and El Tor phages with genotype 5. Results showed that V. cholerae O139 strains of Kolkata, isolated over a decade, harboured CTX prophages in the large chromosome having no RS1 downstream of CTX prophage. During the course of its intermittent incidence over a decade, five types of O139 strains were detected based on CT genotypes. Such abrupt genetic changes in O139 strains might not favour its continued prevalence in human cases in Kolkata, India.

Peruvian Vibrio cholerae O1 El Tor strains possess a distinct region in the Vibrio seventh pandemic island-II that differentiates them from the prototype seventh pandemic El Tor strains

A collection of environmental and clinical strains of Vibrio cholerae O1 isolated from the beginning of the Latin American epidemic of cholera in 1991 to 2003 from multiple locations in Peru were characterized and compared with V. cholerae O1 El Tor strains of the seventh pandemic from the rest of the world (Asia, Africa, Australia and Europe) using a multilocus virulence gene profiling strategy and DNA sequencing. Peruvian strains differed from El Tor strains from the rest of the world by the failure of PCR to amplify genes VC0512, VC0513, VC0514 and VC0515 in the Vibrio seventh pandemic island-II (VSP-II) gene cluster. Sequencing of the VSP-II gene cluster and its flanking regions in one Peruvian strain (PERU-130) confirmed the PCR results, indicating that the Peruvian strain had low DNA homology (46.6 %) compared to the reference strain N16961 within the VSP-II region encompassing genes VC0511 to VC0515. Based on these differences in VSP-II, and based on the overall similarity between the pulsotypes of the Peruvian strains and the El Tor reference strain N16961, we concluded that the Peruvian, Eurasian and African strains belonged to the same clonal complex, and that the Peruvian strains represented variants that had independently evolved for a relatively short time. Since these ORFs in VSP-II of Peruvian strains are unique and conserved, they could form the basis for tracking the origin of the Peruvian strains and therefore of the Latin American pandemic.

Cholera outbreaks caused by an altered Vibrio cholerae O1 El Tor biotype strain producing classical cholera toxin B in Vietnam in 2007 to 2008

Vibrio cholerae O1 isolates collected during cholera outbreaks occurring from late 2007 to early 2008 in northern Vietnam were revealed to represent an altered strain containing the RS1 element followed by a CTX prophage harboring El Tor type rstR and classical ctxB on the large chromosome.

Multilocus genetic analysis reveals that the Australian strains of Vibrio cholerae O1 are similar to the pre-seventh pandemic strains of the El Tor biotype

Episodes of cholera stemming from indigenous Vibrio cholerae strains in Australia are mainly associated with environmental sources. In the present study, 10 V. cholerae O1 strains of Australian origin were characterized. All of the strains were serogroup O1 and their conventional phenotypic traits categorized them as belonging to the El Tor biotype. Genetic screening of 12 genomic regions that are associated with virulence in V. cholerae showed variable results. Analysis of the ctxAB gene showed that the Australian environmental reservoir contains both toxigenic and non-toxigenic V. cholerae strains. DNA sequencing revealed that all of the toxigenic V. cholerae strains examined were of ctxB genotype 2. Whole genome PFGE analysis revealed that the environmental toxigenic V. cholerae O1 strains were more diverse than the non-toxigenic environmental O1 strains, and the absence of genes that make up the Vibrio seventh pandemic island-I and -II in all of the strains indicates their pre-seventh pandemic ancestry.

Incidence, virulence factors, and clonality among clinical strains of non-O1, non-O139 Vibrio cholerae isolates from hospitalized diarrheal patients in Kolkata, India

The incidence of Vibrio cholerae non-O1, non-O139 strains from hospitalized patients with acute diarrhea constituted 27.4% (n = 54) of the total 197 V. cholerae strains isolated from patients in Kolkata, India, in 2003. Of 197 strains, 135 were identified as O1 serotype Ogawa and 2 were identified as O139. In the same time period, six O1 background rough strains that possessed all known virulence factors were identified. Serotype analysis of the non-O1, non-O139 strains placed 42 strains into 19 serogroups, while 12 remained O nontypeable (ONT); the existing serotyping scheme involved antisera to 206 serogroups. Detection of a good number of ONT strains suggested that additional serogroups have arisen that need to be added to the current serotyping scheme. The non-O1, non-O139 strains were nontoxigenic except for an O36 strain (SC124), which regulated expression of cholera toxin as O1 classical strains did. Additionally, strain SC124 carried alleles of tcpA and toxT that were different from those of the O1 counterpart, and these were also found in five clonally related strains belonging to different serogroups. Strains carrying tcpA exhibited higher colonization in an animal model compared to those lacking tcpA. PCR-based analyses revealed remarkable variations in the distribution of other virulence factors, including hlyA, rtxA, Vibrio seventh pandemic island I (VSP-I), VSP-II, and type III secretion system (TTSS). Most strains contained hlyA (87%) and rtxA (81.5%) and secreted cytotoxic factors when grown in vitro. Approximately one-third of the strains (31.5%) contained the TTSS gene cluster, and most of these strains were more motile and hemolytic against rabbit erythrocytes. Partial nucleotide sequence analysis of the TTSS-containing strains revealed silent nucleotide mutations within vcsN2 (type III secretion cytoplasmic ATPase), indicating functional conservation of the TTSS apparatus.

Molecular characterization of recent Vibrio cholerae O1, El Tor, Inaba strains isolated from hospitalized patients in Kolkata, India

OBJECTIVES

To study the phenotypic and genotypic characterization of newly emerged V. cholerae O1, Inaba strains isolated from patients with diarrhoea.

METHODS

Bacterial characterization was made using polymerase chain reaction, ribotyping, PFGE and RFLP.

RESULTS

After its first appearance in July 2004, O1 Inaba became the dominant serotype by March 2005 and totally replaced the former dominant serotype, Ogawa from May 2005. Most of the Inaba isolates belong to a new ribotype RIV. Ogawa and also some Inaba strains isolated during the same period were identified as RIII. Similarly, the majority of the Inaba isolates belong to 'H1' pulsotype and one isolate is type 'H', while the Ogawa isolates were mostly 'H' pulsotype. Presence of CTX prophage was detected in a single site of the chromosome with at least two RS elements.

CONCLUSIONS

There has been a switch of dominant serotype from Ogawa to Inaba in the Kolkata region. This is not necessarily due to emergence of a new clone but does serve as an epidemiological marker. Further analysis at the molecular level will be required to define this trend and to monitor future spread to other regions.

Multiply antibiotic-resistant Vibrio cholerae O1 biotype El Tor strains emerge during cholera outbreaks in Zambia

Antibiotic resistance data, made available from laboratory records during eight cholera outbreaks between 1990 and 2004 showed Vibrio cholerae serogroup O1 to have a low level of resistance (2-3%) to tetracycline during 1990-1991. Resistance increased for tetracycline (95%), chloramphenicol (78%), doxycycline (70%) and trimethoprim-sulphamethoxazole (97%) in subsequent outbreaks. A significant drop in resistance to tetracycline and chloramphenicol followed the adoption of a national policy to replace tetracycline with erythromycin for treating cholera. Sixty-nine strains from cholera outbreaks in Zambia between 1996 and 2004, were examined for antibiotic resistance and basic molecular traits. A 140 MDa conjugative, multidrug-resistant plasmid was found to encode tetracycline resistance in strains from 1996/1997 whereas strains from 2003/2004 were resistant to furazolidone, but susceptible to tetracycline, and lacked this plasmid. PCR revealed 25 of 27 strains from 1996/1997 harboured the intl1 class 1 integron but lacked SXT, a conjugative transposon element. Similar screening of 42 strains from 2003/2004 revealed all carried SXT but not the intl1 class 1 integron. All 69 strains, except two, one lacking ctxA and the other rstR and thus presumably truncated in the CTX prophage region, were positive for important epidemic markers namely rfbO1, ctxA, rstR2, and tcpA of El Tor biotype. Effective cholera management is dependent on updated reports on culture and sensitivity to inform the choice of antibiotic. Since the emergence of antibiotic resistance may significantly influence strategies for controlling cholera, continuous monitoring of epidemic strains is crucial.