Molecular subtyping of Vibrio cholerae O1 and O139 by pulsed-field gel electrophoresis in Hong Kong: correlation with epidemiological events from 1994 to 2002

Molecular subtyping for source tracking of Escherichia coli using core genome multilocus sequence typing at a food manufacturing plant

When harmful bacteria are detected in the final product at a food manufacturing plant, it is necessary to identify and eliminate the source of contamination so that it does not occur again. In the current study, the source of contamination was tracked using core genome multilocus sequence typing (cgMLST) analysis in cases where Escherichia coli was detected in the final product at a food manufacturing plant. cgMLST analysis was performed on 40 strains of E. coli collected from the environment [floor (26 strains), drainage ditch (5 strains), container (4 strains), post-heating production line (1 strain)] and products [final product (3 strains) and intermediate product (1 strain)]. In total, 40 E. coli isolates were classified into 17 genogroups by cgMLST analysis. The 4 E. coli strains isolated from the intermediate and final products were classified into two genogroups (I and II). Certain isolates collected from the environment also belonged to those genogroups, it was possible to estimate the transmission of E. coli in the manufacturing plant. Thus, the dynamics of E. coli in the food manufacturing location were clarified by using cgMLST analysis. In conclusion, our results indicate that cgMLST analysis can be effectively used for hygiene management at food manufacturing locations.

Post-monsoon waterlogging-associated upsurge of cholera cases in and around Kolkata metropolis, 2015

The Infectious Diseases and Beliaghata General Hospital, Kolkata, India witnessed a sudden increase in admissions of diarrhoea cases during the first 2 weeks of August 2015 following heavy rainfall. This prompted us to investigate the event. Cases were recruited through hospital-based surveillance along with the collection of socio-demographic characteristics and clinical profile using a structured questionnaire. Stool specimens were tested at bacteriological laboratory of the National Institute of Cholera and Enteric Diseases (NICED), Kolkata. Admission of 3003 diarrhoea cases, clearly indicated occurrence of outbreak in Kolkata municipal area as it was more than two standard deviation of the mean number (911; s.d. = 111) of diarrhoea admissions during the same period in previous 7 years. Out of 164 recruited cases, 25% were under-5 children. Organisms were isolated from 80 (49%) stool specimens. Vibrio cholerae O1 was isolated from 50 patients. Twenty-eight patients had this organism as the sole pathogen. Among 14 infants, five had cholera. All V. cholerae O1 isolates were resistant to nalidixic acid, followed by co-trimoxazole (96%), streptomycin (92%), but sensitive to fluroquinolones. We confirmed the occurrence of a cholera outbreak in Kolkata during August 2015 due to V. cholerae O1 infection, where infants were affected.

Comparison of virulence gene profiles and genomic fingerprints of Vibrio cholerae O1 and non-O1/non-O139 isolates from diarrheal patients in southern Thailand

BACKGROUND

Vibrio cholerae is associated with severe watery diarrheal disease among people in many parts of the world, including the coastal provinces of Southern Thailand. There are relatively few studies focusing on the genetic characterization among V. cholerae isolates in this region. Therefore, this study aimed at exploring the presence of virulence genes and DNA fingerprints among V. cholerae O1 and non-O1/non-O139 isolates obtained from clinical samples in four southern coastal provinces during the period of 2001-2009 (n = 21).

RESULTS

All V. cholerae O1 isolates possessed ctxA, tcpA, zot, ace, hlyA, and vasH genes. However, only hlyA, vcsV2, and vasH genes were detected in the majority of the non-O1/non-O139 isolates. All O1 isolates showed indistinguishable PCR fingerprints by arbitrarily primed (AP)-PCR and enterobacterial repetitive intergenic consensus (ERIC)-PCR regardless of the geographical area and period of isolation. However, the multi-locus variable-number of tandem-repeat analysis (MLVA) could differentiate these O1 isolates (n = 11) into eight profiles. Isolates exhibiting an undistinguished MLVA profile also showed identical pulsed-field gel electrophoresis (PFGE). In addition, the O1 isolates were grouped into the same cluster by all methods used in this study.

CONCLUSIONS

This study demonstrated the presence of virulence genes and genetic diversity among different serogroups of V. cholerae isolates from clinical samples in southern Thailand. V. cholerae O1 isolated over a period of multiple years were genetically related, suggesting that they had a clonal origin, whereas non-O1/non-O139 isolates could have evolved independently.

Molecular characterization and drug susceptibility of non-O1/O139 V. cholerae strains of seafood, environmental and clinical origin, Italy

Toxigenic and antimicrobial susceptibility patterns and genetic relatedness of 42 non-O1/O139 V. cholerae strains, the majority of them isolated from seafood and marine water of the Adriatic sea, Italy, and 9 clinical strains, two of which with seawater of the Adriatic as the source of infection, were studied. All strains had hlyA El Tor gene but lacked ctxA gene. Four and two isolates, respectively, also had stn/sto and tcpA Class genes. More than 90% of strains showed susceptibility to cefotaxime, ciprofloxacin, cloramphenicol, tetracycline, trimethoprim + sulfamethoxazole and intermediate or full resistance to tetracycline and erythromycin. Six strains of seafood and clinical source were multi-drug resistant. PFGE analysis allowed to type all the strains with 50 banding patterns. Twenty-one strains, 11 and 8 from seafood and seawater, respectively, and 2 of clinical origin, were grouped into 9 different clusters. We report the presence of toxigenic and multidrug resistant non-O1/O139 V. cholerae strains in Adriatic, some of which genetically related, and support that they represent a potential reservoir of toxin and antibiotic resistance genes.

The trend of enteropathogenic Escherichia coli towards atypical multidrug resistant genotypes

The aim of this study was to investigate the prevalence of multidrug resistance (MDR) in atypical enteropathogenic Escherichia coli (EPEC) genotypes among 547 diarrheal children. All E. coli isolates with eae⁺stx1^-stx2^- genotypes included in this study and atypical property of EPEC was characterized by the absence of Bundle-forming pili (bfpA gene). Bacterial pathogens were detected in 70 patients (12.8%) among which atypical EPEC (5.3%) were the most common. The higher resistance rate was seen to tetracycline (70%), cotrimoxazole (60%) and nalidixic acid (53.3%) related to MDR phenotype in 63.3% of isolates. The presence of class 1 and 2 integrons was 30% and 6.6% with the dominance of dfrA, aadA gene cassettes among the isolates. Eleven out of 21 phenotypically tetracycline-resistant isolates (52.38%) harboured one or two tetracycline resistance genes (A-D) which shows the incapability of known data to reasonable tetracycline resistance phenotypes among EPEC isolates. A high level of genotypic diversity was seen among the isolates by Pulsed-field gel electrophoresis method which ranged from 89.7 to 29% and no clear correlation was obtained between tetracycline resistance or integron carriage and specific pulsotypes. In conclusion, the data presented here add to knowledge about the heterogeneous nature of MDR EPEC population in Iran which has a growing tendency towards atypical genotypes. The distribution of integrons among EPEC isolates in Iran is decreasing; although, the resistance gene content is almost stable through years.

Characteristics of Vibrio cholerae O1 isolated from water of the River Ganga, Varanasi, India

BACKGROUND

Vibrio cholerae is an autochthonous inhabitant of fresh and brackish water and estuarine system. Investigation of V. cholerae from the River Ganga seems important to find variation in CTX arrangement and genomic diversity.

OBJECTIVES

To investigate V. cholerae O1 strains for the presence of virulence and regulatory genes, variation in number and organisation of the pre-CTXΦ and/or CTXΦ, and for the genomic diversity.

MATERIALS AND METHODS

Polymerase chain reaction (PCR) was used to detect virulence and regulatory genes, type of rstR and location of CTXΦ on the chromosome. Southern hybridisation was conducted to see the number and arrangement of pre-CTXΦ and CTXΦ. Ribotyping and pulsed-field gel electrophoresis were used to find genetic relatedness.

RESULTS

Seven strains gave positive results by PCR for the gene encoding for ctx A, zot, ace, tcp A (El Tor), omp U, and tox R, except one strain that was negative for the ctx A. Three strains were positive for the tcp A (El Tor), omp U and tox R genes. Determination of CTX organisation showed that among the ctx-positive strains, four harboured two copies of CTXETΦ arranged in tandem and two harboured one copy of CTXETΦ, and one ctx-negative strain harboured only one copy of pre-CTXETΦ. Pulsotype and ribotype analysis showed existence of at least three pulsotype and ribotypes indicating diversity in genomic content among them.

CONCLUSION

This study thus indicates that multiple clones (ribotypes/pulsotypes) of V. cholerae O1 carrying pre-CTXΦ and/or CTXΦ and ctx-negative strains were present in the water of the River Ganga, Varanasi, India.

Genome Wide Analysis for Rapid Identification of Vibrio Species

The highly conserved 16S rRNA (rrs) gene is generally used for bacterial identification. In organisms possessing multiple copies of rrs, high intra-genomic heterogeneity does not allow easy distinction among different species. In order to identify Vibrio species, a wide range of genes have been employed. There is an urgent requirement of a consensus gene, which can be used as biomarker for rapid identification. Eight sequenced genomes of Vibrio species were screened for selecting genes which were common among all the genomes. Out of 108 common genes, 24 genes of sizes varying from 0.11 to 3.94 kb were subjected to in silico digestion with 10 type II restriction endonucleases (RE). A few unique genes-dapF, fadA, hisD, ilvH, lpxC, recF, recR, rph and ruvB in combination with certain REs provided unique digestion patterns, which can be used as biomarkers. This protocol can be exploited for rapid diagnosis of Vibrio species.

Molecular characterization of enterohemorrhagic Escherichia coli isolates from cattle

A total of 21 (4.3%) enterohemorrhagic E. coli strains were isolated by biochemical tests and identification of the eae⁺stx1⁺stx2⁺ genotype from 490 stool samples obtained from calves with diarrhea during 1-year period from a major farm in Tehran, Iran. All of the strains showed resistance to ampicillin, ciprofloxacin, trimethoprim, streptomycin, chloramphenicol and tetracycline, while 19% showed resistance to gentamicin. Out of 21 EHEC strains, 11 (53%) harbored class 1 integron. Two different amplification products, which were approximately 750 and 1,700 bp in size, were obtained from amplified variable regions (in-F/in-R primers) in 3 (14.3%) and 4 (19%) of the EHEC isolates, which corresponded to dfrA7(dihydrofolate reductase type I) and dfrA1/aadA1(dihydrofolate reductase/aminoglycoside adenyltransferase) resistance gene cassettes, respectively, and this was confirmed by sequencing. Genotyping analysis revealed a total of 16 pulsotypes that corresponded to 16 isolates with the similarity indices of 62% and 30% for the most and least similar isolates, respectively, 9 of which harbored class 1 integron. Analysis of pulsotypes showed an extensive diversity among the isolates harboring integron, which is indicative of a lack of any significant genetic relatedness among the isolates. No obvious relation could be deduced between integron content and special pulsotypes. The little data available on the genotyping patterns of EHEC isolates from cattle and their resistance gene contents emphasize the need to establish genotyping databases in order to monitor and source track the source of emergence and spread of new resistant and integron-carrying genotypes.

Survivability and molecular variation in Vibrio cholerae from epidemic sites in China

The survival behaviour of Vibrio cholerae in cholera epidemics, together with its attributes of virulence-associated genes and molecular fingerprints, are significant for managing cholera epidemics. Here, we selected five strains representative of V. cholerae O1 and O139 involved in cholera events, examined their survival capacity in large volumes of water sampled from epidemic sites of a 2005 cholera outbreak, and determined virulence-associated genes and molecular subtype changes of the surviving isolates recovered. The five strains exhibited different survival capacities varying from 17 to 38 days. The virulence-associated genes of the surviving isolates remained unchanged, while their pulsotypes underwent slight variation. In particular, one waterway-isolated strain maintained virulence-associated genes and evolved to share the same pulsotype as patient strains, highlighting its role in the cholera outbreak. The strong survival capacity and molecular attributes of V. cholerae might account for its persistence in environmental waters and the long duration of the cholera outbreak, allowing effective control measures.

A molecular surveillance reveals the prevalence of Vibrio cholerae O139 isolates in China from 1993 to 2012

Vibrio cholerae serogroup O139 was first identified in 1992 in India and Bangladesh, in association with major epidemics of cholera in both countries; cases were noted shortly thereafter in China. We characterized 211 V. cholerae O139 isolates that were isolated at multiple sites in China between 1993 and 2012 from patients (n = 92) and the environment (n = 119). Among clinical isolates, 88 (95.7%) of 92 were toxigenic, compared with 47 (39.5%) of 119 environmental isolates. Toxigenic isolates carried the El Tor CTX prophage and toxin-coregulated pilus A gene (tcpA), as well as the Vibrio seventh pandemic island I (VSP-I) and VSP-II. Among a subset of 42 toxigenic isolates screened by multilocus sequence typing (MLST), all were in the same sequence type as a clinical isolate (MO45) from the original Indian outbreak. Nontoxigenic isolates, in contrast, generally lacked VSP-I and -II, and fell within 13 additional sequence types in two clonal complexes distinct from the toxigenic isolates. In further pulsed-field gel electrophoresis (PFGE) (with NotI digestion) studies, toxigenic isolates formed 60 pulsotypes clustered in one group, while the nontoxigenic isolates formed 43 pulsotypes which clustered into 3 different groups. Our data suggest that toxigenic O139 isolates from widely divergent geographic locations, while showing some diversity, have maintained a relatively tight clonal structure across a 20-year time span. Nontoxigenic isolates, in contrast, exhibited greater diversity, with multiple clonal lineages, than did their toxigenic counterparts.

Study of drug resistance and molecular typing of 59 cholerae01 clinical isolates from 1984 to 2002 in Chongqing, China

OBJECTIVE

To analyze the correlation between drug resistance and Cholerae01 clinical isolates from 1984 to 2002 in Chongqing, China.

METHODS

K-B assay was applied to detect the sensitivity of 59 Cholerae01 clinical isolates (20 Ogawa, 39 Inaba) to 16 kinds of antibiotics. BioNumerics software was used for a cluster analysis of electrophoresis patterns obtained from the Not I enzyme-cutting genomic DNA by Pulsed-field gel electrophoresis (PFGE).

RESULTS

Vibrio cholerae01 in Chongqing area, China were highly resistant to Cotrimoxazole, Furazolidone and Streptomycin. The resistance rates were 28.81% (17/59), 61.02% (36/59) and 30.51% (18/59), respectively. While the isolates from the crowd were sensitive to Amikacin, Gentamicin, Tobramycin, Ampicillin, Neomycin and Doxycycline, and no drug-resistant strains were observed.

CONCLUSION

No significant changes are found in the drug resistance of Vibrio cholerae01 from the crowd in Chongqing, China and the drug resistances of the Ogawa and the Inaba strains are different. Vibrio cholerae01 from the crowd in Chongqing, China are highly homologous, which may be from the epidemic strains with the same source.

Vibrio fluvialis in patients with diarrhea, Kolkata, India

We identified 131 strains of Vibrio fluvialis among 400 nonagglutinating Vibrio spp. isolated from patients with diarrhea in Kolkata, India. For 43 patients, V. fluvialis was the sole pathogen identified. Most strains harbored genes encoding hemolysin and metalloprotease; this finding may contribute to understanding of the pathogenicity of V. fluvialis.

Phenotypic and genetic analyses of 111 clinical and environmental O1, O139, and non-O1/O139 Vibrio cholerae strains from different geographical areas

A total of 111 clinical and environmental O1, O139 and non-O1/O139 Vibrio cholerae strains isolated between 1978 and 2008 from different geographical areas were typed using a combination of methods: antibiotic susceptibility, biochemical test, serogroup, serotype, biotype, sequences containing variable numbers of tandem repeats (VNTRs) and virulence genes ctxA and tcpA amplification. As a result of the performed typing work, the strains were organized into four clusters: cluster A1 included clinical O1 Ogawa and O139 serogroup strains (ctxA(+) and tcpA(+)); cluster A2 included clinical non-O1/O139 strains (ctxA(-) and tcpA(-)), as well as environmental O1 Inaba and non-O1/O139 strains (ctxA(-) and tcpA(-)/tcpA(+)); cluster B1 contained two clinical O1 strains and environmental non-O1/O139 strains (ctxA(-) and tcpA(+)/tcpA(-)); cluster B2 contained clinical O1 Inaba and Ogawa strains (ctxA(+) and tcpA(+)). The results of this work illustrate the advantage of combining several typing methods to discriminate between clinical and environmental V. cholerae strains.

Genotypic characterization of Vibrio cholerae isolates using several DNA fingerprint techniques

Serious pandemics of cholera have occurred throughout the known history of mankind, especially in India, which is a motherland for cholera disease. For the last 20 years several DNA-based typing methods have been employed to study the clonal relatedness between Vibrio cholerae isolates irrespective of their geographical locations. Traditional typing methods, such as biochemical tests, phage typing, serotyping, biotyping and antimicrobial susceptibility tests, have produced reliable and informative data regarding V. cholerae for a long time. Gradually molecular typing techniques have taken the place of traditional typing methods because they produce the same results upon repeat testing of V. cholerae strain. In this article we focus on the discriminatory power of different DNA fingerprint techniques that are generally used to know the homogeneity and heterogeneity among different V. cholerae isolates.

Molecular evidence of cholera outbreak caused by a toxigenic Vibrio cholerae O1 El tor variant strain in Kelantan, Malaysia

A total of 20 Vibrio cholerae isolates were recovered for investigation from a cholera outbreak in Kelantan, Malaysia, that occurred between November and December 2009. All isolates were biochemically characterized as V. cholerae serogroup O1 Ogawa of the El Tor biotype. They were found to be resistant to multiple antibiotics, including tetracycline, erythromycin, sulfamethoxazole-trimethoprim, streptomycin, penicillin G, and polymyxin B, with 35% of the isolates being resistant to ampicillin. All isolates were sensitive to ciprofloxacin, norfloxacin, chloramphenicol, gentamicin, and kanamycin. Multiplex PCR analysis confirmed the biochemical identification and revealed the presence of virulence genes, viz., ace, zot, and ctxA, in all of the isolates. Interestingly, the sequencing of the ctxB gene showed that the outbreak strain harbored the classical cholera toxin gene and therefore belongs to the newly assigned El Tor variant biotype. Clonal analysis by pulsed-field gel electrophoresis demonstrated that a single clone of a V. cholerae strain was responsible for this outbreak. Thus, we present the first molecular evidence that the toxigenic V. cholerae O1 El Tor variant has invaded Malaysia, highlighting the need for continuous monitoring to facilitate early interventions against any potential epidemic by this biotype.

Pulsed field gel electrophoresis: a review of application and interpretation in the molecular epidemiology of infectious disease

Over the years, approaches to the epidemiological analysis of infectious disease have undergone a remarkable evolutionary transition moving from phenotypic to molecular in nature. As discussed here, the quest for a clearer comparison of genomic relatedness between bacterial clinical isolates has involved four generations of molecular iteration. First generation plasmid analysis gave way to a second generation use of restriction enzymes and probes. This was followed by third generation pulsed field gel electrophoresis (PFGE) and PCR-based methods with movement now to fourth-generation DNA sequence-based approaches. Remarkably, despite (or perhaps because of) its more than 20-year history as a typing method, PFGE has demonstrated exceptional staying power. The reasons for this endurance as well as the pros and cons of PFGE use are examined in this review. In broad context the history and technology behind PFGE are considered. Issues commonly influencing the quality of PFGE data and its analysis are discussed. Specifics regarding the mechanics of DNA preparation, restriction-enzyme digestion, and proper conditions for electrophoresis are detailed and, most importantly for any approach to epidemiological assessment, issues regarding the analysis and interpretation of PFGE data are explored.

Multiple-locus variable-number tandem repeat analysis of Vibrio cholerae in comparison with pulsed field gel electrophoresis and virulotyping

Molecular analysis of Malaysian Vibrio cholerae was carried out using a multiple-locus variable-number tandem repeat analysis (MLVA) assay based on 7 loci of V. cholerae. The discriminatory ability of the assay was compared with pulsed-field gel electrophoresis (PFGE) using 43 Malaysian V. cholerae isolated from various sources. In addition, the virulotypes of the strains were determined. Based on MLVA, 38 allelic profiles were obtained (F = 0.63) while PFGE generated 35 pulsotypes (F = 0.71). Simpson's index of diversity for different VNTR loci ranged from 0.59 to 0.92. The combined loci increased the discriminatory index to 0.99 which was comparable with PFGE (D = 0.99). Most of the environmental non-O1/non-O139 strains harbored rtxA, rstR, toxR, and hlyA only, and the virulotype of this serogroup was significantly different (P < .01) from clinical/environmental O1 and environmental O139 strains. In conclusion, the MLVA assay developed in this study was a useful genotyping tool with comparable discriminatory power with PFGE. In addition, the combination of the two approaches can further distinguish the strains from different sources and geographical regions of isolation.

Assessing clonality of Vibrio cholerae strains isolated during four consecutive years (2004 - 2007) in Iran

Genotypic and phenotypic characterizations of 50 clinical Vibrio cholerae isolates obtained during 4 consecutive y from 2004 to 2007 in Iran were studied. Antimicrobial susceptibility test, biochemical fingerprinting with Phene Plate system (PhP-RV) and pulsed-field gel electrophoresis (PFGE) were performed. Antibiotic susceptibility test of all isolates showed 12 different profiles. The predominant antimicrobial resistance profile (62%) was simultaneous resistance to SXT, streptomycin, chloramphenicol and erythromycin. PFGE revealed a total of 9 pulsotypes with a single dominant type in each y under study. PhP-RV revealed 6 common and 10 single types constituting 80% and 20% of the isolates, respectively. Diversity index of PhP-RV was 0.853, indicative of homogeneity among the isolates. Data obtained from PhP-RV was in close agreement with the results of PFGE genotyping. A comparison of the published PFGE patterns performed using the PulseNet protocol revealed the presence of similar patterns between some of our isolates and the isolates from Pakistan, Nepal and India, suggestive of dissemination of common V. cholerae clones in this region of the world. This could, in part, be due to human travel or occurrence of analogous DNA rearrangements, resulting in the emergence of similar V. cholerae genotypes in regional countries.

Comparison of automated ribotyping and pulsed-field gel electrophoresis for subtyping of Vibrio cholerae

AIMS

To compare the discriminatory power of an automated ribotyping method for Vibrio cholerae subtyping with the pulsed-field gel electrophoresis (PFGE), to evaluate the possibility of automated ribotyping in use of outbreak investigations and surveillance of cholera.

METHODS AND RESULTS

Eight-one epidemiologically unrelated isolates of V. cholerae, and 19 isolates from seven cholera outbreaks were used as the panels. When comparing the two methods using the epidemiologically unrelated isolates, automated ribotyping using PvuII distinguished 38 different ribotypes with a D-value of 0.8956. When combined with serotyping, the D-value is 0.9466. However, PFGE with NotI and SfiI digestions had higher D-values of 0.9951 and 0.9948, respectively. PFGE could cluster the isolates from each outbreak into the same pattern, and distinguish different patterns from different outbreaks, whereas automated ribotyping had lower discriminatory ability.

CONCLUSIONS

The automated ribotyping has lower discriminatory ability compared to PFGE, and is limited to application in V. cholerae subtyping and outbreak investigation.

SIGNIFICANCE AND IMPACT OF THE STUDY

The study evaluated the limitation in subtyping of automated ribotyping for V. cholerae, and raise the question of improvement for the automated ribotyping in subtyping.

A molecular survey on virulence associated genotypes of non-O1 non-O139 Vibrio cholerae in aquatic environment of Tehran, Iran

Thirty-seven Vibrio cholerae strains were isolated from surface water sources at 5 different locations in Tehran, Iran during 2006 and were identified as non-O1 and non-O139 isolates. PCR for SXT element and class 1 integron was positive for 19% and 5.4% of isolates, respectively. PCR for virulence associated-genes within the vibrio pathogenicity island (VPI) gene cluster showed the presence of LJ, int and RJ in 8, 59 and 30% of the isolates, respectively. None of the V. cholerae isolates contained the toxin encoding genes (ace, zot, ctx) in the CTX genetic element. Biochemical fingerprinting using PhPlate system (PhP-RV) was able to type all strains and resulted in 8 common types (containing 78% of the isolates) and 8 single types (22%). Out of 37 isolates, only 26 isolates were typeable with pulsed-field gel electrophoresis (PFGE) producing banding patterns. The results presented in this study showed no genotyping correlation between the V. cholerae isolated from surface water and the clinical setting which had been reported previously by this laboratory. Furthermore, combination of PFGE and PhP-RV methods was proved beneficial for non-typeable V. cholerae isolates.

Molecular analyses of Vibrio cholerae O1 clinical strains, including new nontoxigenic variants isolated in Mexico during the Cholera epidemic years between 1991 and 2000

We studied the evolution of Vibrio cholerae O1 during the 1991 to 2000 cholera epidemic in Mexico by biochemical, serological, and molecular characterization of strains collected during this period. Strains were divided into toxigenic and nontoxigenic groups according to the presence or absence of genes encoding cholera toxin. As previously reported, we characterized two populations among toxigenic strains, which were present from the first year of the epidemic. BglI rRNA analysis revealed that these strains had ribotype profiles, denoted M5 and M6 in our study, that were identical to those previously designated Koblavi B5 or Popovic 5 and Popovic 6a or Tamayo B21a, respectively. Ribotype M5 was isolated between 1991 and 1993. This ribotype had a low level of genetic variation as detected by pulsed-field gel electrophoresis (PFGE). Ribotype M6 persisted from 1991 to 2000. However, PFGE profiles suggested that two epidemiologically unrelated strains coexisted within this single ribotype from 1995 until the end of the epidemic. We identified three new BglI ribotypes, Mx1, Mx2, and Mx3, from nontoxigenic V. cholerae O1 strains isolated between 1998 and 2000; one of them grouped strains positive for the toxin-coregulated pilus island. They differed from nontoxigenic clones isolated in Latin America and on the U.S. Gulf Coast and are probably autochthonous Mexican V. cholerae O1 variants. Most of these new variants were isolated from states surrounding the Gulf of Mexico, where the highest incidence of cholera in the country was recorded. Thus, the Mexican Gulf Coast, like the U.S. Gulf Coast, may act as an environmental reservoir of V. cholerae O1.

Comparison of different electrophoretic parameters of Pulse-Field Gel Electrophoresis for Vibrio cholerae subtyping

Molecular subtyping is used to distinguish pathogenic bacterial strains during epidemiological surveys of infectious diseases and the discovery of novel pathogens. There is a need to standardize protocols involving Pulse-Field Gel Electrophoresis (PFGE), a crucial method in molecular subtyping, to make inter-laboratory results comparable and information exchange possible. The PFGE pattern varies with electrophoretic parameters (EPs), so it is important to select the parameter that can distinguish the patterns better. To optimize EPs of PFGE for Vibrio cholerae, we analyzed 24 isolates of V. cholerae O1 biotype El Tor and 26 isolates of O139 by PFGE with Not|. We used four different EPs and compared the similarity coefficients from the four groups by Friedman test. Based on the principle that the electrophoretic parameter producing smaller similarity in the PFGE patterns has higher level of discriminatory power, the one producing the smallest similarity coefficients is considered to be optimal. This method could be applied to determine the optimal molecular subtyping protocol for other bacteria.

Phenotypic and molecular typing of Vibrio cholerae O1 and O139 isolates from India

OBJECTIVES

To determine whether Vibrio cholerae strains with similar phage types are also genetically homogeneous or heterogeneous by molecular typing methods like PFGE and RAPD-PCR employed in this study.

METHODS

A total of 26 V. cholerae O1 and O139 strains received from different parts of India were analyzed by using conventional phage typing method, antibiogram and molecular typing methods such as PFGE and RAPD-PCR.

RESULTS

Both O1 and O139 strains were resistant against two antibiotics (Ampicillin and Furazolidone) were detected. All of these strains were clustered in a single phage type, i.e., Type 27 for V. cholerae O1 and Type 1 for V. cholerae O139. Extensive molecular characterization by RAPD and PFGE showed that six sets of O1 and O139 strains, each comprising two strains, had identical PFGE and RAPD profiles. Only one O139 strain (PL-4) had unique RAPD and PFGE profile among all the 26 V. cholerae strains used in this study.

CONCLUSION

Apart from serology, the strains of V. cholerae can be discriminated by this conventional phage typing system that offers the basic information on identification, biotyping and discrimination of strains. But, a high level of heterogeneity was observed in RAPD and PFGE profiles indicating the clonal diversity of V. cholerae O1 and O139 strains. It was concluded that these strains were phenotypically identical through phage typing system and antibiogram but genetically dissimilar, as shown in molecular typing systems.

DNA fingerprinting ofVibrio cholerae andAeromonas species by pulsed-field minigel electrophoresis

DNA molecules of Vibrio cholerae and Aeromonas species were prepared by incubating immobilized cells for 4 and 2 h, respectively, with a nonenzymatic solution that contains chemical reagents only (NDSUPlus). This method gave results as reproducible as the enzymatic one that uses proteinase K, and rendered DNA molecules suitable for fingerprinting by mini-CHEF electrophoresis. As rapid DNA separations at high electric field are achieved in mini-CHEF chamber with low heat evolution, DNA restriction fragments were separated in 5 h at 10 V/cm in a single resolution window. Then, fragment separations in three resolution windows were done in 15 h. This time is shorter than the one needed by the large CHEF chamber for resolving fragments in a single resolution window. Three windows permitted to include larger numbers of restriction fragments in the calculation of isolate similarities. Both sample preparation and mini-CHEF electrophoresis may represent an alternative for performing massive epidemiological studies of V. cholerae and Aeromonas species.

Development and validation of a PulseNet standardized pulsed-field gel electrophoresis protocol for subtyping of Vibrio cholerae

PulseNet is a network that utilizes standardized pulsed-field gel electrophoresis (PFGE) protocols with the purpose of conducting laboratory-based surveillance of foodborne pathogens. PulseNet standardized PFGE protocols are subject to rigorous testing during the developmental phase and careful evaluation during a validation process assessing its robustness and reproducibility in different laboratories. Here we describe the development and validation of a rapid PFGE protocol for subtyping Vibrio cholerae for use in PulseNet International activities. While the protocol was derived from the existing PulseNet protocol for Escherichia coli O157, various aspects of this protocol were optimized for use with V. cholerae, most notably a change of the primary and secondary restriction enzyme to SfiI and NotI, respectively, and the use of a two-block electrophoresis program. External validation of this protocol was undertaken through a collaboration between three PulseNet Asia Pacific laboratories (Public Health Laboratory Centre, Hong Kong, National Institute of Infectious Diseases, Japan, and International Center for Diarrhoeal Diseases Research-Bangladesh) and PulseNet USA. Comparison of PFGE patterns generated by each of the participating laboratories demonstrated that the protocol is robust and reproducible.