Vibrio parahaemolyticus diarrhea, Chile, 1998 and 2004

Plasmid-related quinolone resistance determinants in epidemic Vibrio parahaemolyticus, uropathogenic Escherichia coli, and marine bacteria from an aquaculture area in Chile

Marine bacteria from aquaculture areas with industrial use of quinolones have the potential to pass quinolone resistance genes to animal and human pathogens. The VPA0095 gene, related to the quinolone resistance determinant qnrA, from clinical isolates of epidemic Vibrio parahaemolyticus conferred reduced susceptibility to quinolone after cloning into Escherichia coli K-12 either when acting alone or synergistically with DNA gyrase mutations. In addition, a plasmid-mediated quinolone resistance gene from marine bacteria, aac(6')-Ib-cr, was identical to aac(6')-Ib-cr from urinary tract isolates of E. coli, suggesting a recent flow of this gene between these bacteria isolated from different environments. aac(6')-Ib-cr from E. coli also conferred reduced susceptibility to quinolone and kanamycin when cloned into E. coli K-12.

Serotype, virulence, and genetic traits of foodborne and clinical Vibrio parahaemolyticus isolates in Shanghai, China

Vibrio parahaemolyticus is a major foodborne pathogen in China and other countries. In this study, a total of 578 clinical V. parahaemolyticus strains and 51 foodborne strains were isolated during the period from 2009 to 2011 in the eastern coastal city of Shanghai, China. Their serotypes, virulence genes, pandemic traits, and genotyping were investigated. A total of nine O groups and 20 K types were identified by serological analysis of all isolates. Six different O groups and 14 different K types were detected among the 578 clinical strains. Eight different O groups and five K types were detected among the 51 foodborne strains. The O3:K6 serotype was the dominant serotype. A total of 200 representative clinical strains and 51 foodborne isolates were analyzed for virulence genes, pandemic traits, and genotyping. Of the clinical strains, 92.5% had the virulence genes tdh and/or trh. Four foodborne isolates had virulence genes; one trh-positive strain was O3:K6 and three tdh-positive strains were either O4:KUT or O3:KUT. Molecular typing by pulsed-field gel electrophoresis also showed divergence among the nonpandemic strains, although the pandemic strains formed a cluster. These results suggest high serodiversity and genetic diversity of V. parahaemolyticus. Pathogenic isolates were present in food, thus representing a public health risk and warranting epidemiological and ecological monitoring to ensure safety.

Pandemic Vibrio parahaemolyticus O3:K6 on the American continent

Vibrio parahaemolyticus is one of the most important seafood-borne bacterial in recent years and is the leading causal agent of human acute gastroenteritis, primarily following the consumption of raw, undercooked or mishandled marine products. Until 1996, infections caused by V. parahaemolyticus were generally associated with diverse serovars. However, in February 1996, a unique serovar (O3:K6) of V. parahaemolyticus with specific genetic markers (tdh, toxRS/New and/or orf8) appeared abruptly in Kolkata, India. In subsequent years, O3:K6 isolates similar to those isolated in Kolkata have been reported from food borne outbreaks in Southeast Asia, as well as in the Atlantic and Gulf coasts of the United States (U.S). More recently, there have been reports in Europe, Africa and Central and South America. Specifically, in the American continent, some countries have reported cases of gastroenteritis due to the pandemic O3:K6 strain and its serovariants; the pandemic strain was first detected in Peru (1996, >100 cases), subsequently spreading to Chile in 1998 (>16,804 human cases), to the U.S. in 1998 (>700 cases), to Brazil in 2001 (>18 cases) and to Mexico in 2004 (>1200 cases). The arrival of the pandemic clone on the American continent may have resulted in a significant shift on the epidemic dynamics of V. parahaemolyticus. However, although O3:K6 is the predominant serovar of the recognized clinical strains in some countries in the Americas, a decrease in clinical cases caused by O3:K6 and an increase in cases associated with a new serotype (O3:K59, Chile) have been recently reported. The emergence and worldwide dissemination of O3:K6 and other pandemic strains since 1996 have come to represent a threat to public health and should concern health authorities. This review focuses on the presence, distribution and virulence factors of the V. parahaemolyticus O3:K6 pandemic clone and its serovariants in clinical and environmental strains on the American continent.

Characterization of all RND-type multidrug efflux transporters in Vibrio parahaemolyticus

Resistance nodulation cell division (RND)-type efflux transporters play the main role in intrinsic resistance to various antimicrobial agents in many gram-negative bacteria. Here, we estimated 12 RND-type efflux transporter genes in Vibrio parahaemolyticus. Because VmeAB has already been characterized, we cloned the other 11 RND-type efflux transporter genes and characterized them in Escherichia coli KAM33 cells, a drug hypersusceptible strain. KAM33 expressing either VmeCD, VmeEF, or VmeYZ showed increased minimum inhibitory concentrations (MICs) for several antimicrobial agents. Additional four RND-type transporters were functional as efflux pumps only when co-expressed with VpoC, an outer membrane component in V. parahaemolyticus. Furthermore, VmeCD, VmeEF, and VmeYZ co-expressed with VpoC exhibited a broader substrate specificity and conferred higher resistance than that with TolC of E. coli. Deletion mutants of these transporter genes were constructed in V. parahaemolyticus. TM32 (ΔvmeAB and ΔvmeCD) had significantly decreased MICs for many antimicrobial agents and the number of viable cells after exposure to deoxycholate were markedly reduced. Strains in which 12 operons were all disrupted had very low MICs and much lower fluid accumulation in rabbit ileal loops. These results indicate that resistance nodulation cell division-type efflux transporters contribute not only to intrinsic resistance but also to exerting the virulence of V. parahaemolyticus.

Contamination by Vibrio parahaemolyticus and Its Virulent Strains in Seafood Marketed in Thailand, Vietnam, Malaysia, and Indonesia

Infections by virulent strains of Vibrio parahaemolyticus are frequently reported in Southeast Asia. This is due to the frequent seafood contamination by virulent strains. In this study conducted from 2008 to 2011, seafood like fish, shrimp, squid, crab, and molluscan shellfish were purchased from provinces in Thailand and three Southeast Asian countries and examined for the prevalence of three genetic markers of V. parahaemolyticus (species-specific gene: toxR gene, virulence genes: tdh and trh genes). An enrichment culture of seafood was examined for these markers using PCR methods. Molluscan shellfish showed a high frequency of contamination in Thailand. The shellfish harvested from the Gulf of Thailand were significantly more contaminated with virulence genes than those from the Andaman Sea. The seafood purchased from three Southeast Asian countries was positive for the three markers of V. parahaemolytcus at differing frequencies. The virulence markers (tdh and trh markers) were frequently detected in molluscan shellfish from Vietnam (17.9 and 8.0%, respectively), Malaysia (11.1 and 16.7%), and Indonesia (9.1 and 13.6%). These data suggest that the molluscan shellfish sold in Southeast Asian markets are highly contaminated with virulent strains of V. parahaemolyticus.

Molecular epidemiology and genetic variation of pathogenic Vibrio parahaemolyticus in Peru

Vibrio parahaemolyticus is a foodborne pathogen that has become a public health concern at the global scale. The epidemiological significance of V. parahaemolyticus infections in Latin America received little attention until the winter of 1997 when cases related to the pandemic clone were detected in the region, changing the epidemic dynamics of this pathogen in Peru. With the aim to assess the impact of the arrival of the pandemic clone on local populations of pathogenic V. parahaemolyticus in Peru, we investigated the population genetics and genomic variation in a complete collection of non-pandemic strains recovered from clinical sources in Peru during the pre- and post-emergence periods of the pandemic clone. A total of 56 clinical strains isolated in Peru during the period 1994 to 2007, 13 strains from Chile and 20 strains from Asia were characterized by Multilocus Sequence Typing (MLST) and checked for the presence of Variable Genomic Regions (VGRs). The emergence of O3:K6 cases in Peru implied a drastic disruption of the seasonal dynamics of infections and a shift in the serotype dominance of pathogenic V. parahaemolyticus. After the arrival of the pandemic clone, a great diversity of serovars not previously reported was detected in the country, which supports the introduction of additional populations cohabitating with the pandemic group. Moreover, the presence of genomic regions characteristic of the pandemic clone in other non-pandemic strains may represent early evidence of genetic transfer from the introduced population to the local communities. Finally, the results of this study stress the importance of population admixture, horizontal genetic transfer and homologous recombination as major events shaping the structure and diversity of pathogenic V. parahaemolyticus.

Ocean warming and spread of pathogenic vibrios in the aquatic environment

Vibrios are among the most common bacteria that inhabit surface waters throughout the world and are responsible for a number of severe infections both in humans and animals. Several reports recently showed that human Vibrio illnesses are increasing worldwide including fatal acute diarrheal diseases, such as cholera, gastroenteritis, wound infections, and septicemia. Many scientists believe this increase may be associated with global warming and rise in sea surface temperature (SST), although not enough evidence is available to support a causal link between emergence of Vibrio infections and climate warming. The effect of increased SST in promoting spread of vibrios in coastal and brackish waters is considered a causal factor explaining this trend. Field and laboratory studies carried out over the past 40 years supported this hypothesis, clearly showing temperature promotes Vibrio growth and persistence in the aquatic environment. Most recently, a long-term retrospective microbiological study carried out in the coastal waters of the southern North Sea provided the first experimental evidence for a positive and significant relationship between SST and Vibrio occurrence over a multidecadal time scale. As a future challenge, macroecological studies of the effects of ocean warming on Vibrio persistence and spread in the aquatic environment over large spatial and temporal scales would conclusively support evidence acquired to date combined with studies of the impact of global warming on epidemiologically relevant variables, such as host susceptibility and exposure. Assessing a causal link between ongoing climate change and enhanced growth and spread of vibrios and related illness is expected to improve forecast and mitigate future outbreaks associated with these pathogens.

Genetic and phylogenetic evidence for horizontal gene transfer among ecologically disparate groups of marine Vibrio

Vibrio represents a diverse bacterial genus found in different niches of the marine environment, including numerous genera of marine sponges (phylum Porifera), inhabiting different depths and regions of benthic seas, that are potentially important in driving adaptive change among Vibrio spp. Using 16S rRNA gene sequencing, a previous study showed that sponge-derived (SD) vibrios clustered with their mainstream counterparts present in shallow, coastal ecosystems, suggesting a genetic relatedness between these populations. Sequences from the topA, ftsZ, mreB, rpoD, rctB and toxR genes were used to investigate the degree of relatedness existing between these two separate populations by examining their phylogenetic and genetic disparity. Phylogenies were constructed from the concatenated sequences of the six housekeeping genes using maximum-parsimony, maximum-likelihood and neighbour-joining algorithms. Genetic recombination was evaluated using the incongruence length difference test, Split decomposition and measuring overall compatibility of sites. This combined technical approach provided evidence that SD Vibrio strains are largely genetically homologous to their shallow-water counterparts. Moreover, the analyses conducted support the existence of extensive horizontal gene transfer between these two groups, supporting the idea of a single panmictic population structure among vibrios from two seemingly distinct, marine environments.

Multi-Probe-Fluorescence in situ Hybridization for the Rapid Enumeration of Viable Vibrio parahaemolyticus

A one-step multi-probe FISH method of detecting viable Vibrio parahaemolyticus was developed. Three candidate regions, corresponding to Helix 440+441, Helix 588, and Helix 1241 in 16S rRNA, were selected for detection, the thermodynamic parameters (ΔG(overall)) of the probes were optimized, and VP437, VP612 and VP1253, whose fluorescence were 1.7 to 11.3 times that of ΔG(overall)-unadjusted sequences, were designed. The addition of competitive oligonucleotides to reactions with VP612 and VP1253 strengthened the specificity of the probes. The three probes were labeled with FITC, TAMRA, and Cy5, respectively, and using a mixture of the probes and six competitive oligonucleotides, one-step FISH was applied to the species-specific detection of V. parahaemolyticus including epidemic strains of O3:K6 and O4:K68 serotypes. V. alginolyticus, V. rotiferianus, and V. campbellii were not detected in the reaction. Microcolonies (30-80 μm in diameter) of V. parahaemolyticus were observed within 6 hours at 37°C on seawater agar plates in both fresh and heat-damaged V. parahaemolyticus. Viable bacterial counts based on the proposed method were significantly different from those measured with typical vibrio selective media (CHROMagar Vibrio and TCBS).

Ecology of Vibrio parahaemolyticus and Vibrio vulnificus in the coastal and estuarine waters of Louisiana, Maryland, Mississippi, and Washington (United States)

Vibrio parahaemolyticus and Vibrio vulnificus, which are native to estuaries globally, are agents of seafood-borne or wound infections, both potentially fatal. Like all vibrios autochthonous to coastal regions, their abundance varies with changes in environmental parameters. Sea surface temperature (SST), sea surface height (SSH), and chlorophyll have been shown to be predictors of zooplankton and thus factors linked to vibrio populations. The contribution of salinity, conductivity, turbidity, and dissolved organic carbon to the incidence and distribution of Vibrio spp. has also been reported. Here, a multicoastal, 21-month study was conducted to determine relationships between environmental parameters and V. parahaemolyticus and V. vulnificus populations in water, oysters, and sediment in three coastal areas of the United States. Because ecologically unique sites were included in the study, it was possible to analyze individual parameters over wide ranges. Molecular methods were used to detect genes for thermolabile hemolysin (tlh), thermostable direct hemolysin (tdh), and tdh-related hemolysin (trh) as indicators of V. parahaemolyticus and the hemolysin gene vvhA for V. vulnificus. SST and suspended particulate matter were found to be strong predictors of total and potentially pathogenic V. parahaemolyticus and V. vulnificus. Other predictors included chlorophyll a, salinity, and dissolved organic carbon. For the ecologically unique sites included in the study, SST was confirmed as an effective predictor of annual variation in vibrio abundance, with other parameters explaining a portion of the variation not attributable to SST.

Characteristics of a sharp decrease in Vibrio parahaemolyticus infections and seafood contamination in Japan

Vibrio parahaemolyticus has been one of the most important foodborne pathogens in Japan since the 1960s, and a large epidemic was caused by the pandemic serotype O3:K6 from 1997 to 2001. V. parahaemolyticus infections, however, have sharply declined since that time. Data on serotypes isolated from 977 outbreaks were collected and analysed. Total and pathogenic, thermostable direct hemolysin (TDH) gene-positive V. parahaemolyticus were qualitatively and quantitatively detected in 842 seafood samples from wholesale markets in 2007-2009. Strains isolated from patients and seafood were analysed by serotyping, tdh-PCR, group-specific PCR for pandemic strains, and pulsed-field gel electrophoresis (PFGE). The sharp decrease in the infections from 1999 onwards was noted not only for O3:K6 infections but also for other serotypes. The change in the seafood contamination situation from 2001 to 2007-2009 was characterised by a decrease to three-fourths in the frequency of tdh-positive samples, although that decrease was small compared to the 18-fold decrease in the cases of V. parahaemolyticus outbreaks. PFGE detected the pandemic O3:K6 serotype in the same profile in seafood and patients from 1998 to the present. Because of no large decrease in seafood contamination by V. parahaemolyticus from the production to distribution stages and the presence of pandemic O3:K6 serotype in seafood to the present, it was suggested that the change of seafood contamination was unrelated to the sharp decrease in V. parahaemolyticus infections. V. parahaemolyticus infections might be prevented at the stages after the distribution stage.

Bacteriostatic anti-Vibrio parahaemolyticus activity of Pseudoalteromonas sp. strains DIT09, DIT44 and DIT46 isolated from Southern Chilean intertidal Perumytilus purpuratus

We characterised the anti-Vibrio parahaemolyticus (anti-V. parahaemolyticus) marine bacteria DIT09, DIT44 and DIT46 isolated from the intertidal mussel Perumytilus purpuratus. The 16S rRNA gene sequences identify a Pseudoalteromonas sp. that form a clade with P. prydzensis and P. mariniglutinosa. The strains produced bacteriostatic anti-V. parahaemolyticus agents during the exponential growth phase, which were also active against V. cholerae and V. anguillarum, but not on other Gram positive and Gram negative bacteria. Bacteriostatic agents could be permeated by analytic ultra-filtration with 3.5 kDa cut-off, partially precipitated with 70 and 90 % ammonium sulphate, but not extracted with ethyl acetate. Reverse-phase HPLC revealed the production of a set of 5-6 active compounds by each strain (elution from 20 to 40 % acetonitrile), with similar but non identical HPLC patterns. Additionally, V. parahaemolyticus was able to progressively overcome the inhibition of antibiotics in trypticase soy agar with Fe(III) 0.5 up to 2 mM, suggesting the involvement of a set of novel siderophore or active molecules targeted at different Fe-siderophore uptake systems. The overall findings suggest that Pseudoalteromonas sp. DIT strains produce a putatively novel class of bacteriostatic and probably amphiphilic anti-Vibrio agents, indicating the need for further studies with chemical purification followed by their structural and functional characterization. Finally, the crude cell-free extracts, as well as the strains incubated at 10(3) and 10(5) c.f.u./mL, did not cause mortality in Artemia franciscana nauplii, suggesting that these bacteria are serious candidates for further probiotic evaluations with shellfish and fish cultures.

Characterization of Vibrio parahaemolyticus isolated from oysters and mussels in São Paulo, Brazil

Vibrio parahaemolyticus is a marine bacterium, responsible for gastroenteritis in humans. Most of the clinical isolates produce thermostable direct hemolysin (TDH) and TDH-related hemolysin (TRH) encoded by tdh and trh genes respectively. In this study, twenty-three V. parahaemolyticus, previously isolated from oysters and mussels were analyzed by PCR using specific primers for the 16S rRNA and virulence genes (tdh, trh and tlh) and for resistance to different classes of antibiotics and PFGE. Nineteen isolates were confirmed by PCR as V. parahaemolyticus. The tlh gene was present in 100% of isolates, the tdh gene was identified in two (10.5%) isolates, whereas the gene trh was not detected. Each isolate was resistant to at least one of the nine antimicrobials tested. Additionally, all isolates possessed the blaTEM-116 gene. The presence of this gene in V. parahaemolyticus indicates the possibility of spreading this gene in the environment. Atypical strains of V. parahaemolyticus were also detected in this study.

Microevolution of pandemic Vibrio parahaemolyticus assessed by the number of repeat units in short sequence tandem repeat regions

The emergence of the pandemic strain Vibrio parahaemolyticus O3:K6 in 1996 caused a large increase of diarrhea outbreaks related to seafood consumption in Southeast Asia, and later worldwide. Isolates of this strain constitutes a clonal complex, and their effectual differentiation is possible by comparison of their variable number tandem repeats (VNTRs). The differentiation of the isolates by the differences in VNTRs will allow inferring the population dynamics and microevolution of this strain but this requires knowing the rate and mechanism of VNTRs' variation. Our study of mutants obtained after serial cultivation of clones showed that mutation rates of the six VNTRs examined are on the order of 10(-4) mutant per generation and that difference increases by stepwise addition of single mutations. The single stepwise mutation (SSM) was deduced because mutants with 1, 2, 3, or more repeat unit deletions or insertions follow a geometric distribution. Plausible phylogenetic trees are obtained when, according to SSM, the genetic distance between clusters with different number of repeats is assessed by the absolute differences in repeats. Using this approach, mutants originated from different isolates of pandemic V. parahaemolyticus after serial cultivation are clustered with their parental isolates. Additionally, isolates of pandemic V. parahaemolyticus from Southeast Asia, Tokyo, and northern and southern Chile are clustered according their geographical origin. The deepest split in these four populations is observed between the Tokyo and southern Chile populations. We conclude that proper phylogenetic relations and successful tracing of pandemic V. parahaemolyticus requires measuring the differences between isolates by the absolute number of repeats in the VNTRs considered.

Association of pandemic Vibrio parahaemolyticus O3:K6 present in the coastal environment of Northwest Mexico with cases of recurrent diarrhea between 2004 and 2010

In 2004, more than 1,230 cases of gastroenteritis due to pandemic O3:K6 strains of Vibrio parahaemolyticus were reported in southern Sinaloa, a state in Northwestern Mexico. Recurrent sporadic cases arose from 2004 to 2010, spreading from the south to the north. In the present study, Vibrio parahaemolyticus was detected in both environmental samples and clinical cases along the Pacific coast of Sinaloa during 2004 to 2010. An evaluation was made of the serotypes, distribution of virulence genes, and presence of pandemic O3:K6 strains. A total of 144 strains were isolated from environmental samples (from sediment, seawater, and shrimp), and 154 clinical strains were isolated. A total of 10 O serogroups and 30 serovars were identified in the strains. Environmental strains (n = 144) belonged to 10 O serogroups and 28 serovars, while clinical strains (n = 154) belonged to 8 O serogroups and 14 serovars. Ten serovars were shared by both environmental and clinical strains. Among 144 environmental isolates, 4.1% (6/144) belonged to the pandemic clone, with 83.3% containing the orf8 gene and with O3:K6 accounting for 67%. On the other hand, pathogenic strains (tdh and/or trh) accounted for 52% (75/144) of the environmental isolates. Interestingly, among 154 clinical isolates, 80.5% (124/154) were pandemic strains, with O3:K6 (tdh, toxRS(new), and orf8) representing the predominant serovar (99.2%, 123/124). Overall, our results indicate that in spite of a high serodiversity and prevalence of pathogenic Vibrio parahaemolyticus in the environment, the pandemic strain O3:K6 caused >79% of reported cases between 2004 and 2010 in Sinaloa, Mexico.

Visual analytics of surveillance data on foodborne vibriosis, United States, 1973-2010

Foodborne illnesses caused by microbial and chemical contaminants in food are a substantial health burden worldwide. In 2007, human vibriosis (non-cholera Vibrio infections) became a notifiable disease in the United States. In addition, Vibrio species are among the 31 major known pathogens transmitted through food in the United States. Diverse surveillance systems for foodborne pathogens also track outbreaks, illnesses, hospitalization and deaths due to non-cholera vibrios. Considering the recognition of vibriosis as a notifiable disease in the United States and the availability of diverse surveillance systems, there is a need for the development of easily deployed visualization and analysis approaches that can combine diverse data sources in an interactive manner. Current efforts to address this need are still limited. Visual analytics is an iterative process conducted via visual interfaces that involves collecting information, data preprocessing, knowledge representation, interaction, and decision making. We have utilized public domain outbreak and surveillance data sources covering 1973 to 2010, as well as visual analytics software to demonstrate integrated and interactive visualizations of data on foodborne outbreaks and surveillance of Vibrio species. Through the data visualization, we were able to identify unique patterns and/or novel relationships within and across datasets regarding (i) causative agent; (ii) foodborne outbreaks and illness per state; (iii) location of infection; (iv) vehicle (food) of infection; (v) anatomical site of isolation of Vibrio species; (vi) patients and complications of vibriosis; (vii) incidence of laboratory-confirmed vibriosis and V. parahaemolyticus outbreaks. The additional use of emerging visual analytics approaches for interaction with data on vibriosis, including non-foodborne related disease, can guide disease control and prevention as well as ongoing outbreak investigations.

Genome sequence of the clinical O4:K12 serotype Vibrio parahaemolyticus strain 10329

Vibrio parahaemolyticus is the leading cause of food-borne illnesses worldwide. Here, we report a draft genome of V. parahaemolyticus strain 10329 of the O4:K12 serotype. It belongs to the main U.S. West Coast clonal complex of V. parahaemolyticus (sequence type 36 [ST36]) causing oyster-associated human illness. It contains the virulence determinants tdh and trh but appears to infect at much lower doses than V. parahaemolyticus strains with these same determinants from other areas, such as the U.S. Gulf and Atlantic coasts.

Origins and colonization history of pandemic Vibrio parahaemolyticus in South America

The dynamics of dissemination of the environmental human pathogen Vibrio parahaemolyticus are uncertain. The O3:K6 clone was restricted to Asia until its detection along the Peruvian coasts and in northern Chile in 1997 in phase with the arrival of El Niño waters. A subsequent emergence of O3:K6 strains was detected in austral Chile in 2004. The origin of these 1997 and 2004 population radiations has not yet been conclusively determined. Multiple loci VNTR analysis using seven polymorphic loci was carried out with a number of representative strains from Asia, Peru and Chile to determine their genetic characteristics and population structure. Asian and Chilean subpopulations were the most genetically distant groups with an intermediate subpopulation in Peru. Population structure inferred from a minimum-spanning tree and Bayesian analysis divided the populations into two genetically distinct groups, consistent with the epidemic dynamics of the O3:K6 clone in South America. One group comprised strains from the original Asiatic population and strains arriving in Peru and Chile in 1997. The second group included the remaining Peruvian Strains and Chilean strains obtained from Puerto Montt in 2004. The analysis of the arrival of the O3:K6 clone at the Pacific coasts of South America has provided novel insights linking the origin of the invasion in 1997 to Asian populations and describing the successful establishment of the O3:K6 populations, first in Peru and subsequently in the South of Chile owing to a possible radiation of Peruvian populations.

Modulation of responses of Vibrio parahaemolyticus O3:K6 to pH and temperature stresses by growth at different salt concentrations

Vibrio parahaemolyticus inhabits marine, brackish, and estuarine waters worldwide, where fluctuations in salinity pose a constant challenge to the osmotic stress response of the organism. Vibrio parahaemolyticus is a moderate halophile, having an absolute requirement for salt for survival, and is capable of growth at 1 to 9% NaCl. It is the leading cause of seafood-related bacterial gastroenteritis in the United States and much of Asia. We determined whether growth in differing NaCl concentrations alters the susceptibility of V. parahaemolyticus O3:K6 to other environmental stresses. Vibrio parahaemolyticus was grown at a 1% or 3% NaCl concentration, and the growth and survival of the organism were examined under acid or temperature stress conditions. Growth of V. parahaemolyticus in 3% NaCl versus that in 1% NaCl increased survival under both inorganic (HCl) and organic (acetic acid) acid conditions. In addition, at 42 degrees C and -20 degrees C, 1% NaCl had a detrimental effect on growth. The expression of lysine decarboxylase (encoded by cadA), the organism's main acid stress response system, was induced by both NaCl and acid conditions. To begin to address the mechanism of regulation of the stress response, we constructed a knockout mutation in rpoS, which encodes the alternative stress sigma factor, and in toxRS, a two-component regulator common to many Vibrio species. Both mutant strains had significantly reduced survival under acid stress conditions. The effect of V. parahaemolyticus growth in 1% or 3% NaCl was examined using a cytotoxicity assay, and we found that V. parahaemolyticus grown in 1% NaCl was significantly more toxic than that grown in 3% NaCl.

Serodiversity and ecological distribution of Vibrio parahaemolyticus in the Venetian Lagoon, Northeast Italy

Vibrio parahaemolyticus is a natural inhabitant of estuarine and marine environments constituting part of the autochthonous microflora. This species is associated with human gastroenteritis caused by ingestion of contaminated water and undercooked seafood. During the past several years, the number of V. parahaemolyticus gastroenteritis cases have increased worldwide, causing over half of all food-poisoning outbreaks of bacterial origin. Vibrio populations in water are known to be influenced by environmental factors. Notably, it has been shown that in different parts of the world the distribution of V. parahaemolyticus in the marine environment is related to the water temperature. In this study, we identified environmental determinants affecting distribution of V. parahaemolyticus in the Venetian Lagoon, in the Italian North Adriatic Sea. Data obtained revealed that sea surface temperature constitutes the key factor influencing occurrence of V. parahaemolyticus, but salinity and chlorophyll concentration are also important. Serotyping of a collection of V. parahaemolyticus environmental isolates revealed high serodiversity, with serotypes O3:KUT and O1:KUT, belonging to the 'pandemic group', occurring with higher frequency. From our results, we conclude that there is no correlation between serotype and specific geographic site or season of the year. However, certain serotypes were isolated in the Lagoon during the entire 18 months of the study, strongly suggesting persistence in this environment.

Prevalence and genetic diversity of pathogenic populations of Vibrio parahaemolyticus in coastal waters of Galicia, Spain

The natural reservoirs and biological characteristics of pathogenic populations of Vibrio parahaemolyticus in marine habitats remain unclear due to difficulties in obtaining pathogenic strains from the environment. The distribution and characteristics of pathogenic V. parahaemolyticus were investigated over 1 year in three coastal environments in Galicia (Spain), including areas of the major international ports in the region. Vibrio parahaemolyticus was present in 35.3% of the samples analysed, and 535 strains were isolated over the period of study. Virulence genes were detected in 94 strains with diverse genetic traits: 66 trh+/tdh-, 24 trh-/tdh+ and 4 trh+/tdh+. Different spatial and seasonal patterns were observed in relation to genetic traits. The trh+/tdh- strains were detected exclusively in northern areas and prevailed in the autumn, when seawater is warmer and less saline, whereas the trh-/tdh+ strains were found in all three areas throughout winter and spring. Characterization of potentially pathogenic strains from the environment revealed an unexpectedly diverse array of serotypes and pulsed-field gel electrophoresis (PFGE) profiles (pulsotypes) that were unrelated to clinical strains of V. parahaemolyticus that are prevalent in Spain. The results of the current study provide a novel view of V. parahaemolyticus in Europe, in which diverse pathogenic groups are constitutive components of the environmental populations in coastal habitats.

Environmental occurrence and clinical impact of Vibrio vulnificus and Vibrio parahaemolyticus: a European perspective

Vibrio vulnificus and Vibrio parahaemolyticus are ubiquitous Gram-negative bacterial pathogens found naturally in marine and estuarine waters, and are a leading cause of seafood-associated bacterial illness. These pathogens are commonly reported in the USA and in many Asian countries, including China, Japan and Taiwan; however, there is growing concern that V. vulnificus and V. parahaemolyticus may represent an important and increasing clinical problem in Europe. Several factors underlie the need for a greater understanding of these non-cholera vibrios within a European context. First, there is a growing body of evidence to suggest that V. vulnificus and V. parahaemolyticus infections are increasing, and tend to follow regional climatic trends, with outbreaks typically following episodes of unusually warm weather. Such findings are especially alarming given current predictions regarding warming of marine waters as a result of global climatic change. Second, a myriad of epidemiological factors may greatly increase the incidence as well as clinical burden of these pathogens - including increasing global consumption and trade of seafood produce coupled to an increase in the number of susceptible individuals consuming seafood produce. Finally, there is currently a lack of detailed surveillance information regarding non-cholerae Vibrio infections in Europe, as these pathogens are not notifiable in many countries, which probably masks the true clinical burden of many human infections. This review will present a pertinent overview of both the environmental occurrence and clinical impact of V. vulnificus and V. parahaemolyticus in Europe.

Correlation between environmental factors and prevalence of Vibrio parahaemolyticus in oysters harvested in the southern coastal area of Sao Paulo State, Brazil

The presence of Vibrio parahaemolyticus in 123 oyster samples collected from an estuary on the southern coast of Sao Paulo state, Brazil, was investigated. Of the 123 samples, 99.2% were positive with densities ranging from < 3 to 10(5) most probable number (MPN)/g. Densities correlated significantly with water temperature (r = 0.48; P < 0.001) but not with salinity (r = -0.09; P = 0.34). The effect of harvest site on counts was not significant (P > 0.05). These data provide information for the assessment of exposure of V. parahaemolyticus in oysters at harvest.

Dynamics of clinical and environmental Vibrio parahaemolyticus strains during seafood-related summer diarrhea outbreaks in southern Chile

Seafood consumption-related diarrhea became prevalent in Chile when the pandemic strain of Vibrio parahaemolyticus serotype O3:K6 reached a region in the south of Chile (Region de los Lagos) where approximately 80% of the country's seafood is produced. In spite of the large outbreaks of clinical infection, the load of V. parahaemolyticus in shellfish of this region is relatively low. The pandemic strain constitutes a small but relatively stable group of a diverse V. parahaemolyticus population, composed of at least 28 genetic groups. Outbreaks in Region de los Lagos began in 2004 and reached a peak in 2005 with 3,725 clinical cases, all associated with the pandemic strain. After 2005, reported cases steadily decreased to a total of 477 cases in 2007. At that time, 40% of the clinical cases were associated with a pandemic strain of a different serotype (O3:K59), and 27% were related to V. parahaemolyticus isolates unrelated to the pandemic strain. In the results published here, we report that in the summer of 2008, when reported cases unexpectedly increased from 477 to 1,143, 98% of the clinical cases were associated with the pandemic strain serotype O3:K6, a change from 2007. Nevertheless, in 2009, when clinical cases decreased to 441, only 64% were related to the pandemic strain; the remaining cases were related to a nonpandemic tdh- and trh-negative strain first identified in shellfish in 2006. Overall, our observations indicate that the pandemic strain has become a relatively stable subpopulation and that when the number of diarrhea cases related to the pandemic strain is low, previously undetected V. parahaemolyticus pathogenic strains become evident.

Serogroup, virulence, and genetic traits of Vibrio parahaemolyticus in the estuarine ecosystem of Bangladesh

Forty-two strains of Vibrio parahaemolyticus were isolated from Bay of Bengal estuaries and, with two clinical strains, analyzed for virulence, phenotypic, and molecular traits. Serological analysis indicated O8, O3, O1, and K21 to be the major O and K serogroups, respectively, and O8:K21, O1:KUT, and O3:KUT to be predominant. The K antigen(s) was untypeable, and pandemic serogroup O3:K6 was not detected. The presence of genes toxR and tlh were confirmed by PCR in all but two strains, which also lacked toxR. A total of 18 (41%) strains possessed the virulence gene encoding thermostable direct hemolysin (TDH), and one had the TDH-related hemolysin (trh) gene, but not tdh. Ten (23%) strains exhibited Kanagawa phenomenon that surrogates virulence, of which six, including the two clinical strains, possessed tdh. Of the 18 tdh-positive strains, 17 (94%), including the two clinical strains, had the seromarker O8:K21, one was O9:KUT, and the single trh-positive strain was O1:KUT. None had the group-specific or ORF8 pandemic marker gene. DNA fingerprinting employing pulsed-field gel electrophoresis (PFGE) of SfiI-digested DNA and cluster analysis showed divergence among the strains. Dendrograms constructed using PFGE (SfiI) images from a soft database, including those of pandemic and nonpandemic strains of diverse geographic origin, however, showed that local strains formed a cluster, i.e., "clonal cluster," as did pandemic strains of diverse origin. The demonstrated prevalence of tdh-positive and diarrheagenic serogroup O8:K21 strains in coastal villages of Bangladesh indicates a significant human health risk for inhabitants.

The linear plasmid prophage Vp58.5 of Vibrio parahaemolyticus is closely related to the integrating phage VHML and constitutes a new incompatibility group of telomere phages

Vibrio parahaemolyticus O3:K6 pandemic strains recovered in Chile frequently possess a 42-kb plasmid which is the prophage of a myovirus. We studied the prototype phage VP58.5 and show that it does not integrate into the host cell chromosome but replicates as a linear plasmid (Vp58.5) with covalently closed ends (telomeres). The Vp58.5 replicon coexists with other plasmid prophages (N15, PY54, and PhiKO2) in the same cell and thus belongs to a new incompatibility group of telomere phages. We determined the complete nucleotide sequence (42,612 nucleotides) of the VP58.5 phage DNA and compared it with that of the plasmid prophage. The two molecules share the same nucleotide sequence but are 35% circularly permuted to each other. In contrast to the hairpin ends of the plasmid, VP58.5 phage DNA contains 5'-protruding ends. The VP58.5 sequence is 92% identical to the sequence of phage VHML, which was reported to integrate into the host chromosome. However, the gene order and termini of the phage DNAs are different. The VHML genome exhibits the same gene order as does the Vp58.5 plasmid. VHML phage DNA has been reported to contain terminal inverted repeats. This repetitive sequence is similar to the telomere resolution site (telRL) of VP58.5 which, after processing by the phage protelomerase, forms the hairpin ends of the Vp58.5 prophage. It is discussed why these closely related phages may be so different in terms of their genome ends and their lifestyle.

Epidemiology of Vibrio parahaemolyticus outbreaks, southern Chile

One-sentence summary for table of contents: Outbreaks are decreasing and the O3:K6 pandemic strain is being replaced by a new serotype and new strains.

Disease outbreaks caused by Vibrio parahaemolyticus in Puerto Montt, Chile, began in 2004 and reached a peak in 2005 at 3,600 clinical cases. Until 2006, every analyzed case was caused by the serovar O3:K6 pandemic strain. In the summer of 2007, only 475 cases were reported; 73% corresponded to the pandemic strain. This decrease was associated with a change in serotype of many pandemic isolates to O3:K59 and the emergence of new clinical strains. One of these strains, associated with 11% of the cases, was genotypically different from the pandemic strain but contained genes that were identical to those found on its pathogenicity island. These findings suggest that pathogenicity-related genes were laterally transferred from the pandemic strain to one of the different V. parahaemolyticus groups comprising the diverse and shifting bacterial population in shellfish in this region.

Multiple-locus variable-number tandem-repeat analysis for clonal identification of Vibrio parahaemolyticus isolates by using capillary electrophoresis

Epidemics of Vibrio parahaemolyticus in Chile have occurred since 1998. Direct genome restriction enzyme analysis (DGREA) using conventional gel electrophoresis permitted discrimination of different V. parahaemolyticus isolates obtained from these outbreaks and showed that this species consists of a highly diverse population. A multiple-locus variable-number tandem-repeat (VNTR) analysis (MLVA) approach was developed and applied to 22 clinical and 91 environmental V. parahaemolyticus isolates from Chile to understand their clonal structures. To this end, an advanced molecular technique was developed by applying multiplex PCR, fluorescent primers, and capillary electrophoresis, resulting in a high-resolution and high-throughput (HRHT) genotyping method. The genomic basis of this HRHT method was eight VNTR loci described previously by Kimura et al. (J. Microbiol. Methods 72:313-320, 2008) and two new loci which were identified by a detailed molecular study of 24 potential VNTR loci on both chromosomes. The isolates of V. parahaemolyticus belonging to the same DGREA pattern were distinguishable by the size variations in the indicative 10 VNTRs. This assay showed that these 10 VNTR loci were useful for distinguishing isolates of V. parahaemolyticus that had different DGREA patterns and also isolates that belong to the same group. Isolates that differed in their DGREA patterns showed polymorphism in their VNTR profiles. A total of 81 isolates was associated with 59 MLVA groups, providing fine-scale differentiation, even among very closely related isolates. The developed approach enables rapid and high-resolution analysis of V. parahaemolyticus with pandemic potential and provides a new surveillance tool for food-borne pathogens.

Characterization of a new plasmid-like prophage in a pandemic Vibrio parahaemolyticus O3:K6 strain

Vibrio parahaemolyticus is a common food-borne pathogen that is normally associated with seafood. In 1996, a pandemic O3:K6 strain abruptly appeared and caused the first pandemic of this pathogen to spread throughout many Asian countries, America, Europe, and Africa. The role of temperate bacteriophages in the evolution of this pathogen is of great interest. In this work, a new temperate phage, VP882, from a pandemic O3:K6 strain of V. parahaemolyticus was purified and characterized after mitomycin C induction. VP882 was a Myoviridae bacteriophage with a polyhedral head and a long rigid tail with a sheath-like structure. It infected and lysed high proportions of V. parahaemolyticus, Vibrio vulnificus, and Vibrio cholerae strains. The genome of phage VP882 was sequenced and was 38,197 bp long, and 71 putative open reading frames were identified, of which 27 were putative functional phage or bacterial genes. VP882 had a linear plasmid-like genome with a putative protelomerase gene and cohesive ends. The genome does not integrate into the host chromosome but was maintained as a plasmid in the lysogen. Analysis of the reaction sites of the protelomerases in different plasmid-like phages revealed that VP882 and PhiHAP-1 were highly similar, while N15, PhiKO2, and PY54 made up another closely related group. The presence of DNA adenine methylase and quorum-sensing transcriptional regulators in VP882 may play a specific role in this phage or regulate physiological or virulence-associated traits of the hosts. These genes may also be remnants from the bacterial chromosome following transduction.

Emergence of Asiatic Vibrio diseases in South America in phase with El Niño

BACKGROUND

The seventh pandemic of Vibrio cholerae unexpectedly reached the coast of Peru in 1991, causing an explosive emergence of infections throughout the American continents. The origin and routes of dissemination are as yet unknown. A new Vibrio epidemic arose in 1997 in South America (northern Chile) when the pandemic clone of Vibrio parahaemolyticus was for the fist time detected outside of Asia. These 2 cases were concurrent with 2 episodes of El Niño.

METHODS

We carried out a survey of records of V. parahaemolyticus infection and of strains existing in the Instituto Nacional de Salud of Peru between 1994 and 2005. Association between the El Niño event and the V. parahaemolyticus disease was analyzed through generalized additive models applied to time-series data with negative binomial response, selecting some oceanographic factors distinctive of the movement of the El Niño waters.

RESULTS

Epidemiologic data and laboratory investigations of the strains showed that V. parahaemolyticus infections caused by the pandemic clone emerged in the coasts of Peru linked to the 1997 El Niño episode. The epidemic dissemination of this clone matched the expansion and dynamics of the poleward propagation and the receding of the El Niño waters. This pattern was similar to previously reported onset of cholera epidemic in 1991.

CONCLUSIONS

These findings identify the El Niño episodes as a reliable vehicle for the introduction and propagation of Vibrio pathogens in South America. The movement of oceanic waters seems to be one of the driving forces of the spread of Vibrio diseases.

Enhancement of UV light sensitivity of a Vibrio parahaemolyticus O3:K6 pandemic strain due to natural lysogenization by a telomeric phage

The Vibrio parahaemolyticus O3:K6 pandemic clonal strain was first observed in southern Chile in 2004 and has since caused approximately 8,000 seafood-related diarrhea cases in this region. The massive proliferation of the original clonal population offers a unique opportunity to study the evolution of a bacterial pathogen in its natural environment by detection and characterization of emerging bacterial variants. Here, we describe a group of pandemic variants characterized by the presence of a 42-kb extrachromosomal DNA that can be recovered by alkaline extraction. Upon treatment with mitomycin C, these variants lyse with production of a myovirus containing DNA of equal size to the plasmid but which cannot be recovered by alkaline extraction. Plasmid and phage DNAs show similar restriction patterns corresponding to enzyme sites in a circular permutation. Sequenced regions showed 81 to 99% nucleotide similarity to bacteriophage VHML of Vibrio harveyi. Altogether these observations indicate that the 42-kb plasmid corresponds to a prophage, consisting of a linear DNA with terminal hairpins of a telomeric temperate phage with a linear genome. Bacteria containing the prophage were 7 to 15 times more sensitive to UV radiation, likely due to phage induction by UV irradiation as plasmid curing restored the original sensitivity. The enhanced UV sensitivity could have a significant role in reducing the survival and propagation capability of the V. parahaemolyticus pandemic strain in the ocean.

Evaluation of the Compact Dry VP method for screening raw seafood for total Vibrio parahaemolyticus

Compact Dry VP (CDVP) is a ready-to-use method for enumerating Vibrio parahaemolyticus in food. The presterilized plates contain a culture medium comprising peptone, NaCl, bile salts, antibiotics, chromogenic substrates, and polysaccharide gum as a cold water-soluble gelling. After diluting raw seafood samples in a phosphate-buffered saline solution, a 1-ml aliquot was inoculated onto the center of the plate and allowed to diffuse by capillary action. Blue-green colonies forming on the plates were counted after 18 to 20 h of incubation at 35 degrees C. A total of 85 V. parahaemolyticus strains (62 tdh+ strains and 23 tdh- strains) were studied for inclusivity, 81 (95.3 %) of which produced blue-green colonies. When 97 strains (14 strains of Vibrio spp., 33 strains of coliform bacteria, and 50 strains of noncoliform bacteria) were assessed for exclusivity, 10 strains of Vibrio spp. produced non-blue-green colonies, and 87 strains failed to grow. The CDVP and U.S. Food and Drug Administration Bacteriological Analytical Manual (FDA-BAM) methods were compared with the use of four different types of raw seafood that were inoculated with four different V. parahaemolyticus strains. For raw tuna and oysters, the FDA-BAM colony lift method was used, whereas the FDA-BAM most-probable-number method was used for salmon and scallop. The linear correlation coefficients between the CDVP and FDA-BAM methods were 0.99 for fresh raw tuna, 0.95 for fresh raw oysters, 0.95 for frozen raw salmon, and 0.95 for frozen raw scallops. These results suggest that the CDVP method is useful for screening raw seafood for V. parahaemolyticus.

Comparison of V. parahaemolyticus isolated from seafoods and cases of gastrointestinal disease in the UK

In this study the prevalence of Vibrio parahaemolyticus in shellfish and estuarine waters from the UK was examined using cultural and nucleic acid hybridisation approaches. Forty-nine isolates derived from environmental sources were characterised using serotyping, PCR, nucleic acid hybridisation and pulsed field gel electrophoresis (PFGE). The serotypic and molecular profiles of these isolates were compared to 20 clinical isolates, including representatives of the pandemic O3:K6 clone. Thirty percent of environmental samples were positive for V. parahaemolyticus. The tdh gene was identified in 12% of samples tested. Environmentally derived tdh+ strains were highly heterogeneous with neither association between isolates from similar origins nor seafood type. Previously uncharacterised clinical strains from UK patients with travel related V. parahaemolyticus associated gastroenteritis, were unrelated to tdh+ or tdh- environmental isolates but 2 were clonally indistinguishable from the pandemic O3:K6 strain responsible for outbreaks in Spain, Korea, Japan and Laos.

On the origin and function of an insertion element VPaI-1 specific to post-1995 pandemic Vibrio parahaemolyticus strains

Since 1995, new virulent strains of Vibrio parahaemolyticus have emerged and spread throughout the world. These "pandemic" strains have four strain specific genomic islands (GIs), which are considered to be potential factors of the pandemicity. We investigated the origin and function of 24 genes in the so-called VPaI-1, one of the four GIs, by searching homologs in various species in Bacteria and Archaea. Of these 24 genes, two are found only in Vibrio vulnificus CMCP6 and Shewanella sp. MR-7. The genomic segment (- 8 kb) encompassing the two genes shows the synteny among the three species. Since many of the Shewanella species can grow at 4 degrees C, these two genes may be candidates of adaptation to temperature stress. Further, we found a candidate for a swarming gene, which is reported as the V. cholerae virulence gene. Based on these findings, we hypothesized the emergence of pandemicity and discuss the mechanism for how these strains spread throughout the world.

Molecular analysis of the emergence of pandemic Vibrio parahaemolyticus

Background

Vibrio parahaemolyticus is abundant in the aquatic environment particularly in warmer waters and is the leading cause of seafood borne gastroenteritis worldwide. Prior to 1995, numerous V. parahaemolyticus serogroups were associated with disease, however, in that year an O3:K6 serogroup emerged in Southeast Asia causing large outbreaks and rapid hospitalizations. This new highly virulent strain is now globally disseminated.

Results

We performed a four-way BLAST analysis on the genome sequence of V. parahaemolyticus RIMD2210633, an O3:K6 isolate from Japan recovered in 1996, versus the genomes of four published Vibrio species and constructed genome BLAST atlases. We identified 24 regions, gaps in the genome atlas, of greater than 10 kb that were unique to RIMD2210633. These 24 regions included an integron, f237 phage, 2 type III secretion systems (T3SS), a type VI secretion system (T6SS) and 7 Vibrio parahaemolyticus genomic islands (VPaI-1 to VPaI-7). Comparative genomic analysis of our fifth genome, V. parahaemolyticus AQ3810, an O3:K6 isolate recovered in 1983, identified four regions unique to each V. parahaemolyticus strain. Interestingly, AQ3810 did not encode 8 of the 24 regions unique to RMID, including a T6SS, which suggests an additional virulence mechanism in RIMD2210633. The distribution of only the VPaI regions was highly variable among a collection of 42 isolates and phylogenetic analysis of these isolates show that these regions are confined to a pathogenic clade.

Conclusion

Our data show that there is considerable genomic flux in this species and that the new highly virulent clone arose from an O3:K6 isolate that acquired at least seven novel regions, which included both a T3SS and a T6SS.

Evaluation and validation of a PulseNet standardized pulsed-field gel electrophoresis protocol for subtyping Vibrio parahaemolyticus: an international multicenter collaborative study

The pandemic spread of Vibrio parahaemolyticus is an international public health issue. Because of the outbreak potential of the organism, it is critical to establish an internationally recognized molecular subtyping protocol for V. parahaemolyticus that is both rapid and robust as a means to monitor its further spread and to guide control measures in combination with epidemiologic data. Here we describe the results of a multicenter, multicountry validation of a new PulseNet International standardized V. parahaemolyticus pulsed-field gel electrophoresis (PFGE) protocol. The results are from a composite analysis of 36 well-characterized V. parahaemolyticus isolates from six participating laboratories, and the isolates represent predominant serotypes and various genotypes isolated from different geographic regions and time periods. The discriminatory power is very high, as 34 out of 36 sporadic V. parahaemolyticus strains tested fell into 34 distinguishable PFGE groups when the data obtained with two restriction enzymes (SfiI and NotI) were combined. PFGE was further able to cluster members of known pandemic serogroups. The study also identified quality measures which may affect the performance of the protocol. Nonadherence to the recommended procedure may lead to high background in the PFGE gel patterns, partial digestion, and poor fragment resolution. When these quality measures were implemented, the PulseNet V. parahaemolyticus protocol was found to be both robust and reproducible among the collaborating laboratories.

Determination of molecular phylogenetics of Vibrio parahaemolyticus strains by multilocus sequence typing

Vibrio parahaemolyticus is an important human pathogen whose transmission is associated with the consumption of contaminated seafood. There is a growing public health concern due to the emergence of a pandemic strain causing severe outbreaks worldwide. Many questions remain unanswered regarding the evolution and population structure of V. parahaemolyticus. In this work, we describe a multilocus sequence typing (MLST) scheme for V. parahaemolyticus based on the internal fragment sequences of seven housekeeping genes. This MLST scheme was applied to 100 V. parahaemolyticus strains isolated from geographically diverse clinical (n = 37) and environmental (n = 63) sources. The sequences obtained from this work were deposited and are available in a public database (http://pubmlst.org/vparahaemolyticus). Sixty-two unique sequence types were identified, and most (50) were represented by a single isolate, suggesting a high level of genetic diversity. Three major clonal complexes were identified by eBURST analysis. Separate clonal complexes were observed for V. parahaemolyticus isolates originating from the Pacific and Gulf coasts of the United States, while a third clonal complex consisted of strains belonging to the pandemic clonal complex with worldwide distribution. The data reported in this study indicate that V. parahaemolyticus is genetically diverse with a semiclonal population structure and an epidemic structure similar to that of Vibrio cholerae. Genetic diversity in V. parahaemolyticus appears to be driven primarily by frequent recombination rather than mutation, with recombination ratios estimated at 2.5:1 and 8.8:1 by allele and site, respectively. Application of this MLST scheme to more V. parahaemolyticus strains and by different laboratories will facilitate production of a global picture of the epidemiology and evolution of this pathogen.