Genotyping of pandemic Vibrio parahaemolyticus O3:K6 still open to question

Epidemiological and Whole-Genome Sequencing Analysis of a Gastroenteritis Outbreak Caused by a New Emerging Serotype of Vibrio parahaemolyticus in China

Vibrio parahaemolyticus is an important foodborne pathogen with diverse serotypes. In May 2021, we investigated a gastroenteritis outbreak that occurred in China, caused by V. parahaemolyticus O10:K4 infection. Based on the epidemiological curve, this outbreak was identified as a homologous exposure event. A case-control study demonstrated that emperor crab with mashed garlic (odds ratio [OR] = 4.60, p = 0.030; 95% confidence interval [95% CI]: 1.11-19.14), goose liver geoduck (OR = 4.50, p = 0.029; 95% CI: 1.12-18.13), shrimp (OR = 4.89, p = 0.021; 95% CI: 1.22-19.65), and sea cucumber (OR = 7.36, p = 0.005; 95% CI: 1.68-32.26) were the potential sources of the food poisoning. V. parahaemolyticus isolates from 18 laboratory-confirmed cases were all serotyped O10:K4, and determined to be sequence type ST3 via multilocus sequence typing. Pulsed field gel electrophoresis and whole-genome sequencing analysis revealed the identical pattern and 0-2 single nucleotide variation among these isolates. tdh was positive in all isolates, while trh and Orf8 were absent. Seven essential base positions in toxRS for pandemic clone identification were identical between the O10:K4 and O3:K6 pandemic clones. Phylogenetic analysis with 45 additional genomes of 13 different serotypes showed the closest genetic relationship between O10:K4 and O1: KUT. O10:K4 was thought to evolve from the O3:K6 pandemic clone. The new serovariant of O3:K6 poses a challenge for the prevention and control of V. parahaemolyticus disease outbreaks, or even epidemics, in the future.

Challenges in Vibrio parahaemolyticus infections caused by the pandemic clone

Vibrio Parahaemolyticus infections caused by the pandemic clone have become a global public health issue. The pandemic clone includes over ten sequence types and 49 serotypes. Several markers such as toxRS/new, orf8 and genomic islands were considered specific for pandemic strains, but subsequent studies later confirmed a lack of specificity. Thus, identifying stable indicators for the pandemic clone is still an open question. In recent years, several environmental pandemic strains are growing, constituting a new threat to seafood safety and human health. Traditional methods show limited discrimination in studying the microevolution of pandemic strains. For example, multilocus sequence typing divides many pandemic strains into ST3 type, making it difficult to further distinguish the variability within ST3 strains from different contexts. When using a whole genome sequencing-based technique, strains including those with the same sequence type, could be well separated. Whole genome sequencing-based technology also played important roles in dissecting the evolution process and revealing the mechanism underlying rapid serotype conversion within pandemic strains. In addition, the emergence of multiple-antibiotic resistant pandemic strains needs attention. Altogether, we are facing many challenges posed by pandemic V. parahaemolyticus strains, which need to be resolved in future studies.

Vibrio Parahaemolyticus: A Review on Distribution, Pathogenesis, Virulence Determinants and Epidemiology

Vibrio parahaemolyticus is a Gram-negative, halophilic bacterium isolated from marine environments globally. After the consumption of contaminated seafood, V. parahaemolyticus causes acute gastroenteritis. To initiate infection, a wide range of virulence factors are required. A complex group of genes is known to participate in the pathogenicity of V. parahaemolyticus; however, to understand the full mechanism of infection, extensive research is yet required. V. parahaemolyticus has become the leading cause of seafood-related gastroenteritis in Japan, the United States and several other parts of the world. In addition, outbreaks caused by the pandemic clone of this organism are escalating and spreading universally. To minimize the risk of V. parahaemolyticus infection and warrant the safety of seafood, collaboration between governments and scientists is required. We herein provide an updated review of the pathogenicity determinants and distribution of V. parahaemolyticus to deliver a better understanding of the significance of V. parahaemolyticus and its host-pathogen interactions.

Vibrio parahaemolyticus strengthens their virulence through modulation of cellular reactive oxygen species in vitro

Vibrio parahaemolyticus (Vp) is one of the emergent food-borne pathogens that are commensally associated with various shellfish species throughout the world. It is strictly environmental and many strains are pathogenic to humans. The virulent strains cause distinct diseases, including wound infections, septicemia, and most commonly, acute gastroenteritis, which is acquired through the consumption of raw or undercooked seafood, especially shellfish. Vp has two type three secretion systems (T3SSs), which triggering its cytotoxicity and enterotoxicity via their effectors. To better understand the pathogenesis of Vp, we established a cell infection model in vitro using a non-phagocytic cell line. Caco-2 cells were infected with different strains of Vp (pandemic and non-pandemic strains) and several parameters of cytotoxicity were measured together with adhesion and invasion indices, which reflect the pathogen's virulence. Our results show that Vp adheres to cell monolayers and can invade non-phagocytic cells. It also survives and persists in non-phagocytic cells by modulating reactive oxygen species (ROS), allowing its replication, and resulting in complete cellular destruction. We conclude that the pathogenicity of Vp is based on its capacities for adhesion and invasion. Surprisingly's; enhanced of ROS resistance period could promote the survival of Vp inside the intestinal tract, facilitating tissue infection by repressing the host's oxidative stress response.

Distribution and dynamics of epidemic and pandemic Vibrio parahaemolyticus virulence factors

Vibrio parahaemolyticus, autochthonous to estuarine, marine, and coastal environments throughout the world, is the causative agent of food-borne gastroenteritis. More than 80 serotypes have been described worldwide, based on antigenic properties of the somatic (O) and capsular (K) antigens. Serovar O3:K6 emerged in India in 1996 and subsequently was isolated worldwide, leading to the conclusion that the first V. parahaemolyticus pandemic had taken place. Most strains of V. parahaemolyticus isolated from the environment or seafood, in contrast to clinical strains, do not produce a thermostable direct hemolysin (TDH) and/or a TDH-related hemolysin (TRH). Type 3 secretion systems (T3SSs), needle-like apparatuses able to deliver bacterial effectors into host cytoplasm, were identified as triggering cytotoxicity and enterotoxicity. Type 6 secretion systems (T6SS) predicted to be involved in intracellular trafficking and vesicular transport appear to play a role in V. parahaemolyticus virulence. Recent advances in V. parahaemolyticus genomics identified several pathogenicity islands (VpaIs) located on either chromosome in both epidemic and pandemic strains and comprising additional colonization factors, such as restriction-modification complexes, chemotaxis proteins, classical bacterial surface virulence factors, and putative colicins. Furthermore, studies indicate strains lacking toxins and genomic regions associated with pathogenicity may also be pathogenic, suggesting other important virulence factors remain to be identified. The unique repertoire of virulence factors identified to date, their occurrence and distribution in both epidemic and pandemic strains worldwide are described, with the aim of highlighting the complexity of V. parahaemolyticus pathogenicity as well as its dynamic genome.

Comparison of the pathogenic potentials of environmental and clinical vibrio parahaemolyticus strains indicates a role for temperature regulation in virulence

Although the presence of pathogenic Vibrio spp. in estuarine environments of northern New England has been known for some time (C. H. Bartley and L. W. Slanetz, Appl. Microbiol. 21: 965-966, 1971, and K. R. O'Neil, S. H. Jones, and D. J. Grimes, FEMS Microbiol. Lett. 60:163-167, 1990), their virulence and the relative threat they may pose to human health has yet to be evaluated. In this study, the virulence potential of 33 Vibrio parahaemolyticus isolates collected from the Great Bay Estuary of New Hampshire was assessed in comparison to that of clinical strains. The environmental isolates lack thermostable direct hemolysin (TDH) and TDH-related hemolysin (TRH), which are encoded by tdh and trh, respectively. Though not hemolytic, they do possess putative virulence factors, such type III secretion system 1, and are highly cytotoxic to human gastrointestinal cells. The expression of known and putative virulence-associated traits, including hemolysin, protease, motility, biofilm formation, and cytotoxicity, by clinical reference isolates correlated with increased temperature from 28°C to 37°C. In contrast, the environmental isolates did not induce their putative virulence-associated traits in response to a temperature of 37°C. We further identified a significant correlation between hemolytic activity and growth phase among clinical strains, whereby hemolysin production decreases with increasing cell density. The introduction of a tdh::gfp promoter fusion into the environmental strains revealed that they regulate this virulence-associated gene appropriately in response to temperature, indicating that their existing regulatory mechanisms are primed to manage newly acquired virulence genes.

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.

Molecular characterization of Vibrio parahaemolyticus strains associated with foodborne illness in Florida

Vibrio parahaemolyticus is the leading cause of bacterial seafood-based illness in the United States. Real-time PCR, pandemic group-specific PCR, ribotyping, and multilocus sequence typing were used to characterize 30 strains of V. parahaemolyticus including 11 strains associated with foodborne outbreaks in Florida and 6 known pandemic strains. Thirteen strains were positive for four pandemic group-specific PCR markers, including 5 strains associated with outbreaks in Florida. Molecular typing methods were used to further define the pandemic status of these strains because the current PCR markers are not sufficient to identify pandemic isolates. Nine of the Florida strains clustered with a majority of the known pandemic strains, based on ribotyping patterns using PvuII, but no isolated pandemic branch was formed. Using multilocus sequence typing, it was determined that 14 strains possess a previously determined pandemic sequence type. This study identified 13 novel sequence types and seven to nine novel alleles for each locus. Furthermore, the results indicate that seven of the strains from recent foodborne outbreaks in Florida are pandemic strains, and that multilocus sequence typing was the most accurate molecular method to identify these strains.

Global dissemination of Vibrio parahaemolyticus serotype O3:K6 and its serovariants

Vibrio parahaemolyticus is recognized as a cause of food-borne gastroenteritis, particularly in the Far East, where raw seafood consumption is high. An unusual increase in admissions of V. parahaemolyticus cases was observed at the Infectious Diseases Hospital in Calcutta, a city in the northeastern part of India, beginning February 1996. Analysis of the strains revealed that a unique serotype, O3:K6, not previously isolated during the surveillance in Calcutta accounted for 50 to 80% of the infections in the following months. After this report, O3:K6 isolates identical to those isolated in Calcutta were reported from food-borne outbreaks and from sporadic cases in Bangladesh, Chile, France, Japan, Korea, Laos, Mozambique, Peru, Russia, Spain, Taiwan, Thailand, and the United States. Other serotypes, such as O4:K68, O1:K25, and O1:KUT (untypeable), that had molecular characteristics identical to that of the O3:K6 serotype were subsequently documented. These serotypes appeared to have diverged from the O3:K6 serotype by alteration of the O:K antigens and were defined as "serovariants" of the O3:K6 isolate. O3:K6 and its serovariants have now spread into Asia, America, Africa, and Europe. This review traces the genesis, virulence features, molecular characteristics, serotype variants, environmental occurrence, and global spread of this unique clone of V. parahaemolyticus.

Variability of properties of Vibrio parahaemolyticus strains isolated from individual patients

Infections by strains belonging to the O3:K6 pandemic clone of Vibrio parahaemolyticus are prevalent in southern Thailand, and serovariants of these strains have also been detected. V. parahaemolyticus strains lacking important virulence genes (tdh and trh) were isolated from 6.5 to 10.9% of clinical specimens during the period from 2000 to 2003. In order to understand whether changes to the characteristics of V. parahaemolyticus occur during infection, 10 isolates collected from each of 63 patients who presented with diarrhea at the Hat Yai hospital from 2003 to 2004 were examined for the presence of the tdh and trh genes, the O:K serotype, and genetic markers for the pandemic clone. A total of 42 patients (66.7%) yielded identical isolates (homogeneous populations), and 21 of the patients (33.3%) yielded isolates that differed in at least one character from the other isolates (heterogeneous populations). The DNA fingerprints (examined by arbitrarily primed PCR and pulsed-field gel electrophoresis) of some, but not all, of the heterogeneous populations from single patients were indistinguishable. The results indicated that some patients were infected with a unique strain and that in vivo changes (tdh deletion or serotype conversion) might have occurred in certain individuals. It is therefore important to bear in mind that epidemiological studies based on the analysis of a single colony from a single patient might lead to misleading conclusions. Finally, the present study did not rule out the possibility that isolates lacking tdh and trh have unknown virulence mechanisms other than the tdh and trh genes.

Identification of a DNA methyltransferase gene carried on a pathogenicity island-like element (VPAI) in Vibrio parahaemolyticus and its prevalence among clinical and environmental isolates

In this study we identified a putative virulence-associated DNA methyltransferase (MTase) gene carried on a novel 22.79-kb pathogenicity island-like element (VPAI) in V. parahaemolyticus. The V. parahaemolyticus MTase gene was shown by PCR to be prevalent (>98%) in pandemic thermostable direct hemolysin gene-positive isolates, which suggests that VPAI may confer unique virulence traits to pandemic strains of V. parahaemolyticus.

Four genomic islands that mark post-1995 pandemic Vibrio parahaemolyticus isolates

Background

Vibrio parahaemolyticus is an aquatic, halophilic, Gram-negative bacterium, first discovered in 1950 in Japan during a food-poisoning outbreak. Infections resulting from consumption of V. parahaemolyticus have increased globally in the last 10 years leading to the bacterium's classification as a newly emerging pathogen. In 1996 the first appearance of a pandemic V. parahaemolyticus clone occurred, a new O3:K6 serotype strain that has now been identified worldwide as a major cause of seafood-borne gastroenteritis.

Results

We examined the sequenced genome of V. parahaemolyticus RIMD2210633, an O3:K6 serotype strain isolated in Japan in 1996, by bioinformatic analyses to uncover genomic islands (GIs) that may play a role in the emergence and pathogenesis of pandemic strains. We identified 7 regions ranging in size from 10 kb to 81 kb that had the characteristics of GIs such as aberrant base composition compared to the core genome, presence of phage-like integrases, flanked by direct repeats and the absence of these regions from closely related species. Molecular analysis of worldwide clinical isolates of V. parahaemolyticus recovered over the last 33 years demonstrated that a 24 kb region named V. parahaemolyticus island-1 (VPaI-1) encompassing ORFs VP0380 to VP0403 is only present in new O3:K6 and related strains recovered after 1995. We investigated the presence of 3 additional regions, VPaI-4 (VP2131 to VP2144), VPaI-5 (VP2900 to VP2910) and VPaI-6 (VPA1254 to VPA1270) by PCR assays and Southern blot analyses among the same set of V. parahaemolyticus isolates. These 3 VPaI regions also gave similar distribution patterns amongst the 41 strains examined.

Conclusion

The 4 VPaI regions examined may represent DNA acquired by the pandemic group of V. parahaemolyticus isolates that increased their fitness either in the aquatic environment or in their ability to infect humans.

Characterization of pathogenic Vibrio parahaemolyticus isolates from clinical sources in Spain and comparison with Asian and North American pandemic isolates

In spite of the potential risk involved with contamination of seafood with Vibrio parahaemolyticus, there is a lack of information on the occurrence of pathogenic V. parahaemolyticus in Europe. This organism was isolated in 1999 from a large outbreak (64 cases admitted to a single hospital) associated with raw oyster consumption in Galicia, Spain, one of the most important regions in shellfish production worldwide. Two V. parahaemolyticus isolates from the 1999 Galicia outbreak, three additional clinical isolates obtained in the same period from hospitals in Spain, two reference strains from clinical sources, and five Spanish environmental isolates were examined. Seventeen isolates belonging to the pandemic clone isolated in Asia and North America were included in the study for comparison. All isolates were characterized by serotyping, PCR for virulence-related genes, pulsed-field gel electrophoresis (PFGE), and plasmid analysis. Four of the five clinical isolates from hospitals in Spain belonged to serotype O4:K11; the remaining isolate was O4:K untypeable (KUT). All five isolates were positive for V. parahaemolyticus toxR and tlh (species-specific genes) and tdh and negative for trh and group-specific PCR (a PCR method for detection of the pandemic clone). PFGE analysis with NotI and SfiI discriminated the European isolates in two closely related PFGE types included in a homogeneous cluster, clearly differentiated from the Asian and North American isolates. The five environmental isolates belonged to serotypes O2:K28, O2:KUT, O3:K53, O4:KUT, and O8:K22 and were negative for all virulence genes. The five isolates were discriminated into five different PFGE types unrelated to any other isolate included in the study. While the virulence characteristics (tdh positive, trh negative) of the Spanish clinical isolates matched those of the O3:K6 clone from Asia and North America, they were clearly excluded from this clone by group-specific PCR, PFGE, and serotyping. The results of this study suggest that a unique and specific clone could be related to the V. parahaemolyticus infections in Europe.

Emergence and serovar transition of Vibrio parahaemolyticus pandemic strains isolated during a diarrhea outbreak in Vietnam between 1997 and 1999

We characterized 523 Vibrio parahaemolyticus strains isolated during a survey of diarrhea patients in Khanh Hoa province, Vietnam between 1997 and 1999. Forty-nine percent of the strains were judged to belong to the pandemic strains that emerged around 1996 and spread to many countries. These strains were positive in the GS-PCR assay and carried the tdh gene. The ORF8 of the f237 phage genome, a possible marker of the pandemic clone, was absent in 10% of these strains. Eleven O: K serovars were detected among the pandemic strains and the strains representing all 11 serovars of pandemic strains were shown to be closely related regardless of the ORF8 genotype using arbitrarily primed PCR and pulsed field gel electrophoresis analyses. It was clear that a transition of major serovars occurred among the pandemic strains represented by the emergence of O3: K6 in 1997, O4: K68 in 1998, and O1: K25 in 1998 and 1999.

Identification of a protein biomarker unique to the pandemic O3:K6 clone of Vibrio parahaemolyticus

The present method of characterizing Vibrio parahaemolyticus strains involves serotyping or detection methods based on assessment of the presence or absence of genes thought to be markers of an organism's pathogenicity. It is unclear whether these assays detect all pathogenic V. parahaemolyticus strains since a clear correlation between the presence of a particular gene and the organism's pathogenicity has not yet been observed. We have described a proteomics-based method to distinguish individual V. parahaemolyticus strains on the basis of their protein profiles and identified a specific protein that is characteristic of the pandemic O3:K6 strain and its clonal derivatives. In the pandemic clone of V. parahaemolyticus, a histone-like DNA-binding protein, HU-alpha, has a C-terminal amino acid sequence different from those of other strains of V. parahaemolyticus. Upon further study, it was discovered that the gene encoding this protein has a 16-kbp insert at the 3' terminus of the open reading frame for this protein. By using the protein sequence of the unique biomarker for the pandemic clone of V. parahaemolyticus, it was possible to rationally design specific PCR-based probes and assays that permit the rapid and precise identification of pandemic strains of V. parahaemolyticus.

Genotypic analyses of Vibrio parahaemolyticus and development of a pandemic group-specific multiplex PCR assay

A total of 54 Vibrio parahaemolyticus strains including pandemic O3:K6 strains and newly emerged O4:K68, O1:K25, O1:K26, and O1:K untypeable strains (collectively referred to as the "pandemic group") were examined for their pulsed-field gel electrophoresis (PFGE) and arbitrarily primed PCR (AP-PCR) profiles and for the presence or absence of genetic marker DNA sequences, toxRS/new or orf8, that had been reported elsewhere to be specific for the pandemic group. Both PFGE and AP-PCR analyses indicated that all strains of the pandemic group formed a distinct genotypic cluster, suggesting that they originated from the same clone. In addition to the pandemic group, four O3:K6 strains that did not possess the thermostable direct hemolysin (tdh) gene also belonged to this cluster and possessed the toxRS/new sequence. However, three O3:K6 strains that clearly belonged to the pandemic group by PFGE and AP-PCR did not possess the orf8 sequence. The evidence suggests that neither the toxRS/new nor the orf8 sequence is a reliable gene marker for definite identification of the pandemic group. We therefore developed a novel multiplex PCR assay specific for the pandemic group. The assay successfully distinguished pandemic group strains from other V. parahaemolyticus strains by yielding two distinct PCR products for tdh (263 bp) and the toxRS/new sequence (651 bp).