Genetic diversity of Vibrio parahaemolyticus strains isolated from farmed Pacific white shrimp and ambient pond water affected by acute hepatopancreatic necrosis disease outbreak in Thailand

Prevalence, virulence characteristics, and antimicrobial resistance of Vibrio parahaemolyticus isolates from raw seafood in a province in Northern Thailand

Vibrio parahaemolyticus (V. parahaemolyticus) is commonly found in seawater and seafood products, but evidence is limited of its presence in seafood marketed in locations very distant from coastal sources. This study determined the prevalence and characterization of V. parahaemolyticus in seafood from markets in landlocked Phayao province, Northern Thailand. Among 120 samples, 26 (21.7%) were positive for V. parahaemolyticus, being highest in shrimp (43.3%), followed by shellfish (36.7%), and squid (6.7%), but was not found in fish. V. parahaemolyticus comprised 33 isolates that were non-pathogenic and non-pandemic. Almost all isolates from shrimp and shellfish samples were positive for T3SS1. Only five isolates (15.2%) showed two antimicrobial resistance patterns, namely, kanamycin-streptomycin (1) carrying sul2 and ampicillin-kanamycin-streptomycin (4) that carried tetA (2), tetA-sul2 (1), as well as one negative. Antimicrobial susceptible V. parahaemolyticus isolates possessing tetA (67.9%) and sul2 (3.5%) were also found. Six isolates positive for integron class 1 and/or class 2 were detected in 4 antimicrobial susceptible and 2 resistant isolates. While pathogenic V. parahaemolyticus was not detected, contamination of antimicrobial resistance V. parahaemolyticus in seafood in locations distant from coastal areas requires ongoing monitoring to improve food safety in the seafood supply chain.

Genomic and Evolutionary Features of Nine AHPND Positive Vibrio parahaemolyticus Strains Isolated from South American Shrimp Farms

Vibrio parahaemolyticus is a bacterial pathogen that becomes lethal to Penaeus shrimps when acquiring the pVA1-type plasmid carrying the PirABvp genes, causing acute hepatopancreatic necrosis disease (AHPND). This disease causes significant losses across the world, with outbreaks reported in Southeast Asia, Mexico, and South America. Virulence level and mortality differences have been reported in isolates from different locations, and whether this phenomenon is caused by plasmid-related elements or genomic-related elements from the bacteria remains unclear. Here, nine genomes of South American AHPND-causing V. parahaemolyticus (VPAHPND) isolates were assembled and analyzed using a comparative genomics approach at (i) whole-genome, (ii) secretion system, and (iii) plasmid level, and then included for a phylogenomic analysis with another 86 strains. Two main results were obtained from our analyses. First, all isolates contained pVA1-type plasmids harboring the toxin coding genes, and with high similarity with the prototypical sequence of Mexican-like origin, while phylogenomic analysis showed some level of heterogeneity with discrete clusters and wide diversity compared to other available genomes. Second, although a high genomic similarity was observed, variation in virulence genes and clusters was observed, which might be relevant in the expression of the disease. Overall, our results suggest that South American pathogenic isolates are derived from various genetic lineages which appear to have acquired the plasmid through horizontal gene transfer. Furthermore, pathogenicity seems to be a multifactorial trait where the degree of virulence could be altered by the presence or variations of several virulence factors. IMPORTANCE AHPND have caused losses of over $2.6 billion to the aquaculture industry around the world due to its high mortality rate in shrimp farming. The most common etiological agent is V. parahaemolyticus strains possessing the pVA1-type plasmid carrying the PirABvp toxin. Nevertheless, complete understanding of the role of genetic elements and their impact in the virulence of this pathogen remains unclear. In this work, we analyzed nine South American AHPND-causing V. parahaemolyticus isolates at a genomic level, and assessed their evolutionary relationship with other 86 strains. We found that all our isolates were highly similar and possessed the Mexican-type plasmid, but their genomic content did not cluster with other Mexican strains, but instead were spread across all isolates. These results suggest that South American VPAHPND have different genetic backgrounds, and probably proceed from diverse geographical locations, and acquire the pVA1-type plasmid via horizontal gene transfer at different times.

Multilocus Sequence Typing and Virulence Potential of Vibrio parahaemolyticus Strains Isolated from Aquatic Bird Feces

Vibrio parahaemolyticus is a Gram-negative, foodborne pathogenic bacterium that causes human gastroenteritis. This organism is ubiquitously present in the marine environment. Detection of V. parahaemolyticus in aquatic birds has been previously reported; however, the characterization of isolates of this bacterium recovered from these birds remains limited. The present study isolated and characterized V. parahaemolyticus from aquatic bird feces at the Bangpu Recreation Center (Samut Prakan province, Thailand) from 2016 to 2017, using multilocus sequence typing (MLST) and genome analysis. The results showed that V. parahaemolyticus was present in 34.9% (76/218) of the collected bird fecal samples. Among the ldh-positive V. parahaemolyticus isolates (n = 308), 1% (3/308) were positive for tdh, 1.3% (4/308) were positive for trh, and 0.3% (1/308) were positive for both tdh and trh. In turn, the MLST analysis revealed that 49 selected V. parahaemolyticus isolates resolved to 36 STs, 26 of which were novel (72.2%). Moreover, a total of 10 identified STs were identical to globally reported pathogenic strains (ST1309, ST1919, ST491, ST799, and ST2516) and environmental strains (ST1879, ST985, ST288, ST1925, and ST260). The genome analysis of isolates possessing tdh and/or trh (ST985, ST1923, ST1924, ST1929 and ST2516) demonstrated that the organization of the T3SS2α and T3SS2β genes in bird fecal isolates were almost identical to those of human clinical strains posing public health concerns of pathogen dissemination in the recreational area. The results of this study suggest that aquatic birds are natural reservoirs of new strains with high genetic diversity and are alternative sources of potentially pathogenic V. parahaemolyticus in the marine environment.

IMPORTANCE To our knowledge, infection of foodborne bacterium V. parahamolyticus occurs via the consumption of undercooked seafood contaminated with pathogenic strains. Aquatic bird is a neglectable source that can transmit V. parahaemolyticus along coastal areas. This study reported the detection of potentially pathogenic V. parahamolyticus harboring virulence genes from aquatic bird feces at the recreational center situated near the Gulf of Thailand. These strains shared identical genetic profile to the clinical isolates that previously reported in many countries. Furthermore, the strains from aquatic birds showed extremely high genetic diversity. Our research pointed out that the aquatic bird is possibly involved in the evolution of novel strains of V. parahaemolyticus and play a role in dissimilation of the potentially pathogenic strains across geographical distance.

Multifunctional carbonized nanogels to treat lethal acute hepatopancreatic necrosis disease

Background

Shrimp aquaculture has suffered huge economic losses over the past decade due to the outbreak of acute hepatopancreatic necrosis disease (AHPND), which is mainly caused by the bacteria Vibrio parahaemolyticus (V. parahaemolyticus) with the virulence pVA1 plasmid, which encodes a secretory photorhabdus insect-related (Pir) toxin composed of PirA and PirB proteins. The Pir toxin mainly attacks the hepatopancreas, a major metabolic organ in shrimp, thereby causing necrosis and loss of function. The pandemic of antibiotic-resistant strains makes the impact worse.

Methods

Mild pyrolysis of a mixture of polysaccharide dextran 70 and the crosslinker 1,8-diaminooctane at 180 ℃ for 3 h to form carbonized nanogels (DAO/DEX-CNGs) through controlled cross-linking and carbonization. The multifunctional therapeutic CNGs inherit nanogel-like structures and functional groups from their precursor molecules.

Results

DAO/DEX-CNGs manifest broad-spectrum antibacterial activity against Vibrio parahaemolyticus responsible for AHPND and even multiple drug-resistant strains. The polymer-like structures and functional groups on graphitic-carbon within the CNGs exhibit multiple treatment effects, including disruption of bacterial membranes, elevating bacterial oxidative stress, and neutralization of PirAB toxins. The inhibition of Vibrio in the midgut of infected shrimp, protection of hepatopancreas tissue from Pir toxin, and suppressing overstimulation of the immune system in severe V. parahaemolyticus infection, revealing that CNGs can effectively guard shrimp from Vibrio invasion. Moreover, shrimps fed with DAO/DEX-CNGs were carefully examined, such as the expression of the immune-related genes, hepatopancreas biopsy, and intestinal microbiota. Few adverse effects on shrimps were observed.

Conclusion

Our work proposes brand-new applications of multifunctional carbon-based nanomaterials as efficient anti-Vibrio agents in the aquatic industry that hold great potential as feed additives to reduce antibiotic overuse in aquaculture.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-021-01194-8.

Distribution of Vibrio parahaemolyticus in Farmed Shrimp Penaeus vannamei, Farm Water and Sediment

The halophilic marine bacterium Vibrio parahaemolyticus is a zoonotic pathogen associated with wild-caught and farmed shrimp. The bacterium is an important cause of gastroenteritis associated with the consumption of raw or undercooked seafood. In the present study, the prevalence and human pathogenic potential of Vibrio parahaemolyticus in Penaeus vannamei (tissue and hepatopancreas) and the farm environment (water and sediment) was investigated by conventional culture and molecular techniques. The total Vibrio counts of P. vannamei ranged from <1 CFU/mL in hemolymph to 7.61 log CFU/g in the hepatopancreas. The sediment samples consistently showed the counts of 6-7 log CFU/g, while the pond water had Vibrio counts in the range of 2-3 log CFU/ml. Of 120 Vibrio isolates identified, 87 were confirmed as V. parahaemolyticus based on the toxR and tlh gene-specific PCR. The virulence marker gene tdh was not detected in any of the isolates, while the trh gene was detected in 3 (3.6%) isolates. Although the incidence of pathogenic V. parahaemolyticus in farmed P. vannamei is low, the high numbers of total vibrios and V. parahaemolyticus demand constant monitoring of animals and the farm environment for human pathogenic strains of V. parahaemolyticus.

Whole genome analysis unveils genetic diversity and potential virulence determinants in Vibrio parahaemolyticus associated with disease outbreak among cultured Litopenaeus vannamei (Pacific white shrimp) in India

Vibrio parahaemolyticus has caused widespread mortality in Indian shrimp aquaculture in recent years. However, there are insufficient genome data for the isolates from Indian shrimp vibriosis to analyze genetic diversity and track the acquisition of genetic features that could be involved in virulence and fitness. In this study, we have performed genome analysis of V. parahaemolyticus isolated from moribund shrimps collected from shrimp farms along coastal Karnataka, India, for better understanding of their diversity and virulence. Five newly sequenced genomes of V. parahaemolyticus along with 40 genomes retrieved from NCBI were subjected to comparative genome analysis. The sequenced genomes had an overall genome size of 5.2 Mb. MLST analysis and core genome phylogenomic analysis revealed considerable genetic diversity among the isolates obtained from the moribund shrimps. Interestingly, none of the V. parahaemolyticus isolates possessed the classical features (PirAB) of the strains associated with Acute Hepatopancreatic Necrosis Disease (AHPND). This study also revealed the presence of multiple virulence attributes, including ZOT, ACE and RTX toxins, secretion systems, and mobile genetic elements. The findings of this study provide insights into the possible transition of an environmental V. parahaemolyticus to emerge as pathogens of aquaculture species by increasing its virulence and host adaptation. Future studies focusing on continuous genomic surveillance of V. parahaemolyticus are required to study the evolution and transmission of new variants in shrimp aquaculture, as well as to design and implement biosecurity programs to prevent disease outbreaks.

Acute Hepatopancreatic Necrosis Disease (AHPND): Virulence, Pathogenesis and Mitigation Strategies in Shrimp Aquaculture

Shrimp, as a high-protein animal food commodity, are one of the fastest growing food producing sectors in the world. It has emerged as a highly traded seafood product, currently exceeding 8 MT of high value. However, disease outbreaks, which are considered as the primary cause of production loss in shrimp farming, have moved to the forefront in recent years and brought socio-economic and environmental unsustainability to the shrimp aquaculture industry. Acute hepatopancreatic necrosis disease (AHPND), caused by Vibrio spp., is a relatively new farmed penaeid shrimp bacterial disease. The shrimp production in AHPND affected regions has dropped to ~60%, and the disease has caused a global loss of USD 43 billion to the shrimp farming industry. The conventional approaches, such as antibiotics and disinfectants, often applied for the mitigation or cure of AHPND, have had limited success. Additionally, their usage has been associated with alteration of host gut microbiota and immunity and development of antibiotic resistance in bacterial pathogens. For example, the Mexico AHPND-causing V. parahaemolyticus strain (13-306D/4 and 13-511/A1) were reported to carry tetB gene coding for tetracycline resistance gene, and V. campbellii from China was found to carry multiple antibiotic resistance genes. As a consequence, there is an urgent need to thoroughly understand the virulence mechanism of AHPND-causing Vibrio spp. and develop novel management strategies to control AHPND in shrimp aquaculture, that will be crucially important to ensure food security in the future and offer economic stability to farmers. In this review, the most important findings of AHPND are highlighted, discussed and put in perspective, and some directions for future research are presented.

Do acute hepatopancreatic necrosis disease-causing PirABVP toxins aggravate vibriosis?

Gram-negative marine bacterium Vibrio parahaemolyticus is an important aquatic pathogen and has been demonstrated to be the causative agent of acute hepatopancreatic necrotic disease (AHPND) in shrimp aquaculture. The AHPND-causing V. parahaemolyticus strains contain a pVA1 plasmid encoding the binary PirA^VP and PirB^VP toxins, are the primary virulence factor that mediates AHPND and mortality in shrimp. Since PirAB^VP toxins are secreted extracellularly, one can hypothesize that PirAB^VP toxins would aggravate vibriosis in the aquatic environment. To address this, in vivo and in vitro experiments were conducted. Germ-free Artemia franciscana were co-challenged with PirAB^VP toxins and 10 Vibrio spp. The in vivo results showed that PirAB^VP toxin interact synergistically with MM30 (a quorum sensing AI-2 deficient mutant) and V. alginolyticus AQ13-91, aggravating vibriosis. However, co-challenge by PirAB^VP toxins and V. campbellii LMG21363, V. parahaemolyticus CAIM170, V. proteolyticus LMG10942, and V. anguillarum NB10 worked antagonistically, increasing the survival of Artemia larvae. The in vitro results showed that the addition of PirAB^VP toxins significantly modulated the production of the virulence factors of studied Vibrio spp. Yet these in vitro results did not help to explain the in vivo results. Hence it appears that PirAB^VP toxins can aggravate vibriosis. However, the dynamics of interaction is strain dependent.

Characterization of Vibrio parahaemolyticus strains isolated from clinically asymptomatic seafood workers

Vibrio parahaemolyticus (VP) is a major cause of gastroenteritis outbreaks in Thailand and other countries due to the consumption of contaminated and undercooked seafood. However, there have been few reports of the molecular epidemiology of VP isolates from asymptomatic seafood handlers. Here, we report the phenotypic and genetic characterization of 61 VP isolates obtained from asymptomatic workers in two seafood-processing plants. We found 24 O:K serotypes, of which O11:KUT, O1:KUT and O3:KUT were the dominant serotypes. Analysis by PCR showed that 12 isolates harbored either tdh or trh genes with the potential to be pathogenic VP strains. The presence of T3SS2α and T3SS2β genes was correlated with the presence of tdh and trh, respectively. Four tdh+ isolates were positive for pandemic marker. In this study, VP isolates were commonly resistant to ampicillin, cephazolin, fosfomycin and novobiocin. Phylogenetic analysis of VP1680 loci in 35 isolates from 17 asymptomatic workers, 6 gastroenteritis patients, 7 environmental samples and 5 genomes from a database showed 22 different alleles. Gene VP1680 was conserved in tdh+ isolates and pandemic strains, while that of trh + isolates was diverse. Asymptomatic workers carrying VP were the most likely source of contamination, which raises concerns over food safety in seafood-processing plants.

Prevalence of Vibrio parahaemolyticus Causing Acute Hepatopancreatic Necrosis Disease of Shrimp in Shrimp, Molluscan Shellfish and Water Samples in the Mekong Delta, Vietnam

Simple Summary

Recently, Vibrio parahaemolyticus has been identified as an important agent of acute hepatopancreatic necrosis disease in shrimp. In Vietnam, this disease has appeared since 2010 and caused a big economic loss for shrimp farming. However, the information of this agent in Vietnam has been not fully understood. This study aims to investigate the prevalence of shrimp pathogenic Vibrio parahaemolyticus and several it’s characteristics in the Mekong Delta of Vietnam. A total of 481 shrimp and molluscan shellfish samples from retail shops and farms and 64 water samples from shrimp and molluscan shellfish farms were examined for the presence of pathogenic strains. The pathogenic strains were isolated in 0.7% of molluscan shellfish samples from retail shops, 9.9% of shrimp samples from shrimp ponds, and 4.8% of water samples from shrimp ponds. These strains were classified into two types of O antigen (O1 and O3), in which O1 was the predominant. They showed resistance to several antimicrobial agents, multidrug resistance and pathogenicity to experimental shrimp. These results indicate that shrimp pathogenic Vibrio parahaemolyticus is widely prevalent in environment in the Mekong Delta, Vietnam. These findings can be used for understanding the risk of shrimp pathogenic Vibrio parahaemolyticus in the Mekong Delta.

Abstract

A total of 481 samples, including 417 shrimp and molluscan shellfish samples from retail shops and farms and 64 water samples from shrimp and molluscan shellfish farms in the Mekong Delta located the southern part of Vietnam, were examined for the presence of Vibrio parahaemolyticus (Vp_AHPND) caused acute haepatopancreatic necrosic disease (AHPND) in shrimp. Vp_AHPND strains were isolated in two of 298 (0.7%) molluscan shellfish samples from retail shops, seven of 71 (9.9%) shrimp samples from shrimp ponds, and two of 42 (4.8%) water samples from shrimp ponds. Vp_AHPND strains were classified into two types of O antigen, including O1 and O3, in which O1 was the predominant. Vp_AHPND strains isolated showed high resistance rates to colistin (100%), ampicillin (93.8%), and streptomycin (87.5%). These results indicate that Vp_AHPND is widely prevalent in environment in the Mekong Delta, Vietnam.

Determination of the Infectious Agent of Translucent Post-Larva Disease (TPD) in Penaeus vannamei

A new emerging disease called “translucent post-larvae disease” (TPD) or “glass post-larvae disease” (GPD) of Penaeus vannamei, characterized by pale or colorless hepatopancreas and digestive tract, has become an urgent threat to the shrimp farming industry. Following this clue that treatment of an antibacterial agent could alleviate the disease, systematic investigation of the potential infectious agent of TPD was conducted using bacterial identification and artificial challenge tests to fulfill Koch’s postulates. A dominant bacterial isolate, Vp-JS20200428004-2, from the moribund individuals was isolated and identified as Vibrio parahaemolyticus based on multi-locus sequence analysis. However, Vp-JS20200428004-2 differed from the V. parahaemolyticus that caused typical acute hepatopancreatic necrosis disease. Immersion challenge tests revealed that Vp-JS20200428004-2 could cause 100% mortality within 40 h at a dose of 1.83 × 10⁶ CFU/mL, and experimental infected shrimp showed similar clinical signs of TPD. The Vp-JS20200428004-2 could be re-isolated and identified from the experimental infected individuals. Moreover, histopathological analysis of diseased samples indicated that Vp-JS20200428004-2 caused severe necrosis and sloughing of epithelial cells of the hepatopancreas and midgut in shrimp individuals both naturally and experimentally infected. Our present results indicated that Vp-JS20200428004-2 is a highly virulent infectious agent associated with the TPD and deserves further attention.

Comparative genomic analysis of Vibrios yields insights into genes associated with virulence towards C. gigas larvae

BACKGROUND

Vibriosis has been implicated in major losses of larvae at shellfish hatcheries. However, the species of Vibrio responsible for disease in aquaculture settings and their associated virulence genes are often variable or undefined. Knowledge of the specific nature of these factors is essential to developing a better understanding of the environmental and biological conditions that lead to larvae mortality events in hatcheries. We tested the virulence of 51 Vibrio strains towards Pacific Oyster (Crassostreae gigas) larvae and sequenced draft genomes of 42 hatchery-associated vibrios to determine groups of orthologous genes associated with virulence and to determine the phylogenetic relationships among pathogens and non-pathogens of C. gigas larvae.

RESULTS

V. coralliilyticus strains were the most prevalent pathogenic isolates. A phylogenetic logistic regression model identified over 500 protein-coding genes correlated with pathogenicity. Many of these genes had straightforward links to disease mechanisms, including predicted hemolysins, proteases, and multiple Type 3 Secretion System genes, while others appear to have possible indirect roles in pathogenesis and may be more important for general survival in the host environment. Multiple metabolism and nutrient acquisition genes were also identified to correlate with pathogenicity, highlighting specific features that may enable pathogen survival within C. gigas larvae.

CONCLUSIONS

These findings have important implications on the range of pathogenic Vibrio spp. found in oyster-rearing environments and the genetic determinants of virulence in these populations.

Comparative genomic provides insight into the virulence and genetic diversity of Vibrio parahaemolyticus associated with shrimp acute hepatopancreatic necrosis disease

Acute hepatopancreatic necrosis disease (AHPND) is an important shrimp disease of economic importance which causes mass mortality of cultivated penaeid shrimps in Southeast Asian countries, Mexico and South America. This disease was originally caused by Vibrio parahaemolyticus (VP_AHPND) which is reported to harbour a transferable plasmid carrying the virulent PirAB-like toxin genes (pirAB^vp). However, little is known about the pathogenicity of VP_AHPND. To extend our understanding, comparative genomic analyses was performed in this study to identify the genetic differences and to understand the phylogenetic relationship of VP_AHPND strains. Seven Vibrio parahaemolyticus strains (five VP_AHPND strains and two non-VP_AHPND strains) were sequenced and 31 draft genomes of V. parahaemolyticus were retrieved from NCBI database and incorporated into the genomic comparison to elucidate their genomic diversity. The study showed that the genome sizes of the VP_AHPND strains were approximately 5 Mbp. Ten sequence types (STs) were identified among the VP_AHPND strains using in silico-Multilocus Sequence Typing analysis (MLST) and ST 970 was the predominant ST. Phylogenetic analysis based on MLST and single nucleotide polymorphisms (SNP) showed that the VP_AHPND strains were genetically diverse. Based on the comparative genomic analysis, several functional proteins were identified from diiferent categories associated with virulence-related proteins, secretory proteins, conserved domain proteins, transporter proteins, and phage proteins. The CRISPR analysis showed that VP_AHPND strains contained less number of CRISPRs elements than non-VP_AHPND strains while six prophages regions were identified in the genomes, suggested the lack of CRISPR might promote prophage insertion. The genomic information in this study provide improved understanding of the virulence of these VP_AHPND strains.

The B Subunit of PirABvp Toxin Secreted from Vibrio parahaemolyticus Causing AHPND Is an Amino Sugar Specific Lectin

Vibrio parahaemolyticus (Vp) is the etiological agent of the acute hepatopancreatic necrosis disease (AHPND) in Penaeus vannamei shrimp. Vp possesses a 63-70 kb conjugative plasmid that encodes the binary toxin PirAvp/PirBvp. The 250 kDa PirABvp complex was purified by affinity chromatography with galactose-sepharose 4B and on a stroma from glutaraldehyde-fixed rat erythrocytes column, as a heterotetramer of PirAvp and PirBvp subunits. In addition, recombinant pirB (rPirBvp) and pirA (rPirAvp) were obtained. The homogeneity of the purified protein was determined by SDS-PAGE analysis, and the yield of protein was 488 ng/100 μg of total protein of extracellular products. The PirABvp complex and the rPirBvp showed hemagglutinating activity toward rat erythrocytes. The rPirAvp showed no hemagglutinating capacity toward the animal red cells tested. Among different mono and disaccharides tested, only GalNH2 and GlcNH2 were able to inhibit hemagglutination of the PirABvp complex and the rPirBvp. Glycoproteins showed inhibitory specificity, and fetuin was the glycoprotein that showed the highest inhibition. Other glycoproteins, such as mucin, and glycosaminoglycans, such as heparin, also inhibited the activity. Desialylation of erythrocytes enhanced the hemagglutinating activity. This confirms that Gal or Gal (β1,4) GlcNAc are the main ligands for PirABvp. The agglutinating activity of the PirABvp complex and the rPirBvp is not dependent on cations, because addition of Mg2+ or Ca2+ showed no effect on the protein capacity. Our results strongly suggest that the PirBvp subunit is a lectin, which is part of the PirA/PirBvp complex, and it seems to participate in bacterial pathogenicity.

Genetically-engineered plants yield an orally immunogenic PirA-like toxin from Vibrio parahaemolyticus

Vibrio parahaemolyticus is the main etiological agent of human gastroenteritis by seafood consumption and some strains from this species causing the Acute Hepatopancreatic Necrosis Disease in shrimp have been recently reported. The PirA-like toxin from V. parahaemolyticus (ToxA) has been recently reported as an attractive antigen implicated in subunit vaccine development. Since plants are attractive hosts for the production and delivery of vaccines in the present study plants expressing ToxA were developed to account with a low cost platform for the production and oral delivery of ToxA. Tobacco plants were genetically engineered by Agrobacterium-mediated transformation to stably integrate the ToxA-coding gene into the nuclear genome. Transgenic lines were rescued in kanamycin-containing medium and analyzed by ELISA to determine ToxA yields observing levels up to 9 μg g-1 FW leaf tissues. Western blot analysis confirmed the presence of the ToxA protein in plant extracts. Immunogenicity assessment of the plant-made ToxA was performed in mice, comprising a 4-dose oral immunization scheme; revealing the induction of anti-ToxA humoral responses (IgG in serum and IgA in feces). This study opens the path for the development of low cost plant-based vaccines against Vibrio parahaemolyticus.

pirABvp -Bearing Vibrio parahaemolyticus and Vibrio campbellii Pathogens Isolated from the Same AHPND-Affected Pond Possess Highly Similar Pathogenic Plasmids

Acute hepatopancreatic necrosis disease (AHPND) is a severe shrimp disease originally shown to be caused by virulent strains of Vibrio parahaemolyticus (VP_AHPND). Rare cases of AHPND caused by Vibrio species other than V. parahaemolyticus were reported. We compared an AHPND-causing V. campbellii (VC_AHPND) and a VP_AHPND isolate from the same AHPND-affected pond. Both strains are positive for the virulence genes pirAB^vp. Immersion challenge test with Litopenaeus vannamei indicated the two strains possessed similar pathogenicity. Complete genome comparison showed that the pirAB^vp-bearing plasmids in the two strains were highly homologous, and they both shared high homologies with plasmid pVA1, the reported pirAB^vp-bearing plasmid. Conjugation and DNA-uptake genes were found on the pVA1-type plasmids and the host chromosomes, respectively, which may facilitate the dissemination of pirAB^vp. Novel variations likely driven by ISVal1 in the genetic contexts of the pirAB^vp genes were found in the two strains. Moreover, the VC_AHPND isolate additionally contains multiple antibiotic resistance genes, which may bring difficulties to control its future outbreak. The dissemination of the pirAB^vp in non-parahaemolyticus Vibrio also rises the concern of missing detection in industrial settings since the isolation method currently used mainly targeting V. parahaemolyticus. This study provides timely information for better understanding of the causes of AHPND and molecular epidemiology of pirAB^vp and also appeals for precautions to encounter the dissemination of the hazardous genes.

A Natural Vibrio parahaemolyticus ΔpirA Vp pirB Vp+ Mutant Kills Shrimp but Produces neither Pir Vp Toxins nor Acute Hepatopancreatic Necrosis Disease Lesions

Acute hepatopancreatic necrosis disease (AHPND) of shrimp is caused by Vibrio parahaemolyticus isolates (VP_AHPND isolates) that harbor a pVA plasmid encoding toxins PirA ^Vp and PirB ^Vp These are released from VP_AHPND isolates that colonize the shrimp stomach and produce pathognomonic AHPND lesions (massive sloughing of hepatopancreatic tubule epithelial cells). PCR results indicated that V. parahaemolyticus isolate XN87 lacked pirA ^Vp but carried pirB ^Vp Unexpectedly, Western blot analysis of proteins from the culture broth of XN87 revealed the absence of both toxins, and the lack of PirB ^Vp was further confirmed by enzyme-linked immunosorbent assay. However, shrimp immersion challenge with XN87 resulted in 47% mortality without AHPND lesions. Instead, lesions consisted of collapsed hepatopancreatic tubule epithelia. In contrast, control shrimp challenged with typical VP_AHPND isolate 5HP gave 90% mortality, accompanied by AHPND lesions. Sequence analysis revealed that the pVA plasmid of XN87 contained a mutated pirA ^Vp gene interrupted by the out-of-frame insertion of a transposon gene fragment. The upstream region and the beginning of the original pirA ^Vp gene remained intact, but the insertion caused a 2-base reading frameshift in the remainder of the pirA ^Vp gene sequence and in the downstream pirB ^Vp gene sequence. Reverse transcription-PCR and sequencing of 5HP revealed a bicistronic pirAB ^Vp mRNA transcript that was not produced by XN87, explaining the absence of both toxins in its culture broth. However, the virulence of XN87 revealed that some V. parahaemolyticus isolates carrying mutant pVA plasmids that produce no Pir ^Vp toxins can cause mortality in shrimp in ponds experiencing an outbreak of early mortality syndrome (EMS) but may not have been previously recognized to be AHPND related because they did not cause pathognomonic AHPND lesions.IMPORTANCE Shrimp acute hepatopancreatic necrosis disease (AHPND) is caused by Vibrio parahaemolyticus isolates (VP_AHPND isolates) that harbor the pVA1 plasmid encoding toxins PirA ^Vp and PirB ^Vp The toxins are produced in the shrimp stomach but cause death by massive sloughing of hepatopancreatic tubule epithelial cells (pathognomonic AHPND lesions). V. parahaemolyticus isolate XN87 harbors a mutant pVA plasmid that produces no Pir toxins and does not cause AHPND lesions but still causes ∼50% shrimp mortality. Such isolates may cause a portion of the mortality in ponds experiencing an outbreak of EMS that is not ascribed to VP_AHPND Thus, they pose to shrimp farmers an additional threat that would be missed by current testing for VP_AHPND Moribund shrimp from ponds experiencing an outbreak of EMS that exhibit collapsed hepatopancreatic tubule epithelial cells can serve as indicators for the possible presence of such isolates, which can then be confirmed by additional PCR tests for the presence of a pVA plasmid.

Actin cytoskeleton-modulating T3SS2 effectors and their contribution to the Vibrio parahaemolyticus-induced diarrhea

To understand how bacterial pathogens cause diseases is the most important step in order to prevent the infection and develop an effective treatment. However, the past proceeding studies make us aware of quite-complicated interactions between the host and pathogenic bacteria. Vibrio parahaemolyticus, a food-born pathogen that is a subject of our study, causes inflammatory diarrhea in human upon ingestion of contaminated raw or undercooked seafood. Many virulence factors has been proposed since its discovery in Osaka around 70 years ago, while our research group has revealed that one of these virulence factors, type 3 secretion system 2 (T3SS2), is necessary for diarrhea induced by this bacterium. In addition, we recently found two novel T3SS2 effectors (VopO and VopV) that manipulate the actin cytoskeleton in infected host cells. In this article, I would like to show our findings with regard to biological activities of the effectors and their contributions to the T3SS2-induced enterotoxicity.