Bile acid and bile acid transporters are involved in the pathogenesis of acute hepatopancreatic necrosis disease in white shrimp Litopenaeus vannamei

Proteomic Analysis Reveals the Phenotypic Heterogeneity and Tolerance Mechanisms of Halophilic Vibrio parahaemolyticus Under Dual Stress of Low Salinity and Bile Salts in the Human Intestine

Vibrio parahaemolyticus, a halophilic Gram-negative bacterium commonly found in aquatic products, can colonize the human small intestine, causing gastroenteritis and potentially leukemia. As a major intestinal pathogen, it poses a significant threat to public health. This study aims to investigate the phenotypic heterogeneity of V. parahaemolyticus in the low-salinity and bile salt environments of the human intestinal tract and to elucidate its mechanisms of tolerance and pathogenicity using proteomics. The experimental results indicated that under the low salinity and bile salts conditions of the human intestinal environment, the growth, motility, and biofilm formation of the strains were significantly inhibited. Proteomics analysis revealed that, under these conditions, the energy metabolism, chemotaxis system, flagellar motor, and ribosome-related proteins of V. parahaemolyticus were significantly affected, thereby influencing its growth, motility, and biofilm formation. Furthermore, the activation of the secretion system, particularly the T2SS, enhanced the virulence of secreted factors on host cells. Additionally, the activation of the β-lactam resistance pathway increased resistance to the intestinal environment, thereby enhancing the pathogenicity of V. parahaemolyticus.

Antibiofilm mechanism of mouse gastrointestinal stimulation against Vibrio parahaemolyticus under bile salt culture

Bile salts are crucial microbe-selective inhibitors present in the intestinal tracts of humans and other animals. Environmental and clinical strains of Vibrio parahaemolyticus (V. parahaemolyticus) exhibited different biofilm-forming abilities under bile salt incubation. In order to find an effective way to eliminate biofilm, in this study, environmental strains were subjected to mouse gastrointestinal (GI) stimulation and cultured in medium containing 0.06 % bile salts. The effects of GI stimulation on V. parahaemolyticus biofilm formation were evaluated by biofilm cells assay, atomic force microscopy (AFM) assay, confocal laser scanning microscopy (CLSM) assay, extracellular polysaccharide (EPS) assay, and salmon surface biofilm formation assay. The results showed that GI stimulation diminished the ability of V. parahaemolyticus to form biofilm, significantly reduced biofilm cells, decreased the level of EPS, and destroyed the biofilm structure. For the biofilm formed by V. parahaemolyticus after GI stimulation, AFM observed that the appearance of the biofilm became inhomogeneous and rough, and CLSM observed that the 3D structure of the biofilm became dispersed and sparse. GI stimulation reduced the ability of V. parahaemolyticus to form biofilms on the surface of salmon containing 0.06 % bile salts at both 12 h and 24 h, as evidenced by a decrease in the number of adherent cells. Comparing biofilms formed by tdh-positive V. parahaemolyticus before and after undergoing GI stimulation, a total of 1169 differentially expressed genes (DEGs) were identified by RNA sequencing. And 10 of the biofilm-related genes displayed significant down-regulation after GI stimulation. Enrichment analysis of DEGs revealed that affecting the switch between succinate and fumarate in the TCA cycle could inhibit biofilm formation. This study offers new insights into strategies for preventing biofilm formation by foodborne pathogens.

ARRDC3, a novel α-arrestin, modulates WSSV replication and AHPND pathogenesis in Litopeneaus vannamei

Although shrimp are a valuable protein source, shrimp aquaculture has numerous challenges from various infectious diseases and understanding molecular mechanisms of disease pathogenesis is crucial for disease management. In this study, a gene-to-gene correlation network generated from a transcriptomic database of the stomach of shrimp infected with acute hepatopancreatic necrosis disease (AHPND) was used to identify a new α-arrestin, termed arrestin domain containing-3 gene (LvARRDC3), with crucial roles in development of both AHPND and white spot disease (WSD). Double stranded RNA-mediated silencing or plasmid-mediated overexpression of LvARRDC3 gene significantly decreased expression of WSSV genes (IE1, VP28, and ICP11) and viral genome copy numbers. Nevertheless, in AHPND, silencing the LvARRDC3 gene increased the AHPND-associated plasmid and Pir toxins copy numbers, whereas overexpression of LvARRDC3 had the opposite effect. An in vitro pathogen binding assay with recombinant LvARRDC3 protein produced robust binding to WSSV virions and AHPND-causing V. parahaemolyticus. Moreover, based on immunofluorescence, LvARRDC3 was localized in the cytoplasm of Spodoptera frugiperda (Sf9) insect cells. Therefore, we inferred that LvARRDC3 has a role in pathogen internalization, making it a valuable target for addressing AHPND and WSD and also a biomarker for marker-associated shrimp breeding.

The QS regulator AphBvp promotes expression of the AHPND PirAvp and PirBvp toxins and may enhance virulence under acidic conditions

Shrimp acute hepatopancreatic necrosis disease (AHPND) is one of the most devastating diseases to impact the global shrimp farming industry, with a mortality rate of 70 %-100 %. The key virulence factors are a pair of Photorhabdus insect-related (Pir)-like toxins, PirAvp and PirBvp. In this study, by using an in vitro transcription and translation assay, we first confirmed that the quorum sensing transcriptional regulator AphBvp could trigger the expression of its downstream genes after binding to the AphBvp binding sequence in the promoter region of the pirAvp/pirBvp operon. Next, we showed that AphBvp was essential for the expression of these toxins by using an aphBvp-deletion mutant (ΔaphBvp) derived from the AHPND-causing Vibrio parahaemolyticus. Lastly, we discovered that the expression levels of PirAvp and PirBvp were up-regulated under acidic conditions (pH 4.5), and further showed that an acidic environment promoted the binding of AphBvp to the pirABvp promoter. We speculate that this was because the acidic environment favored the formation of AphBvp tetramers, which is important for binding to DNA. Taken together, these findings improve our understanding of the gene regulatory mechanisms of pirAvp and pirBvp, and suggest that the pH value of the environment might affect the virulence of AHPND-causing V. parahaemolyticus.

Comparative analysis of transcriptomics and metabolomics provides insights into the mechanisms of VPAHPND invasion and hepatopancreatic damage in Litopenaeus vannamei

Acute hepatopancreatic necrosis disease (AHPND) poses significant threats to the global shrimp farming industry; however, its molecular mechanisms remain largely unknown. Previous research has primarily focused on comparisons between infected and non-infected states, limiting our understanding of VPAHPND mechanisms. We integrated transcriptomic and metabolomic analyses to investigate the pathogenic mechanism underpinning AHPND in highly vulnerable post-larvae (PL) stage shrimp. By comparing shrimp infected with VPAHPND, those infected with non-VPAHPND, and uninfected shrimp (controls), we identified different VPAHPND infection responses, including significant cytoskeleton and metabolic reprogramming changes. Specifically, VPAHPND infection disturbed lipid, glutathione, and bile acid metabolism, while a key regulatory factor Farnesoid X Receptor (FXR) in these pathways was down-regulated. These findings suggest that VPAHPND manipulates host metabolism to enhance infectivity, leading to severe and irreparable hepatopancreas damage. Our study highlights the molecular interactions between VPAHPND and shrimp, and provides potential targets to mitigate the impact of AHPND in aquaculture.

Bile acids activate the antibacterial T6SS1 in the gut pathogen Vibrio parahaemolyticus

The marine bacterium Vibrio parahaemolyticus is a major cause of seafood-borne gastroenteritis in humans and of acute hepatopancreatic necrosis disease in shrimp. Bile acids, produced by the host and modified into secondary bile acids by commensal bacteria in the gastrointestinal tract, induce the virulence factors leading to disease in humans and shrimp. Here, we show that secondary bile acids also activate this pathogen's type VI secretion system 1, a toxin delivery apparatus mediating interbacterial competition. This finding implies that Vibrio parahaemolyticus exploits secondary bile acids to activate its virulence factors and identify the presence of commensal bacteria that it needs to outcompete in order to colonize the host.IMPORTANCEBacterial pathogens often manipulate their host and cause disease by secreting toxic proteins. However, to successfully colonize a host, they must also remove commensal bacteria that reside in it and may compete with them over resources. Here, we find that the same host-derived molecules that activate the secreted virulence toxins in a gut bacterial pathogen, Vibrio parahaemolyticus, also activate an antibacterial toxin delivery system that targets such commensal bacteria. These findings suggest that a pathogen can use one cue to launch a coordinated, trans-kingdom attack that enables it to colonize a host.

Changes of bacterial communities and bile acid metabolism reveal the potential "intestine-hepatopancreas axis" in shrimp

The interaction between the gut and the liver plays a significant role in individual health and diseases. Mounting evidence supports that bile acids are important metabolites in the bidirectional communication between the gut and the liver. Most of the current studies on the "gut-liver axis" have focused on higher vertebrates, however, few was reported on lower invertebrates such as shrimp with an open circulatory system. Here, microbiomic and metabolomic analyses were conducted to investigate the bacterial composition and bile acid metabolism in intestine, hemolymph and hepatopancreas of Penaeus vannamei fed diets supplemented with octanoic acid and oleic acid. After six days of feeding, the bacterial composition in intestine, hemolymph and hepatopancreas changed at different stages, with significant increases in the relative abundance of several genera such as Pseudomonas and Rheinheimera in intestine and hepatopancreas. Notably, there was a more similar bacterial composition in intestine and hepatopancreas at the genus level, which indicated the close communication between shrimp intestine and hepatopancreas. Meanwhile, higher content of some bile acids such as lithocholic acid (LCA) and α-muricholic acid (α-MCA) in intestine and lower content of some bile acids such as taurohyocholic acids (THCA) and isolithocholic acid (IsoLCA) in hepatopancreas were detected. Furthermore, Spearman correlation analysis revealed a significant correlation between bacterial composition and bile acid metabolism in intestine and hepatopancreas. The microbial source tracking analysis showed that there was a high proportion of intestine and hepatopancreas bacterial community as the source of each other. Collectively, these results showed a strong crosstalk between shrimp intestine and hepatopancreas, which suggests a unique potential "intestine-hepatopancreas axis" in lower invertebrate shrimp with an open circulatory system. Our finding contributed to the understanding of the interplay between shrimp intestine and hepatopancreas in the view of microecology and provided new ideas for shrimp farming and disease control.

Gut microbiota of Pacific white shrimp (Litopenaeus vannamei) exhibits distinct responses to pathogenic and non-pathogenic Vibrio parahaemolyticus

Acute hepatopancreatic necrosis disease (AHPND), a high-mortality-rate shrimp disease, is caused by specific Vibrio parahaemolyticus (Vp) strains with a plasmid encoding the PirABVp toxins. As a bacterial pathogen, the invasion of AHPND-causing Vp might impose pressure on commensal microbiota in the shrimp gut, while the relationship between the pathogenesis of AHPND and the dysbiosis of gut bacterial communities remains unclear. Here we explored the temporal changes of shrimp gut microbiota in response to AHPND-causing and non-AHPND-causing Vp strains, with the non-infected controls as a baseline of the shrimp gut microbiota. The diversity and composition of bacterial communities from 168 gut samples (covering three treatments at seven time points with eight individuals per set) were investigated using 16S rRNA gene metabarcoding with high-throughput sequencing. The results showed that (i) species diversity of gut bacterial communities declined in Vp-infected shrimp, independent of the strain pathogenicity; (ii) taxonomic compositions of gut bacterial communities were significantly different between shrimp infected by AHPND-causing and non-AHPND-causing Vp strains; (iii) short-term (within 6 hours) compositional shifts in the gut microbiota were found only in AHPND-causing Vp-infected shrimp; (iv) the gut microbiota of AHPND-causing Vp-infected shrimp was enriched with genera Photobacterium and Vibrio, with a decline in Candidatus Bacilliplasma; and (v) functional predictions suggested the loss of normal metabolism due to compositional shifts in the gut microbiota. Our work reveals distinct features of community dynamics in shrimp gut microbiota, associated with pathogenic versus non-pathogenic Vibrio infections, providing a new perspective of the pathogenesis of AHPND. IMPORTANCE Shrimp production is continually threatened by newly emerging diseases, such as AHPND, which is caused by specific Vp strains. Previous studies on the pathogenesis of AHPND have mainly focused on the histopathology and immune responses of the host. However, more attention needs to be paid to the gut microbiota, which acts as the first barrier to pathogen colonization. In this study, we revealed that shrimp gut microbiota responded differently to pathogenic and non-pathogenic Vp strains, with bacterial genera Photobacterium and Vibrio enriched in pathogenic Vp-infected shrimp, and Candidatus Bacilliplasma enriched in non-pathogenic Vp-infected shrimp. Moreover, functional predictions suggested that changes in taxonomic compositions would further affect normal metabolic functions, emphasizing the importance of sustaining an equilibrium in the gut microbiota. Several biomarkers associated with specific microbial taxa and functional pathways were identified in our data sets, which help predict the incidence of disease outcomes.

Resilience and probiotic interventions to prevent and recover from shrimp gut dysbiosis

To counter the recurrent outbreaks of bacterial (acute hepatopancreatic necrosis disease; AHPND) and viral (white spot disease; WSD) shrimp diseases, which still remain a threat to the global industry, shrimp gut microbiota research has been gaining more attention in recent years, and the use of probiotics in aquaculture has had promising results in improving shrimp gut health and immunity. In this review based on our studies on AHPND and WSD, we summarize our current understanding of the shrimp gastrointestinal tract and the role of the microbiota in disease, as well as effects of probiotics. We focus particularly on the concept of microbiota resilience, and consider strategies that can be used to restore shrimp gut health by probiotic intervention at a crucial time during gut microbiota dysbiosis. Based on the available scientific evidence, we argue that the use of probiotics potentially has an important role in controlling disease in shrimp aquaculture.

A bacterial binary toxin system that kills both insects and aquatic crustaceans: Photorhabdus insect-related toxins A and B

Photorhabdus insect-related toxins A and B (PirA and PirB) were first recognized as insecticidal toxins from Photorhabdus luminescens. However, subsequent studies showed that their homologs from Vibrio parahaemolyticus also play critical roles in the pathogenesis of acute hepatopancreatic necrosis disease (AHPND) in shrimps. Based on the structural features of the PirA/PirB toxins, it was suggested that they might function in the same way as a Bacillus thuringiensis Cry pore-forming toxin. However, unlike Cry toxins, studies on the PirA/PirB toxins are still scarce, and their cytotoxic mechanism remains to be clarified. In this review, based on our studies of V. parahaemolyticus PirAvp/PirBvp, we summarize the current understanding of the gene locations, expression control, activation, and cytotoxic mechanism of this type of toxin. Given the important role these toxins play in aquatic disease and their potential use in pest control applications, we also suggest further topics for research. We hope the information presented here will be helpful for future PirA/PirB studies.

Taurine metabolism is modulated in Vibrio-infected Penaeus vannamei to shape shrimp antibacterial response and survival

BACKGROUND

Numerous microorganisms are found in aquaculture ponds, including several pathogenic bacteria. Infection of cultured animals by these pathogens results in diseases and metabolic dysregulation. However, changes in the metabolic profiles that occur at different infection stages in the same ponds and how these metabolic changes can be modulated by exogenous metabolites in Penaeus vannamei remain unknown.

RESULTS

Here, we collected gastrointestinal tract (GIT) samples from healthy, diseased, and moribund P. vannamei in the same aquaculture pond for histological, metabolic, and transcriptome profiling. We found that diseased and moribund shrimp with empty GITs and atrophied hepatopancreas were mainly infected with Vibrio parahaemolyticus and Vibrio harveyi. Although significant dysregulation of crucial metabolites and their enzymes were observed in diseased and moribund shrimps, diseased shrimp expressed high levels of taurine and taurine metabolism-related enzymes, while moribund shrimp expressed high levels of hypoxanthine and related metabolism enzymes. Moreover, a strong negative correlation was observed between taurine levels and the relative abundance of V. parahaemolyticus and V. harveyi. Besides, exogenous taurine enhanced shrimp survival against V. parahaemolyticus challenge by increasing the expression of key taurine metabolism enzymes, mainly, cysteine dioxygenase (CDO) and cysteine sulfinic acid decarboxylase (CSD).

CONCLUSIONS

Our study revealed that taurine metabolism could be modulated by exogenous supplementation to improve crustacean immune response against pathogenic microbes. Video Abstract.

Shrimp Lipid Droplet Protein Perilipin Involves in the Pathogenesis of AHPND-Causing Vibrio parahaemolyticus

Acute hepatopancreatic necrosis disease (AHPND), caused by a unique strain of Vibrio parahaemolyticus (Vp (AHPND)), has become the world’s most severe debilitating disease in cultured shrimp. Thus far, the pathogenesis of AHPND remains largely unknow. Herein, in Litopenaeus vannamei, we found that a Vp (AHPND) infection significantly increased the expression of lipid droplets (LDs) protein LvPerilipin, as well as promoted the formation of LDs. In addition, the knockdown of LvPerilipin increased the shrimp survival rate in response to the Vp (AHPND) infection, and inhibited the proliferation of Vp (AHPND). Furthermore, we demonstrated that LvPerilipin depletion could increase the production of reactive oxygen species (ROS), which may be responsible for the decreased Vp (AHPND) proliferation. Taken together, our current data for the first time reveal that the shrimp lipid droplets protein Perilipin is involved in the pathogenesis of Vp (AHPND) via promoting LDs accumulation and decreasing ROS production.

Effects of dietary chenodeoxycholic acid supplementation in a low fishmeal diet on growth performance, lipid metabolism, autophagy and intestinal health of Pacific white shrimp, Litopenaeus vannamei

An 8-week feeding trial was conducted to evaluate the effects of chenodeoxycholic acid (CDCA) on growth performance, body composition, lipid metabolism, and intestinal health of juvenile white shrimp, Litopenaeus vannamei fed a low fishmeal diet. Four practical diets were formulated: HFM (25% fishmeal), LFM (15% fishmeal), LB1 (LFM + 0.04% CDCA), LB2 (LFM + 0.08% CDCA). Each diet was assigned to four tanks with forty shrimp (initial weight 0.33 ± 0.03 g) per tank. The results indicated that the growth performance of shrimp were similar between the four groups; the crude lipid content of shrimp fed the LB2 diet was significantly lower than those fed the HFM diet (P < 0.05). The lipase activity content in hepatopancreatic were significantly higher in the two CDCA supplemented groups than that in LFM group; the contents of total cholesterol, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol in hemolymph were significantly lower in LFM group, LB1 group and LB2 group than that in HFM group (P < 0.05). The shrimp fed LB1 diet was significantly decreased the intestinal expression levels of tube than those fed in HFM diet; the intestinal gene expression of imd and toll were significantly lower in LB2 group than those in HFM group (P < 0.05). The results of hepatopancreas gene expression suggest that shrimp fed the LFM diet showed significantly upregulated expression levels of sterol regulatory element-binding protein (srebp), acetyl-CoA carboxylase (acc), and carnitine palmitoyltransferase 1 (cpt-1) than those fed the HFM diet; shrimp fed the LB1 diet showed significantly upregulated expression levels of srebp, acc, and AMP-activated protein kinase (ampk) than those fed the HFM diet; shrimp fed the LB2 diet had higher expression levels of srebp, acc, and cpt-1 than those fed the HFM diet (P < 0.05). In the hepatopancreas, the shrimp fed the LFM diet shown significantly up-regulated the expression levels of beclin1 compared to those fed HFM diet; the expression levels of autophagy-related protein13 (atg3), autophagy-related protein 12 (atg12) of in shrimp fed the LB1 diet were significantly higher than those fed the HFM diet; and the expression levels of autophagy-related protein13 (atg13), beclin1, atg3, atg12, autophagy-related protein 9 (atg9) of shrimp fed LB2 diet were significantly higher than those fed the HFM diet (P < 0.05). The atg3 in intestine of shrimp fed the LB2 diet were significantly higher than those fed the HFM diet (P < 0.05). Intestinal mucous fold were damaged, hepatic tubules were disorganized and B cells appeared to be swollen in LFM group. The fold height and width of shrimp fed the diets supplemented with CDCA increased significantly than those fed the LFM diet (P < 0.05), the hepatic tubules were neatly arranged, and R cells increased. In conclusion, supplementary CDCA in a low fishmeal diet promoted lipid metabolism, enhanced autophagy of shrimp, also improved the health of the intestine and hepatopancreas.

Litopenaeus vannamei peritrophin interacts with WSSV and AHPND-causing V. parahaemolyticus to regulate disease pathogenesis

Peritrophins are peritrophic membrane (PM) proteins that can interact with chitin fibers via chitin-binding domains. Peritrophins have essential roles in providing porosity and strength to the PM that lines the shrimp midgut. Acute hepatopancreatic necrosis disease (AHPND), caused by strains of V. parahaemolyticus, is known to initially colonize the shrimp stomach and simultaneously disrupt its structural barriers (e.g., cuticle or epithelial tissues) to reach the hepatopancreas. Although stomach and hepatopancreas were identified as target tissues involved in AHPND pathogenesis, our results indicated that peritrophin in peritrophic membrane has a crucial role in determining not only colonization of AHPND-causing bacteria but also their tissue distribution. As the interaction between LvPeritrophin (LvPT) and WSSV (white spot syndrome virus) is not well understood, we noted that LvPT expression was upregulated in shrimp stomach challenged with either WSSV or AHPND. In an in vitro pathogen binding assay, there was strong binding of recombinant LvPT WSSV and AHPND-causing V. parahaemolyticus, and various bacteria. Furthermore, dsRNA-mediated LvPT silencing inhibited WSSV gene expression and viral genome replication. However, downregulation of LvPT gene expression increased copies of AHPND-causing bacteria in shrimp digestive tract, and facilitated bacterial colonization in stomach. In conclusion, we speculated that LvPT might regulate bacterial colonization during AHPND, whereas in WSSV infection, LvPT silencing favored the host. Although recombinant LvPT had strong binding with WSSV, the precise role of LvPT in WSSV infection needs further investigation. These findings increased our understanding of host-pathogen interactions in AHPND and WSSV infection that can be applied in shrimp aquaculture for developing effective antibacterial and antiviral strategies.

A novel C-type lectin LvCTL 4.2 has antibacterial activity but facilitates WSSV infection in shrimp (L. vannamei)

Glycan-binding protein C-type lectin (CTL), one of the pattern recognition receptors (PRRs), binds to carbohydrates on the surface of pathogens and elicits antimicrobial responses in shrimp innate immunity. The objective was to identify and characterize a novel C-type lectin LvCTL 4.2 in Litopenaeus vannamei. The LvCTL 4.2 protein consisted of a signal peptide at the N terminal and a carbohydrate-recognition domain (CRD) with a mutated mannose-binding (Glu-Pro-Ala; EPA) motif at the C terminal, and thereby has a putative secreted mannose-binding C-type lectin architecture. LvCTL 4.2 was highly expressed in nervous tissue and stomach. Infection with white spot syndrome virus (WSSV) induced expression of LvCTL 4.2 in shrimp stomach at 12 h post infection. Conversely, there was no obvious upregulation in expression of LvCTL 4.2 in stomach or hepatopancreas of shrimp with AHPND (acute hepatopancreas necrosis disease). Pathogen binding assays confirmed recombinant LvCTL 4.2 protein (rLvCTL 4.2) had significant binding ability with the WSSV virion, Gram-negative, and Gram-positive bacteria. Moreover, rLvCTL 4.2 had strong growth inhibition of Vibrio parahaemolyticus. Silencing LvCTL 4.2 suppressed WSSV replication, whereas pretreatment of WSSV with rLvCTL 4.2 facilitated viral replication in vivo. In conclusion, LvCTL 4.2 acted as a PRR that inhibited AHPND-causing bacteria, but facilitated WSSV pathogenesis.

Metabolic Alterations in Shrimp Stomach During Acute Hepatopancreatic Necrosis Disease and Effects of Taurocholate on Vibrio parahaemolyticus

Acute hepatopancreatic necrosis disease (AHPND), a recently emerged bacterial shrimp disease, has increased shrimp mortality and caused huge economic losses in many Asian countries. However, molecular factors underlying pathogenesis of this disease remain largely unknown. Our objective was to characterize metabolic alterations in shrimp stomach during AHPND and determine effects of taurocholate on AHPND-causing Vibrio parahaemolyticus. Based on metabolomics, pathways for lipid metabolism and for primary bile acid (BA) synthesis were majorly affected following AHPND infection. Bile acid metabolites, namely taurocholate, were downregulated in the metabolomics database. This prompted us to study effects of taurocholate on biofilm formation, PirAB ^vp toxin release and biofilm detachment capabilities in AHPND-causing V. parahaemolyticus. Treatment of this bacterium with high concentration of taurocholate, a primary bile acid, induced biofilm formation, PirAB ^vp toxin release and facilitated the dispersion of bacterial cells. Taken together, our findings suggest that AHPND infection can affect the lipid metabolites in shrimp stomach, and further suggest that the primary bile acid taurocholate is important for the virulence of AHPND-causing V. parahaemolyticus.

Cytotoxicity of Vibrio parahaemolyticus AHPND toxin on shrimp hemocytes, a newly identified target tissue, involves binding of toxin to aminopeptidase N1 receptor

Acute hepatopancreatic necrosis disease (AHPND) caused by PirAB^VP-producing strain of Vibrio parahaemolyticus, VP_AHPND, has seriously impacted the shrimp production. Although the VP_AHPND toxin is known as the VP_AHPND virulence factor, a receptor that mediates its action has not been identified. An in-house transcriptome of Litopenaeus vannamei hemocytes allows us to identify two proteins from the aminopeptidase N family, LvAPN1 and LvAPN2, the proteins of which in insect are known to be receptors for Cry toxin. The membrane-bound APN, LvAPN1, was characterized to determine if it was a VP_AHPND toxin receptor. The increased expression of LvAPN1 was found in hemocytes, stomach, and hepatopancreas after the shrimp were challenged with either VP_AHPND or the partially purified VP_AHPND toxin. LvAPN1 knockdown reduced the mortality, histopathological signs of AHPND in the hepatopancreas, and the number of virulent VP_AHPND bacteria in the stomach after VP_AHPND toxin challenge. In addition, LvAPN1 silencing prevented the toxin from causing severe damage to the hemocytes and sustained both the total hemocyte count (THC) and the percentage of living hemocytes. We found that the rLvAPN1 directly bound to both rPirA^VP and rPirB^VP toxins, supporting the notion that silencing of LvAPN1 prevented the VP_AHPND toxin from passing through the cell membrane of hemocytes. We concluded that the LvAPN1 was involved in AHPND pathogenesis and acted as a VP_AHPND toxin receptor mediating the toxin penetration into hemocytes. Besides, this was the first report on the toxic effect of VP_AHPND toxin on hemocytes other than the known target tissues, hepatopancreas and stomach.

A specific strain of Vibrio parahaemolyticus causing acute hepatopancreatic necrosis disease (AHPND) in shrimp or VP_AHPND produces a binary toxin (PirAB^vp toxin) that is previously known to induce cell death of stomach and hepatopancreas but the molecular mechanism has not been defined. Similar to Cry toxin receptor in insects, a novel aminopeptidase N1 protein from L. vannamei (LvAPN1) was identified as a putative receptor of VP_AHPND toxin. Suppression of LvAPN1 reduced the number of AHPND virulence plasmids in stomach and occurrence of AHPND clinical sign, sustained the number of total hemocyte count, and elevated the number of viable hemocyte. We demonstrated that VP_AHPND toxin challenge induces hemocyte cell damage and it interacts with LvAPN1 in vitro. Collectively, our finding suggested that not only stomach and hepatopancreas but also hemocyte are the VP_AHPND target tissues where LvAPN1 serves as a VP_AHPND toxin receptor. This study provides novel insight into the contributions of LvAPN1 receptor towards the AHPND pathogenesis in shrimp and may extend to the development of AHPND preventive measure in shrimp.

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.

A Review of the Functional Annotations of Important Genes in the AHPND-Causing pVA1 Plasmid

Acute hepatopancreatic necrosis disease (AHPND) is a lethal shrimp disease. The pathogenic agent of this disease is a special Vibrio parahaemolyticus strain that contains a pVA1 plasmid. The protein products of two toxin genes in pVA1, pirAvp and pirBvp, targeted the shrimp's hepatopancreatic cells and were identified as the major virulence factors. However, in addition to pirAvp and pirBvp, pVA1 also contains about ~90 other open-reading frames (ORFs), which may encode functional proteins. NCBI BLASTp annotations of the functional roles of 40 pVA1 genes reveal transposases, conjugation factors, and antirestriction proteins that are involved in horizontal gene transfer, plasmid transmission, and maintenance, as well as components of type II and III secretion systems that may facilitate the toxic effects of pVA1-containing Vibrio spp. There is also evidence of a post-segregational killing (PSK) system that would ensure that only pVA1 plasmid-containing bacteria could survive after segregation. Here, in this review, we assess the functional importance of these pVA1 genes and consider those which might be worthy of further study.