Expression of immune-related genes in the digestive organ of shrimp, Penaeus monodon, after an oral infection by Vibrio harveyi

Microbiome analysis reveals the intestinal microbiota characteristics and potential impact of Procambarus clarkii

The intestinal microbiota interacts with the host and plays an important role in the immune response, digestive physiology, and regulation of body functions. In addition, it is also well documented that the intestinal microbiota of aquatic animals are closely related to their growth rate. However, whether it resulted in different sizes of crayfish in the rice-crayfish coculture model remained vague. Here, we analyzed the intestinal microbiota characteristics of crayfish of three sizes in the same typical rice-crayfish coculture field by high-throughput sequencing technology combined with quantitative real-time polymerase chain reaction (qRT-PCR) and enzyme activity, investigating the relationship between intestinal microbiota in crayfish and water and sediments. The results showed that the dominant intestinal microbiota of crayfish was significantly different between the large size group (BS), normal size group (NS), and small size group (SS), where Bacteroides and Candidatus_Bacilloplasma contributed to the growth of crayfish by facilitating food digestion through cellulolysis, which might be one of the potential factors affecting the difference in sizes. Follow-up experiments confirmed that the activity of lipase (LPS) and protease was higher in BS, and the relative expression of development-related genes, including alpha-amylase (α-AMY), myocyte-specific enhancer factor 2a (MEF2a), glutathione reductase (GR), chitinase (CHI), and ecdysone receptor (EcR), in BS was significantly higher than that in SS. These findings revealed the intestinal microbiota characteristics of crayfish of different sizes and their potential impact on growth, which is valuable for managing and manipulating the intestinal microbiota in crayfish to achieve high productivity in practice. KEY POINTS: • Significant differences in the dominant microflora of BS, NS, and SS in crayfish. • Cellulolysis might be a potential factor affecting different sizes in crayfish. • Adding Bacteroides and Candidatus_Bacilloplasma helped the growth of crayfish.

Transcriptome-based network analysis reveals hub immune genes and pathways of hepatopancreas against LPS in Amphioctopus fangsiao

The hepatopancreas is the biggest digestive organ in Amphioctopus fangsiao (A. fangsiao), but also undertakes critical functions like detoxification and immune defense. Generally, pathogenic bacteria or endotoxin from the gut microbiota would be arrested and detoxified in the hepatopancreas, which could be accompanied by the inevitable immune responses. In recent years, studies related to cephalopods immune have been increasing, but the molecular mechanisms associated with the hepatopancreatic immunity are still unclear. In this study, lipopolysaccharide (LPS), a major component of the cell wall of Gram-negative bacteria, was used for imitating bacteria infection to stimulate the hepatopancreas of A. fangsiao. To investigate the immune process happened in A. fangsiao hepatopancreas, we performed transcriptome analysis of hepatopancreas tissue after LPS injection, and identified 2615 and 1943 differentially expressed genes (DEGs) at 6 and 24 h post-injection, respectively. GO and KEGG enrichment analysis showed that these DEGs were mainly involved in immune-related biological processes and signaling pathways, including ECM-receptor interaction signaling pathway, Phagosome signaling pathway, Lysosome signaling pathway, and JAK-STAT signaling pathways. The function relationships between these DEGs were further analyzed through protein-protein interaction (PPI) networks. It was found that Mtor, Mapk14 and Atm were the three top interacting DEGs under LPS stimulation. Finally, 15 hub genes involving multiple KEGG signaling pathways and PPI relationships were selected for qRT-PCR validation. In this study, for the first time we explored the molecular mechanisms associated with hepatopancreatic immunity in A. fangsiao using a PPI networks approach, and provided new insights for understanding hepatopancreatic immunity in A. fangsiao.

Structural and immunohistochemical characterization of pancreas of Molly fish (Poecilia sphenops), with a special reference to its immune role

Recently, teleost species have been considered important model systems for investigating different research areas including immunologic one. The available literature provides poor data about the localization and the structure of pancreas in Molly fish. Moreover, little attention has been paid to the immunologic role of pancreatic tissue of teleost, particularly Molly fish; therefore, this study aimed to highlights the description of pancreatic tissue in Molly fish using light- and electron- microscopy, focusing on the role of pancreatic immune cells and pancreatic acinar cells in immune responses. Microscopic analysis revealed that the pancreas of Molly fish was composed of intrahepatic, disseminated and compact parts. Exocrine pancreatic tissue was diffusely extended within the hepatic tissue forming hepatopancreas. The disseminated pancreas appeared as several irregular nodules of pancreatic tissue localized within the mesenteric adipose tissue. The compact pancreas appeared as an oval shaped body embedded within the mesenteric adipose tissue between the spleen and the intestinal loops. Several telocytes and melanomacrophages were detected within the disseminated pancreatic nodules. Moreover, dendritic cells were found in a close association to the exocrine pancreatic acini. The pancreatic acinar cells showed strong immunoreactivity to APG5, TGF-β, IL-1β, NF-κB, Nrf2, and SOX9 in both hepatopancreas and disseminated pancreas of Molly fish. S100 protein revealed a strong expression in the exocrine pancreatic acinar cells of disseminated pancreas and also in the endocrine cells of the compact pancreas. In conclusion, findings of this study suggest the potential role of the pancreas of the Molly fish in cell proliferation and differentiation, proinflammatory cytokines stimulation, and regulation of both innate and adaptive immunity. RESEARCH HIGHLIGHTS: Telocytes and melanomacrophages were detected in the disseminated pancreatic nodules of the Molly fish. In Molly fish, dendritic cells were found in a close association to the exocrine pancreatic acini. Strong immunoreactivity of the pancreatic acinar cells of the Molly fish to APG5, TGF-β, IL-1β, NF-κB, Nrf2, SOX9, and S100.

Effects of β-1,3-glucan on growth, immune responses, and intestinal microflora of the river prawn (Macrobrachium nipponense) and its resistance against Vibrio parahaemolyticus

In this study, we investigated the impact of β-1,3-glucan on the immune responses and gut microbiota of the river prawn (Macrobrachium nipponense) in the presence of Vibrio parahaemolyticus stress. Shrimps were fed one of the following diets: control (G1), 0.2% curdlan (G2), 0.1% β-1,3-glucan (G3), 0.2% β-1,3-glucan (G4), or 1.0% β-1,3-glucan (G5) for 6 weeks and then challenged with V. parahaemolyticus for 96 h. Under Vibrio stress, shrimps in G4 exhibited the highest length gain rate, weight gain rate, and survival rate. They also showed increased intestinal muscle thickness and villus thickness compared to the control and 0.2% curdlan groups. The apoptosis rate was lower in G4 than in the control group, and the digestive enzyme activities (pepsin, trypsin, amylase, and lipase), immune enzyme activities (acid phosphatase, alkaline phosphatase, lysozyme, and phenoxidase), and energy metabolism (triglyceride, cholesterol, glycogen, and lactate dehydrogenase) were enhanced. Expression levels of growth-related genes (ecdysone receptor, calmodulin-dependent protein kinase I, chitin synthase, and retinoid X receptor) and immune-related genes (toll-like receptor 3, myeloid differentiation primary response 88, mitogen-activated protein kinase 7, and mitogen-activated protein kinase 14) were higher in G4 than in the control. Microbiota analysis indicated higher bacterial abundance in shrimps fed β-1,3-glucan, as evidenced by Sob, Chao1, and ACE indices. Moreover, 0.2% β-1,3-glucan increased the relative abundances of Bacteroidota and Firmicutes while reducing those of Corynebacteriales and Lactobacillales. In summary, β-1,3-glucan enhances immune enzyme activities, alters immune-related gene expression, and impacts gut microbial diversity in shrimp. These findings provide valuable insights into the mechanisms underlying β-1,3 glucan's immune-enhancing effects.

Shrimp microbiome and immune development in the early life stages

With its contribution to nutrition, development, and disease resistance, gut microbiome has been recognized as a crucial component of the animal's health and well-being. Microbiome in the gastrointestinal tract constantly interacts with the host animal's immune systems as part of the normal function of the intestines. Interactions between the microbiome and the immune system are complex and dynamic, with the microbiome shaping immune development and function. In contrast, the immune system modulates the composition and activity of the microbiome. In shrimp, as with all other aquatic animals, the interaction between the microbiome and the animals occurs at the early developmental stages. This early interaction is likely essential to the development of immune responses of the animal as well as many key physiological developments that further contribute to the health of shrimp. This review provides background knowledge on the early developmental stage of shrimp and its microbiome, examines the interaction between the microbiome and the immune system in the early life stage of shrimp, and discusses potential pitfalls and challenges associated with microbiome research. Understanding the interaction between the microbiome and shrimp immune system at this crucial developmental stage could have the potential to aid in the establishment of a healthy microbiome, improve shrimp survival, and provide ways to shape the microbiome with feed supplements or other strategies.

Molecular characterization and functional analysis of TRIM37 from black tiger shrimp (Penaeus monodon)

The family of TRIM proteins with E3 ubiquitin ligase activity served important roles in the regulation of innate immune processes, in particular antiviral and proinflammatory cytokine responses. In this study, a novel TRIM37 homolog was identified from Penaeus monodon (named PmTRIM37). The PmTRIM37 protein contained three conserved domains (one RING finger domain, a B-box, and one Coiled-coil region) at its N-terminal and one Meprin and MATH domain at its C-terminal. The MATH domain was the characteristic of TRIM37 family. PmTRIM37 has relatively high expression in immune-related tissues such as hepatopancreas, gills, lymphoid organs and hemocytes. The expression levels of PmTRIM37 in hepatopancreas and lymphoid organs were significantly up-regulated after white spot syndrome virus (WSSV) infection. Knock down of PmTRIM37 promoted WSSV replication and VP28 expression, suggesting that PmTRIM37 played a negative role in WSSV infection. Further studies revealed that PmTRIM37 positively regulated the NF-κB pathway and Antimicrobial peptides (AMP) expression during WSSV infection. These findings indicated that PmTRIM37 might restrict WSSV replication by positively regulating NF-κB pathway during WSSV infection in P. monodon.

A novel perlucin-like protein (PLP) protects Litopenaeus vannamei against Vibrio harveyi infection

C-type lectins (CTLs), as pattern recognition receptors (PRRs), play an important role in the innate immunity of Litopenaeus vannamei. In this study, a novel CTL, named perlucin-like protein (PLP), was identified from L. vannamei, which shared homology sequences of PLP from Penaeus monodon. PLP from L. vannamei was expressed in the hepatopancreas, eyestalk, muscle and brain and could be activated in the tissues (hepatopancreas, muscle, gill and intestine) after infection with the pathogen Vibrio harveyi. Bacteria (Vibrio alginolyticus, V. parahaemolyticus, V. harveyi, Streptococcus agalactiae and Bacillus subtilis) could be bound and agglutinated by the PLP recombinant protein in a Ca2+-dependent manner. Moreover, PLP could stabilise the expression of the immune-related genes (ALF, SOD, HSP70, Toll4 and IMD) and apoptosis gene (Caspase2). The RNAi of PLP could remarkably affect the expression of antioxidant gene, antimicrobial peptide genes, other CTLs, apoptosis genes, Toll signaling pathways, and IMD signaling pathways. Moreover, PLP reduced the bacterial load in the hepatopancreas. These results suggested that PLP was involved in the innate immune response against V. harveyi infection by recognising bacterial pathogens and activating the expression of immune-related and apoptosis genes.

Comparative effects of different bacterial lipopolysaccharides on modulation of immune levels to improve survival of the black tiger shrimp

To prevent loss from disease, immunostimulants have been used as dietary supplements to improve immunity and survival of shrimps. Among the various types of immunostimulants, there is increasing evidence that a diet enriched with bacterial lipopolysaccharide can reduce the mortality rate of shrimp under exposure to pathogens. Here, the immunostimulatory effects of bacterial lipopolysaccharide (LPS) from various bacterial sources were explored. Bacterial LPS was extracted from a shrimp pathogen, Vibrio harveyi and its effects were compared with the commercially available LPS from the non-shrimp pathogen, Escherichia coli. Our results revealed that the LPS from V. harveyi was different in molecular size but contained similar functional groups to that from E. coli. To understand their molecular mechanisms, bacterial LPS from the two sources were applied as a supplementary diet and fed to juvenile shrimp for 4-week feeding period before tissue samples were collected for transcriptomic analysis by next generation sequencing. Gene expression profiling revealed that major immune-related genes such as pattern recognition proteins (PRPs), proteinases and proteinase inhibitors, prophenoloxidase systems (proPO system), antimicrobial peptides (AMPs), signaling transduction pathways, heat shock proteins (HSPs), oxidative stress responses, and other immune-related molecules such as mucins and peritrophins were modulated in the groups of shrimp fed with bacterial LPS from both sources, but at different levels. The results suggest that bacterial LPS could modulate shrimp immune system, and different LPS sources led to different activation of immune pathways. Additionally, metabolic-related genes were affected by LPS, suggesting that energy was required for immune stimulation. In the V. harveyi pathogen challenge trial, all shrimp groups fed with diets containing LPS from both bacterial sources showed better survival than the control group without LPS. When comparing groups fed with LPS supplemented diets, the higher concentration of LPS (8 μg/body weight) from E. coli resulted in a better survival rate than a lower concentration (4 μg/body weight). Conversely, shrimp fed with a diet containing LPS from V. harveyi showed a lower survival rate when a higher dose of LPS (8 μg/body weight) was administered than the group fed with a lower concentration of LPS (4 μg/body weight). This could be due to overstimulation of shrimp immune responses, especially by LPS derived from shrimp pathogens, resulting in a reverse effect. These results confirm that immunity in shrimp upon administration of bacterial LPS depends on the origin and dose of the LPS administered.

Transcriptome analysis provides novel insights into the immune mechanisms of Macrobrachium nipponense during molting

Molting is a basic physiological behavior of the Oriental river prawn (Macrobrachium nipponense), however, the gene expression patterns and immune mechanisms during the molting process of Oriental river prawn are unclear. In the current study, the gene expression levels of the hepatopancreas of the Oriental river prawn at different molting stages (pre-molting, Prm; mid-molting, Mm; and post-molting, Pom) were detected by mRNA sequencing. A total of 1721, 551, and 1054 differentially expressed genes (DEGs) were identified between the Prm hepatopancreas (PrmHe) and Mm hepatopancreas (MmHe), MmHe and Pom hepatopancreas (PomHe) and PrmHe and PomHe, respectively. The results showed that a total of 1151 DEGs were annotated into 316 signaling pathways, and the significantly enriched immune-related pathways were "Lysosome", "Hippo signaling pathway", "Apoptosis", "Autophagy-animal", and "Endocytosis". The qRT-PCR verification results of 30 randomly selected DEGs were consistent with RNA-seq. The expression patterns of eight immune related genes in different molting stages of the Oriental river prawn were analyzed by qRT-PCR. The function of Caspase-1 (CASP1) was further investigated by bioinformatics, qRT-PCR, and RNAi analysis. CASP1 has two identical conserved domains: histidine active site and pentapeptide motif, and the expression of CASP1 is the highest in ovary. The expression levels of triosephosphate isomerase (TPI), Cathepsin B (CTSB) and Hexokinase (HXK) were evaluated after knockdown of CASP1. This research provides a valuable basis to improve our understanding the immune mechanisms of Oriental river prawns at different molting stages. The identification of immune-related genes is of great significance for enhancing the immunity of the Oriental river prawn, or other crustaceans, by transgenic methods in the future.

A new SVWC protein functions as a pattern recognition protein in antibacterial responses in Chinese mitten crab (Eriocheirsinensis)

Because invertebrates lack acquired immunity, they rely primarily on the innate immune system to defend themselves against viral and bacterial infections. SVWC, also called Vago, is a class of small-molecule proteins characterized by a single von Willebrand factor C-domain and appears to be restricted to arthropods. It has been reported that SVWC is involved in antiviral immunity in invertebrates, but whether it is involved in antimicrobial immunity and the mechanism of its involvement in antimicrobial immunity remains unclear. In this study, we identified a novel SVWC gene in Eriocheir sinensis and named it EsSVWC. EsSVWC was found to respond positively to bacterial stimulation and to regulate the expression of related antimicrobial peptides (AMPs). The EsSVWC protein recognized and bound to a variety of pathogen-associated molecular patterns (PAMPs) but did not exhibit direct bactericidal effects. Thus, the EsSVWC protein in crabs helps resist bacterial infection and improve survival rates. In summary, EsSVWC may regulate the innate immune system of crabs in response to microbial invasion in an indirect manner.

Squid industry by-product hydrolysate supplementation enhances growth performance of Penaeus monodon fed plant protein-based diets without fish meal

The poor growth of aquatic animals fed with diets containing high plant proteins has been attributed to low diet acceptability and feed value. Supplementation of protein hydrolysate, with high contents of free amino acids and soluble low molecular weight peptides, may increase the acceptability and feed value of a plant protein-based diet. In the present work, squid processing by-products were enzymatically hydrolyzed and used as a supplement in a plant protein-based diet, without fish meal, of Penaeus monodon to fully maximize the utilization of this marine resource. The hydrolysate was incorporated at 0, 0.5, and 1% levels in P. monodon diets containing 0 and 10% fish meal levels. Growth, digestive enzyme activities, muscle growth-, gut pro-inflammatory and immune-related gene expressions, and muscle morphometric measurements were evaluated as biological indices in an 8-week feeding trial. The squid by-product hydrolysate produced in the present study contains 90.25% protein, 5.84% lipid, and 3.91% ash, and has a molecular weight of 3.76 kDa. Supplementation at 1% hydrolysate in the experimental shrimp diet without fish meal resulted in the highest growth performance associated with increased feed intake, efficient feed and nutrient conversion and retention, enhanced digestive enzyme activities, upregulation of muscle growth- and immune-related genes, and suppression of the gut pro-inflammatory gene. The growth promotion is also linked with a significant increase in muscle mean fiber area, which suggests hypertrophic growth in shrimp. Generally, the supplementation of 1% squid by-product hydrolysate supported the growth of P. monodon fed on a plant protein-based diet without fish meal.

Microbiome analysis reveals microecological advantages of emerging ditchless rice-crayfish co-culture mode

Ditchless rice-crayfish co-culture is an emerging model of rice-crayfish farming that circumvents the potential hazards of digging ditches in traditional rice-crayfish farming. However, due to the complex interactions among crayfish, ambient microbiota, and environmental variables, it is necessary to assess the differences in bacterial structure between ditchless and traditional rice-crayfish culture. In this study, the crayfish culture area in the Sichuan basin was selected as the study area, and the bacterial communities of two rice-crayfish culture systems were compared by high-throughput sequencing of 16S rDNA. The results showed that the ditchless system had lower water depth, higher dissolved oxygen, lower total ammonia nitrogen and lower morbidity. There are intuitive differences in the composition of environmental bacterial communities due to environmental changes, even if they are similar in composition at the phylum level. Microbiota in sediments from ditchless systems appear to produce less ammonia nitrogen. The abundance of the pathogens colonizing the intestine of ditchless crayfish was lower than ditched one, and the composition was similar to water. Ditch-farmed crayfish appear to be more susceptible to environmental microbes and have a more fragile intestinal structure. Water depth and dissolved oxygen are the main environmental factors that determine the distribution of microbiota. This study is the first to investigate the bacterial ecology of a ditchless rice- crayfish farming system. The results show that the ditchless rice-crayfish culture model has a more superior bacterial system than the traditional rice-crayfish culture.

Effects of Nano-Aerators on Microbial Communities and Functions in the Water, Sediment, and Shrimp Intestine in Litopenaeus vannamei Aquaculture Ponds

Nanobubble technology has promising development and application prospects in the fields of sewage treatment, soil and groundwater remediation, animal and plant growth, and biomedicine. However, few studies have investigated its effect on shrimp aquaculture. In this study, we investigated the effect of nano-aerators on microbial communities of the water, sediment, and shrimp gut in a Litopenaeus vannamei aquaculture pond using 16S rRNA high-throughput sequencing. The results indicated that the nano-aerator significantly increased the microbial community diversity and species abundance in the pond, and the microbial community diversity of the pond sediment increased under short-term aeration conditions. Compared to that with ordinary aerators, nano-aerators increased the proportion of beneficial bacteria, such as Exiguobacterium and Acinetobacter, in the water and sediment microbial communities. Moreover, the proportions of beneficial bacteria in the gut, including Rhodobacter, Oscillospira, and Faecalibacterium, were all increased by using the nano-aerator. Therefore, our findings suggest that nano-aerators could promote the activity of beneficial bacteria in aquaculture ecosystems, thereby regulating water quality, reducing disease incidence, and improving aquaculture efficiency and benefits. Our findings provide new insights into the effects of nano-aerators on microbes in crustacean culture ponds.

New Insights into the Mechanism of Action of PirAB from Vibrio Parahaemolyticus

PirAB toxins secreted by Vibrio parahaemolyticus (Vp) harbor the pVA1 virulence plasmid, which causes acute hepatopancreatic necrosis disease (AHPND), an emerging disease in Penaeid shrimp that can cause 70–100% mortality and that has resulted in great economic losses since its first appearance. The cytotoxic effect of PirAB^Vp on the epithelial cells of the shrimp hepatopancreas (Hp) has been extensively documented. New insights into the biological role of the PirB^Vp subunit show that it has lectin-like activity and recognizes mucin-like O-glycosidic structures in the shrimp Hp. The search for toxin receptors can lead to a better understanding of the infection mechanisms of the pathogen and the prevention of the host disease by blocking toxin–receptor interactions using a mimetic antagonist. There is also evidence that Vp AHPND changes the community structure of the microbiota in the surrounding water, resulting in a significant reduction of several bacterial taxa, especially Neptuniibacter spp. Considering these findings, the PirAB^vp toxin could exhibit a dual role of damaging the shrimp Hp while killing the surrounding bacteria.

Different expression pattern of thrombospondin gene in the presence and absence of β-glucan fed Penaeus monodon challenged with white spot syndrome virus

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Thrombospondin (TSP) gene expression in response to WSSV infection in shrimp, Penaeus monodon.

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Expression pattern of TSP gene in the presence and absence of β-glucan fed shrimp.

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High-level expression of TSP gene in various tissues of WSSV-challenged shrimp at 2 h.

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Altered gene expression pattern in WSSV-challenged shrimp, fed with and without β-glucan.

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TSP could play a role as an acute inducible phase response protein to WSSV infection.

Thrombospondins (TSPs) are extracellular, calcium-binding glycoproteins that play an essential role in cell homeostasis and development, wound-healing, angiogenesis, connective tissue organization, immune response etc. and it conserves from sea sponges to mammals. However, their role in shrimp immunity is poorly understood. In the present study, the differential expression profiling of TSP transcripts in Penaeus monodon tissues such as gills, lymphoid organs, hepatopancreas, and hemolymph challenged with white spot syndrome virus (WSSV), were studied by quantitative real-time PCR. Further, shrimps fed with the immunostimulant (β-glucan) when challenged with WSSV showed significant upregulation of TSP expression in gills, hepatopancreas, and lymphoid organ at the early phase of WSSV infection. The results suggest that TSP may be an inducible acute phase response protein to WSSV infection. The possibility of differences in mRNA expression pattern seen in immunostimmulated shrimp after the viral challenge, possibility due to altered immune mechanisms getting triggered during immunostimulant administration and virus infections in the host.

Effects of Dietary Phaffia rhodozyma Astaxanthin on Growth Performance, Carotenoid Analysis, Biochemical and Immune-Physiological Parameters, Intestinal Microbiota, and Disease Resistance in Penaeus monodon

The present study aimed to investigate the effect of dietary astaxanthin (Ast) from Phaffia rhodozyma on growth performance, survival, carotenoid content, the activity of antioxidant and immune-related enzymes, intestinal microbiota comparison, and disease resistance against Vibrio parahaemolyticus in Penaeus monodon. Juveniles (average weight 3.15 ± 0.12 g) were fed with six experimental diets supplemented with 0 (Control), 20.5, 41, 61.5, 82, and 102.5 mg/kg of Ast (defined as diet A–D) in triplicate for 56 days. The results indicated that shrimp fed with Ast supplementation significantly (p < 0.05) improved growth performance compared with the control. Furthermore, significantly (p < 0.05) increased survival and decreased feed conversion ratio (FCR) demonstrated the beneficial effects of dietary Ast on enhancing nutrient utilization and ultimately improving the growth and survival of shrimp. Furthermore, shrimp fed with Ast including diet developed a deeper red color than the control, consistent with the significantly (p < 0.05) increased Ast deposition in the shrimp shell. Hemolymph-immunological parameters [aspartate aminotransferase (AST), alanine aminotransferase (ALT), and alkaline phosphatase (AKP)] and hepatopancreatic antioxidant status [total antioxidant capacity (T-AOC), malondialdehyde (MDA), catalase (CAT), and superoxide dismutase (SOD)] were significantly (p < 0.05) affected by dietary Ast supplementation. Dietary increasing Ast levels significantly (p < 0.05) increased shrimp resistance performance to V. parahaemolyticus according to the LT₅₀ results in the current study, which may be caused by increased total carotenoid contents in shrimp tissues from all the Ast-supplemented treatments. Conversely, intestinal microbiota biodiversity and richness were not affected by dietary Ast. The best performances of growth, antioxidant status, immunological response, and carotenoid deposition were observed in diets E and F among all the Ast-supplemented treatments. Overall, all the data suggested that dietary P. rhodozyma Ast played a critical role in improving growth performance, achieving the desired coloration, increasing carotenoid content, and keeping better health status of shrimp. Based on these positive performances, P. rhodozyma Ast could gain the trust of the consumers as a natural source and provide a potential alternative for synthetic Ast using in the Penaeus monodon culture industry.

Identification of a TRIM32 from Penaeus monodon is involved in autophagy and innate immunity during white spot syndrome virus infection

Many tripartite motif (TRIM) family proteins played an important role in regulating innate immune and autophagy pathway and were important for host defenses against viral pathogens. However, the role of TRIM proteins in autophagy and innate immunity during virus infection was seldom studied in crustaceans. In this study, a novel TRIM32 homolog was identified from Penaeus monodon (named PmTRIM32). PmTRIM32 was significantly upregulated by rapamycin stimulation and WSSV infection. RNA interference experiments showed that PmTRIM32 could restrict WSSV replication and lead P. monodon more resistance to WSSV challenge. Autophagy could be induced by WSSV or rapamycin challenge and has been proved to play a positive role in restricting WSSV replication in P. monodon. The autophagy activity induced by WSSV or rapamycin challenge could be obviously inhibited by silence of PmTRIM32 in P. monodon. Further studies revealed that PmTRIM32 positively regulated the expression of nuclear transcription factor (NF-κB) and it mediated antimicrobial peptides. Moreover, Pull-down and in vitro ubiquitination assay demonstrated that PmTRIM32 could interact with WSSV envelope protein and target it for ubiquitination in vitro. Collectively, this study demonstrated that PmTRIM32 restricted WSSV replication and was involved in positively regulating autophagy and NF-κB pathway during WSSV infection in P. monodon.

Comparative transcriptome analysis of Chinese grass shrimp (Palaemonetes sinensis) hepatopancreas under ectoparasitic isopod (Tachaea chinensis) infection

Tachaea chinensis, a parasitic isopod, negatively affects the production of several commercially important shrimp species. To better understand the interaction between shrimp immunity and isopod infection, we performed a transcriptome analysis of the hepatopancreas of Palaemonetes sinensis challenged with T. chinensis. After assembly and annotation, 75,980 high-quality unigenes were obtained using RNA-seq data. Differential gene expression analysis revealed 896 significantly differently expressed genes (DEGs) after infection, with 452 and 444 upregulated and downregulated genes, respectively. Specifically, expression levels of genes involved in detoxification, such as the interferon regulatory factor, venom carboxylesterase-6, serine proteinase inhibitor, and cytochrome P450, were upregulated. Furthermore, expression levels of genes corresponding to retinol dehydrogenase, triosephosphate isomerase, variant ionotropic glutamate receptor, and phosphoenolpyruvate carboxykinase were significantly upregulated after isopod parasitization, indicating that the shrimp's visual system was influenced by isopod parasitization. Moreover, quantitative real-time PCR of 10 DEGs helped validate the RNA-seq findings. These results provide a valuable basis for future studies on the elucidation of immune responses of P. sinensis to T. chinensis infection.

Toll protein family structure, evolution and response of the whiteleg shrimp (Litopenaeus vannamei) to exogenous iridescent virus

Whiteleg shrimp is a widely cultured crustacean, but frequent disease outbreaks have decreased production and caused significant losses. Toll-like receptors (TLRs) comprise a large innate immune family that is involved in the innate immune response. However, understanding of their regulatory mechanism is limited. In this study, PacBio sequencing and Illumina sequencing were applied to the gill and hepatopancreas tissues of whiteleg shrimp and an integrated transcript gene set was established. The upregulation of Toll1, Toll2 and Toll3 transcripts in the hepatopancreas tissue of whiteleg shrimp after iridescent virus infection implies that these proteins are involved in the immune response to the virus; simultaneously, the TRAF6 and relish transcripts in the Toll pathway were also upregulated, implying that the Toll pathway was activated. We predicted the three-dimensional structure of the five Toll proteins in whiteleg shrimp and humans and constructed a phylogenetic tree of the Toll protein family. In addition, there was a large discrepancy of Toll1 between invertebrates and vertebrates, presumably because of the loss of Toll1 protein sequence during the evolution process from invertebrates to vertebrates. Our research will improve the cognition of Toll protein family in invertebrates in terms of evolution, structure and function and provide theoretical guidance for researchers in this field.

Identification of a Shrimp E3 Ubiquitin Ligase TRIM50-Like Involved in Restricting White Spot Syndrome Virus Proliferation by Its Mediated Autophagy and Ubiquitination

Most tripartite motif (TRIM) family proteins are critical components of the autophagy machinery and play important roles in host defense against viral pathogens in mammals. However, the roles of TRIM proteins in autophagy and viral infection have not been studied in lower invertebrates, especially crustaceans. In this study, we first identified a TRIM50-like gene from Penaeus monodon (designated PmTRIM50-like), which, after a white spot syndrome virus (WSSV) challenge, was significantly upregulated at the mRNA and protein levels in the intestine and hemocytes. Knockdown of PmTRIM50-like led to an increase in the WSSV quantity in shrimp, while its overexpression led to a decrease compared with the controls. Autophagy can be induced by WSSV or rapamycin challenge and has been shown to play a positive role in restricting WSSV replication in P. monodon. The mRNA and protein expression levels of PmTRIM50-like significantly increased with the enhancement of rapamycin-induced autophagy. The autophagy activity induced by WSSV or rapamycin challenge could be inhibited by silencing PmTRIM50-like in shrimp. Further studies showed that rapamycin failed to induce autophagy or inhibit WSSV replication after knockdown of PmTRIM50-like. Moreover, pull-down and in vitro ubiquitination assays demonstrated that PmTRIM50-like could interact with WSSV envelope proteins and target them for ubiquitination in vitro. Collectively, this study demonstrated that PmTRIM50-like is required for autophagy and is involved in restricting the proliferation of WSSV through its ubiquitination. This is the first study to report the role of a TRIM family protein in virus infection and host autophagy in crustaceans.

Altered oral microbiota composition associated with recurrent aphthous stomatitis in young females

Oral microbiota has been implicated in pathogenesis of recurrent aphthous stomatitis (RAS), which is a common mucosal disorder with unclear etiology. This study has explored the association between oral microbiota disorder and RAS in high-risk young female population.Forty-five young females were enrolled, including 24 RAS patients and 21 healthy individuals. Oral microbiome was analyzed by Illumina Miseq sequencing.Oral microbiota associated with RAS was characterized by the lower alpha-diversity indices (Chao1 and ACE). Several infectious pathogens increased in RAS, such as genera Actinobacillus, Haemophilus, Prevotella and Vibrio. The PICRUSt analysis indicated that the oral microbiota might be related with the up-regulation of genes involving infectious and neurodegenerative diseases, environmental adaptation, the down-regulation of genes involving basal metabolism, such as carbohydrate, energy, and amino acid metabolism.This study indicated that oral microbiota may play a significant role in RAS development.

TRIM9 is involved in facilitating Vibrio parahaemolyticus infection by inhibition of relish pathway in Penaeus monodon

Tripartite motif-containing 9 (TRIM9) has been demonstrated to exert important roles in regulation of innate immune signaling. In this study, a novel TRIM9 homolog was identified from Penaeus monodon (named PmTRIM9). The open reading frame (ORF) of PmTRIM9 was 2064 bp, which encoding a 687-amino-acid polypeptide. Following Vibrio parahaemolyticus challenge, the expression levels of PmTRIM9 mRNA were significantly down-regulated in tested tissues. RNA interference and recombinant protein injection experiments were performed to explore the function of PmTRIM9, and the results showed it could facilitate V. parahaemolyticus replication and lead P. monodon more vulnerable to V. parahaemolyticus challenge. The dual-luciferase reporter assay showed that PmTRIM9 possessed the ability to inhibit the promoter activity in HEK293 T cells. Silencing of PmTRIM9 could increase the expression of the major NF-κB transcription factor, PmRelish. Further studies showed that knockdown of PmRelish promoted the V. parahaemolyticus infection and decreased the expression of specific antimicrobial peptides (AMPs), including PmCRU5, PmCRU7, PmALF6, PmALF3, PmLYZ and PmPEN5. However, knockdown of PmTRIM9 increased expression levels of the same AMPs, but except for PmCRU5, indicating that PmTRIM9 may negatively regulate the PmRelish-mediated expression of AMPs. All these results suggest that PmTRIM9 was involved in facilitating V. parahaemolyticus infection by inhibition of Relish pathway in P. monodon.

Vibrio and major commercially important vibriosis diseases in decapod crustaceans

Bacteria fromthe Vibriogenus are autochthonous to aquatic environments and ubiquitous in aquaculture production systems. Many Vibrio species are non-pathogenic and can be commonly found in healthy farmed aquatic animals. However, some Vibrio species and strains are pathogenic leading to a variety of 'vibriosis' diseases. These diseases can have a significant negative impact on animal production, including farmed crustaceans such as shrimps, lobsters, and crabs. As such, vibriosis can pose a threat to meeting growing food demand and global food security. Preventive management is essential to avoid the onset of vibriosis. This includes a robust health management plan, the use of prophylaxis and treatment measures, and enhancing animal health through nutrition. Furthermore, the use of probiotics, prebiotics, synbiotics, quorum sensing disruption, green water, biofloc, bacteriophages, and immune priming could also play a role in preventing and controlling a vibriosis outbreak. This review aims to inform and update the reader about the current state of knowledge about Vibrio and associated vibriosis in farmed crustaceans (i.e. shrimp, lobster, and crabs). Furthermore, the review will identify potential knowledge gaps in the literature, which serves as a basis for future research priorities.

Vibrio parahaemolyticus Infection Influenced Trace Element Homeostasis, Impaired Antioxidant Function, and Induced Inflammation Response in Litopenaeus vannamei

Vibrio parahaemolyticus (V. parahaemolyticus) caused huge diseases and economic losses in shrimp aquaculture. Understanding the infection mechanism might help develop new strategies for controlling pathogen outbreak. Redistribution of trace element homeostasis, accompanied by impairment of antioxidant status and immune response, was observed during various infections. Accordingly, we hypothesized that V. parahaemolyticus infection might influence trace element homeostasis, impair antioxidant function, and induce inflammation response in shrimp. In the present study, the aim of this study was to investigate the influence of V. parahaemolyticus infection on trace element homeostasis, antioxidant status, and inflammation response in Litopenaeus vannamei (L. vannamei). The results showed that compared with the control group, V. parahaemolyticus infection significantly increased (P < 0.05) intestinal V. parahaemolyticus number, serum copper (Cu) concentration at 24, 48, and 72 h and significantly increased (P < 0.05) serum zinc (Zn), iron (Fe), and manganese (Mn) concentrations at 24 h but decreased (P < 0.05) at 72 h. The intestinal gene expressions of metal transporters ZIP13, CTR1, and MT1 were significantly decreased at 24, 48, and 72 h, and DMT1 was significantly decreased at 48 h and 72 h in the infection group. The activities of superoxide dismutase (SOD) and glutathione peroxidase (GPx) were suppressed at 48 h and 72 h, and the malondialdehyde (MDA) content was increased at 24, 48, and 72 h in the infection group; the pro-inflammatory genes including necrosis factor-α (TNF-α), lipopolysaccharide-induced TNF-α factor (LITAF), and Ras-related protein Rab6A (RAB6A) were significantly upregulated at 48 and 72 h in the infection group. These results suggest that V. parahaemolyticus infection influenced trace element homeostasis, impaired antioxidant function, and induced inflammation response in L. vannamei, which might help understand the infection mechanism. The results provide a better understanding of the L. vannamei and V. parahaemolyticus interactions and may deliver the basis for further research in preventing the bacterial diseases.

B-type allatostatin modulates immune response in hepatopancreas of the mud crab Scylla paramamosain

B-type allatostatin (AST-B) is a pleiotropic neuropeptide, widely found in arthropods. However, the information about its immune effect in crustaceans is unknown. In this study, we identified the nervous tissue as the main site for Sp-AST-B expression, while its receptor gene (Sp-AST-BR) is widely expressed in various tissues, including the hepatopancreas. This suggests the peptide's potential role in diverse physiological processes in the mud crab Scylla paramamosain. In situ hybridization revealed that Sp-AST-BR is mainly localized in the F-cell of hepatopancreas. Furthermore, we found a significant up-regulation of Sp-AST-BR transcripts in the hepatopancreas following exposure to lipopolysaccharide (LPS) or polyriboinosinic polyribocytidylic acid (Poly (I:C)). Results from in vitro and in vivo experiments revealed that treatment with a synthetic AST-B peptide mediated significant upregulation in expression of AST-BR, nuclear factor-κB (NF-κB) pathway components (Dorsal and Relish), pro-inflammatory cytokine (IL-16) and antimicrobial peptides (AMPs) in the hepatopancreas. In addition, AST-B treatment mediated significant elevation of nitric oxide (NO) production and enhanced the bacteriostasis capacity of the hepatopancreas tissue in vitro. Taken together, these findings reveal the existence of a basic neuroendocrine-immune (NEI) network in crabs, and indicate that AST-B could couple with its receptor to trigger downstream signaling pathways and induce immune responses in the hepatopancreas.

Hepatopancreas immune response during molt cycle in the mud crab, Scylla paramamosain

Molt is a critical developmental process in crustaceans. Recent studies have shown that the hepatopancreas is an important source of innate immune molecules, yet hepatopancreatic patterns of gene expression during the molt cycle which may underlie changes in immune mechanism are unknown. In this study, we performed Illumina sequencing for the hepatopancreas of the mud crab, Scylla paramamosain during molt cycle (pre-molt stage, post-molt stage, and inter-molt stage). A total of 44.55 Gb high-quality reads were obtained from the normalized cDNA of hepatopancreas. A total of 70,591 transcripts were assembled; 55,167 unigenes were identified. Transcriptomic comparison revealed 948 differentially expressed genes (DEGs) in the hepatopancreas from the three molt stages. We found that genes associated with immune response patterns changed in expression during the molt cycle. Antimicrobial peptide genes, inflammatory response genes, Toll signaling pathway factors, the phenoloxidase system, antioxidant enzymes, metal-binding proteins and other immune related genes are significantly up-regulated at the post-molt stage and inter-molt stage compared with the pre-molt stage, respectively. These genes are either not expressed or are expressed at low levels at the pre-molt stage. To our knowledge, this is the first systematic transcriptome analysis of genes capable of mobilizing a hepatopancreas immune response during the molt cycle in crustaceans, and this study will contribute to a better understanding of the hepatopancreas immune system and mud crab prophylactic immune mechanisms at the post-molt stage.

A stomach-specific lectin plays a crucial role in the innate immunity of red swamp crayfish, Procambarus clarkii

C-type lectins (CTLs) play important roles in innate immune system of crustaceans as pattern recognition receptors (PRRs). In this study, a novel CTL gene was obtained from the red swamp crayfish, Procambarus clarkii, designated as PcLec. PcLec encodes a peptide with 175 amino acids, with a signal peptide and a single carbohydrate recognition domain (CRD). The PcLec transcripts were specifically expressed in crayfish stomach and were induced by bacterial challenge. In vitro assays with recombinant PcLec protein revealed that it had bacterial binding activity, polysaccharide binding activity, bacterial agglutination activity, and antimicrobial activity. Most importantly, PcLec knockdown significantly impaired the survivability of crayfish upon oral infection with its pathogen A. hydrophila. According to these results, we infer that the PcLec plays a crucial role in antibacterial defense of crayfish.

Molecular and Functional Diversity of Crustin-Like Genes in the Shrimp Litopenaeus vannamei

Crustins are crustacean cationic cysteine-rich antimicrobial peptides that contain one or two whey acidic protein (WAP) domain(s) at the carboxyl terminus and mainly show antimicrobial and/or proteinase inhibitory activities. Here, we performed genome and transcriptome screening and identified 34 full-length crustin-like encoding genes in Litopenaeus vannamei. Multiple sequence analysis of the deduced mature peptides revealed that these putative crustins included 10 type Ia, two type Ib, one type Ic, 11 type IIa, three type IIb, four type III, one type IV, one type VI, and one type VII. These putative crustins were clustered into different groups. Phylogenetic analysis, considering their domain composition, showed that different types of crustin-like genes in crustaceans might be originated from the WAP core region, along with sequence insertion, duplication, deletion, and amino acid substitution. Tissue distribution analysis suggested that most crustin-like genes were mainly detected in immune-related tissues while several crustin-like genes exhibited tissue-specific expression patterns. Quantitative PCR analysis on 15 selected crustin-like genes showed that most of them were apparently upregulated after Vibrio parahaemolyticus or white spot syndrome virus (WSSV) infection. One type Ib crustin-like gene, mainly expressed in the ovary, showed the highest expression levels before the gastrula stage and was hardly detected after the limb bud stage, suggesting that it was a maternal immune effector. Collectively, the present data revealed the molecular and functional diversity of crustins and their potential evolutionary routes in crustaceans.

Vibrio harveyi Infection Significantly Alters Amino Acid and Carbohydrate Metabolism in Whiteleg Shrimp, Litopenaeus vannamei

Vibrio harveyi is one of the pathogens that threaten the shrimp farming industry. However, metabolic changes induced by V. harveyi infection in shrimp remain unknown. In this study, we first conducted high resolution-magic angle spinning (HR-MAS) nuclear magnetic resonance (NMR)-based metabolomics studies on gill, hepatopancreas, and haemolymph of V. harveyi-infected white leg shrimp, Litopenaeus vannamei. Using multivariate statistical analysis, we observed a clear separation between the early (3 and 9 h post-injection (hpi)) and late phases (24, 72 and 144 hpi) of the infection in all tissues. Moreover, metabolic changes in response to V. harveyi infection were faster in the haemolymph in the early phase and significantly changed in the late phase of the infection in the gills. Extensive changes were observed in the hepatopancreas, with 24 hpi being the turning point of progression from early to late phase infection in the hepatopancreas. V. harveyi infection increased the energy demand in L. vannamei and the amino acid and carbohydrate metabolism pathways also exhibited significant changes depending on the tissue. Thus, each tissue displayed different metabolic changes, depending on the progress of the infection.

Short neuropeptide F enhances the immune response in the hepatopancreas of mud crab (Scylla paramamosain)

Short neuropeptide F (sNPF), a highly conserved neuropeptide, displays pleiotropic functions on multiple aspects of physiological processes, such as feeding, metabolic stress, locomotion, circadian clock and reproduction. However, to date there has no any report on the possible immunoregulation of sNPF in crustaceans. In the present study, we found that the Sp-sNPF was mainly expressed in the nervous tissue in the mud crab Scylla paramamosain, while the sNPF receptor gene (Sp-sNPF-R) was expressed in a wide variety of tissues, including the hepatopancreas. In situ hybridization further showed that the Sp-sNPF-R positive signal mainly localized in the F-cells of the hepatopancreas. Moreover, the Sp-sNPF-R transcription could be significantly up-regulated after the challenge of bacteria-analog LPS or virus-analog Poly (I:C). Both in vitro and in vivo experiments showed that the synthetic sNPF peptide significantly increased the gene expressions of sNPF-R, nuclear factor-κB (NF-κB) signaling genes and antimicrobial peptides (AMPs) in the hepatopancreas. Simultaneously, the administration of sNPF peptide in vitro also increased the concentration of nitric oxide (NO) and the bacteriostasis of the culture medium of hepatopancreas. These results indicated that sNPF up-regulated hepatopancreas immune responses, which may bring new insight into the neuroendocrine-immune regulatory system in crustacean species, and could potentially provide a new strategy for disease prevention and control for mud crab aquaculture.

A unique lectin composing of fibrinogen-like domain from Fenneropenaeus merguiensis contributed in shrimp immune defense and firstly found to mediate encapsulation

In invertebrates, both fibrinogen-related proteins (FREPs) and C-type lectins are acknowledged to act as pattern recognition receptors (PRRs) to participate particularly in an innate immunity. Hereby, a unique C-type lectin designated as FmLFd was isolated from the hemocytes of Fenneropenaeus merguiensis. FmLFd contained one open reading frame which encoding a peptide of 312 amino acid residues and a signal peptide of 18 amino acids. The primary sequence of FmLFd was composed of a fibrinogen-like domain (Fd) with a Ca²⁺-binding site and possessing specificity to bind N-acetyl glucosamine (GlcNAc). The FmLFd transcripts were detected mainly in hemocytes of healthy shrimp. The expression of FmLFd was significantly up-regulated upon challenge shrimp with Vibrio parahaemolyticus and Vibrio harveyi which more potent than by white spot syndrome virus (WSSV). The knocking down shrimp with FmLFd double-stranded RNA caused dramatical gene down-regulation. The gene silencing with co-injection of pathogens resulted in reduction of the shrimp survival rate. Recombinant protein of FmLFd (rFmLFd) could agglutinate and bind directly to both Gram-negative and Gram-positive bacteria in a Ca²⁺-dependent manner and showed the sugar specificity to GlcNAc and bacterial saccharides; peptidoglycan (PGN), lipopolysaccharide (LPS) and lipoteichoic acid (LTA). Recombinant protein of Fd domain (rFd) displayed the lower activity and specificity only to PGN. The binding between recombinant proteins of FmLFd and its domain confirming by ELISA demonstrated that both rFmLFd and rFd could bind to PGN, LPS and LTA with the highest affinity respected to PGN including a less extent of rFd. Besides, rFmLFd but not rFd could bind to WSSV proteins with the highest binding affinity to capsid VP15 and decreasing in order to envelope VP28 and tegument VP39A, respectively. It was presumed that entire molecule of FmLFd exhibited the antimicrobial ability by inhibiting the growth of pathogenic V. parahaemolyticus and this action was not affected by GlcNAc. Otherwise, FmLFd, a lectin containing fibrinogen-like domain, was firstly reported to be capable of promoting encapsulation by hemocytes. Altogether, we concluded that FmLFd belonged to a FREP family indentified by the existence of a conserved fibrinogen-like domain with possessing an ability to bind GlcNAc. It was a new C-type lectin existed in F. merguiensis and might presumably act as a kind of PRRs to participate in the shrimp immune defense towards bacterial and viral pathogens.

Potential immunomodulatory and protective effects of the Arthrospira-based dietary supplement on shrimp intestinal immune defenses

Herein, we evaluated the immunomodulatory and the antiviral protective properties of a cyanobacteria-enriched diet on the immune responses of the Pacific white shrimp Litopenaeus vannamei challenged with the White spot syndrome virus (WSSV). Shrimp were fed with an Arthrospira platensis supplemented feed during 20 days, and its effects were examined by evaluating well-known standardized shrimp immune parameters (total hemocyte counts, total protein concentration, phenoloxidase activity, and serum agglutination titer). Additionally, we assessed the expression of crucial genes involved in both hemolymph- and gut-based immunities related to the shrimp capacity to circumvent viral and microbial infections. Dietary supplementation improved shrimp survival rates after challenge with a median lethal dose of WSSV. From all immune parameters tested, only the serum agglutination titer was higher in treated animals. On the other hand, the expression of some representative marker genes from different immune response pathways was only modulated in the midgut and not in the circulating hemocytes, suggesting that this feed supplementation can be used as an attractive strategy to enhance immunity in shrimp gut. Altogether, our results evidence the immunomodulatory properties of A. platensis supplemented feed in shrimp humoral and intestinal defenses and highlight the potential use of cyanobacteria-based immunostimulants in shrimp farming for protection against infectious diseases.

Comparative analysis of the intestinal bacterial community and expression of gut immunity genes in the Chinese Mitten Crab (Eriocheir sinensis)

Remarkably little information is available about the interaction between the gut microbiota and intestinal immunity in fish and crustaceans. In our study, we used Illumina MiSeq sequencing and real-time quantitative PCR to compare the microbial community and immunity genes expression in the foregut, midgut and hindgut of Chinese mitten crab (Eriocheir sinensis). Our results indicated that the community richness of the midgut is higher than in the foregut or the hindgut, although the bacterial diversity in the hindgut is higher. The predominant phyla were Tenericutes and Firmicutes in the foregut, Tenericutes and Proteobacteria in the midgut and Proteobacteria, Tenericutes and Bacteroidetes in the hindgut. When compared with the midgut, the expression of antimicrobial peptides (AMPs) were significantly elevated in the hindgut (P < 0.05), and the gene expression of EsRelish (IMD pathway) was higher than the Toll signaling pathway genes. Actinobacteria and Lactobacillus had negative correlationship with the expression of AMPs, although Acinetobacter, Bacteroides, Flavobacterium can up-regulate the expression of AMP genes. Collectively, our data indicate that microbiota are site-specific within the digestive tracts of crabs and the bacterial community and intestinal immunity have a close relationship in E. sinensis.

Dietary yeast hydrolysate and brewer's yeast supplementation could enhance growth performance, innate immunity capacity and ammonia nitrogen stress resistance ability of Pacific white shrimp (Litopenaeus vannamei)

An 8-week feeding trial was conducted to evaluate the effects of dietary yeast hydrolysate and brewer's yeast supplementation on growth, immune-related genes expression and ammonia nitrogen stress resistance of Pacific white shrimp (Litopenaeus vannamei). Three isonitrogenous and isolipidic practical diets were formulated to contain 0% (control diet), 1% yeast hydrolysate and 1% brewer's yeast, respectively. 360 juvenile L. vannamei with an initial weight (0.88 ± 0.01 g) was randomly divided into 3 treatments in four replicates (30 shrimp per replicate). The results indicated that shrimp fed the diet containing 1% yeast hydrolysate had a significantly higher weight gain (WG), and specific growth rate (SGR) than that fed the control diet, and the lowest feed conversion ratio (FCR) was occurred in the 1% yeast hydrolysate supplementation group. Proximate composition in whole body and muscle among all treatments was not significantly influenced by the dietary yeast hydrolysate or brewer's yeast supplementation. The challenge test with ammonia nitrogen showed that lower cumulative survival was observed in those fed the control diet, and the highest cumulative survival was occurred at shrimp fed the 1% yeast hydrolysate supplementation. Shrimp fed the control diet had higher inflammation-related genes expression levels of tnf-α and il-1β in the intestine than those fed the diets supplemented with 1% yeast hydrolysate or 1% brewer's yeast, however, there was no significant difference in expression level of alp in intestine among all treatments. The relative expression levels of mTOR signal pathway genes (eif4ebp, eif4e1a, eif4e2 and p70s6k) were significantly up-regulated in the shrimp fed the diets supplemented with 1% yeast hydrolysate, and the lowest gene expression levels of eif4ebp, eif4e1a, eif4e2 and p70s6k in the intestine were occurred at the control diet. The highest expression levels of the immune-related genes (dorsal, relish, and proPO) in the intestine were observed at shrimp fed the 1% yeast hydrolysate supplementation, and the lowest expression levels of these genes were occurred at shrimp fed the control diet, however, there was no significant difference in gene expression of lysozyme among all treatments. The expression levels of penaeidin3a, crustin, proPO, and IMD in the hepatopancreas were significantly influenced by the dietary yeast hydrolysate, brewer's yeast or no yeast product supplementation, shrimp fed the 1% yeast hydrolysate supplementation had higher expression levels of these genes than those fed the control diet. The present study indicated that dietary 1% yeast hydrolysate or brewer's yeast supplementation could improve growth performance, enhance innate immunity, and strengthen resistance of ammonia nitrogen stress, and dietary 1% yeast hydrolysate supplementation provides better immunostimulatory effects than brewer's yeast of L. vannamei.

Microbiota assemblages of water, sediment, and intestine and their associations with environmental factors and shrimp physiological health

Microorganisms play crucial roles in nutrient cycling, water quality maintenance, and farmed animal health. Increasing evidences have revealed a close association between unstable microbial environments and disease occurrences in aquaculture. Thereupon, we used high-throughput sequencing technology to comprehensively compare the bacterial communities of water, sediment, and intestine in mariculture ponds at the middle and late stages of Litopenaeus vannamei farming and analyzed whether changes of their microbiota assemblages were associated with environmental factors and shrimp physiological health. Results showed that bacterial community structures were significantly distinct among water, sediment, and intestine; meanwhile, the relative abundances of intestinal dominant taxa were significantly changed between different rearing stages. Compared with intestine and water, shrimp intestine and sediment had a similar profile of the dominant bacterial genera by cluster analysis, and the observed species, diversity indexes, and shared OTUs of bacterial communities in intestine and sediment were simultaneously increased after shrimp were farmed for 90 days. These results reflected a closer relationship between microbiotas in sediment and intestine, which was further proved by nonmetric multidimensional scaling analysis. However, bacterial communities in water, sediment, and intestine responded differently to environmental variables by redundancy and correlation analysis. More importantly, shrimp physiological parameters were closely associated with bacterial variations in the gut and/or ambient, especially the gut microbiota owning significantly high levels of predicted functional pathways involved in disease emergence. These findings may greatly add to our understanding of the microbiota characteristics of the shrimp pond ecosystem and the complex interactions among shrimp, ambient microflora, and environmental variables.

A comparative study on oxidative stress response in the hepatopancreas and midgut of the white shrimp Litopenaeus vannamei under gradual changes to low or high pH environment

White shrimp Litopenaeus vannamei were reared under conditions of gradual changes to a low pH (gradual-low pH, 6.65-8.20) or a high pH (gradual-high pH, 8.20-9.81) versus a normal pH environment (8.14-8.31) during a 28-day period. Survival of shrimp, and ROS production, antioxidant responses and oxidative damage in the hepatopancreas and midgut were investigated. Consequently, shrimp enhanced MnSOD, GPx, and Hsp70 transcripts as early defense mechanism in the hepatopancreas and midgut to scavenge excessive ROS during short-term (≤ 7 days) gradual-low and high pH stress. Meanwhile, the hepatopancreas was more sensitive to ROS than midgut because of earlier ROS production increase, antioxidant response and oxidative damage. Then, suppressed antioxidant response in the hepatopancreas and midgut of shrimp suggested a loss of antioxidant regulatory capacity caused by aggravated oxidative damage after long-term (≥ 14 days) gradual-high pH stress, leading to continuous death. However, enhanced GPx, GST, and Hsp70 transcripts in the hepatopancreas and midgut might be long-term(≥ 14 days) antioxidant adaptation mechanism of shrimp to gradual-low pH stress, which could prevent further ROS perturbation and weaken oxidative damage to achieve a new immune homeostasis, contributing to stable survival rate. Therefore, we have a few insights that it is necessary to protect hepatopancreas for controlling shrimp death under gradual-high pH stress.

Multiple Isoforms of Anti-Lipopolysaccharide Factors and Their Antimicrobial Functions in the Ridgetail Prawn Exopalaemon carinicauda

As a kind of antimicrobial peptides (AMP) in crustacean, anti-lipopolysaccharide factors (ALFs) have broad spectrum antimicrobial activities. In the present study, we identified four ALF genes, EcALF2-5, from the ridgetail prawn Exopalaemon carinicauda. Tissue distribution analysis showed that EcALF2 and EcALF4 transcripts were mainly located in gill, epidermis, and stomach, while EcALF3 and EcALF5 were mainly in hemocytes. Peptides corresponding to the LPS binding domain (LBD) of EcALFs were synthesized for analyzing their antimicrobial activities. Minimal inhibitory concentration (MIC) analysis showed that the synthetic LBD peptides of EcALF3 and EcALF4 could inhibit the growth of Gram-positive and Gram-negative bacteria, while the synthetic LBD peptides of EcALF2 and EcALF5 showed antibacterial activity against Vibrio. Incubation of white spot syndrome virus (WSSV) with the synthetic LBD peptides of EcALF3, EcALF4, and EcALF5 could reduce the in vivo viral copy number in WSSV-infected prawns. After silencing of EcALFs, Vibrio exhibited a rapid proliferation in the hepatopancreas of the prawn. The present data showed the important function of different EcALFs in modulating the in vivo bacterial and viral propagation in E. carinicauda. This study will provide new clues into the disease control in aquaculture.

An immune-related gene expression atlas of the shrimp digestive system in response to two major pathogens brings insights into the involvement of hemocytes in gut immunity

Much of our current knowledge on shrimp immune system is restricted to the defense reactions mediated by the hemocytes and little is known about gut immunity. Here, we have investigated the transcriptional profile of immune-related genes in different organs of the digestive system of the shrimp Litopenaeus vannamei. First, the tissue distribution of 52 well-known immune-related genes has been assessed by semiquantitative analysis in the gastrointestinal tract (foregut, midgut and hindgut) and in the hepatopancreas and circulating hemocytes of shrimp stimulated or not with heat-killed bacteria. Then, the expression levels of 18 genes from key immune functional categories were quantified by fluorescence-based quantitative PCR in the midgut of animals experimentally infected with the Gram-negative Vibrio harveyi or the White spot syndrome virus (WSSV). Whereas the expression of some genes was induced at 48 h after the bacterial infection, any of the analyzed genes showed to be modulated in response to the virus. Whole-mount immunofluorescence assays confirmed the presence of infiltrating hemocytes in the intestines, indicating that the expression of some immune-related genes in gut is probably due to the migratory behavior of these circulating cells. This evidence suggests the participation of hemocytes in the delivery of antimicrobial molecules into different portions of the digestive system. Taken all together, our results revealed that gut is an important immune organ in L. vannamei with intimate association with hemocytes.

Specific Molecular Signatures for Type II Crustins in Penaeid Shrimp Uncovered by the Identification of Crustin-Like Antimicrobial Peptides in Litopenaeus vannamei

Crustins form a large family of antimicrobial peptides (AMPs) in crustaceans composed of four sub-groups (Types I-IV). Type II crustins (Type IIa or “Crustins” and Type IIb or “Crustin-like”) possess a typical hydrophobic N-terminal region and are by far the most representative sub-group found in penaeid shrimp. To gain insight into the molecular diversity of Type II crustins in penaeids, we identified and characterized a Type IIb crustin in Litopenaeus vannamei (Crustin-like Lv) and compared Type II crustins at both molecular and transcriptional levels. Although L. vannamei Type II crustins (Crustin Lv and Crustin-like Lv) are encoded by separate genes, they showed a similar tissue distribution (hemocytes and gills) and transcriptional response to the shrimp pathogens Vibrio harveyi and White spot syndrome virus (WSSV). As Crustin Lv, Crustin-like Lv transcripts were found to be present early in development, suggesting a maternal contribution to shrimp progeny. Altogether, our in silico and transcriptional data allowed to conclude that (1) each sub-type displays a specific amino acid signature at the C-terminal end holding both the cysteine-rich region and the whey acidic protein (WAP) domain, and that (2) shrimp Type II crustins evolved from a common ancestral gene that conserved a similar pattern of transcriptional regulation.

Transcriptomic and morphological analyses of Litopenaeus vannamei intestinal barrier in response to Vibrio paraheamolyticus infection reveals immune response signatures and structural disruption

The white shrimp Litopenaeus vannamei has been greatly impacted by Vibrio infection. In this study, we investigated the intestinal barrier response of L vannamei following challenge with Vibrio parahaemolyticus E1, by examining morphological changes and transcriptome expression levels. A total of 16,4420 unigenes were obtained from RNAseq data after quality control and assembly, and 4646 differentially expressed genes (DEGs) were identified following Vibrio challenge, of which 2469 unigenes were significantly up-regulated and 2177 were significantly down-regulated. DEGs were determined to be involved in various physical, chemical and immunological intestinal barrier functions, including peritrophin, cytoskeleton and cell junction, pattern recognition receptors, antimicrobial peptide and immune signaling pathways, serine protease/protease inhibitor and prophenoloxidase system, apoptosis and phagocytosis, and antioxidant systems. Fifteen DEGs were randomly selected for validation by real-time quantitative PCR (RT-qPCR) and showed results consistent with the RNA-seq data. Intestinal epithelial cell morphology was also affected by Vibrio challenge, showing epithelial detachment, nuclear pyknosis, and destruction of cell junctions. These results improve our current understanding of the intestinal barrier function in the shrimp response to bacterial infection.

Pathogenicity profile of Vibrio parahaemolyticus in farmed Pacific white shrimp, Penaeus vannamei

A pathobiological study was conducted using Vibrio parahaemolyticus (V_P) strain isolated from vibriosis affected shrimp (Penaeus vannamei) farms in Kancheepuram and Thiruvallur districts of Tamil Nadu during August 2014 to February 2015. The isolate was identified based on the morphological, physiological, biochemical and molecular characters. LD₅₀ value with intramuscular injection was determined as 2.6 × 10⁴ cfu/shrimp and sequential pathology was studied giving 6.1 × 10³ cfu/shrimp (LD₂₅). Total plate count (TPC) and total Vibrio count (TVC) in water, pond sediment, haemolymph, muscle, HP and gut were found significantly (P < 0.01) higher in natural cases than the experimental set up. Clinical signs and lesions observed in the natural and experimental cases were anorexia, lethargy, cuticle softening, loose shells, abdominal muscle cramp, red discoloration, opaque and whitish abdominal and tail musculature, necrosis of exoskeleton or splinter burns, reddish pleural borders of antennae, uropods and telson, swollen tail fan, ulcers, moribund shrimp sinking to bottom, and mortalities with shrunken discoloured HP with empty gut. Total haemocyte count (THC), small nongranular haemocyte (SNGH), large nongranular haemocyte (LNGH), small granular haemocyte (SGH) and large granular haemocyte (LGH) counts lowered significantly (P < 0.01) at 3, 6, 12, 24, 48, 96 and 192 h post injection (p.i). No LGH were found after 96 h of challenge. The post injection qPCR analyses of haemocytes showed up-regulations of penaeidin-3a, lysozyme, prophenoloxidase I, prophenoloxidase II and serine protein at 3 and 6 h of infection. There was total down-regulation of crustin from 3 to 192 h p.i. There was a remarkable elevation in the level of proPO I with concomitant depletion of proPO II. The pattern of up- and down-regulations in proPO I and SP were similar. The post infection qPCR analyses showed that these immune related genes could be used as markers for assessing the immune status of P. vannamei. Major histopathological manifestations observed were haemocyte infiltration/nodule in the epidermis, skeletal and cardiac muscles, atrophy of the excretory organ, and disrupted HP tubules with diffuse interstitial edema and haemocytic infiltration. Further HP showed that there was thickening of intertubular space, karyomegaly with prominent nucleoli, rounding and sloughing of HP tubular epithelium, many mitotic figures with bacterial colonies and apoptotic bodies, separation of shrunken tubule epithelium from myoepithelial fibers, regeneration of tubules, cystic, dilated and vacuolated appearance of HP tubules, hypoplastic changes in the tubules with no B, R and F cells, granuloma formation, concretions in tubules, calcification, necrosis, and washed out appearance with complete loss of architecture. The progression of the degenerative changes in the HP tubular epithelial cells was from proximal to distal end. In haematopoietic organ, increased mitotic activities with focal to extensive depletion and degeneration were observed. Degeneration of the stromal matrix with spheroid formation in lymphoid organ was observed among the Vp infected natural and experimental animals. Degeneration of glandular structures in the prehensile appendages with bacterial colonies, melanization and loss of epithelial layer in oesophagus, swelling and loss of architecture with mucinous secretion in the stomach, degeneration of peritrophic membrane in the lumen of intestine were observed in field cases but not in the experimental studies. Further, this study established the pathobiology of the Vp isolate to P. vannamei.

Characterization and function analysis of interleukin-1 receptor-associated kinase-1 (IRAK-1) from Fenneropenaeus penicillatus

The interleukin-1 receptor-associated kinase-1 (IRAK-1) is an important adapter protein which links downstream of MyD88, and involved in the complex composed of MyD88 and TRAF6 to activate TLRs signaling pathway. In this study, an IRAK-1 homolog (FpIRAK-1) was cloned from the red tail shrimp Fenneropenaeus penicillatus. The ORF of FpIRAK-1 consisted of 2874 bp encoding a protein of 957 amino acids which contains a death domain (DD) and a catalytic domain of serine/threonine kinases (STKc). Homology analysis revealed that the predicted amino acid sequence of FpIRAK-1 shared 71% similarities with IRAK-1 of Litopenaeus vannamei. Real-time RT-PCR indicated that FpIRAK-1 was constitutively expressed in various tissues of F. penicillatus. The expression level of FpIRAK-1 mRNA was significantly up-regulated and then decreased gradually after white spot syndrome virus (WSSV) and Vibrio alginolyticus challenge. Gene knockdown of FpIRAK-1 enhanced the sensitivity of shrimps to WSSV and V. alginolyticus challenge, suggesting FpIRAK-1 could play a positive role against bacterial and viral pathogens. In conclusion, the results of this study provide some insights into the function of FpIRAK-1 in activating Toll signaling pathway and the host defense against invading pathogens.

Tumor necrosis factor receptor-associated factor 6 (TRAF6) participates in anti-lipopolysaccharide factors (ALFs) gene expression in mud crab

Tumor necrosis factor receptor-associated factor 6 (TRAF6) is a key cytoplasm signal adaptor that mediates signals activated by tumor necrosis factor receptor (TNFR) superfamily and the Interleukin-1 receptor/Toll-like receptor (IL-1/TLR) superfamily. The full-length 2492 bp TRAF6 (Sp-TRAF6) from Scylla paramamosain contains 1800 bp of open reading frame (ORF) encoding 598 amino acids, including an N-terminal RING-type zinc finger, two TRAF-type zinc fingers and a conserved C-terminal meprin and TRAF homology (MATH) domain. Multiple alignment analysis shows that the putative amino acid sequence of Sp-TRAf6 has highest identity of 88% with Pt-TRAF6 from Portunus trituberculatus, while the similarity of Sp-TRAF6 with other crustacean sequences was 54-55%. RT-PCR analysis indicated that Sp-TRAF6 transcripts were predominantly expressed in the hepatopancreas and stomach, whereas it was barely detected in the heart and hemocytes in our study. Moreover, Sp-TRAF6 transcripts were significantly up-regulated after Vibrio parahemolyticus and LPS challenges. RNA interference assay was carried out used by siRNA to investigate the genes expression patterns regulated by Sp-TRAF6. The qRT-PCR results showed that silencing Sp-TRAF6 gene could inhibit SpALF1, SpALF2, SpALF5 and SpALF6 expression in hemocytes, while inhibit SpALF1, SpALF3, SpALF4, SpALF5 and SpALF6 expression in hepatopancreas. Taken together, the acute-phase response to immune challenges and the inhibition of SpALFs gene expression indicate that Sp-TRAF6 plays an important role in host defense against pathogen invasions via regulation of ALF gene expression in S. paramamosain.

Differentially expressed transcripts in stomach of Penaeus monodon in response to AHPND infection

Acute Hepatopancreatic Necrosis Disease (AHPND) is an emerging disease in aquacultured shrimp caused by a pathogenic strain of Vibrio parahaemolyticus. As with several pathogenic bacteria, colonization of the stomach appeared to be the initial step of the infection for AHPND-causing Vibrio. To understand the immune responses in the stomach of black tiger shrimp (Penaeus monodon), differentially expressed transcripts (DETs) in the stomach during V. parahaemolyticus strain 3HP (VP3HP) infection was examined using Ion Torrent sequencing. From the total 42,998 contigs obtained, 1585 contigs representing 1513 unigenes were significantly differentially expressed with 1122 and 391 unigenes up- and down-regulated, respectively. Among the DETs, there were 141 immune-related unigenes in 10 functional categories: antimicrobial peptide, signal transduction pathway, proPO system, oxidative stress, proteinases/proteinase inhibitors, apoptotic tumor-related protein, pathogen recognition immune regulator, blood clotting system, adhesive protein and heat shock protein. Expression profiles of 20 of 22 genes inferred from RNA sequencing were confirmed with the results from qRT-PCR. Additionally, a novel isoform of anti-lipopolysaccharide factor, PmALF7 whose transcript was induced in the stomach after challenge with VP3HP was discovered. This study provided a fundamental information on the molecular response in the shrimp stomach during the AHPND infection that would be beneficial for future research.

A fibrinogen-related protein identified from hepatopancreas of crayfish is a potential pattern recognition receptor

Fibrinogen-related protein (FREP) family is a large group of proteins containing fibrinogen-like (FBG) domain and plays multiple physiological roles in animals. However, their immune functions in crayfish are not fully explored. In the present study, a novel fibrinogen-like protein (designated as PcFBN1) was identified and characterized from hepatopancreas of red swamp crayfish Procambarus clarkii. The cDNA sequence of PcFBN1 contains an open reading frame (ORF) of 1353 bp encoding a protein of 450 amino acids. Sequence and structural analysis indicated that PcFBN1 contains an FBG domain in C-terminal and a putative signal peptide of 19 amino acids in N-terminal. Semi-quantitative PCR revealed that the main expression of PcFBN1 was observed in hepatopancreas and hemocyte. Temporal expression analysis exhibited that PcFBN1 expression could be significantly induced by heat-killed Aeromonas hydrophila. Tissue distribution and temporal change of PcFBN1 suggested that PcFBN1 may be involved in immune responses of red swamp crayfish. Recombinant PcFBN1 protein binds and agglutinates both gram-negative bacteria Escherichia coli and gram-positive bacteria Micrococcus lysodeikticus. Moreover, binding and agglutination is Ca(2+) dependent. Further analysis indicated that PcFBN1 recognizes some acetyl group-containing substance LPS and PGN. RNAi experiment revealed that PcFBN1 is required for bacterial clearance and survival from A. hydrophila infection. Reduction of PcFBN1 expression significantly decreased the survival and enhanced the number of A. hydrophila in the hemolymph. These results indicated that PcFBN1 plays an important role in the innate immunity of red swamp crayfish as a potential pattern recognition receptor.

The role of oncoprotein NM23 gene from Exopalaemon carinicauda is response to pathogens challenge and ammonia-N stress

Oncoprotein NM23, as a family of genes encoding the nucleoside diphosphate (NDP) kinase, plays important roles in bioenergetics, DNA replication, differentiation and tumor metastasis. In this study, a full-length cDNA of NM23 (designated EcNM23) was cloned from Exopalaemon carinicauda by using rapid amplification of cDNA ends (RACE) approaches. The full-length cDNA of EcNM23 was 755 bp, which contains an open reading frame (ORF) of 518 bp, encoding a 175 amino-acid polypeptide with the predicted molecular weight of 19.60 kDa and estimated isoelectric point of 7.67. The deduced amino acid sequence of EcNM23 shared high identity (86%-93%) with that of other crustaceans. a NDP kinase super family signature was identified in E. carinicauda EcNM23. Quantitative real-time RT-qPCR analysis indicated that EcNM23 was expressed in all the examined tissues with the high expression level in hemocytes and ovary. The EcNM23 expression in immune-related tissues changed rapidly and reached peak at different time after pathogens (Vibrio parahaemolyticus and WSSV) challenge and ammonia-N stress treatment. The results suggested that EcNM23 might be associated with the immune defenses to pathogens infection and ammonia-N stress in E. carinicauda.

Oxidative stress response of the black tiger shrimp Penaeus monodon to Vibrio parahaemolyticus challenge

Vibrio parahaemolyticus is a virulent pathogen that affects shrimp aquaculture. Reactive oxygen species are produced by the immune system that defends the host against foreign microorganisms. In the present study, the oxidative stress response in hepatopancreas and gills of Penaeus monodon to V. parahaemolyticus challenge were studied, such as respiratory burst, ROS production (·O2(-) and ·OH), activities of antioxidant enzymes (CAT, GPx, SOD, POD and GST) and oxidative damage to lipid and protein (indexed by contents of MDA). Compared with the control group, after V. parahaemolyticus challenge, respiratory burst and ROS production were up-regulated significantly. GPx and POD activity increased significantly in hepatopancreas and gills of the shrimps at 12 h, but CAT activity decreased markedly at 12 h and 24 h. SOD and GST activity in hepatopancreas of the shrimps increased significantly at 1.5 h, but decreased markedly at 12 h-48 h. MDA content increased significantly after 6 h-24 h challenge. HE staining showed that V. parahaemolyticus challenge induced damage symptoms in hepatopancreas of P. monodon. Our study revealed that V. parahaemolyticus influenced the antioxidative status and caused oxidative stress and tissue damage via confusion of antioxidant enzymes in P. monodon.

A new method to evaluate the effects of bacterial dosage, infection route and Vibrio strain in experimental challenges of Litopenaeus vannamei, based on the Cox proportional hazard model

In the shrimp challenge test the Vibrio dosage, infection route, and strain are considered as risk factors that result in mortality. Assessment of Vibrio/shrimp interactions, and disease dynamics following infection by Vibrio, are useful techniques needed for detailed studies on the control of risk factors. In this paper we used an application of the Cox proportional hazard model to assess relative survival probability, estimate mortality risk, and construct a prognostic model to assess predictions of estimated time to death. Results indicate that infection route was the most important prognostic factor contributing to mortality in the challenge test (β = 3.698, P < 0.000). The shrimp infection rate following injection was found to be 40.4 times greater than that following immersion (hazard ratio (HR) = 40.4; p = 0.000). Our results also indicated that the HR resulting in shrimp mortality following a high dose of 10(8) cfu/shrimp was significantly greater (HR = 5.9, P < 0.000), than that following a baseline dosage of 10(7) cfu/shrimp. Strain Vh was found to be more virulent than Strain Vp (HR = 4.8; P < 0.000). The prognostic index also indicated that the infection route is the most important prognostic factor contributing to mortality in the challenge test.