Crustacean hemolymph microbiota: Endemic, tightly controlled, and utilization expectable

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.

Host-related traits influence the microbial diversity of the invasive signal crayfish Pacifastacus leniusculus

The microbiome influences a variety of host-environment interactions, and there is mounting evidence of its significant role in biological invasions. During invasion, shifts in microbial diversity and function can occur due to both changing characteristics of the novel environment and physiological condition of the host. The signal crayfish (Pacifastacus leniusculus) is one of the most successful crayfish invaders in Europe. During range expansion, its populations often exhibit differences in many traits along the invasion range, including sex-composition, size-structure and aggressiveness, but to date it was not studied whether crayfish traits can also drive changes in the host microbiome. Thus, we used 16S rRNA gene amplicon sequencing to examine the effects of host-related traits, namely total length (TL), body condition index (FCF), hepatosomatic index (HSI) and sex on the microbial diversity of the signal crayfish. We examined both external (exoskeletal) and internal (intestinal, hepatopancreatic, hemolymph) microbiomes of 110 signal crayfish individuals from four sites along its invasion range in the Korana River, Croatia. While sex did not exhibit a significant effect on the microbial diversity in any of the examined tissues, exoskeletal, intestinal and hemolymph microbial diversity significantly decreased with increasing crayfish size. Additionally, significant effects of signal crayfish condition (FCF, HSI) on microbial diversity were recorded in the hepatopancreas, a main energy storage organ in crayfish that supports reproduction and growth and also regulates immune response. Our findings provide a baseline for evaluating the contribution of microbiome to an invader's overall health, fitness and subsequent invasion success.

Assessing the microbiota of the snail intermediate host of trematodes, Galba truncatula

BACKGROUND

The microbiome is known to play key roles in health and disease, including host susceptibility to parasite infections. The freshwater snail Galba truncatula is the intermediate host for many trematode species, including the liver and rumen flukes Fasciola hepatica and Calicophoron daubneyi, respectively. The snail-parasite system has previously been investigated. However, the specific interaction between the snail-associated microbiota and intra-snail developmental stages of trematodes has yet to be explored.

METHODS

Galba truncatula snails were collected from farms in Northern Ireland and trematode infection was diagnosed using PCR. High-throughput sequencing analysis of the bacterial 16S ribosomal DNA V3-V4 hypervariable regions was subsequently applied to characterise the microbiota of both uninfected and infected snails.

RESULTS

We first showed that the snail harboured microbiota that was distinct for its environment. The microbiota of infected snails was found to differ significantly from that of uninfected snails. In particular, the bacterial genera Mycoplasma and Methylotenera were significantly more abundant in infected snails, while genera Sphingomonas and Nocardioides were predominantly associated with uninfected snails.

CONCLUSION

These findings pave the way to future studies on the functional roles of bacteria in host-parasite relationships.

Symbiotic hemolymph bacteria reduce hexavalent chromium to protect the host from chromium toxicity in Procambarus clarkii

Hexavalent chromium (Cr(VI)) is a cytotoxic heavy metal pollutant that adversely affects all life forms. Interestingly, the crustacean Procambarus clarkii exhibits a relatively high tolerance to heavy metals. The underlying mechanisms remain unclear. In this study, we investigated the role of symbiotic bacteria in P. clarkii in alleviating Cr(VI)-induced damage and explored their potential mechanisms of action. Through transcriptomic analysis, we observed that Cr(VI) activated P. clarkii's antimicrobial immune responses and altered the bacterial composition in the hemolymph. After antibiotic treatment to reduce bacterial populations, Cr(VI)-induced intestinal and liver damage worsened, and crayfish exhibited lower levels of GSH/CAT/SOD activity. The Exiguobacterium, the symbiotic bacteria in the hemolymph of P. clarkii, were proved to be primary contributor to Cr(VI) tolerance. Further investigation suggested that it resists Cr(VI) through the activation of the ABC transporter system and the reduction of Cr(VI) via the reductase gene nfsA. To validate the role of Exiguobacterium in Cr(VI) tolerance, crayfish treated with antibiotics then supplemented with Exiguobacterium H6 and recombinant E. coli (with the nfsA gene), reduced Cr(VI)-induced ovarian damage. Overall, this study revealed that the symbiotic bacteria Exiguobacterium can absorb and reduce hexavalent chromium, mitigating Cr(VI)-induced damage in P. clarkii. These findings provide new insights into hexavalent chromium tolerance mechanisms in crustaceans.

Molecular cloning and functional characterization of peroxiredoxin 4 (prx 4) in freshwater crayfish, Procambarus clarkii

Peroxiredoxin (Prx), which is a newly discovered member of the antioxidant protein family, performs important biological functions in intracellular signal transduction. In the present study, a peroxiredoxin 4 gene was cloned from crayfish for the first time and named Pc-prx 4. According to the amino acid sequence signature, Pc-Prx 4 was identified as the typical 2-Cys Prx molecule, which possessed two conserved cysteines (Cys98 and Cys219). Time-course expression patterns post V. harveyi infection revealed that Pc-prx 4 was likely related to crayfish innate immune defense responses. In particular, the highest fold upregulation of the Pc-prx 4 mRNA transcript reached approximately 170 post V. harveyi infection in the crayfish hepatopancreas. The results of the mixed functional oxidase assay showed that rPc-Prx 4^△ could resist the damaging effect of reactive oxygen species generated from the thiol/Fe³⁺/O₂^- reaction system to some extent. In addition, the results of the RNAi assay revealed that the crayfish survival rate was obviously increased post injection of V. harveyi when Pc-prx 4 was knocked down. Further study revealed that both hemolymph melanization and PO activity were strengthened to different degrees in the RNAi assay. Therefore, we speculated that the increase in the crayfish survival rate was likely due to the increase in hemolymph melanization. The obviously reinforced hemolymph melanization was directly caused by the upregulation of hemolymph PO activity, which was induced by the knockdown of Pc-prx 4. However, further studies are still indispensable for illuminating the molecular mechanism of Pc-prx 4 in the crayfish innate immune defense system.

The mycobiome of a successful crayfish invader and its changes along the environmental gradient

BACKGROUND

The microbiome plays an important role in biological invasions, since it affects various interactions between host and environment. However, most studies focus on the bacteriome, insufficiently addressing other components of the microbiome such as the mycobiome. Microbial fungi are among the most damaging pathogens in freshwater crayfish populations, colonizing and infecting both native and invasive crayfish species. Invading crayfish may transmit novel fungal species to native populations, but also, dispersal process and characteristics of the novel environment may affect the invaders' mycobiome composition, directly and indirectly affecting their fitness and invasion success. This study analyzes the mycobiome of a successful invader in Europe, the signal crayfish, using the ITS rRNA amplicon sequencing approach. We explored the mycobiomes of crayfish samples (exoskeletal biofilm, hemolymph, hepatopancreas, intestine), compared them to environmental samples (water, sediment), and examined the differences in fungal diversity and abundance between upstream and downstream segments of the signal crayfish invasion range in the Korana River, Croatia.

RESULTS

A low number of ASVs (indicating low abundance and/or diversity of fungal taxa) was obtained in hemolymph and hepatopancreas samples. Thus, only exoskeleton, intestine, sediment and water samples were analyzed further. Significant differences were recorded between their mycobiomes, confirming their uniqueness. Generally, environmental mycobiomes showed higher diversity than crayfish-associated mycobiomes. The intestinal mycobiome showed significantly lower richness compared to other mycobiomes. Significant differences in the diversity of sediment and exoskeletal mycobiomes were recorded between different river segments (but not for water and intestinal mycobiomes). Together with the high observed portion of shared ASVs between sediment and exoskeleton, this indicates that the environment (i.e. sediment mycobiome) at least partly shapes the exoskeletal mycobiome of crayfish.

CONCLUSION

This study presents the first data on crayfish-associated fungal communities across different tissues, which is valuable given the lack of studies on the crayfish mycobiome. We demonstrate significant differences in the crayfish exoskeletal mycobiome along the invasion range, suggesting that different local environmental conditions may shape the exoskeletal mycobiome during range expansion, while the mycobiome of the internal organ (intestine) remained more stable. Our results provide a basis for assessing how the mycobiome contributes to the overall health of the signal crayfish and its further invasion success.

scRNA-seq analysis reveals toxicity mechanisms in shrimp hemocytes subjected to nitrite stress

In shrimp, hemocytes play an important role in detoxification and immune defense, and are where nitrite accumulates during exposure to this toxic environmental pollutant. However, the heterogeneity mechanisms of toxicity have not been reported under nitrite expose in shrimp. Here, we used single-cell RNA-seq to resolve 24,000 cells, which the responses of different cell populations of hemocytes under nitrite exposure in Penaeus vannamei. We identified 394 specific nitrite-responsive genes in 9 clusters of hemocytes, and found heterogeneity in the nitrite response of the three subpopulations of hemocytes (hyaline, semi-granular and granular cells). In hyaline, the response appeared modest, whereas nitrite-related dysregulation of metabolic processes in granular and semi-granular was pronounced. Ammonia nitrogen will rapidly accumulate in hemocytes of shrimp under nitrite stress. In semi-granular, excessive ammonia will interfere with oxidative phosphorylation and antioxidant system, thus inducing the production of reactive oxygen species. In granular, the abnormality of urea cycle caused by ammonia accumulation is the main toxic factor, which by inhibits arginase and arginine kinase. Collectively, our data provide a single-cell atlas for the dissection of shrimp hemocyte complexity, and reveal the toxicity mechanisms associated with nitrite exposure.

Comparative Analysis of the Symbiotic Microbiota in the Chinese Mitten Crab (Eriocheir sinensis): Microbial Structure, Co-Occurrence Patterns, and Predictive Functions

Symbiotic microorganisms in the digestive and circulatory systems are found in various crustaceans, and their essential roles in crustacean health, nutrition, and disease have attracted considerable interest. Although the intestinal microbiota of the Chinese mitten crab (Eriocheir sinensis) has been extensively studied, information on the symbiotic microbiota at various sites of this aquatic economic species, particularly the hepatopancreas and hemolymph, is lacking. This study aimed to comprehensively characterize the hemolymph, hepatopancreas, and intestinal microbiota of Chinese mitten crab through the high-throughput sequencing of the 16S rRNA gene. Results showed no significant difference in microbial diversity between the hemolymph and hepatopancreas (Welch t-test; p > 0.05), but their microbial diversity was significantly higher than that in the intestine (p < 0.05). Distinct differences were found in the structure, composition, and predicted function of the symbiotic microbiota at these sites. At the phylum level, the hemolymph and hepatopancreas microbiota were dominated by Proteobacteria, Firmicutes, and Acidobacteriota, followed by Bacteroidota and Actinobacteriota, whereas the gut microbiota was mainly composed of Firmicutes, Proteobacteria, and Bacteroidota. At the genus level, Candidatus Hepatoplasma, Shewanella, and Aeromonas were dominant in the hepatopancreas; Candidatus Bacilloplasma, Roseimarinus, and Vibrio were dominant in the intestine; Enterobacter, norank_Vicinamibacterales, and Pseudomonas were relatively high-abundance genera in the hemolymph. The composition and abundance of symbiotic microbiota in the hemolymph and hepatopancreas were extremely similar (p > 0.05), and no significant difference in functional prediction was found (p > 0.05). Comparing the hemolymph in the intestine and hepatopancreas, the hemolymph had lower variation in bacterial composition among individuals, having a more uniform abundance of major bacterial taxa, a smaller coefficient of variation, and the highest proportion of shared genera. Network complexity varied greatly among the three sites. The hepatopancreas microbiota was the most complex, followed by the hemolymph microbiota, and the intestinal microbiota had the simplest network. This study revealed the taxonomic and functional characteristics of the hemolymph, hepatopancreas, and gut microbiota in Chinese mitten crab. The results expanded our understanding of the symbiotic microbiota in crustaceans, providing potential indicators for assessing the health status of Chinese mitten crab.

Crustin Defense against Vibrio parahaemolyticus Infection by Regulating Intestinal Microbial Balance in Litopenaeus vannamei

Crustins are a kind of antimicrobial peptide (AMP) that exist in crustaceans. Some crustins do not have direct antimicrobial activity but exhibit in vivo defense functions against Vibrio. However, the underlying molecular mechanism is not clear. Here, the regulatory mechanism was partially revealed along with the characterization of the immune function of a type I crustin, LvCrustin I-2, from Litopenaeus vannamei. LvCrustin I-2 was mainly detected in hemocytes, intestines and gills and was apparently up-regulated after Vibrio parahaemolyticus infection. Although the recombinant LvCrustin I-2 protein possessed neither antibacterial activity nor agglutinating activity, the knockdown of LvCrustin I-2 accelerated the in vivo proliferation of V. parahaemolyticus. Microbiome analysis showed that the balance of intestinal microbiota was impaired after LvCrustin I-2 knockdown. Further transcriptome analysis showed that the intestinal epithelial barrier and immune function were impaired in shrimp after LvCrustin I-2 knockdown. After removing the intestinal bacteria via antibiotic treatment, the phenomenon of impaired intestinal epithelial barrier and immune function disappeared in shrimp after LvCrustin I-2 knockdown. This indicated that the impairment of the shrimp intestine after LvCrustin I-2 knockdown was caused by the dysbiosis of the intestinal microbiota. The present data suggest that crustins could resist pathogen infection through regulating the intestinal microbiota balance, which provides new insights into the functional mechanisms of antimicrobial peptides during pathogen infection.

Deacetylation of K481 and K484 on Penaeid Shrimp Hemocyanin Is Critical for Antibacterial Activity

Although invertebrates' innate immunity relies on several immune-like molecules, the diversity of these molecules and their immune response mechanisms are not well understood. Here, we show that Penaeus vannamei hemocyanin (PvHMC) undergoes specific deacetylation under Vibrio parahaemolyticus and LPS challenge. In vitro deacetylation of PvHMC increases its binding capacity with LPS and antibacterial activity against Gram-negative bacteria. Lysine residues K481 and K484 on the Ig-like domain of PvHMC are the main acetylation sites modulated by the acetyltransferase TIP60 and deacetylase HDAC3. Deacetylation of PvHMC on K481 and K484 allows PvHMC to form a positively charged binding pocket that interacts directly with LPS, whereas acetylation abrogates the positive charge to decrease PvHMC-LPS attraction. Besides, V. parahaemolyticus and LPS challenge increases the expression of Pvhdac3 to induce PvHMC deacetylation. This work indicates that, during bacterial infections, deacetylation of hemocyanin is crucial for binding with LPS to clear Gram-negative bacteria in crustaceans.

Metabolomic Investigation of Ultraviolet Ray-Inactivated White Spot Syndrome Virus-Induced Trained Immunity in Marsupenaeus japonicus

Trained immunity is driven by metabolism and epigenetics in innate immune cells in mammals. The phenomenon of trained immunity has been identified in invertebrates, including shrimp, but the underlying mechanisms remain unclear. To elucidate mechanisms of trained immunity in shrimp, the metabolomic changes in hemolymph of Marsupenaeus japonicus trained by the UV-inactivated white spot syndrome virus (UV-WSSV) were analyzed using tandem gas chromatography–mass/mass spectrometry. The metabolomic profiles of shrimp trained with UV-WSSV followed WSSV infection showed significant differences comparison with the control groups, PBS injection followed WSSV infection. 16 differential metabolites in total of 154 metabolites were identified, including D-fructose-6-phosphate, D-glucose-6-phosphate, and D-fructose-6-phosphate, and metabolic pathways, glycolysis, pentose phosphate pathway, and AMPK signaling pathway were enriched in the UV-WSSV trained groups. Further study found that histone monomethylation and trimethylation at H3K4 (H3K4me1 and H3K4me3) were involved in the trained immunity. Our data suggest that the UV-WSSV induced trained immunity leads to metabolism reprogramming in the shrimp and provide insights for WSSV control in shrimp aquaculture.

A g-type lysozyme from the scallop Argopecten purpuratus participates in the immune response and in the stability of the hemolymph microbiota

Lysozymes are antimicrobial acid hydrolases widely distributed in nature. They are located inside the cells in lysosomes, or they are secreted to the extracellular space, where they can lyse the cell wall of certain species of bacteria via hydrolysis of the peptidoglycan. Thus, lysozymes are bacteriolytic enzymes and play a major biological role in biodefense, as these enzymes can act as antibacterial and immune-modulating agents. In this study, we characterized a g-type lysozyme from the scallop Argopecten purpuratus named ApGlys. The cDNA sequence comprises an open reading frame (ORF) of 600 nucleotides, codifying for a putative protein of 200 amino acids with a signal peptide of 18 amino acids. The deduced mature protein sequence displays a molecular weight of 20.07 kDa and an isoelectric point (pI) of 6.49. ApGlys deduced protein sequence exhibits conserved residues associated with catalytic activity and substrate fixation in other g-type lysozymes. The phylogenetic analysis revealed a high degree of identity of ApGlys with other mollusk g-type lysozymes, which form a restricted and separated clade from the vertebrate lysozymes. ApGlys transcripts were constitutively and highly expressed in the digestive gland, and it was induced in hemocytes and gills of scallops after an immune challenge. Furthermore, the ApGlys protein was located inside hemocytes of immunostimulated scallops, determined by immunofluorescence analysis. Finally, the transcript silencing of ApGlys by RNA interference led to an increase of total culturable bacteria from the scallop hemolymph. Furthermore, we detected a higher diversity of the bacterial community in ApGlys-silenced scallops and an imbalance of certain bacterial groups present in the hemolymph by 16S rDNA deep amplicon sequencing. Overall, our results showed that ApGlys is a new member of scallop lysozymes that is implicated in the immune response and in the microbial homeostasis of A. purpuratus hemolymph.

Characterization of novel L-asparaginases having clinically safe profiles from bacteria inhabiting the hemolymph of the crab, Scylla serrata (Forskål, 1775)

L-asparaginase (ASNase) is the principal chemotherapeutic agent against different blood cancers. The risks associated with current clinical preparations demand screening for novel ASNases. Accordingly, the study was conducted to shortlist ASNases having clinically safer profiles from a novel niche, namely, microbes in the gut and hemolymph of apparently healthy Scylla serrata. A four-step strategic approach incorporating the essential requirements for clinically safer profiles was followed. The initial step through plate assay showed five (9.61%) potential ASNase producers. The relative prevalence of ASNase producers was higher in hemolymph (13.33%) than gut (4.5%). The positive isolates were identified as Priestia aryabhattai, Priestia megaterium, Bacillus altitudinis, Shewanella decolorationis, and Chryseomicrobium amylolyticum. Quantitative profiles revealed high ASNase production (114.29 to 287.36 U/mL) without any optimization, with an added advantage of the extracellular production. The second step for substrate specificity studies revealed the absence of L-glutaminase and urease activities in ASNases from C. amylolyticum and P. megaterium, the most desirable properties for safe clinical applications. This is the first report of glutaminase and urease-free ASNase from these two bacteria. The third step ensured type II nature of selected ASNases, the targeted form in clinical applications. The fourth step confirmed the activity and stability in human physiological conditions. Altogether, the results revealed two potential ASNases with clinically compatible profiles.

Microbiome of the Successful Freshwater Invader, the Signal Crayfish, and Its Changes along the Invasion Range

Increasing evidence denotes the role of the microbiome in biological invasions, since it is known that microbes can affect the fitness of the host. Here, we demonstrate differences in the composition of an invader’s microbiome along the invasion range, suggesting that its microbial communities may affect and be affected by range expansion. Using a 16S rRNA gene amplicon sequencing approach, we (i) analyzed the microbiomes of different tissues (exoskeleton, hemolymph, hepatopancreas, and intestine) of a successful freshwater invader, the signal crayfish, (ii) compared them to the surrounding water and sediment, and (iii) explored their changes along the invasion range. Exoskeletal, hepatopancreatic, and intestinal microbiomes varied between invasion core and invasion front populations. This indicates that they may be partly determined by population density, which was higher in the invasion core than in the invasion front. The highly diverse microbiome of exoskeletal biofilm was partly shaped by the environment (due to the similarity with the sediment microbiome) and partly by intrinsic crayfish parameters (due to the high proportion of exoskeleton-unique amplicon sequence variants [ASVs]), including the differences in invasion core and front population structure. Hemolymph had the most distinct microbiome compared to other tissues and differed between upstream (rural) and downstream (urban) river sections, indicating that its microbiome is potentially more driven by the effects of the abiotic environment. Our findings offer an insight into microbiome changes during dispersal of a successful invader and present a baseline for assessment of their contribution to an invader’s overall health and its further invasion success.

IMPORTANCE Invasive species are among the major drivers of biodiversity loss and impairment of ecosystem services worldwide, but our understanding of their invasion success and dynamics still has many gaps. For instance, although it is known that host-associated microbial communities may significantly affect an individual’s health and fitness, the current studies on invasive species are mainly focused on pathogenic microbes, while the effects of the remaining majority of microbial communities on the invasion process are almost completely unexplored. We have analyzed the microbiome of one of the most successful crayfish invaders in Europe, the signal crayfish, and explored its changes along the signal crayfish invasion range in the Korana River, Croatia. Our study sets the perspective for future research required to assess the contribution of these changes to an individual’s overall health status and resilience of dispersing populations and their impact on invasion success.

Molting Alters the Microbiome, Immune Response, and Digestive Enzyme Activity in Mud Crab (Scylla paramamosain)

Molting is a crucial lifelong process in the growth, development, and reproduction of crustaceans. In mud crab (Scylla paramamosain), new exoskeleton, gills, and appendages are formed after a molting, which contributes to a 40 to 90% increase in body weight. However, little is currently known about the associations between molting and the dynamic changes of microbiota and physiological characteristics in mud crabs. In this study, the effects of molting on changes of the microbiome, immune response, and digestive enzyme activities in mud crabs were investigated. The results showed dynamic changes in the abundances and community compositions of crab-associated microbiota harboring the gills, subcuticular epidermis, hepatopancreas, midgut, and hemolymph during molting. Renewed microbiota was observed in the gills and midgut of crabs at the postmolt stages, which seems to be related to the formation of a new exoskeleton after the molting. A significant positive correlation between the expression of two antimicrobial peptide (AMP) genes (SpALF5 and SpCrustin) and the relative abundance of two predominant microorganisms (Halomonas and Shewanella) in hemolymph was observed in the whole molt cycle, suggesting that AMPs play a role in modulating hemolymph microbiota. Furthermore, digestive enzymes might play a vital role in the changes of microbiota harboring the hepatopancreas and midgut, which provide suitable conditions for restoring and reconstructing host-microbiome homeostasis during molting. In conclusion, this study confirms that molting affects host-associated microbiota and further sheds light on the effects on the immune response and the digestive systems as well. IMPORTANCE Molting is crucial for crustaceans. In mud crab, its exoskeleton is renewed periodically during molting, and this process is an ideal model to study the effects of host development on its microbiota. Here, multiple approaches were used to investigate the changes in microbial taxa, immune response, and digestive enzyme activity with respect to molting in mud crab. The results found that a renewed microbiota was generated in the gills and midgut of crab after a molt. A significant positive correlation between changes in the relative abundances of microbes (such as Halomonas and Shewanella) and the expression of AMP genes (SpALF5 and SpCrustin) was observed in the hemolymph of crabs during the whole molt cycle, suggesting the modulation of hemolymph microbes by AMPs. Furthermore, the digestive enzymes were found to participate in the regulation of microbiota in hepatopancreas and midgut, consequently providing a suitable condition for the restoration and reconstruction of host-microbiome homeostasis during the molting. This study confirms that molting affects the microbial communities and concomitantly influences the immune and digestive systems in mud crabs. This is also the first time the homeostasis of the host and microbiome, and the associations between molting and physiological characteristics in crustaceans, have been revealed.

Exosomal miR-224 contributes to hemolymph microbiota homeostasis during bacterial infection in crustacean

It is well known that exosomes could serve as anti-microbial immune factors in animals. However, despite growing evidences have shown that the homeostasis of the hemolymph microbiota was vital for immune regulation in crustaceans, the relationship between exosomes and hemolymph microbiota homeostasis during pathogenic bacteria infection has not been addressed. Here, we reported that exosomes released from Vibrio parahaemolyticus-infected mud crabs (Scylla paramamosain) could help to maintain the homeostasis of hemolymph microbiota and have a protective effect on the mortality of the host during the infection process. We further confirmed that miR-224 was densely packaged in these exosomes, resulting in the suppression of HSP70 and disruption of the HSP70-TRAF6 complex, then the released TRAF6 further interacted with Ecsit to regulate the production of mitochondrial ROS (mROS) and the expression of Anti-lipopolysaccharide factors (ALFs) in recipient hemocytes, which eventually affected hemolymph microbiota homeostasis in response to the pathogenic bacteria infection in mud crab. To the best of our knowledge, this is the first document that reports the role of exosome in the hemolymph microbiota homeostasis modulation during pathogen infection, which reveals the crosstalk between exosomal miRNAs and innate immune response in crustaceans.

Exosomes are small membrane vesicles of endocytic origin which are widely involved in the regulation of a variety of pathological processes in mammals. Yet, although the antibacterial function of exosomes has been discovered for many years, the relationship between exosomes and hemolymph microbiota homeostasis remains unknown. In the present study, we identified the miRNAs packaged by exosomes that were possibly involved in Vibrio parahaemolyticus infection by modulating hemolymph microbiota homeostasis in crustacean mud crab Scylla paramamosain. Moreover, it was found that miR-224 was densely packaged in exosomes after Vibrio parahaemolyticus challenge, resulting in the suppression of HSP70 and disruption of the HSP70-TRAF6 complex in recipient hemocytes, then the released TRAF6 was further interacted with Ecsit to regulate ROS and ALFs levels, which eventually affected hemolymph microbiota homeostasis to cope with pathogenic bacteria infection. Our finding is the first to reveal the relationship between exosomes and hemolymph microbiota homeostasis in animals, which shows a novel molecular mechanism of invertebrate resistance to pathogenic microbial infection.

Hepatopancreas-Specific Lectin Participates in the Antibacterial Immune Response by Regulating the Expression of Antibacterial Proteins

The hepatopancreas is an important digestive and immune organ in crustacean. There were low but stable numbers of microbes living in the hemolymph of crustacean, whereas the organs (including hepatopancreas) of crustacean were immersed in the hemolymph. It is very important to study the immune mechanism of the hepatopancreas against bacteria. In this study, a novel CTL (HepCL) with two CRDs, which was mainly expressed in the hepatopancreas, was identified in red swamp crayfish (Procambarus clarkii). HepCL binds to bacteria in vitro and could enhance bacterial clearance in vivo. Compared with the C-terminal CRD of HepCL (HepCL-C), the N-terminal CRD (HepCL-N) showed weaker bacterial binding ability in vitro and stronger bacterial clearance activity in vivo. The expression of some antimicrobial proteins, such as FLP, ALF1 and ALF5, was downregulated under knockdown of HepCL or blocked with Anti-HepCL after challenge with Vibrio in crayfish. These results demonstrated that HepCL might be involved in the antibacterial immune response by regulating the expression of antimicrobial proteins.

Cellular Immune Response of an Endemic Lake Baikal Amphipod to Indigenous Pseudomonas sp

Studies of invertebrates have shown that the internal environment of crustaceans is not always sterile in normal conditions, and in many species, it can be populated by microorganisms even in the absence of any visible pathological processes in the body. This observation raises the question of whether genetically modified indigenous hemolymph microorganisms can be used for biotechnological purposes inside the crustacean either as local producers of some compounds or as sensors to physiological parameters. In this study, we tested the ability of the bacteria isolated from the hemolymph of the amphipod Eulimnogammarus verrucosus to hide from the cellular immune response of the host as the most important feature for their potential long-term application in vivo. 16S rDNA amplicon sequencing revealed five common bacterial genera in all analyzed samples of the amphipod hemolymph, among which Pseudomonas is most easily subjected to genome modification and, thus, the most prospective for biotechnological application. Cultivation of Pseudomonas gave us a number of strains undoubtedly derived from the amphipod hemolymph, and one of them (belonging to the Pseudomonas fluorescens group) was chosen for further tests. The primary culture of amphipod hemocytes was used to analyze the immunogenicity of the strain and showed a pronounced reaction of the immune cells to a high amount of the bacteria within six hours. This result indicates that modulation of cellular immune response to metabolically active bacterial cells is not mandatory for the survival and wide distribution of these microorganisms in the hemolymph of numerous amphipod individuals.

Metabolomic Profiles in the Intestine of Shrimp Infected by White Spot Syndrome Virus and Antiviral Function of the Metabolite Linoleic Acid in Shrimp

White spot syndrome virus (WSSV) is a threatening pathogenic virus in shrimp culture, and at present, no effective strategy can prevent and control the disease. Intestinal flora and its metabolites are important for the resistance of shrimp to lethal pathogenic viruses. However, the changes of metabolites in the shrimp intestines after WSSV infection remain unclear. We established an artificial oral infection method to infect shrimp with WSSV and analyzed the metabolites in intestinal content of shrimp by HPLC and tandem mass spectrometry. A total of 78 different metabolites and five different metabolic pathways were identified. Among them, we found that the content of linoleic acid, an unsaturated fatty acid, increased significantly after WSSV infection, indicating that linoleic acid might be involved in antiviral immunity in shrimp. Further study showed that, after oral administration of linoleic acid, WSSV proliferation decreased evidently in the shrimp, and survival rate of the shrimp increased significantly. Mechanical analysis showed that linoleic acid directly bound to WSSV virions and inhibited the viral replication. Linoleic acid also promoted the expression of antimicrobial peptides and IFN-like gene Vago5 by activating the ERK-NF-κB signaling pathway. Our results indicated that WSSV infection caused metabolomic transformation of intestinal microbiota and that the metabolite linoleic acid participated in the immune response against WSSV in shrimp.

FOXO regulates the expression of antimicrobial peptides and promotes phagocytosis of hemocytes in shrimp antibacterial immunity

Invertebrates rely on innate immunity, including humoral and cellular immunity, to resist pathogenic infection. Previous studies showed that forkhead box transcription factor O (FOXO) participates in mucosal immune responses of mammals and the gut humoral immune regulation of invertebrates. However, whether FOXO is involved in systemic and cellular immunity regulation in invertebrates remains unknown. In the present study, we identified a FOXO from shrimp (Marsupenaeus japonicus) and found that it was expressed at relatively basal levels in normal shrimp, but was upregulated significantly in shrimp challenged by Vibrio anguillarum. FOXO played a critical role in maintaining hemolymph and intestinal microbiota homeostasis by promoting the expression of Relish, the transcription factor of the immune deficiency (IMD) pathway for expression of antimicrobial peptides (AMPs) in shrimp. We also found that pathogen infection activated FOXO and induced its nuclear translocation by reducing serine/threonine kinase AKT activity. In the nucleus, activated FOXO directly regulated the expression of its target Amp and Relish genes against bacterial infection. Furthermore, FOXO was identified as being involved in cellular immunity by promoting the phagocytosis of hemocytes through upregulating the expression of the phagocytotic receptor scavenger receptor C (Src), and two small GTPases, Rab5 and Rab7, which are related to phagosome trafficking to the lysosome in the cytoplasm. Taken together, our results indicated that FOXO exerts its effects on homeostasis of hemolymph and the enteric microbiota by activating the IMD pathway in normal shrimp, and directly or indirectly promoting AMP expression and enhancing phagocytosis of hemocytes against pathogens in bacteria-infected shrimp. This study revealed the different functions of FOXO in the mucosal (local) and systemic antibacterial immunity of invertebrates.

Accumulation of paralytic shellfish toxins by Southern Rock lobster Jasus edwardsii causes minimal impact on lobster health

Recurrent dinoflagellate blooms of Alexandrium catenella expose the economically and ecologically important Southern Rock Lobster in Tasmania to paralytic shellfish toxins (PST), and it is unknown if PST accumulation adversely affects lobster performance, health and catchability. In a controlled aquaculture setting, lobsters were fed highly contaminated mussels to accumulate toxin levels in the hepatopancreas (mean of 6.65 mg STX.2HCl equiv. kg^-1), comparable to those observed in nature. Physiological impact of PST accumulation was comprehensively assessed by a range of behavioural (vitality score, righting ability and reflex impairment score), health (haemocyte count, bacteriology, gill necrosis and parasite load), nutritional (hepatopancreas index and haemolymph refractive index) and haemolymph biochemical (21 parameters including electrolytes, metabolites, and enzymes) parameters during a 63 day period of uptake and depuration of toxins. Exposure to PST did not result in mortality nor significant changes in the behavioural, health, or nutritional measures suggesting limited gross impact on lobster performance. Furthermore, most haemolymph biochemical parameters measured exhibited no significant difference between control and exposed animals. However, the concentration of potassium in the haemolymph increased with PST, whilst the concentration of lactate and the sodium:potassium ratio decreased with PST. In addition, exposed lobsters showed a hyperglycaemic response to PST exposure, indicative of stress. These findings suggest that PST accumulation results in some measurable indicators of stress for lobsters. However, these changes are likely within the adaptive range for Jasus edwardsii and do not result in a significant impairment of gross performance. Our findings support previous conclusions that crustaceans are relatively tolerant to PST and the implications for the lobster fishery are discussed.

The hemolymph of Biomphalaria snail vectors of schistosomiasis supports a diverse microbiome

The microbiome - the microorganism community that is found on or within an organism's body - is increasingly recognized to shape many aspects of its host biology and is a key determinant of health and disease. Microbiomes modulate the capacity of insect disease vectors (mosquitoes, tsetse flies, sandflies) to transmit parasites and disease. We investigate the diversity and abundance of microorganisms within the hemolymph (i.e. blood) of Biomphalaria snails, the intermediate host for Schistosoma mansoni, using Illumina MiSeq sequencing of the bacterial 16S V4 rDNA. We sampled hemolymph from five snails from six different laboratory populations of B. glabrata and one population of B. alexandrina. We observed 279.84 ± 0.79 amplicon sequence variants per snail. There were significant differences in microbiome composition at the level of individual snails, snail populations and species. Snail microbiomes were dominated by Proteobacteria and Bacteroidetes while water microbiomes from snail tank were dominated by Actinobacteria. We investigated the absolute bacterial load using qPCR: hemolymph samples contained 2784 ± 339 bacteria/μl. We speculate that the microbiome may represent a critical, but unexplored intermediary in the snail-schistosome interaction as hemolymph is in very close contact with the parasite at each step of its development.

Host Defense Effectors Expressed by Hemocytes Shape the Bacterial Microbiota From the Scallop Hemolymph

The interaction between host immune response and the associated microbiota has recently become a fundamental aspect of vertebrate and invertebrate animal health. This interaction allows the specific association of microbial communities, which participate in a variety of processes in the host including protection against pathogens. Marine aquatic invertebrates such as scallops are also colonized by diverse microbial communities. Scallops remain healthy most of the time, and in general, only a few species are fatally affected on adult stage by viral and bacterial pathogens. Still, high mortalities at larval stages are widely reported and they are associated with pathogenic Vibrio. Thus, to give new insights into the interaction between scallop immune response and its associated microbiota, we assessed the involvement of two host antimicrobial effectors in shaping the abundances of bacterial communities present in the scallop Argopecten purpuratus hemolymph. To do this, we first characterized the microbiota composition in the hemolymph from non-stimulated scallops, finding both common and distinct bacterial communities dominated by the Proteobacteria, Spirochaetes and Bacteroidetes phyla. Next, we identified dynamic shifts of certain bacterial communities in the scallop hemolymph along immune response progression, where host antimicrobial effectors were expressed at basal level and early induced after a bacterial challenge. Finally, the transcript silencing of the antimicrobial peptide big defensin ApBD1 and the bactericidal/permeability-increasing protein ApLBP/BPI1 by RNA interference led to an imbalance of target bacterial groups from scallop hemolymph. Specifically, a significant increase in the class Gammaproteobacteria and the proliferation of Vibrio spp. was observed in scallops silenced for each antimicrobial. Overall, our results strongly suggest that scallop antimicrobial peptides and proteins are implicated in the maintenance of microbial homeostasis and are key molecules in orchestrating host-microbiota interactions. This new evidence depicts the delicate balance that exists between the immune response of A. purpuratus and the hemolymph microbiota.

IgIT-Mediated Signaling Inhibits the Antimicrobial Immune Response in Oyster Hemocytes

The long-term evolutionary interaction between the host and symbiotic microbes determines their cooperative relationship. It is well known that the symbiotic microbes have evolved various mechanisms to either benefit or exploit the mammalian host immune system to maintain homeostasis. However, the strategies employed by the symbiotic microbes to overcome host immune responses in invertebrates are still not clear. In the current study, the hemolymph microbes in oyster Crassostrea gigas were found to be able to directly bind an oyster Ig superfamily member (IgSF) (designated as CgIgIT) to inhibit the immune responses of hemocytes. The mRNA transcripts of CgIgIT in hemocytes increased significantly after the stimulation with hemolymph microbes. CgIgIT was found to be located on the hemocyte membrane and it was able to directly bind the hemolymph microbes and polysaccharides via its three Ig domains and recruited the protein tyrosine phosphatase CgSHP2 through its ITIM. The recruited CgSHP2 inhibited the activities of CgERK, CgP38 and CgJNK proteins to reduce the productions of dual oxidase 2 (CgDuox2) and defensin 2 (CgDef2), which eventually protected the hemolymph microbes from CgDuox2/CgDef2-mediated elimination. Collectively, the results suggest that the oyster IgIT-SHP2 signaling pathway can recognize bacteria capable of residing in oyster hemolymph and inhibit innate immune responses, which contributes to the maintenance, colonization, and survival of hemolymph microbes.

C-type lectin-mediated microbial homeostasis is critical for Helicoverpa armigera larval growth and development

The immune system of a host functions critically in shaping the composition of the microbiota, and some microbes are involved in regulating host endocrine system and development. However, whether the immune system acts on endocrine and development by shaping the composition of the microbiota remains unclear, and few molecular players or microbes involved in this process have been identified. In the current study, we found that RNA interference of a C-type lectin (HaCTL3) in the cotton bollworm Helicoverpa armigera suppresses ecdysone and juvenile hormone signaling, thus reducing larval body size and delaying pupation. Depletion of HaCTL3 also results in an increased abundance of Enterocuccus mundtii in the hemolymph, which may escape from the gut. Furthermore, HaCTL3 and its controlled antimicrobial peptides (attacin, lebocin, and gloverin) are involved in the clearance of E. mundtii from the hemolymph via phagocytosis or direct bactericidal activity. Injection of E. mundtii into larval hemocoel mimics HaCTL3-depleted phenotypes and suppresses ecdysone and juvenile hormone signaling. Taken together, we conclude that HaCTL3 maintains normal larval growth and development of H. armigera via suppressing the abundance of E. mundtii in the hemolymph. Our results provide the first evidence of an immune system acting on an endocrine system to modulate development via shaping the composition of microbiota in insect hemolymph. Thus, this study will deepen our understanding of the interaction between immunity and development.

Considering that a large number of hemocytes and multiple soluble effectors are present in insect hemolymph, it is conventionally believed that healthy insect hemolymph is a hostile environment for bacteria and is, therefore, sterile. However, increasing evidences disprove this opinion, although the interactive mechanism between hemolymph microbiota and insect host, as well as the function of hemolymph microbiota, remain unclear. C-type lectin (CTL), as pattern recognition receptor (PRR), plays important roles in defending against various pathogens. Here we found that various bacteria colonized the hemolymph of the cotton bollworm Helicoverpa armigera. We first reported that an H. armigera CTL (HaCTL3) was involved in modulating larval growth and development. Further study indicated that HaCTL3-mediated homeostasis of Enterocuccus mundtii in the hemolymph is critical for normal larval growth and development. Our study demonstrated that this PRR modulated insect development through shaping hemolymph microbiota, which may represent a novel mechanism of immune system regulation during insect development.

SpATF2 participates in maintaining the homeostasis of hemolymph microbiota by regulating dual oxidase expression in mud crab

Activating transcription factors 2 (ATF2) is a transcription factor of the members of ATF/CREB family that is phosphorylated and activated by the mitogen-activated protein kinase (MAPK) in responding to the stimulation of stimuli. In present study, SpATF2 from mud crab (Scylla paramamosain) was identified and studied. The open reading frame of SpATF2 with 2136 bp in length encodes a protein with 711 amino acids. The SpATF2 protein includes the putative zinc finger domain in the N-terminus and bZIP type DNA-binding domain in the C-terminal. Tissue distribution of SpATF2 transcripts showed that SpATF2 was ubiquitously expressed in all examined tissues of the untreated mud crabs, with the highest expression levels in muscle and hepatopancreas. The transcriptional level of SpATF2 was up-regulated in the hemocytes after Vibrio parahemolyticus or WSSV infection. Reporter gene assays indicated that SpATF2 could activate the expression of dual oxidase (SpDuox1) in S. paramamosain. The RNA interference (RNAi) of SpATF2 significantly decreased the expression of SpDuox1, and consequently reduced reactive oxygen species production thereby significantly increased the bacterial load in the hemolymph of mud crabs. Similarly, significant reduction in bacterial clearance of hemolymph was observed after the V. parahemolyticus infection in SpATF2 knockdown mud crabs. This study showed that SpATF2 played a vital role in maintaining homeostasis of the hemolymph microbiota through regulating the expression of dual oxidase of mud crab.

The functional relevance of shrimp C-type lectins in host-pathogen interactions

C-type lectins (CTLs) are key recognition proteins in shrimp immunity. A few years ago we reviewed sequence information, ligand specificity, expression profiles and specific functions of the shrimp CTLs. Since then, multiple integrated studies that implemented biochemical approaches using both the native and recombinant proteins, functional genetic approaches using RNA interference, and mechanistic studies by analyzing protein-protein interactions were carried out. Results from these rigorous studies revealed the functions and mechanisms of action of selected members of the shrimp CTL family. This review focuses on this new knowledge, that includes unique structural aspects, functions, and mechanisms in host-pathogen interactions, the functional relevance of regions other than the C-type lectin domain, and the regulation of transcription of shrimp CTLs. Thus, this review aims to provide a detailed update of recent studies that have contributed to our better understanding of the shrimp immune events that involve CTL functions.

Induction of innate immune response in whiteleg shrimp (Litopenaeus vannamei) embryos

The immune response of commercially relevant marine invertebrates has been extensively studied, in search of new disease-control strategies. Immune training is considered a novel approach that could help improve resistance to different pathogens. Here, we stimulated the white shrimp (Litopenaeus vannamei) during embryo development by exposure to heat-killed bacteria and evaluated their effect on hatching, larval development, and the expression of immune-related genes. In addition, we evaluated its impact on the response of shrimp nauplii during a challenge with Vibrio parahaemolyticus. We observed that the percentage of hatching and the resistance to bacterial infection increased due to the treatment of embryos with heat-killed cells of Vibrio and Bacillus. Apparently different stimuli could generate a differential pattern of gene expression, e.g., Vibrio induced a strong effector immune response whereas Bacillus elicited a protective immune profile. In addition, each response was triggered by molecular patterns detected in the environment. The results obtained in this study provide new insights for immune training to improve shrimp farming.

Source of hemolymph microbiota and their roles in the immune system of mud crab

Special innate immune mechanisms against pathogens are developed in marine invertebrates such as mud crab, which is also an economically important aquaculture species in many coastal countries. Hemolymph is a critical site in host immune response, but its source of microorganisms is less known. In this study, we provided a detailed investigation of the microorganisms inhabiting various body sites of healthy mud crabs, including hemolymph, midgut, gill, subcuticular epidermis and hepatopancreas. By using fluorescence microscopy and high-throughput sequencing of the bacterial 16S rRNA genes, various abundances and kinds of microorganisms were observed in the healthy mud crabs, of which some are potential pathogens to mud crab and human. The SourceTracker analysis and oral injection experiment confirm the hypothesis that hemolymph microorganisms are derived from the digestive systems of invertebrates with open circulatory systems, indicating that these microorganisms play vital roles in crab immune response. Moreover, physiological differences (gut length), behavioral characteristics (foraging behavior), diet preferences (herbivory), and/or sex hormones (testosterone) possibly determine the unique features of the crab-associated microbiota for both sexes. These findings also contribute to the development of appropriate microbial immunoenhancers, which has potential applications for improving quality and yield during crab aquaculture.

Temperature elevation and Vibrio cyclitrophicus infection reduce the diversity of haemolymph microbiome of the mussel Mytilus coruscus

Haemolymph microbiome was considered to be unique to healthy invertebrates and beneficial to the host against external pathogens, including disease resistance and maintenance of homeostasis. Here, we investigated the effects of elevated water temperature on infection of haemolymph microbiome of the hard-shelled mussel (Mytilus coruscus). Exposure to Vibrio. cyclitrophicus resulted in high mortality of mussels on day nine at 27 °C. The haemolymph was collected to determine the microbiota by 16 S rRNA gene sequencing. Exposure to waterborne V. cyclitrophicus increased the mortality of mussels that was associated with a reduction in the diversity of their microbial community. Principal coordinate analysis (PCoA) revealed that temperature was an essential factor in shaping microbial communities in mussel haemolymph. Vibrio exposure promoted the proliferation of opportunistic pathogens (e.g., Arcobacter and Francisella) at a lower temperature. A high abundance of Vibrio present in live and dead mussels, at 27 °C might contribute greatly to mortality, as indicated by linear discriminant analysis effect size (LEfSe). These data suggested that the dynamics of microbial community have unique biomarker species in mussel haemolymph that could be used as health indicators. An elevated temperature may reduce the ability of bacterial elimination function against infection in mussel haemolymph.

Protective Efficacy of a Pseudoalteromonas Strain in European Abalone, Haliotis tuberculata, Infected with Vibrio harveyi ORM4

The hemolymph of healthy marine invertebrates is known to harbor antibiotic-producing bacteria belonging to the genus Pseudoalteromonas. Such strains are potential probiotics to control infectious diseases in aquaculture. In the present study, we screened a collection of Pseudoalteromonas strains isolated from the hemolymph of oyster and mussel for antimicrobial activity against Vibrio harveyi, a pathogenic species responsible for high mortality in abalone. Subsequently, the protective efficacy of the most active strain named hCg-6 was investigated in abalone culture faced with a Vibrio harveyi ORM4 infection. First, we have controlled the Pseudoalteromonas hCg-6 safety for abalone health. To that end, animals were immersed for 4 h in Pseudoalteromonas hCg-6 suspensions in seawater. The abalone viability was monitored and Pseudoalteromonas hCg-6 was tracked by quantitative-PCR in abalone hemolymph. After immersion, no abalone death occurred while the strain hCg-6 was significantly detected in hemolymph. Therefore, the strain hCg-6 was considered safe for abalone and evaluated for its ability to protect abalone against V. harveyi (injection of 1 × 10³Vibrio per animal). A 4-h long immersion of abalone in a seawater suspension of Pseudoalteromonas hCg-6 (1 × 10⁶ CFU mL^-1) prior to infection with Vibrio harveyi significantly improved the abalone viability. Indeed, 15 days post infection, the hCg-6 treatment used increased the abalone survival rate from 16% in untreated animals to 40% in treated abalone. We hypothesized that Pseudoalteromonas hCg-6 antibacterial activity increased the hemomicrobiota shielding effect. In conclusion, Pseudoalteromonas hCg-6 is a promising anti-Vibrio strain for abalone culture.

Spp38 MAPK participates in maintaining the homeostasis of hemolymph microbiota in Scylla paramamosain

The p38 mitogen-activated protein kinases (MAPKs) are evolutionally conserved from yeasts to mammals, and are involved in the regulation of cells response to various extracellular stimuli. In this study, the p38 MAPK gene (designated as Spp38) of mud crab (Scylla paramamosain) was identified and studied. Spp38 contained the conserved Thr-Gly-Tyr (TGY) motif and a Ala-Thr-Arg-Trp (ATRW) substrate-binding site. Spp38 transcript was ubiquitously expressed in all tissues examined, with the highest expression found in muscle and hepatopancras. Quantitative real-time PCR revealed that Spp38 was upregulated in hemocytes and hepatopancras after infection with Vibrio parahemolyticus and Lipopolysaccharides (LPS). Reporter gene assays indicated that Spp38 activated the expression of anti-lipopolysaccharides (SpALF1 - SpALF6) in S. paramamosian. RNA interference (RNAi)-mediated knockdown of Spp38 or inhibition of Spp38 by SB203580 decreased the expression levels of SpALF1-6 and dual oxidase (SpDuox1 and SpDuox2) in S. paramamosian, which consequently reduced reactive oxygen species (ROS) production thereby significantly increasing the bacterial count in the hemolymph of mud crabs. Similarly, there was a significant reduction in bacterial clearance ability of hemolymph after Spp38 knockdown followed by V. parahemolyticus infection. Taken together, the current data indicated that Spp38 could play a vital role in maintaining the homeostasis of hemolymph microbiota in S. paramamosain.

Haemolymph microbiome of the cultured spiny lobster Panulirus ornatus at different temperatures

Lobsters have an open circulatory system with haemolymph that contains microorganisms even in the healthy individuals. Understanding the role of these microorganisms becomes increasingly important particularly for the diagnosis of disease as the closed life-cycle aquaculture of the spiny lobster Panulirus ornatus nears commercial reality. This study aimed to characterise haemolymph responses of healthy cultured P. ornatus juveniles at control (28 °C) and elevated (34 °C) temperatures. This was assessed by measuring immune parameters (total granulocyte counts, total haemocyte counts, clotting times), and culture-independent (pyrosequencing of haemolymph DNA) and culture-dependent (isolation using nonselective growth medium) techniques to analyse bacterial communities from lobster haemolymph sampled on days 0, 4 and 6 post-exposure to the temperature regimes. Elevated temperature (34 °C) affected lobster survival, total granulocyte counts, and diversity, load and functional potential of the haemolymph bacterial community. Pyrosequencing analyses showed that the core haemolymph microbiome consisted of phyla Proteobacteria and Bacteriodetes. Overall, culture-independent methods captured a higher bacterial diversity and load when compared to culture-dependent methods, however members of the Rhodobacteraceae were strongly represented in both analyses. This is the first comprehensive study providing comparisons of haemolymph bacterial communities from healthy and thermally stressed cultured juvenile P. ornatus and has the potential to be used in health monitoring programs.

New insights into the haemo- and coelo-microbiota with antimicrobial activities from Echinodermata and Mollusca

AIMS

The aim of this study was to investigate the diversity of bacteria with antimicrobial activity present in the coelomic fluid and haemolymph of wild and healthy echinodermata and mollusca.

METHODS AND RESULTS

Collection expeditions of healthy marine molluscs and echinoderms were conducted in the Glenan archipelago in spring 2014. Members of the culturable microbiota present in the haemolymph, (haemo-microbiota) of Haliotis tuberculata (gastropoda, abalone) and Mytilus edulis (bivalvia, mussel), as well as in the coelomic fluid (coelo-microbiota) of Echinus esculentus (echinoidea, sea urchin) and Holothuria forskali (Holothuroidea, holothurian) were screened for antimicrobial activity, and further identified using 16S rRNA sequencing. Except for E. esculentus, culturable bacteria in the internal fluids of all studied organisms (mussel, abalone and holothurian) were more abundant than in seawater. The haemo- and coelo-microbiota with antimicrobial activity differed significantly between host species, in terms of abundance and diversity. Indeed, higher numbers were isolated from mussel than from abalone haemolymph. Moreover, in mussels and holothurians, bacteria with antimicrobial activities were predominantly Vibrio spp. (respectively 55 and 45%), while Pseudoalteromonas spp. were the most abundant (50%) in abalone haemolymph. Nevertheless, the activity spectra of these bacteria mainly included marine pathogens affiliated to the Vibrio genus.

CONCLUSION

The haemo- and coelo-microbiota with antimicrobial activities were significantly related to their host species and differed in terms of abundance and diversity. These bacteria may play a key role in host homeostasis against pathogens.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study brings new knowledge on the diversity of bacteria present in the internal fluids of two marine molluscs and two echinoderms and their antimicrobial activities towards marine pathogens.

A prokaryotic-eukaryotic relation in the fat body of Bombus terrestris

The interaction between the insect host and its microbiota plays a central role in insect health and is mostly studied in relation to the digestive system. Nonetheless, there are numerous microorganisms occupying multiple habitats in and on insects. We studied microbial communities in the gut and fat body of bumblebees (Bombus terrestris) using the V4 region of the 16S rRNA gene on the Illumina MiSeq platform. In one of the two study locations, the fat body microbial composition was marked by the dominant presence of Arsenophonus sp. and Phyllobacterium sp. Bumblebees infected with Apicystis bombi, a eukaryotic parasite multiplying in the fat body, had a significant higher relative abundance of Arsenophonus sp. compared with the non-infected individuals. In general, the infection of A. bombi correlated with a more interlinked microbial association network, as we observed an increase of significant associations between the relative abundance of bacteria present in the gut and fat body of infected bumblebees. The causality within this potential prokaryotic-eukaryotic relation is important when assessing the health impact on bees.

Dual oxidases participate in the regulation of hemolymph microbiota homeostasis in mud crab Scylla paramamosain

Dual oxidases (DUOXs) were originally identified as NADPH oxidases (NOXs), found to be associated with the reactive oxygen species (ROS) hydrogen peroxide (H₂O₂) production at the plasma membrane and crucial in host biological processes. In this study, SpDUOX1 and SpDUOX2 of mud crab (Scylla paramamosain) were identified and studied. Both SpDUOX1 and SpDUOX2 are transmembrane proteins, including an N-signal peptide region and a peroxidase homology domain in the extracellular region, transmembrane regions, and three EF (calcium-binding region) domains, a FAD-binding domain, and a NAD binding domain in the intracellular region. The SpDUOXs were expressed in all tissues examined, but mainly in hepatopancreas, heart, and mid-intestine. The expression of the SpDUOXs in the hemolymph of mud crabs was up-regulated after challenge with Vibrio parahemolyticus or LPS. RNA interference (RNAi) of the SpDUOXs resulted in reduced ROS production in hemolymph. The bacterial count increased in the hemolymph of mud crabs injected with SpDUOX1 or SpDUOX2-RNAi, while the bacterial clearance ability of hemolymph significantly reduced. At the phylum level, the phyla Bacteroidetes and Actinobacteria were significantly increased, while Proteobacteria were significantly reduced following SpDUOX2 knockdown. There was a significant increase in the relative abundance of the genera Marinomonas, Pseudoalteromonas, Shewanella, and Hydrogenoph in SpDUOX2 depleted mud crabs compared with the controls. Our current findings therefore indicated that SpDUOXs might play important roles in maintaining the homeostasis in the hemolymph microbiota of mud crab.

Microbiome composition within a sympatric species complex of intertidal isopods (Jaera albifrons)

The increasingly recognised effects of microbiomes on the eco-evolutionary dynamics of their hosts are promoting a view of the "hologenome" as an integral host-symbiont evolutionary entity. For example, sex-ratio distorting reproductive parasites such as Wolbachia are well-studied pivotal drivers of invertebrate reproductive processes, and more recent work is highlighting novel effects of microbiome assemblages on host mating behaviour and developmental incompatibilities that underpin or reinforce reproductive isolation processes. However, examining the hologenome and its eco-evolutionary effects in natural populations is challenging because microbiome composition is considerably influenced by environmental factors. Here we illustrate these challenges in a sympatric species complex of intertidal isopods (Jaera albifrons spp.) with pervasive sex-ratio distortion and ecological and behavioural reproductive isolation mechanisms. We deep-sequence the bacterial 16S rRNA gene among males and females collected in spring and summer from two coasts in north-east Scotland, and examine microbiome composition with a particular focus on reproductive parasites. Microbiomes of all species were diverse (overall 3,317 unique sequences among 3.8 million reads) and comprised mainly Proteobacteria and Bacteroidetes taxa typical of the marine intertidal zone, in particular Vibrio spp. However, we found little evidence of the reproductive parasites Wolbachia, Rickettsia, Spiroplasma and Cardinium, suggesting alternative causes of sex-ratio distortion. Notwithstanding, a significant proportion of the variance in microbiome composition among samples was explained by sex (14.1 %), nested within geographic (26.9 %) and seasonal (39.6 %) variance components. The functional relevance of this sex signal was difficult to ascertain given the absence of reproductive parasites, the ephemeral nature of the species assemblages and substantial environmental variability. These results establish the Jaera albifrons species complex as an intriguing system for examining the effects of microbiomes on reproductive processes and speciation, and highlight the difficulties associated with snapshot assays of microbiome composition in dynamic and complex environments.

Hemolymph Microbiomes of Three Aquatic Invertebrates as Revealed by a New Cell Extraction Method

Symbiotic microorganisms have been found in the hemolymph (blood) of many aquatic invertebrates, such as crabs, shrimp, and oysters. Hemolymph is a critical site in the host immune response. Currently, studies on hemolymph microorganisms are mostly performed with culture-dependent strategies using selective media (e.g., thiosulfate-citrate-bile salts-sucrose [TCBS], 2216E, and LB) for enumerating and isolating microbial cells. However, doubts remain about the "true" representation of the microbial abundance and diversity of symbiotic microorganisms in hemolymph, particularly for uncultivable microorganisms, which are believed to be more abundant than the cultured microorganisms. To explore this, we developed a culture-independent cell extraction method for separating microbial cells from the hemolymph of three aquatic invertebrates (Scylla paramamosain [mud crab], Litopenaeus vannamei [whiteleg shrimp], and Crassostrea angulata [Portuguese oysters]) involving filtration through a 5-μm-pore-size mesh filter membrane (the filtration method). A combination of the filtration method with fluorescence microscopy and high-throughput sequencing technique provides insight into the abundances and diversity of the total microbiota in the hemolymph of these three invertebrates. More than 2.6 × 10⁴ cells/ml of microbial cells dominated by Escherichia-Shigella and Halomonas, Photobacterium and Escherichia-Shigella, and Pseudoalteromonas and Arcobacter were detected in the hemolymph of Scylla paramamosain, Litopenaeus vannamei, and Crassostrea angulata, respectively. A parallel study for investigating the hemolymph microbiomes by comparing the filtration method and a culture-dependent method (the plate count method) showed significantly higher microbial abundances (between 26- and 369-fold difference; P < 0.05) and less biased community structures of the filtration method than those of the plate count method. Furthermore, hemolymph of the three invertebrates harbored many potential pathogens, including Photobacterium, Arcobacter, and Vibrio species. Finally, the filtration method provides a solution that improves the understanding of the metabolic functions of uncultivable hemolymph microorganisms (e.g., metagenomics) devoid of host hemocyte contamination.IMPORTANCE Microorganisms are found in the hemolymph of invertebrates, a critical site in the host immune response. Currently, studies on hemolymph microorganisms are mostly performed with culture-dependent strategies. However, doubts remain about the "true" representation of the hemolymph microbiome. This study developed a culture-independent cell extraction method that could separate microbial cells from the hemolymph of three aquatic invertebrates (S. paramamosain, L. vannamei, and C. angulata) based on filtration through a 5-μm-pore-size mesh filter membrane (the filtration method). A combination of the filtration method with fluorescence microscopy and a high-throughput sequencing technique provides insight into the abundances and diversity of the total microbiota in the hemolymph of these three invertebrates. Our results demonstrate that the hemolymph of aquatic invertebrates harbors a much higher microbial abundance and more distinct microbial community composition than previously estimated. Furthermore, this work provides a less biased solution for studying the metabolic functions of uncultivable hemolymph microbiota devoid of host hemocyte contamination.

Identification and functional analysis of a TEP gene from a crustacean reveals its transcriptional regulation mediated by NF-κB and JNK pathways and its broad protective roles against multiple pathogens

Thioester-containing proteins (TEPs) are present in a wide range of species from deuterostomes to protostomes and are thought to be involved in innate immunity. In the current study, a TEP gene homologous to insect TEPs (iTEP) from the crustacean Litopenaeus vannamei, named LvTEP1, is cloned and functionally characterized. The open reading frame (ORF) of LvTEP1 is 4383 bp in length, encoding a polypeptide of 1460 amino acids with a calculated molecular weight of 161.1 kDa LvTEP1, which is most similar to other TEPs from insects, contains some conserved sequence features, including a N-terminal signal peptide, a canonical thioester (TE) motif, and a C-terminal distinctive cysteine signature. LvTEP1 is expressed in most immune-related tissues, such as intestine, epithelium, and hemocytes, and the mRNA level of LvTEP1 is upregulated in hemocytes after bacterial and viral challenges, indicating its involvement in the shrimp innate immune response. An expression assay in Drosophila S2 cells shows LvTEP1 to be a full-length secretory protein, and processed forms are present in the supernatant. Of note, only the processed form of LvTEP1 protein can bind to both the gram-negative bacterium Vibrio parahaemolyticus and the gram-positive bacterium Staphylococcus aureus in vitro, and its abundance can be induced after bacterial treatment. Moreover, knockdown of LvTEP1 renders shrimps more susceptible to both V. parahaemolyticus and S. aureus, as well as white spot syndrome virus (WSSV) infection, suggesting its essential defensive role against these invading microbes. We also observe that the expression of LvTEP1 is regulated in a manner dependent on both NF-κB and AP-1 transcription factors in naive shrimps and in vitro, suggesting that LvTEP1 could be poised in the body cavity prior to infection and thus play an important role in basal immunity. Taken together, our findings provide some in vitro and in vivo evidence for the involvement of LvTEP1 in shrimp innate immunity and provide some insight into its expression regulation mediated by multiple transcription factors or signaling pathways.