Structure-Functional Activity Relationship of β-Glucans From the Perspective of Immunomodulation: A Mini-Review

β-Glucans are a heterogeneous group of glucose polymers with a common structure comprising a main chain of β-(1,3) and/or β-(1,4)-glucopyranosyl units, along with side chains with various branches and lengths. β-Glucans initiate immune responses via immune cells, which become activated by the binding of the polymer to specific receptors. However, β-glucans from different sources also differ in their structure, conformation, physical properties, binding affinity to receptors, and thus biological functions. The mechanisms behind this are not fully understood. This mini-review provides a comprehensive and up-to-date commentary on the relationship between β-glucans' structure and function in relation to their use for immunomodulation.

The blue mussel inside: 3D visualization and description of the vascular-related anatomy of Mytilus edulis to unravel hemolymph extraction

The blue mussel Mytilus edulis is an intensely studied bivalve in biomonitoring programs worldwide. The lack of detailed descriptions of hemolymph-withdrawal protocols, particularly with regard to the place from where hemolymph could be perfused from, raises questions regarding the exact composition of aspirated hemolymph and does not exclude the possibility of contamination with other body-fluids. This study demonstrates the use of high resolution X-ray computed tomography and histology combined with 3D-reconstruction using AMIRA-software to visualize some important vascular-related anatomic structures of Mytilus edulis. Based on these images, different hemolymph extraction sites used in bivalve research were visualized and described, leading to new insights into hemolymph collection. Results show that hemolymph withdrawn from the posterior adductor muscle could be extracted from small spaces and fissures between the muscle fibers that are connected to at least one hemolymph supplying artery, more specifically the left posterior gastro-intestinal artery. Furthermore, 3D-reconstructions indicate that puncturing hemolymph from the pericard, anterior aorta, atria and ventricle in a non-invasive way should be possible. Hemolymph withdrawal from the heart is less straightforward and more prone to contamination from the pallial cavity. This study resulted simultaneously in a detailed description and visualization of the vascular-related anatomy of Mytilus edulis.

Gut Microbiota of Migrating Wild Rabbit Fish (Siganus guttatus) Larvae Have Low Spatial and Temporal Variability

We investigated the gut microbiota of rabbit fish larvae at three locations in Vietnam (ThuanAn-northern, QuangNam-intermediate, BinhDinh-southern sampling site) over a three-year period. In the wild, the first food for rabbit fish larvae remains unknown, while the juveniles and adults are herbivores, forming schools near the coasts, lagoons, and river mouths, and feeding mainly on filamentous algae. This is the first study on the gut microbiota of the wild fish larvae and with a large number of individuals analyzed spatially and temporally. The Clostridiales order was the most predominant in the gut, and location-by-location alpha diversity showed significant differences in Chao-1, Hill number 1, and evenness. Analysis of beta diversity indicated that the location, not year, had an effect on the composition of the microbiota. In 2014, the gut microbiota of fish from QuangNam was different from that in BinhDinh; in 2015, the gut microbiota was different for all locations; and, in 2016, the gut microbiota in ThuanAn was different from that in the other locations. There was a time-dependent trend in the north-south axis for the gut microbiota, which is considered to be tentative awaiting larger datasets. We found limited variation in the gut microbiota geographically and in time and strong indications for a core microbiome. Five and fifteen OTUs were found in 100 and 99% of the individuals, respectively. This suggests that at this life stage the gut microbiota is under strong selection due to a combination of fish-microbe and microbe-microbe interactions.

Recombinant DnaK Orally Administered Protects Axenic European Sea Bass Against Vibriosis

Vibrio anguillarum causes high mortality in European sea bass (Dicentrarchus labrax) larviculture and is a hindering factor for successful sustainable aquaculture of this commercially valuable species. Priming of the innate immune system through administration of immunostimulants has become an important approach to control disease outbreaks in marine fish larviculture. This study was conducted to evaluate immunostimulation by Escherichia coli HSP70 (DnaK) in axenic European sea bass larvae in order to protect the larvae against vibriosis. DnaK stimulates the immune response in crustaceans and juvenile fish against bacterial infections. The use of axenic fish larvae allows to study immunostimulation in the absence of an interfering microbial community. At 7 days post-hatching, larvae received a single dose of alginate encapsulated recombinant DnaK. Two non-treated control groups in which animals either received empty alginate microparticles (C1) or no alginante microparticles (C2 and C3) were included in the study. Eighteen hours later, all larvae, except the ones from group C3 (non-infected control) were challenged with V. anguillarum (10⁵ CFU, bath infection). Mortality was daily recorded until 120 h post infection and at 18, 24, and 36 h post infection, larvae were sampled for expression of immune related genes. Results showed that V. anguillarum induced an immune response in axenic sea bass larvae but that the innate immune response was incapable to protect the larvae against deadly septicaemic disease. In addition, we showed that administration of alginate encapsulated DnaK to axenic European sea bass larvae at DAH7 resulted in a significant, DnaK dose dependent, upreglation of immune sensor, regulatory and effector genes. Significant upregulation of cxcr4, cas1 and especially of hep and dic was correlated with significant higher survival rates in V. anguillarum infected larvae. In the future recombinant DnaK might perhaps be used as a novel immunostimulant in sea bass larviculture.

Environmental conditions steer phenotypic switching in acute hepatopancreatic necrosis disease-causing Vibrio parahaemolyticus, affecting PirAVP /PirBVP toxins production

Bacteria in nature are widely exposed to differential fluid shears which are often a trigger for phenotypic switches. The latter mediates transcriptional and translation remodelling of cellular metabolism impacting among others virulence, antimicrobial resistance and stress resistance. In this study, we evaluated the role of fluid shear on phenotypic switch in an acute hepatopancreatic necrosis disease (AHPND)-causing Vibrio parahaemolyticus M0904 strain under both in vitro and in vivo conditions. The results showed that V. parahaemolyticus M0904 grown at lower shaking speed (110 rpm constant agitation, M0904/110), causing low fluid shear, develop cellular aggregates or floccules. These cells increased levan production (as verified by concanavalin binding) and developed differentially stained colonies on Congo red agar plates and resistance to antibiotics. In addition, the phenotypic switch causes a major shift in the protein secretome. At 120 rpm (M0904/120), PirA^VP /PirB^VP toxins are mainly produced, while at 110 rpm PirA^VP /PirB^VP toxins production is stopped and an alkaline phosphatase (ALP) PhoX becomes the dominant protein in the protein secretome. These observations are matched with a very strong reduction in virulence of M0904/110 towards two crustacean larvae, namely, Artemia and Macrobrachium. Taken together, our study provides substantial evidence for the existence of two phenotypic forms in AHPND V. parahaemolyticus strain displaying differential phenotypes. Moreover, as aerators and pumping devices are frequently used in shrimp aquaculture facilities, they can inflict fluid shear to the standing microbial agents. Hence, our study could provide a basis to understand the behaviour of AHPND-causing V. parahaemolyticus in aquaculture settings and open the possibility to monitor and control AHPND by steering phenotypes.

Archivory in hypersaline aquatic environments: haloarchaea as a dietary source for the brine shrimp Artemia

Archaea have been the most overlooked and enigmatic of the three domains of life for decades. Knowledge of key ecological interactions, such as trophic links between this domain and higher level organisms, remains extremely limited. The co-occurrence of halophilic Archaea (haloarchaea) and the non-selective filter feeder, brine shrimp Artemia under the unique ecological characteristics of hypersaline aquatic environments, constitutes an excellent opportunity to further unravel the ecological role of the Archaea domain as a source of food to zooplankton metazoans. In the present study, we combine the use of haloarchaea biomass assimilation experiments using 13C isotope as tracer, with gnotobiotic Artemia culture tests using haloarchaea mono-diets, to investigate potential trophic links between the organisms. Our results demonstrated the ability of Artemia to assimilate nutrients from mono-diets of haloarchaea biomass in order to survive and grow, providing clear indications that archivory may occur in hypersaline aquatic environments. Additionally, our study highlights the use of stable isotopes labelling as a potential tool to further disentangle the specific pathways by which archaeal cellular constituents are digested by consumers.

Phloroglucinol Treatment Induces Transgenerational Epigenetic Inherited Resistance Against Vibrio Infections and Thermal Stress in a Brine Shrimp (Artemia franciscana) Model

Emerging, infectious diseases in shrimp like acute hepatopancreatic necrosis disease (AHPND) caused by Vibrio parahaemolyticus and mortality caused by other Vibrio species such as Vibrio harveyi are worldwide related to huge economic losses in industrial shrimp production. As a strategy to prevent disease outbreaks, a plant-based phenolic compound could be used as a biocontrol agent. Here, using the brine shrimp (Artemia franciscana) as a model system, we showed that phloroglucinol treatment of the parental animals at early life stages resulted in transgenerational inherited increased resistance in their progeny against biotic stress, i.e., bacteria (V. parahaemolyticus AHPND strain and V. harveyi) and abiotic stress, i.e., lethal heat shock. Increased resistance was recorded in three subsequent generations. Innate immune-related gene expression profiles and potential epigenetic mechanisms were studied to discover the underlying protective mechanisms. Our results showed that phloroglucinol treatment of the brine shrimp parents significantly (P < 0.05) enhanced the expression of a core set of innate immune genes (DSCAM, proPO, PXN, HSP90, HSP70, and LGBP) in subsequent generations. We also demonstrated that epigenetic mechanisms such as DNA methylation, m6A RNA methylation, and histone acetylation and methylation (active chromatin marker i.e., H3K4Me3, H3K4me1, H3K27me1, H3 hyperacetylation, H3K14ac and repression marker, i.e., H3K27me3, H4 hypoacetylation) might play a role in regulation of gene expression leading toward the observed transgenerational inheritance of the resistant brine shrimp progenies. To our knowledge, this is the first report on transgenerational inheritance of a compound-induced robust protected phenotype in brine shrimp, particularly protected against AHPND caused by V. parahaemolyticus and vibriosis caused by V. harveyi. Results showed that epigenetic reprogramming is likely to play a role in the underlying mechanism.

Does Ralstonia eutropha, rich in poly-β hydroxybutyrate (PHB), protect blue mussel larvae against pathogenic vibrios?

The natural amorphous polymer poly-β-hydroxybutyrate (PHB-A: lyophilized Ralstonia eutropha containing 75% PHB) was used as a biological agent to control bacterial pathogens of blue mussel (Mytilus edulis) larvae. The larvae were supplied with PHB-A at a concentration of 1 or 10 mg/L for 6 or 24 hr, followed by exposure to either the rifampicin-resistant pathogen Vibrio splendidus or Vibrio coralliilyticus at a concentration of 10⁵ CFU/ml. Larvae pretreated 6 hr with PHB-A (1 mg/L) survived a Vibrio challenge better relative to 24 hr pretreatment. After 96 hr of pathogen exposure, the survival of PHB-A-treated mussel larvae was 1.41- and 1.76-fold higher than the non-treated larvae when challenged with V. splendidus and V. coralliilyticus, respectively. Growth inhibition of the two pathogens at four concentrations of the monomer β-HB (1, 5, 25 and 125 mM) was tested in vitro in LB₃₅ medium, buffered at two different pH values (pH 7 and pH 8). The highest concentration of 125 mM significantly inhibited the pathogen growth in comparison to the lower levels. The effect of β-HB on the production of virulence factors in the tested pathogenic Vibrios revealed a variable pattern of responses.

The chloride channel cystic fibrosis transmembrane conductance regulator (CFTR) controls cellular quiescence by hyperpolarizing the cell membrane during diapause in the crustacean Artemia

Cellular quiescence, a reversible state in which growth, proliferation, and other cellular activities are arrested, is important for self-renewal, differentiation, development, regeneration, and stress resistance. However, the physiological mechanisms underlying cellular quiescence remain largely unknown. In the present study, we used embryos of the crustacean Artemia in the diapause stage, in which these embryos remain quiescent for prolonged periods, as a model to explore the relationship between cell-membrane potential (V _mem) and quiescence. We found that V _mem is hyperpolarized and that the intracellular chloride concentration is high in diapause embryos, whereas V _mem is depolarized and intracellular chloride concentration is reduced in postdiapause embryos and during further embryonic development. We identified and characterized the chloride ion channel protein cystic fibrosis transmembrane conductance regulator (CFTR) of Artemia (Ar-CFTR) and found that its expression is silenced in quiescent cells of Artemia diapause embryos but remains constant in all other embryonic stages. Ar-CFTR knockdown and GlyH-101-mediated chemical inhibition of Ar-CFTR produced diapause embryos having a high V _mem and intracellular chloride concentration, whereas control Artemia embryos released free-swimming nauplius larvae. Transcriptome analysis of embryos at different developmental stages revealed that proliferation, differentiation, and metabolism are suppressed in diapause embryos and restored in postdiapause embryos. Combined with RNA sequencing (RNA-Seq) of GlyH-101-treated MCF-7 breast cancer cells, these analyses revealed that CFTR inhibition down-regulates the Wnt and Aurora Kinase A (AURKA) signaling pathways and up-regulates the p53 signaling pathway. Our findings provide insight into CFTR-mediated regulation of cellular quiescence and V _mem in the Artemia model.

Effect of catecholamine stress hormones (dopamine and norepinephrine) on growth, swimming motility, biofilm formation and virulence factors of Yersinia ruckeri in vitro and an in vivo evaluation in rainbow trout

In this study, we evaluated the impact of the catecholamines on growth, swimming motility, biofilm formation and some virulence factors activities of pathogenic Yersinia ruckeri. Norepinephrine and dopamine (at 100 µM) significantly increased the growth of Y. ruckeri in culture media containing serum. An increase in swimming motility of the pathogen was found following the exposure to the hormones; however, no effect was seen on caseinase, phospholipase and haemolysin productions. Further, antagonists for the catecholamine receptors were observed to block some of the influences of the catecholamines. Indeed, the effects of catecholamines were inhibited by chlorpromazine (the dopaminergic receptor antagonist) for dopamine, labetalol (α-and β-adrenergic receptor antagonist) and phenoxybenzamine (the α-adrenergic receptor antagonist) for norepinephrine, but propranolol (the β-adrenergic receptor antagonist) showed no effect. Pretreatment of Y. ruckeri with the catecholamines resulted in a significant enhancement of its virulence towards rainbow trout and the antagonists could neutralize the effect of the stress hormones in vivo. In summary, our results show that the catecholamines increase the virulence of Y. ruckeri which is pathogenic to trout through increasing the motility, biofilm formation and growth.

High doses of sodium ascorbate act as a prooxidant and protect gnotobiotic brine shrimp larvae (Artemia franciscana) against Vibrio harveyi infection coinciding with heat shock protein 70 activation

Ascorbate is an essential nutrient commonly regarded as an antioxidant. In this study, using axenic brine shrimp and pathogenic strain Vibrio harveyi as the host-pathogen model, we confirmed that pretreatment of sodium ascorbate (NaAs), at an optimum concentration, was a prooxidant by generation of hydrogen peroxide, inducing protective effects in the brine shrimp against V. harveyi infection. Such a protective effect could be neutralized by the addition of an antioxidant enzyme catalase. We further showed that generation of oxygen radicals is linked to the induction of heat shock protein 70 (Hsp70), which is involved in eliciting the antioxidant protection system including superoxidase dismutase (SOD) and possibly many other immune responses. Furthermore, using RNA interference technique, we found that the pretreatment of sodium ascorbate increased the survival significantly in the control knockdown groups (using green fluorescent protein, GFP) but not in Hsp70 knockdown groups and the result directly suggested that the up-regulated Hsp70 induced by sodium ascorbate pretreatment induced the protective effect. These results provide a mechanistic rationale for exploring the further use of ascorbate for antimicrobial therapy in aquaculture.

Can epigenetics translate environmental cues into phenotypes?

Living organisms are constantly exposed to wide ranges of environmental cues. They react to these cues by undergoing a battery of phenotypic responses, such as by altering their physiological and behavioral traits, in order to adapt and survive in the changed environments. The adaptive response of a species induced by environmental cues is typically thought to be associated with its genetic diversity such that higher genetic diversity provides increased adaptive potential. This originates from the general consensus that phenotypic traits have a genetic basis and are subject to Darwinian natural selection and Mendelian inheritance. There is no doubt about the validity of these principles, supported by the successful introgression of specific traits during (selective) breeding. However, a range of recent studies provided fascinating evidences suggesting that environmental effects experienced by an organism during its lifetime can have marked influences on its phenotype, and additionally the organism can pass on the acquired phenotypes to its subsequent generations through non-genetic mechanisms (also termed as epigenetic mechanism) - a notion that dates back to Lamarck and has been controversial ever since. In this review, we describe how the epigenetics has reshaped our long perception about the inheritance/development of phenotypes within organisms, contrasting with the classical gene-based view of inheritance. We particularly highlighted recent developments in our understanding of inheritance of parental environmental induced phenotypic traits in multicellular organisms under different environmental conditions, and discuss how modifications of the epigenome contribute to the determination of the adult phenotype of future generations.

Ralstonia eutropha, containing high poly-β-hydroxybutyrate levels, regulates the immune response in mussel larvae challenged with Vibrio coralliilyticus

Marine invertebrates rely mainly on innate immune mechanisms that include both humoral and cellular responses. Antimicrobial peptides (AMPs), lysozyme and phenoloxidase activity, are important components of the innate immune defense system in marine invertebrates. They provide an immediate and rapid response to invading microorganisms. The impact of amorphous poly-β-hydroxybutyrate (PHB-A) (1 mg PHB-A L^-1) on gene expression of the AMPs mytimycin, mytilinB, defensin and the hydrolytic enzyme lysozyme in infected blue mussel larvae was investigated during "in vivo" challenge tests with Vibrio coralliilyticus (10⁵ CFU mL^-1). RNAs were isolated from mussel larvae tissue, and AMPs were quantified by q-PCR using the 18srRNA gene as a housekeeping gene. Our data demonstrated that AMPs genes had a tendency to be upregulated in challenged mussel larvae, and the strongest expression was observed from 24 h post-exposure onwards. The presence of both PHB-A and the pathogen stimulated the APMs gene expression, however no significant differences were noticed between treatments or between exposure time to the pathogen V. coralliilyticus. Looking at the phenoloxidase activity in the infected mussels, it was observed that the addition of PHB-A significantly increased the activity.

The 9H-Fluoren Vinyl Ether Derivative SAM461 Inhibits Bacterial Luciferase Activity and Protects Artemia franciscana From Luminescent Vibriosis

Vibrio campbellii is a major pathogen in aquaculture. It is a causative agent of the so-called “luminescent vibriosis,” a life-threatening condition caused by bioluminescent Vibrio spp. that often involves mass mortality of farmed shrimps. The emergence of multidrug resistant Vibrio strains raises a concern and poses a challenge for the treatment of this infection in the coming years. Inhibition of bacterial cell-to-cell communication or quorum sensing (QS) has been proposed as an alternative to antibiotic therapies. Aiming to identify novel QS disruptors, the 9H-fluroen-9yl vinyl ether derivative SAM461 was found to thwart V. campbellii bioluminescence, a QS-regulated phenotype. Phenotypic and gene expression analyses revealed, however, that the mode of action of SAM461 was unrelated to QS inhibition. Further evaluation with purified Vibrio fischeri and NanoLuc luciferases revealed enzymatic inhibition at micromolar concentrations. In silico analysis by molecular docking suggested binding of SAM461 in the active site cavities of both luciferase enzymes. Subsequent in vivo testing of SAM461 with gnotobiotic Artemia franciscana nauplii demonstrated naupliar protection against V. campbellii infection at low micromolar concentrations. Taken together, these findings suggest that suppression of luciferase activity could constitute a novel paradigm in the development of alternative anti-infective chemotherapies against luminescent vibriosis, and pave the ground for the chemical synthesis and biological characterization of derivatives with promising antimicrobial prospects.

Effect of quorum quenching bacteria on growth, virulence factors and biofilm formation of Yersinia ruckeri in vitro and an in vivo evaluation of their probiotic effect in rainbow trout

Five N-acyl homoserine lactone-degrading bacteria (quorum quenching (QQ) strains) were selected to evaluate their impacts on growth, virulence factors and biofilm formation in Yersinia ruckeri in vitro. No difference was observed among the growth pattern of Y. ruckeri in monoculture and coculture with the QQ strains. To investigate the regulation of virulence factors by quorum sensing in Y. ruckeri, cultures were supplemented with 3oxo-C8-HSL. The results indicated that swimming motility and biofilm formation are positively regulated by QS (p < 0.05), whereas caseinase, phospholipase and haemolysin productions are not influenced by 3oxo-C8-HSL (p > 0.05). The QQs were able to decrease swimming motility and biofilm formation in Y. ruckeri. QQ bacteria were supplemented to trout feed at 10⁸  CFU/g (for 40 days). Their probiotic effect was verified by Y. ruckeri challenge either by immersion or injection in trout. All strains could significantly increase fish survival with Bacillus thuringiensis and Citrobacter gillenii showing the highest and lowest relative percentage survival (RPS) values (respectively, 85% and 38%). Besides, there was no difference between the RPS values by either immersion or injection challenge expect for B. thuringiensis. The putative involvement of the QQ capacity in the protection against Yersinia is discussed.

Managing the Microbial Community of Marine Fish Larvae: A Holistic Perspective for Larviculture

The availability of high-quality juveniles is a bottleneck in the farming of many marine fish species. Detrimental larvae-microbe interactions are a main reason for poor viability and quality in larval rearing. In this review, we explore the microbial community of fish larvae from an ecological and eco-physiological perspective, with the aim to develop the knowledge basis for microbial management. The larvae are exposed to a huge number of microbes from external and internal sources in intensive aquaculture, but their relative importance depend on the rearing technology used (especially flow-through vs. recirculating systems) and the retention time of the water in the fish tanks. Generally, focus has been on microbes entering the system, but microbes from growth within the system is normally a substantial part of the microbes encountered by larvae. Culture independent methods have revealed an unexpected high richness of bacterial species associated with larvae, with 100-250 operational taxonomic units associated with one individual. The microbiota of larvae changes rapidly until metamorphosis, most likely due to changes in the selection pressure in the digestive tract caused by changes in host-microbe and microbe-microbe interactions. Even though the microbiota of larvae is distinctly different from the microbiota of the water and the live food, the microbiota of the water strongly affects the microbiota of the larvae. We are in the early phase of understanding larvae-microbe interactions in vivo, but some studies with other animals than fish emphasize that we so far have underestimated the complexity of these interactions. We present examples demonstrating the diversity of these interactions. A large variety of microbial management methods exist, focusing on non-selective reduction of microbes, selective enhancement of microbes, and on improvement of the resistance of larvae against microbes. However, relatively few methods have been studied extensively. We believe that there is a lot to gain by increasing the diversity of approaches for microbial management. As many microbial management methods are perturbations of the microbial community, we argue that ecological theory is needed to foresee and test for longer term consequences in microbe-microbe and microbe-larvae interactions. We finally make some recommendations for future research and development.

Teaching Shrimps Self-Defense to Fight Infections

A paradigm shift in our understanding of shrimp immunity offers the potential to develop novel disease-control strategies. We summarize cutting-edge findings on the phenomenon of trained immunity in shrimps and discuss how it may contribute to new avenues for controlling disease in these aquaculturally important animals.

Identification of N-acyl homoserine lactone-degrading bacteria isolated from rainbow trout (Oncorhynchus mykiss)

AIMS

A variety of pathogens use quorum sensing (QS) to control the expression of their virulence factors. QS interference has hence been proposed as a promising antivirulence strategy. The specific aim of this study was to isolate bacteria from trout tissue able to degrade N-acyl homoserine lactones (AHL), a QS molecule family.

METHODS AND RESULTS

In total 132 isolates were screened for AHL degradation using Chromobacterium violaceum CV026 as a biosensor. Twenty-four quorum-quenching (QQ) isolates were identified biochemically and characterized using 16S rDNA sequencing. They belong to Bacillus, Enterobacter, Citrobacter, Acinetobacter, Agrobacterium, Pseudomonas and Stentrophomonas genera. Four Bacillus spp. showed the highest and fastest QQ activity. AHL degradation proved to be enzymatic in most isolates (except for Stentrophomonas spp. and Pseudomonas sp.) as QQ activity could be destroyed by heat and/or proteinase K treatments. All QQ activity proved to be cell-bound except for Pseudomonas sp., where it could be detected in the supernatant. The results of aiiA gene homology analysis revealed the presence of aiiA gene encoding AHL lactonase in all examined isolates except Pseudomonas syringae and Enterobacter cloacae. The HXHXDH motif conserved in all AHL lactonases and considered to be essential for AHL degradation was detected in all AiiAs after sequence alignment.

CONCLUSIONS

Some known and novel QQ bacteria were isolated from trouts and characterized in terms of enzymatic or nonenzymatic AHL degradation activity and their extracellular or intracellular location. In addition, an aiiA gene and its HXHXDH motif were detected in most isolates.

SIGNIFICANCE AND IMPACT OF THE STUDY

We could isolate and identify some novel QQ bacteria including Enterobacter hormaechei, Acinetobacter radioresistens and Citrobacter gillenii. The aiiA gene was detected for the first time in these strains as well as in Stenotrophomonas maltophilia. Our QQ isolates could be used for biocontrol of bacterial infections in aquaculture.

Phloroglucinol-Mediated Hsp70 Production in Crustaceans: Protection against Vibrio parahaemolyticus in Artemia franciscana and Macrobrachium rosenbergii

The halophilic aquatic bacterium, Vibrio parahaemolyticus, is an important aquatic pathogen, also capable of causing acute hepatopancreatic necrosis disease (AHPND) in shrimp resulting in significant economic losses. Therefore, there is an urgent need to develop anti-infective strategies to control AHPND. The gnotobiotic Artemia model is used to establish whether a phenolic compound phloroglucinol is effective against the AHPND strain V. parahaemolyticus MO904. We found that pretreatment with phloroglucinol, at an optimum concentration (30 µM), protects axenic brine shrimp larvae against V. parahaemolyticus infection and induced heat shock protein 70 (Hsp70) production (twofolds or more) as compared with the control. We further demonstrated that the Vibrio-protective effect of phloroglucinol was caused by its prooxidant effect and is linked to the induction of Hsp70. In addition, RNA interference confirms that phloroglucinol-induced Hsp70 mediates the survival of brine shrimp larvae against V. parahaemolyticus infection. The study was validated in xenic Artemia model and in a Macrobrachium rosenbergii system. Pretreatment of xenic brine shrimp larvae (30 µM) and Macrobrachium larvae (5 µM) with phloroglucinol increases the survival of xenic brine shrimp and Macrobrachium larvae against subsequent V. parahaemolyticus challenge. Taken together, our study provides substantial evidence that the prooxidant activity of phloroglucinol induces Hsp70 production protecting brine shrimp, A. franciscana, and freshwater shrimp, M. rosenbergii, against the AHPND V. parahaemolyticus strain MO904. Probably, phloroglucinol treatment might become part of a holistic strategy to control AHPND in shrimp.

Modulation of Innate Immune-Related Genes and Glucocorticoid Synthesis in Gnotobiotic Full-Sibling European Sea Bass (Dicentrarchus labrax) Larvae Challenged With Vibrio anguillarum

Although several efforts have been made to describe the immunoendocrine interaction in fish, there are no studies to date focusing on the characterization of the immune response and glucocorticoid synthesis using the host-pathogen interaction on larval stage as an early developmental stage model of study. Therefore, the aim of this study was to evaluate the glucocorticoid synthesis and the modulation of stress- and innate immune-related genes in European sea bass (Dicentrarchus labrax) larvae challenged with Vibrio anguillarum. For this purpose, we challenged by bath full-sibling gnotobiotic sea bass larvae with 10⁷ CFU mL^-1 of V. anguillarum strain HI 610 on day 5 post-hatching (dph). The mortality was monitored up to the end of the experiment [120 hours post-challenge (hpc)]. While no variations were registered in non-challenged larvae maintained under gnotobiotic conditions (93.20% survival at 120 hpc), in the challenged group a constant and sustained mortality was observed from 36 hpc onward, dropping to 18.31% survival at 120 hpc. Glucocorticoid quantification and expression analysis of stress- and innate immunity-related genes were carried out in single larvae. The increase of cortisol, cortisone and 20β-dihydrocortisone was observed at 120 hpc, although did not influence upon the modulation of stress-related genes (glucocorticoid receptor 1 [gr1], gr2, and heat shock protein 70 [hsp70]). On the other hand, the expression of lysozyme, transferrin, and il-10 differentially increased at 120 hpc together with a marked upregulation of the pro-inflammatory cytokines (il-1β and il-8) and hepcidin, suggesting a late activation of defense mechanisms against V. anguillarum. Importantly, this response coincided with the lowest survival observed in challenged groups. Therefore, the increase in markers associated with glucocorticoid synthesis together with the upregulation of genes associated with the anti-inflammatory response suggests that in larvae infected with V. anguillarum a pro-inflammatory response at systemic level takes place, which then leads to the participation of other physiological mechanisms at systemic level to counteract the effect and the consequences of such response. However, this late systemic response could be related to the previous high mortality observed in sea bass larvae challenged with V. anguillarum.

Physiological and immune response of juvenile rainbow trout to dietary bovine lactoferrin

Lactoferrin, a large multifunctional glycoprotein, is involved in many physiological functions but its immunomodulatory pathways are not well characterized in fish. The objective of the present study was to investigate the temporal effect of dietary bovine lactoferrin (BLf) at low (0.1%) and high (1%) on immunological organs of rainbow trout juveniles. BLf diets did not affect specific growth rate, haematocrit, splenic index, spleen respiratory burst activity as well as humoral (mIgM) and neutrophils (MPO) gene expressions after short term - 35 days (D35) and long term nutrient test - 51 days (D51) of feeding. Both low and high BLf doses induced enhanced level of plasma alternative complement activity, plasma total immunoglobulin on D35 and D51, lymphocyte plus thrombocyte cell proportion on D35 and monocyte cell proportion in total blood leukocyte cells on D51. On D51 but not on D35, BLf diets upregulated the expression of inflammatory genes in kidney for il-1 at the low BLf dose, il-8 at both BLf doses and il-6 at the high BLf dose in spleen, and il-10 at both BLf doses in kidney. Moreover, the expression of T helper (cd4-2α; cd4-2β) genes was significantly upregulated only on D51 by both BLf doses in both spleen and kidney tissues. On D51, controls and BLf treated fish were intraperitoneally injected with A. salmonicida achromogenes. The expression of 13 immune genes was evaluated at 44 h post-injection (D54). The expression of lysozyme gene was upregulated by both BLf doses after bacterial infection both in spleen and kidney. The expression of mcsfrα (spleen) and tgf-β1 (kidney) was also modulated by both BLf doses. Low and high BLf doses enhanced disease resistance of rainbow trout juveniles with the cumulative survival rate of 36% and 38% respectively while those of the control was 19% after 14 days challenged with bacteria. The results indicate that BLf diets activated the humoral immunity, associated to blood leukocyte cells of rainbow trout after short term BLf administration, and the long term BLf administration was necessary for sensitizing other lymphoid organs such as in spleen and kidney. Only after long term test, BLf diets induced significantly higher levels of innate and adaptive immune gene expressions than those of the control. Dietary BLf activated more markedly the expression of innate immune genes than the adaptive ones; this upregulation of some immune genes could explain the high disease resistance observed in rainbow trout juveniles fed BLf.

Poly-β-hydroxybutyrate administration during early life: effects on performance, immunity and microbial community of European sea bass yolk-sac larvae

The reliable production of marine fish larvae is one of the major bottlenecks in aquaculture due to high mortalities mainly caused by infectious diseases. To evaluate if the compound poly-β-hydroxybutyrate (PHB) might be a suitable immunoprophylactic measure in fish larviculture, its capacity to improve immunity and performance in European sea bass (Dicentrarchus labrax) yolk-sac larvae was explored. PHB was applied from mouth opening onwards to stimulate the developing larval immune system at the earliest possible point in time. Larval survival, growth, microbiota composition, gene expression profiles and disease resistance were assessed. PHB administration improved larval survival and, furthermore, altered the larva-associated microbiota composition. The bacterial challenge test using pathogenic Vibrio anguillarum revealed that the larval disease resistance was not influenced by PHB. The expression profiles of 26 genes involved e.g. in the immune response showed that PHB affected the expression of the antimicrobial peptides ferritin (fer) and dicentracin (dic), however, the response to PHB was inconsistent and weaker than previously demonstrated for sea bass post-larvae. Hence, the present study highlights the need for more research focusing on the immunostimulation of different early developmental stages for gaining a more comprehensive picture and advancing a sustainable production of high quality fry.

A probiotic Bacillus strain containing amorphous poly-beta-hydroxybutyrate (PHB) stimulates the innate immune response of Penaeus monodon postlarvae

In this study, the PHB-accumulating Bacillus sp. JL47 strain (capable of accumulating 55% PHB on cell dry weight) was investigated for its effects on the immune response of giant tiger shrimp (Penaeus monodon) postlarvae (PL) before and after the Vibrio campbellii challenge. Briefly, shrimp PL were cultured and fed with Artemia nauplii enriched with Bacillus sp. JL47. Shrimp receiving the Artemia nauplii without JL47 enrichment were used as control. After 15 days of feeding, the shrimp were challenged with pathogenic V. campbellii LMG 21363 at 10⁶ cells mL^-1 by immersion. Relative expression of the immune related genes encoding for prophenoloxidase (proPO), transglutaminase (TGase) and heat shock protein 70 (Hsp70) in the shrimp were measured before (0 h) and after (3, 6, 9, 12, 24 h) the Vibrio challenge by quantitative real-time PCR using β-actin as the reference gene. The expressions of TGase and proPO were significantly up-regulated (p < 0.05) within 9 h and 12 h, respectively after challenge in shrimp receiving the Bacillus sp. JL47 as compared to the challenged and non-challenged controls. Hsp70 expression was significantly increased (p < 0.05) at 3 h post-challenge in all challenged shrimp. Interestingly, proPO and TGase genes were significantly up-regulated (p < 0.05) in Bacillus sp. JL47 treated shrimp even before the Vibrio challenge was applied. No up-regulation in the Hsp70 gene, however, was observed under these conditions. The data suggest that the protective effect of the PHB-accumulating Bacillus sp. JL47 in shrimp was due to its capacity to stimulate the innate immune related genes of the shrimp, specifically the proPO and TGase genes. The application of probiotic Bacillus species, capable of accumulating a significant amount of PHB, is suggested as potential immunostimulatory strategy for aquaculture.