Probing the protective mechanism of poly-ß-hydroxybutyrate against vibriosis by using gnotobiotic Artemia franciscana and Vibrio campbellii as host-pathogen model

Polyhydroxyalkanoates for Sustainable Aquaculture: A Review of Recent Advancements, Challenges, and Future Directions

Polyhydroxyalkanoates (PHA) are biodegradable biopolymers produced by prokaryotic microbes, which, at the same time, can be applied as single-cell proteins (SCPs), growing on renewable waste-derived substrates. These PHA polymers have gained increasing attention as a sustainable alternative to conventional plastics. One promising application of PHA and PHA-rich SCPs lies within the aquaculture food industry, where they hold potential as feed additives, biocontrol agents against diseases, and immunostimulants. Nevertheless, the cost of PHA production and application remains high, partly due to expensive substrates for cultivating PHA-accumulating SCPs, costly sterilization, energy-intensive SCPs harvesting techniques, and toxic PHA extraction and purification processes. This review summarizes the current state of PHA production and its application in aquaculture. The structure and classification of PHA, microbial sources, cultivation substrates, biosynthesis pathways, and the production challenges and solutions are discussed. Next, the potential of PHA application in aquaculture is explored, focusing on aquaculture challenges, common and innovative PHA-integrated farming practices, and PHA mechanisms in inhibiting pathogens, enhancing the immune system, and improving growth and gut health of various aquatic species. Finally, challenges and future research needs for PHA production and application in aquaculture are identified. Overall, this review paper provides a comprehensive overview of the potential of PHA in aquaculture and highlights the need for further research in this area.

Silver nanoparticle toxicity on Artemia parthenogenetica nauplii hatched on axenic tryptic soy agar solid medium

The use of gnobiotic brine shrimp (Artemia spp.) for ecotoxicology and bacteria-host interaction studies is common. However, requirements for axenic culture and matrix effects of seawater media can be an obstacle. Thus, we investigated the hatching ability of Artemia cysts on a novel sterile Tryptic Soy Agar (TSA) medium. Herein, we demonstrate for the first time that Artemia cysts can hatch on a solid medium without liquid, which offers practical advantages. We further optimized the culture conditions for temperature and salinity and assessed this culture system for toxicity screening of silver nanoparticles (AgNPs) across multiple biological endpoints. Results revealed that maxima hatching (90%) of embryos occurred at 28 °C and without addition of sodium chloride. When capsulated cysts were cultured on TSA solid medium Artemia were negatively impacted by AgNPs at 30-50 mgL^-1 in terms of the embryo hatching ratio (47-51%), umbrella- to nauplii-stage transformation ratio (54-57%), and a reduction in nauplii-stage growth (60-85% of normal body length). At 50-100 mgL^-1 AgNPs and higher, evidence of damage to lysosomal storage was recorded. At 500 mgL^-1 AgNPs, development of the eye was inhibited and locomotory behavior impeded. Our study reveals that this new hatching method has applications in ecotoxicology studies and provides an efficient means to control axenic requirements to produce gnotobiotic brine shrimp.

Aggregation propensities of proteins with varying degrees of disorder

The hydration thermodynamics of a globular protein (AcP), three intrinsically disordered protein regions (1CD3, 1MVF, 1F0R) and a fully disordered protein (α-synuclein) is studied by an approach that combines an all-atom explicit water molecular dynamics simulations and three-dimensional reference interaction site model (3D-RISM) theory. The variation in hydration free energy with percentage disorder of the selected proteins is investigated through its nonelectrostatic and electrostatic components. A decrease in hydration free energy is observed with an increase in percentage disorder, indicating favorable interactions of the disordered proteins with the solvent. This confirms the role of percentage disorder in determining the aggregation propensity of proteins which is measured in terms of the hydration free energy in addition to their respective mean net charge and mean hydrophobicity. The hydration free energy is decoupled into energetic and entropic terms. A residue-wise decomposition analysis of the hydration free energy for the selected proteins is evaluated. The decomposition shows that the disordered regions contribute more than the ordered ones for the intrinsically disordered protein regions. The dominant role of electrostatic interactions is confirmed from the residue-wise decomposition of the hydration free energy. The results depict that the negatively charged residues contribute more to the total hydration free energy for the proteins with negative mean net charge, while the positively charged residues contribute more for proteins with positive mean net charge.

Valorization of agro-industrial wastes into polyhydroxyalkanoates-rich single-cell proteins to enable a circular waste-to-feed economy

Recovering and converting carbon and nutrients from waste streams into healthy single-cell proteins (SCPs) can be an effective strategy to address costly waste management and support the increasing animal feed demand for the global food supply. Recently, SCPs rich in polyhydroxybutyrate (PHB) have been identified as an effective biocontrol healthy feed to replace conventional antibiotics-supplemented aquaculture feed. PHB, an intercellular polymer of short-chain-length (SCL) hydroxy-fatty acids, is a common type of polyhydroxyalkanoates (PHA) that can be microbially produced from various organics, including agro-industrial wastes. The complex chemical properties of agro-industrial wastes might produce SCPs containing PHA with SCL and/or medium chain-length (MCL) hydroxy-fatty acids. However, the effects of MCL-PHA-containing SCPs on aqua species' health and disease-fighting ability remains poorly understood. This study investigated the feasibility of producing various PHA-containing SCPs from renewable agro-industrial wastes/wastewaters, the effectiveness of SCL- and MCL-PHA as biocontrol agents, and the effects of these PHA-rich SCPs on the growth and disease resistance of an aquaculture animal model, brine shrimp Artemia. Zobellella denitrificans ZD1 and Pseudomonas oleovorans were able to grow on different pure substrates and agro-industrial wastes/wastewaters to produce various SCL- and/or MCL-PHA-rich SCPs. Low doses of MCL-fatty acids (i.e., PHA intermediates) efficiently suppressed the growth of aquaculture pathogens. Moreover, MCL-PHA-rich SCPs served as great food/energy sources for Artemia and improved Artemia's ability to fight pathogens. This study offers a win-win approach to address the challenges of wastes/wastewater management and feed supply faced by the aquaculture industry.

Novel Production Methods of Polyhydroxyalkanoates and Their Innovative Uses in Biomedicine and Industry

Polyhydroxyalkanoate (PHA), a biodegradable polymer obtained from microorganisms and plants, have been widely used in biomedical applications and devices, such as sutures, cardiac valves, bone scaffold, and drug delivery of compounds with pharmaceutical interests, as well as in food packaging. This review focuses on the use of polyhydroxyalkanoates beyond the most common uses, aiming to inform about the potential uses of the biopolymer as a biosensor, cosmetics, drug delivery, flame retardancy, and electrospinning, among other interesting uses. The novel applications are based on the production and composition of the polymer, which can be modified by genetic engineering, a semi-synthetic approach, by changing feeding carbon sources and/or supplement addition, among others. The future of PHA is promising, and despite its production costs being higher than petroleum-based plastics, tools given by synthetic biology, bioinformatics, and machine learning, among others, have allowed for great production yields, monomer and polymer functionalization, stability, and versatility, a key feature to increase the uses of this interesting family of polymers.

Heat Shock Proteins (Hsps) in Cellular Homeostasis: A Promising Tool for Health Management in Crustacean Aquaculture

Heat shock proteins (Hsps) are a family of ubiquitously expressed stress proteins and extrinsic chaperones that are required for viability and cell growth in all living organisms. These proteins are highly conserved and produced in all cellular organisms when exposed to stress. Hsps play a significant role in protein synthesis and homeostasis, as well as in the maintenance of overall health in crustaceans against various internal and external environmental stresses. Recent reports have suggested that enhancing in vivo Hsp levels via non-lethal heat shock, exogenous Hsps, or plant-based compounds, could be a promising strategy used to develop protective immunity in crustaceans against both abiotic and biotic stresses. Hence, Hsps as the agent of being an immune booster and increasing disease resistance will present a significant advancement in reducing stressful conditions in the aquaculture system.

Protective Effect of Ultrasound-Processed Amazonian Sapota-do-Solimões (Quararibea cordata) Juice on Artemia salina Nauplii

Juice processing by non-thermal technology has been extensively studied, aiming at microbial inactivation and quality improvement. However, the knowledge about the possible toxic effects that those technologies can produce in foodstuffs due to the production of reactive oxygen species is still unknown. In this study, sapota-do-Solimões juice processed by ultrasound (2, 6, and 10 min) was evaluated by a toxicity test and protective effect through stress biomarkers (catalase, superoxide dismutase, and lipid peroxidation) using Artemia salina nauplii. The non-thermal processed juice was nontoxic to A. salina. However, the juice fibers imparted some damage to the animal’s body. The ultrasound-processed juice (2 and 6 min) decreased the A. salina mortality to 30% compared to the control assay with H2O2 where mortality was 80% after 48 h of exposure. However, after 72 h of exposure, the A. salina was entirely degraded by H2O2-induced toxicity. Furthermore, the catalase and superoxide dismutase presented the highest activity after A. salina was exposed to the unprocessed juice. Thus, sapota-do-Solimões juice processed by the ultrasound could promote a protective effect on A. salina, revealing this technology’s potential to enhance juice features without toxicity.

Essential Oils Improve the Survival of Gnotobiotic Brine Shrimp (Artemia franciscana) Challenged With Vibrio campbellii

The halophilic aquatic bacterium Vibrio campbellii is an important aquatic pathogen, capable of causing vibriosis in shrimp and fish resulting in significant economic losses. In a previous work, essential oils (EOs) extracts from Melaleuca alternifolia, Litsea citrata, and Eucalyptus citriodora were found to inhibit the growth of V. campbellii in vitro. This study aimed to determine in vivo EOs’ potential protective effect towards gnotobiotic brine shrimp Artemia franciscana, challenged with V. campbellii. The study showed that brine shrimp larvae supplemented with EOs of M. alternifolia (0.0008%) and L. citrata (0.002%) displayed significantly increased survival against V. campbellii. The results indicated that supplementation of these EOs increased the expression of immune-related genes (either in the presence or absence of the pathogen), probably contributing to enhanced protection. Furthermore, in vitro studies indicated that some EOs modulated the expression of virulence factors including swimming motility, biofilm formation, and gelatinase and lipase activity, while flow cytometry data and regrowth assay indicated that these EOs do not exhibit antimicrobial activity as V. campbellii grew at the tested concentrations [M. alternifolia (0.0008%) and L. citrata (0.002%)]. Our findings suggest that EOs extracted from M. alternifolia and L. citrata, can modulate virulence factor production and immunological responses and might hence become part of an intervention strategy to control vibriosis in a fish or shrimp aquaculture setting, a hypothesis that needs to be validated in the future.

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

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

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

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

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.

PirABVP Toxin Binds to Epithelial Cells of the Digestive Tract and Produce Pathognomonic AHPND Lesions in Germ-Free Brine Shrimp

Acute hepatopancreatic necrosis disease (AHPND), a newly emergent farmed penaeid shrimp bacterial disease originally known as early mortality syndrome (EMS), is causing havoc in the shrimp industry. The causative agent of AHPND was found to be a specific strain of bacteria, e.g., Vibrio and Shewanella sps., that contains pVA1 plasmid (63–70 kb) encoding the binary PirA^VP and PirB^VP toxins. The PirAB^VP and toxins are the primary virulence factors of AHPND-causing bacteria that mediates AHPND and mortality in shrimp. Hence, in this study using a germ-free brine shrimp model system, we evaluated the PirAB^VP toxin-mediated infection process at cellular level, including toxin attachment and subsequent toxin-induced damage to the digestive tract. The results showed that, PirAB^VP toxin binds to epithelial cells of the digestive tract of brine shrimp larvae and produces characteristic symptoms of AHPND. In the PirAB^VP-challenged brine shrimp larvae, shedding or sloughing of enterocytes in the midgut and hindgut regions was regularly visualized, and the intestinal lumen was filled with moderately electron-dense cells of variable shapes and sizes. In addition, the observed cellular debris in the intestinal lumen of the digestive tract was found to be of epithelial cell origin. The detailed morphology of the digestive tract demonstrates further that the PirAB^VP toxin challenge produces focal to extensive necrosis and damages epithelial cells in the midgut and hindgut regions, resulting in pyknosis, cell vacuolisation, and mitochondrial and rough endoplasmic reticulum (RER) damage to different degrees. Taken together, our study provides substantial evidence that PirAB^VP toxins bind to the digestive tract of brine shrimp larvae and seem to be responsible for generating characteristic AHPND lesions and damaging enterocytes in the midgut and hindgut regions.

Antioxidant system of soiny mullet (Liza haematocheila) is responsive to dietary poly-β-hydroxybutyrate (PHB) supplementation based on immune-related enzyme activity and de novo transcriptome analysis

As a dietary supplement, poly-β-hydroxybutyrate (PHB) has been reported to positively influence growth, boost the immune system and enhance disease resistance in fish and shellfish. However, the protective mechanism is little known. Thus, the present study was conducted to evaluate the effect of PHB supplementation on immune-related enzyme activity and transcriptome-based gene expression in soiny mullet (Liza haematocheila). Results showed that dietary PHB supplementation could increase antioxidant enzyme activity, including total antioxidant capacity, catalase and superoxide dismutase. A total of 7,082,094,175 and 7,650,341,357 raw reads with mean length of 757 bp were obtained from control and PHB (dietary PHB supplementation at 2%) groups, respectively. There were 46,106 differentially expressed genes (DEGs) between control and PHB groups, including 21,828 upregulated and 24,278 downregulated DEGs. All the DEGs were classified into three gene ontology categories, and 312 DEGs related with immune system process and 760 with the response to a stimulus. Additionally, all DEGs were allocated to 261 Kyoto Encyclopedia of Gene and Genome pathways, and major immune-related pathways were detected, including MAPK/PI3K-Akt/TNF/NF-κB/TCR/TLR signaling pathways. Moreover, the regulation of several observed immune-related genes was confirmed by qRT-PCR. Altogether, this study suggests that antioxidant system is more effective for dietary PHB supplementation and lays the foundation for further study on the precise immunostimulatory mechanism of PHB. Hopefully, it provides insights into exploring biomarker for assessment of immunostimulants in fish culture.

Sequence and expression analysis of HSP70 family genes in Artemia franciscana

Thus far, only one gene from the heat shock protein 70 (HSP70) family has been identified in Artemia franciscana. Here, we used the draft Artemia transcriptome database to search for other genes in the HSP70 family. Four novel HSP70 genes were identified and designated heat shock cognate 70 (HSC70), heat shock 70 kDa cognate 5 (HSC70-5), Immunoglobulin heavy-chain binding protein (BIP), and hypoxia up-regulated protein 1 (HYOU1). For each of these genes, we obtained nucleotide and deduced amino acid sequences, and reconstructed a phylogenetic tree. Expression analysis revealed that in the juvenile state, the transcription of HSP70 and HSC70 was significantly (P < 0.05) higher in a population of A. franciscana selectively bred for increased induced thermotolerance (TF12) relative to a control population (CF12). Following non-lethal heat shock treatment at the nauplius stage, transcription of HSP70, HSC70, and HSC70-5 were significantly (P < 0.05) up-regulated in TF12. In contrast, transcription of the other HSP70 family members in A. franciscana (BIP, HYOU1, and HSPA4) showed no significant (P > 0.05) induction. Gene expression analysis demonstrated that not all members of the HSP70 family are involved in the response to heat stress and selection and that especially altered expression of HSC70 plays a role in a population selected for increased thermotolerance.

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.

Engineering the Dark Food Chain

Meeting global food needs in the face of climate change and resource limitation requires innovative approaches to food production. Here, we explore incorporation of new dark food chains into human food systems, drawing inspiration from natural ecosystems, the history of single cell protein, and opportunities for new food production through wastewater treatment, microbial protein production, and aquaculture. The envisioned dark food chains rely upon chemoautotrophy in lieu of photosynthesis, with primary production based upon assimilation of CH₄ and CO₂ by methane- and hydrogen-oxidizing bacteria. The stoichiometry, kinetics, and thermodynamics of these bacteria are evaluated, and opportunities for recycling of carbon, nitrogen, and water are explored. Because these processes do not require light delivery, high volumetric productivities are possible; because they are exothermic, heat is available for downstream protein processing; because the feedstock gases are cheap, existing pipeline infrastructure could facilitate low-cost energy-efficient delivery in urban environments. Potential life-cycle benefits include: a protein alternative to fishmeal; partial decoupling of animal feed from human food; climate change mitigation due to decreased land use for agriculture; efficient local cycling of carbon and nutrients that offsets the need for energy-intensive fertilizers; and production of high value products, such as the prebiotic polyhydroxybutyrate.

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.

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.

Virulence-inhibitory activity of the degradation product 3-hydroxybutyrate explains the protective effect of poly-β-hydroxybutyrate against the major aquaculture pathogen Vibrio campbellii

The bacterial storage compound poly-β-hydroxybutyrate, a polymer of the short-chain fatty acid 3-hydroxybutyrate, has been reported to protect various aquatic animals from bacterial disease. In order to obtain a better mechanistic insight, we aimed to (1) investigate whether 3-hydroxybutyrate is released from poly-β-hydroxybutyrate within sterile brine shrimp larvae, (2) determine the impact of 3-hydroxybutyrate on the virulence of Vibrio campbellii to brine shrimp larvae and on its cell density in the shrimp, and (3) determine the impact of this compound on virulence factor production in the pathogen. We detected 3-hydroxybutyrate in poly-β-hydroxybutyrate-fed brine shrimp, resulting in 24 mM 3-hydroxybutyrate in the intestinal tract of shrimp reared in the presence of 1000 mg l-1 poly-β-hydroxybutyrate. We further demonstrate that this concentration of 3-hydroxybutyrate does not affect the growth of V. campbellii, whereas it decreases the production of different virulence factors, including hemolysin, phospholipase and protease activities, and swimming motility. We hypothesize that by affecting all these virulence factors at once, 3-hydroxybutyrate (and thus also poly-β-hydroxybutyrate) can exert a significant impact on the virulence of V. campbellii. This hypothesis was confirmed in a challenge test showing that 3-hydroxybutyrate protected gnotobiotic brine shrimp from pathogenic V. campbellii, without affecting the number of host-associated vibrios.

Effects of dietary poly-β-hydroxybutyrate (PHB) on microbiota composition and the mTOR signaling pathway in the intestines of litopenaeus vannamei

Poly-β-hydroxybutyrate (PHB) is a natural polymer of the short chain fatty acid β-hydroxybutyrate, which acts as a microbial control agent. The mammalian target of the rapamycin (mTOR) signaling pathway plays a crucial role in intestine inflammation and epithelial morphogenesis. In this study, we examined the composition of intestine microbiota, and mTOR signaling-related gene expression in Pacific white shrimp Litopenaeus vannamei fed diets containing different levels of PHB: 0% (Control), 1% (PHB1), 3% (PHB3), and 5% (PHB5) (w/w) for 35 days. High-throughput sequencing analysis revealed that dietary PHB altered the composition and diversity of intestine microbiota, and that the microbiota diversity decreased with the increasing doses of PHB. Specifically, dietary PHB increased the relative abundance of Proteobacteria and Tenericutes in the PHB1 and PHB5 groups, respectively, and increased that of Gammaproteobacteria in the three PHB groups. Alternatively, PHB decreased Alphaproteobacteria in the PHB3 and PHB5 groups. At the genus level, dietary PHB increased the abundance of beneficial bacteria, such as Bacillus, Lactobacillus, Lactococcus, Clostridium, and Bdellovibrio. The relative mRNA expression levels of the mTOR signaling-related genes TOR, 4E-BP, eIF4E1α, and eIF4E2 all increased in the three PHB treatment groups. These results revealed that dietary PHB supplementation had a beneficial effect on intestine health of L. vannamei by modulating the composition of intestine microbiota and activating mTOR signaling.

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.

Enhanced resistance against Vibrio harveyi infection by carvacrol and its association with the induction of heat shock protein 72 in gnotobiotic Artemia franciscana

Induction of HSP72 is a natural response of stressed organisms that protects against many insults including bacterial diseases in farm (aquatic) animals. It would therefore be of great health benefit to search for natural compounds that are clinically safe yet able to induce HSP72 in animals. The phenolic compound carvacrol, an approved food component, had been shown in in vitro study to act as a co-inducer of HSP72, enhancing HSP72 production only in combination with a bona fide stress compared to the compound alone. However, in vitro model systems do not completely represent an in vivo physiology. Here, using the well-established gnotobiotic Artemia model system, we determined whether carvacrol could induce HSP72 in vivo, whether this putative effect could generate resistance in Artemia against biotic/abiotic stress and also unraveled the mechanism behind the possible HSP72-inducing effect of carvacrol. The gnotobiotic system is crucial for such studies because it avoids the interference of any extraneous factors on host-compound interaction. Here, carvacrol was shown to be a potent HSP72 inducer. Induction of HSP72 was associated with the generation of resistance in Artemia larvae against subsequent lethal heat stress or pathogenic Vibrio harveyi. Our results also provided new insight on the mode of HSP72 inducing action of carvacrol, in which the initial generation of reactive molecule H2O2 by the compound plays a key role. Overall results add new information about the bioactivity of carvacrol and advance our knowledge of this compound as potential prophylactic agent for controlling Vibrio infection in aquaculture animals.

Effect of dietary poly-β-hydroxybutyrate (PHB) on growth performance, intestinal health status and body composition of Pacific white shrimp Litopenaeus vannamei (Boone, 1931)

In the present study, the effect of dietary supplementation of poly-β-hydroxybutyrate (PHB) on the growth performance, intestinal digestive and immune function, intestinal short-chain fatty acids (SCFA) content and body composition of Pacific white shrimp Litopenaeus vannamei (Boone, 1931) was evaluated. The shrimp was fed for 35 days with four different diets: 0%, 1%, 3% and 5% PHB supplemented feed. The results indicated that supplementation of PHB significantly increased the growth performance of the shrimp, and the feed conversion rate (FCR) in 3%PHB treatment group was significantly lower than the control (P < 0.05). The intestinal amylase, lipase and trypsin activity in the three PHB treatment groups were all significantly higher than that of the control (P < 0.05), but the pepsin activity were only significantly affected by 3%PHB treatment (P > 0.05). The activities of intestinal immune enzymes such as total antioxidant capacity (T-AOC) and inducible nitric oxide synthase (iNOS) was significantly induced by 3%PHB treatment (P < 0.05), while lysozyme (LSZ) activity was significantly affected by 5%PHB treatment and nitric oxide (NO) content was significantly induced in three PHB treatments. Meanwhile, PHB induced significantly the expression level of intestinal heat shock protein 70 (HSP70), Toll and immune deficiency (Imd) gene. HE staining showed that PHB induced the intestinal health status of L. vannamei. Intestinal SCFA content analysis revealed that the content of propionic and butyric acid of 3%PHB treatment were significantly higher than that of the control (P < 0.05). Body composition analysis showed that the crude protein in 3% and 5%PHB treatments, and the crude lipid in 1% and 5%PHB treatments were all significantly higher than the control (P < 0.05). These results revealed that PHB could improve the growth performance, modulated intestinal digestive and immune function, increased intestinal SCFA content and body composition in L. vannamei, and the optimum dietary PHB requirement by L. vannamei was estimated at 3% (w/w) diet.

Degradation intermediates of polyhydroxy butyrate inhibits phenotypic expression of virulence factors and biofilm formation in luminescent Vibrio sp. PUGSK8

Luminescent vibrios are ubiquitous in the marine environment and are the causative agents of vibriosis and mass mortality in many aquatic animals. In aquatic environments, treatments cannot be limited to the diseased population alone, therefore treatment of the entire aquatic system is the only possible approach. Thus, the use of antibiotics to treat part of the infected animals requires a dose based on the entire biomass, which results in the treatment of uninfected animals as well as non-target normal microbial flora. A treatment method based on anti-virulence or quorum quenching has recently been proposed as an effective treatment strategy for aquatic animals. Polyhydroxy butyrates (PHB) are bacterial storage molecules, which accumulate in cells under nutritional stress. The degradation of PHB releases short-chain β-hydroxy butyric acid, which may act as anti-infective molecule. To date, there is very limited information on the potential anti-infective and anti-virulence mechanisms involving PHB. In this study, we aim to examine the effect of PHB on inhibition of the virulence cascade of Vibrio such as biofilm formation, luminescence, motility behaviour, haemolysin and quorum sensing. A luminescent Vibrio PUGSK8, tentatively identified as Vibrio campbellii PUGSK8 was tested in vitro for production of extracellular virulence factors and then established as a potential shrimp pathogen based on in vivo challenge experiments. The ability of Vibrio PUGSK8 to form biofilms and the effect of PHB on biofilm formation was tested in a 96-well microtitre-plate assay system. The motility behaviour of Vibrio PUGSK8 was evaluated using twitching, swimming and swarming plate assays. Reporter strains such as Chromobacterium violaceum CV026 and Agrobacterium tumefaciens were used to detect quorum-sensing molecules. Gas chromatography-mass spectrometry spectral analysis was performed to elucidate the fragmentation pattern and structure of N-hexanoyl homoserine lactone. PHB depolymerase activity in Vibrio PUGSK8 was quantified as the amount of the enzyme solution to hydrolyse 1 μg of PHB per min. An in vivo challenge experiment was performed using a gnotobiotic Artemia assay. Of the 27 isolates tested, the Vibrio PUGSK8 strain was selected for target-specific assays based on the high intensity of luminescence and production of virulence factors. The virulence cascade detected in Vibrio PUGSK8 include luminescence, motility behaviour, biofilm formation, quorum sensing and haemolysin production. Thus inhibition/degradation of the virulence cascade would be an effective approach to contain Vibrio infections in aquatic animals. In this report, we demonstrate that the degradation intermediate of PHB effectively inhibits biofilm formation, luminescence, motility behaviour, haemolysin production and the N-acyl-homoserine lactone (AHL)-mediated quorum-sensing pathway in PUGSK8. Interestingly, the growth of Vibrio PUGSK8 remains unaffected in the presence of PHB, with PHB degradation being detected in the media. PHB depolymerase activity in Vibrio PUGSK8 results in the release of degradation intermediates include a short-chain β-hydroxy butyric acid, which inhibits the virulence cascade in Vibrio PUGSK8. Thus, a molecule that targets quorum sensing and the virulence cascade and which is species/strain-specific could prove to be an effective alternative to antimicrobial agents to control the pathogenesis of Vibrio, and thereby help to contain Vibrio outbreaks in aquatic systems.

Individual Apostichopus japonicus fecal microbiome reveals a link with polyhydroxybutyrate producers in host growth gaps

Gut microbiome shapes various aspects of a host’s physiology, but these functions in aquatic animal hosts have yet to be fully investigated. The sea cucumber Apostichopus japonicus Selenka is one such example. The large growth gap in their body size has delayed the development of intensive aquaculture, nevertheless the species is in urgent need of conservation. To understand possible contributions of the gut microbiome to its host’s growth, individual fecal microbiome comparisons were performed. High-throughput 16S rRNA sequencing revealed significantly different microbiota in larger and smaller individuals; Rhodobacterales in particular was the most significantly abundant bacterial group in the larger specimens. Further shotgun metagenome of representative samples revealed a significant abundance of microbiome retaining polyhydroxybutyrate (PHB) metabolism genes in the largest individual. The PHB metabolism reads were potentially derived from Rhodobacterales. These results imply a possible link between microbial PHB producers and potential growth promotion in Deuterostomia marine invertebrates.

Probing the phenomenon of trained immunity in invertebrates during a transgenerational study, using brine shrimp Artemia as a model system

The invertebrate’s innate immune system was reported to show some form of adaptive features, termed trained immunity. However, the memory characteristics of innate immune system and the mechanisms behind such phenomena remain unclear. Using the invertebrate model Artemia, we verified the possibility or impossibility of trained immunity, examining the presence or absence of enduring memory against homologous and heterologous antigens (Vibrio spp.) during a transgenerational study. We also determined the mechanisms behind such phenomenon. Our results showed the occurrence of memory and partial discrimination in Artemia’s immune system, as manifested by increased resistance, for three successive generations, of the progenies of Vibrio-exposed ancestors towards a homologous bacterial strain, rather than to a heterologous strain. This increased resistance phenotype was associated with elevated levels of hsp70 and hmgb1 signaling molecules and alteration in the expression of key innate immunity-related genes. Our results also showed stochastic pattern in the acetylation and methylation levels of H4 and H3K4me3 histones, respectively, in the progenies whose ancestors were challenged. Overall results suggest that innate immune responses in invertebrates have the capacity to be trained, and epigenetic reprogramming of (selected) innate immune effectors is likely to have central place in the mechanisms leading to trained immunity.

The gnotobiotic brine shrimp (Artemia franciscana) model system reveals that the phenolic compound pyrogallol protects against infection through its prooxidant activity

The phenolic compound pyrogallol is the functional unit of many polyphenols and currently there has been a growing interest in using this compound in human and animal health owing to its health-promoting effects. The biological actions of pyrogallol moiety (and polyphenols) in inducing health benefitting effects have been studied; however, the mechanisms of action remain unclear yet. Here, we aimed at unravelling the underlying mechanism of action behind the protective effects of pyrogallol against bacterial infection by using the gnotobiotically-cultured brine shrimp Artemia franciscana and pathogenic bacteria Vibrio harveyi as host-pathogen model system. The gnotobiotic test system represents an exceptional system for carrying out such studies because it eliminates any possible interference of microbial communities (naturally present in the experimental system) in mechanistic studies and furthermore facilitates the interpretation of the results in terms of a cause effect relationship. We provided clear evidences suggesting that pyrogallol pretreament, at an optimum concentration, induced protective effects in the brine shrimp against V. harveyi infection. By pretreating brine shrimp with pyrogallol in the presence or absence of an antioxidant enzyme mixture (catalase and superoxide dismutase), we showed that the Vibrio-protective effect of the compound was caused by its prooxidant action (e.g. generation of hydrogen peroxide, H2O2). We showed further that generation of prooxidant is linked to the induction of heat shock protein Hsp70, which is involved in eliciting the prophenoloxidase and transglutaminase immune responses. The ability of pyrogallol to induce protective immunity makes it a potential natural protective agent that might be a potential preventive modality for different host-pathogen systems.

Effect of poly-β-hydroxybutyrate on growth and disease resistance of Nile tilapia Oreochromis niloticus juveniles

The growth promoting effect of the bacterial storage compound poly-β-hydroxybutyrate (PHB) has been studied for young fish of high trophic level (European sea bass) and intermediate trophic level (Siberian sturgeon). Here, the effect of PHB on growth, digestive enzyme activities, body composition and diseases resistance of juvenile Nile tilapia (Oreochromis niloticus) of low trophic level was investigated. Although dietary PHB supplementation (5, 25 and 50 g PHB kg(-1) formulated semi-purified diet) during 28 days resulted in a trend of increased weight gain, there was no significant difference in the mean final body weight (258-284 mg) when compared to the fish from the control group (on average 218 mg). Lipase activity increased significantly with about 20-40% by the supplementation of PHB in the diet, which may have led to the significant increase in total lipid content with about 10% in the PHB treatment groups. However, the profile of total (n-6) fatty acids (FAs), total monounsaturated FAs and total saturated FAs relative to the total lipid was similar among various PHB treatments. An additional challenge test on gnotobiotic Nile tilapia larvae using the pathogen Edwardsiella ictaluri gly09R showed that feeding challenged larvae with PHB-enriched Artemia nauplii resulted in a 20% higher survival as compared to the challenged control larvae. Overall, it is suggested that the trend of increased body weight gain resulted from intestinal lipid digestion, absorption and deposition and that PHB is effective as an antimicrobial agent for application in Nile tilapia larviculture.