Effects of different curing concentrations and drying times on the microbial community structure and metabolites of dried Spanish mackerel

Cured Spanish mackerel has a promising market owing to its nutritious nature as well as ease of transportation and preservation. However, the nutritional and flavor formation mechanism of Spanish mackerel after curing and drying is unclear. To overcome this problem, the effects of different processing conditions on the free amino acid, microbial community, and flavor of Spanish mackerel were explored. Staphylococcus and Cobetia are the main microorganisms in cured mackerel and are closely associated with the formation of their quality. Compared with fresh mackerel, cured mackerel contains increased levels of protein, fat, and chloride, contributing to its distinctive flavor. The contents of free amino acids in the BA64 group were substantially higher than those in other groups, particularly the contents of threonine, glycine, and tyrosine. These findings will contribute to the development of high-quality cured Spanish mackerel products and cured aquatic products.

Transcriptome profiling of lumpfish (Cyclopterus lumpus) head kidney to Renibacterium salmoninarum at early and chronic infection stages

Renibacterium salmoninarum causes Bacterial Kidney Disease (BKD) in several fish species. Atlantic lumpfish, a cleaner fish, is susceptible to R. salmoninarum. To profile the transcriptome response of lumpfish to R. salmoninarum at early and chronic infection stages, fish were intraperitoneally injected with either a high dose of R. salmoninarum (1 × 109 cells dose-1) or PBS (control). Head kidney tissue samples were collected at 28- and 98-days post-infection (dpi) for RNA sequencing. Transcriptomic profiling identified 1971 and 139 differentially expressed genes (DEGs) in infected compared with control samples at 28 and 98 dpi, respectively. At 28 dpi, R. salmoninarum-induced genes (n = 434) mainly involved in innate and adaptive immune response-related pathways, whereas R. salmoninarum-suppressed genes (n = 1537) were largely connected to amino acid metabolism and cellular processes. Cell-mediated immunity-related genes showed dysregulation at 98 dpi. Several immune-signalling pathways were dysregulated in response to R. salmoninarum, including apoptosis, alternative complement, JAK-STAT signalling, and MHC-I dependent pathways. In summary, R. salmoninarum causes immune suppression at early infection, whereas lumpfish induce a cell-mediated immune response at chronic infection. This study provides a complete depiction of diverse immune mechanisms dysregulated by R. salmoninarum in lumpfish and opens new avenues to develop immune prophylactic tools to prevent BKD.

spaB-positive Erysipelothrix rhusiopathiae, a novel teleost pathogen isolated from cultured barramundi

Members of the genus Erysipelothrix are emergent pathogens of cultured eels, as well as several characid and cyprinid species. Since 2013, E. rhusiopathiae has been reported from diseased barramundi (Lates calcarifer) cultured in North America; we recovered 8 E. rhusiopathiae isolates from diseased fish during different outbreaks from the same farm. The E. rhusiopathiae isolates from barramundi were compared phenotypically and genetically to E. piscisicarius isolates characterized from ornamental fish and E. rhusiopathiae recovered from aquatic and terrestrial animals. All barramundi isolates were PCR-positive for the surface protective antigen type B (spaB) gene, and shared ≥ 99.7% sequence similarity among concatenated multilocus sequence analysis gene sequences, indicating a high degree of genetic homogeneity. These isolates were > 99% similar to other spaB-positive isolates from marine invertebrates and marine mammals, consistent with findings for other spa types. The spaA and spaB isolates shared < 98% similarity, as well as < 90% similarity with spaC-positive E. piscisicarius. Similar clonality among the spaB isolates was observed using repetitive element palindromic PCR. In experimental intracoelomic injection challenges conducted to fulfill Koch postulates, 67% of exposed tiger barbs (Puntigrus tetrazona) died within 14 d of challenge. Our study supports previous work citing the genetic variability of Erysipelothrix spp. spa types and the emergence of members of the genus Erysipelothrix as nascent fish pathogens.

Strain-level diversity of symbiont communities between individuals and populations of a bioluminescent fish

The bioluminescent symbiosis involving the urchin cardinalfish, Siphamia tubifer, and Photobacterium mandapamensis, a luminous member of the Vibrionaceae, is highly specific compared to other bioluminescent fish-bacteria associations. Despite this high degree of specificity, patterns of genetic diversity have been observed for the symbionts from hosts sampled over relatively small spatial scales. We characterized and compared sub-species, strain-level symbiont diversity within and between S. tubifer hosts sampled from the Philippines and Japan using PCR fingerprinting. We then carried out whole genome sequencing of the unique symbiont genotypes identified to characterize the genetic diversity of the symbiont community and the symbiont pangenome. We determined that an individual light organ contains six symbiont genotypes on average, but varied between 1-13. Additionally, we found that there were few genotypes shared between hosts from the same location. A phylogenetic analysis of the unique symbiont strains indicated location-specific clades, suggesting some genetic differentiation in the symbionts between host populations. We also identified symbiont genes that were variable between strains, including luxF, a member of the lux operon, which is responsible for light production. We quantified the light emission and growth rate of two strains missing luxF along with the other strains isolated from the same light organs and determined that strains lacking luxF were dimmer but grew faster than most of the other strains, suggesting a potential metabolic trade-off. This study highlights the importance of strain-level diversity in microbial associations and provides new insight into the underlying genetic architecture of intraspecific symbiont communities within a host.

The galU gene is required for in vivo survival of pseudomonas plecoglossicida in large yellow croaker (Larimichthys crocea)

Pseudomonas plecoglossicida is an important pathogenic bacterium in aquaculture that causes visceral granulomas in large yellow croaker (Larimichthys crocea). Uridine diphosphate glucose phosphorylase encoded by galU plays a key role in biosynthesis of the bacterial envelope, particularly lipopolysaccharide and the capsule. In this study, we inactivated the galU gene in the P. plecoglossicida isolate XSDHY-P. The galU mutant strain showed impaired growth in the early exponential stage and lacked the O polysaccharide side chain in lipopolysaccharide, but almost no defect in biofilm formation was detected. The galU mutant strain also exhibited significantly more sensitivity to the bactericidal action of normal fish serum mediated by the complement system compared to the wild-type strain. In a cell model originating from the head kidney of large yellow croaker, the galU mutant strain showed lower capacities of adhesion, invasion, and intracellular survival compared to the wild-type strain. In addition, the deficiency of the galU mutant drastically decreased bacterial loads in tissues and attenuated P. plecoglossicida virulence in fish. These results suggest that the galU gene of P. plecoglossicida is required for in vivo survival in large yellow croaker.

Effect of Ferredoxin Receptor FusA on the Virulence Mechanism of Pseudomonas plecoglossicida

Pseudomonas plecoglossicida is an aerobic Gram-negative bacterium, which is the pathogen of “Visceral white spot disease” in large yellow croaker. P. plecoglossicida is a temperature-dependent bacterial pathogen in fish, which not only reduces the yield of large yellow croaker but also causes continuous transmission of the disease, seriously endangering the healthy development of fisheries. In this study, a mutant strain of fusA was constructed using homologous recombination technology. The results showed that knockout of P. plecoglossicida fusA significantly affected the ability of growth, adhesion, and biofilm formation. Temperature, pH, H₂O₂, heavy metals, and the iron-chelating agent were used to treat the wild type of P. plecoglossicida; the results showed that the expression of fusA was significantly reduced at 4°C, 12°C, and 37°C. The expression of fusA was significantly increased at pH 4 and 5. Cu²⁺ has a significant inducing effect on the expression of fusA, but Pb²⁺ has no obvious effect; the expression of fusA was significantly upregulated under different concentrations of H₂O₂. The expression of the fusA gene was significantly upregulated in the 0.5~4-μmol/l iron-chelating agent. The expression level of the fusA gene was significantly upregulated after the logarithmic phase. It was suggested that fusA included in the TBDR family not only was involved in the transport of ferredoxin but also played important roles in the pathogenicity and environment adaptation of P. plecoglossicida.

Robust host source tracking building on the divergent and non-stochastic assembly of gut microbiomes in wild and farmed large yellow croaker

BACKGROUND

Given the lack of genetic background, the source tracking unknown individuals of fish species with both farmed and wild populations often cannot be robustly achieved. The gut microbiome, which is shaped by both deterministic and stochastic processes, can serve as a molecular marker of fish host source tracking, particularly as an alternative to the yet-to-be-established host genetic marker. A candidate for testing the feasibility is the large yellow croaker, Larimichthys crocea, which is carnivorous and ranks the top mariculture fish in China. Wild resource of this fish was depleted decades ago and might have potential problematic estimation because of escaping of farmed individuals.

RESULTS

The rectums of wild (n = 212) and farmed (n = 79) croakers from multiple batches were collected for the profiling of their gut bacterial communities. The farmed individuals had a higher alpha diversity and lower bacterial load than the wild individuals. The gut microbiota of the two sources exhibited divergence and high inter-batch variation, as featured by the dominance of Psychrobacter spp. in the wild group. Predicted functional capacity of the gut microbiome and representative isolates showed differences in terms of host source. This difference can be linked to the potential diet divergence between farmed and wild fishes. The non-stochastic distribution pattern of the core gut microbiota of the wild and farmed individuals supports the feasibility of microbiota-based host source tracking via the machine learning algorithm. A random forest classifier based on the divergence and non-stochastic assembly of the gut microbiome was robust in terms of host source tracking the individuals from all batches of croaker, including a newly introduced batch.

CONCLUSIONS

Our study revealed the divergence of gut microbiota and related functional profiles between wild and farmed croakers. For the first time, with representative datasets and non-stochastic patterns, we have verified that gut microbiota can be robustly applied to the tracking of host source even in carnivorous fish. Video abstract.

Choreftria shiranui, a new genus and species of cyclopoid copepod (Crustacea: Copepoda) associated with the worm goby from southern Japan, with the proposal of a new family

A new genus and species of cyclopoid copepod, Choreftria shiranui n. g., n. sp. (Crustacea: Copepoda) is described based on an adult female found from the worm goby, Taenioides snyderi Jordan & Hubbs (Actinopterygii: Perciformes: Gobiidae), from mud flats in the Yatsushiro Sea, southern Japan. The new genus is characterized by bearing the following characters in the female: body distinctly segmented cyclopiform with 4-segmented prosome and 6-segmented urosome; genital somite clearly separated from first abdominal somite; antennule 7-segmented; antenna 4-segmented with one fused serrated claw and long claw on second endopodal segment; mandible reduced with one serrated blade; maxillule rod-like with one element; maxilla 2-segmented with recurved terminal claw; maxilliped 2-segmented; legs 1 to 4 biramous with 3-segmented rami. Since the copepod is not attributable to any of the known cyclopoid families, a new family, Choreftriidae n. fam. is established to accommodate Choreftria n. g. LSID urn:lsid:zoobank.org:pub:C1FB9F60-9871-4D4B-A1BF-3202BA24189F.

Brain immunity response of fish Eleginops maclovinus to infection with Francisella noatunensis

The brain's immune system is selective and hermetic in most species, including fish, favoring immune responses mediated by soluble immunomodulatory factors such as serotonin and the availability of nutrients against infectious processes. Francisella noatunensis coexist with fish such as Eleginops maclovinus, which raises questions about the susceptibility and immune response of the brain of E. maclovinus against Francisella. In this study, we inoculated fish with different doses of Francisella and took samples for 28 days. We detected bacteria in the brain of fish injected with a high concentration of Francisella at all time points. qPCR analysis of immune genes indicated a response mainly in the medium-dose and early expression of genes involved in iron metabolism. Finally, brain serotonin levels were higher than in uninfected fish in all conditions, suggesting possible immunomodulatory participation in an infectious process.

Polyactin A and CpG enhance inactivated Pseudomonas plecoglossicida vaccine potency in large yellow croaker (Larimichthys crocea)

Pseudomonas plecoglossicida is the causative agent of visceral granulomas disease (VGD) in large yellow croaker (LYC, Larimichthys crocea) farming. However, multi-antibiotic resistant of P. plecoglossicida creates an urgent need of an efficient vaccine to combat this pathogen. In this study, an inactivated vaccine added polyactin (PA), CpG-riched plasmid (pCpG) and aluminum adjuvant (Al) was developed. As a result, its relative percentage survival (RPS) against P. plecoglossicida were up to 64%. Comparatively, RPS of groups that vaccinated with vaccines adjuvanted with PA and Al or CpG and Al were 49% and 39%. However, an interesting result that the vaccine combined with PA, CpG and Al did not show the strongest activation of total serum protein and antibody levels in serum among three vaccinated groups. According to expressions of some cellular immune related genes, we found that the inactivated vaccine combined with PA, CpG and Al was more likely to induce a cellular immune response rather than humoral immune response. Totally, our study demonstrated that the mixture of PA, CpG and aluminum adjuvant is a potential adjuvant system for LYC vaccine development.

Multilocus sequence analysis reveals different lineages of Pseudomonas anguilliseptica associated with disease in farmed lumpfish (Cyclopterus lumpus L.)

The bacterium Pseudomonas anguilliseptica has in recent years emerged as a serious threat to production of lumpfish in Norway. Little is known about the population structure of this bacterium despite its association with disease in a wide range of different fish species throughout the world. The phylogenetic relationships between 53 isolates, primarily derived from diseased lumpfish, but including a number of reference strains from diverse geographical origins and fish species, were reconstructed by Multi-Locus Sequence Analysis (MLSA) using nine housekeeping genes (rpoB, atpD, gyrB, rpoD, ileS, aroE, carA, glnS and recA). MLSA revealed a high degree of relatedness between the studied isolates, altough the seven genotypes identified formed three main phylogenetic lineages. While four genotypes were identified amongst Norwegian lumpfish isolates, a single genotype dominated, irrespective of geographic origin. This suggests the existence of a dominant genotype associated with disease in production of lumpfish in Norwegian aquaculture. Elucidation of the population structure of the bacterium has provided valuable information for potential future vaccine development.

Preliminary studies on the different roles of T6SSs in pathogenicity of Pseudomonas plecoglossicida NB2011

Pseudomonas plecoglossicida, the causative agent of visceral granulomas in the large yellow croaker (Larimichthys crocea) in China, encodes three sets of type Ⅵ secretion systems (T6SS1-3). The purpose of this study was to characterize the different roles of T6SSs involved in infection. In-frame deletion of T6SSs was constructed, which resulted in 8 mutants. Competition against E. coli DH5α, virulence against the croaker and in vivo survival ability of the mutants were tested. The expression and secretion of Hcp by P. plecoglossicida NB2011 were investigated. The results showed T6SS2 mutant failed to inhibit the growth of E. coli, which is an indication of T6SS2 acting against environmental bacteria. The LD₅₀ value of T6SS1 mutant strongly increased; T6SS2 and T6SS3 mutants were similar to that of the wild type; and the virulence of double deletion or triple deletion mutant was drastically alleviated, indicating that T6SS1 being one of the major virulence factors, and T6SS2 and T6SS3 directly or indirectly being involved in the pathogenicity. T6SS1 mutant disappeared in the fish spleen in 3 days, while other strains kept increasing, indicating the T6SS1 stimulation bacteria replication in vivo. Hcp1 secreted at 12-28°C and Hcp2 secreted at 12-35°C, while Hcp3 secretion not detected in vitro. This study has thrown some insights on the understanding of pathogenicity mechanisms of this pathogen.

Impact of dietary porcine blood by-products in meagre (Argyrosomus regius) physiology, evaluated by welfare biomarkers and the antibacterial properties of the skin mucus

Tools are required for quick and easy preliminary evaluation of functional feeds efficiency on fisheries. The analysis of skin mucus biomarkers is a recent alternative approach providing a faster feed-back from the laboratory which is characterized by being less invasive, more rapid and with reduced costs. The effect of replacing fishmeal and fish protein hydrolysates by means of two porcine by-products, the porcine spray-dried plasma (SDPP) and pig protein hydrolysate (PPH), in compound diets (50.4% crude protein, 16.2% crude protein, 22.1 MJ/kg feed) was evaluated in juvenile meagre (Argyrosomus regius) during a two-months period. To determine the impact of these dietary replacements, growth and food performance were measured together with digestive enzymes activities and filet proximal composition. Additionally, skin mucus was collected and characterized by determining main mucus biomarkers (protein, glucose, lactate, cortisol, and antioxidant capacity) and its antibacterial properties, measured by the quick in vitro co-culture challenges. In comparison to the control group, the inclusion of PPH and SDPP, in meagre diets reduced growth (7.4-8.8% in body weight), increased feed conversion ratios (9.0-10.0%), results that were attributed to a reduction in feed intake values (24.2-33.0%) (P < 0.05). Porcine blood by-products did not modify the activity of gastric and pancreatic digestive enzymes as well as those involved in nutrient absorption (alkaline phosphatase) nor liver oxidative stress condition (P > 0.05). In contrast, a reduction in fillet lipid content associated to an increase in fillet protein levels were found in fish fed SDPP and PPH diets (P < 0.05). As compared to the control diet, the dietary replacement did not alter the levels of the skin mucus biomarkers related to stress (cortisol and antioxidant capacity) or nutritional status (soluble protein, glucose and lactate) (P > 0.05). Interestingly, regardless of the worst performance in somatic growth, meagre fed diets containing both tested porcine by-products showed a significantly improved antibacterial capacity of their skin mucus. This enhancement was more prominent for fish fed with the PPH diet, which may be attributed to a higher content of immunomodulatory bioactive compounds in PPH. Further research will be necessary to provide insights on how the inclusion of SDPP and PPH, at the expense of dietary fishmeal and fish protein hydrolysates, affects feed intake and growth performance in meagre. However, the use of skin mucus biomarkers has been demonstrated to be an excellent methodology for a preliminary characterization of the functional feeds, in particular for their prophylactic properties by the study of mucus antibacterial activity.

The moonlighting protein fructose 1,6-bisphosphate aldolase as a potential vaccine candidate against Photobacterium damselae subsp. piscicida in Asian sea bass (Lates calcarifer)

Vaccination is the most effective, safe, and environmentally friendly method to prevent the outbreak of Photobacterium damselae subsp. piscicida (Phdp), a dangerous pathogen in aquaculture worldwide. Here, recombinant proteins of catalase, superoxide dismutase, isocitrate dehydrogenase, fructose 1,6-bisphosphate aldolase (Fba), and a mixture of all four proteins were investigated for their immunoprotective effects against photobacteriosis in Asian sea bass (Lates calcarifer). After immunization, experimental fish showed an increase in specific antibody levels and lysozyme activities, especially the Fba group. After a lethal challenge with Phdp strain AOD105021, the Fba group achieved the highest relative percentage of survival rate (70.21%) and a significantly lower bacterial load in the spleens than other groups 3 days after infection. The results suggest that Fba is a good candidate for subunit vaccine development against photobacteriosis in fish.

The circular RNA circBCL2L1 regulates innate immune responses via microRNA-mediated downregulation of TRAF6 in teleost fish

Growing numbers of studies have shown that circular RNAs (circRNAs) can function as regulatory factors to regulate the innate immune response, cell proliferation, cell migration, and other important processes in mammals. However, the function and regulatory mechanism of circRNAs in lower vertebrates are still unclear. Here, we discovered a novel circRNA derived from the gene encoding Bcl-2-like protein 1 (BCL2L1) gene, named circBCL2L1, which was related to the innate immune responses in teleost fish. Results indicated that circBCL2L1 played essential roles in host antiviral immunity and antibacterial immunity. Our study also identified a microRNA, miR-30c-3-3p, which could inhibit the innate immune response by targeting inflammatory mediator TRAF6. And TRAF6 is a key signal transduction factor in innate immune response mediated by TLRs. Moreover, we also found that the antiviral and antibacterial effects inhibited by miR-30c-3-3p could be reversed with the expression of circBCL2L1. Our data revealed that circBCL2L1 functioned as a competing endogenous RNA (ceRNA) of TRAF6 by competing for binding with miR-30c-3-3p, leading to activation of the NF-κB/IRF3 inflammatory pathway and then enhancing the innate immune responses. Our results suggest that circRNAs can play an important role in the innate immune response of teleost fish.

Dietary supplementation of fermented lemon peel enhances lysozyme activity and susceptibility to Photobacterium damselae for orange-spotted grouper, Epinephelus coioides

The waste recycling of lemon peel, as a functional feed additive in aquafeed was evaluated by estimating the effects of fermented lemon peel (FLP) supplementation in diet on growth performance, innate immune responses, and susceptibility to Photobacterium damselae of grouper, Epinephelus coioides. A basal diet was added FLP at 0%, 1%, 3%, and 5%. Four tested diets were each fed to juvenile grouper (initial weight: 15.89 ± 0.10 g, triplicate groups) in a recirculation rearing system for eight weeks. Fish fed diets with 0%-3% FLP exhibited higher (p < 0.05) final weight, weight gain, and feed efficiency than fish fed the 5% FLP-diet. After challenge test, fish fed the 3% FLP-diet appeared the lowest mortality, followed by fish fed the 1% FLP-diet, and lowest in fish fed 0% and 5% FLP-diets. Plasma lysozyme activities were higher in fish fed diets with FLP than in fish fed the FLP-free control diet before challenge test. After challenge, fish fed diets with 1% and 3% FLP showed highest lysozyme activities, followed by fish fed the diet with 5% FLP, and lowest in fish fed the control diet. Hepatic malondialdehyde content was higher in fish fed the control diet than in fish fed diets with 1%-3% FLP. Results found that diets supplemented with 1%-3% fermented lemon peel can enhance lysozyme activity and resistance to pathogen P. damselae of grouper.

MAVS splicing variants associated with TRAF3 and TRAF6 in NF-κB and IRF3 signaling pathway in large yellow croaker Larimichthys crocea

Mitochondrial antiviral signaling protein (MAVS) acts as an essential adaptor in host RIG-I-like receptors (RLRs) mediated antiviral signaling pathway. In the present study, two MAVS transcript variants, the typical form and a splicing variant, namely Lc-MAVS_tv1 and Lc-MAVS_tv2 were characterized in large yellow croaker (Larimichthys crocea). The putative Lc-MAVS_tv1 protein contains 512 aa, with an N-terminal CARD domain, a central proline-rich region, and a C-terminal transmembrane (TM) domain, whereas Lc-MAVS_tv2 contains 302 aa and lacks the C-terminal TM domain due to a premature stop in the 102 bp intron fragment insertion. Lc-MAVS_tv1 was identified as a mitochondrion localized protein whereas Lc-MAVS_tv2 exhibited an entire cytosolic distribution. Quantitative real-time PCR revealed that Lc-MAVS_tv1 mRNA was broadly expressed in examined organs/tissues and showed extremely higher level than that of Lc-MAVS_tv2, and both of them could be up-regulated under poly I:C, LPS, PGN, and Pseudomonas plecoglossicida stimulation in vivo. Interestingly, overexpression of Lc-MAVS_tv2 could induce the activation of NF-κB but not IRF3, and Lc-MAVS_tv2 co-transfected with Lc-MAVS_tv1 induced a significantly higher level of NF-κB and IRF3 promoter activity. In addition, Lc-MAVS_tv2 overexpression could enhance TRAF3 and TRAF6 mediated NF-κB activation, but suppress TRAF3 and TRAF6 mediated IRF3 activation, implying that the splicing variant Lc-MAVS_tv2 may function as an important regulator in MAVS mediated signaling pathway.

Horizontal transmission of Piscirickettsia salmonis from the wild sub-Antarctic notothenioid fish Eleginops maclovinus to rainbow trout (Oncorhynchus mykiss) under experimental conditions

Piscirickettsia salmonis is the aetiological agent of piscirickettsiosis, a bacterial disease that affects farmed salmonids, causing high mortalities and significant economic losses in the Chilean salmon farm industry. Given the Chilean native fish species Patagonian blenny, Eleginops maclovinus, lives in the vicinity of salmon farms, it is relevant to clarify the epidemiological role that this species could play in the transmission and/or dissemination of this pathogen. This study aimed to evaluate the bidirectional transmission of P. salmonis between the Patagonian blenny and Oncorhynchus mykiss (rainbow trout), via a cohabitation challenge model. The results of this study demonstrated the transmission of the bacteria from Patagonian blennies to rainbow trout, considering the specific mortality in cohabitant rainbow trout, reaching 46%: the necropsy of these specimens, evidencing the characteristic pathological lesions of the disease and the positive results of the qPCR analysis for P. salmonis, in the same individuals. In contrast, no mortalities of Patagonian blenny specimens were recorded in the challenged experimental groups. This study is the first report showing the horizontal transmission of P. salmonis from a native non-salmonid species, such as the Patagonian blenny, to a salmonid species, generating the disease and specific mortality in rainbow trout, using a cohabitation challenge.

Molecular characterization, expression and function analysis of Epinephelus coioides MKK4 response to SGIV and Vibrio alginolyticus infection

Mitogen-activated protein kinase 4 (MKK4), a member of the MAP kinase family, play important roles in response to many environmental and cellular stresses in mammals. In this study, three MKK4 subtypes, EcMKK4-1, EcMKK4-2 and EcMKK4-3, were obtained from grouper Epinephelus coioides. The open reading frame (ORF) of EcMKK4s are obtained and the EcMKK4s proteins contain highly conserved domains: a S_TKc domain, a canonical diphosphorylation group and two conserved MKKK ATP binding motifs, Asp-Phe-Gly (DFG) and Ala-Pro-Glu (APE). EcMKK4s could be found both in the cytoplasmic and nuclear. The EcMKK4s mRNA were detected in all E. coioides tissues examined with the different expression levels, and the expression were up-regulated during SGIV (Singapore grouper iridescent virus) or Vibrio alginolyticus infection. EcMKK4 could significantly reduce the activation of AP-1 reporter gene. The results suggested that EcMKK4s might play important roles in pathogen-caused inflammation.

Molecular cloning, inducible expression with SGIV and Vibrio alginolyticus challenge, and function analysis of Epinephelus coioides PDCD4

Programmed cell death 4 (PDCD4) in mammals, a gene closely associated with apoptosis, is involved in many biological processes, such as cell aging, differentiation, regulation of cell cycle, and inflammatory response. In this study, grouper Epinephelus coioides PDCD4, EcPDCD4-1 and EcPDCD4-2, were obtained. The open reading frame (ORF) of EcPDCD4-1 is 1413 bp encoding 470 amino acids with a molecular mass of 52.39 kDa and a theoretical pI of 5.33. The ORF of EcPDCD4-2 is 1410 bp encoding 469 amino acids with a molecular mass of 52.29 kDa and a theoretical pI of 5.29. Both EcPDCD4-1 and EcPDCD4-2 proteins contain two conserved MA3 domains, and their mRNA were detected in all eight tissues of E. coioides by quantitative real-time PCR (qRT-PCR) with the highest expression in liver. The expressions of two EcPDCD4s were significantly up-regulated after Singapore grouper iridovirus (SGIV) or Vibrio alginolyticus infection. In addition, over-expression of EcPDCD4-1 or EcPDCD4-2 can inhibit the activity of the nuclear factor-κB (NF-κB) and activator protein-1 (AP-1), and regulate SGIV-induced apoptosis. The results demonstrated that EcPDCD4s might play important roles in E. coioides tissues during pathogen-caused inflammation.

Protective effects of β-glucan as adjuvant combined inactivated Vibrio harveyi vaccine in pearl gentian grouper

Vaccination is one of the strategies for preventing Vibrio harveyi infection in marine-cultured animals. In this study, we prepared a formalin-killed cells of V. harveyi ZJ0603 vaccine (FKC) combined with β-glucan to immune pearl gentian grouper. The results indicated that the expression levels of IgM, TNF-α, MHC-Iα, IL-1β and IL-16 significantly increased in the spleen of the vaccinated fish. Antibody titers, activities of lysozyme and superoxide dismutase were significantly prompted in blood of the vaccinated fish. After 35 d post-vaccination, all fish were challenged intraperitoneally by virulent V. harveyi, and the relative percentage of survival (RPS) of FKC+β-glucan, FKC, β-glucan and PBS were 68 ± 5.7%, 55 ± 8.5%, 42 ± 7.5% and 32 ± 6.9%, respectively. These results demonstrated that β-glucan could be as a potential adjuvant of FKC and provide good protective effect against V. harveyi infection in the pearl gentian grouper culture.

Impact of varied combinatorial mixture of non-fishmeal ingredients on growth, metabolism, immunity and gut microbiota of Lates calcarifer (Bloch, 1790) fry

The search for suitable fish meal replacements in aqua-diets is a salient agenda in the constant effort of making aquaculture practices more sustainable. In this study, we tested four customised diets composed by systematic inclusion of pre-selected fish meal substitutes, lupin kernel meal, BSF meal, TH and PBM on growth, metabolism, cytokine profile, gut morphology and microbiota of juvenile Lates calcarifer. Five isoproteic and isoenergetic diets were prepared viz. FM100 as a control (without fish meal substitute), while FM75, FM50, FM25 and FM0 indicates replacement of fish meal (FM) at 25%, 50%, 75%, and 100%, respectively by a mixture of four different pre-selected non-fish meal (NFM) ingredients. Fish fed FM100, FM75, FM50, FM25 exhibited consistent growth and haematological response, while the fish fed no fishmeal (FM0) showed significant decline in final body weight (FBW) and specific growth rate (SGR). The poor growth performance was correlated with a decrease in villous width, microvilli height and goblet cells density. A significant shift in abundance profile of Psychrobacter in the gut microbial profile of fish fed FM50 was noticed compared to fish fed FM100. The results of qRT-PCR showed up-regulated expression of innate immune responsive genes in the FM50 group. The adverse impacts on growth performance and gut health of fish fed FM0 suggest that the complete substitution of fishmeal is not advisable and the inclusion range of these alternatives should be decided for a species only after examining their effect on maximal physiological performance.

Distribution of Mpeg1+ cells in healthy grouper (Epinephelus coioides) and after Cryptocaryon irritans infection

Cryptocaryon irritans is an extremely harmful ciliated obligate parasite that is responsible for large economic losses in aquaculture. C. irritans infection can cause an insect-resistant immune response in fish, and many immune cells can be observed in the local infection site. However, it is unclear whether macrophages are involved in the host defense against C. irritans infection. The Mpeg1 protein can form pores and destroy the cell membrane of invading pathogens, and is also used as a macrophage-specific marker in mammals. Therefore, a polyclonal antibody against grouper recombinant Mpeg1a was produced to mark macrophages in this study, which could recognize both isoforms of Mpeg1 (Mpeg1a/b). Immunofluorescence revealed that EcMpeg1 positive cells were mostly distributed in the head kidney and spleen in healthy grouper. Immunofluorescence and immunohistochemistry showed that the number of EcMpeg1 positive cells increased in the gills after infection with C. irritans, implying that EcMpeg1 positive cells may be involved in the process of grouper resistance against C. irritans infection.

Temporal, Environmental, and Biological Drivers of the Mucosal Microbiome in a Wild Marine Fish, Scomber japonicus

Changing ocean conditions driven by anthropogenic activities may have a negative impact on fisheries by increasing stress and disease. To understand how environment and host biology drives mucosal microbiomes in a marine fish, we surveyed five body sites (gill, skin, digesta, gastrointestinal tract [GI], and pyloric ceca) from 229 Pacific chub mackerel, Scomber japonicus, collected across 38 time points spanning 1 year from the Scripps Institution of Oceanography Pier (La Jolla, CA). Mucosal sites had unique microbial communities significantly different from the surrounding seawater and sediment communities with over 10 times more total diversity than seawater. The external surfaces of skin and gill were more similar to seawater, while digesta was more similar to sediment. Alpha and beta diversity of the skin and gill was explained by environmental and biological factors, specifically, sea surface temperature, chlorophyll a, and fish age, consistent with an exposure gradient relationship. We verified that seasonal microbial changes were not confounded by regional migration of chub mackerel subpopulations by nanopore sequencing a 14,769-bp region of the 16,568-bp mitochondria across all temporal fish specimens. A cosmopolitan pathogen, Photobacterium damselae, was prevalent across multiple body sites all year but highest in the skin, GI, and digesta between June and September, when the ocean is warmest. The longitudinal fish microbiome study evaluates the extent to which the environment and host biology drives mucosal microbial ecology and establishes a baseline for long-term surveys linking environment stressors to mucosal health of wild marine fish.IMPORTANCE Pacific chub mackerel, Scomber japonicus, are one of the largest and most economically important fisheries in the world. The fish is harvested for both human consumption and fish meal. Changing ocean conditions driven by anthropogenic stressors like climate change may negatively impact fisheries. One mechanism for this is through disease. As waters warm and chemistry changes, the microbial communities associated with fish may change. In this study, we performed a holistic analysis of all mucosal sites on the fish over a 1-year time series to explore seasonal variation and to understand the environmental drivers of the microbiome. Understanding seasonality in the fish microbiome is also applicable to aquaculture production for producers to better understand and predict when disease outbreaks may occur based on changing environmental conditions in the ocean.

Molecular and immunological characterizations of interleukin-11 in large yellow croaker (Larimichthys crocea)

Interleukin (IL)-11 is a multifunctional cytokine that exerts a series of important immunomodulatory effects and exists in many tissues and cells. A 1106-bp nucleotide sequence representing the complete cDNA of IL-11 was obtained from large yellow croaker (Larimichthys crocea), containing an open reading frame (ORF) of 603 bp encoding for 200 amino acids (aa). The predicted LcIL-11 protein included a 12aa signal peptide and a conserved IL-11 domain. The polypeptide sequence identities between LcIL-11 and its counterparts in mammals and other fish are from 84% to 92% with known fish IL-11a and 22%-27% with fish IL-11b. LcIL-11 mRNA existed in most tissues with the most predominant expression in the gill. After immune challenge, the expression levels of LcIL-11 were induced largely in vivo and in vitro, with the peak-value of 32 times as much as the control in the liver at 24 h after Vibrio parahaemolyticus injection (p < 0.05) and the greatest value of 13.9 times as much as the control in LCK cells at 12 h after poly I:C stimulation (p < 0.05). Furthermore, the overexpression vector pcDNA3.1-LcIL-11 was constructed and transfected to LCK cells. Our results showed that the transcriptional expression levels of tumor necrosis factor (TNF)-α and myxovirus resistant protein (Mx) significantly up-regulated in LCK cells after LcIL-11 overexpression (p < 0.05). However, no significant changes of IL-1β, janus kinase (JAK)2 and signal transducers and activators of transcription (STAT)5 was detected. Our finding indicated that LcIL-11 might enhance TNF-α and antiviral protein Mx expression in large yellow croaker.

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.

Immune and gut bacterial successions of large yellow croaker (Larimichthys crocea) during Pseudomonas plecoglossicida infection

Large yellow croaker (Larimichthys crocea, LYC) aquaculture is being threatened by intensive infectious diseases. Relevant studies have focused on LYC immune responses to infection. By contrast, little is known how and to what extent the gut microbiota responds to infection. Here, we explored the interactions between LYC immune responses and gut bacterial communities during Pseudomonas plecoglossicida infection. P. plecoglossicida successfully colonized into LYC gut microbiota, resulting in an increasing mortality rate. Relative gene expressions of pro-inflammatory cytokines (TNF-α1, TNF-α2 and IL-1β) and anti-inflammatory cytokine (IL-10) were consistently and significantly induced by P. plecoglossicida infection, whereas non-specific immune enzymes activities were only enhanced at the early infection stages. P. plecoglossicida infection caused an irreversible disruption in the gut microbiota, of which infection and hours post infection constrained 16.2% and 5.6% variations, respectively. In addition, top 18 discriminatory taxa that were responsible for the difference between treatments were identified, whose abundances were significantly associated with the immune activities of LYC. Using a structural equation modeling (SEM), we found that gut bacterial communities were primarily governed by the conjointly direct (-0.33) and indirect (0) effects of infection, which subsequently affect host immune responses. Our results suggest that an irreversible dysbiosis in gut microbiota could be the causality of increasing mortality. To our knowledge, this is the first study to provide an integrated overview among pathogen infection, immune response and gut microbiota of LYC.

Pseudomonas piscis sp. nov., isolated from the profound head ulcers of farmed Murray cod (Maccullochella peelii peelii)

A Gram-stain-negative, strictly aerobic, motile, rod-shaped bacterium with monopolar flagella, designated as MC042^T, was isolated from the profound head ulcers of farmed Murray cod sampled from Zhejiang Province, China. Analysis of its 16S rRNA gene sequence and multilocus sequence analysis phylogeny showed that strain MC042^T belonged to the genus Pseudomonas, showing the highest 16S rRNA gene sequence similarity to Pseudomonas juntendi BML3^T (98.9 %), and less than 98.8 % similarity to other Pseudomonas species with validly published names. Whole-genome sequencing and phylogenetic reconstruction based on a core set of 1563 Pseudomonas genes further indicated that strain MC042^T was most closely related to the clade formed by Pseudomonas protegens CHA0^T and Pseudomonas saponiphila DSM 975^T and distantly related to any of the validly published species of the genus Pseudomonas. Furthermore, strain MC042^T could be distinguished from its closely related species of the genus Pseudomonas by its ability to assimilate maltose, d-xylose and melibiose, but not d-mannitol. The principal fatty acids were C_16 : 0, summed feature 3 (iso-C_15 : 0 2-OH and/or C_16 : 1ω7c) and summed feature 8 (C_18 : 1 ω7c and/or C_18 : 1 ω6c). The respiratory quinone was Q-9. Polar lipids of strain MC042^T comprised diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylglycerol, unidentified glycolipids, an unidentified lipid, an unknown glycolipid and aminolipid. Based on its phenotypic, chemotaxonomic and phylogenetic features, strain MC042^T is considered to represent a novel species, for which the name Pseudomonas piscis sp. nov. is proposed. The type strain is MC042^T (=KCTC 72033^T=MCCC 1K03575^T).

TRAF3 enhances TRIF-mediated signaling via NF-κB and IRF3 activation in large yellow croaker Larimichthys crocea

As a member of tumor necrosis factor receptor (TNFR)-associated factor (TRAF) family, TRAF3 is an important regulator of NF-κB and type I interferon (IFN) activation, especially in Toll-like receptors (TLRs)- and retinoic acid inducible gene I (RIG-I)-like receptors (RLRs)-mediated signaling pathway. In the present study, a TRAF3 homologue named Lc-TRAF3 was characterized in large yellow croaker (Larimichthys crocea). The open reading frame (ORF) of Lc-TRAF3 contains 1788 bp encoding a protein of 595 amino acids (aa). Sequence analysis indicated that Lc-TRAF3 is conserved in vertebrates, constituted with a N-terminal RING finger, two TRAF-type zinc fingers, and a C-terminal TRAF-MATH domain. The genome organization of Lc-TRAF3 is conserved in fish, with 13 exons and 12 introns, but different from that in birds or mammals, which contains 10 exons and 9 introns. Lc-TRAF3 was identified as cytosolic protein base on fluorescence microscopy analysis. Expression analysis revealed that Lc-TRAF3 was broadly distributed in examined organs/tissues, with the highest expression level in gill and weakest in brain, and could be up-regulated under poly I:C, LPS, PGN, and Pseudomonas plecoglossicida stimulation in vivo. Interestingly, overexpression Lc-TRAF3 could induce the activation of NF-κB, and Lc-TRAF3 co-transfected with Lc-TRIF induced a significantly higher level of NF-κB and IRF3 promoter activity, implying that Lc-TRAF3 may function as an enhancer in Lc-TRIF-mediated signaling pathway.

Quorum Quenching Properties and Probiotic Potentials of Intestinal Associated Bacteria in Asian Sea Bass Lates calcarifer

Quorum quenching (QQ), the enzymatic degradation of N-acyl homoserine lactones (AHLs), has been suggested as a promising strategy to control bacterial diseases. In this study, 10 AHL-degrading bacteria isolated from the intestine of barramundi were identified by 16S rDNA sequencing. They were able to degrade both short and long-chain AHLs associated with several pathogenic Vibrio species (spp.) in fish, including N-[(RS)-3-Hydroxybutyryl]-l-homoserine lactone (3-oh-C₄-HSL), N-Hexanoyl-l-homoserine lactone (C₆-HSL), N-(β-Ketocaproyl)-l-homoserine lactone (3-oxo-C₆-HSL), N-(3-Oxodecanoyl)-l-homoserine lactone (3-oxo-C₁₀-HSL), N-(3-Oxotetradecanoyl)-l-homoserine lactone (3-oxo-C₁₄-HSL). Five QQ isolates (QQIs) belonging to the Bacillus and Shewanella genera, showed high capacity to degrade both synthetic AHLs as well as natural AHLs produced by Vibrio harveyi and Vibrio alginolyticus using the well-diffusion method and thin-layer chromatography (TLC). The genes responsible for QQ activity, including aiiA, ytnP, and aaC were also detected. Analysis of the amino acid sequences from the predicted lactonases revealed the presence of the conserved motif HxHxDH. The selected isolates were further characterized in terms of their probiotic potentials in vitro. Based on our scoring system, Bacillus thuringiensis QQ1 and Bacillus cereus QQ2 exhibited suitable probiotic characteristics, including the production of spore and exoenzymes, resistance to bile salts and pH, high potential to adhere on mucus, appropriate growth abilities, safety to barramundi, and sensitivity to antibiotics. These isolates, therefore, constitute new QQ probiotics that could be used to control vibriosis in Lates calcalifer.

Sea anemone and clownfish microbiota diversity and variation during the initial steps of symbiosis

Clownfishes and sea anemones form an intriguing long-term association, but the mechanism underlying this symbiosis is not well understood. Since clownfishes seem to cover themselves with sea anemone mucus, we investigated the microbiomes of the two partners to search for possible shifts in their compositions. We used a 16S rRNA gene sequencing strategy to study the dynamics of the microbiota during the association between the clownfish Amphiprion ocellaris and its host Heteractis magnifica under laboratory conditions. The experiment conducted in aquaria revealed that both clownfish and sea anemone mucus had specific signatures compared to artificial sea water. The microbiomes of both species were highly dynamic during the initiation of the symbiosis and for up to seven days after contact. Three families of bacteria (Haliangiaceae, Pseudoalteromonadacae, Saprospiracae) were shared between the two organisms after symbiosis. Once the symbiosis had been formed, the clownfishes and sea anemone then shared some communities of their mucus microbiota. This study paves the way for further investigations to determine if similar microbial signatures exist in natural environments, whether such microbial sharing can be beneficial for both organisms, and whether the microbiota is implicated in the mechanisms that protect the clownfish from sea anemone stinging.

Seasonal intestinal coccidiosis in wild bluegill Lepomis macrochirus is associated with a spring bacterial epizootic

Wild bluegill, Lepomis macrochirus Rafinesque, succumb to seasonal mortality in the early spring during cool water temperatures, shown previously to be related to bacteraemia caused by a psychrotrophic bacterium, Pseudomonas mandelii. In the study herein, intestinal coccidiosis in wild bluegill had seasonal prevalence causing heavy intestinal infections and sloughing of intestinal epithelium occurring in late winter/early spring. Infections were predominantly related to two different species, Goussia washuti n. sp., an epicellular coccidium, and a coccidium closely resembling Goussia desseri Molnár 1996, previously only described in percid fish in Europe. In 2019, co-infections of bacteraemia and intestinal coccidiosis occurred in bluegills. Evaluating coccidium infection intensity by fresh parasite examination and histology, an association was observed in which fish with moderate-to-heavy intestinal coccidiosis were 8-12 times more likely to have bacteraemia compared to fish with no or light coccidiosis. The association of these co-infections suggests that intestinal coccidiosis could contribute to seasonal bacterial epizootics of wild bluegill.

Resistance to Streptococcus iniae and its genetic associations with traits of economic importance in Asian seabass (Lates calcarifer)

Streptococcus iniae is one of the most serious aquatic pathogens, causing significant economic losses in marine and freshwater species, including Asian seabass (Lates calcarifer). Controlling this gram-positive bacterial pathogen has been an issue in aquaculture systems, due to the combined effects of aquaculture intensification and climatic impacts. To date, there have not been any genetic parameter estimates for S. iniae resistance in Asian seabass. The main aim of this study was to examine genetic variation in S. iniae resistance and its genetic correlations with growth and cannibalism in Asian seabass families produced from a breeding programme for high growth in 2016 and 2017. The study included a total of 5,835 individual fish that were offspring of 41 sires and 60 dams (31 half-sib and 34 full-sib families). The experimental fish were challenged by intraperitoneal injection with a volume containing 10⁵  CFU (colony-forming unit)/fish. Resistance to S. iniae was measured as survival rate at 6 hr, 3, 5, 7, 10 and 15 days post-challenge test. There were significant variations in S. iniae resistance among families at different observation periods (ranging from 24.4% to 80%). Restricted maximum-likelihood method and mixed model analysis were applied to estimate heritability for S. iniae resistance. The heritability for S. iniae resistance ranged from 7% to 18% across different statistical models used. The common full-sib effects accounted for 0.1%-2% of the total variation in resistance to S. iniae. Genetic correlations of the S. iniae resistance at 6 hr and 3 days with later post-challenge test periods were low to moderate. However, these estimates for S. iniae resistance between successive measurement times (5, 7, 10 and 15 days) were high and close to 1. The genetic correlations of resistance with body weights at 180, 270 and 360 days post-hatch were not significant as well with cannibalism. It is concluded that there is substantial additive genetic variation in resistance to S. iniae, suggesting there is potential for genetic improvement of Asian seabass for resistance to S. iniae through selective breeding.

Phenotypic and Molecular Characterization of Lacinutrix venerupis Isolated from Atlantic Horse Mackerel Trachurus trachurus

The aim of the present study was to characterize two gram-negative bacterial strains that were isolated from diseased Atlantic Horse Mackerel Trachurus trachurus in 2017. Based on the results obtained from the biochemical and chemotaxonomic characterization, the isolates were identified as Lacinutrix spp. The highest similarity of the 16S rRNA gene sequences was obtained with the strain L. venerupis CECT 8573^T (99.1%), while other species showed similarities of 98% (L. jangbogonensis) and 97% (L. algicola and L. mariniflava). Molecular characterization by repetitive element (REP)-PCR and enterobacterial repetitive intergenic consensus (ERIC)-PCR, as well as proteomic characterization by matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF-MS), demonstrated heterogeneity between the strains from the Atlantic Horse Mackerel and the type strain, CECT 8573^T . The virulence of one of the isolates for Turbot Scophthalmus maximus, European Sea Bass Dicentrarchus labrax, Senegalese Sole Solea senegalensis, and Rainbow Trout Oncorhynchus mykiss was assessed under experimental conditions. No mortalities were recorded after intraperitoneal injections with high doses of bacteria (1 × 10⁹  CFU/mL). Thus, further studies are necessary to elucidate the impact of this bacterial species as a fish pathogen.

Protection and antibody reactivity in lumpsucker (Cyclopterus lumpus L.) following vaccination against Pasteurella sp

Two monovalent vaccines against pasteurellosis were developed and tested for efficacy using a previously established bath challenge model. High levels of specific antibodies were detected following vaccination. While the vaccine efficacy trial indicated that some level of protection was obtained, high mortality was still observed. qPCR analysis of head kidney tissues from surviving fish post challenge showed no difference in bacterial numbers in vaccinated and non-vaccinated fish. Clinical symptoms observed in moribund and diseased fish included white spots on the skin and around the eyes, frayed fins and redness around the mouth and fin bases. Despite production of specific antibodies, the protection against experimental challenge was relatively weak. A reason for this could potentially be that the specific antibodies produced are not alone enough to provide complete protection against pasteurellosis in lumpsuckers. Confocal and scanning electron microscopy of head kidney leucocytes exposed to Pasteurella sp. in vitro gave indications of the interactions between the pathogen and leucocytes. The results indicate that parts of the immune system other than humoral antibodies could be important for protection against pasteurellosis. Our combined results highlight the need for further work on host-pathogen interaction between Pasteurella sp. and lumpsuckers.

Study on Schizochytrium sp. improving the growth performance and non-specific immunity of golden pompano (Trachinotus ovatus) while not affecting the antioxidant capacity

A feeding experiment was conducted to determine the effects of Schizochytrium sp. on growth performance, antioxidant capacity and non-specific immunity in golden pompano (Trachinotus ovatus).Two diets were formulated with or without Schizochytrium sp. supplemented (D1:0% and D2: 3%) to feed fish for 8 weeks. Results showed that growth performance, feed intake and survival rate increased significantly with Schizochytrium sp. supplemented (P < 0.05). Feed coefficient rate (FCR) of golden pompano fed the diet supplemented with Schizochytrium sp. was significantly lower than that of fish fed the control diet (P < 0.05). No significant differences were found in antioxidant capacity both in transcriptional level, including nclear factor erythroid-2-related factor-2 (Nrf2), Kelch-like-ECH-associated protein (keap1), catalase (CAT), glutathione peroxidase (GSH-PX) and heme oxygenase 1 (HO-1) and enzyme activity, such as total antioxidant capacity (T-AOC), malondialdehyde (MDA) and superoxide dismutase (SOD) (P > 0.05). Gut amylase and lipase were significantly higher in dietary Schizochytrium sp. supplemented treatment than that in control group (P < 0.05). The relative peroxisome proliferator-activated receptor-α (PPARα) expression level in liver was significantly higher in Schizochytrium sp supplemented treatment than that in control one (P < 0.05). The mRNA expression of myeloid differentiation factor 88 (MyD88), IL-1R-associated kinases 4 (IRAK4), interferon regulating Factor 3 (IRF3), interferon regulating Factor 3(IRF7) and heat shock protein 70 (HSP70) were significantly lower in Schizochytrium sp. supplemented treatment than that in control one (P < 0.05). In Schizochytrium sp. supplemented diet, golden pompano had significantly longer villi length than that in control diet (P < 0.05); muscle thickness in Schizochytrium sp. supplemented diet was thicker than that in control one (P < 0.05) and there were more goblet cells in Schizochytrium sp. treatment (P < 0.05). After the rearing trial, an air exposure trial was conducted. Results showed that the air-exposure mortality (AEM) and mRNA expression level of Nrf2, keap1, CAT, GSH-PX and HO-1 showed no significant difference (P > 0.05). These results indicated that dietary Schizochytrium sp. improved the growth performance and non-specific immunity of golden pompano while made no difference to antioxidant capacity.

Diketopiperazines Synthesis Gene in Shewanella baltica and Roles of Diketopiperazines and Resveratrol in Quorum Sensing

The synthesis pathways of quorum sensing (QS) signal molecules and the mechanism of action of quorum sensing inhibitors (QSIs) have gained considerable attention as research topics in the field of food preservation. Here, Shewanella baltica was detected as the specific spoilage organism in large yellow croaker during 4 °C storage, and it produced the QS signal molecules autoinducer-2 (AI-2) and diketopiperazines (DKPs). Then, a cyclodipeptide synthase (CDPS) homologous gene, sb1370, was screened, and knockout and rescue results revealed that this gene was involved in DKP synthesis but not in AI-2 synthesis, and it also played an important role in QS. Furthermore, fish fillets and mutant strains were treated with resveratrol, and the results suggested that resveratrol was an ideal QSI for inhibition of DKPs production via the sb1370 gene and reduced QS in S. baltica, thus delaying the process of fish spoilage during chilling storage.

clpV is a key virulence gene during in vivo Pseudomonas plecoglossicida infection

Interaction between bacterial pathogen and aquatic animal host is exceedingly complex, which involves large dynamic changes in gene expression during different stages of the disease. However, research on identifying key virulence genes based on the dynamics of gene expression changes of a one-sided bacterial pathogen in tissue has not been reported so far across different stages of infectious disease. The clpV for the T6SS of Pseudomonas plecoglossicida was identified for a candidate for key virulence gene based on dynamic changes of gene expression. For the Epinephelus coioides infected using clpV-RNAi strain, no deaths were observed up to 20 dpi. The spleens, kidneys and livers of all the E. coioides that received clpV-RNAi strain failed to develop visible nodules at 5-8 dpi, with the swelling gradually disappearing. The burdens of clpV-RNAi strain in the spleen and blood were greatly reduced at most of the time points after injection, and the burdens of clpV-RNAi strain in the head kidneys and trunk kidneys also had a sharp reduction from 72 to 120 hpi. This paper provides a new insight into the discovery of key virulence genes of pathogens in infected tissue systems.

High doses of Francisella noatunensis induces an immune response in Eleginops maclovinus

Francisella noatunensis subsp. noatunensis, the etiological agent of Francisellosis, affects a large number of farmed species such as Salmo salar. This species coexists with several native species in the same ecosystem, including Eleginops maclovinus. Our objective was to evaluate the susceptibility, presence of clinical symptoms, and the ability of Eleginops maclovinus to respond to Francisella infection. For this, healthy individuals were inoculated with 1.5 × 10¹, 1.5 × 10⁵, and 1.5 × 10¹⁰ bact/μL of Francisella by intraperitoneal injection, subsequently the fish were sampled on days 1, 3, 7, 14, 21, and 28 post injection (dpi). At the end of the experiment, no mortality, nor internal and external clinical signs were observed, although in the high dose anaemia was detected. Additionally, bacteria were detected in all three doses, however there was replication at day 28 only in the liver in the high dose. Analysis of gene expression by qPCR showed that the spleen generated an immune response against infection from day 1 dpi, however at day 7 dpi most of the genes suffered repressed expression; observing over expression of the genes C3, NLRC3, NLRC5, MHCI, IgM. In contrast, expression in the anterior kidney did not vary significantly during the challenge. IgM quantification showed the production of antibodies in the medium and high doses. This study provides new knowledge about Francisella infection and the long-lasting and specific immune response generated by Eleginops maclovinus. It also demonstrates its susceptibility to Francisellosis where there is a difference in the immune response according to the tissue.

Effects of prebiotic mixtures on growth performance, intestinal microbiota and immune response in juvenile chu's croaker, Nibea coibor

Prebiotics has been known to be growth promoter and immunostimulant in aquatic animals. In this study, we investigated the effects of prebiotics on growth performance, intestinal microbiota, short-chain fatty acids (SCFAs) production and immune response of the marine fish, juvenile chu's croaker (Nibea coibor). The fish were fed IG (including 0.5% inulin and 0.5% GOS), GS (0.5% GOS and 0.5% D-sorbitol), IGS (0.33% inulin, 0.33% GOS and 0.33% D-sorbitol) or control diets for 8 weeks. The results showed that the growth performance of the fish was promoted by IG and GS, but not by IGS. The intestinal microbiota in NDC (non-digestible carbohydrates, NDC)-supplemented groups was clearly separated from that of the control, and the highest Shannon and Simpson diversity indices were observed in the IGS group. In the intestine of the croaker, Proteobacteria, Firmicutes, and Bacteroidetes were dominant; among them, 24 taxa revealed a significant difference among groups. Most of these bacteria are able to produce SCFAs, which were significantly increased in all NDC-supplemented groups. Moreover, NDCs were found to activate the immune system of the fish by modulating the serum complements, cytokine levels, lysozyme activities and antioxidant capacity. Furthermore, the results of this study revealed correlations among intestinal microbiota, SCFAs production, innate immunity, antioxidant capacity and digestive enzymes in the croaker fed NDCs. Taken together, our results demonstrated that NDC mixtures might promote growth performance, antioxidant capacity and immune responses of the croaker through modulating the composition of intestinal microbiota and the subsequent SCFAs production, which suggest that NDCs were efficient feed additives for marine fish.

The gut microbiota community and antioxidant enzymes activity of barramundi reared at seawater and freshwater

To understand the physiological responses of euryhaline fish to reared salinity, the gut microbiota composition and antioxidant enzymes activity of same batch barramundi Lates calcarifer reared in two extreme salinity condition (seawater and freshwater) were studied, under laboratory condition. The gut bacterial composition was analyzed using high-throughput Illumina sequencing. Acid phosphatase (ACP), catalase (CAT), glutathione peroxidase (GSH-Px), lipid peroxide (LPO), lysozyme (LZM), malonaldehyde (MDA), peroxidase (POD), and superoxide dismutase (SOD) were used as biochemical indicators. The reared salinity did not change the major composition of barramundi gut bacteria under circulating aquaculture system. Proteobacteria, Firmicutes were the most phylum in barramundi gut microbiota community, and Exiguobacterium, Citrobacter, Acinetobacter, Pseudomonas were the dominate genus. CAT and ACP activity in barramundi liver were found significantly different between freshwater and seawater group. GSH-Px, LZM, POD, SOD activity and MDA, LPO levels were not significantly affected by reared salinity. This study is the first high-throughput analyses of the gut microbiota diversity in barramundi from same batch of siblings reared under two extreme salinity condition. And the findings in the present study can be instructive to the management of animal health in barramundi circulating farming activities, and further euryhaline fish gut microecology research.

Effects of dietary lipid sources on the intestinal microbiome and health of golden pompano (Trachinotus ovatus)

Replacement of fish oil (FO) with vegetable oils (VO) in diets is economically desirable for the sustainable development of the aquaculture industry. However, inflammation provoked by FO replacement limited its widely application in fish industry. In order to understand the mechanism of VO-induced inflammation, this study investigated the impact of different dietary vegetable oils on the intestinal health and microbiome in carnivorous marine fish golden pompano (Trachinotus ovatus). Three diets supplemented with fish oil (FO, rich in long-chain polyunsaturated fatty acids), soybean oil (SO, rich in 18:2n-6) and linseed oil (LO, rich in 18:3n-3), respectively, were fed on juvenile golden pompano for 8 weeks, and the intestinal histology, digestive enzymes activities, immunity and antioxidant indices as well as intestinal microbiome were determined. The results showed that dietary SO significantly impaired intestinal health, and decreased the number and height of intestinal folds, and muscle thickness, as well as the zonula occludens-1 (zo-1) mRNA expression in intestine. Moreover, the two dietary VO significantly decreased the amylase and lipase activities in intestine, and reduced the trypsin activity in the dietary SO group. Furthermore, the two VO diets increased intestinal acid phosphatase (ACP) activity, while intestinal lysozyme (LZM) activity and serum diamine oxidase (DAO) activity in the SO group were also significantly increased (P < 0.05). Analysis of the intestinal microbiota showed that the two VO diets significantly increased the abundance of intestinal potentially pathogenic bacteria (Mycoplasma and Vibrio) and decreased proportions of intestinal probiotics (Bacillus and Lactococcus), especially in the dietary SO group. These results indicate that complete replacement of FO with VO in diets would induce intestinal inflammation and impair intestinal function, which might be due to changes in intestinal microbiota profiles, and that dietary SO would have a more negative effect compared to dietary LO on intestinal health in T. ovatus.

A novel C1q domain containing protein in black rockfish (Sebastes schlegelii) serves as a pattern recognition receptor with immunoregulatory properties and possesses binding activity to heat-aggregated IgG

C1q-domain-containing (C1qDC) proteins, which are involved in a series of immune responses, are important pattern recognition receptors in innate immunity in vertebrates and invertebrates. Functional studies of C1qDC proteins in vertebrates are scarce. In the present study, a C1qDC protein (SsC1qDC) from the teleost black rockfish (Sebastes schlegelii) was identified and examined at expression and functional levels. The open reading frame of SsC1qDC is 636 bp, and the predicted amino acid sequence of SsC1qDC shares 62%-69% overall identity with the C1qDC proteins of several fish species. SsC1qDC possesses conserved C1qDC features, including a signal sequence and a C1q domain. SsC1qDC was expressed in different tissues and its expression was up-regulated by bacterial and viral infection. Recombinant SsC1qDC (rSsC1qDC) exhibited apparent binding activities against PAMPs including LPS and PGN. rSsC1qDC had antibacterial activity against Vibrio parahaemolyticus, and was able to enhance the phagocytic activity of macrophages towards Vibrio anguillarum. rSsC1qDC interacted with human heat-aggregated IgG. Furthermore, in the presence of rSsC1qDC, fish exhibited enhanced resistance against bacterial infection. Collectively, these results indicated that SsC1qDC serves as a pattern recognition receptor and plays a vital role in the defense system of black rockfish.

Shifts in the Skin-Associated Microbiota of Hatchery-Reared Common Snook Centropomus undecimalis During Acclimation to the Wild

The skin-associated microbiota of fish competes against pathogens for space and nutrients, preventing colonization by harmful bacteria encountered during environmental transitions such as those faced during stock enhancement. Thus, alterations in bacterial community structure during release of cultured fish have important implications for health of these individuals. This study investigated microbiota structure during acclimation of juvenile hatchery-reared common snook Centropomus undecimalis to the wild by comparing skin-associated microflora among snook in captivity, after 48 h of acclimation at release sites, and from the wild. After two days of acclimation, the microbiota of hatchery-reared snook mirrored that observed on wild snook. Relative abundances of potential pathogens were higher in captive fish, whereas acclimated and wild fish harbored bacterial taxa influenced by geographical factors and water quality at release sites. Predicted microbiota function of acclimated and wild fish showed higher production of protective amino acids and antimicrobials, identifying a mechanism for microbial supplementation of the immune defense of these fish. The two-day transition to wild-type microbiota suggests a temporal scale of hours associated with bacterial succession indicating that the microbiota, whose structure is vital to fish health, aids in acclimation of fish to new environments during stock enhancement efforts.

A metabolomic investigation into the temperature-dependent virulence of Pseudomonas plecoglossicida from large yellow croaker (Pseudosciaena crocea)

Pseudomonas plecoglossicida is associated with multiple fish diseases, and temperature is one of the most important environmental factors related to its outbreak. To elucidate the influence of temperature variation on the pathogen, the global metabolomics of P. plecoglossicida (NZBD9) were analysed at the virulent (18°C) and avirulent (12°C and 28°C) temperatures. The result showed that the levels of Phosphoric acid, Tyrosine, Spermidine and Sucrose were significantly reduced，while Itaconic acid, Glucaric acid and Isomaltose were increased in P. plecoglossicida at 18°C. These metabolic adjustments assist P. plecoglossicida to survive in adverse environments, proliferate in the host, colonize and resist host immune clearance during the initial steps of infection. The results suggested that L321_03626 and L321_18122 genes played a key role in the regulation of these metabolic adaptions and thus regulated P. plecoglossicida virulence at virulent temperature, which was proved by further gene silencing and artificial infection. The present study, for the first time, determines the P. plecoglossicida metabolomic responses to temperature variation, which is helpful to explore its pathogenic mechanism and provides reference for disease control.

Effect of cold plasma on maintaining the quality of chub mackerel (Scomber japonicus): biochemical and sensory attributes

BACKGROUND

Atmospheric cold plasma (ACP) has emerged as a potential alternative to traditional methods for non-thermal food decontamination. However, few data are available about ACP treatment for seafood. In this study, dielectric barrier discharge (DBD) was applied to generate CP, and the aim of the study was to investigate the effectiveness of DBD-ACP on improving the quality of chub mackerel on the basis of chemical, microbial and sensory characteristics.

RESULTS

The effect of DBD-ACP on the quality of chub mackerel (Scomber japonicus) during storage was examined. Results revealed that the optimal voltage level and exposure time of this treatment were 60 kV and 60 s respectively, and such conditions exhibited excellent inactivation efficacy and weak influence on proximate chemical compositions. Variations in total viable count (TVC), sensory scores and total volatile basic nitrogen (TVB-N) indicated that ACP treatment extended the shelf life of chub mackerel to 14 days, whereas samples without this treatment exceeded the limits of the three parameters after 6 days. The slow development rates of peroxide value (PV) and thiobarbituric acid (TBA) value implied that lipid oxidation was also effectively retarded by ACP exposure. Scanning electron microscopy confirmed that CP could effectively delay the degradation of myofibrillar proteins and enhance the stability of tissue structures.

CONCLUSION

The excellent antimicrobial efficacy of ACP treatment makes it a potential and promising alternative to other seafood preservation technology. This is the first report on the application of ACP to seafood, which is essential to perishable food storage. © 2018 Society of Chemical Industry.

First Report of Achlya oblongata Infection in Freshwater-Reared Asian Seabass Lates calcarifer

In September 2014, a freshwater oomycete was first isolated from Asian Seabass Lates calcarifer fry that were reared in freshwater at a fish hatchery in Sabah, Malaysia. A fungal strain was isolated from infected fry by using glucose yeast extract (GY) agar. From morphological identification, the strain belonged to the genus Achlya based on the mode of zoospore release. Molecular phylogenetic analysis of the internal transcribed spacer region sequences from the strain showed high similarity (99-100%) to Achlya oblongata. The isolate was able to grow on GY agar incubated at 15-35°C, in GY broth adjusted to pH 3.0-11.0, and in up to 1.0% NaCl. This is the first report of Achlya infection in freshwater-reared Asian Seabass in Malaysia.

Health Characteristics and Blood Bacterial Assemblages of Healthy Captive Red Drum: Implications for Aquaculture and Fish Health Management

The newly emerging tissue microbiota hypothesis suggests that bacteria found in blood and tissues play a role in host health, as these bacterial communities have been associated with various noncommunicable diseases such as obesity, liver disease, and cardiovascular disease. Numerous reports have identified bacteria in the blood of healthy finfish, indicating bacteremia may not always indicate disease. Current research priorities in aquaculture include the development of technologies and practices that will allow for an effective reduction in antibiotic use for the prevention and treatment of disease. Overall, a better understanding of fish health is needed, particularly among species selected for commercial-scale production. This study investigated blood characteristics of cultured Red Drum Sciaenops ocellatus with the tissue microbiota hypothesis in mind. Bacterial assemblages within the blood were characterized using next-generation sequencing and compared with other various blood characteristics, including innate immune function enzymes, between two fish cohorts reared in aquaculture. A total of 137 prokaryotic operational taxonomic units (OTUs) were identified from the blood of Red Drum. Microbiota diversity and structure varied greatly among individuals, for which the number of OTUs ranged from 4 to 58; however, predicted metagenomic function was highly similar between individuals and was dominated by the metabolism of carbohydrates and amino acids and membrane transport. Communities were dominated by Proteobacteria, followed by Bacteroidetes, Firmicutes, and Actinobacteria. The most commonly identified genera included Acinetobacter, Bacillus, Corynebacterium, and Pseudomonas. Three genera previously identified as containing marine fish pathogens were detected: Corynebacterium, Pantoea, and Chryseobacterium. A subset of bacterial OTUs were positively correlated with superoxide dismutase activity and negatively correlated with lysozyme activity, indicating a relationship between blood microbiota and the innate immune system. The results of this study provide further evidence for the tissue microbiota hypothesis and demonstrate the potential for these bacterial communities to be linked to immunological characteristics often used as biomarkers for fish health.

Deepwater Horizon oil alone and in conjunction with Vibrio anguillarum exposure modulates immune response and growth in red snapper (Lutjanus campechanus)

This study examined the impacts of Macondo oil from the Deepwater Horizon oil spill, both alone and in conjunction with exposure to the known fish pathogen Vibrio anguillarum, on the expression of five immune-related gene transcripts of red snapper (il8, il10, tnfa, il1b, and igm). In order to elucidate this impact, six different test conditions were used: one Control group (No oil/No pathogen), one Low oil/No pathogen group (tPAH50 = 0.563 μg/L), one High oil/No pathogen group (tPAH50 = 17.084 μg/L, one No oil/Pathogen group, one Low oil/Pathogen group (tPAH50 = 0.736 μg/L), and one High oil/Pathogen group (tPAH50 = 15.799 μg/L). Fish were exposed to their respective oil concentrations for one week. On day 7 of the experiment, all fish were placed into new tanks (with or without V. anguillarum) for one hour. At three time points (day 8, day 10, and day 17), fish organs were harvested and placed into RNAlater, and qPCR was run for examination of the above specific immune genes as well as cyp1a1. Our results suggest that cyp1a1 transcripts were upregulated in oil-exposed groups throughout the experiment, confirming oil exposure, and that all five immune gene transcripts were upregulated on day 8, but were generally downregulated or showed no differences from controls on days 10 and 17. Finally, both oil and pathogen exposure had impacts on growth.