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Clostridium butyricum (CB) is known to promote growth, enhance immunity, promote digestion, and improve intestinal health. In this study, we investigated the effects of CB in the feed on growth performance, digestion, and intestinal health of juvenile spotted sea bass. To provide a theoretical basis for the development and application of CB in the feed of spotted sea bass, a total of 450 spotted sea bass with an initial body weight of (9.58 ± 0.05) g were randomly divided into six groups. Gradient levels with 0, 0.1%, 0.2%, 0.3%, 0.4%, and 0.5% of CB (1×10 9 cfu/g) were supplemented into diets, designated as CC, CB1, CB2, CB3, CB4, and CB5, respectively. Each group was fed for 54 days. Our results suggest that dietary 0.2% and 0.3% of CB can significantly increase the weight gain (WG) and specific growth rate (SGR) of spotted sea bass. The addition of CB significantly increased intestinal amylase activity, intestinal villus length, intestinal villus width, and intestinal muscle thickness. Similarly, CB supplementation increased the expression of tumor necrosis factor- α ( TNF-α ) and interleukin-8 ( IL-8 ). Sequence analysis of the bacterial 16S rDNA region showed that dietary CB altered the intestinal microbiota profile of juvenile spotted sea bass, increasing the dominant bacteria in the intestine and decreasing the harmful bacteria. Overall, dietary addition of CB can improve growth performance, enhance intestinal immunity, improve intestinal flora structure, and comprehensively improve the health of spotted sea bass.

Streptococcal disease results in major mortality events of both marine and freshwater fishes worldwide. Streptococcus iniae is among the prominent causative bacterial strains as it has been found to cause a higher incidence of mortality and act as a zoonotic pathogen. Here, we examine the susceptibility of two important aquaculture species in the USA, striped bass ( Morone saxatilis ) and white bass ( Morone chrysops ) to S. iniae . A high incidence of mortality was observed in both species, although striped bass succumbed more rapidly than white bass. Spleen gene expression at three time points following infection was analyzed to further elucidate the mechanisms underlying these observations. The down-regulation of gene transcripts associated with pathogen detection ( tlr1 , tlr8 , tlr9 ), antigen processing ( cd74a ), immune cell recruitment and migration ( ccr6b , ccr7 ), macrophage function ( csf1ra ), T-cell signaling, and NF-kB activation ( card11 , fyna , tirap ) was detected in both species. These findings potentially indicate impairment in these critical early immune system processes such that both species were ultimately highly susceptible to S. iniae infection despite the detected up-regulation of transcripts typically associated with effective immune response, such as cytokines ( il1β , il8 , il12b2 , il17rc , tnfα ) and hepcidins ( hamp , hamp2 ). The presented results collectively identify several candidate genes and associated pathways for further investigation to characterize the vulnerability of striped bass and white bass to S. iniae and that may be considered for selective breeding efforts, biotechnological intervention, and/or exploitation in the development of vaccines and alternative treatments.

The mucosal surfaces of fish harbour microbial communities that can act as the first-line of defense against pathogens. Infectious diseases are one of the main constraints to aquaculture growth leading to huge economic losses. Despite their negative impacts on microbial diversity and overall fish health, antibiotics are still the method of choice to treat many such diseases. Here, we use 16 rRNA V4 metataxonomics to study over a 6 week period the dynamics of the gill and skin microbiomes of farmed seabass before, during and after a natural disease outbreak and subsequent antibiotic treatment with oxytetracycline. Photobacterium damselae was identified as the most probable causative agent of disease. Both infection and antibiotic treatment caused significant, although asymmetrical, changes in the microbiome composition of the gills and skin. The most dramatic changes in microbial taxonomic abundance occurred between healthy and diseased fish. Disease led to a decrease in the bacterial core diversity in the skin, whereas in the gills there was both an increase and a shift in core diversity. Oxytetracycline caused a decrease in core diversity in the gill and an increase in the skin. Severe loss of core diversity in fish mucosae demonstrates the disruptive impact of disease and antibiotic treatment on the microbial communities of healthy fish.

There is an increasing interest from the aquafeed industry in functional feeds containing selected additives that improve fish growth performance and health status. Functional feed additives include probiotics, prebiotics, organic acids, and phytogenics (substances derived from plants and their extracts). This study evaluated the effects of dietary inclusion of a mucilage extract rich in galactomannan oligosaccharides (GMOS), a mixture of garlic and labiatae-plants oils (PHYTO), and a combination of them (GMOSPHYTO), on gut microbiota composition of European sea bass (Dicentrarchus labrax) fed with a low fishmeal (FM) and fish oil (FO) diet. Three experimental diets and a control diet (plant-based formulation with 10% FM and 6% FO) were tested in a 63-days feeding trial. To analyze the microbiota associated to feeds and the intestinal autochthonous (mucosa-adhered) and allochthonous (transient) microbial communities, the Illumina MiSeq platform for sequencing of 16S rRNA gene and QIIME2 pipeline were used. Metabarcoding analysis of feed-associated bacteria showed that the microbial communities of control (CTRL) feed deeply differed from those of experimental diets. The number of reads was significantly lower in CTRL feed than in other feeds. The OTU (operational taxonomic unit) number was instead similar between the feeds, ranging from 42 to 50 OTUs. The variation of resident gut microbiota induced by diet was lower than the variation of transient intestinal microbiota, because feedstuffs are a major source of allochthonous bacteria, which can temporarily integrate into the gut transient microbiome. However, the composition of transient bacterial communities was not simply a mirror of feed-borne bacteria. Indeed, the microbial profile of feeds was different from both faecal and mucosa profiles. Our findings suggest that the dietary inclusion of GMOS (0.5%) and PHYTO (0.02%) in a low FM and FO diet induces changes in gut microbiota composition of European sea bass. However, if on allochthonous microbiota the combined inclusion of GMOS and PHYTO showed an antagonistic effect on bactericidal activity against Vibrionales, at mucosa level, only GMOSPHYTO diet increased the relative abundance of Bacteroidales, Lactobacillales, and Clostridiales resident bacterial orders. The main beneficial effects of GMOS and PHYTO on gut microbiota are the reduction of coliforms and Vibrionales bacteria, which include several potentially pathogenic species for fish, and the enrichment of gut microbiota composition with butyrate producer taxa. Therefore, these functional ingredients have a great potential to be used as health-promoting agents in the farming of European sea bass and other marine fish.

The European sea bass ( Dicentrarchus labrax ), one of the most produced marine fish species in Europe, is acutely vulnerable to multiple infectious hazards. In this study, we investigated the potential probiotic effect of some marine Pseudoalteromonas bacterial strains against two major pathogens of this species, Vibrio harveyi and the nervous necrosis virus (NNV), and examined their antibiofilm effect. Impregnation phase was done by repeated immersion of juvenile’s sea bass during 8 to 12 weeks in seawater containing the probiotic candidates at a concentration of 10 6  CFU/mL. Four candidates were tested: (1) a combination of two strains producing antimicrobial compounds, h Cg -42 and h Oe -125; (2) strain 3J6, with known antibiofilm properties; (3) strain RA15, from the same genus, but with no identified probiotic effect; and (4) a control group without probiotics. At the end of the impregnation phase, fish underwent an infection challenge with V. harveyi or with a pathogenic strain of NNV and mortality was monitored. For the V. harveyi challenge, improved survival rates of 10 and 25% were obtained for the RA15 and the mix h Cg -42 + h Oe -125-impregnated groups, respectively. For the NNV challenge, no significant benefic effect of the probiotics on infection kinetics or cumulative mortality was observed. At the end of the impregnation phase, the maximal thickness of biofilm was significantly lower in the 3J6, double strain, and RA15 groups, compared with the non-impregnated control group. This study highlights the interesting probiotic potential of marine bacteria to limit mortalities induced by bacterial pathogens as well as biofilm development.

Nile tilapia (Oreochromis niloticus) and European sea bass (Dicentrarchus labrax) are economically significant species in Mediterranean countries, serving essential roles in the aquaculture industry due to high market demand and nutritional value. They experience substantial losses from bacterial pathogens Vibrio anguillarum and Streptococcus iniae, particularly at the onset of the summer season. The immune mechanisms involved in fish infections by V. anguillarum and S. iniae remain poorly understood. This study investigated their impact through experiments with control and V. anguillarum- and S. iniae-infected groups for each species. Blood samples were collected at 1, 3, and 7 days post bacterial injection to assess biochemical and immunological parameters, including enzyme activities (AST and ALT), oxidative markers (SOD, GPX, CAT, and MDA), and leukocyte counts. Further analyses included phagocyte activity, lysozyme activity, IgM levels, and complement C3 and C4 levels. Muscle tissues were sampled at 1, 3, and 7 days post injection to assess mRNA expression levels of 18 immune-relevant genes. The focus was on cytokines and immune-related genes, including pro-inflammatory cytokines (TNF-α, TNF-β, IL-2, IL-6, IL-8, IL-12, and IFN-γ), major histocompatibility complex components (MHC-IIα and MHC-IIβ), cytokine receptors (CXCL-10 and CD4-L2), antimicrobial peptides (Pleurocidin and β-defensin), immune regulatory peptides (Thymosin β12, Leap 2, and Lysozyme g), and Galectins (Galectin-8 and Galectin-9). β-actin was used as the housekeeping gene for normalization. Significant species-specific responses were observed in N. Tilapia and E. Sea Bass when infected with V. anguillarum and S. iniae, highlighting differences in biochemical, immune, and gene expression profiles. Notably, in N. Tilapia, AST levels significantly increased by day 7 during S. iniae infection, reaching 45.00 ± 3.00 (p < 0.05), indicating late-stage acute stress or tissue damage. Conversely, E. Sea Bass exhibited a significant rise in ALT levels by day 7 in the S. iniae group, peaking at 33.5 ± 3.20 (p < 0.05), suggesting liver distress or a systemic inflammatory response. On the immunological front, N. Tilapia showed significant increases in respiratory burst activity on day 1 for both pathogens, with values of 0.28 ± 0.03 for V. anguillarum and 0.25 ± 0.02 for S. iniae (p < 0.05), indicating robust initial immune activation. Finally, the gene expression analysis revealed a pronounced peak of TNF-α in E. Sea Bass by day 7 post V. anguillarum infection with a fold change of 6.120, suggesting a strong species-specific pro-inflammatory response strategy. Understanding these responses provides critical insights for enhancing disease management and productivity in aquaculture operations.

Outbreaks of diseases in farmed fish remain a recurring problem despite the development of vaccines and improved hygiene standards on aquaculture farms. One commonly observed bacterial disease in tropical aquaculture of the South-East Asian region is tenacibaculosis, which is attributed to members of the genus Tenacibaculum (family Flavobacteriaceae, phylum Bacteroidetes), most notably Tenacibaculum maritimum. The impact of tenacibaculosis on the fish microbiota remains poorly understood. In this study, we analysed the microbiota of different tissues of commercially reared Asian seabass (Lates calcarifer) that showed symptoms of tenacibaculosis and compared the microbial communities to those of healthy and experimentally infected fish that were exposed to diseased farmed fish. The relative abundance of Tenacibaculum species in experimentally infected fish was significantly lower than in commercially reared diseased fish and revealed a higher prevalence of different Tenacibaculum species. One isolated strain, TLL-A2T, shares 98.7% 16S rRNA gene identity with Tenacibaculum mesophilum DSM 13764T. The genome of strain TLL-A2T was sequenced and compared to that of T. mesophilum DSM 13764T. Analysis of average nucleotide identity and comparative genome analysis revealed only 92% identity between T. mesophilum DSM 13764T and strain TLL-A2T and differences between the two strains in predicted carbohydrate activating enzymes respectively. Phenotypic comparison between strain TLL-A2T and T. mesophilum DSM 13764T indicated additional differences, such as growth response at different salt concentrations. Based on molecular and phenotypic differences, strain TLL-A2T (=DSM 106434T, KCTC 62393T) is proposed as the type strain of Tenacibaculum singaporense sp. nov.

European sea bass were fed four low FM/FO (10%/6%) diets containing galactomannan oligosaccharides (GMOS), a mixture of garlic oil and labiatae plants oils (PHYTO), or a combination of both functional products (GMOSPHYTO) for 63 days before exposing the fish to an intestinal Vibrio anguillarum infection combined with crowding stress. In order to evaluate functional diets efficacy in terms of gut health maintenance, structural, cellular, and immune intestinal status were evaluated by optical and electron microscopy and gene expression analyses. A semi-automated software was adapted to determine variations in goblet cell area and mucosal mucus coverage during the challenge test. Feeding with functional diets did not affect growth performance; however, PHYTO and GMOS dietary inclusion reduced European sea bass susceptibility to V. anguillarum after 7 days of challenge testing. Rectum (post-ileorectal valve) showed longer (p = 0.001) folds than posterior gut (pre-ileorectal valve), whereas posterior gut had thicker submucosa (p = 0.001) and higher mucus coverage as a result of an increased cell density than rectum. Functional diets did not affect mucosal fold length or the grade of granulocytes and lymphocytes infiltration in either intestinal segment. However, the posterior gut fold area covered by goblet cells was smaller in fish fed GMOS (F = 14.53; p = 0.001) and PHYTO (F = 5.52; p = 0.019) than for the other diets. PHYTO (F = 3.95; p = 0.049) reduced posterior gut goblet cell size and increased rodlet cell density (F = 3.604; p = 0.068). Dietary GMOS reduced submucosal thickness (F = 51.31; p = 0.001) and increased rodlet cell density (F = 3.604; p = 0.068) in rectum. Structural TEM analyses revealed a normal intestinal morphological pattern, but the use of GMOS increased rectum microvilli length, whereas the use of PHYTO increased (p≤0.10) Ocln, N-Cad and Cad-17 posterior gut gene expression. After bacterial intestinal inoculation, posterior gut of fish fed PHYTO responded in a more controlled and belated way in terms of goblet cell size and mucus coverage in comparison to other treatments. For rectum, the pattern of response was similar for all dietary treatments, however fish fed GMOS maintained goblet cell size along the challenge test.

Vertebrate nasal microbiota (NM) plays a key role regulating host olfaction, immunity, neuronal differentiation, and structuring the epithelium. However, little is known in fish. This study provides the first comprehensive analysis of the NM in two marine fish species, the European seabass and the Atlantic cod. Given its direct environmental exposure, fish NM is likely influenced by seawater fluctuations. We analysed the community structure, specificity regarding seawater, and interindividual variability of 32–38 fish reared under ambient conditions. Additionally, we conducted an experiment to investigate the influence of acidification and a simplified heatwave on cod NM (three fish per replicate). High-throughput 16S rRNA sequencing revealed species-specific NM communities at the genus-level with Stenotrophomonas and Ralstonia dominating seabass and cod NM, respectively. This suggests potential habitat- or physiology-related adaptations. The most abundant bacterial genera in seabass NM were also present in seawater, suggesting environmental acquisition. Alpha diversity was highest in Brest seabass NM and variability greatest in Tromsø cod NM. Simulated climate change-related scenarios did not significantly alter cod NM structure. We propose a minimum of 13 cod rosettes per replicate for future studies. This research establishes a foundation for understanding marine fish NM and its response to environmental changes.

Background Apoptosis, a programmed cell death process mediated by caspases, is crucial for immunity and development. Caspases are categorized into initiator and effector types, regulating apoptosis via intrinsic and extrinsic pathways, and are also involved in pyroptosis and other cellular processes. In fish, diverse caspase genes have been identified and shown to participate in embryonic development and immune responses against bacterial infection. However, their role in largemouth bass against Aeromonas hydrophila —a pathogen causing severe intestinal inflammation and septicemia—remains unclear. This study identifies caspase genes in largemouth bass and investigates their expression and evolution post-infection, providing insights into their immune functions. Results This study presents the first comprehensive genomic analysis of caspases in largemouth bass, identifying 19 caspase genes: 3 inflammatory ( caspase-1 orthologs), 6 effector ( caspase-3/6/7 ), 8 initiator ( caspase-2/8/9/10/20 ), and 2 unclassified ( caspase-17 ) caspases. Functional and evolutionary analyses demonstrated a high level of homology between largemouth bass caspases and those of other teleosts. The sequence diversity analysis revealed that caspase genes in largemouth bass are subject to extensive mutational changes, predominantly through nonsynonymous single nucleotide variants, which may play a role in immune adaptation and resistance to disease. Alternative splicing analysis revealed multiple splicing sites and events in caspases of largemouth bass. Differential alternative splicing analysis showed significant induction of MsCASP8_1 variants after infection with Aeromonas veronii . Gene structure and sequence alignment analyses demonstrated that these isoforms result from alternative splicing events involving the 5th exon. Transcriptome profiling and qRT-PCR analyses revealed that most caspase genes in largemouth bass are responsive to Aeromonas hydrophila infection. This study systematically analyzed the characteristics of the caspase gene family in largemouth bass and identified associative patterns consistent with potential roles in innate immune responses, providing testable hypotheses for future functional validation. Conclusions This study comprehensively identified the caspase gene family in largemouth bass, analyzing their genomic organization, evolutionary relationships, sequence diversity, alternative splicing patterns, and expression responses to Aeromonas hydrophila infection. The findings provide valuable insights for further investigation into caspase-mediated apoptosis and immune mechanisms in teleost fish under Aeromonas hydrophila stress.