Bacillus subtilis AB1 controls Aeromonas infection in rainbow trout (Oncorhynchus mykiss, Walbaum)

The interactions between the host immunity and intestinal microorganisms in fish

There is a huge quantity of microorganisms in the gut of fish, which exert pivotal roles in maintaining host intestinal and general health. The fish immunity can sense and shape the intestinal microbiota and maintain the intestinal homeostasis. In the meantime, the intestinal commensal microbes regulate the fish immunity, control the extravagant proliferation of pathogenic microorganisms, and ensure the intestinal health of the host. This review summarizes developments and progress on the known interactions between host immunity and intestinal microorganisms in fish, focusing on the recent advances in zebrafish (Danio rerio) showing the host immunity senses and shapes intestinal microbiota, and intestinal microorganisms tune host immunity. This review will offer theoretical references for the development, application, and commercialization of intestinal functional microorganisms in fish. KEY POINTS: • The interactions between the intestinal microorganisms and host immunity in zebrafish • Fish immunity senses and shapes the microbiota • Intestinal microbes tune host immunity in fish.

Autochthonous probiotic bacteria improve intestinal pathology and histomorphology, expression of immune and growth-related genes and resistance against Vibrio alginolyticus in Asian seabass (Lates calcarifer)

The study isolated two strains of intestinal autochthonous bacteria Lactiplantibacillus plantarum1 (MH155966.1) (L1) and Lactiplantibacillus plantarum2 (MH105076.1) (L2) from the Choobdeh Abadan region. The aim of this study was to investigate the effects of different strains of probiotic bacteria on the growth performance, digestive enzyme activity, histopathologic and histomorphometric characterization of the intestine, expression of immune and growth related genes, and evaluate Lates calcarifer resistance against Vibrio alginolyticus. To achieve this, for each treatment 60 L. calcarifer juveniles (75 ± 12 g) were randomly distributed in three fiberglass tanks (300 L) and fed for 45 days. The treatments were established as Diet 1 (control diet); L1 (diet with Lb. plantarum isolated 1); L2 (diet with Lb. plantarum isolated 2) with a bacterial concentration of 1 × 109 CFU/g. Nine fish from each treatment were sampled and examined, after euthanasia. The fish were placed 2 cm from the beginning of the intestine for microscopic sampling of villi height, villi width and thickness of the epithelium, with 3 treatments: The result showed differences in the mean values of total weight were found at the end of the experiment. After 45 days of culture, the fish fed with L1 had higher (P < 0.05) growth performance than the other treatment groups. But at the end of the trial, in L2, the digestive enzyme activities were higher (P < 0.05) than the other treatment groups. The fishes fed diets supplemented with the L2 group, like the digestive enzyme activities test, presented an increase in the thickness of the epithelium of the intestine, and villus height, and villus width were greatest in L2. Fish feeding with L1 and L2 probiotics induced higher transcription levels of interleukin-10 (IL-10), granulocyte-macrophage colony-forming cells (GMCFC), epidermal growth factor (EGF), and Transforming Growth Factor Beta (TGF-β) genes in the gut, which may correlate with better immune and hematological parameters in these groups. The results of the challenge test revealed that the percentage of survival was significantly higher in L1 (76.2%) and L2 (80.95%) treatments than in the control (P < 0.05). These results indicate that host-derived probiotics (Lb. plantarum) have significant potential as important probiotics to enhance nutrient utilization, Digestive enzymes, and metabolism by increasing the gut surface area of Lates calcarifer juveniles at 45 days of culture.

Harnessing paraprobiotics and postbiotics for enhanced immune function in Asian seabass (Lates calcarifer): Insights into pattern recognition receptor signaling

The Asian seabass, Lates calcarifer, is a key species in Asian aquaculture due to its nutritional value and adaptability. However, disease outbreaks, particularly viral and bacterial infections, pose significant challenges to its production. Immunostimulants offer promising solutions but raise safety concerns. Paraprobiotics and postbiotics (CPP) emerge as safer alternatives, exerting health benefits without live microorganisms. This study investigated the potential of probiotic paraprobiotic and postbiotic supplements derived from Bacillus subtilis to enhance the immune response and antioxidant capacity of Asian seabass and improve their resistance to Streptococcus iniae infection. Analysis of antioxidant activity and lipid peroxidation revealed significant improvements in fish supplemented with CPP, indicating their effectiveness in mitigating oxidative stress. Immunological assays demonstrated enhanced growth performance and serum immunity, including increased alternative complement activity, immunoglobulin levels, and phagocytic activity, in supplemented fish. Furthermore, upregulated expression of proinflammatory cytokines (TNF-α, IL-6, IL-1β) and pattern recognition receptors (NLRC3, TLR22, MDA5) in immune tissues. Fish supplemented with CPP exhibited higher resistance and survival rates against S. iniae infection challenge compared to control groups. The study elucidates the mechanisms underlying the immunomodulatory effects of CPP, shedding light on their potential applications in aquaculture.

The alleviative effect of Bacillus subtilis-supplemented diet against Vibrio cholerae infection in Nile tilapia (Oreochromis niloticus)

Bacterial illness causes detrimental impacts on fish health and survival and finally economic losses for the aquaculture industry. Antibiotic medication causes microbial resistance, so alternative control strategies should be applied. In this work, we investigated the probiotic-medicated diet as an alternative control approach for antibiotics in treating Vibrio cholerae infection in Nile tilapia (Oreochromis niloticus). One hundred eighty fish (50 ± 2.5 g Mean ± SD) were allocated into six groups in glass aquariums (96 L) in triplicate for 10 days. Groups 1 (G1), G2, and G 3 were intraperitoneally (IP) injected with 0.5 mL sterilized tryptic soy broth and fed on a basal diet, basal diet contained B. subtilis (BS) (1 × 10 5 CFU/ kg-1 diet), and basal diet contained trimethoprim-sulfamethoxazole (TMP-SMX) (1.5 g/kg-1 diet), respectively. Additionally, G4, G5, and G6 were IP challenged with 0.5 mL of V. cholerae (1.5 × 107 CFU) and received the same feeding regime as G 1 to 3, respectively. The results exhibited that the V. cholera-infected fish exhibited skin hemorrhage, fin rot, and the lowest survival (63.33%). Additionally, lowered immune-antioxidant biomarkers (white blood cells count, serum bactericidal activity, phagocytic activity, phagocytic index, and lysozymes) with higher lipid peroxidation marker (malondialdehyde) were consequences of V. cholerae infection. Noteworthy, fish-fed therapeutic diets fortified with BS and TMP-SMX showed a substantial amelioration in the clinical signs and survival. The BS diet significantly improved (P < 0.05) the immune-antioxidant indices of the infected fish compared to the TMP-SMX diet. The current findings supported the use of a BS-enriched diet as an eco-friendly approach for the control of V. cholerae in O. niloticus.

Present Status, Limitations, and Prospects of Using Streptomyces Bacteria as a Potential Probiotic Agent in Aquaculture

Streptomyces is a Gram-positive bacterium, belonging to the family Streptomycetaceae and order Streptomycetales. Several strains from different species of Streptomyces can be used to promote the health and growth of artificially cultured fish and shellfish by producing secondary metabolites including antibiotics, anticancer agents, antiparasitic agents, antifungal agents, and enzymes (protease and amylase). Some Streptomyces strains also exhibit antagonistic and antimicrobial activity against aquaculture-based pathogens by producing inhibitory compounds such as bacteriocins, siderophores, hydrogen peroxide, and organic acids to compete for nutrients and attachment sites in the host. The administration of Streptomyces in aquaculture could also induce an immune response, disease resistance, quorum sensing/antibiofilm activity, antiviral activity, competitive exclusion, modification in gastrointestinal microflora, growth enhancement, and water quality amelioration via nitrogen fixation and degradation of organic residues from the culture system. This review provides the current status and prospects of Streptomyces as potential probiotics in aquaculture, their selection criteria, administrative methods, and mechanisms of action. The limitations of Streptomyces as probiotics in aquaculture are highlighted and the solutions to these limitations are also discussed.

Effects of Bacillus subtilis as a single strain probiotic on growth, disease resistance and immune response of striped catfish (Pangasius hypophthalmus)

The present study investigated the potential role of Bacillus subtilis as probiotic in striped catfish (Pangasius hypophthalmus). Fish (initial weight = 150.00±2.63g n = 180) were stocked in circular tanks. Four isonitrogenous (30%) and isolipidic (3.29%) diets were formulated having supplementation of B. subtilis at four different levels (P0; 0, P1: 1×106, P2: 1×108 and P3: 1×1010 CFU/g). Each treatment had three replicates, while each replicate had fifteen fish. The trial started on second week of July and continued for eight weeks. Growth, feed conversion ratio, crude protein content, the concentration of amylase and protease, the profile of both dispensable and non-dispensable amino acids in all four dietary groups increased with a gradual increase of B. subtilis in the diet. At the end of growth experiment, fish in all four groups were exposed to Staphylococcus aureus (5×105 CFU/ml). After S. aureus challenge, fish fed with B. subtilis responded better to damage caused by reactive oxygen species and lipid peroxidation and better survival rate. The catalase and superoxide dismutase level also increased in response to bacterial challenge in B. subtilis fed groups. On the other hand, the concentration of malondialdehyde gradually decreased in these groups (+ve P0 >P1>P2>P3). It is concluded that supplementation of B. subtilis as a probiotic improved the growth, protein content, antioxidant response and immunocompetency against S. aureus in striped catfish. The optimum dosage of B. subtilis, at a concentration of 1×1010 CFU/g, resulted in the most favorable outcomes in striped catfish. This single bacterial strain can be used as an effective probiotic in large scale production of aquafeed for striped catfish. Future studies can investigate this probiotic's impact in the intensive culture of the same species.

Synbiotic Agents and Their Active Components for Sustainable Aquaculture: Concepts, Action Mechanisms, and Applications

Aquaculture is a fast-emerging food-producing sector in which fishery production plays an imperative socio-economic role, providing ample resources and tremendous potential worldwide. However, aquatic animals are exposed to the deterioration of the ecological environment and infection outbreaks, which represent significant issues nowadays. One of the reasons for these threats is the excessive use of antibiotics and synthetic drugs that have harmful impacts on the aquatic atmosphere. It is not surprising that functional and nature-based feed ingredients such as probiotics, prebiotics, postbiotics, and synbiotics have been developed as natural alternatives to sustain a healthy microbial environment in aquaculture. These functional feed additives possess several beneficial characteristics, including gut microbiota modulation, immune response reinforcement, resistance to pathogenic organisms, improved growth performance, and enhanced feed utilization in aquatic animals. Nevertheless, their mechanisms in modulating the immune system and gut microbiota in aquatic animals are largely unclear. This review discusses basic and current research advancements to fill research gaps and promote effective and healthy aquaculture production.

Dietary Administration Effects of Exopolysaccharide Produced by Bacillus tequilensis PS21 Using Riceberry Broken Rice, and Soybean Meal on Growth Performance, Immunity, and Resistance to Streptococcus agalactiae of Nile tilapia (Oreochromis niloticus)

Overuse of antibiotics in aquaculture has generated bacterial resistance and altered the ecology. Aquacultural disease control requires an environmentally sustainable approach. Bacterial exopolysaccharides (EPSs) as bioimmunostimulants have not been extensively explored in aquaculture. This study investigated EPS produced from 5% w/v riceberry broken rice as a carbon source and 1% w/v soybean meal as a nitrogen source by Bacillus tequilensis PS21 from milk kefir grain for its immunomodulatory, antioxidant activities and resistance to pathogenic Streptococcus agalactiae in Nile tilapia (Oreochromis niloticus). The FTIR spectrum of EPS confirmed the characteristic bonds of polysaccharides, while the HPLC chromatogram of EPS displayed only the glucose monomer subunit, indicating its homopolysaccharide feature. This EPS (20 mg/mL) exhibited DPPH scavenging activity of 65.50 ± 0.31%, an FRAP value of 2.07 ± 0.04 mg FeSO4/g DW, and antimicrobial activity (14.17 ± 0.76 mm inhibition zone diameter) against S. agalactiae EW1 using the agar disc diffusion method. Five groups of Nile tilapia were fed diets (T1 (Control) = 0.0, T2 = 0.1, T3 = 0.2, T4 = 1.0, and T5 = 2.0 g EPS/kg diet) for 90 days. Results showed that EPS did not affect growth performances or body composition, but EPS (T4 + T5) significantly stimulated neutrophil levels and serum lysozyme activity. EPS (T5) significantly induced myeloperoxidase activity, catalase activity, and liver superoxide dismutase activity. EPS (T5) also significantly increased the survival of fish at 80.00 ± 5.77% at 14 days post-challenge with S. agalactiae EW1 compared to the control (T1) at 53.33 ± 10.00%. This study presents an efficient method for utilizing agro-industrial biowaste as a prospective source of value-added EPS via a microbial factory to produce a bio-circular green economy model that preserves a healthy environment while also promoting sustainable aquaculture.

Aerobiology in High Latitudes: Evidence of Bacteria Acting as Tracer of Warm Air Mass Advection reaching Northern Antarctic Peninsula

Despite the extent use of geochemical tracers to track warm air mass origin reaching the Antarctic continent, we present here evidences that microorganisms being transported by the atmosphere and deposited in fresh snow layers of Antarctic ice sheets do act as tracers of air mass advection from the Southern Patagonia region to Northern Antarctic Peninsula. We combined atmospheric circulation data with microorganism content in snow/firn samples collected in two sites of the Antarctic Peninsula (King George Island/Wanda glacier and Detroit Plateau) by using flow cytometer quantification. In addition, we cultivated, isolated and submitted samples to molecular sequencing to precise species classification. Viable gram-positive bacteria were found and recovered in different snow/firn layers samples, among dead and living cells, their number concentration was compared to northern wind component, stable isotopes of oxygen, d18O, and the concentration of crustal elements (Fe, Ti and Ca). Use of satellite images combined with air mass back-trajectory analysis obtained from the NOAA/ HYSPLIT model corroborated the results.

Eubiotic Effect of a Dietary Bio-Aqua® and Sodium Diformate (NaDF) on Salmo trutta caspius: Innate Immune System, Biochemical Indices, Antioxidant Defense, and Expression of Immunological and Growth-Related Genes

The present study investigated the effects of combined and singular oral administration of Bio-Aqua® with different dosages of sodium diformate (NaDF) on biochemical indices, innate immune responses, antioxidant effects, and expressions of immunological related genes of Caspian brown trout (Salmo trutta caspius). Fingerlings Salmo trutta caspius (n = 1800; initial weight 15 ± 3 g) were randomly allocated into five groups (120 fish group-1 in triplicates). Control diet: without any addition, G1, G2, G3, and G4 received diets containing 0.2 g kg-1 commercial probiotic Bio-Aqua® combined with 0, 0.5, 1.0, and 1.5% NaDF to the basal diet for 60 days according to recommended dosages reported in previous studies. Results indicated that serum bactericidal activity (G3 on day 60 and G1 on day 30) and classic complement in all groups (on day 60) (G1 and G2 on day 30) were significantly elevated (P < 0.05). The serum lysozyme, glucose, globulin, and albumin levels showed no significant differences between all groups compared to the control group (P > 0.05). On days 30 and 60 of the sampling, no significant difference was observed in the amount of superoxide disotase (SOD) and catalase (CAT) between the treatments (P > 0.05) but activity of malondialdehyde (MDA) was lower in G1 than the control (P < 0.05). The expression of the immune-regulating genes IL-10, IL-1β, GTP, FATP, and IGF was significantly improved in all probiotic + acidifier-treated groups (P < 0.05). The current findings showed that mixture of Bio-Aqua® and NaDF (1.5% + pro) is beneficial, as it effectively improves some immune parameters and expression of immunological and growth-related genes in Caspian brown trout.

Histomorphological Changes in Fish Gut in Response to Prebiotics and Probiotics Treatment to Improve Their Health Status: A Review

The gastrointestinal tract (GIT) promotes the digestion and absorption of feeds, in addition to the excretion of waste products of digestion. In fish, the GIT is divided into four regions, the headgut, foregut, midgut, and hindgut, to which glands and lymphoid tissues are associated to release digestive enzymes and molecules involved in the immune response and control of host-pathogens. The GIT is inhabited by different species of resident microorganisms, the microbiota, which have co-evolved with the host in a symbiotic relationship and are responsible for metabolic benefits and counteracting pathogen infection. There is a strict connection between a fish's gut microbiota and its health status. This review focuses on the modulation of fish microbiota by feed additives based on prebiotics and probiotics as a feasible strategy to improve fish health status and gut efficiency, mitigate emerging diseases, and maximize rearing and growth performance. Furthermore, the use of histological assays as a valid tool for fish welfare assessment is also discussed, and insights on nutrient absorptive capacity and responsiveness to pathogens in fish by gut morphological endpoints are provided. Overall, the literature reviewed emphasizes the complex interactions between microorganisms and host fish, shedding light on the beneficial use of prebiotics and probiotics in the aquaculture sector, with the potential to provide directions for future research.

Effect of yeast species and processing on intestinal microbiota of Atlantic salmon (Salmo salar) fed soybean meal-based diets in seawater

BACKGROUND

Yeasts are gaining attention as alternative ingredients in aquafeeds. However, the impact of yeast inclusion on modulation of intestinal microbiota of fish fed plant-based ingredients is limited. Thus, the present study investigates the effects of yeast and processing on composition, diversity and predicted metabolic capacity of gut microbiota of Atlantic salmon smolt fed soybean meal (SBM)-based diet. Two yeasts, Cyberlindnera jadinii (CJ) and Wickerhamomyces anomalus (WA), were produced in-house and processed by direct heat-inactivation with spray-drying (ICJ and IWA) or autolyzed at 50 °C for 16 h, followed by spray-drying (ACJ and AWA). In a 42-day feeding experiment, fish were fed one of six diets: a fishmeal (FM)-based diet, a challenging diet with 30% SBM and four other diets containing 30% SBM and 10% of each of the four yeast products (i.e., ICJ, ACJ, IWA and AWA). Microbial profiling of digesta samples was conducted using 16S rRNA gene sequencing, and the predicted metabolic capacities of gut microbiota were determined using genome-scale metabolic models.

RESULTS

The microbial composition and predicted metabolic capacity of gut microbiota differed between fish fed FM diet and those fed SBM diet. The digesta of fish fed SBM diet was dominated by members of lactic acid bacteria, which was similar to microbial composition in the digesta of fish fed the inactivated yeasts (ICJ and IWA diets). Inclusion of autolyzed yeasts (ACJ and AWA diets) reduced the richness and diversity of gut microbiota in fish. The gut microbiota of fish fed ACJ diet was dominated by the genus Pediococcus and showed a predicted increase in mucin O-glycan degradation compared with the other diets. The gut microbiota of fish fed AWA diet was highly dominated by the family Bacillaceae.

CONCLUSIONS

The present study showed that dietary inclusion of FM and SBM differentially modulate the composition and predicted metabolic capacity of gut microbiota of fish. The inclusion of inactivated yeasts did not alter the modulation caused by SBM-based diet. Fish fed ACJ diet increased relative abundance of Pediococcus, and mucin O-glycan degradation pathway compared with the other diets.

Effects of Bacillus spp. Mixture on Growth, Immune Responses, Expression of Immune-Related Genes, and Resistance of Nile Tilapia Against Streptococcus agalactiae Infection

The purpose of this study was to evaluate the effect of Bacillus spp. mixture (Bacillus subtilis TISTR001, Bacillus megaterium TISTR067, and Bacillus licheniformis DF001) (1 × 10⁶ CFU/g) on growth, immune parameters, immune-related gene expression, and resistance of Nile tilapia against Streptococcus agalactiae AAHM04. Fish were fed different concentrations of Bacillus spp. 0 (control; T1), 1 (T2), 3 (T3), and 5 (T4) g/kg diets for 120 days. The results showed that weight gain, average daily gain, specific growth rate, feed conversion ratio in T3 diet were significantly higher than the control group and other tested diets (p < 0.05). Immune parameters, such as myeloperoxidase and lysozyme, were significantly higher in the T3 and T4 diets compared to the control group (p < 0.05). Similarly, IL-1β and TNF-α gene expressions in the spleen of fish fed T2, T3, and T4 diets were significantly higher than the control group (p < 0.05). However, no significant differences in survival rate, hematology, blood chemical indices, malondialdehyde (MDA) levels, body chemical composition, and organosomatic indices (p > 0.05) were noticed in all treatments. No significant differences in survival rate after the challenge test with S. agalactiae AAHM04 were found in fish fed Bacillus spp. mixture diets, except for the T3 diet. These results suggest that Bacillus spp. mixture diet at 3 g/kg diet (T3) could improve growth, immune response, and disease resistance of Nile tilapia.

The Genetic and Phenotypic Diversity of Bacillus spp. from the Mariculture System in China and Their Potential Function against Pathogenic Vibrio

Bacillus spp. could be one of the most suitable substitutes for the control and prevention of aquatic diseases. The occurrence of species population, antimicrobial character, and virulence diversity in Bacillus spp. recovered from the mariculture system in China between 2009 and 2021 were investigated, screening for probiotic Bacillus strains with good biological safety that can inhibit Vibrio parahaemolyticus, V. alginolyticus, V. harveyi, V. owensii, V. campbellii. The results showed that 116 Bacillus isolates were divided into 24 species, and the top three species were B. subtilis (37/116), B. velezensis (28/116), and B. amyloliquefaciens (10/116). Among the 116 Bacillus isolates, 32.8% were effective against V. parahaemolyticus, 30.1% for V. alginolyticus, 60.3% for V. harveyi, 69.8% for V. owensii and 74.1% for V. campbellii. More than 62% of Bacillus isolates were susceptible to florfenicol, doxycycline and tetracycline, etc., and 26/116 Bacillus isolates were found to be multiple-antibiotic-resistant (MAR), with MARI values ranging from 0 to 0.06. Eighteen kinds of antibiotic resistance genes were tested; only tetB, blaTEM, and blaZ were detected. And 9 isolates in 2 Bacillus species were excluded by 6/10 kinds of Bacillus-related toxin gene (hblA, hblC, nheB, nheC, entFM, cykK). Bio-safety testing indicated that three kinds of probiotics were good probiotic candidates to prevent Vibriosis. These results provide comprehensive genetic diversity, potential risks, and probiotic characteristics of Bacillus in the mariculture system in China, and provide basic support for green and healthy development of aquatic industry.

Exopolysaccharides-Mediated ZnO Nanoparticles for the Treatment of Aquatic Diseases in Freshwater Fish Oreochromis mossambicus

Bacterial fish disease outbreaks are a key concern for aquaculture. Complementary feed additives such as immunostimulants can serve as an ideal solution for disease prevention. Herein, we scrutinized the efficacy of exopolysaccharides (EPSs) from probiotic Bacillus licheniformis and EPS-mediated zinc oxide nanoparticles (EPS-ZnO NPs) for a diet to evaluate growth parameters, antioxidant enzyme activities, and immune stimulation together with disease resistance against Aeromonas hydrophila and Vibrio parahaemolyticus in Mozambique tilapia Oreochromis mossambicus. Fish were separated into seven groups, with six experimental groups fed with EPS and EPS-ZnO NPs at 2, 5, and 10 mg/g and a control fed a basal diet. The fish ingesting feed supplemented with EPS and EPS-ZnO NPs at 10 mg/g showed improved growth performance. Cellular and humoral-immunological parameters were tested in serum and mucus after 15 and 30 days of feeding. These parameters were substantially enriched with a 10 mg/g diet (p < 0.05) of EPS and EPS-ZnO NPs in comparison with the control. Furthermore, the EPS and EPS-ZnO NP supplemental diet actively enhanced the antioxidant response (glutathione peroxidase, superoxide dismutase, and catalase). In addition, the supplemental diet of EPS and EPS-ZnO NPs lowered the death rate and improved the disease resistance of O. mossambicus following assessment with A. hydrophila and V. parahaemolyticus at 50 µL. Hence, the overall results suggest that the supplemental diet of EPS and EPS-ZnO NPs might be used to ensure aquaculture feed additives.

Bacillus Species as Direct-Fed Microbial Antibiotic Alternatives for Monogastric Production

Antibiotic growth promoters have been utilized for long time at subtherapeutic levels as feed supplements in monogastric animal rations. Because of their side-effects such as antibiotic resistance, reduction of beneficial bacteria in the gut, and dysbiosis, it is necessary to look for non-therapeutic alternatives. Probiotics play an important role as the key substitutes to antibacterial agents due to their many beneficial effects on the monogastric animal host. For instance, enhancement of the gut microbiota balance can contribute to improvement of feed utilization efficiency, nutrients absorption, growth rate, and economic profitability of livestock. Probiotics are defined as "live microorganisms that, when administered in adequate amounts, confer a health benefit on the host." They are available in diverse forms for use as feed supplements. Their utilization as feed additives assists in good digestion of feed ingredients and hence, making the nutrients available for promoting growth. Immunity can also be enhanced by supplementing probiotics to monogastrics diets. Moreover, probiotics can help in improving major meat quality traits and countering a variety of monogastric animals infectious diseases. A proper selection of the probiotic strains is required in order to confer optimal beneficial effects. The present review focuses on the general functional, safety, and technological screening criteria for selection of ideal Bacillus probiotics as feed supplements as well as their mechanism of action and beneficial effects on monogastric animals for improving production performance and health status.

Individual and combined effects of the dietary Spirulina platensis and Bacillus licheniformis supplementation on growth performance, antioxidant capacity, innate immunity, relative gene expression and resistance of goldfish, Carassius auratus to Aeromonas hydrophila

This study evaluated the individual and combined effects of the dietary Spirulina platensis (SP) and probiotic bacterium Bacillus licheniformis (BL) on the growth performance, immune responses, and disease resistance in goldfish (Carassius auratus). A total of 216 fish (3.39 ± 0.24 g) were randomly distributed in 12 tanks with 18 fish per tank (4 treatments with 3 replications) and fed with diets containing 0% S. platensis and B. licheniformis (T0), 10⁸ CFU/g B. licheniformis (T1), 2.5% S. platensis (T2), and 10⁸ CFU/g B. licheniformis + 2.5% S. platensis (T3(. There were no significant differences in growth parameters. The alternative complement pathway (ACH50) and lysozyme activity were significantly increased in T2 and T3 treatments. No marked differences were observed in total immunoglobulin and protease activity among treatments (P > 0.05). The relative expression of IGF-1 was not affected by experimental diets (P > 0.05). Ghrelin gene showed significantly higher mRNA levels in fish fed with SP and BL (P < 0.05). The relative expression of catalase (CAT), and glutathione reductase (GSR) significantly increased in fish fed with the SP and BL (P < 0.05). No marked difference in glutathione peroxidase (GPX) gene expression was seen between the treatments (P > 0.05). The mRNA levels of lysozyme, IL6, IL-1β, TGF, and TNF2 transcription were higher in fish fed with SP and BL (P < 0.05). No notable difference was observed in TNF1 and IL10 gene expression between treatments (P > 0.05). Moreover, the result of the challenge test with A. hydrophila showed that goldfish fed with SP and BL had a lower mortality rate than the control. In conclusion, the supplementation of SP and BL can be used as feed additives to enhance disease resistance against A. hydrophila infection by stimulating the immune system in goldfish.

Isolation of Chitinolytic Bacteria from European Sea Bass Gut Microbiota Fed Diets with Distinct Insect Meals

Insect meal (IM), recently authorized for use in aquafeeds, positions itself as a promising commodity for aquafeed inclusion. However, insects are also rich in chitin, a structural polysaccharide present in the exoskeleton, which is not digested by fish, resulting in lower fish performance. Through the application of a dietary pressure, this study aimed to modulate European sea bass gut microbiota towards the enrichment of chitinolytic bacteria to allow the isolation of novel probiotics capable of improving the use of IM-containing diets, overcoming chitin drawbacks. Five isoproteic (44%) and isolipidic (18%) diets were used: a fish meal (FM)-based diet (diet CTR), a chitin-supplemented diet (diet CHIT5), and three diets with either 25% of Hermetia illucens and Tenebrio molitor larvae meals (HM25 and TM25, respectively) or H. illucens exuviae meal (diet HEM25) as partial FM substitutes. After an 8-week feeding trial, the results showed a clear modulatory effect towards spore-forming bacteria by HM25 and HEM25 diets, with the latter being responsible for the majority of the chitinolytic fish isolates (FIs) obtained. Sequential evaluation of the FI hemolytic activity, antibiotic resistance, total chitinolytic activity, sporulation, and survival in gastrointestinal-like conditions identified FI645 and FI658 as the most promising chitinolytic probiotics for in vivo application.

Bacillus subtilis Expressing the Infectious Pancreatic Necrosis Virus VP2 Protein Retains Its Immunostimulatory Properties and Induces a Specific Antibody Response

Bacillus subtilis has been documented in the past years as an effective probiotic for different aquacultured species, with recognized beneficial effects on water quality, fish growth and immune status. Furthermore, its potential as a vaccine adjuvant has also been explored in different species. In the current work, we have used B. subtilis spores as delivery vehicles for the presentation of the VP2 protein from infectious pancreatic necrosis virus (IPNV). For this, the VP2 gene was amplified and translationally fused to the crust protein CotY. The successful expression of VP2 on the spores was confirmed by Western blot. We then compared the immunostimulatory potential of this VP2-expressing strain (CRS208) to that of the original B. subtilis strain (168) on rainbow trout (Oncorhynchus mykiss) leukocytes obtained from spleen, head kidney and the peritoneal cavity. Our results demonstrated that both strains significantly increased the percentage of IgM⁺ B cells and the number of IgM-secreting cells in all leukocyte cultures. Both strains also induced the transcription of a wide range of immune genes in these cultures, with small differences between them. Importantly, specific anti-IPNV antibodies were detected in fish intraperitoneally or orally vaccinated with the CRS208 strain. Altogether, our results demonstrate B. subtilis spores expressing foreign viral proteins retain their immunomodulatory potential while inducing a significant antibody response, thus constituting a promising vaccination strategy.

Inhibitory effect of probiotic Bacillus spp. isolated from the digestive tract of Rhynchocypris Lagowskii on the adhesion of common pathogenic bacteria in the intestinal model

Diseases of fish caused by pathogenic bacteria are an important constraint on aquaculture production. Antibiotics have been widely used to control infectious diseases, but this has led to the emergence of drug-resistant bacteria and affected human health. In this context, probiotics are used as an alternative to antibiotics for the prevention and control of diseases in aquaculture. The aim of this study was to obtain probiotic candidate strains of Bacillus spp. from the gut of Rhynchocypris Lagowskii. Strains were screened by enzyme-producing ability, antagonism assay and antibiotic susceptibility. The safety of the strains to host fish has also been established. The isolated Bacillus licheniformis (LSG1-1) and Bacillus subtilis (LSG2-1) were characterized and performed well in tolerance experiments. In addition, LSG1-1 and LSG2-1 were detected to have higher self-aggregation ability and surface hydrophobicity. In the in vitro adhesion model, LSG1-1 and LSG2-1 showed good adhesion ability and had obvious adhesion inhibitory effect on three pathogens of Aeromonas. Based on the characteristics observed so far, Bacillus licheniformis LSG1-1 and Bacillus subtilis LSG2-1 could form potential probiotic candidates in the digestive tract of R. lagowskii to help combat diseases in aquaculture.

The optimized inclusion level of Bacillus subtilis fermented Azolla pinnata in Nile tilapia (Oreochromis niloticus) diets: immunity, antioxidative status, intestinal digestive enzymes and histomorphometry, and disease resistance

A 95-day feeding trial was conducted to evaluate the outcomes of feeding Bacillus subtilis fermented Azolla (BSFA) on nonspecific immunity, antioxidative status, intestinal digestive enzymes and histomorphometry, and disease resistance in the Nile tilapia. We formulated five isonitrogenous and isocaloric experimental diets to incorporate BSFA at level of (0%, 15%, 30%, 45%, 60%). The growth performance parameters (FBW, BWG, SGR, PER, and FCR) revealed a significant increase in the BSFA30 tilapia group compared to the control group followed by BSFA45 (P < 0.05). The BSFA30 group exhibited the highest nonspecific immunity parameters including (lysozyme activity, phagocytic index, and phagocytic activity) compared to other groups (P < 0.05). SOD and GPx reported the highest values in the BSFA60 group. Nile tilapia carcass composition was not influenced by BSFA inclusion level (P > 0.05). Interestingly, Nile tilapia fed with BSFA15 diet exhibited the highest protease activity level (P < 0.05), while those fed on BSFA30 documented the highest amylase activity. Intestinal histomorphology was significantly enhanced with the gradual increase of administrated BSFA. Regarding the tilapia disease resistance against Aeromonas septicemia, BSFA significantly diminished the cumulative mortality compared to the control group. To sum up, BSFA was more effective in improving the growth performance and immunity of Nile tilapia.

High antimicrobial activity of lactoferricin-expressing Bacillus subtilis strains

The lactoferricin expressed in Bacillus subtilis is relatively low in yield, making it hard to apply in industrial settings. We constructed a six tandem repeat of lactoferricin cDNA driven by promoter PtrnQ. After transformation, two transformants P245 and P263 possessing a stable inheritance of plasmid and high expression of lactoferricin were selected. The bactericidal activities, 1 μl of aliquot of a total 5.5 ml of solution extracted from 5 ml of cultured P245 and P263, were equivalent to the efficacy of 238.25 and 322.7 ng of Ampicillin against Escherichia coli, respectively, and 366.4 and 452.52 ng of Ampicillin against Staphylococcus epidermidis respectively. These extracts were able to kill an Ampicillin‐resistant E. coli strain. The bactericidal activities of P245 and P263 equivalent to the efficacy of Tetracycline against Vibrio parahaemolyticus and V. alginolyticus were also determined. Moreover, the bactericidal activities of P245 and P263 were 168.04 and 249.94 ng of Ampicillin against Edwardsiella tarda, respectively, and 219.7 and 252.43 ng of Tetracycline against Streptococcus iniae respectively. Interestingly, the survival rate of E. tarda‐infected tilapia fry fed the P263 extract displayed a significantly greater than that of the fry‐fed control strain. Collectively, these B. subtilis transgenic strains are highly promising for use in animal husbandry during a disease outbreak.

Influence of extracellular protein isolated from fish gut associated bacteria as an enhancer of growth and innate immune system in Mugil cephalus

The cultural microbiomes of 27 bacteria colonies were isolated from Mugil cephalus for analysis of the antibacterial and antagonistic activities. A potent probiotic bacterium was characterized using16S r RNA sequencing. The potent strain was added to fish diet to perform the challenge test and to study the growth and immunological parameter. The extracellular proteins from the probiotic were collected and characterized using MALDI TOF/TOF. Out of G27, G9 strain inhibited all the five pathogenic strains. An isolated bacterium was identified as Bacillus subtilis PRBD09 with accession number KF765648. After 35 days of feeding period B. subtilis PRBD09 enhance the both cellular and humoral immune responses, which responsible for survive of the Mugil cephalus against Aeromonas hydrophila infection. The MALDI TOF sample 08 and 09 were recognized as hypothetical proteins based on the MALDI TOF sample. A cytidinedeaminase was found in samples 10, 11, and 12. Extracellular proteins may be involved for the immunological increase in Mugil cephalus against Aeromonas hydrophila, according to the current research.

Colonization of Lactobacillus rhamnosus GG in Cirrhinus molitorella (Mud Carp) Fingerling: Evidence for Improving Disease Resistance and Growth Performance

The use of probiotic bacteria can not only enhance the nutritional utilization of fish feeds to produce more biomass but can also provide a practically “safer” alternative to the fish farming industry to reduce the abuse of antibiotics and drugs. This study investigated the possibility of colonizing Lactobacillus rhamnosus strain GG (LGG) to the intestine of Cirrhinus molitorella (mud carp) fingerling. Colonization of LGG was observed in gut tissue after 14 days of administration with a diet supplemented with 1 × 108 CFU/mL LGG. Moreover, growth performance parameters of the LGG-supplemented diet group, including relative weight gain, feed conversion ratio and feed efficiency, were found about two-fold higher than the control group after 60 days. In addition, fish fed with an LGG-supplemented diet for 60 days showed substantial resistance against the infection of pathogenic bacterial Aeromonas hydrophila, with a relative survival rate of up to 57% compared to the control group. In summary, the results indicated that LGG as dietary supplement for mud carp fingerling can enhance nutrition utilization and better protect fish against the infection of Aeromonas hydrophila. The results provide an insight to the fish farming industry, encouraging a reduction in the use of antibiotics and drugs and the production of “safer” mud carp for the market at a manageable cost.

In vitro modulation of gilthead seabream (Sparus aurata L.) leukocytes by Bacillus spp. extracellular molecules upon bacterial challenge

Stimulation of the fish immune system using immunostimulants is an environmentally friendly strategy to minimize bacterial outbreaks in aquaculture. Different biological and synthetic immunostimulants can enhance non-specific innate immune responses by directly activating immune cells. An example are Bacillus spp., known for their immunostimulatory effects, although the exact mechanisms by which Bacillus spp. offer protection against diseases remains to be elucidated. Furthermore, most studies have focused on Bacillus spp. cells, while the immunostimulant effect of their extracellular metabolome, known to harbour biologically important metabolites, including antimicrobial molecules, has been scarcely evaluated. Here, we evaluated the in vitro immune-modulatory properties of extracellular extracts of three Bacillus spp. strains (B. subtilis FI314, B. vezelensis FI436 and B. pumilus FI464), previously isolated from fish-guts and characterized for their in vitro and in vivo antimicrobial activity against a wide range of fish pathogens. Bacillus spp. extracellular extracts did not affect immune cells viability, but remarkably increased pathogens' phagocytosis when seabream head-kidney leukocytes were challenged with Vibrio anguillarum and Edwardsiella tarda. All extracts significantly increased the engulfment of bacterial pathogens 1 h post-infection. Cells stimulated with the extracellular extracts showed an up-regulation of the expression of immune-relevant genes associated with inflammation, including IL-1β, IL-6, and COX-2. In cells challenged with E. tarda, FI314 extracellular extract significantly increased the expression of IL-1β, IL-6, and COX-2, while FI436 and FI464 significantly increased IL-6 expression. The results of this study revealed that the extracellular molecules from Bacillus spp. fish isolates improved the in vitro response of gilthead seabream immune cells and are thus promising candidates to act as immunostimulants, helping fish fight diseases.

Stabilized fermentation product of Cetobacterium somerae improves gut and liver health and antiviral immunity of zebrafish

Probiotics are widely used in aquafeeds and exhibited beneficial effects on fish by improving host health and resisting pathogens. However, probiotics applied to aquaculture are mainly from terrestrial sources instead of the host animal. The purpose of the work was to evaluate the effects of stabilized fermentation product of commensal Cetobacterium somerae XMX-1 on gut, liver health and antiviral immunity of zebrafish. A total of 240 zebrafish were assigned to the control (fed a basal diet) and XMX-1 group (fed a basal diet with 10 g XMX-1/kg diet). After four weeks feeding, growth performance, feed utilization, hepatic steatosis score, TAG, lipid metabolism related genes and serum ALT were evaluated. Furthermore, serum LPS, the expression of Hif-1α, intestinal inflammation score, antioxidant capability and gut microbiota were tested. The survival rate and the expression of antiviral genes were analyzed after challenge by spring viremia of carp virus (SVCV). Results showed that dietary XMX-1 did not affect growth of zebrafish. However, dietary XMX-1 significantly decreased the level of serum LPS, intestinal inflammation score and intestinal MDA, as well as increased T-AOC and the expression of Hif-1α in zebrafish intestine (p < 0.05). Furthermore, XMX-1 supplementation decreased the relative abundance of Proteobacteria and increased Firmicutes and Actinobacteria. Additionally, XMX-1 supplementation significantly decreased hepatic steatosis score, hepatic TAG, serum ALT and increased the expression of lipolysis genes versus control (p < 0.05). Zebrafish fed XMX-1 diet exhibited higher survival rate after SVCV challenge. Consistently, dietary XMX-1 fermentation product increased the expression of IFNφ2 and IFNφ3 after 2 days of SVCV challenge and the expression of IFNφ1, IFNφ2 and MxC after 4 days of SVCV challenge in the spleen in zebrafish versus control (p < 0.05). In conclusion, our results indicate that dietary XMX-1 can improve liver and gut health, while enhancing antiviral immunity of zebrafish.

Efficiency of a probiotic in carp lactococcosis in an in vitro experiment

The purpose of this article is to study the effect of the probiotic on experimental infections of carp's fingerlings with Lactococcus garvieae. Lactic acid bacteria (LAB) (Lactobacillus fermentum 24с, Pediococcus pentosaceus 10/9к, Lactobacillus paracasei 9c) for the probiotic were previously isolated from the intestines of mature carps from Maybalyk commercial fisheries, which provided fingerlings for this experiment too as well. The feed-contained probiotic was given to fish in the experimental group for 14 days before challenge with pathogen L. garvieae. Throughout ten days after the infection, death of the fish was regularly recorded in the group, where the probiotic was not given with the feeding. Ten days after, all fish in this group died. In the probiotic group, the mortality on the tenth day after the challenge with pathogen was 10%. It was concluded the effect of the probiotic is not due to antibacterial action to the pathogen. The effectiveness of the probiotic can be associated with the displacement of the pathogen, due to the competitive adhesion and/or more likely, with the activation of the immune response from the fish organism due to the addition of the probiotic to the feed.

Probiotics for periodontal health-Current molecular findings

Dysbiosis of the oral microbiome is associated with a variety of oral and systemic diseases, including periodontal disease. Oral dysbiosis in periodontal disease leads to an exacerbated host immune response that induces progressive periodontal tissue destruction and ultimately tooth loss. To counter the disease‐associated dysbiosis of the oral cavity, strategies have been proposed to reestablish a “healthy” microbiome via the use of probiotics. This study reviews the literature on the use of probiotics for modifying the oral microbial composition toward a beneficial state that might alleviate disease progression. Four in vitro and 10 preclinical studies were included in the analysis, and these studies explored the effects of probiotics on cultured biofilm growth and bacterial gene expressions, as well as modulation of the host response to inflammation. The current molecular findings on probiotics provide fundamental evidence for further clinical research for the use of probiotics in periodontal therapy. They also point out an important caveat: Changing the biofilm composition might alter the normal oral flora that is beneficial and/or critical for oral health.

The potentials of secondary metabolites from Bacillus cereus SN7 and Vagococcus fluvialis CT21 against fish pathogenic bacteria

One of the major factors that affect the total production of fisheries is the declining number of catches and aquaculture production due to the high pathogenicity in aquatic environment. This enforces the need to find anti-pathogenic agents that could solve the problem. In addition, the application of potential Bacillus cereus SN7 and Vagococcus fluvialis CT21 isolated and identified from the sea water of Siak, Riau, Indonesia need to be optimally exploited. The aim of this study, therefore, is to determine the component of bioactive compounds present in Bacillus cereus SN7 and Vagococcus fluvialis CT21, and also to explore their intrinsic potential as a biological control agent in fisheries, especially for inhibiting the growth of pathogenic bacteria (Vibrio alginolyticus, Aeromonas hydrophila and Pseudomonas aeruginosa). The method used was experimental, where the ethyl acetate crude extracts of both samples were analyzed for their phytochemical content, followed by thin layer chromatography analysis and Liquid Chromatography Mass Spectrometry. In addition, anti-pathogenic activity test was performed using the Kirby-Bauer method, minimum inhibitory and bactericidal concentration analysis. The results showed alkaloids, flavonoids, and saponins were the potential bioactive components in the crude extracts of Bacillus cereus SN7 and Vagococcus fluvialis CT21. Furthermore, the anti-pathogenic activity test demonstrated the ability for both bacteria to inhibit three types of pathogens with the following inhibitory zone values: Vibrio alginolyticus (10-11 mm), Aeromonas hydrophila (8-12 mm), and Pseudomonas aeruginosa (8-10 mm). In conclusion secondary metabolite compounds produced by Bacillus cereus SN7 and Vagococcus fluvialis CT21 possess the capacity to inhibit pathogenic bacteria. Hence, both samples are potential candidates for anti-pathogen development, especially in fisheries.

Fishing for the right probiotic: Host-microbe interactions at the interface of effective aquaculture strategies

Effective aquaculture management strategies are paramount to global food security. Growing demands stimulate the intensification of production and create the need for practices that are both economically viable and environmentally sustainable. Importantly, pathogenic microbes continue to be detrimental to fish growth and survival. In terms of host health, the intestinal mucosa and its associated consortium of microbes have a critical role in modulating fitness and present an attractive opportunity to promote health at this interface. In light of this, the administration of probiotic microorganisms is being considered as a means to restore and sustain health in fish. Current evidence suggests that certain probiotic strains might be able to augment immunity, enhance growth rate, and protect against infection in salmonids, the most economically important family of farmed finfish. This review affirms the relevance of host-microbe interactions in salmonids in light of emerging evidence, with an emphasis on intestinal health. In addition, the current understanding of the mode of action of probiotics in salmonid fish is discussed, along with delivery systems that can effectively carry the living microbes.

Mechanisms Used by Probiotics to Confer Pathogen Resistance to Teleost Fish

Probiotics have been defined as live microorganisms that when administered in adequate amounts confer health benefits to the host. The use of probiotics in aquaculture is an attractive bio-friendly method to decrease the impact of infectious diseases, but is still not an extended practice. Although many studies have investigated the systemic and mucosal immunological effects of probiotics, not all of them have established whether they were actually capable of increasing resistance to different types of pathogens, being this the outmost desired goal. In this sense, in the current paper, we have summarized those experiments in which probiotics were shown to provide increased resistance against bacterial, viral or parasitic pathogens. Additionally, we have reviewed what is known for fish probiotics regarding the mechanisms through which they exert positive effects on pathogen resistance, including direct actions on the pathogen, as well as positive effects on the host.

Scope of Archaea in Fish Feed: a New Chapter in Aquafeed Probiotics?

The outbreak of diseases leading to substantial loss is a major bottleneck in aquaculture. Over the last decades, the concept of using feed probiotics was more in focus to address the growth and health of cultivable aquatic organisms. The objective of this review is to provide an overview of the distinct functionality of archaea from conventional probiotics in nutrient utilization, specific caloric contribution, evading immune response and processing thermal resistance. The prime limitation of conventional probiotics is the viability of desired microbes under harsh feed processing conditions. To overcome the constraints of commercial probiotics pertaining to incompatibility towards industrial processing procedure, a super microbe, archaea, appears to be a potential alternative approach in aquaculture. The peculiarity of the archaeal cell wall provides them with heat stability and rigidity under industrial processing conditions. Besides, archaea being one of the gut microbial communities participates in various health-oriented biological functions in animals. Thus, the current review devoted that administration of archaea in aquafeed could be a promising strategy in aquaculture. Archaea may be used as a potential probiotic with the possible modes of functions and advantages over conventional probiotics in aquafeed preparation. The present review also provides the challenges associated with the use of archaea for aquaculture and a brief outline of the patents on archaea to highlight the various use of archaea in different sectors.

Isolation and Characterization of a Bacillus velezensis D-18 Strain, as a Potential Probiotic in European Seabass Aquaculture

Within the food-producing sectors, aquaculture is the one that has developed the greatest growth in recent decades, currently representing almost 50% of the world's edible fish. The diseases can affect the final production in intensive aquaculture; in seabass, aquaculture vibriosis is one of the most important diseases producing huge economical losses in this industry. The usual methodology to solve the problems associated with the bacterial pathology has been the use of antibiotics, with known environmental consequences. This is why probiotic bacteria are proposed as an alternative fight against pathogenic bacteria. The aim of this study was to analyse a strain of Bacillus velezensis D-18 isolated from a wastewater sample collected from a fish farm, for use as probiotics in aquaculture. The strain was evaluated in vitro through various mechanisms of selection, obtaining as results for growth inhibition by co-culture a reduction of 30%; B. velezensis D-18 was able to survive at 1.5-h exposure to 10% seabass bile, and at pH 4, its survival is 5% and reducing by 60% the adhesion capacity of V. anguillarum 507 to the mucus of seabass and in vivo by performing a challenge. Therefore, in conclusion, we consider B. velezensis D-18 isolate from wastewater samples collected from the farms as a good candidate probiotic in the prevention of the infection by Vibrio anguillarum 507 in European seabass after in vitro and biosafety assays.

Intestinal microbial communities of rainbow trout (Oncorhynchus mykiss) may be improved by feeding a Hermetia illucens meal/low-fishmeal diet

With demands and reliance on aquaculture still growing, there are various challenges to allow sustainable growth and the shift from fishmeal (FM) to other protein sources in aquafeed formulations is one of the most important. In this regard, interest in the use of insect meal (IM) in aquafeeds has grown rapidly. Accordingly, the aim of the present study was to assess the effects of dietary IM from Hermetia illucens (Hi) larvae included in a low-FM diet on gut microbial communities of rainbow trout (Oncorhynchus mykiss), in terms of both composition and function of microbiome. A feeding trial was conducted using 192 trout of about 100-g mean initial weight. Fish were fed in quadruplicate (4 tanks/diet) for 131 days with two diets: the control (Ctrl) contained 20% of FM as well as other protein sources, whereas the Hi diet contained 15% of Hi larvae meal to replace 50% of the FM contained in the Ctrl diet. High-throughput sequencing of 16S rRNA gene was used to identify the major feed and gut bacterial taxa, whereas Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt) analysis was performed on gut bacterial genomes to identify the major active biological pathways. The inclusion of IM led to an increase in Firmicutes, mainly represented by Bacilli class and to a drastic reduction of Proteobacteria. Beneficial genera, such as Lactobacillus and Bacillus, were enriched in the gut of fish fed with the Hi diet, whereas the number of bacteria assigned to the pathogenic Aeromonas genus was drastically reduced in the same fish group. The metagenome functional data provided evidence that dietary IM inclusion can shape the metabolic activity of trout gut microbiota. In particular, intestinal microbiome of fish fed with IM may have the capacity to improve dietary carbohydrate utilization. Therefore, H. illucens meal is a promising protein source for trout nutrition, able to modulate gut microbial community by increasing the abundance of some bacteria taxa that are likely to play a key role in fish health.

Effects of Potential Probiotic Enterococcus casseliflavus (EC-001) on Growth Performance, Immunity, and Resistance to Aeromonas hydrophila Infection in Common Carp (Cyprinus carpio)

The effects of different levels of dietary Enterococcus casseliflavus (EC-001), as a potential probiotic, were investigated on the growth performance, hemato-biochemical parameters, immune responses, and resistance to Aeromonas hydrophila infection in common carp (Cyprinus carpio) fingerlings. Accordingly, fish (N = 720; 12.0 ± 0.5 g) were distributed into four treatments receiving different dietary levels of E. casseliflavus, EC-001 (0 [control], 1 × 10⁷, 10⁸, and 10⁹ CFU g^-1 feed), for 8 weeks. The fish fed with a diet containing 10⁹ CFU g^-1 showed the highest weight gain and specific growth rate, along with the lowest feed conversion ratio, compared with the control group (P < 0.05). Red and white blood cells, hemoglobin, hematocrit, neutrophils, and monocytes significantly increased in the fish fed with 1 × 10⁸ and 10⁹ CFU g^-1 (P < 0.05). Dietary inclusion of 1 × 10⁸ and 10⁹ CFU g^-1 significantly increased serum total protein, albumin, and immunoglobulin content (P < 0.05). Feeding the fish with 1 × 10⁹ CFU g^-1 resulted in a significant increase in serum and skin mucus lysozyme activity compared with the other groups (P < 0.05). Complement component 3 and skin mucus protease activity were also significantly higher in all the fish treated with dietary E. casseliflavus (EC-001) compared with the control group (P < 0.05). The cumulative mortality in the treated fish was lower (ranging from 10 to 22%) than the control group (31%) after challenging the fish with A. hydrophila infection, while the fish fed with E. casseliflavus (EC-001) at 1 × 10⁹ CFU g^-1 exhibited the lowest mortality rate (P < 0.05). In conclusion, our results revealed the potential probiotic effects of E. casseliflavus (EC-001) for enhancing growth performance, immunity, and disease resistance of common carp.

The use of probiotics in animal feeding for safe production and as potential alternatives to antibiotics

Although the production of safe food for human consumption is the primary purpose for animal rearing, the environment and well-being of the animals must also be taken into consideration. Based on microbiological point of view, the production of healthy food from animals involves considering foodborne pathogens, on the one hand and on the other hand, the methods used to fight against germs during breeding. The conventional method to control or prevent bacterial infections in farming is the use antibiotics. However, the banning of these compounds as growth promoters caused many changes in animal breeding and their use has since been limited to the treatment and prevention of bacterial infections. In this function, their importance no longer needs to be demonstrated, but unfortunately, their excessive and abusive use have led to a double problem which can have harmful consequences on consumer health: Resistance to antibiotics and the presence of antibiotic residues in food. The use of probiotics appears to be a suitable alternative to overcome these problems because of their ability to modulate the immune system and intestinal microflora, and further considering their antagonistic role against certain pathogenic bacteria and their ability to play the role of growth factor (sometimes associated with prebiotics) when used as feed additives. This review aims to highlight some of the negative effects of the use of antibiotics in animal rearing as well as emphasize the current knowledge on the use of probiotics as a feed additive, their influence on animal production and their potential utility as an alternative to conventional antibiotics, particularly in poultry, pig, and fish farming.

The effect of Pediococcus acidilactici on mucosal immune responses, growth, and reproductive performance in zebrafish (Danio rerio)

A completely randomized experimental design carried out to investigate the effects of different levels of Pediococcus acidilactici (PA) including 0 (basal diet as a control diet), 1 × 10⁶, 2 × 10⁶, 4 × 10⁶, and 8 × 10⁶ colony-forming unit (CFU) per gram of the diet for 60 days on the mucosal immunity responses, growth, and reproductive performance, in zebrafish, Danio rerio (with mean weigh ± SE: 120 ± 10 mg). The obtained results revealed that the best growth and reproduction indices were related to the concentration of 4 × 10⁶ CFU PA g^-1 diet (P < 0.05). The maximum activities of mucosal immune responses including total protein, alternative complement system, IgM, and lysozyme were observed in the fish fed with 4 × 10⁶ CFU PA g^-1 diet (P < 0.05). Furthermore, the maximum alkaline phosphatase activity of skin mucus was recorded in the fish fed with 8 × 10⁶ CFU PA g^-1 diet (P < 0.05). Fish fed with 4 × 10⁶ CFU PA g^-1 diet had the highest villus length and width of the intestine (P < 0.05). Supplementing the diet with 4 × 10⁶ CFU PA g^-1 diet more significantly enhanced Cyp19a gene expression in comparison with this in other groups. Hence, PA with a concentration of 4 × 10⁶ CFU g^-1 diet can be considered as a proper level of probiotic for improving the health, growth, and reproductive performance of the D. rerio.

Isolation of probiotics and their effects on growth, antioxidant and non-specific immunity of sea cucumber Apostichopus japonicus

Probiotics play vital roles in controlling diseases, enhancing specific and non-specific immunity and stimulating growth in the aquaculture industry. However, the effect of fermentation of feed by probiotics on the immune ability of sea cucumber has not been reported to date. Here, three candidate probiotic strains (Bacillus species) were isolated from the culture seawater and sediment of sea cucumber, and fishmeal and scallop mantle fermented by the candidate probiotic strains were used to feed sea cucumber. The results showed that the free amino acid and small peptide contents of the fishmeal and scallop mantle were significantly increased after fermentation for 72 h. However, the weight gain (WG) and specific growth rate (SGR) of sea cucumber showed no significant differences among the fermented fishmeal, fermented scallop mantle and control groups. Scallop mantle fermented by the three candidate probiotics could increase the coelomocyte number and respiratory burst activity. The immune-related enzymatic activity was increased after consuming the fermented fishmeal and scallop mantle, while the activity of antioxidant enzymes was reduced. The expression levels of immune- and antioxidant-related genes were changed after consuming the fermented fishmeal and scallop mantle. Taken together, our results suggest that probiotics could increase the immunocompetence of sea cucumber, and fermented scallop mantle might be a potential substitute for fishmeal during feed preparation. Our results lay a foundation for further understanding the relationship between probiotics and the non-specific immunity of sea cucumber.

Salmonid Antibacterial Immunity: An Aquaculture Perspective

The aquaculture industry is continuously threatened by infectious diseases, including those of bacterial origin. Regardless of the disease burden, aquaculture is already the main method for producing fish protein, having displaced capture fisheries. One attractive sector within this industry is the culture of salmonids, which are (a) uniquely under pressure due to overfishing and (b) the most valuable finfish per unit of weight. There are still knowledge gaps in the understanding of fish immunity, leading to vaccines that are not as effective as in terrestrial species, thus a common method to combat bacterial disease outbreaks is the use of antibiotics. Though effective, this method increases both the prevalence and risk of generating antibiotic-resistant bacteria. To facilitate vaccine design and/or alternative treatment efforts, a deeper understanding of the teleost immune system is essential. This review highlights the current state of teleost antibacterial immunity in the context of salmonid aquaculture. Additionally, the success of current techniques/methods used to combat bacterial diseases in salmonid aquaculture will be addressed. Filling the immunology knowledge gaps highlighted here will assist in reducing aquaculture losses in the future.

Improvement of growth, intestinal short-chain fatty acids, non-specific immunity and ammonia resistance in Pacific white shrimp (Litopenaeus vannamei) fed dietary water-soluble chitosan and mixed probiotics

This study was to explore the impacts of water-soluble chitosan and mixed probiotics on growth performance, intestinal short-chain fatty acids (SCFAs) and immunity and ammonia resistance in Litopenaeus vannamei. Shrimp were fed one of four experimental diets including basal diet (CON), 0.10% water-soluble chitosan diet (WSC), 0.30% mixed probiotics (MP) and 0.10% water-soluble chitosan +0.30% mixed probiotics (SYN) for 8 weeks. Results showed shrimp fed with dietary MP and SYN diets could significantly improve growth performance and feed utilization in comparison with those of shrimp fed with CON diet (P < 0.05). Acetic acid content was significantly higher in shrimp fed with all supplemented diets compared to that in shrimp fed with CON diet (P < 0.05). Compared to shrimp fed with CON diet, dietary WSC and MP significantly influenced the contents and/or activities of aspartate aminotransferase (AST), total protein (TP), superoxide dismutase (SOD), lysozyme (LZM) in serum, SOD, malondialdehyde (MDA), acid phosphatase (ACP) in hepatopancreas and SOD and MDA in intestine. In addition, the gene expression levels of prophenoloxidase (proPO), penaiedin 3a (Pen-3a), crustin (Crustin), serine proteinase (SP), GPX and SOD in hepatopancreas, were significantly upregulated compared to those in CON diet at some time points (P < 0.05). Significantly higher survival rate in all supplemented diets were observed after ammonia challenge (P < 0.05). Therefore, the above results indicated dietary WSC and MP or SYN could enhance intestinal SCFAs content, stimulated antioxidant capacity and immune response, and increase the ammonia resistance of Litopenaeus vannamei. Besides, the growth performance was also improved by dietary MP and SYN.

Dietary administration of β-1,3/1,6-glucan and Lactobacillus plantarum improves innate immune response and increases the number of intestine immune cells in roach (Rutilus rutilus)

BACKGROUND

The aim of the study has been to compare the effect of dietary supplementation of β-1,3/1,6-glucan, Lactobacillus plantarum bacteria or their mixture on the growth performance, selected parameters of the immune system as well as the liver and intestinal histology of roach. Fish were fed for 14 days with four different diets, each treatment being carried out in triplicate. In control group, fish were fed dry commercial starter feed Aller Performa 2 (Aller Aqua, Denmark). The other experimental fish groups received the same commercial starter feed supplemented with: 1% β-1,3/1,6-glucan (Leiber® Beta-S) in group G; 108 cfu L. plantarum g- 1 in group L; 1% β-1,3/1,6-glucan + 108 cfu L. plantarum g- 1 in group G + L. The stimulating effect of the tested preparations was evaluated once the feeding with commercial feed exclusively was resumed and 2 weeks afterwards.

RESULTS

No effect on the survivability and growth performance of the fish was observed in any of the groups. Supplementation of feed with β-1,3/1,6-glucan improved (P < 0.05) selected parameters of innate humoral immunity and the pinocytotic activity of phagocytes. Increased respiratory burst activity of head kidney phagocytes (RBA) was observed in groups L and G + L (P < 0.05), and the effect persisted for 2 weeks after the commercial feed regime was resumed. An analogous tendency was determined for the killing activity of phagocytes (PKA) of the head kidney with respect to Aeromonas hydrophila, although this effect appeared only during the feed supplementation period. Supplying roach with β-1,3/1,6-glucan, singly or with L. plantarum, had no effect (P > 0.05) on the proliferation of mitogen-activated lymphocytes. However, an increase in the number of CD3-positive cells and goblet cells was noticed in the digestive system of the L group fish (P < 0.05).

CONCLUSIONS

The results show that feeding fish with added L. plantarum and β-1,3/1,6-glucan stimulates the non-specific resistance mechanisms and raises the counts of intestinal immune cells. Synbiotic may help to control serious bacterial diseases and offer an alternative to antibiotics commonly used in fish farming, and its prolonged immunostimulatory effect could increase fish surviving after release to the natural environment.

Mechanisms and the role of probiotic Bacillus in mitigating fish pathogens in aquaculture

Diseases are natural components of the environment, and many have economic implications for aquaculture and fisheries. Aquaculture is a fast-growing industry with the aim to meet the high protein demand of the ever-increasing global population; however, the emergence of diseases is a major setback to the industry. Probiotics emerged as a better solution to curb the disease problem in aquaculture among many alternatives. Probiotic Bacillus has been proven to better combat a wide range of fish pathogens relative to other probiotics in aquaculture; therefore, understanding the various mechanisms used by Bacillus in combating diseases will help improve their mode of action hence yielding better results in their combat against pathogens in the aquaculture industry. Thus, an overview of the mechanisms (production of bacteriocins, suppression of virulence gene expression, competition for adhesion sites, production of lytic enzymes, production of antibiotics, immunostimulation, competition for nutrients and energy, and production of organic acids) used by Bacillus probiotics in mitigating fish pathogens ranging from Aeromonas, Vibrio, Streptococcus, Yersinia, Pseudomonas, Clostridium, Acinetobacter, Edwardsiella, Flavobacterium, white spot syndrome virus, and infectious hypodermal and hematopoietic necrosis virus proven to be mitigated by Bacillus have been provided.

Effects of hydrolyzed fish protein and autolyzed yeast as substitutes of fishmeal in the gilthead sea bream (Sparus aurata) diet, on fish intestinal microbiome

Background

This study evaluated the effects of partial substitution of dietary fishmeal (FM) with either fish protein hydrolysate (FPH) or autolysed dried yeast (HiCell®, Biorigin, Brazil) on intestinal microbiota of gilthead sea bream (Sparus aurata). A total number of 720 fish of 122.18 ± 6.22 g were fed for 92 days with three different diets in triplicate (3 tanks/diet). A diet based on FM/vegetable meal was used as control. The other two diets were formulated by replacing FM with 5% of either FPH or HiCell®. To analyze the gut microbiota associated to autochthonous and allochthonous microbial communities, the Illumina MiSeq platform for sequencing of 16S rRNA gene and QIIME pipeline were used.

Results

A total number of 102 OTUs (operational taxonomic units) at 97% identity were identified in fish gut samples collected at the end of feeding trial. Fourteen OTUs constituted the core gut microbiota, i.e. those OTUs found in at least nine out of fifteen samples per group and shared regardless of the diet. Eight OTUs were assigned to Firmicutes represented by Lactobacillus, Staphylococcus, and Bacillus genera, and six to Proteobacteria phylum. Dietary dried yeast autolysate modulated the intestinal microbiota by promoting the growth of some beneficial bacteria. At order level, fish fed yeast showed an enrichment in Bacillales and Clostridiales as compared to the control group, whereas fish fed FPH showed a significantly lower amount of bacteria belonging to Alteromonadales and Enterobacteriales than the other two feeding groups. Although we did not observe any effect of 5% FM replacement with alternative nitrogen sources at phylum level, at lower taxonomical levels, the composition of gut microbiota, in terms of relative abundance of specific taxa, was significantly influenced by the dietary treatment.

Conclusions

The metabarcoding analysis revealed a clearly intestinal microbiota modulation in response to dietary autolyzed yeast. The abundance of some beneficial bacteria, i.e. indigestible carbohydrate degrading- and SCFA producing bacteria, was positively affected. Brewer’s yeast autolysate could be a valid alternative protein source to FM as well as a valid functional ingredient for aquafeed production.

Influence of skin wounds on the intestinal inflammatory response and barrier function: Protective role of dietary Shewanella putrefaciens SpPdp11 administration to gilthead seabream (Sparus aurata L.)

The effects of skin wounds on the intestinal barrier function and the beneficial effects of the dietary administration of Shewanella putrefaciens (known as SpPdp11) in gilthead seabream (Sparus aurata L.) were studied. Two replicates of fish were fed a commercial diet (control, CON) or CON diet enriched with 10⁹ cfu g^-1 SpPdp11 (SP diet) for 30 days. After this time, half of the fish were sampled, while the others were injured below the lateral line (wounded fish, W) and fed the same diets for an extra week before sampling (CON + W and SP + W groups). The intestinal histology and gene expression of different genes relevant for the intestinal barrier function were studied. The results showed that injured fish had a disordered enterocyte nucleus disposition, a more intense infiltration of mixed leucocytes and a thicker lamina propria in the intestine compared to the control fish. However, the fish in the SP + W group did not present these pathological symptoms in the intestine. No significant variations in the number of goblet cells were detected among the different experimental groups. Pro-inflammatory cytokines (colony-stimulating factor receptor 1, CSF1R, myeloperoxidase, MPO and interleukin-1β, IL-1β), mucins (intestinal mucin, IMUC and mucin 2, MUC2), and immunoglobulin T heavy chain (IGHT) were up-regulated, while tight junction protein occludin was down-regulated in the intestine from fish of the CON + W group. Similarly, the dietary administration of SpPdp11 markedly depressed the gene expression of pro-inflammatory cytokines, MUC2 and IGHT, but increased the gene expression of anti-inflammatory cytokine transforming growth factor-β1 (TGF-β1) and the tight junction proteins tricellulin and occluding after wounding. In brief, the skin wounds provoked an intestinal inflammatory response that included changes in the mucus layer and tight junction disruptions. Besides this, preventive administration of SpPdp11 alleviated the intestinal dysfunctions caused by skin wounds in gilthead seabream.

Effects of three host-associated Bacillus species on mucosal immunity and gut health of Nile tilapia, Oreochromis niloticus and its resistance against Aeromonas hydrophila infection

Skin and intestinal mucosa lymphoid tissues are known to be the fish's first line of defence since they serve as the first point of contact for pathogens. Only few studies have investigated the influence of host-associated Bacillus on mucosal immunity. In this study, the effects of three host-associated Bacillus species on mucosal immunity, intestinal morphology, intestinal digestive enzymes activity, intestinal microbiome and resistance of Nile tilapia against Aeromonas hydrophila infection was evaluated. The fish were divided into five treatment groups and fed with diets containing no bacteria denoted as Control, Bacillus velezensis TPS3N denoted as group V, Bacillus subtilis TPS4 denoted as group S, Bacillus amyloliquefaciens TPS17 denoted as group A and a 5^th group containing the three Bacillus species at a ratio 1:1:1 denoted as group CB. At the end of the feeding trial, significant enhancement of both skin mucus and intestinal immune titres were recorded in terms of nitric oxide (NO) (except in the mucus of V and S groups), immunoglobulin M (IgM) (except in the intestine of group V), lysozyme (LZM), and alkaline phosphatase (AKP) in all fish fed the Bacillus supplemented groups relative to the untreated group. Intestinal antioxidant enzymes (catalase (CAT) (except in the intestine of group S) and superoxide dismutase (SOD)) capacity of Nile tilapia were higher in the Bacillus groups. Intestinal lipase activity was elevated in the Bacillus supplemented groups. The intestinal morphological parameters (villus height, villus width, goblet cells count (except in group S and A), and intestinal muscle thickness) were significantly enhanced in the Bacillus supplemented groups relative to the Control group. Dietary probiotic supplementation also influenced the intestinal microflora composition of Nile tilapia. Proteobacteria recorded the highest abundance followed by Firmicutes, Fusobacteria, and Bacteroidetes at the phylum level in this study. At the genus level, the abundance of pathogenic bacteria viz Staphylococcus and Aeromonas were reduced in the Bacillus supplemented groups in comparison to the Control group. A challenge test with A. hydrophila resulted in lower mortalities (%) in the Bacillus treated groups thus 86.67%, 50.00%, 43.33%, 63.33%, and 30.00% for Nile tilapia fed Control, V, S, A, and CB diets respectively. In conclusion, the inclusion of B. velezensis TPS3N, B. subtilis TPS4, and B. amyloliquefaciens TPS17 in the diet of Nile tilapia singularly or in combination, could enhance the mucosal immunity, intestinal health, and resistance of Nile tilapia against A. hydrophila infection.

Effects of Bacillus subtilis WB60 and Lactococcus lactis on Growth, Immune Responses, Histology and Gene Expression in Nile tilapia, Oreochromis niloticus

An eight-week feeding trial was conducted to evaluate the effects of a basal control diet (CON), Bacillus subtilis at 107 (BS7) and at 108 CFU/g diet (BS8), Lactococcus lactis at 107 CFU/g (LL7) and at 108 CFU/g diet (LL8), and oxytetracycline (OTC) at 4 g/kg diet on Nile tilapia. Fish with initial body weight of 2.83 ± 0.05 g (mean ± SD) were fed two times a day. Weight gain, specific growth rate, feed efficiency, protein efficiency ratio and lysozyme activity of fish fed BS8, LL8 and LL7 diets were significantly higher than those of fish fed CON diet (p < 0.05). Superoxide dismutase and myeloperoxidase activity of fish fed BS8, LL8, BS7, LL7 and OTC diets were significantly higher than those of fish fed CON diet. Intestinal villi length and muscular layer thickness of fish fed BS8, LL8 and LL7 diets were significantly higher than those of fish fed CON and OTC diets. Also, heat shock protein 70 (HSP70), interleukin (IL-1β), interferon-gamma (IFN-γ) and tumour necrosis factor (TNF-α) gene expression of fish fed BS8 and LL8 diets were significantly higher than those of fish fed CON diet. After 13 days of challenge test, cumulative survival rate of fish fed BS8 and LL8 diets were significantly higher than those of fish fed CON, BS7 and OTC diets. Based on these results, B. subtilis and L. lactis at 108 (CFU/g) could replace antibiotics, and have beneficial effects on growth, immunity, histology, gene expression, and disease resistance in Nile tilapia.

Effects of Bacillus lincheniformis feeding frequency on the growth, digestion and immunity of Haliotis discus hannai

To study the effects of Bacillus lincheniformis feeding frequency on the survival and growth of Haliotis discus hannai abalone, we measured the expression levels of nonspecific immune genes and monitored the anti-Vibrio parahaemolyticus immune reaction. H. discus hannai (shell length: 32.75 ± 2.63 mm, body weight: 4.91 ± 0.34 g) was selected to perform a 70 d laboratory culture experiment including a 14 d V. parahaemolyticus artificial infection experiment. The control group (C) was fed normal commercial feed every day. The M1 experimental group was given experimental feed and basal feed on alternating days until the end of the experiment. The M2 experimental group was given experimental feed for 4 d and basal feed for 3 d, and this cycle was repeated every 7 d until the end of the experiment. The M3 experimental group was given experimental feed for 2 d and basal feed for 5 d, and this cycle was repeated every 7 d until the end of the experiment. The M4 group was continuously given experimental feed for the duration of the experiment. The concentration of added B. lincheniformis in each experimental group was 10⁵ cfu/g (according to the quantity of viable bacteria). The specific growth rate (as measured by body weight) and the feed conversion efficiency of the abalone in M1 and M2 were significantly higher than those in M4 and C (P < 0.05). The cellulose and lipase activities of abalone in M1, M2 or M4 were significantly higher than those in M3 or C (P < 0.05). The acid phosphatase, superoxide dismutase, total haemocyte counts, O₂^- levels generated by respiratory bursts, and the expression levels of Mn-SOD, TP_x, GST_s and GST_m in abalone in the M2 group were significantly higher than those in any other feeding frequency group (P < 0.05). At the end of the V. parahaemolyticus infection, the cumulative mortality of the abalone in M2 was significantly lower than that in any other group (P < 0.05). Consequently, given the growth advantages and the enhancement of immune function, the feeding plan in which B. lincheniformis was applied for 4 d per week, and basal feed was then applied for 3 d, did not lead to a high level of immune reaction, immune fatigue or waste of resources, but increased the growth rate of individuals and their resistance to V. parahaemolyticus infection.

Effects of the probiotic Bacillus amyloliquefaciens on the growth, immunity, and disease resistance of Haliotis discus hannai

The effects of a diet containing the probiotic Bacillus amyloliquefaciens on the survival and growth of Haliotis discus hannai were evaluated by measuring growth and hematological parameters and the expression levels of nonspecific immune genes. In addition, the abalone's response to Vibrio parahaemolyticus infection was assessed. H. discus hannai (shell length: 29.35 ± 1.81 mm, body weight: 4.28 ± 0.23 g) were exposed to an 8-week culture experiment in indoor aquariums and a 2-week V. parahaemolyticus artificial infection experiment. In each experiment, the control group (C) was fed daily with the basal feed; the experimental groups were fed daily with the experimental feed, prepared by spraying B. amyloliquefaciens onto the basal feed at final concentrations of 10³ (group A1), 10⁵ (A2), and 10⁷ (A3) cfu/g. The survival rate, body weight specific growth rate, and food conversion efficiency in A2 and A3 were significantly higher than those in A1 and C (P < 0.05). The total number of blood lymphocytes, the O₂^- and NO levels produced from respiratory burst, the activities of acid phosphatase, superoxide dismutase, and catalase, and the expression levels of catalase and thiol peroxidase in A2 were not significantly different from those in A3, but these factors were significantly higher in A2 compared to A1 and C (P < 0.05). The total antioxidant capacity and expression levels of glutathione S-transferase in A1, A2 and A3 were significantly higher than those in C (P < 0.05). At day 9 after infection with V. parahaemolyticus, all abalone in C were dead; at the end of the experiment, the cumulative mortality of abalone in A2 was significantly lower than that in any other group (P < 0.05). Thus, the experimental feed containing 10⁵ cfu/g B. amyloliquefaciens not only facilitated the food intake and growth of abalone, but also effectively enhanced their non-specific immunity and resistance to V. parahaemolyticus infection. In this regard, B. amyloliquefaciens may be a useful probiotic strain for abalone aquaculture.