Effects of dietary chitosan and Bacillus subtilis on the growth performance, non-specific immunity and disease resistance of cobia, Rachycentron canadum

Potential biocontrol for bacterial and viral disease treatment in aquaculture: a minireview

Aquaculture is part of the crucial industry that supplies food, especially for the global human population that is gradually increasing annually. Innovations of culture techniques have been improved throughout the years but aquaculture is regularly susceptible to bacterial and viral diseases. Numerous factors could contribute to occurrence of disease and usually they are from environmental or human stressors on the cultured animals. Synthetic chemicals in commercial treatments may yield fast results however, the side effects are usually unknown until it has taken effect. Therefore, biological control methods to treat diseases in aquaculture are preferred. This mini review provides an overview of different potential biocontrol practices for treatment of bacterial and viral diseases. Bacteriophage causes death of pathogenic bacteria by killing the cell and continue to multiply until all targeted pathogenic bacteria are eliminated. Probiotic, prebiotic, synbiotic, biofloc, and immunostimulants are beneficial products from the respective organisms that are effective in inhibiting pathogens. Vaccines introduce inactivated pathogen into the body to stimulate the immune system, while genetic modifications involve alteration and selection of disease resistant genetics.

Chitin, chitosan and chitooligosaccharides as potential growth promoters and immunostimulants in aquaculture: A comprehensive review

There is a stable growth in aquaculture production to avoid seafood scarcity. The usage of eco-friendly feed additives is not only associated with aquatic animal health but also reduces the risk of deleterious effects to the environment and consumers. Aquaculture researchers are seeking dietary solutions to improve the growth performance and yield of target organisms. A wide range of naturally derived compounds such as probiotics, prebiotics, synbiotics, complex carbohydrates, nutritional factors, herbs, hormones, vitamins, and cytokines was utilized as immunostimulants in aquaculture. The use of polysaccharides derived from natural resources, such as alginate, agar, laminarin, carrageenan, fucoidan, chitin, and chitosan, as supplementary feed in aquaculture species has been reported. Polysaccharides are prebiotic substances which are enhancing the immunity, disease resistance and growth of aquatic animals. Further, chitin (CT), chitosan (CTS) and chitooligosaccharides (COS) were recognized for their biodegradable properties and unique biological functions. The dietary effects of CT, CTS and COS at different inclusion levels on growth performance, immune response and gut microbiota in aquaculture species has been reviewed. The safety regulations, challenges and future outlooks of CT, CTS and COS in aquatic animals have been discussed in this review.

Fructooligosaccharide and Bacillus subtilis synbiotic combination promoted disease resistance, but not growth performance, is additive in fish

Species diversification from major to minor carps for their sturdiness and initial higher growth, and also a quest for antibiotic-free aqua farming in the subcontinent, mandates search for and evaluation of alternatives. An experiment was performed to investigate the potential of fructooligosaccharide (FOS) and Bacillus subtilis (BS) (alone or as synbiotics) in promoting growth and immunity against infections in Labeo fimbriatus fingerlings. Six iso-nitrogenous and iso-lipidic diets containing combinations of two levels of FOS (0% and 0.5%) and three levels of BS (0, 104, 106 CFU/g feed) were fed to fish for 60 days. At the end of the feeding trial, twenty-four fish from each group were injected intra-peritoneally with pathogenic strain of Aeromonas hydrophila O:18 to test the immunoprotective efficacy of the supplements against bacterial infection. BS, but not FOS, significantly improved (P < 0.05) growth and feed utilisation attributes like percentage weight gain (PWG), specific growth rate (SGR) and feed conversion ratio (FCR). There were interactive effects of FOS and BS on PWG, SGR and FCR; however, the effects were not additive in nature. These beneficial effects of BS, alone or in combination with FOS, were corroborated by increased protease activity, microvilli density and diameter and number of goblet cells. Overall beneficial effects of FOS and BS included improved erythrocyte (RBC), hemoglobin (Hb), total protein and globulin levels. Total leucocyte (WBC) count and immunological parameters like respiratory burst activity of leucocytes (NBT reduction), lysozyme activity, albumin: globulin ratio and post-challenge survival were significantly improved by both FOS and BS, and their dietary combination yielded the highest improvement in these parameters. Synergistic effects of FOS and BS as dietary supplements indicate that a combination of 106 CFU/g BS and 0.5% FOS is optimal to improve growth, feed utilisation, immune functions, and disease resistance in L. fimbriatus fingerlings.

Synthesis of L-methionine-loaded chitosan nanoparticles for controlled release and their in vitro and in vivo evaluation

Therapeutically popular controlled release-enabling technology has forayed into the nutrition sector. Polymer coated forms of L-methionine used in soy protein diets, and its intermediate metabolite, S-adenosyl-L-methionine, used in myriad of medical conditions have proved more efficacious over (highly catabolized) free forms. In this premier study, L-methionine-loaded chitosan nanoparticles (M-NPs) were synthesized using ionic gelation method and their efficacy was evaluated. Biophysical characterization of the NPs was done using a Nanopartica SZ 100 analyser, transmission electron microscopy, and Fourier transform infrared spectroscopy. The M-NPs were spherical and smooth and 218.9 ± 7.4 nm in size and in vitro testing confirmed the controlled release of methionine. A 60-days feeding trial in L. rohita fish fingerlings was conducted. A basal diet suboptimal (0.85%) in methionine was provided with one of the supplements as under: none (control), 0.8% chitosan NPs (0.8% NPs), 1.2% L-methionine (1.2% M) (crystalline free form), 0.6% M-NPs and 1.2% M-NPs. While the addition of 0.6% M-NPs to the basal diet complemented towards meeting the established dietary requirement and resulted in significantly highest (P < 0.05) growth and protein efficiency and sero-immunological test scores (serum total protein, serum globulin, serum albumin: globulin ratio, phagocytic respiratory burst/NBT reduction and lysozyme activity), 1.2% supplementation in either form (free or nano), for being 0.85% excess, was counterproductive. Liver transaminases and dehydrogenases corroborated enhanced growth. It was inferred that part of the methionine requirement in nano form (M-NPs) can confer intended performance and health benefits in animals relying on plant proteins-based diets limiting in this essential amino acid. The study also paves the way for exploring chitosan NPs-based sustained delivery of amino acids in human medical conditions.

Effects of supplemental fulvic acid on survival, growth performance, digestive ability and immunity of large yellow croaker (Larimichthys crocea) larvae

A 30-day feeding trial was designed to evaluate the effect of supplemental fulvic acid (FA) on survival, growth performance, digestive ability and immunity of large yellow croaker (Larimichthys crocea) larvae (initial body weight 11.33 ± 0.57 mg). Four isonitrogenous and isolipids diets containing 0.00%, 0.01%, 0.02% and 0.04% FA were formulated, respectively. Results showed that the supplementation of 0.04% FA significantly improved survival rate of large yellow croaker larvae. Meanwhile, supplemental FA significantly increased final body weight and specific growth rate. Based on the specific growth rate, the optimal supplementation was 0.0135% FA. Larvae fed the diet with 0.01% FA had significantly higher villus height than the control. The supplementation of 0.01%–0.02% FA significantly increased the muscular thickness of intestine. Moreover, supplementation of FA significantly increased mRNA expression of intestinal epithelial proliferation and barrier genes (pcna, zo-1 and zo-2). Diets supplemented with 0.02%–0.04% FA significantly increased the activity of trypsin in the intestinal segment, while 0.01%–0.02% FA significantly increased the activity of trypsin in the pancreatic segment. Compared with the control, supplementation of FA remarkably increased activities of alkaline phosphatase and leucine aminopeptidase in the brush border membrane of intestine. Larvae fed the diet with 0.01% FA significantly increased activities of lysozyme and total nitric oxide synthase. Furthermore, the supplementation of 0.01% to 0.02% FA significantly decreased the mRNA expression of pro-inflammatory cytokines (tnf-α and il-6). Concurrently, supplemental FA significantly increased anti-inflammatory cytokine (il-10) mRNA expression level. In conclusion, this study indicated that the supplementation of FA could improve the survival rate and growth performance of larvae by promoting intestinal development, digestive enzymes activities and innate immunity.

Bacillus subtilis Effects on Growth Performance and Health Status of Totoaba macdonaldi Fed with High Levels of Soy Protein Concentrate

T. macdonaldi is a carnivorous species endemic to the Gulf of California. Indiscriminate exploitation has put totoaba at risk, inducing the development of aquaculture procedures to grow it without affecting the wild population. However, aquafeeds increasing cost and low yields obtained with commercial feeds have motivated researchers to look for more nutritious and cheaper alternatives. Soybean (SB) is the most popular alternative to fishmeal (FM); however, antinutritional factors limit its use in carnivorous species. In this study, we analyzed B. subtilis 9b probiotic capacity to improve growth performance and health status of T. macdonaldi fed with formulations containing 30% and 60% substitution of fish meal with soy protein concentrate (SPC). In addition, we investigated its effect on internal organs condition, their capacity to modulate the intestinal microbiota, and to boost the immunological response of T. macdonaldi against V. harveyi infections. In this sense, we found that T. macdonaldi fed with SPC30Pro diet supplemented with B. subtilis 9b strain and 30% SPC produced better results than SPC30C control diet without B. subtilis and DCML commercial diet. Additionally, animals fed with SPC60Pro diet supplemented with B. subtilis 9b strain and 60% SPC doubled their weight and produced 20% more survival than SPC60C control diet without B. subtilis. Thus, B. subtilis 9b improved T. macdonaldi growth performance, health status, modulated intestinal microbiota, and increased animal's resistance to V. harveyi infections, placing this bacterium as an excellent candidate to produce functional feeds with high levels of SPC.

Biotherapeutic microbial supplementation for ameliorating fish health: developing trends in probiotics, prebiotics, and synbiotics use in finfish aquaculture

Nutrition demands in aquaculture can be realized through quality aquafeeds as compounded diets that contribute to the growth and health of aquaculture species. Functional additives in feed, notably probiotics, prebiotics, and their admixture synbiotics, have been recently recognized for their biotherapeutic role as immunostimulants capable of conferring disease resistance, stress tolerance, and gastrointestinal health; counteracting the negative effects of anti-nutrients, pathogenic prevalence, and antimicrobials in finfish aquaculture. Formulated diets based on probiotics, prebiotics, and as a supplemental combination for synbiotics can significantly influence fish gut microbiomes, establishing the modalities of microbial dynamics to maximize host-associated benefits. These microbial functional-feed supplements are acclaimed to be biocompatible, biodegradable, and safe for dietary consumption as well as the environment. In fed fish aquaculture, prebiotic appended probiotic diet 'synbiotic' has propounded larger attention for its additional health and nutritional benefits. Synbiotic, prebiotic, and probiotic usage as functional feeds for finfish aquaculture thus provides promising prospects. Developing trends in their intended application are reviewed here forth.

Effects of dietary Bacillus amyloliquefaciens on the growth, immune responses, intestinal microbiota composition and disease resistance of yellow catfish, Pelteobagrus fulvidraco

The influence of dietary probiotic Bacillus amyloliquefaciens on the growth performance, digestive enzyme activity, immune parameters and disease resistance of yellow catfish (Pelteobagrus fulvidraco) was evaluated. Commercial diet (C) or diet containing 106 cfu/g B. amyloliquefaciens (T) was fed for 4 weeks, and final weight (FW), specific growth rate (SGR) and feed conversion ratio (FCR) were improved (p<0.05) in the T group. Dietary B. amyloliquefaciens increased protease and amylase activities in the digestive tract after 2 and 4 weeks, respectively. Respiratory burst (RB), plasma lysozyme (LZM) activity, total antioxidant capacity (T-AOC) and superoxide dismutase (SOD) activity were also elevated (p<0.05). Immune-related genes signal transducer and activator of transcription 1 (STATA-1), immunoglobulin M (IgM) and C-type lectin (CTL) were upregulated (p<0.05), but interleukin-1 β (IL-1β) was not (p >0.05). Intestinal microbiota analysis showed that the community structure was significantly different between the two groups; the relative abundance of Cetobacterium was increased but Plesiomonas was decreased in T. Moreover, challenge tests showed that the resistance of fish fed B. amyloliquefaciens against Aeromonas veronii and Edwardsiella ictaluri was significantly enhanced (p<0.05). In conclusion, dietary supplementation of B. amyloliquefaciens can effectively improve the growth performance, digestive enzyme activity, immune responses, intestinal microbiota composition and disease resistance of yellow catfish.

Microbiome composition and autochthonous probiotics from contrasting probiosis/dysbiosis states in cobia (Rachycentron canadum) fish epitheliocystis

Microbiome components and bacterial isolates related to healthy and epitheliocystis states in aquaculture cycles of cobia fish were studied. We detected well-defined 16S rRNA amplicon gene sequence variants showing differential abundance in healthy or diseased cycles. Isolation trials were performed, and experimental tests were used to determine probiotic potential of the bacterial strains obtained from water, tissues or live food used in this aquaculture model. The taxonomic affiliation of these strains was cross-compared against microbiome components, finding that some of them had close or identical affiliation to the abundant types found in healthy cycles. Strains belonging to the groups already identified as predominant by culture-independent means were screened as potential probiotics based on desirable activities such as antagonism and antibiosis against marine pathogenic bacteria, quorum quenching, bile acid resistance, antibiotic sensitivity and enzymatic activities for improved nutrient digestion. We have also found that in the tracking of microbiome composition across different developmental stages of cobia, healthy cycles exhibited a consistent high relative abundance of a Mesobacillus sp., while in the diseased cycle the emergence of a Vibrio sp. was observed. Our study suggests that epithelocystis in cobia is associated with a displacement of a symbiotic microbiome community linked to the increase frequency of Vibrio species.

Modulation of growth performance, antioxidant capacity, non-specific immunity and disease resistance in largemouth bass (Micropterus salmoides) upon compound probiotic cultures inclusion

An 8-week feeding trial was conducted to evaluate the effects of dietary supplementation of compound probiotic cultures (CPC; Bacillus subtilis, Lactobacillus plantarum and Saccharomyces cerevisiae) on the growth performance, antioxidant capacity, non-specific immunity and disease resistance of juvenile largemouth bass. Triplicate groups of largemouth bass (average weight 42.05 ± 0.02 g), with a destiny of 30 individuals per tank, were fed diets supplemented with different concentration of compound probiotic cultures (CPC) (0%, CPC (0.0); 0.5%, CPC (0.5); 1.0%, CPC (1.0); 2.0%, CPC (2.0)). After the feeding trial, tissue samples of largemouth bass were collected and the challenge test with Aeromonas hydrophila was performed. Results indicated that the CPC supplementation produced no significant difference on the growth performance, feed utilization and body composition of largemouth bass, while significantly increased the cumulative survival rate in the Aeromonas hydrophila challenge test. Meanwhile, the inclusion of CPC elevated the hepatic antioxidant capacity, and the highest activity of antioxidant enzymes, including T-AOC, CAT, GPx and T-SOD, was observed in the CPC (2.0) group. Meanwhile, the transcription of Nrf2/keap1 and antioxidant related genes, including CAT, GPx, GST, SOD1 and SOD2, was significantly elevated with the inclusion of CPC. In addition, the inclusion of CPC improved the non-specific immunity of largemouth bass. The activity of serum lysozyme was significantly elevated in the CPC (2.0) group, while the transcription of RelA and pro-inflammatory factors, including TNF-α and IL-1β, was inhibited with the inclusion of CPC. Meanwhile, related genes potentially linked to RelA, including TLR2 and p38 MAPK, were detected that their relative expression was significantly inhibited with the inclusion of CPC. The current findings indicated that the inclusion of 2% CPC improved the antioxidant capacity, non-specific immunity and disease resistance of juvenile largemouth bass, and suggested that 2% CPC as a functional additive could be applied to the diet of juvenile largemouth bass in aquaculture practice.

Dietary chitosan modulates the growth performance, body composition and nonspecific immunity of juvenile yellow catfish (Pelteobagrus fulvidraco)

This study investigated the effects of feeding different concentrations of chitosan on the growth performance, body composition and non-specific immune function of juvenile yellow catfish (Pelteobagrus fulvidraco). Four kinds of experimental diets were respectively prepared by adding 0 (control group), 5, 10 and 15 g/kg of chitosan to the basal feed and fed to juvenile yellow catfish for 8 weeks. Results show that the body weight gain rate, specific growth rate, survival rate, body protein content, serum superoxide dismutase activity, catalase activity, glutathione peroxidise activity, lysozyme activity and disease resistance ability against Aeromonas hydrophila of the experimental group with chitosan added to its diet were significantly higher than those of the control group optimally by 36.22 %, 14.37 %, 9.46 %, 8.97 %, 50.89 %, 33.15 %, 21.52 %, 40.80 %, 41.09 %, and 79.71 %, respectively (P < 0.05). No significant differences in feed efficiency among all groups (P > 0.05) were observed. The optimum dose of dietary chitosan required for the maximum growth of juvenile yellow catfish was 8.95 g/kg. Therefore, adding an appropriate amount of chitosan (8.95 g/kg) to the feed of yellow catfish can significantly improve its growth performance, ameliorate body composition and enhance its non-specific immunity.

Boosted growth performance, immunity, antioxidant capacity and disease resistance of crucian carp (Carassius auratus) by single or in combination dietary Bacillus subtilis and xylo-oligosaccharides

In this study, a total of 420 healthy crucian carp (9.77 ± 0.04 g) were randomly divided into CK, B·S, XOS and B·S + XOS group, and cultured for 8 weeks. Results showed that the dietary Bacillus subtilis (B. subtilis) and xylo-oligosaccharides (XOS) can significantly increased the final weight, weight gain, specific growth rate, feed efficiency, protein efficiency and survival rate of crucian carp. Dietary B. subtilis and XOS can significantly increased the activities of catalase, glutathione, superoxide dismutase and total antioxidant capacity, significantly decreased the contents of malondialdehyde, and significantly increased the activities of alkaline phosphatase, acid phosphatase, lysozyme and the contents of complement component 3,4 and immunoglobulin M in crucian carp serum. In addition, compared with CK group, the expression levels of TGF-β and IL-10 in B·S, XOS and B·S + XOS group were significantly increased, and the expression levels of TNF-α, HSP90, IL-1β, TLR4 and MyD88 were significantly decreased. Supplementation of B. subtilis and XOS can also improve the intestinal tissue morphology of crucian carp. After injection of 1 × 10⁷ CFU/mL Aeromonas hydrophila (A. hydrophila), compared with CK group, the survival rates of the B·S group, the XOS group and the B·S + XOS group were increased by 13.98%, 10.56% and 30.74%, respectively. These results show that dietary B. subtilis and XOS can significantly improve the growth performance, antioxidant capacity, immunity and resistance to A. hydrophila of crucian carp, and the combined effect is better than that of single addition.

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.

Effects of Chitosan-Coated Microdiet on Dietary Physical Properties, Growth Performance, Digestive Enzyme Activities, Antioxidant Capacity, and Inflammation Response of Large Yellow Croaker (Larimichthys crocea) Larvae

A 30-day feeding trial was designed to investigate the physical properties of chitosan-coated microdiet (CCD) and the effect of CCD on survival, growth performance, activities of digestive enzymes, intestinal development, antioxidant capacity, and inflammatory response of large yellow croaker larvae (initial weight: 3.81 ± 0.20 mg). Four isonitrogenous (50% crude protein) and isolipidic (20% crude lipid) microdiets were prepared with different concentrations of chitosan wall material by spray drying method (0.00%, 0.30%, 0.60%, and 0.90%, weight (chitosan) : volume (acetic acid)). Results showed that the lipid encapsulation efficiency (control: 60.52%, Diet1: 84.63%, Diet2: 88.06%, Diet3: 88.65%) and nitrogen retention efficiency (control: 63.76%, Diet1: 76.14%, Diet2: 79.52%, Diet3: 84.68%) correlated positively with the concentration of wall material (P < 0.05). Furthermore, the loss rate of CCD was significantly lower than the uncoated diet. Larvae fed the diet with 0.60% CCD had significantly higher specific growth rate (13.52 and 9.95%/day) and survival rate (14.73 and 12.58%) compared to the control group (P < 0.05). Larvae fed the diet with 0.30% CCD had significantly higher trypsin activity in pancreatic segments than the control group (4.47 and 3.05 U/mg protein) (P < 0.05). Larvae fed the diet with 0.60% CCD had significantly higher activity of leucine aminopeptidase (7.29 and 4.77 mU/mg protein) and alkaline phosphatase (83.37 and 46.09 U/mg protein) in the brush border membrane than those of the control group (P < 0.05). The intestinal epithelial proliferation- and differentiation-related factors (zo-1, zo-2, and pcna) in larvae fed the diet with 0.30% CCD had higher expression than those of the control group (P < 0.05). When the concentration of wall material reached 0.90%, the larvae had significantly higher superoxide dismutase activity than that of the control group (27.27 and 13.72 U/mg protein) (P < 0.05). Meanwhile, malondialdehyde contents were significantly lower in larvae fed the diet with 0.90% CCD than that of the control group (8.79 and 6.79 nmol/mg protein) (P < 0.05). 0.30%~0.60% CCD significantly increased the activity of total nitric oxide synthase (2.31, 2.60, and 2.05 mU/mg protein) and inducible nitric oxide synthase (1.91, 2.01, and 1.63 mU/mg protein) and had significantly higher transcriptional levels of inflammatory factor genes (il-1β, tnf-α, and il-6) than those of the control group (P < 0.05). The results indicated chitosan-coated microdiet had great potential in feeding large yellow croaker larvae in addition to reducing nutrition loss.

Probiotics, prebiotics and synbiotics improved the functionality of aquafeed: Upgrading growth, reproduction, immunity and disease resistance in fish

Aquaculture plays an increasingly significant role in improving the sustainability of global fish production. This sector has been intensified with the advent of new husbandry practices and the development of new technology. However, the increasing intensification and indiscriminate commercialized farming has enhanced the vulnerability of cultivated aquatic species to damage from pathogens. In efforts to confront these various diseases, frequent use of drugs, antibiotics, chemotherapeutics, and agents for sterilization have unintentionally added to the risk of transmission of pathogens and harmful chemical compounds to consumers. Some natural dietary supplements are believed to have the potential to offset this setback in aquaculture. Application of bio-friendly feed additives such as probiotics, prebiotics and synbiotics are becoming popular dietary supplements with the potential to not only improve growth performance, but in some cases can also enhance immune competence and the overall well-being of fish and crustaceans. The present review discusses and summarizes the effects of probiotics, prebiotics and synbiotics application on growth, stress mitigation, microbial composition of intestine, immune system and health condition of aquatic animals in association with existing constraints and future perspectives in aquaculture.

Dietary immunostimulants reduced infectivity of Diplostomum spp. eye fluke in common carp, Cyprinus carpio L

Juvenile common carp Cyprinus carpio L. (5.52 ± 1.66 cm, TL) were fed on four diets containing either beta-glucan (MacroGard, 1 g kg -1), nucleotides (Optimûn, 0.2 g kg - 1), chitosan (deacetylated chitin ≥75% shrimp shells, 10 g kg -1) or a basal control diet for 35 days to test whether these so-called "immunostimulants" could affect eye fluke Diplostomum spp. infection success. The immunostimulants diets reduced the number of eye fluke infecting the eyes of C. carpio, with significantly higher infections in the control diet (4.78 ± 1.27) compared with the chitosan (2.08 ± 0.87), nucleotide (2.98 ± 1.01), and beta-glucan (1.41 ± 0.79) diets. To our knowledge, this is the first study to provide evidence that beta-glucan, nucleotides, and chitosan diets can aid against a Diplostomum infection and provides valuable preliminary knowledge on the use of immunostimulants to ameliorate parasitic infections.

Potential of Bacterial Strains Isolated from Coastal Water for Wastewater Treatment and as Aqua-Feed Additives

Bacteria have various and sustained effects on humans in various fields: molecular biology, biomedical science, environmental/food industry, etc. This study was conducted to evaluate the wastewater treatment capacity and feed-additive fish-growth effect of four strains of bacteria: Pseudoalteromonas mariniglutinosa, Psychrobacter celer, Bacillus albus, and Bacillus safensis. In a wastewater degradation experiment, (i) nitrate-N and nitrite-N were removed within 1 h in all of the 4 bacterial strains; (ii) the removal rates of TAN and TN were higher in all of the strains relative to the B. subtilis. In a feed-additive experiment (5% Kg-1), (i) the growth of fish was higher in all of the 4 bacterial strains with the B. subtilis relative to the commercial feed; (ii) there was no significant growth difference for B. albus and B. safensis relative to the B. subtilis, but growth was higher in P. mariniglutinosa and P. celer. The results indicated that the 4 bacterial strains can be effectively utilized for biological wastewater treatment processes and as aqua-feed.

In vitro Assessment of the Safety and Potential Probiotic Characteristics of Three Bacillus Strains Isolated From the Intestine of Hybrid Grouper (Epinephelus fuscoguttatus♀ × Epinephelus lanceolatus♂)

Probiotics serving as an alternative to the criticized antibiotics mainly focus on improving animal's growth and health. After realizing the dangers posed by diseases that have led to lots of economic losses, aquaculture scientists have sought the usage of probiotics. However, most probiotics are ineffective in eliciting aquatic animals' preferred effects, since they are from non-fish sources. Again, there are even a few marine aquatic probiotics. Given this, a study was conducted to investigate the probiotic potential of the bacteria species isolated from the digestive tract of hybrid grouper (Epinephelus fuscoguttatus♀ × Epinephelus lanceolatus♂). Based on the morphological, biochemical, 16S rRNA sequencing analysis and evolutionary relationships, the isolated species were identified as Bacillus tequilensis GPSAK2 (MW548630), Bacillus velezensis GPSAK4 (MW548635), and Bacillus subtilis GPSAK9 (MW548634), which were designated as GPSAK2, GPSAK4, and GPSAK9 strains, respectively. Their probiotic potentials including their ability to tolerate high bile salt concentration, low pH, high temperatures, adhesion ability (auto-aggregation and cell-surface hydrophobicity), antimicrobial activity and biosafety test, compatibility test, hemolytic activity, and antibiotic susceptibility test were evaluated. While GPSAK2 and GPSAK9 strains were γ-hemolytic, that of GPSAK4 was α-hemolytic. All the isolates were resistant to low pH (1) and higher bile salt concentration (0.5%), showed higher viability ability after higher temperature exposure (80, 90, and 100°C), as well as higher cell-surface percentage hydrophobicity and auto-aggregation. All isolates exhibited positive compatibility with each other, signifying their ability to be used as multispecies. The three strains were susceptible to ampicillin (except GPSAK9, which was resistant), penicillin, kanamycin, ceftriaxone, chloramphenicol, erythromycin, clindamycin, furazolidone (except GPSAK2 and GPSAK9, which were moderately susceptible and resistant, respectively), polymyxin B, vancomycin (except GPSAK9, which was resistant), sulfamethoxazole (except GPSAK9, which was moderately susceptible), amikacin, minocycline, ofloxacin, norfloxacin, doxycycline, neomycin, gentamicin, tetracycline, carbenicillin, midecamycin (except GPSAK9, which was moderately susceptible), ciprofloxacin, piperacillin, and cefoperazone. All isolates demonstrated good antimicrobial activity against four pathogens, viz. Streptococcus agalactiae, Streptococcus iniae, Vibrio harveyi, and Vibrio alginolyticus. The results collectively suggest that Bacillus strains GPSAK2, GPSAK4, and GPSAK9 could serve as potential probiotic candidates that can be used to improve the growth and health status of aquatic animals, especially grouper.

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.

Synergistic effect of durian fruit rind polysaccharide gel encapsulated prebiotic and probiotic dietary supplements on growth performance, immune-related gene expression, and disease resistance in Zebrafish (Danio rerio)

The present study investigated the effect of polysaccharide gel (PG) extracted from the rind of durian fruit which is encapsulated with Bacillus subtilis as a feed and co-inoculation with Artemia nauplii in the induction of immune response in Danio rerio after Vibrio immersion challenge (5 days). The total RBC count, lysozyme activity test, weight, and length analysis revealed that the zebra fishes fed with the PG encapsulated probiotics had more immune induction activity, survival, and growth than the non-encapsulated groups. When the expression of the immune-related genes was measured, the studies revealed the significant upregulation (P < 0 .05) of interleukin 1 beta (Il1b), lysozyme (lyz), tumor necrosis factor-alpha (TNF-α), superoxide dismutase (SOD) genes in fish fed with PG encapsulated probiotics with A. nauplii showed an immense effect on the induction of immune response compared to other feeds.

Effects of a Potential Host Gut-Derived Probiotic, Bacillus subtilis 6-3-1, on the Growth, Non-specific Immune Response and Disease Resistance of Hybrid Grouper (Epinephelus fuscoguttatus♀ × Epinephelus lanceolatus♂)

A potential host-derived probiotic, Bacillus subtilis 6-3-1, was successfully screened from 768 isolates from the intestines of healthy hybrid grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂) based on multiple probiotic characteristics in vitro assays, such as, non-hemolytic activity, extracellular enzyme activity, inhibitory activity against pathogens, tolerance to gastrointestinal stress, cell surface hydrophobicity, autoaggregation, and antibiotic susceptibility. Eight weeks of feeding trial revealed that dietary supplementation of B. subtilis 6-3-1 at all three concentrations (1 × 10⁶ CFU g^-1 as BS6; 1 × 10⁷ CFU g^-1 as BS7; 1 × 10⁸ CFU g^-1 as BS8) could promote the growth performance of hybrid groupers to a certain extent at different time points. At the end of 8th week, BS6 and BS8 significantly promoted the weight gain rate (WGR), specific growth rate (SGR) of hybrid groupers. The digestive enzyme activities were also increased in BS6 and BS8 groups comparing with those in control group, except that the increase of amylase activities in BS6 was not significant (P > 0.05). However, BS7 showed the best non-specific immunity stimulating effects among the three concentration groups. While BS7 significantly boosted serum total protein contents, lysozyme (LZM), total antioxidant capacity (T-AOC), superoxide dismutase (SOD), catalase (CAT), and acid phosphatase (ACP) levels, BS6 significantly enhanced serum total protein, LZM activity, and BS8 significantly improved LZM, respiratory bursts activity. B. subtilis 6-3-1 up-regulated the expression of MyD88 in head kidney and intestine and increased villi length (VL) in intestine of BS7 group. It also up-regulated the expression of IgM in head kidney in BS6 group and IgM and TLR1 in intestine of BS8 group. Though all B. subtilis 6-3-1 supplemented groups reduced the cumulative mortality rate post-Vibro harveyi-challenge, BS7 showed the best protection effects among the three concentration groups. In conclusion, with its immune promoting, intestine health enhancing, and V. harveyi resisting effects, BS7 show great potential to be used as a probiotic in hybrid grouper culture.

Chitosan and chitooligosaccharides attenuate soyabean meal-induced intestinal inflammation of turbot (Scophthalmus maximus): possible involvement of NF-кB, activator protein-1 and mitogen-activated protein kinases pathways

An 8-week feeding experiment was conducted to investigate and confront the putative functions of chitosan (CTS) and chitooligosaccharide (COS) in the growth and homoeostasis of distal intestine in juvenile turbots fed diets containing soyabean meal (SBM). Three isolipidic and isonitrogenous diets were formulated by supplemented basal diet (based on a 400 g/kg SBM) with 7·5 g/kg CTS or with 2·0 g/kg COS. Our results indicated that both CTS and COS supplementation could significantly improve (i) the growth performance and feed efficiency ratio; (ii) antioxidant activity driven by metabolic enzymes (i.e. catalase, glutathione reductase, glutathione peroxidase and superoxide dismutase); (iii) glutathione levels; (iv) acid phosphatase and lysozyme activity and (v) IgM content. As a result, these two particular prebiotics were able to significantly attenuate the histological alterations due to local inflammation as well as to decrease the transcriptional levels of proinflammatory cytokines (i.e. IL-1β, IL-8 and TNF-α) and major pathway effectors (i.e. activator protein-1 (AP-1), NF-кB, p38 mitogen-activated protein kinase, c-Jun N-terminal kinase and extracellular regulated kinase). High-throughput sequencing data indicated that dietary CTS and COS could significantly decrease the diversity of intestinal bacteria but elevate the relative abundances of Bacillus, Lactobacillus and Pseudomonas genera. Altogether, these findings suggest that CTS and COS can improve growth of turbot, enhance intestinal immune and anti-oxidant systems and promote the balance of intestinal microbiota. The protective effects, elicited by these two prebiotics, against SBM-induced inflammation could be attributed to their roles in alleviating the overexpression of inflammatory cytokines by possibly down-regulating NF-кB, AP-1 and/or mitogen-activated protein kinases pathways.

Phenol-rich fulvic acid as a water additive enhances growth, reduces stress, and stimulates the immune system of fish in aquaculture

Aquaculture has become imperative to cover the demands for dietary animal protein. Simultaneously, it has to overcome prejudices from excessive use of antibiotics and environmental impacts. Natural supplements are traditionally applied orally. In this study, we demonstrated another pathway: the gills. Humic substances are immunostimulants and a natural part of every aquatic ecosystem, making them ideal to be used as bath stimulants. Five and 50 mg C/L of a fulvic acid-rich humic substance was added for 28 days to the water of juvenile rainbow trout (Oncorhynchus mykiss). This fulvic acid is characterized by a high content of phenolic moieties with persistent free radicals and a high electron exchange capacity. The high concentration of the fulvic acid significantly increased growth and reduced the food conversion ratio and the response to a handling-stressor. Phagocytosis and potential killing activity of head kidney leukocytes were increased, as well as the total oxyradical scavenging capacity (TOSC) and lysozyme activity in the gills. In conclusion, immunostimulation via gills is possible with our fulvic acid, and the high phenolic content improved overall health and stress resistance of fish.

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.

Modulatory role of dietary Thymus vulgaris essential oil and Bacillus subtilis against thiamethoxam-induced hepatorenal damage, oxidative stress, and immunotoxicity in African catfish (Clarias garipenus)

Thiamethoxam (TMX) is a widely used neonicotinoid insecticide for its effective potential for controlling insects from the agricultural field, which might induce toxicity to the aquatic biota. In this study, the role of the probiotic Bacillus subtilis (BS) and a phytogenic oil extract of Thymus vulgaris essential oil (TVEO) in the modulation of thiamethoxam (TMX)-induced hepatorenal damage, oxidative stress, and immunotoxicity in African catfish (Clarias garipenus) has been evaluated. Fish were subjected to TMX (5 mg L^-1) and fed with a diet either supplemented with BS (1000 ppm) or TVEO (500 ppm). The experiment lasted for 1 month. By the end of the experiment, blood was sampled for biochemical analysis and fish organs and tissues were collected for histopathological and immunohistochemical examinations. Results showed a substantial increase of serum markers of hepatorenal damage such as the activities of aspartate transaminase (AST), alanine transaminase (ALT), and alkaline phosphatase (ALP) and levels of blood urea nitrogen (BUN) and creatinine with an obvious decrease of serum protein levels in the TMX-intoxicated group. Also, there was a considerable increase in malondialdehyde (MDA) levels and glutathione-S-transferase (GST) activity. TMX remarkably suppressed serum lysozyme activity, respiratory burst activity, and phagocytosis with a conspicuous elevation of the levels of interleukins (interleukin-1 beta (IL-1β) and interleukin-6 IL-6). The histopathological findings showed that TMX induced degenerative changes and necrosis in the gills, liver, head kidneys, and spleen of the intoxicated fish. Significant alterations of frequency, size, and area percentage of melanomacrophage centers (MMCs), decreased splenocyte proliferation, and increased number of caspase-3 immunopositive cells were also observed. Contrariwise, the concurrent supplementation of either BS or TVEO in the diets of catfish partially mitigated both the histopathological and histomorphometric lesions of the examined tissues. Correspondingly, they improved the counts of proliferating cell nuclear antigen (PCNA) and caspase-3 immunopositive splenocytes. In conclusion, the co-administration of either BS or TVEO in catfish diets partially diminished the toxic impacts of TMX. Nonetheless, the inclusion of TVEO in the diets of catfish elicited better protection than BS against TMX-induced toxicity in response to its potential anti-inflammatory, antioxidant, anti-apoptotic, and immune-stimulant effects.

Dietary Supplementation of Probiotic Bacillus subtilis Affects Antioxidant Defenses and Immune Response in Grass Carp Under Aeromonas hydrophila Challenge

This study investigated whether Bacillus subtilis can provide protection for grass carp against oxidative stress damage induced by Aeromonas hydrophila. A total of 240 healthy grass carp (Ctenopharyngodon idellus) (average weight of 71.42 ± 4.36g) were randomly divided into four groups with three replicates: control group, A. hydrophila group, B. subtilis + A. hydrophila group, and A. hydrophila + B. subtilis group. After challenge with A. hydrophila, the lipid oxidative damage, antioxidant defenses, and the gene expression of inflammatory cytokines of the grass carp were investigated. Our results showed that A. hydrophila caused lipid oxidative damage, led to significant decreases in antioxidant defenses, and induced inflammatory responses of grass carp. However, the grass carp group fed the probiotic B. subtilis diet for 42 days before the challenge and the group fed the probiotic B. subtilis diet immediately after the challenge both showed (i) a reduced level of oxidative stress with a decrease in the level of MDA; (ii) an increase in antioxidant defenses, including an increase in total antioxidant capacity (T-AOC), increased activities of SOD and CAT, increased levels of GSH, and upregulated gene expression of antioxidant enzymes (SOD, CAT, and Gpx); and (iii) an improved immune response with the level of antiinflammatory cytokines IL-10 messenger RNA (mRNA) upregulated and the levels of pro-inflammatory cytokines TNF-α, IL-1β, and IL-8 mRNA downregulated. Based on this study, B. subtilis can provide effective protection of fish against oxidative stress damage induced by A. hydrophila infection.

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.

The effect of dietary supplementation with Clostridium butyricum on the growth performance, immunity, intestinal microbiota and disease resistance of tilapia (Oreochromis niloticus)

This study was conducted to assess the effects of dietary Clostridium butyricum on the growth, immunity, intestinal microbiota and disease resistance of tilapia (Oreochromis niloticus). Three hundreds of tilapia (56.21 ± 0.81 g) were divided into 5 groups and fed a diet supplemented with C. butyricum at 0, 1 x 10⁴, 1 x 10⁵, 1 x 10⁶ or 1 x 10⁷ CFU g^-1 diet (denoted as CG, CB1, CB2, CB3 and CB4, respectively) for 56 days. Then 45 fish from each group were intraperitoneally injected with Streptococcus agalactiae, and the mortality was recorded for 14 days. The results showed that dietary C. butyricum significantly improved the specific growth rate (SGR) and feed intake in the CB2 group and decreased the cumulative mortality post-challenge with S. agalactiae in the CB2, CB3 and CB4 groups. The serum total antioxidant capacity and intestinal interleukin receptor-associated kinase-4 gene expression were significantly increased, and serum malondialdehyde content and diamine oxidase activity were significantly decreased in the CB1, CB2, CB3 and CB4 groups. Serum complement 3 and complement 4 concentrations and intestinal gene expression of tumour necrosis factor α, interleukin 8, and myeloid differentiation factor 88 were significantly higher in the CB2, CB3 and CB4 groups. Intestinal toll-like receptor 2 gene expression was significantly upregulated in the CB3 and CB4 groups. Dietary C. butyricum increased the diversity of the intestinal microbiota and the relative abundance of beneficial bacteria (such as Bacillus), and decreased the relative abundance of opportunistic pathogenic bacteria (such as Aeromonas) in the CB2 group. These results revealed that dietary C. butyricum at a suitable dose enhanced growth performance, elevated humoral and intestinal immunity, regulated the intestinal microbial components, and improved disease resistance in tilapia. The optimal dose was 1 x 10⁵ CFU g^-1 diet.

Effect of Bacillus velezensis on Aeromonas veronii-Induced Intestinal Mucosal Barrier Function Damage and Inflammation in Crucian Carp (Carassius auratus)

Aeromonas veronii is an emerging aquatic pathogen causing hemorrhagic septicemia in humans and animals. Probiotic is an effective strategy for controlling enteric infections through reducing intestinal colonization by pathogens. Here we report that the consumption of Bacillus velezensis regulated the intestinal innate immune response and decreased the degree of intestinal inflammation damage caused by the A. veronii in Crucian carp. In this study, we isolated four strains of B. velezensis, named C-11, S-22, L-17 and S-14 from apparently healthy Crucian carp, which exerted a broad-spectrum antimicrobial activity inhibiting both Gram-positive and Gram-negative bacteria especially the fish pathogens. B. velezensis isolates showed typical Bacillus characteristics by endospore staining, physiological and biochemical test, enzyme activity analysis (amylase, protease, and lipase), and molecular identification. Here, Bacillus-containing dietary was orally administrated to Crucian carp for 8 weeks before A. veronii challenge. Immunological parameters and the expression of immune-related genes were measured at 2, 4, 6, 8, and 10 weeks post-administration. The results showed that B. velezensis was found to promote the increase in the phagocytic activities of peripheral blood leukocytes (PBLs) and head kidney leukocytes (HKLs), as well as the increase in interleukin 1β (IL-1β), IL-10 and tumor necrosis factor α (TNF-α) concentration of serum. Lysozyme levels (113.76 U/mL), ACP activity (25.32 U/mL), AKP activity (130.08 U/mL), and SOD activity (240.63 U/mL) were maximum (P < 0.05) in the B. velezensis C-11 treated group at 8 week. Our results showed that Crucian carp fed with the diet containing B. velezensis C-11 and S-22 developed a strong immune response with significantly higher (P < 0.05) levels of IgM in samples of serum, mucus of skin and intestine compared to B. velezensis L-17 and S-14 groups. Moreover, B. velezensis spores appeared to show no toxicity and damage in fish, which could inhabit the gut of Crucian carp. B. velezensis restrained up-regulation of pro-inflammation cytokines (IL-1β, IFN-γ, and TNF-α) mRNA levels in the intestine and head kidney at final stage of administration, and the expression of IL-10 was increased throughout the 10-week trial. A. veronii infection increased the population of inflammatory cells in the intestinal villi in the controls. In contrast, numerous goblet cells and few inflammatory cells infiltrated the mucosa in the B. velezensis groups after challenge with A. veronii. Compared with A. veronii group, B. velezensis could safeguard the integrity of intestinal villi. The highest post-challenge survival rate (75.0%) was recorded in B. velezensis C-11 group. The present data suggest that probiotic B. velezensis act as a potential gut-targeted therapy regimens to protecting fish from pathogenic bacteria infection.

IMPORTANCE

In this work, four Bacillus velezensis strains isolated from apparently healthy Crucian carp, which exhibited a broad-spectrum antibacterial activity especially the fish pathogens. Administration of B. velezensis induced the enhancement of the intestinal innate immune response through reducing intestinal colonization by pathogens. The isolation and characterization would help better understand probiotic can be recognized as an alternative of antimicrobial drugs protecting human and animal health.

Acinetobacter Strain KUO11TH, a Unique Organism Related to Acinetobacter pittii and Isolated from the Skin Mucus of Healthy Bighead Catfish and Its Efficacy Against Several Fish Pathogens

The bacterial strain KU011TH was isolated from the skin mucus of healthy bighead catfish. The strain is a Gram-negative coccobacillus that is nonmotile, aerobic, catalase positive, oxidase negative, and nonhemolytic. Sequence analyses of the housekeeping genes 16S rRNA, gyrB and rpoB indicate that this strain is a new member of the Acb complex of the genus Acinetobacter and is closely related to Acinetobacter pittii and Acinetobacter lactucae. In addition, the genome relatedness-associated ANIb (<95–96%) and in silico DDH (<70%) values clearly supported the new member of the genus Acinetobacter and the Acb complex. The genome of the strain KU011TH was approximately 3.79 Mbp in size, comprising 3619 predicted genes, and the DNA G+C content was 38.56 mol%. The major cellular fatty acids were C18:1ω9c, C16:0, C16:1, C20:2, C18:2ω6c and C18:1ω9t. The whole-genome sequences and phenotypic, phylogenetic, and chemotaxonomic data clearly support the classification of the strain KU011TH as a new member in the genus Acinetobacter which is closest to A. pittii. Additionally, the new bacterial strain exhibited strong activity against a broad range of freshwater fish pathogens in vitro.

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.

Effects of different molar mass chitooligosaccharides on growth, antioxidant capacity, non-specific immune response, and resistance to Aeromonas hydrophila in GIFT tilapia Oreochromis niloticus

A feeding trial was conducted to evaluate the effects of different molar mass chitooligosaccharides (1000 Da, 3000 Da and 8000 Da) on growth, antioxidant capacity, non-specific immune response, and resistance to Aeromonas hydrophila in GIFT tilapia (Oreochromis niloticus). A total of 600 fish were divided into four treatments with five replicates of thirty fish per tank. The results showed that the supplementation of 1000 Da and 3000 Da COS significantly improved the growth performance and feed utilization in GIFT tilapia. The trend of decreasing total cholesterol, triglyceride, ALT, and ACP activity was observed in fish fed diet supplemented COS. The supplementation of 1000 Da and 3000 Da COS significantly improved the serum TAC activity, and decreased the serum MDA and catalase activities (P < 0.05). The lysozyme activity of blood, liver, and gills in fish fed diets supplemented with 1000 Da and 3000 Da COS was significantly higher than that of fish fed control diet after 56 days of feeding (P < 0.05). The phagocytic activity and phagocytic index of fish fed diets supplemented with 1000 Da and 3000 Da COS were significantly higher than those of fish fed control diet. Post-challenge test showed that fish mortality in 1000 Da, 3000 Da, and 8000 Da COS groups were significantly lower than that of fish in control group (P < 0.05). In conclusion, the present study indicated that dietary 1000 Da and 3000 Da COS supplementation could enhance more performance and immune response of GIFT tilapia than 8000 Da COS.

Effects of a Sudden Drop in Salinity on Immune Response Mechanisms of Anadara kagoshimensis

In this experiment, the effects of a sudden drop of salinity on the immune response mechanisms of the ark shell Anadara kagoshimensis were examined by simulating the sudden drop of salinity that occurs in seawater after a rainstorm. Additionally, the differentially expressed genes (DEGs) were identified using transcriptome sequencing. When the salinity dropped from 30‱ (S30) to 14‱ (S14), the phagocytic activity of blood lymphocytes, the O₂^- levels produced from respiratory burst, the content of reactive oxygen species, and the activities of lysozymes and acid phosphatases increased significantly, whereas the total count of blood lymphocytes did not increase. Total count of blood lymphocytes in 22‱ salinity (S22) was significantly higher than that in any other group. The raw data obtained from sequencing were processed with Trimmomatic (Version 0.36). The expression levels of unigenes were calculated using transcripts per million (TPM) based on the effects of sequencing depth, gene length, and sample on reads. Differential expression analysis was performed using DESeq (Version 1.12.4). Transcriptome sequencing revealed 269 (101 up-regulated, 168 down-regulated), 326 (246 up-regulated, 80 down-regulated), and 185 (132 up-regulated, 53 down-regulated) significant DEGs from comparison of the S14 vs. S22, S22 vs. S30, and S14 vs. S30 groups, respectively. Gene Ontology enrichment analysis of the DEGs in these salinity comparison groups revealed that the cellular amino acid metabolic process, the regulation of protein processing, the regulation of response to stress, and other terms were significantly enriched. Kyoto Encyclopedia of Genes and Genomes enrichment analysis showed that nucleotide-binding, oligomerization domain (NOD)-like receptor signaling pathway (ko04621), apoptosis-multiple species (ko04215), Toll and Imd signaling pathway (ko04624), NF-κB signaling pathway (ko04064), apoptosis (ko04210), and focal adhesion (ko04510) were significantly enriched in all salinity comparison groups. qRT-PCR verification of 12 DEGs in the above six pathways was conducted, and the results were consistent with the transcriptome sequencing results in terms of up-regulation and down-regulation, which illustrates that the transcriptome sequencing data are credible. These results were used to preliminarily explore the effects of a sudden drop of salinity on blood physiological and biochemical indexes and immunoregulatory mechanisms of A. kagoshimensis. They also provide a theoretical basis for the selection of bottom areas optimal for release and proliferation of A. kagoshimensis required to restore the declining populations of this species.

Effects of chitooligosaccharides supplementation with different dosages, molecular weights and degrees of deacetylation on growth performance, innate immunity and hepatopancreas morphology in Pacific white shrimp (Litopenaeus vannamei)

Three trials were conducted to comprehensively evaluate the application of chitooligosaccharides (COSs) different dosages, molecular weights (MW) and degrees of deacetylation (DD) as a feed additive for Pacific white shrimp (Litopenaeus vannamei). In trial 1, COSs (3 KDa and 85% DD) at four different dosages (0.05%, 0.1%, 0.15% and 0.2%) were added to feed to investigate the appropriate dosages. The survival ratio (SR) of the shrimps was not significantly different (P > 0.05) between the control and treatment groups. The shrimps fed with 0.1% COSs supplementation exhibited the highest wet body weight (FBW), specific growth ratio (SGR), and weight gain (WG) and the lowest feed conversion ratio (FCR). In trial 2, COSs with different MW (85% DD and 0.1% dosage) were tested. Except for the group treated with the 12 KDa COSs, all shrimps fed with COSs had remarkably higher (P < 0.05) FBW, WG, and SGR and lower FCR (P < 0.05) than the control group, and shrimps fed with the 1 KDa COSs showed most positive effects. In trial 3, COSs with different DD (MW of 1 KDa and 0.1% dosage) were further studied. The different DD were DD5, DD25, DD50, DD75, and DD95. Shrimps fed COSs-supplemented diets of DD75 or DD95 exhibited higher (P < 0.05) FBW, WG and SGR and lower FCR than the other groups. The DD95 group had the highest FBW, WG and SGR and the lowest FCR, but there were no significant differences (P > 0.05) between the DD75 and DD95 groups. Moreover, for COSs supplementation, especially for the DD75 and DD95 groups, the antioxidant parameters were significantly different from those of the other groups. Furthermore, groups fed diets with COSs supplementation had higher (P < 0.05) trypsin activity than those fed control diets. In addition, immune and antioxidant gene expression and the morphology of the hepatopancreas were affected by the DD of COSs. Therefore, the additive dosages, molecular weights and degrees of deacetylation of COSs significantly affected the growth performance of the shrimps; therefore, it is particularly important to determine the optimum parameters of COSs.

Effects of chitosan-supplemented diets on the growth performance, nonspecific immunity and health of loach fish (Misgurnus anguillicadatus)

This study investigated the effects of dietary chitosan on the growth performance and nonspecific immunity of loaches (Misgurnus anguillicadatus). Four practical diets at three levels of chitosan (1, 5 and 10 g/kg) were fed to loach (3.13 ± 0.02 g) in triplicate groups (20 fish per replicate) for 10 weeks. Contrary to high-chitosan treatment, the administration of low or moderate levels of oral chitosan (1 or 5 g/kg) significantly increased the body weight gain, specific growth rate and condition factor. The oral administration of chitosan significantly increased the survival rate, phenoloxidase, superoxide dismutase, glutathione peroxidase, lysozyme, acid phosphatase and alkaline phosphatase activities, as well as the immunoglobulin M and complement 3 contents and the disease resistance against Aeromonas hydrophila. The optimum dose of dietary chitosan required for the maximum growth of loach was 5 g/kg. These results indicated that chitosan exerted immunostimulatory effects on loaches (M. anguillicadatus) and can thus be used as a dietary supplement.

Immunostimulatory effect of dietary chitosan nanoparticles on the performance of Nile tilapia, Oreochromis niloticus (L.)

Feed supplements to fish are generally used to overcome any expected diseases and stressors and to sustain eco-friendly fish farming. One of these feed supplements is chitosan, which stimulated growth and immune properties for many aquatic organisms. It is expected that the nano-sized materials may have stronger immune activation in fish than the ordinary size. Therefore, the current study was conducted to evaluate the effect of dietary chitosan nanoparticles (CNP) on growth performance, antioxidant activity, and innate immunity of Nile tilapia, Oreochromis niloticus (L.). Fish (19.8 ± 0.59 g) were fed on diets enriched with 0.0, 0.25, 0.5, 1.0, and 2.0 g CNP/kg diet for 45 days. Fish performance was significantly improved with increasing CNP levels over the control diet with optimum level of 1.0 g CNP/kg diet. Antioxidant-stimulated activity was observed due to dietary CNP supplementation over the control diet in a dose-dependent manner. However, malondialdehyde level decreased significantly, whereas activities of catalase, superoxide dismutase, lysozyme, and respiratory burst increased significantly due to CNP supplementation in a dose-dependent manner. The current study evoked that dietary CNP showed strong immune modulatory properties and enhanced significantly the performance and health of Nile tilapia with optimum level of 1.0 g CNP/kg diet.

Dietary chitosan-selenium nanoparticle (CTS-SeNP) enhance immunity and disease resistance in zebrafish

Selenium (Se) is an essential micronutrient for human and animals. It plays an important role in antioxidative stress, selenoenzymes regulation and immunomodulation. In this study, two common immunostimulants chitosan (CTS) and Se were used to synthesize nanoparticles (CTS-SeNP). Immunomodulation of CTS-SeNP were explored in wild-type zebrafish (Danio rerio). Dietary supplementation of CTS-SeNP enhanced lysozyme activity, phagocytic respiratory burst as well as splenocytes proliferation stimulated by LPS and ConA. CTS-SeNP showed immunomodulation effect from 5 to 20 μg/g but the best outcome was observed at 10 μg/g. Immunomodulation effect were rapidly induced after 3-9d and can sustain to 60. The zebrafish fed with 10 μg/g CTS-SeNP also showed 26.7% higher survival rate than the control after intraperitoneal injection of common bacterium Aeromonas hydrophila. Our results suggested that CTS-SeNP is an effective immunostimulant to fish and has potential application in aquaculture.

Application of marine-derived polysaccharides as immunostimulants in aquaculture: A review of current knowledge and further perspectives

The use of antibiotics in the feeds of cultivable aquatic animals has been generally practised to reduce infectious diseases as well as to improve the survival and growth. In recent years, many countries ban to aquatic animals due to the use of large amount of antibiotics and chemotherapies, thus alternative novel strategies are need to promote the growth of aquatic animals and control the pathogens. Dietary supplementation of marine-derived polysaccharides (MDPs) is one of the potential substitutes for antibiotics in aquatic animal feeds. Recently, the use of dietary MDPs in the aquaculture animals has been focused with much interest. In aquaculture, MDPs are used as prebiotic substance which is mostly accepted as a nutritional component for improving the growth performance and health conditions. Hence, present review is a comprehensive and an updated collection of available research reports on different MDPs (alginate, fucoidan, carrageenan, laminarin, ulvan, galactan, agar, chitin and chitosan), route of administration, dosage and applications for improving aqua feeds with emphasis on its effects on growth, biochemical indices, immune response, gut microbiota and disease resistance of aquaculture animals. This review describes the sustainability of global aquaculture production by providing a best alternative to harmful antibiotics, thereby meeting the emerging consumer demand for antibiotic-free aquatic food products.

Use of probiotics in aquaculture of China-a review of the past decade

China is the largest aquaculture producer in the world. Antibiotics were extensively used to ensure the development of the intensive aquaculture; however, the use of antibiotics causes safety- and environment-associated problems. As an alternative strategy to antibiotics, aquatic probiotics have attracted attention. The microbial organisms used as probiotics or tested as potential probiotics in Chinese aquaculture belong to various taxonomic divisions, including Actinobacteria, Bacteroidetes, Firmicutes, Proteobacteria and yeast. Moreover, the mixture of probiotic strains and synbiotics are also widely used. Studies on the mode of action of aquatic probiotics have extended our understanding of the probiotic effects, and novel mechanisms have been discovered, such as interference of quorum sensing. However, use of probiotics in Chinese aquaculture is still at an initial stage, and there are potential risks for some probiotic applications in aquaculture. Further regulation and management are required to normalize the production and usage of aquatic probiotics. In this review, we discuss species, effects, and mode of actions of probiotics in Chinese aquaculture since 2008. Challenges and future directions for research are also discussed.

Effect of Lactobacillus casei on Innate Immunity Responses and Aeromonas hydrophila Resistance in Shabot, Tor grypus

For investigating the possible effect of Lactobacillus casei on the immune status of Shabot, Tor grypus, 480 fish weighing 40 ± 10 g were randomly divided into four groups in triplicate. The first group (control) was fed on a commercial diet without probiotic supplementation. Other groups (A, B, and C) were received diet supplemented by different levels of L. casei, i.e., 5 × 10⁶, 5 × 10⁷, and 5 × 10⁸ CFU g^-1, respectively. The feeding trial was performed for 60 continuous days followed by 15 days as withdraw period, i.e., day 75. Serum immune responses were determined on days 0, 30, 60, and 75 of feeding trial. Gene expression in the kidneys was analyzed on days 0, 60, and 75 as well as following bacterial challenge. The results showed that dietary L. casei supplementation significantly (P < 0.05) increased serum lysozyme, serum bactericidal, complement, and respiratory burst activities as compared with control group, while it was observed that treated fish did not show any significant differences in immune responses on day 75 (P < 0.05). Results of gene expression (TNF-α, IL-1β, and IL-8) showed significant increases among different experimental times (P < 0.05). According to obtained results, it might be concluded that the feeding by L. casei could likely enhance the immune responses and gene expression in T. grypus. Treatments A and C were better than B, but the lower concentration is better as an economic viewpoints and biosecurity. Therefore, concentration A is suggested.

Synergistic effects of dietary supplementation of Bacillus subtilis WB60 and mannanoligosaccharide (MOS) on growth performance, immunity and disease resistance in Japanese eel, Anguilla japonica

This study evaluated the synergistic effects of dietary Bacillus subtilis WB60 and mannanoligosaccharide (MOS) in juvenile Japanese eel, Anguilla japonica. Seven treatment diets were formulated to contain three different levels of B. subtilis (0.0, 0.5, and 1.0 × 10⁷ CFU/g diet denoted as BS₀, BS_0.5, and BS₁, respectively) with two MOS levels (0 and 5 g/kg diet denoted as M₀ and M₅, respectively), and one diet with oxytetracycline (OTC) at 5 g/kg diet. Each diet (BS₀M₀ (CON), BS₀M₅, BS_0.5M_0, BS_0.5M_5, BS₁M_0, BS₁M₅, and OTC) was fed to triplicate groups of 20 fish averaging 9.00 ± 0.11 g (mean ± SD) for eight weeks. Average weight gain, feed efficiency, specific growth rate and protein efficiency ratio of fish fed the BS_0.5M₅ and BS₁M₅ diets were significantly higher than those of fish fed CON, BS_0.5M₀ and OTC diets (P < 0.05). Significant increases in the nonspecific enzymatic activities (e.g., lysozyme and myeloperoxidase) were detected from fish fed the BS_0.5M₅, BS₁M₅, and OTC diets compared to the CON, BS_0.5M₀, and BS₀M₅ diets (P < 0.05). Whereas, immunoglobulin M expressions were recorded significantly higher for fish fed the BS_0.5M₅ and BS₁M₅ diets compared to those of fish fed the other diets (P < 0.05). Also, heat shock protein 70 mRNA levels of fish fed BS_0.5M₅ and BS₁M₅ diets were significantly higher than those of fish fed the CON diet (P < 0.05). Histological observations of the intestinal morphology showed healthier gut for fish fed BS_0.5M₅ and BS₁M₅ diets than those fed CON, BS₀M₅, and OTC diets. Additionally, resistance to bacterial challenge with Vibrio anguillarum was recorded significantly lower for fish fed the CON diet than those fed other diets (P > 0.05). Therefore, the results for growth performance, non-specific immune responses, intestinal morphology, and disease resistance demonstrated that supplementation of B. subtilis at 0.5 × 10⁷ CFU/g diet and mannanoligosaccharide at 5 g/kg diet could have beneficial synergistic effects in Japanese eel. The isolated probiotic from eel and the selected prebiotic could lead to the development of a specific and potential synbiotic in Japanese eel aquaculture.

Effects of dietary synbiotic on innate immunity, antioxidant activity and disease resistance of Cirrhinus mrigala juveniles

The dietary supplementation of synbiotic in Cirrhinus mrigala juvenile (with initial body weight ranging from 2.87 ± 0.01 g to 3.26 ± 0.05 g) was evaluated in terms of changes in innate immunity, antioxidant activity and disease resistance against Aeromonas hydrophilla infection. One hundred eighty acclimatized juveniles of mrigal were randomly distributed in the three replicates of each of four experimental groups i.e. control (without Probiotic and Prebiotic), T₁ (High Probiotic + Low Prebiotic), T₂ (Low Probiotic + High Prebiotic) and T₃ (High Probiotic + High Prebiotic), using completely randomized design (CRD). At the end of the feeding trial for 60 days, fish were challenged by Aeromonas hydrophila and survival rate was recorded for the next 15 days. Bacillus subtilis used as a probiotic source and MOS used as a prebiotic source in the experiment. Results showed that innate immunity was comparatively improved in T₃ group. Lysozyme activity and respiratory burst activity (NBT) were significantly (P < 0.05) affected in T₃ group. Highest activities of antioxidant enzymes (P < 0.05) were reported in T₃ group. Cumulative mortality % was found to be lower in the fish fed dietary synbiotic on T₃ group after challenging with Aeromonas hydrophilla infection. The results of this study showed that under the experimental conditions, dietary supplementation of synbiotic had a synergestic effect on enhancing innate immunity and disease resistance of Cirrhinus mrigala (P < 0.05).

The effects of feeding Lactobacillus pentosus on growth, immunity, and disease resistance in Haliotis discus hannai Ino

To study the effects of probiotic-added food on the survival and growth of abalone (Haliotis discus hannai Ino), the expression levels of nonspecific immune genes and the anti-Vibrio parahaemolyticus infection were examined. During an 8-week culturing experiment in an indoor aquarium and a 2-week V. parahaemolyticus artificial infection experiment, the control group was fed with untreated food once a day, while the experimental groups (L1, L2 and L3) were fed with Lactobacillus pentosus added food. The concentration of probiotics in the experimental food was 10³ cfu/g (L1), 10⁵ cfu/g (L2) and 10⁷ cfu/g (L3), respectively. The results showed that the survival rate, shell length-specific growth rate, and the food conversion rate (FCR) of abalones in L1 and L2 were significantly higher than the control group. The food intake of abalones in L3 was significantly lower than that in L1, L2 and the control group, but there was no significant difference in FCR identified between L1, L2 and L3. In the L. pentosus-added groups, the total number of blood lymphocytes, lysozyme activity, acid phosphatase, superoxide dismutase, and expression levels of Mn-superoxide dismutase (Mn-SOD) and thioredoxin peroxidase (TP_x) were significantly higher than the control group, while the malondialdehyde (MDA) content was significantly lower than the control group. The phagocytic activity of blood lymphocytes, catalase activity and the expression levels of heat shock protein 70 (HSP70) of abalones in the control group were significantly lower than that in L1 and L2, but there was no significant difference when compared with L3. The levels of O₂^-, NO produced by respiratory burst of blood lymphocytes and the expression levels of catalase (CAT) in L1 and L2 were significantly higher than both L3 and the control group. Seven days after infection with V. parahaemolyticus, all abalones in the control group were dead. After 14 days the cumulative mortality rate of abalones in the L. pentosus-added groups was significantly lower than that in the control group. Therefore, the 10³ cfu/g and 10⁵ cfu/g L. pentosus-added food not only promoted food intake and growth of abalones, but also improved their non-specific immunity and reduced V. parahaemolyticus infection, indicating that this strain is a good potential candidate for probiotic added food in the aquaculture industry.

The Gut Microbiota of Marine Fish

The body of work relating to the gut microbiota of fish is dwarfed by that on humans and mammals. However, it is a field that has had historical interest and has grown significantly along with the expansion of the aquaculture industry and developments in microbiome research. Research is now moving quickly in this field. Much recent focus has been on nutritional manipulation and modification of the gut microbiota to meet the needs of fish farming, while trying to maintain host health and welfare. However, the diversity amongst fish means that baseline data from wild fish and a clear understanding of the role that specific gut microbiota play is still lacking. We review here the factors shaping marine fish gut microbiota and highlight gaps in the research.

Effects of a probiotic (Bacillus licheniformis) on the growth, immunity, and disease resistance of Haliotis discus hannai Ino

To study the effects of a probiotic (Bacillus lincheniformis) on the survival and growth of Haliotis discus hannai Ino, the expression levels of nonspecific immune genes and the resistance to Vibrio parahaemolyticus infection were assessed. Abalones (shell length: 27.64 ± 1.59 mm, body weight: 4.17 ± 0.32 g) were selected for use in an 8-week culture experiment and a 2-week V. parahaemolyticus artificial infection experiment. In both experiments, the control group (C) was fed with a basal feed and the experimental groups were fed with experimental food prepared by spraying the probiotic on the basal feed at different concentrations: 10³ (B1), 10⁵ (B2), and 10⁷ (B3) cfu/mL. The survival rate, total number of blood lymphocytes, activity of acid phosphatase, and expression level of heat shock protein 70 were significantly higher in B1, B2, and B3 than in C (P < 0.05). The specific growth rate of shell length, food intake, food conversion rate, phagocytic activity of blood lymphocytes, activities of myeloperoxidase and catalase (CAT), and expression levels of CAT and thioredoxin peroxidase of abalones in B2 were significantly higher than those in B1 and C (P < 0.05). Although the level of O₂^- produced by the respiratory burst of blood lymphocytes in B2 was not significantly different from those in B1 and B3, they were significantly higher than that in C (P < 0.05). The activity of superoxide dismutase (SOD), the nitric oxide levels produced by the respiratory burst of blood lymphocytes, and the expression levels of Mn-SOD in B1 and B3 were significantly higher than those in C but significantly lower than those in B2 (P < 0.05). Fourteen days after infection with V. parahaemolyticus, the cumulative mortality of abalones in B2 was significantly lower than those in B1 and C (P < 0.05). These results indicate that the food containing 10⁵ cfu/mL Bacillus licheniformis promoted food intake and growth of abalones and also improved their resistance to V. parahaemolyticus infection. Thus, B. licheniformis is a good potential probiotic.