Effects of dietary Lactobacillus rhamnosus JCM1136 and Lactococcus lactis subsp. lactis JCM5805 on the growth, intestinal microbiota, morphology, immune response and disease resistance of juvenile Nile tilapia, Oreochromis niloticus

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.

Oral administration of recombinant Lactococcus lactis expressing largemouth bass (Micropterus salmoides) IFNa3 protein enhances immune response against largemouth bass virus (LMBV) infection

Largemouth bass virus (LMBV) is a highly pathogenic pathogen that often causes high mortality of affected largemouth bass and significant financial losses. Type I interferon as an effective and broad spectrum tool has been successfully used for therapeutic or prophylactic treatment some viral infections. However, the implementation of immunotherapies based on interferon administration to combat LMBV infections has not been reported. And Lactic Acid Bacteria (LAB) are a powerful vehicle for expressing cytokines or immunostimulant peptides at the gastrointestinal level after oral administration. In this study, Lactococcus lactis (L. lactis) expression system with lactose as a screening marker was utilized to express the Micropterus salmoides interferon a3 (IFNa3) protein and orally administered to largemouth bass. The genetically engineered strain pNZ8149-Usp45-IFNa3-6His/L. lactis NZ3900 was successfully constructed, and its potential to elicit immune protection response by oral administration was evaluated. After orally administration, the recombinant L. lactis was detected in guts of experimental fish and remained detectable for 72 h. Additionally, IFNa3 was able to enhance the test fish's immune response, as determined by the relatively increased mRNA relative expression of immune-related genes in the liver, spleen, and kidney tissues, including IFN-γ, TNF-α, IL-1β, IL-8, IgM and IgT. Following LMBV challenge, the experiment group of pNZ8149-Usp45-IFNa3-6His/L. lactis NZ3900 exhibited a 70 % survival rate, while survival rate were 15 % in the PBS control group, 45 % in the pNZ8149/L. lactis NZ3900 group. Furthermore, the viral load in the surviving fish was significantly lower than that of the control groups. These findings suggest that oral administration of recombinant L. lactis producing IFNa3 induces largemouth bass immune responses at a systemic level to effective prevent and combat of LMBV infection.

The dietary effects of two strain probiotics (Leuconostoc mesenteroides, Lactococcus lactis) on growth performance, immune response and gut microbiota in Nile tilapia (Oreochromis niloticus)

The aquaculture industry has been growing rapidly over the past few decades to meet future animal protein demands. However, intensive aquaculture industry faces challenges such as growth abnormalities, high mortality rates, water quality and intestinal health deterioration. Administering probiotics can serve as a nutritional strategy to enhance the immune system and growth performance of fish influxes of gut microbiota. This study aimed to evaluate the impact of two dietary probiotic strains L. mesenteroides and L. lactis on the growth performance, immunity, and gut microbiota of Nile tilapia (Oreochromis niloticus). Fish were fed with basal and experimental diet supplemented by both L. mesenteroides and L. lactis bacteria at 106 cell/g for 8 weeks. Feeding a combination of L. mesenteroides and L. lactis resulted in significant improvements in feed utilization parameters (PER and FER) (P < 0.001), alternative complement pathway activity, intestinal lactic acid bacteria count (P < 0.012), mucus secretion (P < 0.002) and peroxidase activity (P < 0.001) compared to the control groups. Serum lysozyme activity also exhibited a significant increase in the L. mesenteroides and L. lactis dietary group (P < 0.011) compared to the control and single probiotic supplemented diet groups. Furthermore, Nile tilapia fed the L. mesenteroides and L. lactis supplemented diet showed enhanced growth performance metrics (weight gain, final weight and specific growth rate) compared to those fed control and single probiotic supplemented diets (P < 0.022). Additionally, superoxide dismutase activity was significantly elevated in the L.mesenteroides and L. lactis supplemented diet groups compared to the control and single L.mesenteroides supplemented diet groups (P < 0.017). These findings strongly indicate that a dietary combination of L. mesenteroides and L. lactis probiotics could function as a beneficial immunostimulant feed supplement in Nile tilapia aquaculture.

Probiotic Strain Lactobacillus plantarum GCLP4 Derived From the Gut of Ctenopharyngodon idella Modulates Growth, Digestive Enzymes and Immune-Biochemical Parameters of Labeo rohita Fingerlings

The present study was carried out to isolate, identify, and characterize bacterial probiotic strain from the gut of Ctenopharyngodon idella (Grass carp) and then to study its effect on growth, digestive enzymes and immunity of Labeo rohita fingerlings. A total of 6 gram-positive bacteria belonging to the genus Lactobacillus spp. (3), Bacillus spp. (2) and Staphylococcus spp. (1), were identified biochemically. Based on the biochemical results, the isolate GCLP4 was selected for molecular confirmation and BLAST analysis showed maximum homology with Lactobacillus plantarum (100% ident). Fish were fed for 60 days with diet containing 0 (T0), 105 (T1), 107 (T2), 109 (T3) cfu/g diet of L. plantarum GCLP4 and 105 (T4) of commercial probiotics. Results shows that supplementation of GCLP4 had significantly (p < 0.05) improve weight gain (%), specific growth rate (SGR) and feed conversion ratio (FCR) of L. rohita with better values in T3 group. The immunological parameters (white blood cell, red blood cell, haemoglobin, total protein, albumin and globulin were significantly higher (p < 0.05) with 107 and 109 Lactobacillus GCLP4 cfu/g diet. The digestive enzyme activities (protease, amylase and lipase) were significantly (p < 0.05) higher, particularly, with 109 Lactobacillus GCLP4 cfu/g of diet. All the groups supplemented with GCLP4 including the commercial probiotics have lower (p < 0.05) activities of serum transaminase enzymes along with lower (p < 0.005) level of glucose as compared to the control group. The results of the study collectively suggest that dietary L. plantarum GCLP4 at 109 cfu/g is an effective probiotic obtained from grass carp having potency to promote growth, digestive enzymes and immune-biochemical indices of L. rohita fingerlings in present culture condition.

The Indigenous Probiotic Lactococcus lactis PH3-05 Enhances the Growth, Digestive Physiology, and Gut Microbiota of the Tropical Gar (Atractosteus tropicus) Larvae

Probiotics in aquaculture hold promise for enhancing fish health and growth. Due to their increased specificity and affinity for their host, indigenous probiotics may offer isolated and potentially amplified benefits. This study investigated the effects of Lactococcus lactis PH3-05, previously isolated from adults of tropical gar (Atractosteus tropicus), on the growth, survival, digestive enzyme activity, intestinal morphology, expression of barrier and immune genes, and intestinal microbiota composition in the larvae of tropical gar. Larvae were fed with live L. lactis PH3-05 concentrations of 104, 106, and 108 CFU/g for 15 days alongside a control diet without probiotics. Higher concentrations of L. lactis PH3-05 (106 and 108 CFU/g) positively influenced larval growth, increasing hepatocyte area and enterocyte height. The 106 CFU/g dose significantly enhanced survival (46%) and digestive enzyme activity. Notably, the 108 CFU/g dose stimulated increased expression of muc-2 and il-10 genes, suggesting enhanced mucosal barrier function and anti-inflammatory response. Although L. lactis PH3-05 did not significantly change the diversity, structure, or Phylum level composition of intestinal microbiota, which was constituted by Proteobacteria, Bacteroidota, Chloroflexi, and Firmicutes, an increase in Lactobacillus abundance was observed in fish fed with 106 CFU/g, suggesting enhanced probiotic colonization. These results demonstrate that administering L. lactis PH3-05 at 106 CFU/g promotes growth, survival, and digestive health in A. tropicus larvae, establishing it as a promising indigenous probiotic candidate for aquaculture applications.

Probiotic, fructooligosaccharide and yeast extract mixture improves gut health in rainbow trout, Oncorhynchus mykiss

The present study investigated the effects of a feed additive (FA) containing a probiotic consortium, fructooligosaccharide and yeast extract on growth performance, humoral immune responses, hepatic antioxidant parameters and intestine digestive enzymes, morphology and transcripts in rainbow trout, Oncorhynchus mykiss. The fish were reared for 8 weeks, feeding on diets containing 0 (CNT), 0.3 (0.3FA), 1 (1FA) and 2 (2FA) g/kg FA. The results showed that fish growth parameters were significantly and quadratically related to FA levels and FA treatments had better growth performance than CNT treatment. Intestinal amylase activity significantly increased in 2FA, whereas, intestinal protease activity increased in all FA treatments. Intestinal villus length and muscular layer thickness significantly increased in 0.3FA treatment. Blood leucocyte and lymphocyte counts, plasma lysozyme activity and hepatic glutathione content significantly increased in 0.3FA and 1FA treatments; whereas hepatic malondialdehyde significantly decreased in these treatments. Blood neutrophil and monocyte counts significantly increased in 0.3FA treatment, while plasma alternative complement activity significantly increased in 1FA treatments. Plasma bactericidal activities against Aeromonas hydrophila, Yersinia ruckeri and Streptococcus iniae, and intestinal expression of heat shock protein 70 and beta-defensin significantly increased in all FA treatments. The abundance of A. hydrophila, Y. ruckeri and S. iniae in fish gut significantly decreased in 0.3FA treatment; these bacteria were absent in the intestines of 1FA and 2FA treatments. The present results suggest that dietary 0.3-1 g/kg of FA can significantly improve growth performance, immune response, intestinal health and hepatic antioxidant capacity in rainbow trout.

Assessment of Encapsulated Probiotic Lactococcus lactis A12 Viability Using an In Vitro Digestion Model for Tilapia

Probiotics face harsh conditions during their transit through the gastrointestinal tract (GIT) of fish because of low-pH environments and intestine fluid. Therefore, the evaluation of probiotic viability under simulated gastrointestinal conditions is an important step to consider for probiotic supplementation in fish feed prior to in vivo trials. Therefore, this study aimed to evaluate the effect of stomach and intestinal simulated conditions on the viability of encapsulated Lactococcus lactis A12 using an in vitro digestion model for tilapia. A Box Behnken design was used to evaluate the potential effect of three factors, namely stomach pH, residence time in the stomach, and enzyme quantity, on the viability of encapsulated Lactococcus lactis A12. As the main results, low pH (4.00), long residence time (4 h), and enzyme quantity (2.68 U of total protease activity) led to lower final cell counts after the phases of the stomach and intestine. Encapsulated probiotic bacteria showed higher viability (p < 0.05) and antibacterial activity (p < 0.05) against the pathogen Streptococcus agalactiae than non-encapsulated bacteria. The results suggest that L. lactis A12 survives in GIT conditions and that the proposed in vitro model could be used to explore the viability of probiotic bacteria intended for fish feed supplementation.

Effects of Dietary Multi-Strain Probiotics on Growth Performance, Antioxidant Status, Immune Response, and Intestinal Microbiota of Hybrid Groupers (Epinephelus fuscoguttatus ♀ × E. lanceolatus ♂)

This study aims to examine the effects of the mixture of Bacillus cereus G1-11 and Exiguobacterium acetylicum G1-33, isolated from the gut of hybrid groupers (Epinephelus fuscoguttatus ♀ × E. lanceolatus ♂), on the host. The hybrid groupers were divided into a control (C, without any probiotics), B. cereus (BC, 1010 cfu/g), E. acetylicum (EA, 108 cfu/g), compound (mix, a 1:1 mixture of B. cereus and E. acetylicum), and positive reference group (P, Lactobacillus acidophilus, 5 × 108 cfu/L). Each group had four replicates, with 30 fish per replicate (53.30 ± 0.50 g), and were fed for 60 days. The results showed that adding probiotics to the feed significantly improved the weight gain, weight growth rate, specific growth rate, and digestive enzyme activities of hybrid groupers compared to the C group. The compound group was the most significant. In addition, composite probiotics added to feed significantly upregulated the expression levels of several growth-related genes in the liver and muscles. The activities of alkaline phosphatase, catalase, glutathione peroxidase, glutathione transferase, lysozyme, and total antioxidant capacity in the serum and liver were significantly influenced through mixed probiotic feeding. Moreover, the expression levels of several immune-related genes in the liver, spleen, and head kidney were significantly enhanced by adding single and mixed probiotics to feed, with the synergy of mixed probiotics being the best. An analysis of the gut microbiota showed that adding composite bacteria enhanced the richness and diversity of the gut microbiota, significantly increasing the relative abundance of potential probiotics (Cetobacterium and Microbacterium) while decreasing the presence of potential pathogens (Mycoplasma). Overall, our findings highlighted the efficacy of mixed probiotics (B. cereus and E. acetylicum) in enhancing growth performance, nutritional value of hybrid grouper feed, antioxidant capacity, immune response, and intestinal health, in finding the best combination of functional feed additives.

Commensal Bacillus pumilus SE5-Derived Peptidoglycan and Lipoteichoic Acid Showed Synergistic Effects in Improving Growth, Immunity, and Intestinal Health of Grouper (Epinephelus coioides)

Commensal-derived peptidoglycan (PG) or lipoteichoic acid (LTA) can improve the growth, immunity, and intestinal health of fish, but it is not clear whether the two components have synergistic effects. To clarify this, grouper (Epinephelus coioides) was fed basal diet (CG) or diets containing 1.0 × 108 CFU/g heat-inactivated SE5 (HIB), PG (21.30 mg/kg), LTA (6.70 mg/kg), mixture (PL1) of PG (10.65 mg/kg) and LTA (3.35 mg/kg), and mixture (PL2) of PG (21.30 mg/kg) and LTA (6.70 mg/kg). Improved growth performance and feed utilization were observed in groups PG, LTA, PL1, and PL2, and the optimum growth performance was recorded in group PL1. Furthermore, improved serum alkaline phosphatase (AKP) activity and immunoglobulin M (IgM) and complement C3 (C3) contents were observed in all treatments, and the AKP activity in group PL1 was significantly superior to that of groups PG and LTA. Although PG and LTA alone or in combination exert comparable effects on intestinal microbiota and physical structure, obviously enhanced intestinal protease activity was observed in group PL1. The combined efficacy of PL1 could further potentiate the immune response by modulating the nucleotide-binding oligomerization domain-containing protein 2 (NOD2) and upregulating the expression of antimicrobial peptides (epinecidin-1, hepcidin-1, and β-defensin) as well as IgM. At the same time, group PL1 could further mitigate intestinal inflammation by downregulating pro-inflammatory cytokines and upregulating anti-inflammatory cytokines. In conclusion, probiotic B. pumilus SE5-derived PG and LTA mixture (10.65 mg/kg PG and 3.35 mg/kg LTA) exhibits better potential for improving the growth performance, intestinal health, and immune function compared to another mixture (21.30 mg/kg PG and 6.70 mg/kg LTA) and PG or LTA alone in grouper.

From growth promotion to intestinal inflammation alleviation: Unraveling the potential role of Lactobacillus rhamnosus GCC-3 in juvenile grass carp (Ctenopharyngodon idella)

Lactobacillus rhamnosus is a probiotic, which not only promotes the growth of animals, but also has anti-inflammatory effects. However, the mechanism by which Lactobacillus rhamnosus regulates intestinal immunity is not well comprehended. Hence, the study aimed to research how Lactobacillus rhamnosus affects the intestinal immunity using juvenile grass carp (Ctenopharyngodon idella) as a model. We selected 1800 juvenile grass carp for testing. They were divided into six treatments and fed with six gradients of Lactobacillus rhamnosus GCC-3 (0.0, 0.5, 1.0, 1.5, 2.0, 2.5 g/kg) for 70 days. Enteritis was subsequently induced with dextroside sodium sulfate. Results indicated that dietary Lactobacillus rhamnosus GCC-3 addition improved growth performance. Meanwhile, appropriate levels of Lactobacillus rhamnosus GCC-3 alleviated excessive inflammatory response by down-regulating the expression of TLR4 and NOD receptors, up-regulating the expression of TOR, and then down-regulating the expression of NF-κB. Additionally, appropriate Lactobacillus rhamnosus GCC-3 improved intestinal immunity by reducing pyroptosis triggered by NLRP3 inflammasome and mediated by GSDME. Furthermore, 16 S rRNA sequencing showing appropriate levels of Lactobacillus rhamnosus GCC-3 increased Lactobacillus and Bifidobacterium abundance and decreased Aeromonas abundance. These results suggest that Lactobacillus rhamnosus GCC-3 can alleviate intestinal inflammation through down-regulating NF-κB and up-regulating TOR signaling pathways, as well as by inhibiting pyroptosis.

The effects of Bacillus subtilis on the immunity, mucosal tissue morphology, immune-related gene transcriptions, and intestinal microbiota in flounder (Paralichthys olivaceus) with two feeding methods: Continuous versus discontinuous feeding

Probiotics as dietary additives can improve weight gain, feed efficiency, and disease resistance in cultured fish. In this research, we evaluated and compared the effects of Bacillus subtilis on immunity, mucosal tissue morphology, immune-related gene transcriptions, and intestinal microbiota in flounder (Paralichthys olivaceus) by a 30-day feeding experiment based on a continuous feeding schedule (E1) and a discontinuous feeding schedule (E2). As a result, the use of B. subtilis exerted the best positive effects on survival rate, enzyme activity, mucosal tissue morphology, immune-related gene transcriptions, and intestinal microbiota in flounders. Alkaline phosphatase (AKP), lysozyme (LZM), and superoxide dismutase (SOD) activities in the liver of E2 were higher than those of E1 (P < 0.05). Furthermore, the villi length in the intestinal tract and the fold length in the stomach of E2 were also higher than in E1 (P < 0.05). The il-1 expression levels in the spleen were significantly increased in E2 (P < 0.05) compared to E1. We performed 16 S rRNA sequencing analysis to find that Bacillus in E1 (1.06%) and E2 (1.01%) had higher relative abundances than in E0 (0.053%) at the end of the experiments, indicating that short-term application of B. subtilis with the continuous or discontinuous feeding method can allow both the adaptation of the ecosystem to the presence of probiotics by the establishment of new species in the gut microbiota and the ability these new probiotic species to perform corresponding functions. No significant differences in the ability of probiotic establishment were observed between E1 and E2. Our findings provided a unique perspective to explore the mechanism of immune enhancement with probiotics and to screen the optimal administration strategy in aquaculture application for probiotic use. Together, these results point to some level of enhancement in immune status by continuous and discontinuous feeding after a short-term feeding period, which could be used as a prophylactic strategy for flounder health management.

[Cu(NN1)2]ClO4, a Copper (I) Complex as an Antimicrobial Agent for the Treatment of Piscirickettsiosis in Atlantic Salmon

Piscirickettsia salmonis is the pathogen that most affects the salmon industry in Chile. Large quantities of antibiotics have been used to control it. In search of alternatives, we have developed [Cu(NN1)2]ClO4 where NN1 = 6-((quinolin-2-ylmethylene)amino)-2H-chromen-2-one. The antibacterial capacity of [Cu(NN1)2]ClO4 was determined. Subsequently, the effect of the administration of [Cu(NN1)2]ClO4 on the growth of S. salar, modulation of the immune system and the intestinal microbiota was studied. Finally, the ability to protect against a challenge with P. salmonis was evaluated. The results obtained showed that the compound has an MIC between 15 and 33.9 μg/mL in four isolates. On the other hand, the compound did not affect the growth of the fish; however, an increase in the transcript levels of IFN-γ, IL-12, IL-1β, CD4, lysozyme and perforin was observed in fish treated with 40 μg/g of fish. Furthermore, modulation of the intestinal microbiota was observed, increasing the genera of beneficial bacteria such as Lactobacillus and Bacillus as well as potential pathogens such as Vibrio and Piscirickettsia. Finally, the treatment increased survival in fish challenged with P. salmonis by more than 60%. These results demonstrate that the compound is capable of protecting fish against P. salmonis, probably by modulating the immune system and the composition of the intestinal microbiota.

Effects of dietary supplementation of Bacillus subtilis DSM 32315 on growth, immune response and acute ammonia stress tolerance of Nile tilapia (Oreochromis niloticus) fed with high or low protein diets

Aquatic animals have benefited from Bacillus subtilis-based probiotics over the past few decades. This study evaluated the effects of B. subtilis DSM 32315 probiotics as a feed additive on growth, immune response and resistance to acute ammonia challenge in Nile tilapia. Specifically, four supplemental levels (0%, 0.1%, 0.2%, and 0.3%) of B. subtilis probiotics were tested under two dietary protein levels (32% and 28%). Five replicate tanks were randomly allotted to each dietary treatment, with each tank containing 30 Nile tilapia. After 8 weeks of feeding, Nile tilapia in each tank were exposed to 43.61 mg/L of total ammonia nitrogen for 48 h. The results revealed that reducing protein levels from 32% to 28% did not affect growth performance or antioxidant capacity. However, the low protein diet tended to induce an inflammatory effect shown by increased expressions of TGF-β and IFN-γ genes (P < 0.05) in the liver. The impact was alleviated by the probiotic supplementation. Compared with the non-supplemented group, 0.1% probiotic supplementation remarkably increased plasma lysozyme activity, total antioxidant capacity and complement C3 and interleukin-10 mRNA levels (P < 0.05) in the 28% protein diet, while higher supplementation of probiotics (0.3%) was shown to be beneficial for the high protein diet (32%). In both the dietary protein levels, 0.1% supplementation of probiotics promoted the antioxidant capacity of Nile tilapia before exposure to ammonia stress but higher probiotic supplementation (0.3%) proved to be necessary under ammonia stress as evidenced by higher fish survival rate. Results exhibited that supplementation with B. subtilis probiotics had a better effect on the intestinal morphology (villi height and width) regardless of protein levels. In conclusion, dietary supplementation of B. subtilis DSM 32315 probiotics at 0.1% in the low protein diet and up to 0.3% in the high protein diet showed beneficial effects on the growth, immunity, and antioxidant capacity of Nile tilapia. Under ammonia stress conditions, the higher supplementation of B. subtilis DSM 32315 probiotics at 0.3% improves stress tolerance of Nile tilapia despite the two dietary protein levels (32%; 28%).

Microbiota plasticity in tilapia gut revealed by meta-analysis evaluating the effect of probiotics, prebiotics, and biofloc

Tilapia species are among the most cultivated fish worldwide due to their biological advantages but face several challenges, including environmental impact and disease outbreaks. Feed additives, such as probiotics, prebiotics, and other microorganisms, have emerged as strategies to protect against pathogens and promote immune system activation and other host responses, with consequent reductions in antibiotic use. Because these additives also influence tilapia's gut microbiota and positively affect the tilapia culture, we assume it is a flexible annex organ capable of being subject to significant modifications without affecting the biological performance of the host. Therefore, we evaluated the effect of probiotics and other additives ingested by tilapia on its gut microbiota through a meta-analysis of several bioprojects studying the tilapia gut microbiota exposed to feed additives (probiotic, prebiotic, biofloc). A total of 221 tilapia gut microbiota samples from 14 bioprojects were evaluated. Alpha and beta diversity metrics showed no differentiation patterns in relation to the control group, either comparing additives as a group or individually. Results also revealed a control group with a wide dispersion pattern even when these fish did not receive additives. After concatenating the information, the tilapia gut core microbiota was represented by four enriched phyla including Proteobacteria (31%), Fusobacteria (23%), Actinobacteria (19%), and Firmicutes (16%), and seven minor phyla Planctomycetes (1%), Chlamydiae (1%), Chloroflexi (1%), Cyanobacteria (1%), Spirochaetes (1%), Deinococcus Thermus (1%), and Verrucomicrobia (1%). Finally, results suggest that the tilapia gut microbiota is a dynamic microbial community that can plastically respond to feed additives exposure with the potential to influence its taxonomic profile allowing a considerable optimal range of variation, probably guaranteeing its physiological function under different circumstances.

Lactococcus lactis, a bacterium with probiotic functions and pathogenicity

Lactococcus lactis (L. lactis) is the primary organism for lactic acid bacteria (LAB) and is a globally recognized safe microorganism for the regulation of the intestinal micro-ecological balance of animals and improving the immune performance of the host. L. lactis is known to play a commercially important role in feed fortification, milk fermentation, and vaccine production, but pathogenic L. lactis has been isolated from many clinical cases in recent years, such as the brain of silver carp with Lactococcosis, the liver and spleen of diseased waterfowl, milk samples and padding materials with cow mastitis, and blood and urine from human patients with endocarditis. In dairy farming, where L. lactis has been used as a probiotic in the past, however, some studies have found that L. lactis can cause mastitis in cows, but the lack of understanding of the pathogenesis of mastitis in cows caused by L. lactis has become a new problem. The main objective of this review is to analyze the increasingly serious clinical mastitis caused by L. lactis and combined with the wide application of L. lactis as probiotics, to comprehensively discuss the characteristics and diversity of L. lactis.

Interactive Effect of Dietary Heat-Killed Lactobacillus Plantarum L-137 and Berberine Supplementation on Intestinal Mucosa and Microbiota of Juvenile Black Sea Bream (Acanthopagrus Schlegelii)

To compare the synergistic impact of dietary heat-killed Lactobacillus plantarum and berberine supplementation on intestinal health of juvenile black sea bream, the test fish (5.67 ± 0.05 g) were fed three diets: a basal control diet designated as Con; basal diet supplemented with 400 mg/kg L. plantarum, labelled LP; and basal diet supplemented with 400 mg/kg L. plantarum + 50 mg/k berberine, labelled LPBB. After 56 days of feeding, the control fish had significantly lower intestinal villus height (VH), villus surface area (VSA), and muscularis mucosae (MS) thickness than the rest of the groups (P < 0.05). The LPBB fish had significantly higher VH than the control fish, and wider MS and VSA than the rest of the groups (P < 0.05). Occludin was significantly upregulated in the LPBB fish, and heat shock protein 90 was upregulated in the control fish (P < 0.05). The abundance of Proteobacteria family was significantly higher in the intestinal microbiome of the control and LP fish, the LPBB fish had higher abundance of Cyanobacteria and Spirochaetes, and the LP group had higher Bacteroidetes abundance (P < 0.05). Potentially beneficial Delftia and Brevinema were the significantly abundant genera in the LP and LPBB fish, respectively; potentially pathogenic Elizabethkingia was abundant in the LP fish; and the control fish had higher abundance of potentially pathogenic Burkholderia-Caballeronia-Paraburkholderia (P < 0.05). According to these results, there is possible synergy between L. plantarum and berberine as dietary supplements in fostering healthy intestine for black sea bream than L. plantarum alone.

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.

Evaluation of dietary single probiotic isolates and probiotic multistrain consortia in growth performance, gut histology, gut microbiota, immune regulation, and infection resistance of Nile tilapia, Oreochromis niloticus, shows superior monostrain performance

The probiotic potential of a designed bacterial consortia isolated from a competitive exclusion culture originally obtained from the intestinal contents of tilapia juveniles were evaluated on Nile tilapia alevins. The growth performance, intestinal histology, microbiota effects, resistance to Streptococcus agalactiae challenge, and immune response were assessed. In addition, the following treatments were included in a commercial feed: A12+M4+M10 (Lactococcus lactis A12, Priestia megaterium M4, and Priestia sp. M10), M4+M10 (P. megaterium M4, and Priestia sp. M10) and the single bacteria as controls; A12 (L. lactis A12), M4 (P. megaterium M4), M10 (Priestia sp. M10), also a commercial feed without any probiotic addition was included as a control. The results showed that all probiotic treatments improved the growth performance, intestinal histology, and resistance during experimental infection with S. agalactiae in comparison to the control fish. Also, the administration of probiotics resulted in the modulation of genes associated with the innate and adaptive immune systems that were non-dependent on microbial colonization. Surprisingly, L. lactis A12 alone induced benefits in fish compared to the microbial consortia, showing the highest increase in growth rate, survival during experimental infection with S. agalactiae, increased intestinal fold length, and the number of differentially expressed genes. Lastly, we conclude that a competitive exclusion culture is a reliable source of probiotics, and monostrain L. lactis A12 has comparable or even greater probiotic potential than the bacterial consortia.

Sustainable Fish Feeds with Insects and Probiotics Positively Affect Freshwater and Marine Fish Gut Microbiota

Aquaculture is the fastest-growing agricultural industry in the world. Fishmeal is an essential component of commercial fish diets, but its long-term sustainability is a concern. Therefore, it is important to find alternatives to fishmeal that have a similar nutritional value and, at the same time, are affordable and readily available. The search for high-quality alternatives to fishmeal and fish oil has interested researchers worldwide. Over the past 20 years, different insect meals have been studied as a potential alternate source of fishmeal in aquafeeds. On the other hand, probiotics-live microbial strains-are being used as dietary supplements and showing beneficial effects on fish growth and health status. Fish gut microbiota plays a significant role in nutrition metabolism, which affects a number of other physiological functions, including fish growth and development, immune regulation, and pathogen resistance. One of the key reasons for studying fish gut microbiota is the possibility to modify microbial communities that inhabit the intestine to benefit host growth and health. The development of DNA sequencing technologies and advanced bioinformatics tools has made metagenomic analysis a feasible method for researching gut microbes. In this review, we analyze and summarize the current knowledge provided by studies of our research group on using insect meal and probiotic supplements in aquafeed formulations and their effects on different fish gut microbiota. We also highlight future research directions to make insect meals a key source of proteins for sustainable aquaculture and explore the challenges associated with the use of probiotics. Insect meals and probiotics will undoubtedly have a positive effect on the long-term sustainability and profitability of aquaculture.

Microbiome analysis of thai traditional fermented soybeans reveals short-chain fatty acid-associated bacterial taxa

Thua Nao is a Thai traditional fermented soybean food and low-cost protein supplement. This study aimed to evaluate the bacterial community in Thua Nao from northern Thailand and assess potentially active short-chain fatty acids (SCFAs)-related bacteria. Sixty-five Thua Nao consisting of 30 wet and 35 dried samples were collected from six provinces: Chiang Rai, Chiang Mai, Mae Hong Son, Lampang, Lamphun, and Phayao. Bacterial diversity was significantly higher in the wet samples than in the dried samples. The dominant phyla were Firmicutes (92.7%), Proteobacteria (6.7%), Actinobacteriota (0.42%), and Bacteroidota (0.26%). The genus Bacillus (67%) was the most represented in all samples. Lactobacillus, Enterococcus, and Globicatella were enriched in the wet samples. Assessment of the SCFA-microbiota relationships revealed that high butyrate and propionate concentrations were associated with an increased Clostridiales abundance, and high acetate concentrations were associated with an increased Weissella abundance. Wet products contained more SCFAs, including acetate (P = 2.8e-08), propionate (P = 0.0044), butyrate (P = 0.0021), and isovalerate (P = 0.017), than the dried products. These results provide insight into SCFA-microbiota associations in Thua Nao, which may enable the development of starter cultures for SCFA-enriched Thua Nao production.

Screening of host gut-derived probiotics and effects of feeding probiotics on growth, immunity, and antioxidant enzyme activity of hybrid grouper (Epinephelus fuscoguttatus♀ × E. lanceolatus♂)

In recent years, the widespread use of antibiotics in intensive grouper mariculture has resulted in the ineffectiveness of antibiotic treatment, leading to an increasing incidence of diseases caused by bacteria, viruses, and parasites, causing serious economic losses. Hence, it is crucial to develop alternative strategies to antibiotics for healthy and sustainable development of the mariculture industry. Here, we aimed to screen host gut-derived probiotics and evaluate its effects on growth and immunity of grouper. In this study, 43 bacterial strains were isolated from the intestine of the hybrid grouper (Epinephelus fuscoguttatus♀ × E. lanceolatus♂), and a potential probiotic strain G1-26, which can efficiently secrete amylase, protease, and lipase, was obtained using different screening mediums. Based on 16S rDNA sequencing, the potential probiotic strain G1-26 was identified as Vibrio fluvialis. The results of a biological characteristic evaluation showed that V. fluvialis G1-26 could grow at 25-45 °C, pH 5.5-7.5, salinity 10-40, and bile salt concentration 0-0.030%, and produce amylase, lipase, and protease under different culture conditions. Additionally, V. fluvialis G1-26 is sensitive to many antibiotics and does not exhibit aquatic biotoxicity. Subsequently, hybrid groupers were fed diets containing V. fluvialis G1-26 at different concentrations (0, 10⁶, 10⁸, and 10¹⁰ CFU/g) for 60 d. The results showed that V. fluvialis G1-26 at 10⁸ CFU/g did not significantly affect the growth performance of the hybrid grouper (P > 0.05). V. fluvialis G1-26 supplementation at 10⁸ and 10¹⁰ CFU/g significantly promoted the relative expression of immune-related genes in hybrid groupers (TLR3, TLR5, IL-1β, IL-8, IL-10, CTL, LysC, TNF-2, and MHC-2) and improved the activities of alkaline phosphatase, acid phosphatase, total superoxide dismutase, and total protein in the liver. In conclusion, V. fluvialis G1-26, a potential probiotic strain isolated from the intestine of the hybrid grouper, can be used as an effective immunopotentiator at an optimal dose of 10⁸ CFU/g diet. Our results provide a scientific basis for the development and utilization of probiotics in the grouper mariculture industry.

Identification and evaluation of potential probiotics against skin-ulceration disease in the Chinese tongue sole (Cynoglossus semilaevis)

In this study, three highly pathogenic bacterial strains (Vibrio harveyi TB6, Vibrio alginolyticus TN1, and Vibrio parahaemolyticus TN3) were isolated from skin ulcers and intestines of diseased Chinese tongue sole (Cynoglossus semilaevis). The bacteria were investigated using hemolytic activity tests, in vitro co-culture with intestinal epithelial cells, and artificial infection of C. semilaevis. A further 126 strains were isolated from the intestines of healthy C. semilaevis. The three pathogens were used as indicator bacteria, and the antagonistic strains were identified from the 126 strains. The activities of exocrine digestive enzymes in the strains were also tested. Four strains with antibacterial and digestive enzyme activities were obtained and the best strains, Bacillus subtilis Y2 and Bacillus amyloliquefaciens Y9, were selected according to their ability to protect epithelial cells from infection. In addition, the effects of strains Y2 and Y9 at the individual level were investigated, finding that the activities of the immune-related enzymes superoxide dismutase, catalase, acid phosphatase, and peroxidase were significantly increased in the sera of the treatment group compared with the control group (p < 0.05). The specific growth rate (SGR, %) was also increased, especially in the Y2 group, and was significantly higher compared with the controls (p < 0.05). The result of the artificial infection test showed that the cumulative mortality within 72 h in the Y2 group was the lowest (50.5%), and in the Y9 group (68.5%) it was significantly lower than that in the control group (100%) (p < 0.05). Analysis of the intestinal microbial communities indicated that Y2 and Y9 could alter the composition of the intestinal flora, increasing both species richness and evenness, and inhibiting the growth of Vibrio in the intestine. These results suggested food supplemented with Y2 and Y9 could improve both immune function and disease resistance, as well as have a positive effect on the growth performance and the intestinal morphology of C. semilaevis.

Interspecies Interactions within the Host: the Social Network of Group B Streptococcus

Group B Streptococcus (GBS) is a pervasive neonatal pathogen accounting for a combined half a million deaths and stillbirths annually. The most common source of fetal or neonatal GBS exposure is the maternal microbiota. GBS asymptomatically colonizes the gastrointestinal and vaginal mucosa of 1 in 5 individuals globally, although its precise role in these niches is not well understood. To prevent vertical transmission, broad-spectrum antibiotics are administered to GBS-positive mothers during labor in many countries. Although antibiotics have significantly reduced GBS early-onset neonatal disease, there are several unintended consequences, including an altered neonatal microbiota and increased risk for other microbial infections. Additionally, the incidence of late-onset GBS neonatal disease remains unaffected and has sparked an emerging hypothesis that GBS-microbe interactions in developing neonatal gut microbiota may be directly involved in this disease process. This review summarizes our current understanding of GBS interactions with other resident microbes at the mucosal surface from multiple angles, including clinical association studies, agriculture and aquaculture observations, and experimental animal model systems. We also include a comprehensive review of in vitro findings of GBS interactions with other bacterial and fungal microbes, both commensal and pathogenic, along with newly established animal models of GBS vaginal colonization and in utero or neonatal infection. Finally, we provide a perspective on emerging areas of research and current strategies to design microbe-targeting prebiotic or probiotic therapeutic intervention strategies to prevent GBS disease in vulnerable populations.

Heavy metals (HMs) pollution in the aquatic environment: Role of probiotics and gut microbiota in HMs remediation

The presence of heavy metals (HMs) in aquatic ecosystems is a universal concern due to their tendency to accumulate in aquatic organisms. HMs accumulation has been found to cause toxic effects in aquatic organisms. The common HMs-induced toxicities are growth inhibition, reduced survival, oxidative stress, tissue damage, respiratory problems, and gut microbial dysbiosis. The application of dietary probiotics has been evolving as a potential approach to bind and remove HMs from the gut, which is called "Gut remediation". The toxic effects of HMs in fish, mice, and humans with the potential of probiotics in removing HMs have been discussed previously. However, the toxic effects of HMs and protective strategies of probiotics on the organisms of each trophic level have not been comprehensively reviewed yet. Thus, this review summarizes the toxic effects caused by HMs in the organisms (at each trophic level) of the aquatic food chain, with a special reference to gut microbiota. The potential of bacterial probiotics in toxicity alleviation and their protective strategies to prevent toxicities caused by HMs in them are also explained. The dietary probiotics are capable of removing HMs (50-90%) primarily from the gut of the organisms. Specifically, probiotics have been reported to reduce the absorption of HMs in the intestinal tract via the enhancement of intestinal HM sequestration, detoxification of HMs, changing the expression of metal transporter proteins, and maintaining the gut barrier function. The probiotic is recommended as a novel strategy to minimize aquaculture HMs toxicity and safe human health.

Effects of enrofloxacin's exposure on the gut microbiota of Tilapia fish (Oreochromis niloticus)

Enrofloxacin (ENFX) has a broad-spectrum antibiotic activity, which is widely used in aquaculture. The effect of different ENFX exposure ways on the gut microbiota of tilapia is unclear. This study was conducted to investigate the effects of ENFX exposure on the gut microbiota of tilapia fish (Oreochromis niloticus). Three methods of ENFX exposure were selected: injection (IEG), oral administration (OEG) and soaking (SEG). After 48 h of exposure period, the intestine of tilapia was collected for high-throughput sequencing. PCoA analysis revealed a distinct clustering of control group, and which was located rather far away from ENFX exposure groups. The dominant phyla in the gut microbiota of tilapia fish were Proteobacteria, Actinobacteriota, Fusobacteria and Firmicutes. Compared to the control group, phylum Fusobacteriota was increased in SEG and IEG while decreased in OEG. ENFX treatment led to a decline in Corynebacterium, Clostridium sensu stricto_3 and Bacillus in treated fish compared with control fish, accompanied by an increase in Akkermansia, Ralstonia and Romboutsia. IEG had the least effect on gut microbiota of tilapia because it retained more microbes among treatment groups. Alpha- diversity decreased the most in SEG, but retained more probiotics such as Cetobacterium and Akkermansia. We assessed the effect of enrofloxacin on tilapia by changes in intestinal flora. The result indicated that either exposure method significantly reduced the diversity of tilapia gut microbiota. It may provide basic data for the scientific use of ENFX in aquaculture.

Immunization effect of recombinant Lactobacillus casei displaying Aeromonas veronii Aha1 with an LTB adjuvant in carp

Aeromonas veronii is an important aquatic zoonotic, which elicits a range of diseases, such as haemorrhagic septicemia. To develop an effective oral vaccine against Aeromonas veronii infection in carp, the Aeromonas veronii adhesion (Aha1) gene was used as a target molecule to attach to intestinal epithelial cells. Two anchored recombinant. Lactic acid bacteria strains (LC-pPG-Aha1 1038 bp and LC-pPG-Aha1-LTB 1383 bp) were constructed by fusing them with the E. coli intolerant enterotoxin B subunit (LTB) gene and using Lactobacillus casei as antigen delivery vector to evaluate immune effects of these in carp. Western blotting and immunofluorescence were used to confirm that protein expression was successful. Additionally, levels of specific IgM in serum and the activities of ACP, AKP, SOD, LYS, C3, C4, and lectin enzymes-were assessed. Cytokines IL-10, IL-1β, TNF-α, IgZ1, and IgZ2 were measured in the liver, spleen, kidney, intestines, and gills tissue by qRT-PCR, which showed an increasing trend compared with the control group (P < 0.05). A colonization assay showed that the two L. casei recombinants colonized the middle and hind intestines of immunized fish. When immunized carp were experimentally challenged with Aeromonas veronii the relative percentage protection of LC-pPG-Aha1 was 53.57%, and LC-pPG-Aha1-LTB was 60.71%. In conclusion, these results demonstrate that Aha1 is a promising candidate antigen when it is displayed on lactic acid bacteria (Lc-pPG-Aha1 and Lc-pPG-Aha1-LTB) seems promising for a mucosal therapeutic approach. We plan to investigate the molecular mechanism of the L. casei recombinant in regulating the intestinal tissue of carp in future studies.

Rosmarinic acid alone or in combination with Lactobacillus rhamnosus ameliorated ammonia stress in the rainbow trout, Oncorhynchus mykiss: growth, immunity, antioxidant defense and liver functions

Rosmarinic acid (RS) and Lactobacillus rhamnosus (LR) were added singularly or in combination to rainbow trout (Oncorhynchus mykiss) diets to test their efficacy in the protection against ammonia stress. Fish (31.4±0.6 g) were randomly allocated to six groups in three replicates, as follows: T1: basic food as control, T2: LR with a concentration of 1.5 × 108 CFU/g, T3: LR with a concentration of 3 × 108 CFU/g, T4: 1 g RS/kg, T5: 3 g RS/kg, and T6: 1.5 × 108 CFU/g LR + 1 g RS/kg and T7: 3 × 108 CFU/g LR + 3 g RS/kg. After 60 days feeding, fish exposed to 0ammonia stress. After the feeding period, the supplemented fish had the highest final body weight (FW), weight gain (WG), and specific growth rate (SGR), and the lowest feed conversion ratio (FCR) as compared with the control group (P<0.05). Amylase, protease and lipase activities were noticed markedly higher in fish supplemented with 1.5 × 108 CFU/g LR + 1 g RS/kg and 1.5 × 108 CFU/g LR diets compared to the control (P<0.05). Generally, fish in supplemented diets, particularly T2 and T6 groups, had the highest lysozyme, alternative complement activity (ACH50), total Ig, nitroblue tetrazolium test (NBT), myeloperoxidase (MPO), complement component 3 (C3), complement component 4 (C4), superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx). On the other hand, T2 and T6 groups had the lowest malondialdehyde (MDA), glucose, and cortisol concentrations as well as alanine aminotransferase (ALT), alkaline phosphatase (ALP), lactate dehydrogenase (LDH) enzyme levels when were compared with the control (P<0.05). After ammonia stress, fish in the supplemented groups, particularly T2 and T6, generally showed significantly higher values of lysozyme, ACH50, total Ig, NBT, MPO, C3, C4, SOD, CAT, GPx and lower levels of MDA, glucose, cortisol, ALT, ALP, LDH when compared with the control (P<0.05). In conclusion, a combined administration of RS and L. rhamnosus effectively improved growth performance and health status as well as enhanced the resistance of rainbow trout against ammonia toxicity.

Dietary Effects of Probiotic Lactobacillus casei on Some Immunity Indices of Common Carp (Cyprinus carpio) Exposed to Cadmium

The present study investigated the effect of feeding common carp with Lactobacillus probiotic supplement on growth, blood indices, and immunity of serum and mucus exposed to cadmium. A total of 250 common carp with an average weight of 15 ± 0.62 were divided into three groups: fish without probiotics, and fish with two levels of probiotic. At the end of a 42-day feeding period, 50% of the lethal concentrations of cadmium were exposed for 14 days and blood and immune indices of serum and mucus were measured. The results showed that growth indices, blood indices, and serum and mucus immunity were significantly different from the control group (P < 0.05). Finally, this study showed probiotics could improve the decreasing effect of cadmium in BWI and PBWI indices and the additive effect of cadmium in FCR and ALT. Probiotic was also able to completely improve the decreasing effect of cadmium on lymphocyte indices and the additive effect of cadmium on ALP and AST and monocyte.

Partial Substitution of Fish Meal with Soy Protein Concentrate on Growth, Liver Health, Intestinal Morphology, and Microbiota in Juvenile Large Yellow Croaker (Larimichthys crocea)

The present study investigated the growth performance, feed utilization, intestinal morphology, and microbiota communities of juvenile large yellow croaker (Larimichthys crocea) fed diets containing different proportions of soy protein concentrate (SPC) (0, 15%, 30%, and 45%, namely FM, SPC15, SPC30, and SPC45) as a substitute for fish meal (FM) for 8 weeks. The weight gain (WG) and specific growth rate (SGR) in fish fed SPC45 were significantly lower than those fed FM and SPC15 but not differ with these fed SPC30. The feed efficiency (FE) and protein efficiency ratio (PER) decreased sharply when the dietary SPC inclusion level was higher than 15%. The activity of alanine aminotransferase (ALT) and expression of alt and aspartate aminotransferase (ast) were significantly higher in fish fed SPC45 than those fed FM. The activity and mRNA expression of acid phosphatase were opposite. The villi height (VH) in distal intestine (DI) showed a significant quadratic response to increasing dietary SPC inclusion levels and was highest in SPC15. The VH in proximal intestine, middle intestine decreased significantly with increasing dietary SPC levels. The 16S rRNA sequences in intestine revealed that fish fed SPC15 had higher bacterial diversity and abundance of Phylum Firmicutes such as order Lactobacillales and order Rhizobiaceae than those fed other diets. Genus vibrio, family Vibrionaceae and order Vibrionales within phylum Proteobacteria were enriched in fish fed FM and SPC30 diets. Tyzzerella and Shewanella that belongs to phylum Firmicutes and Proteobacteria, respectively, were enriched in fish fed SPC45 diet. Our results indicated that SPC replacing more than 30% FM could lead to lower quality diet, retard growth performance, ill health, disordered intestine structure, and microbiota communities. Tyzzerella could be the bacteria indicator of intestinal in large yellow croaker fed low quality diet due to high SPC content. Based on the quadratic regression analysis of WG, the best growth performance could be observed when the replacement of FM with SPC was 9.75%.

Systematic evaluation of genome-wide metabolic landscapes in lactic acid bacteria reveals diet- and strain-specific probiotic idiosyncrasies

Lactic acid bacteria (LAB) are well known to elicit health benefits in humans, but their functional metabolic landscapes remain unexplored. Here, we analyze differences in growth, intestinal persistence, and postbiotic biosynthesis of six representative LAB and their interactions with 15 gut bacteria under 11 dietary regimes by combining multi-omics and in silico modeling. We confirmed predictions on short-term persistence of LAB and their interactions with commensals using cecal microbiome abundance and spent-medium experiments. Our analyses indicate that probiotic attributes are both diet and species specific and cannot be solely explained using genomics. For example, although both Lacticaseibacillus casei and Lactiplantibacillus plantarum encode similarly sized genomes with diverse capabilities, L. casei exhibits a more desirable phenotype. In addition, "high-fat/low-carb" diets more likely lead to detrimental outcomes for most LAB. Collectively, our results highlight that probiotics are not "one size fits all" health supplements and lay the foundation for personalized probiotic design.

Single and combined effects of microplastics and cadmium on juvenile grass carp (Ctenopharyngodon idellus)

Microplastics (MPs) have received extensive attention as a new type of environmental pollutants with potential ecological risks. However, there are still few studies on the physiological stress response of aquatic organisms under the interaction of MPs and heavy metals. In this study, grass carp (Ctenopharyngodon idellus) were chosen as experimental fish and were exposed to 5 μm polystyrene microplastics (PS - MPs, 700 μg/L) and cadmium (Cd, 100 μg/L) individually or in combination. The results indicated that the presence of Cd didn't affect the accumulation of MPs in the intestines of grass carp. On the contrary, the concentration of Cd in the intestines of grass carp was higher in the MPs - Cd combined exposure group than in the Cd alone exposure group. Histological analysis revealed multiple abnormalities in the intestines after acute exposure, and the damage in the MPs - Cd combined exposure group was particularly severe. After 24 h of exposure, the expression of pro-inflammatory cytokines was significantly up-regulated in all exposed groups. However, after 48 h of exposure, the expression of inflammatory cytokines was significantly down-regulated, which may be related to intestinal damage. Our results deepen the significance of toxicological studies of MPs exposure, highlight their interaction with heavy metal toxicants, and provide important data for assessing the risk of MPs and heavy metals to grass carp.

Lactococcus lactis' Effect on the Intestinal Microbiota of Streptococcus agalactiae-Infected Zebrafish (Danio rerio)

Streptococcus agalactiae is a common pathogen in aquaculture that disrupts the balance of the intestinal microbiota and threatens fish health, causing enormous losses to the aquaculture industry. In this study, we isolated and screened a Lactococcus lactis KUST48 (LLK48) strain with antibacterial effect against S. agalactiae in vitro and used it as a potential probiotic to explore its therapeutic effect on zebrafish (Danio rerio) infected with S. agalactiae. This study divided zebrafish into 3 groups: control group, injected with phosphate-buffered saline; infection group, injected with S. agalactiae; and treatment group, treated with LLK48 after S. agalactiae injection. Then, the 16S rRNA gene sequences of the intestinal microbiota of these 3 groups were sequenced using Illumina high-throughput sequencing technology. The results showed that the relative abundance of intestinal bacteria was significantly decreased in the infection group, and a high relative abundance of S. agalactiae was observed. The relative abundance of the intestinal microbiota was increased in the treatment group, with a decrease in the relative abundance of S. agalactiae compared to that in the control group. In the Cluster of Orthologous Groups of proteins function classification, the relative abundance of each biological function in the infection group was significantly lower than that of the control and treatment groups, showing that LLK48 has a positive therapeutic effect on zebrafish infected with S. agalactiae. This study provides a foundation for exploring the pathogenic mechanism of S. agalactiae on fish and their intestinal symbionts, and also presents a new approach for the treatment of S. agalactiae infections in fish aquaculture systems. IMPORTANCE L. lactis KUST48 (LLK48) with a bacteriostatic effect against S. agalactiae was isolated from tilapia intestinal tracts. S. agalactiae infection significantly reduced the relative abundance of intestinal bacteria and various physiological functions in zebrafish intestines. LLK48 demonstrated infection and subsequent therapeutic effects on the S. agalactiae infection in the zebrafish intestine. Therefore, the potential probiotic LLK48 can be considered as a therapeutic treatment for S. agalactiae infections in aquaculture, which can reduce the use of antibiotics and help maintain fish health.

Competitive Exclusion Bacterial Culture Derived from the Gut Microbiome of Nile Tilapia (Oreochromis niloticus) as a Resource to Efficiently Recover Probiotic Strains: Taxonomic, Genomic, and Functional Proof of Concept

This study aims to mine a previously developed continuous-flow competitive exclusion culture (CFCEC) originating from the Tilapia gut microbiome as a rational and efficient autochthonous probiotic strain recovery source. Three isolated strains were tested on their adaptability to host gastrointestinal conditions, their antibacterial activities against aquaculture bacterial pathogens, and their antibiotic susceptibility patterns. Their genomes were fully sequenced, assembled, annotated, and relevant functions inferred, such as those related to pinpointed probiotic activities and phylogenomic comparative analyses to the closer reported strains/species relatives. The strains are possible candidates of novel genus/species taxa inside Lactococcus spp. and Priestia spp. (previously known as Bacillus spp.) These results were consistent with reports on strains inside these phyla exhibiting probiotic features, and the strains we found are expanding their known diversity. Furthermore, their pangenomes showed that these bacteria have indeed a set of so far uncharacterized genes that may play a role in the antagonism to competing strains or specific symbiotic adaptations to the fish host. In conclusion, CFCEC proved to effectively allow the enrichment and further pure culture isolation of strains with probiotic potential.

Effects of probiotics on Zebrafish model infected with Aeromonas hydrophila: spatial distribution, antimicrobial, and histopathological investigation

Usage of “probiotics” for treatment of food-borne pathogens associated diseases, makes a significant reduction in transmission of resistant bacteria, and antimicrobial resistance genes from aquaculture environments to humans. In this research, the authors aim to evaluate the immunomodulatory, and histological effects of two probiotic strains on the Zebrafish model. Fish models were treated with Lactobacillus delbrueckii (G2), Lactobacillus acidophilus (G3) and both probiotics (G4) and compared with the control group (G1) (only infected by pathogen and receiving no probiotic). Biometric tests, height, weight, and mortality rate of the fishes were assessed. Afterward, RT-PCR was conducted for bacterial existence of probiotic strains, and quantitative assessment of alterations in targeted immune genes. Subsequently, histological sampling was done for investigation of spatial distribution, and villus length in proximal, middle, and distal sections of intestinal tissues. Based on the results, G4 showed the highest gene expression for Lactobacillus acidophilus after 28 days (P < 0.05). G4 also showed an increase in the number of goblet cells and villus length in the middle and distal sections of intestinal tissue after 56 days. Furthermore, after 56 days, the highest number of intraepithelial cells was observed in the proximal sections of intestinal tissue in G4. G2 and G3 showed significant differences in comparison with G1 (P < 0.05). After 60 days, the highest gene expression for Lactobacillus bulgaricus was found in group treated with only this probiotic bacteria. The highest expression level of IL-1β and TNF-α were found in G1. The highest survival rate was in the case of groups only treated with Lactobacillus bulgaricus (G2). To sum up, it seems that usage of probiotics for the improvement of public health and fisheries industries can be helpful.

Oral vaccination with recombinant Lactobacillus casei expressing Aha1 fused with CTB as an adjuvant against Aeromonas veronii in common carp (Cyprinus carpio)

Aeromonas veronii (A. veronii) is a pathogenic that can infect human, animal and aquatic organisms, in which poses a huge threat to the health of many aquatic organisms such as Cyprinus carpio. In this study, Lactobacillus casei (L. casei) strain CC16 was used as antigen deliver carrier and fused with cholera toxin B subunit (CTB) as an adjuvant to construct the recombinant L. casei pPG-Aha1/Lc CC16(surface-displayed) and pPG-Aha1-CTB/Lc CC16(surface-displayed) expressing Aha1 protein of A. veronii, respectively. And the immune responses in Cyprinus carpio by oral route was explored. Our results demonstrated that the recombinant strains could stimulate high serum specific antibody immunoglobulin M (IgM) and induce a stronger acid phosphatase (ACP), alkaline phosphatase (AKP), C3, C4, lysozyme (LZM), Lectin and superoxide dismutase (SOD) activity in Cyprinus carpio compared with control groups. Meanwhile, the expression of Interleukin-10 (IL-10), Interleukin-1β (IL-1β), Tumor Necrosis Factor-α (TNF-α), immunoglobulin Z1 (IgZ1) and immunoglobulin Z2 (IgZ2) in the tissues were significantly upregulated compared with Lc-pPG or PBS groups, indicating that humoral and cell immune response were triggered. Additionally, recombinant L. casei could survive and colonize in fish intestine. Significantly, recombinant L. casei provides immune protection against A. veronii infection, which Cyprinus carpio received pPG-Aha1-CTB/Lc CC16 (64.29%) and pPG-Aha1/Lc CC16 (53.57%) had higher survival rates compared with the controls. Thus, we demonstrated that recombinant pPG-Aha1/Lc CC16 and pPG-Aha1-CTB/Lc CC16 may be the promising strategy for the development of an oral vaccine against A. veronii.

Modulation of humoral immunological and antioxidant responses and gut bacterial community and gene expression in rainbow trout, Oncorhynchus mykiss, by dietary lactic acid supplementation

The present study was conducted to find the effects of dietary lactic acid (LA) supplementation on growth performance, hematological characteristics, humoral antioxidant and immunological factors, intestinal gene expression and bacteriological parameters in rainbow trout, Oncorhynchus mykiss. The fish (104 ± 0.90 g) were randomly distributed in twelve tanks (150 L water) as four treatments, receiving diets supplemented with 0, 5, 10, and 20 g/kg LA for 70 days. At the end of the feeding trial, no significant differences were observed in the fish growth performance and survival among the treatments. Blood/plasma red blood cell (5-20 g/kg LA), white blood cell (10 g/kg LA), hematocrit (5 and 10 g/kg LA), hemoglobin (5 and 10 g/kg LA), superoxide dismutase (10 g/kg LA), glutathione peroxidase (10 g/kg LA), total antioxidant capacity (10 g/kg LA), lysozyme (5-20 g/kg LA), and alternative complement activity (10 g/kg LA) significantly increased, as malondialdehyde (10 g/kg LA) significantly decreased in the LA-treated fish. Dietary LA supplementation induced no significant changes in the intestinal defensin gene expression, but significantly up-regulated the expression of toll-like receptor-3 (5-20 g/kg LA), Mx-9 (5-20 g/kg LA), and heat shock protein-70 (5 g/kg LA) genes; the highest expressions were observed at 5 g/kg LA supplementation. Dietary LA supplementation significantly increased total bacterial count (5 and 10 g/kg LA), Lactobacillus sp. (5-20 g/kg LA), and Bacteroides sp. (5-20 g/kg LA), but decreased Streptococcus iniae (5-20 g/kg LA) in the fish intestine; the highest Lactobacillus sp. And Bacteroides sp. Were observed in 10 and 5 g/kg LA supplementation, respectively. Overall, dietary LA supplementation, although has no significant effects on the fish growth performance, is beneficial to improve humoral antioxidant parameters, humoral and intestinal immunological parameters and intestinal microflora. According to the results, dietary 5-10 g/kg LA is recommended for the formulation of rainbow trout feed.

Natural Feed Supplements Improve Growth, Non-Specific Immune Responses and Resistance against Vibrio alginolyticus in Lates calcarifer

This study aimed to investigate the effects of dietary natural feed supplement on the growth performance, non-specific responses, and disease resistance in Lates calcarifer. Three commercial products (A, B, and C) containing a basal probiotic mixture were tested. Product A contained a basal mixture of Paenibacillus sp., Bacillus subtilis, Bacillus amyloliquefaciens, and Lactobacillus rhamnosus (107 cfu/g for each probiotic); product B contained additional Lactobacillus plantarum (1010 cfu/g); and product C contained additional soybean peptides (500 g/kg) and garlic powder (1 g/kg). Each product was supplemented into subject diets at dosages of 1 or 2 g/kg (designated as the A1, A2, B1, B2, C1, and C2 groups, respectively). Following an eight-week trial, growth parameters (specific growth rate and feed conversion ratio), non-specific immune responses (O2− production, phagocytic rate, and phagocytic index), and the results of a challenge test against Vibrio alginolyticus were evaluated. The results show that all probiotic supplement groups exhibited an improvement in growth performance compared to the control group (non-probiotic diet). In terms of non-specific immunity parameters, a significant improvement in O2− production was found in the C2 group, whereas significant improvements in phagocytic activity were found in all the B and C groups. The C2 group displayed optimal O2− production, phagocytic rate, and phagocytic index results. For the challenge test, the C groups showed higher Vibrio resistance than the other experimental groups and the control group. These results suggest that product C, given at dosages of 2 g/kg, may serve as a growth-promoting and immunostimulatory additive for the cultivation of Asian seabass.

Assessing the Influence of Dietary Pediococcus acidilactici Probiotic Supplementation in the Feed of European Sea Bass (Dicentrarchus labrax L.) (Linnaeus, 1758) on Farm Water Quality, Growth, Feed Utilization, Survival Rate, Body Composition, Blood Biochemical Parameters, and Intestinal Histology

The probiotics are being used as ecofriendly and bioremediation tools for developing sustainability to aquaculture. The present study was conducted to explore the practical capability of using dietary lactic acid bacteria (Pediococcus acidilactici) probiotics and see how its dose variation affected the water quality, growth performance, survival rate, body composition, blood biochemical parameters, and intestinal histology of European sea bass (Dicentrarchus labrax L.). A total of 120 fingerlings with an initial weight of $9\pm 0.2$  g were divided into four groups, each with three replicates. The feeding experiment lasted for 60 days. In addition to the control (without probiotics) (T0), fish were fed diets containing (T1) 2.0, (T2) 2.5, and (T3) 3.0 g of probiotics per kg of diet twice a day. When compared to the control, sea bass fed probiotic-supplemented diets had significantly higher growth parameters, fish body “crude lipid,” and villi height ( $p<0.05$ , $p<0.01$ , and $p<0.001$ ). The P. acidilactici probiotic treatments improved survival rate, feed conversion ratio, body composition, and blood biochemical markers, but not statistically significant ( $p>0.05$ ). Also, in regard to water quality, P. acidilactici drastically reduced ammonia and pH levels. In this experiment, fish fed with a dosage of 3.0 g of this commercial probiotic per kg of probiotics performed better. The study found that including probiotics in the diets of European sea bass improved growth, body composition, survival rate, blood biochemical markers, intestinal histology, and some water quality parameters.

Research Progress of the Gut Microbiome in Hybrid Fish

Fish, including hybrid species, are essential components of aquaculture, and the gut microbiome plays a vital role in fish growth, behavior, digestion, and immune health. The gut microbiome can be affected by various internal and/or external factors, such as host development, diet, and environment. We reviewed the effects of diet and dietary supplements on intestinal microorganisms in hybrid fish and the difference in the gut microbiome between the hybrid and their hybrids that originate. Then, we summarized the role of the gut microbiome in the speciation and ecological invasion of hybrid fish. Finally, we discussed possible future studies on the gut microbiome in hybrid fish, including the potential interaction with environmental microbiomes, the effects of the gut microbiome on population expansion, and fish conservation and management.

The Effect of Dietary Lactic Acid Bacteria on Intestinal Microbiota and Immune Responses of Crucian Carp (Carassius auratus) Under Water Temperature Decrease

Temperature changes have a great impact on fish feeding, intestinal microorganisms, metabolism, and immune function. Therefore, it is necessary to develop effective methods to enhance the survival rates and growth of fish under water temperature changes. Lactic acid bacteria (LAB) are promising immunostimulatory feed additive, as demonstrated by their beneficial effects in several fish species. This study investigated the short-term effects of dietary LAB on intestinal microbiota composition and immune responses of crucian carp (Carassius auratus) when water temperature decreased from 30 ± 1°C to 18 ± 1°C. Lactococcus (L.) lactis 1,209 and L. lactis 1,242 with potential probiotics isolated from the intestine of Qinghai naked carp (Gymnocypris przewalskii) were selected as feed additives for the crucian carp feeding experiment. A total of 225 commercially available healthy crucian carp (250 ± 10 g) of similar age were kept in 30°C water for a week and then immediately transferred to 18 ± 1°C water, assigned to three dietary treatments for a 16-day feeding trial randomly: (1) HC, diets without additives (the control group); (2) HT, diets with 10⁶ CFU/ml L. lactis 1,209; and (3) HL, with 10⁶ CFU/ml L. lactis 1,242. Each group was set up with 3 replicates and each with 25 fish. The results showed that the mortality rate of crucian carp in HC, HT, and HL group was 50, 27, and 33%, respectively. High-throughput sequencing results displayed that the composition of the intestinal microorganism varied dynamically in response to different treatments and water temperature decrease. Among them, compared with the HC group, a higher abundance of Firmicutes and Proteobacteria, and a lower of Actinobacteria appeared in HT and HL. The cytokines heat shock protein 70 (HSP-70) in crucian carp intestinal tract significantly decreased when water temperature decreased (p < 0.05).

Fermentation of Soybean Meal with Lactobacillus acidophilus Allows Greater Inclusion of Vegetable Protein in the Diet and Can Reduce Vibrionacea in the Intestine of the South American Catfish (Rhamdia quelen)

Simple Summary

There is a demand to replace fishmeal with protein sources of plant origin in fish feeds. Biotechnology strategies, such as fermentation, can improve the bioavailability of plant proteins and decrease the presence of antinutrients, optimizing the results obtained. Fermented soybean meal has already been evaluated for different fish species as a replacement for fishmeal, and there is evidence that it can improve the intestinal health of animals. Lactobacillus acidophilus is a strain used as a probiotic in fish feeding but it remains to be evaluated as a potential fermentation bacterium for feed ingredients. This study aimed to evaluate the effect of diets containing different inclusion levels (0%, 7%, 14%, 21% and 28%) of soybean meal fermented by L. acidophilus (SMFL) on the zootechnical performance and intestinal health of South American catfish juveniles (Rhamdia quelen). The inclusion of SMFL up to 21% in replacement of fish meal did not affect the zootechnical performance of fish and also decreased the concentration of Vibrionaceae bacteria present in the intestine compared to the control group. The results demonstrate that fermentation with L. acidophilus enables greater inclusion of soybean protein in South American catfish diets and promotes the control of intestinal pathogenic bacteria.

Abstract

The objective of this study was to evaluate the effect of diets containing different inclusion levels (0%, 7%, 14%, 21% and 28%) of soybean meal fermented by Lactobacillus acidophilus (SMFL) on the zootechnical performance and intestinal health of South American catfish juveniles (Rhamdia quelen). The experimental design was completely randomized with five treatments and four replications and lasted 56 days. Five isoproteic (39% crude protein) and isoenergetic (4300 kcal of gross energy kg⁻¹) diets were formulated where SMFL was included in replacement of fish meal. Two hundred forty South American catfish juveniles (3.0 ± 0.5 g) were distributed in 20 tanks (70 L) connected in a recirculation aquaculture system. At the end of the experiment, the inclusion of SMFL up to 21% in replacement of fish meal did not affect the zootechnical performance and also decreased the concentration of Vibrionaceae bacteria present in the intestine compared to the control group. The amount of total lactic and heterotrophic bacteria, the enzymatic activity and the intestinal morphometry did not differ between dietary treatments. The results demonstrate that fermentation with Lactobacillus acidophilus enables greater inclusion of soybean protein in South American catfish diets and promotes the control of intestinal pathogenic bacteria.

Evaluation of Dietary Probiotic Bacteria and Processed Yeast (GroPro-Aqua) as the Alternative of Antibiotics in Juvenile Olive Flounder Paralichthys olivaceus

We investigated the three probiotic bacteria and a processed yeast (GroPro-Aqua) as the replacers of antibiotics in juvenile olive flounder. A total of seven diets were used, that is, one basal or control (CON) diet; and six other diets, of which, three diets were prepared by supplementing probiotic bacteria such as Bacillus subtilis WB60 (BSWB60) at 1 × 10⁸ CFU/g diet, Bacillus subtilis SJ10 (BSSJ10) at 1 × 10⁸ CFU/g diet, and Enterococcus faecium SH30 (EFSH30) at 1 × 10⁷ CFU/g diet; one diet with processed yeast (GRO) at 0.35% diet; and two other diets were supplemented with oxytetracycline (OTC) and amoxicillin (AMO) at 4 g/kg of each. Triplicate groups of fish (average 12.1 g) were fed one of the diets for eight weeks. At the end of the feeding trial, the fish that were fed the probiotic bacteria-supplemented diets had a significantly higher final weight, weight gain, and specific growth rate compared to the CON, OTC, and AMO diets. Fish that were fed the GRO diet had significantly higher feed efficiency and protein efficiency ratios than those of the fish that were fed the CON diet. Serum glutamic pyruvic transaminase, glutamic oxaloacetic transaminase, glucose, and total protein were not affected by the diets. Lysozyme activity in fish that were fed the BSSJ10, BSWB60, and EFSH30 diets were significantly higher compared to the CON and OTC diets, whereas myeloperoxidase activity of fish fed the BSWB60 and EFSH30 diets were significantly higher than those of fish fed the CON and AMO diets. Flounder growth hormone gene expressions of fish that were fed BSWB60 and GRO diets were significantly higher compared to the CON, OTC, and AMO diets. The interleukin-1β gene expression of fish that were fed the BSSJ10, BSWB60, EFSH30, OTC, and GRO diets was significantly higher than those of fish fed the CON diet. The interleukin-10 gene expression of fish that were fed the BSSJ10, EFSH30, and GRO diets was significantly higher than those of fish fed the CON and AMO diets. Posterior intestinal histology of fish showed significantly higher villi length in fish that were fed the BSSJ10, BSWB60, EFSH30, and GRO diets compared to the CON diet. After 15 days of challenge test with pathogenic bacteria Edwardsiella tarda, the cumulative survival rate of fish that were fed the BSSJ10, BSWB60, EFSH30, and GRO diets were significantly higher than those of fish that were fed the CON diet. Overall, the results indicate that dietary supplementation of B. subtilis (10⁸ CFU/g diet), E. faecium (10⁷ CFU/g diet), and processed yeast (GroPro-Aqua at 0.35% diet) could replace the antibiotics in terms of improving growth, immunity, gut health, and disease resistance in juvenile olive flounder.

Dietary application of Lactococcus lactis alleviates toxicity and regulates gut microbiota in Cyprinus carpio on exposure to heavy metals mixture

Heavy metals (HMs) contaminated fish is a threat to humans when consumed. Dietary probiotics have evolved as a successful HMs removal approach. In this study, probiotics Enterococcus (EC) sp. and Lactococcus (LC) sp. were evaluated for toxicity alleviation and gut microbiota maintenance in Cyprinus carpio (single and combined approach) on Cr, Cd, and Cu mixture (0.8 mg/L and 1.6 mg/L) exposure (28 days). HMs removal, oxidative stress, cytokines response, histology, and gut microbiota were investigated. LC alone showed remarkable HMs removal for Cr (62.28%-87.57%), Cd (89%-90.42%), and Cu (72%-88%) than LC + EC. Probiotics up-regulated superoxide dismutase and total protein levels, while decreased the activity of malondialdehyde than the control. Pro-inflammatory cytokine (TNF-α) and chemokine (IL-8) expressions were higher at 1.6 mg/L concentration, whereas anti-inflammatory cytokine (IL-10) was higher in the 0.8 mg/L group. LC mitigated the histological alterations of gills, kidneys, and intestines, particularly at the lower concentration. Sequencing results revealed that Proteobacteria (44%-61%) was the most dominant phylum in all groups, followed by Fusobacteria (34%-36%) at 0.8 mg/L and Firmicutes (19%-34%) at 1.6 mg/L. The current study presented LC and EC potential separately and in combination to countermeasure HMs mixture induced toxicity and gut microbial dysbiosis, in which the conjoint group was less effective.

Probiotics in tilapia (Oreochromis niloticus) culture: Potential probiotic Lactococcus lactis culture conditions

Tilapia is one of the most extensively farmed fish on a global scale. Lately, many studies have been carried out to select and produce probiotics for cultured fish. Bacteria from the genera Bacillus, Lactiplantibacillus (synonym: Lactobacillus), and Lactococcus are the most widely studied with respect to their probiotic potential. Among these microorganisms, Lactococcus lactis has outstanding prospects as a probiotic because it is generally recognized as safe (GRAS) and has previously been shown to exert its probiotic potential in aquaculture through different mechanisms, such as competitively excluding pathogenic bacteria, increasing food nutritional value, and enhancing the host immune response against pathogenic microorganisms. However, it is not sufficient to simply select a microorganism with significant probiotic potential for commercial probiotic development. There are additional challenges related to strategies involving the mass production of bacterial cultures, including the selection of production variables that positively influence microorganism metabolism. Over the last ten years, L. lactis production in batch and fed-batch processes has been studied to evaluate the effects of culture temperature and pH on bacterial growth. However, to gain a deeper understanding of the production processes, the effect of hydrodynamic stress on cells in bioreactor production and its influence on the probiotic potential post-manufacturing also need to be determined. This review explores the trends in tilapia culture, the probiotic mechanisms employed by L. lactis in aquaculture, and the essential parameters for the optimal scale-up of this probiotic.

Autochthonous Probiotics Alleviate the Adverse Effects of Dietary Histamine in Juvenile Grouper (Epinephelus coioides)

High dose (0.3%) of dietary histamine can cause adverse effects on growth performance, innate immunity, and gut health in juvenile grouper (Epinephelus coioides). In the present study, three autochthonous probiotics (Bacillus pumilus SE5, Psychrobacter sp. SE6, and Bacillus clausii DE5) were supplemented separately to diets containing 0.3% of histamine and their effects on growth performance, innate immunity, and gut health of grouper (E. coioides) were evaluated in a 56-day feeding trial. The results showed considerable increase in weight gain, specific growth rate, hepatosomatic index, and decreased feed conversion rate in groupers fed with probiotic-supplemented diets. Supplementation of autochthonous probiotics has improved antioxidant capacity and innate immunity of E. coioides by measuring correlative parameters, such as total antioxidant capacity, superoxide dismutase activity, malondialdehyde content, and so on. Additionally, dietary probiotics have significantly reduced the levels of serum interleukin-1β (at days 28 and 56), fatty acid-binding protein 2, and intestinal trefoil factor (at day 28), and promoted intestinal integrity following remarkably increased muscle thickness and mucosal fold height at day 56, especially in grouper fed with B. pumilus SE5 containing diet (P < 0.05). On day 56, the gut microbial composition of E. coioides was positively shaped by autochthonous probiotics, the relative abundance of potentially pathogenic Photobacterium decreased while beneficial Lactobacillus increased in fish fed with probiotic strains, especially with B. pumilus SE5 and B. clausii DE5. These results suggest that among the three autochthonous probiotic strains tested, B. pumilus SE5 is showing better efficiency in alleviating the adverse effects of (high levels) dietary histamine by decreasing the expression of inflammatory markers and by improving the growth, innate immunity, and gut health of juvenile grouper E. coioides.

Probiotic Bacillus safensis NPUST1 Administration Improves Growth Performance, Gut Microbiota, and Innate Immunity against Streptococcus iniae in Nile tilapia (Oreochromis niloticus)

Probiotics are considered ecofriendly alternatives to antibiotics as immunostimulants against pathogen infections in aquaculture. In the present study, protease-, amylase-, cellulase-, and xylanase-producing Bacillus safensis NPUST1 were isolated from the gut of Nile tilapia, and the beneficial effects of B. safensis NPUST1 on growth, innate immunity, disease resistance and gut microbiota in Nile tilapia were evaluated by feeding tilapia a basal diet or basal diet containing 105 and 106-107 CFU/g for 8 weeks. The results showed that the weight gain, feed efficiency and specific growth rate were significantly increased in tilapia fed a diet containing 106 CFU/g and 107 CFU/g B. safensis NPUST1. Intestinal digestive enzymes, including protease, amylase and lipase, and hepatic mRNA expression of glucose metabolism and growth-related genes, such as GK, G6Pase, GHR and IGF-1, were also significantly increased in the 106 CFU/g and 107 CFU/g B. safensis NPUST1 treated groups. Immune parameters such as phagocytic activity, respiratory burst and superoxide dismutase activity in head kidney leukocytes, serum lysozyme, and the mRNA expression of IL-1β, IL-8, TNF-α and lysozyme genes were significantly induced in the head kidney and spleen of 106 CFU/g and 107 CFU/g B. safensis NPUST1 treated fish. The cumulative survival rate was significantly increased in fish fed a diet containing 106 CFU/g and 107 CFU/g B. safensis NPUST1 after challenge with Streptococcus iniae. Dietary supplementation with B. safensis NPUST1 improves the gut microbiota of Nile tilapia, which increases the abundance of potential probiotics and reduces the abundance of pathogenic pathogens. The present study is the first to report the use of B. safensis as a potential probiotic in aquaculture, and a diet containing 106 CFU/g B. safensis NPUST1 is adequate for providing beneficial effects on growth performance and health status in tilapia.

Effects of Supplementing Intestinal Autochthonous Bacteria in Plant-Based Diets on Growth, Nutrient Digestibility, and Gut Health of Bullfrogs (Lithobates catesbeianus)

Poor utilization efficiency of plant protein diets always leads to intestinal barrier dysfunction and growth inhibition in animals. Probiotics have shown promise in improving growth performance and gut health of the host. However, obtaining the host-beneficial probiotic from thousands of bacterial phylotypes is challenging. Here, four intestinal autochthonous bacteria were isolated from fast-growing bullfrog after a 60-day feeding on a soybean meal (SM)-based diet. Another feeding trial was conducted to evaluate the effects of supplementing these strains in an SM-based diet on growth, nutrient digestibility, immunity, and gut health of bullfrog. A high-SM basal diet was used as a non-supplemented control group (NC), and four other diets were prepared by supplementing the basal diet with 1 × 10⁷ CFU/g of Bacillus siamensis, Bacillus tequilensis (BT), Bacillus velezensis, and Lactococcus lactis (LL). Results showed that weight gain, feed efficiency, nitrogen retention, and apparent digestibility coefficients of dry matter and protein were significantly higher in the LL group compared with the NC group (p < 0.05). Furthermore, compared with the NC group, both BT and LL groups showed markedly higher jejunal protease and amylase activities, serum complement 4 and immunoglobulin M levels, jejunal muscularis thickness (p < 0.05), and up-regulated expression of il-10 and zo-1 genes (p < 0.05). High-throughput sequencing revealed higher abundances of Bacillus and Cetobacterium in BT and LL groups, respectively, accompanied with decreased abundances of Enterobacter and Escherichia-Shigella. Besides, KEGG pathways related to metabolisms were significantly enhanced by the LL diet relative to the NC diet (p < 0.05). Overall, the beneficial effects of two frog-derived probiotics were determined: supplementation of L. lactis in SM-based diet promoted growth and nutrient digestibility; both B. tequilensis and L. lactis supplementation improved immune response and intestinal barrier function of bullfrogs.