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♂)

Strain-Specific Benefits of Bacillus Probiotics in Hybrid Grouper: Growth Enhancement, Metabolic Health, Immune Modulation, and Vibrio harveyi Resistance

In the realm of modern aquaculture, the utilization of probiotics has gained prominence, primarily due to their ability to enhance growth, boost immunity, and prevent diseases in aquatic species. This study primarily investigates the efficacy of Bacillus subtilis strains, both host-derived and from other sources, in influencing fish growth, immunity, lipid metabolism, and disease resistance. Employing a 42-day feeding trial, we divided hybrid grouper into four distinct groups: a control group on a basal diet and three experimental groups supplemented with 1 × 108 CFU/g of different Bacillus subtilis strains-BS, 6-3-1, and HAINUP40. Remarkably, the study demonstrated that the 6-3-1 and HAINUP40 groups exhibited significant enhancements across key growth parameters: final body weight (FBW), weight gain rate (WGR), feed intake (FI), feed efficiency ratio (FER), and feed conversion ratio (FCR). The investigation into lipid metabolism revealed that the 6-3-1 strain upregulated seven metabolism-related genes, HAINUP40 affected four metabolism-related genes, and the BS strain influenced two metabolism-related genes, indicating diverse metabolic impacts by different strains. Further, a notable reduction in liver enzymes AST and ALT was observed across all supplemented groups, implying improved liver health. Noteworthy was the BS strain's superior antioxidative capabilities, positively affecting all four measured parameters (CAT, GSH-Px, MDA). In the sphere of immune-related gene expression, the BS strain significantly decreased the expression of both inflammation and apoptosis-related genes, whereas the HAINUP40 strain demonstrated an upregulation in these genes. The challenge test results were particularly telling, showcasing improved survival rates against Vibrio harveyi infection in the BS and 6-3-1 groups, unlike the HAINUP40 group. These outcomes highlight the strain-specific nature of probiotics and their varying mechanisms of action within the host. In conclusion, this study reveals that probiotic strains, varying by source, demonstrate unique, strain-specific effects in promoting growth and modulating immunity in hybrid grouper. This research highlights the promise of tailored probiotic applications in improving aquaculture practices. Such advancements contribute to more sustainable and efficient fish farming methods.

Host gut-derived Bacillus probiotics supplementation improves growth performance, serum and liver immunity, gut health, and resistive capacity against Vibrio harveyi infection in hybrid grouper (♀Epinephelus fuscoguttatus × ♂Epinephelus lanceolatus)

Several reports have revealed the vital role that probiotics play in fish growth and health. However, few works are available for host gut-derived probiotics on the growth, immunity, and gut microbiota of fish, especially in hybrid grouper (♀Epinephelus fuscoguttatus × ♂Epinephelus lanceolatus) due to their isolation difficulty and functional verification. This study aimed at assessing 3 host gut-derived Bacillus species' effects on the growth, immune and antioxidant-biochemical responses, haematological parameters, intestinal morphology, immune-related gene expression, gut microbiota, and disease resistance against Vibrio harveyi in hybrid grouper. A total of 480 hybrid grouper (initial weight = 9.03 ± 0.02 g) were randomly allotted into 4 groups, namely, the group fed a basal diet without probiotic inclusion (control, B0), the group fed the basal diet with Bacillus velezensis GPSAK4 (BV), the group fed the basal diet with Bacillus subtilis GPSAK9 (BS), and the group fed the basal diet with Bacillus tequilensis GPSAK2 (BT) strains at 1.0 × 10⁹ CFU/g. After a 6-week feeding trial, the results revealed significant improvements (P < 0.05) in the growth performance, whole fish-body proximate composition, blood haematological parameters, serum, liver, and intestinal biochemical indexes, intestinal morphology, and protection against V. harveyi pathogen in the probiotic-treated groups compared with the untreated. Additionally, the expressions of intestinal tight junction genes (occludin and ZO1), pro- and anti-inflammatory genes, including IL1β, IL6, IL8, TNFα, MyD88, IL10, and TGFβ, were upregulated (P < 0.05) after Bacillus species administration. Host gut-derived Bacillus supplementation shaped the gut microbiota by significantly increasing (P < 0.05) the relative abundance of Proteobacteria, Bacteroidetes, Actinobacteria (except the BS group), Acidobacteria (except the BT group), Cyanobacteria (except the BV and BT groups), and Verrucomicrobia phyla, as well as known beneficial genera (Romboutsia, Turicibacter, Epulopiscium, Clostridium_sensu_stricto 1 and 13, Lactobacillus, and Bacillus), but significantly decreased (P < 0.05) the abundance of Firmicutes, Chloroflexi, and Fusobacteria phyla, and purported pathogenic genera (Staphylococcus and Photobacterium) compared with the control group. Collectively, the results suggest that B. velezensis GPSAK4, B. subtilis GPSAK9 (especially this strain), B. tequilensis GPSAK2 dietary supplementation at 1.0 × 10⁹ CFU/g has positive effects on the intestinal health of hybrid grouper via microbial composition modulation, thus enhancing the assimilation and absorption of nutrients to boost fish growth, immunity, and disease resistance.

Autochthonous Bacillus subtilis and Enterococcus faecalis improved liver health, immune response, mucosal microbiota and red-head disease resistance of yellow drum (Nibea albiflora)

Yellow drum (Nibea albiflora), a commercially important fish species in the coastal regions of southeast China, is highly susceptible to red-head disease caused by Vibrio harveyi B0003. Probiotics have been shown to enhance disease resistance in fish, but whether commensal probiotics could improve of the resistance to red-head disease in yellow drum and possible mechanisms has yet not been reported. A six-week feeding trial was conducted to investigate the red-head disease resistance potentials of five probiotic candidates (Bacillus megaterium B1M2, B. subtilis B0E9, Enterococcus faecalis AT5, B. velezensis DM5 and B. siamensis B0E14), and the liver health, serum and skin immunities, gut and skin mucosal microbiota of yellow drum were determined to illustrate the possible mechanisms. The results showed that autochthonous B. subtilis B0E9 and E. faecalis AT5 (particularly E. faecalis AT5, P < 0.05) effectively improved red-head disease resistance in yellow drum. Furthermore, B. subtilis B0E9 and E. faecalis AT5 (particularly E. faecalis AT5) efficiently improve liver health by improving liver morphology and decreasing serum glutamic oxaloacetic transaminase and glutamic propylic transaminase activities pre and post challenged with V. harveyi B0003 (P < 0.05). B. subtilis B0E9 and E. faecalis AT5 led to significant improvement (P < 0.05) in the serum complement 3 content (un-detected after challenged with V. harveyi B0003), lysozyme activity and skin mucosal immunity (such as IL-6, IL-10 and lysozyme expression) pre and post challenged with V. harveyi B0003, which was generally consistent with the cumulative mortality after challenged with V. harveyi B0003. This induced activations of serum and skin mucosal immunities were consistent with the microbiota data showing that B. subtilis B0E9 and E. faecalis AT5 modulated the overall structure of intestinal and skin mucosal microbiota, and in particular, the relative abundance of potentially pathogenic Achromobacter decreased while beneficial Streptococcus, Rothia, and Lactobacillus increased in fish fed with B. subtilis B0E9 and E. faecalis AT5. Overall, autochthonous B. subtilis B0E9 and E. faecalis AT5 (particularly E. faecalis AT5) can improve liver health, serum and skin immunities (especially up-regulated lysozyme activity and inflammation-related genes expression), positively shape gut and skin mucosal microbiota, and enhance red-head disease resistance of yellow drum.

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

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

Mucosal and systemic immune effects of Bacillus subtilis in rainbow trout (Oncorhynchus mykiss)

Bacillus spp. are well known for their probiotic properties. Hence, the long-term feeding of Bacillus spp. strains to different fish species has been proved to confer beneficial effects regarding growth or pathogen resistance, among others. However, whether these strains could function as mucosal adjuvants, up-regulating immune responses after a single administration, has not yet been investigated in fish. Thus, in the current work, we have performed a series of experiments in rainbow trout (Oncorhynchus mykiss) aimed at establishing the potential of two Bacillus subtilis spore-forming strains, designated as ABP1 and ABP2, as oral adjuvants/immunostimulants. As an initial step, we evaluated their transcriptional effects on the rainbow trout intestinal epithelial cell line RTgutGC, and in gut tissue explants incubated ex vivo with the two strains. Their capacity to adhere to RTgutGC cells was also evaluated by flow cytometry. Although both strains had the capacity to modulate the transcription of several genes related to innate and adaptive immune responses, it was the ABP1 strain that led to stronger transcriptional effects, also exerting a higher binding capacity to intestinal epithelial cells. Consequently, we selected this strain to establish its effects on splenic B cells upon in vitro exposure as well as to determine the transcriptional effects exerted in the spleen, kidney, and gut after a single oral administration of the bacteria. Our results showed that B. subtilis ABP1 had the capacity to modulate the proliferation, IgM secreting capacity and MHC II surface expression of splenic B cells. Finally, we confirmed that this strain also induced the transcription of genes involved in inflammation, antimicrobial genes, and genes involved in T cell responses upon a single oral administration. Our results provide valuable information regarding how B. subtilis modulates the immune response of rainbow trout, pointing to the usefulness of the ABP1 strain to design novel oral vaccination strategies for aquaculture.

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.