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

Characterization and application of Bacillus velezensis D6 co-producing α-amylase and protease

BACKGROUND

Research on the co-production of multiple enzymes by Bacillus velezensis as a novel species is still a topic that needs to be studied. This study aimed to investigate the fermentation characteristics of B. velezensis D6 co-producing α-amylase and protease and to explore their enzymatic properties and applications in fermentation.

RESULTS

The maximum co-production of α-amylase and protease reached 13.13 ± 0.72 and 2106.63 ± 64.42 U mL-1, respectively, under the optimal fermented conditions (nutrients: 20.0 g L-1 urea, 20.0 g L-1 glucose, 0.7 g L-1 MnCl2; incubation conditions: initial pH 7.0, temperature 41 °C, 8% inoculation size and 30% working volume). Moreover, the genetic co-expression of α-amylase and protease increased from 0 to 24 h and then decreased after 36 h at the transcriptional level, which coincided with the growth trend of B. velezensis D6. The optimal reaction temperature of α-amylase was 55-60 °C, while that of protease was 35-40 °C. The activities of α-amylase and protease were retained by over 80% after thermal treatment (90 °C, 1 h), which indicated that two enzymes co-produced by B. velezensis D6 demonstrated excellent thermal stability. Moreover, the two enzymes were stable over a wide pH range (pH 4.0-8.0 for α-amylase; pH 4.0-9.0 for protease). Finally, the degrees of hydrolysis of corn, rice, sorghum and soybeans by α-amylase from B. velezensis D6 reached 44.95 ± 2.95%, 57.16 ± 2.75%, 52.53 ± 4.01% and 20.53 ± 2.42%, respectively, suggesting an excellent hydrolysis effect on starchy raw materials. The hydrolysis degrees of mackerel heads and soybeans by protease were 43.93 ± 2.19% and 26.38 ± 1.72%, respectively, which suggested that the protease from B. velezensis D6 preferentially hydrolyzed animal-based protein.

CONCLUSION

This is a systematic study on the co-production of α-amylase and protease by B. velezensis D6, which is crucial in widening the understanding of this species co-producing multi-enzymes and in exploring its potential application. © 2024 Society of Chemical Industry.

An In-Depth Study on the Inhibition of Quorum Sensing by Bacillus velezensis D-18: Its Significant Impact on Vibrio Biofilm Formation in Aquaculture

Amid growing concerns about antibiotic resistance, innovative strategies are imperative in addressing bacterial infections in aquaculture. Quorum quenching (QQ), the enzymatic inhibition of quorum sensing (QS), has emerged as a promising solution. This study delves into the QQ capabilities of the probiotic strain Bacillus velezensis D-18 and its products, particularly in Vibrio anguillarum 507 communication and biofilm formation. Chromobacterium violaceum MK was used as a biomarker in this study, and the results confirmed that B. velezensis D-18 effectively inhibits QS. Further exploration into the QQ mechanism revealed the presence of lactonase activity by B. velezensis D-18 that degraded both long- and short-chain acyl homoserine lactones (AHLs). PCR analysis demonstrated the presence of a homologous lactonase-producing gene, ytnP, in the genome of B. velezensis D-18. The study evaluated the impact of B. velezensis D-18 on V. anguillarum 507 growth and biofilm formation. The probiotic not only controls the biofilm formation of V. anguillarum but also significantly restrains pathogen growth. Therefore, B. velezensis D-18 demonstrates substantial potential for preventing V. anguillarum diseases in aquaculture through its QQ capacity. The ability to disrupt bacterial communication and control biofilm formation positions B. velezensis D-18 as a promising eco-friendly alternative to conventional antibiotics in managing bacterial diseases in aquaculture.

Optimization of degradation conditions and analysis of degradation mechanism for nitrite by Bacillus aryabhattai 47

Excessive nitrite levels cause significant damage to aquaculture, making it crucial to explore green and reliable nitrite removal technologies. In this study, A Bacillus aryabhattai (designated as the strain 47) isolated from aquaculture wastewater was used as the experimental strain. The nitrite degradation conditions of the strain 47 were optimized, and the optimal conditions are: glucose was 12.74 g/L, fermented special soybean meal was 21.27 g/L, MgCl2 369 mg/L, pH 7.0, incubated at 30 °C with the inoculum size of 2 % and the rotation speed of 170 rpm. Under the optimal conditions, the nitrite concentration of the culture solution was 200 mg/L, and the nitrite removal rate reached 91.4 %. Meanwhile, the mechanism by which Mg2+ enhanced the nitrite degradation ability of the strain 47 was investigated by transcriptomics. An operon structure directed cellular trafficking of Mg2+, and then, the Mg2+-mediated catalytic reaction of multiple enzymes enhanced and improved cellular metabolic processes (e.g. the transport and metabolism of nitrite, central carbohydrate metabolism oxidative phosphorylation). At the same time, with the progress of cell metabolism, cells secreted a series of enzymes related to nitrite transport and metabolism to promote the metabolism of nitrite. And the process of the assimilated nitrate reduction pathway of nitrite degradation in the strain 47 was elaborated at the transcriptome level. This study provided a new insight into nitrite treatment mediated by microbial organisms.

Current Status of Probiotics in European Sea Bass Aquaculture as One Important Mediterranean and Atlantic Commercial Species: A Review

European sea bass production has increased in recent decades. This increase is associated with an annually rising demand for sea bass, which encourages the aquaculture industries to increase their production to meet that demand. However, this intensification has repercussions on the animals, causing stress that is usually accompanied by dysbiosis, low feed-conversion rates, and immunodepression, among other factors. Therefore, the appearance of pathogenic diseases is common in these industries after immunodepression. Seeking to enhance animal welfare, researchers have focused on alternative approaches such as probiotic application. The use of probiotics in European sea bass production is presented as an ecological, safe, and viable alternative in addition to enhancing different host parameters such as growth performance, feed utilization, immunity, disease resistance, and fish survival against different pathogens through inclusion in fish diets through vectors and/or in water columns. Accordingly, the aim of this review is to present recent research findings on the application of probiotics in European sea bass aquaculture and their effect on growth performance, microbial diversity, enzyme production, immunity, disease resistance, and survival in order to help future research.

Dietary supplementation of Bacillus velezensis improves Vibrio anguillarum clearance in European sea bass by activating essential innate immune mechanisms

Bacillus spp. supplementation as probiotics in cultured fish diets has a long history of safe and effective use. Specifically, B. velezensis show great promise in fine-tuning the European sea bass disease resistance against the pathogenicity caused by several members of the Vibrio family. However, the immunomodulatory mechanisms behind this response remain poorly understood. Here, to examine the inherent immune variations in sea bass, two equal groups were fed for 30 days with a steady diet, with one treatment supplemented with B. velezensis. The serum bactericidal capacity against live cells of Vibrio anguillarum strain 507 and the nitric oxide and lysozyme lytic activities were assayed. At the cellular level, the phagocytic response of peripheral blood leukocytes against inactivated Candida albicans was determined. Moreover, head-kidney (HK) total leukocytes were isolated from previously in vivo treated fish with LPS of V. anguillarum strain 507. Mechanistically, the expression of some essential proinflammatory genes (interleukin-1 (il1b), tumor necrosis factor-alpha (tnfa), and cyclooxygenase 2 (cox2) and the sea bass specific antimicrobial peptide (AMP) dicentracin (dic) expressions were assessed. Surprisingly, the probiotic supplementation significantly increased all humoral lytic and cellular activities assayed in the treated sea bass. In addition, time-dependent differences were observed between the control and probiotic treated groups for all the HK genes markers subjected to the sublethal LPS dose. Although the il1b was the fastest responding gene to a significant level at 48 h post-injection (hpi), all the other genes followed 72 h in the probiotic supplemented group. Finally, an in vivo bacteria challenge against live V. anguillarum was conducted. The probiotic fed fish observed a significantly higher survival. Overall, our results provide clear vertical evidence on the beneficial immune effects of B. velezensis and unveil some fundamental immune mechanisms behind its application as a probiotic agent in intensively cultured European sea bass.

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.

Phage Therapy as a Focused Management Strategy in Aquaculture

Therapeutic bacteriophages, commonly called as phages, are a promising potential alternative to antibiotics in the management of bacterial infections of a wide range of organisms including cultured fish. Their natural immunogenicity often induces the modulation of a variated collection of immune responses within several types of immunocytes while promoting specific mechanisms of bacterial clearance. However, to achieve standardized treatments at the practical level and avoid possible side effects in cultivated fish, several improvements in the understanding of their biology and the associated genomes are required. Interestingly, a particular feature with therapeutic potential among all phages is the production of lytic enzymes. The use of such enzymes against human and livestock pathogens has already provided in vitro and in vivo promissory results. So far, the best-understood phages utilized to fight against either Gram-negative or Gram-positive bacterial species in fish culture are mainly restricted to the Myoviridae and Podoviridae, and the Siphoviridae, respectively. However, the current functional use of phages against bacterial pathogens of cultured fish is still in its infancy. Based on the available data, in this review, we summarize the current knowledge about phage, identify gaps, and provide insights into the possible bacterial control strategies they might represent for managing aquaculture-related bacterial diseases.