Characterization and mechanism of anti-Aeromonas salmonicida activity of a marine probiotic strain, Bacillus velezensis V4

Identification and characterization of four Bacillus species from the intestine of hybrid grouper (Epinephelus fuscoguttatus♀ × E. lanceolatus♂), their antagonistic role on common pathogenic bacteria, and effects on intestinal health

As an alternative to the criticized antibiotics, probiotics have been adopted for their eco-friendly nature and ability to enhance host growth and immunity. Nevertheless, reports suggest ineffectiveness in commercially available probiotics since most are from non-fish sources; thus, this study was envisaged to isolate and characterize new Bacillus spp. from the gut of hybrid grouper (Epinephelus fuscoguttatus♀ × Epinephelus lanceolatus♂) which could serve as potential probiotics. The isolation and characterization were performed based on their morphological and biochemical properties, and 16S rRNA sequencing homology analysis. A subsequent 30-day in vivo biosafety feeding trial was conducted to ascertain isolates' non-pathogenicity, as well as their effects on fish growth, and intestinal mucosal microvilli via scanning electron microscopy (SEM) analysis. Four Bacillus spp. strains, namely, B. velezensis strain PGSAK01 (accession number OQ726606), B. stercoris strain PGSAK05 (accession number OQ726607), B. velezensis strain PGSAK17 (accession number OQ726601), and B. subtilis strain PGSAK19 (accession number OQ726605), were identified and characterized in the current study. The strains showed promising probiotic properties such higher adhesion capability, higher thermotolerance, displaying higher survivability to 0.5 % bile, lower pH tolerance, γ-haemolytic activity, and multispecies characteristics. Among the 24 antibiotics tested, while all isolates showed susceptibility to 21, the PGSAK01 strain showed resistance to furazolidone antibiotics. None of the isolates showed possession of i) virulence factor genes encoding enterotoxigenic (hblA, hblC, hblD, nheA, nheB, and entFM) and emetic (cereulide synthetase gene, ces) genes, and ii) streptomycin resistance gene (vat c), ampicillin-resistant genes (mecA and bla), and vancomycin-resistant gene (van B). Nevertheless, the PGSAK01 and PGSAK17 strains showed possession of tek K, cat, and ant(4')-Ia (adenylyltransferase) (except the PGSAK01) resistant genes. All isolates displayed better antimicrobial effects against pathogenic bacteria Streptococcus agalactiae, S. iniae, Vibrio harveyi, and V. alginolyticus. The in vivo biosafety trial involved hybrid grouper fish being grouped into five (average weight 32 ± 0.94 g), namely, the group fed the basal diet void of isolate's supplementation (control), and the remaining four groups fed the basal diet with 1 × 108 CFU/g diet of individual strain PGSAK01, PGSAK05, PGSAK17, and PGSAK19 supplementation. At the end of the study, a significantly higher WGR, K (except the PGSAK01 group), VSI; lysozyme (except PGSAK01 group), total antioxidant activity, alkaline phosphatase, superoxide dismutase enzyme activities; highly dense intestinal mucosal villi (based on the scanning electron microscopy analysis); and significantly lower malondialdehyde levels were witnessed in the isolated treated groups compared to the control, supporting the results obtained in the auto-aggregation and cell-surface hydrophobicity test. This work's results have provided thought-provoking targets; thus, studies involving extensive genome sequencing and functional annotation analysis will be explored to offer unfathomable insights into their mechanisms of action and potential health benefits, further establishing the four Bacillus strains' (PGSAK01, PGSAK05, PGSAK17, and PGSAK19) potential role in probiotic fields and functional foods.

Inhibitory effect of marine Bacillus sp. and its biomineralization on the corrosion of X65 steel in offshore oilfield produced water

The issue of material failure attributed to microbiologically influenced corrosion (MIC) is escalating in seriousness. Microorganisms not only facilitate corrosion but certain beneficial microorganisms also impede its occurrence. This study explored the impact of marine B. velezensis on the corrosion behavior of X65 steel in simulated offshore oilfield produced water. B. velezensis exhibited rapid growth in the initial stages, and the organic acid metabolites were found to promote corrosion. Subsequently, there was an increase in cross-linked "networked" biofilms products, a significant rise in the prismatic shape of corrosion products, and a tendency for continuous development in the middle and late stages. The organic/inorganic mineralized film layer formed on the surface remained consistently complete. Metabolic products of amino acid corrosion inhibitors were also observed to be adsorbed into the film. B. velezensis altered the kinetics of the X65 steel cathodic reaction, resulting in a deceleration of the electrochemical reaction rate. The mineralization induced by B. velezensis effectively slowed down the corrosion rate of X65 steel.

Introduction of Cellulolytic Bacterium Bacillus velezensis Z2.6 and Its Cellulase Production Optimization

Enzyme-production microorganisms typically occupy a dominant position in composting, where cellulolytic microorganisms actively engage in the breakdown of lignocellulose. Exploring strains with high yields of cellulose-degrading enzymes holds substantial significance for the industrial production of related enzymes and the advancement of clean bioenergy. This study was inclined to screen cellulolytic bacteria, conduct genome analysis, mine cellulase-related genes, and optimize cellulase production. The potential carboxymethylcellulose-hydrolyzing bacterial strain Z2.6 was isolated from the maturation phase of pig manure-based compost with algae residuals as the feedstock and identified as Bacillus velezensis. In the draft genome of strain Z2.6, 31 related cellulolytic genes were annotated by the CAZy database, and further validation by cloning documented the existence of an endo-1,4-β-D-glucanase (EC 3.2.1.4) belonging to the GH5 family and a β-glucosidase (EC 3.2.1.21) belonging to the GH1 family, which are predominant types of cellulases. Through the exploration of ten factors in fermentation medium with Plackett-Burman and Box-Behnken design methodologies, maximum cellulase activity was predicted to reach 2.98 U/mL theoretically. The optimal conditions achieving this response were determined as 1.09% CMC-Na, 2.30% salinity, and 1.23% tryptone. Validation under these specified conditions yielded a cellulose activity of 3.02 U/mL, demonstrating a 3.43-fold degree of optimization. In conclusion, this comprehensive study underscored the significant capabilities of strain Z2.6 in lignocellulolytic saccharification and its potentialities for future in-depth exploration in biomass conversion.

Characterization of Bacillus velezensis TJS119 and its biocontrol potential against insect pathogens

Introduction

The white-spotted flower chafer (Protaetia brevitarsis seulensis), which is widely distributed in Asian countries, is traditionally used in oriental medicine. However, its larvae are prone to severe damage by green muscardine disease (caused by Metarhizium anisopliae) during breeding. The aim of this study was to characterize Bacillus velezensis TJS119, which has been isolated from freshwater, and investigate its potential as a biocontrol agent against M. anisopliae in insects.

Methods

TJS119 was obtained from freshwater samples in the Republic of Korea and was classified as B. velezensis. We evaluated its in vitro antifungal effect, sequenced the bacterial whole genome, mined genes responsible for the synthesis of secondary metabolites, performed secondary metabolite analysis Ultra performance liquid chromatography-mass spectrometry (UPLC-MS/MS), and conducted bioassays for determining green muscardine disease control ability.

Results

Bacillus velezensis TJS119 inhibited the mycelial growth of M. anisopliae in vitro. The size of the B. velezensis TJS119 genome was estimated to be 3,890,913 bp with a GC content of 46.67% and 3,750 coding sequences. Biosynthetic gene clusters for secondary metabolites with antifungal activity were identified in the genome. Lipopeptides, including fengycin secreted by TJS119 exhibit antifungal activity. Application of TJS119 for the biocontrol against green muscardine disease increased the viability of white-spotted flower chafer by 94.7% compared to the control.

Discussion

These results indicate that B. velezensis TJS119 is a potential biocontrol agent for insect pathogens.

Bacillus velezensis LT1: a potential biocontrol agent for southern blight on Coptis chinensis

Introduction

Southern blight, caused by Sclerotium rolfsii, poses a serious threat to the cultivation of Coptis chinensis, a plant with significant medicinal value. The overreliance on fungicides for controlling this pathogen has led to environmental concerns and resistance issues. There is an urgent need for alternative, sustainable disease management strategies.

Methods

In this study, Bacillus velezensis LT1 was isolated from the rhizosphere soil of diseased C. chinensis plants. Its biocontrol efficacy against S. rolfsii LC1 was evaluated through a confrontation assay. The antimicrobial lipopeptides in the fermentation liquid of B. velezensis LT1 were identified using Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF-MS). The effects of B. velezensis LT1 on the mycelial morphology of S. rolfsii LC1 were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

Results

The confrontation assay indicated that B. velezensis LT1 significantly inhibited the growth of S. rolfsii LC1, with an inhibition efficiency of 78.41%. MALDI-TOF-MS analysis detected the presence of bacillomycin, surfactin, iturin, and fengycin in the fermentation liquid, all known for their antifungal properties. SEM and TEM observations revealed that the mycelial and cellular structures of S. rolfsii LC1 were markedly distorted when exposed to B. velezensis LT1.

Discussion

The findings demonstrate that B. velezensis LT1 has considerable potential as a biocontrol agent against S. rolfsii LC1. The identified lipopeptides likely contribute to the antifungal activity, and the morphological damage to S. rolfsii LC1 suggests a mechanism of action. This study underscores the importance of exploring microbial biocontrol agents as a sustainable alternative to chemical fungicides in the management of plant diseases. Further research into the genetic and functional aspects of B. velezensis LT1 could provide deeper insights into its biocontrol mechanisms and facilitate its application in agriculture.

Antimicrobial peptides from Bacillus spp. and strategies to enhance their yield

Antibiotic resistance is a growing concern that is affecting public health globally. The search for alternative antimicrobial agents has become increasingly important. Antimicrobial peptides (AMPs) produced by Bacillus spp. have emerged as a promising alternative to antibiotics, due to their broad-spectrum antimicrobial activity against resistant pathogens. In this review, we provide an overview of Bacillus-derived AMPs, including their classification into ribosomal (bacteriocins) and non-ribosomal peptides (lipopeptides and polyketides). Additionally, we delve into the molecular mechanisms of AMP production and describe the key biosynthetic gene clusters involved. Despite their potential, the low yield of AMPs produced under normal laboratory conditions remains a challenge to large-scale production. This review thus concludes with a comprehensive summary of recent studies aimed at enhancing the productivity of Bacillus-derived AMPs. In addition to medium optimization and genetic manipulation, various molecular strategies have been explored to increase the production of recombinant antimicrobial peptides (AMPs). These include the selection of appropriate expression systems, the engineering of expression promoters, and metabolic engineering. Bacillus-derived AMPs offer great potential as alternative antimicrobial agents, and this review provides valuable insights on the strategies to enhance their production yield, which may have significant implications for combating antibiotic resistance. KEY POINTS: • Bacillus-derived AMP is a potential alternative therapy for resistant pathogens • Bacillus produces two main classes of AMPs: ribosomal and non-ribosomal peptides • AMP yield can be enhanced using culture optimization and molecular approaches.

Strong Opponent of Walnut Anthracnose-Bacillus velezensis and Its Transcriptome Analysis

Walnut is a significant economic tree species worldwide. Walnut anthracnose, caused by the pathogen Colletotrichum gloeosporioides, greatly reduces walnut production and economic benefits. Our study showed that Bacillus velezensis effectively halted the growth of C. gloeosporioides, inducing noticeable abnormalities such as hyphal breakage and distortion, thereby curtailing the pathogen's virulence. A 50-100 times dilution of B. velezensis fermentation broth, applied every two to three days, served as an efficient protective layer for walnut leaves and fruits against C. gloeosporioides infection. Transcriptomic analysis of B. velezensis unveiled its dynamic response against C. gloeosporioides. On the second day, B. velezensis upregulated a significant number of differentially expressed genes related to the synthesis of metabolic products, amino acid biosynthesis, and motility. On the fourth day, continuous synthesis of metabolic products and amino acids, along with differential expression of spore-related genes, was observed. By the sixth day, the focus shifted towards environmental adaptation and carbon source utilization. Throughout the process, B. velezensis likely employed strategies such as the release of metabolic products, increased chemotaxis, and nutrient competition to exert its antagonistic effect on C. gloeosporioides. Fluorescence quantitative results showed that 15 primer pairs were up-regulated and 15 were down-regulated, with a 100% similarity rate to transcriptome sequencing results, confirming their authenticity. These findings provided a foundation for the widespread application of B. velezensis as a biocontrol agent in agriculture and forestry.

Characterization of Bacillus spp. isolated from the intestines of Rhynchocypris lagowskii as a potential probiotic and their effects on fish pathogens

Probiotics sourced from fish intestinal microbiota have a merit over other bacterial sources due to colonization ability and effective time. This study aimed to evaluate the bacilli isolated from the Rhynchocypris lagowskii intestines and their validity as a probiotic. Three isolates were selected (LSG 2-5, LSG 3-7, and LSG 3-8) and defined by morphological and 16S rRNA analysis as Bacillus velezensis, Bacillus aryabhattai, and Bacillus mojavensis, respectively. Results showed the strain tolerant abilities to gastrointestinal fluid, bile salt, pH, and temperature expotures. Additionally, all bacterial strains showed anti-pathogenic activity against at least four strains out of six tested pathogen strains (Staphylococcus aureus, Aeromonas hydrophila, Escherichia coli, Aeromonas veronii, Edwardsiella, and Aeromonas sobria). The bacterial strains also showed a high percentage of co-aggregation activity, more than 70%, with Aer. hydrophile, Staph. epidermidis, and Klebsiella aerogenes. At the same time, the results of competition, rejection, and substitution activity with Aer. hydrophila and Aer. veronii indicated the ability of the isolated strains to reduce the adhesion of pathogens to mucin. All strains showed safety properties, non-hemolytic, and sensitivity characteristics for most of tested antibiotics. In vivo test after injecting these strains into fish at various concentrations showed no side effects in the internal or external organs of fish compared to controls, proving that this is safe for these fish. Furthermore, the three strains produced lipase, amylase, and protease enzymes. The strains also showed bile salt hydrolase activity and biofilm formation, allowing them to tolerate stressful conditions. Conclusion: Based on these strains characteristics and features, they could be considered a promising candidate probiotic and can be used as an anti-pathogenic, especially in aquaculture.

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

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

Alcaligenes ammonioxydans HO-1 antagonizes Bacillus velezensis via hydroxylamine-triggered population response

Antagonism is a common behavior seen between microbes in nature. Alcaligenes ammonioxydans HO-1 converts ammonia to nitrogen under aerobic conditions, which leads to the accumulation of extracellular hydroxylamine (HA), providing pronounced growth advantages against many bacterial genera, including Bacillus velezensis V4. In contrast, a mutant variant of A. ammonioxydans, strain 2-29, that cannot produce HA fails to antagonize other bacteria. In this article, we demonstrate that cell-free supernatants derived from the antagonistic HO-1 strain were sufficient to reproduce the antagonistic behavior and the efficiency of this inhibition correlated strongly with the HA content of the supernatant. Furthermore, reintroducing the capacity to produce HA to the 2-29 strain or supplementing bacterial co-cultures with HA restored antagonistic behavior. The HA-mediated antagonism was dose-dependent and affected by the temperature, but not by pH. HA caused a decline in biomass, cell aggregation, and hydrolysis of the cell wall in exponentially growing B. velezensis bulk cultures. Analysis of differential gene expression identified a series of genes modulating multicellular behavior in B. velezensis. Genes involved in motility, chemotaxis, sporulation, polypeptide synthesis, and non-ribosomal peptide synthesis were all significantly downregulated in the presence of HA, whereas autolysis-related genes showed upregulation. Taken together, these findings indicate that HA affects the population response of coexisting strains and also suggest that A. ammonioxydans HO-1 antagonize other bacteria by producing extracellular HA that, in turn, acts as a signaling molecule.

Comparative genomics analysis of Bacillus velezensis LOH112 isolated from a nonagenarian provides insights into its biocontrol and probiotic traits

Bacillus velezensis has recently received increasing attention as a biological fungicide and a potential probiotic agent because of its broad spectrum of antibacterial and antifungal activities. Here, we evaluated the beneficial traits of a newly isolated B. velezensis strain LOH112 using comprehensive bioinformatics and comparative genomic analyses and in vitro experimental approaches. Whole genome sequencing and assembly results showed that the genome of LOH112 consists of a circular chromosome and a circular plasmid, which encodes proteins involved in important biological processes such as sporulation, quorum sensing, and antibiotic synthesis. LOH112 contains 13 secondary metabolism gene clusters responsible for the production of antimicrobial compounds. In vitro experiments showed that LOH112 effectively inhibits several fungi and Gram-positive pathogenic bacteria, hydrolyzes protein and cellulose, and is capable of forming strong adhesive biofilms. Furthermore, comparative genomics revealed that LOH112 contains 34 strain-specific orthologous gene clusters, including two caseinolytic protease P (clpP) genes responsible for proteomic homeostasis. Selective pressure analysis indicated that the transmembrane transporter and ATP-dependent alanine/valine adenylase genes were strongly positively selected, which may endow LOH112 with better biocontrol ability and potential probiotic properties. Collectively, these results not only provide insights into a deeper understanding of the genomic characterization of LOH112 but also imply the potential application of LOH112 as biocontrol and probiotic agents.

Antibacterial Activity of Two Metabolites Isolated From Endophytic Bacteria Bacillus velezensis Ea73 in Ageratina adenophora

Ageratina adenophora, as an invasive and poisonous weed, seriously affects the ecological diversity and development of animal husbandry. Weed management practitioners have reported that it is very difficult to control A. adenophora invasion. In recent years, many researchers have focused on harnessing the endophytes of the plant as a useful resource for the development of pharmacological products for human and animal use. This study was performed to identify endophytes with antibacterial properties from A. adenophora. Agar well diffusion method and 16S rRNA gene sequencing technique were used to screen and identify endophytes with antibacterial activity. The response surface methodology and prep- high-performance liquid chromatography were used to determine the optimizing fermentation conditions and isolate secondary metabolites, respectively. UV-visible spectroscopy, infrared spectroscopy, nuclear magnetic resonance, and high-resolution mass spectrum were used to determine the structures of the isolated metabolites. From the experiment, we isolated a strain of Bacillus velezensis Ea73 (GenBank no. MZ540895) with broad-spectrum antibacterial activity. We also observed that the zone of inhibition of B. velezensis Ea73 against Staphylococcus aureus was the largest when fermentation broth contained 6.55 g/L yeast extract, 6.61 g/L peptone, 20.00 g/L NaCl at broth conditions of 7.95 pH, 51.04 h harvest time, and a temperature of 27.97°C. Two antibacterial peptides, Cyclo (L-Pro-L-Val) and Cyclo (L-Leu-L-Pro), were successfully extracted from B. velezensis Ea73. These two peptides exhibited mild inhibition against S. aureus and Escherichia coli. Therefore, we isolated B. velezensis Ea73 with antibacterial activity from A. adenophora. Hence, its metabolites, Cyclo (L-Pro-L-Val) and Cyclo (L-Leu-L-Pro), could further be developed as a substitute for human and animal antibiotics.

Antimicrobial Bacillus: Metabolites and Their Mode of Action

The agricultural industry utilizes antibiotic growth promoters to promote livestock growth and health. However, the World Health Organization has raised concerns over the ongoing spread of antibiotic resistance transmission in the populace, leading to its subsequent ban in several countries, especially in the European Union. These restrictions have translated into an increase in pathogenic outbreaks in the agricultural industry, highlighting the need for an economically viable, non-toxic, and renewable alternative to antibiotics in livestock. Probiotics inhibit pathogen growth, promote a beneficial microbiota, regulate the immune response of its host, enhance feed conversion to nutrients, and form biofilms that block further infection. Commonly used lactic acid bacteria probiotics are vulnerable to the harsh conditions of the upper gastrointestinal system, leading to novel research using spore-forming bacteria from the genus Bacillus. However, the exact mechanisms behind Bacillus probiotics remain unexplored. This review tackles this issue, by reporting antimicrobial compounds produced from Bacillus strains, their proposed mechanisms of action, and any gaps in the mechanism studies of these compounds. Lastly, this paper explores omics approaches to clarify the mechanisms behind Bacillus probiotics.

Pharmacological Effects of Marine-Derived Enterococcus faecium EA9 against Acute Lung Injury and Inflammation in Cecal Ligated and Punctured Septic Rats

Microorganisms obtained from the marine environment may represent a potential therapeutic value for multiple diseases. This study explored the possible protective role of marine-derived potential probiotic Enterococcus faecium EA9 (E. faecium) against pulmonary inflammation and oxidative stress using the cecal ligation and puncture (CLP) model of sepsis in Wistar rats. Animals were pretreated with E. faecium for 10 days before either sham or CLP surgeries. Animals were sacrificed 72 hours following the surgical intervention. The histological architecture of lung tissues was evaluated as indicated by the lung injury score. In addition, the extend of pulmonary edema was determined as wet/dry weight ratio. The inflammatory cytokines were estimated in lung tissues, including tumor necrosis factor-alpha (TNF-α), interleukin-6 (IL-6), and interleukin-1 beta (IL-1β) using the enzyme-linked-immunosorbent-assay (ELISA) technique. Moreover, markers for lipid peroxidation such as thiobarbituric acid reaction substances (TBARs), and endogenous antioxidants, including reduced glutathione (GSH) were determined in lung tissues. Finally, the enzymatic activities of antioxidant enzymes such as catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase (GPx), and glutathione reductase (GR) were assayed in the lungs. Pretreatment with E. faecium markedly attenuated CLP-induced lung injury and pulmonary edema. Markers for inflammation, including TNF-α, IL-6, and IL-1β were augmented in the lung tissues of CLP animals, while E. faecium ameliorated their augmented levels. E. faecium pretreatment also restored the elevated TBARS levels and the prohibited CAT, SOD, and GPx enzymatic activities in CLP animals. GSH levels were corrected by E. faecium in CLP animals. The inflammatory and lipid peroxidation mediators were positively correlated, while antioxidant enzymatic activities were negatively correlated with CLP-induced lung injury and pulmonary edema. Collectively, marine-derived Enterococcus faecium EA9 might be considered as a prospective therapeutic tool for the management of pulmonary dysfunction associated with sepsis.

The antagonistic mechanism of Bacillus velezensis ZW10 against rice blast disease: Evaluation of ZW10 as a potential biopesticide

Rice blast, caused by the fungus Magnaporthe oryzae, is one of the three major diseases affecting rice production and quality; it reduces rice grain yield by nearly 30%. In the early stage of this study, a strain of Bacillus velezensis with strong inhibition of M. oryzae was isolated and named ZW10. In vitro assays indicated prolonged germination time of conidia of M. oryzae treated with the antifungal substances of ZW10, 78% of the conidia could not form appressorium, and the conidial tubes expanded to form vacuolar structure and then shrank. The results of FDA-PI composite dyes showed that the antifungal substances of ZW10 inhibited the normal activity of M. oryzae hyphae that were rarely able to infect the epidermal cells of rice leaf sheath in vivo tests. In addition, rice treated with the antifungal substances of ZW10 showed a variety of defense responses, including activation of defense-related enzymes, increased expression of the salicylic acid pathway genes, and accumulation of hydrogen peroxide (H2O2), which might function directly or indirectly in resistance to pathogen attack. The field experiment with rice blast infection in different periods showed that the antifungal substances of ZW10 had the same control effect as carbendazim. The significant biological control activity of ZW10 and its capacity to stimulate host defenses suggest that this B. velezensis strain has the potential to be developed into a biopesticide for the biocontrol of rice blast.

In vitro Assessment of the Probiotic Characteristics of Three Bacillus Species from the Gut of Nile Tilapia, Oreochromis niloticus

Probiotics used in aquaculture are mostly from non-fish sources, as a result ineffective in eliciting the desired effects in aquatic animals. In this study, three Bacillus species were isolated from the digestive tract of freshwater fish Oreochromis niloticus and characterised based on their morphological, biochemical and evolutionary relationships. Their probiotic potentials were evaluated based on their ability to tolerate low pH, bile salt concentration, high temperatures, adhesion ability (auto-aggregation and hydrophobicity), haemolytic activity and antimicrobial activity including biosafety assay. Three Bacillus strains identified as Bacillus velezensis TPS3N (MK130897), Bacillus subtilis TPS4 (MK130899) and Bacillus amyloliquefaciens TPS17 (MK130898) were designated as TPS3N, TPS4 and TPS17, respectively. TPS3N and TPS17 were α-haemolytic, while TPS4 was γ-haemolytic. The three isolates had higher viability ability after exposure to higher temperatures (80 °C, 90 °C and 100 °C) and were resistant to low pH (1) and bile salt concentration (0.5%) as well as high cell surface hydrophobicity and auto-aggregation. The three isolates were compatible with one another and thus can be used in consortia. These strains were susceptible to gentamicin, cephalexin, ampicillin, ceftriaxone, kanamycin, amikacin, penicillin, cefoperazone, chloramphenicol, erythromycin, tetracycline, doxycycline, ciprofloxacin, clindamycin (except TPS4) and furazolidone (except TPS17). The antimicrobial assessment showed that among the three isolates, TPS3N and TPS17 exhibited good antimicrobial activity against the three fish pathogens (Streptococcus agalactiae, Aeromonas hydrophila, Vibrio harveyi), while TPS4 was effective against Streptococcus agalactiae only. The results of this work suggest that Bacillus strains TPS3N, TPS4 and TPS17 could be considered as potential probiotics in tilapia aquaculture.

Potential of Bacillus velezensis as a probiotic in animal feed: a review

Bacillus velezensis is a plant growth-promoting bacterium that can also inhibit plant pathogens. However, based on its properties, it is emerging as a probiotic in animal feed. This review focuses on the potential characteristics of B. velezensis for use as a probiotic in the animal feed industry. The review was conducted by collecting recently published articles from peer-reviewed journals. Google Scholar and PubMed were used as search engines to access published literature. Based on the information obtained, the data were divided into three groups to discuss the (i) probiotic characteristics of B. velezensis, (ii) probiotic potential for fish, and (iii) the future potential of this species to be developed as a probiotic for the animal feed industry. Different strains of B. velezensis isolated from different sources were found to have the ability to produce antimicrobial compounds and have a beneficial effect on the gut microbiota, with the potential to be a candidate probiotic in the animal feed industry. This review provides valuable information about the characteristics of B. velezensis, which can provide researchers with a better understanding of the use of this species in the animal feed industry.

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

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

Bacillus velezensis WLYS23 strain possesses antagonistic activity against hybrid snakehead bacterial pathogens

AIM

The aims of this study were to screen an antagonistic probiotic for the prevention and control of bacterial diseases in snakehead fish and to evaluate the antimicrobial activities, biosafety and biocontrol effect of the antagonistic strain.

METHODS AND RESULTS

In this study, the WLYS23 strain exhibiting the most effective antagonistic properties against several fish pathogens was selected from the intestine of healthy snakehead fish. The strain was identified as Bacillus velezensis based on morphological, physiological and biochemical characteristics, and phylogenetic analysis. This strain showed amylase, cellulase, protease and lipase activities according to extracellular enzyme activity assays. Moreover, the cell-free extract of the WLYS23 strain exhibited strong antibacterial activity, with MICs of ≤64 μg ml^-1 for most fish pathogens. Additionally, the cell-free extract was heat and pH stable, and resistant to protease, whereas their antimicrobial activities were significantly influenced by metal ions at high concentration. The WLYS23 strain and its cell-free extract were safe for snakehead and zebrafish. The disease resistance of snakehead fish was significantly increased following administration of the WLYS23 strain and its cell-free extract respectively. The complete genome of the WLYS23 strain was sequenced and found to harbour seven gene clusters encoding secondary metabolites with antimicrobial activity.

CONCLUSION

The WLYS23 strain possesses great potential as a biocontrol agent, which can be commercially developed to improve disease control in freshwater aquaculture.

SIGNIFICANCE AND IMPACT OF THE STUDY

Snakehead fish are important commercially farmed fish worldwide. However, the bacterial diseases caused by Aeromonas sp. and Nocardia seriolae in farmed snakehead fish lead to huge economic losses. Safe, economical and efficient probiotics are limited to prevent and control these diseases. Here, we provide a promising biocontrol agent with antagonistic activity against bacterial diseases of snakehead.

Isolation and Characterization of Fish-Gut Bacillus spp. as Source of Natural Antimicrobial Compounds to Fight Aquaculture Bacterial Diseases

Aquaculture is responsible for more than 50% of global seafood consumption. Bacterial diseases are a major constraint to this sector and associated with misuse of antibiotics, pose serious threats to public health. Fish-symbionts, co-inhabitants of fish pathogens, might be a promising source of natural antimicrobial compounds (NACs) alternative to antibiotics, limiting bacterial diseases occurrence in aquafarms. In particular, sporeforming Bacillus spp. are known for their probiotic potential and production of NACs antagonistic of bacterial pathogens and are abundant in aquaculture fish guts. Harnessing the fish-gut microbial community potential, 172 sporeforming strains producing NACs were isolated from economically important aquaculture fish species, namely European seabass, gilthead seabream, and white seabream. We demonstrated that they possess anti-growth, anti-biofilm, or anti-quorum-sensing activities, to control bacterial infections and 52% of these isolates effectively antagonized important fish pathogens, including Aeromonas hydrophila, A. salmonicida, A. bivalvium, A. veronii, Vibrio anguillarum, V. harveyi, V. parahaemolyticus, V. vulnificus, Photobacterium damselae, Tenacibaculum maritimum, Edwardsiela tarda, and Shigella sonnei. By in vitro quantification of sporeformers' capacity to suppress growth and biofilm formation of fish pathogens, and by assessing their potential to interfere with pathogens communication, we identified three promising candidates to become probiotics or source of bioactive molecules to be used in aquaculture against bacterial aquaculture diseases.

A Review on the Biotechnological Applications of the Operational Group Bacillus amyloliquefaciens

Bacteria under the operational group Bacillus amyloliquefaciens (OGBa) are all Gram-positive, endospore-forming, and rod-shaped. Taxonomically, the OGBa belongs to the Bacillus subtilis species complex, family Bacillaceae, class Bacilli, and phylum Firmicutes. To date, the OGBa comprises four bacterial species: Bacillus amyloliquefaciens, Bacillus siamensis, Bacillus velezensis and Bacillus nakamurai. They are widely distributed in various niches including soil, plants, food, and water. A resurgence in genome mining has caused an increased focus on the biotechnological applications of bacterial species belonging to the OGBa. The members of OGBa are known as plant growth-promoting bacteria (PGPB) due to their abilities to fix nitrogen, solubilize phosphate, and produce siderophore and phytohormones, as well as antimicrobial compounds. Moreover, they are also reported to produce various enzymes including α-amylase, protease, lipase, cellulase, xylanase, pectinase, aminotransferase, barnase, peroxidase, and laccase. Antimicrobial compounds that able to inhibit the growth of pathogens including non-ribosomal peptides and polyketides are also produced by these bacteria. Within the OGBa, various B. velezensis strains are promising for use as probiotics for animals and fishes. Genome mining has revealed the potential applications of members of OGBa for removing organophosphorus (OPs) pesticides. Thus, this review focused on the applicability of members of OGBa as plant growth promoters, biocontrol agents, probiotics, bioremediation agents, as well as producers of commercial enzymes and antibiotics. Here, the bioformulations and commercial products available based on these bacteria are also highlighted. This review will better facilitate understandings of members of OGBa and their biotechnological applications.

Genomic and phenotypic characterization of Bacillus velezensis AMB-y1; a potential probiotic to control pathogens in aquaculture

The aim of this investigation was to isolate and identify Bacillus species isolated from the internal microbiota of Red sea stingrays as potential probiotics. An initial assay on the ability of the isolates to control stingray pathogens of Vibrio species led to the selection of one highly antagonistic isolate. The most potent isolate was identified based on whole genome phylogeny as Bacillus velezensis AMB-y1. Genome mining for secondary metabolites identified five antibacterial biosynthetic clusters that produce, bacilysin, bacillaene, difficidin, macrolactin and mersacidin. Genome mining also identified two antifungal biosynthetic clusters which encode genes to produce bacillomycin D and fengycin. The genome mining also identified an unknown NRPS-transAT-PKS cluster that likely produced another compound with antibiotic activity. The strain was further characterized by the assessment of abiotic stress tolerances that are required in potential probiotic agents. The selected isolate had promising results in abiotic stress tolerance; pH tolerance within the range from 4.0 to 8.0, able to survive concentrations of bile salt up to 0.4% and sodium chloride from 0 to 6.5%. In addition, the strain showed a value of hydrophobicity (31%) along with a higher value of auto-aggregation (49.9%), which demonstrates its potential ability to adhere to the intestinal wall on the basis of its cell surface traits. The strain was evaluated for susceptibility to antimicrobials and the novel B. velezensis AMB-y1 has potential to be used as a probiotic in aquaculture to control marine fish and stingray pathogens.

Antibacterial Activity of Bacillus inaquosorum Strain T1 against pirABVp -Bearing Vibrio parahaemolyticus: Genetic and Physiological Characterization

Acute hepatopancreatic necrosis disease (AHPND) is caused by PirAB toxin-producing Vibrio parahaemolyticus and has devastated the global shrimp aquaculture industry. One approach for preventing the growth of AHPND-producing Vibrio spp. is through the application of beneficial bacteria capable of inhibiting these pathogens. In this study, we focused on the inhibitory activity of Bacillus inaquosorum strain T1, which hinders V. parahaemolyticus growth in coculture experiments in a density-dependent manner; inhibition was also observed using cell-free supernatants from T1 stationary-phase cultures. Using mariner-based transposon mutagenesis, 17 mutants having a complete or partial loss of inhibitory activity were identified. Of those displaying a total loss of activity, 13 had insertions within a 42.6-kb DNA region comprising 15 genes whose deduced products were homologous to nonribosomal polypeptide synthetases (NRPSs), polyketide synthases (PKSs), and related activities, which were mapped as one transcriptional unit. Mutants with partial activity contained insertions in spo0A and oppA, indicating stationary-phase control. The levels of expression of NRPS and PKS lacZ transcriptional fusions were negligible during growth and were the highest during early stationary phase. Inactivation of sigH resulted in a loss of inhibitor activity, indicating a role for σ^H in transcription. Disruption of abrB resulted in NRPS and PKS gene overexpression during growth as well as enhanced growth inhibition. Our characterization of the expression and control of an NRPS-PKS gene cluster in B. inaquosorum T1 provides an understanding of the factors involved in inhibitor production, enabling this strain's development for use as a tool against AHPND-causing Vibrio pathogens in shrimp aquaculture.IMPORTANCE The shrimp aquaculture industry has been significantly impacted by acute hepatopancreatic necrosis disease (AHPND), resulting in significant financial losses annually. AHPND is caused by strains of the bacterial pathogen Vibrio parahaemolyticus, and treatment of AHPND involves the use of antibiotics, which leads to a rise in the number of antibiotic-resistant strains. Alternative treatments include the application of beneficial microorganisms having inhibitory activities against pathogens causing AHPND. In this study, we examined the ability of Bacillus inaquosorum strain T1 to inhibit the growth of an AHPND-causing Vibrio strain, and we show that this activity involves a gene cluster associated with antibacterial compound production. We found that gene expression is under stationary-phase control and that enhanced activity occurs upon inactivation of a global transition state regulator. Our approach for understanding the factors involved in producing B. inaquosorum strain T1 inhibitory activity will allow for the development of this strain as a tool for AHPND prevention and treatment.

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

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

Complete genome sequence and antimicrobial activity of Bacillus velezensis JT3-1, a microbial germicide isolated from yak feces

Bacillus velezensis JT3-1 was isolated from feces of the domestic yak (Bos grunniens) in Gansu province of China. Aim to know whether B. velezensis JT3-1 has the potency to be developed as a probiotic bacterium, works on the complete genome sequence, antimicrobial activity, growth performance in calves, and treatment effect on calf diarrhea of B. velezensis JT3-1 were carried out. The results showed that the complete genome of B. velezensis JT3-1 contains one gapless circular chromosome which is 3,929,799 bp, and has 3761 protein-encoding genes with an average GC content of 46.50%. From the antimicrobial activity results, B. velezensis JT3-1 has shown strong antagonistic activities against Listeria monocytogenes, Staphylococcus aureus, Escherichia coli, Salmonella Typhimurium, Mannheimia haemolytica, Staphylococcus hominis, Clostridium perfringens, and Mycoplasma bovis. Compared with the control group, the average weight of the experiment animals from Bv1 group and Bv2 group which were supplemented with B. velezensis JT3-1 was significantly increased (P < 0.05). Meanwhile, the Bv1 and Bv2 supplement significantly improved the level of IgA, IgG, IgM, and IFN-γ in calves as compared with the controls (P < 0.05), but the IL-2 level was not obviously changed between the three groups. In addition, B. velezensis JT3-1 showed a good effect against diarrhea, as the cure rate reached 95.0% (171/180) in newborn calves (Angus cattle) in Xinjiang, and 100.0% (149/149) in yak calves in Qinghai, respectively. Our study will lay a good foundation for the elucidation of the molecular mechanisms of its antimicrobial activity, and supports the hypothesis that JT3-1 has the potential to be developed as a probiotic bacterium in cattle.

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

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

IMPORTANCE

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

Genome Sequence of Lipopeptide- and Antioxidant-Producing Strain Bacillus velezensis NWUMFkBS10.5

Candidate biocontrol agent Bacillus velezensis NWUMFkBS10.5 possesses unique genomic characteristics revealed by antiSMASH analysis and in vitro metabolomic elucidation. Besides its capability to produce antimicrobial lipopeptides, further in silico genome profiling predicted the presence of metabolic pathways for synthesizing antioxidants like lampranthin-2, miraxanthin V, and 2-decarboxybetanidin.

Bacillus velezensis: phylogeny, useful applications, and avenues for exploitation

Some members of the Bacillus velezensis (Bv) group (e.g., Bv FZB42T and AS3.43) were previously assigned grouping with B. subtilis and B. amyloliquefaciens, based on the fact that they shared a 99% DNA-DNA percentage phylogenetic similarity. However, hinging on current assessments of the pan-genomic reassignments, the differing phylogenomic characteristics of Bv from B. subtilis and B. amyloliquefaciens are now better understood. Within this re-grouping/reassignment, the various strains within the Bv share a close phylogenomic resemblance, and a number of these strains have received a lot of attention in recent years, due to their genomic robustness, and the growing evidence for their possible utilization in the agricultural industry for managing plant diseases. Only a few applications for their use medicinally/pharmaceutically, environmentally, and in the food industry have been reported, and this may be due to the fact that the majority of those strains investigated are those typically occurring in soil. Although the intracellular unique biomolecules of Bv strains have been revealed via in silico genome modeling and investigated using transcriptomics and proteomics, a further inquisition into the Bv metabolome using newer technologies such as metabolomics could elucidate additional applications of this economically relevant Bacillus species, beyond that of primarily the agricultural sector.

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

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

Isolation of Marine Bacillus sp. with Antagonistic and Organic-Substances-Degrading Activities and Its Potential Application as a Fish Probiotic

We report on the isolation and characterization of an acid- and bile-tolerant bacterial strain, Bacillus sp. YB1701 with antibacterial and quorum-quenching activity. Strain YB1701 was isolated from coastal sediment samples and characterized by biochemical tests and 16S rRNA sequencing. In vitro study indicated that strain YB1701 can survive at pH 2.0 for up to 3 h and tolerate bile up to 2.0% concentration even after 12 h of exposure. Strain YB1701 showed antimicrobial activity against fish pathogens Aeromonas hydrophila and Vibrio parahemolyticus using an agar well diffusion assay. The trial test showed dietary supplementation of YB1701 significantly improved the resistance of Carassius auratus gibelio against A. hydrophila challenge. The safety assessment revealed that the isolate Bacillus sp. YB1701 was not cytotoxic to Carassius auratus gibelio or mice and did not exhibit hemolytic activity on rabbit blood agar plate. Disc-diffusion assays using a panel of antibiotics listed by the European Food Safety Authority (EFSA) showed that YB1701 was susceptible to selected antibiotics. Under laboratory conditions, the degradation rate of organic waste (predominately fish excrement) for 14 days by YB1701 was 79.69%. Results from the present study suggest that strain YB1701 is a potential probiotic strain and can be used in aquaculture for degrading organic waste and improving disease resistance of fish against bacterial infection. Further study is needed to assess the utility of strain YB1701 on a commercial scale.

Characteristics and Application of a Novel Species of Bacillus: Bacillus velezensis

Bacillus velezensis has been investigated and applied more and more widely recently because it can inhibit fungi and bacteria and become a potential biocontrol agent. In order to provide more clear and comprehensive understanding of B. velezensis for researchers, we collected the recent relevant articles systematically and reviewed the discovery and taxonomy, secondary metabolites, characteristics and application, gene function, and molecular research of B. velezensis. This review will give some direction to the research and application of this strain for the future.

Strong Genomic and Phenotypic Heterogeneity in the Aeromonas sobria Species Complex

Aeromonas sobria is a mesophilic motile aeromonad currently depicted as an opportunistic pathogen, despite increasing evidence of mutualistic interactions in salmonid fish. However, the determinants of its host-microbe associations, either mutualistic or pathogenic, remain less understood than for other aeromonad species. On one side, there is an over-representation of pathogenic interactions in the A. sobria literature, of which only three articles to date report mutualistic interactions; on the other side, genomic characterization of this species is still fairly incomplete as only two draft genomes were published prior to the present work. Consequently, no study specifically investigated the biodiversity of A. sobria. In fact, the investigation of A. sobria as a species complex may have been clouded by: (i) confusion with A. veronii biovar sobria because of their similar biochemical profiles, and (ii) the intrinsic low resolution of previous studies based on 16S rRNA gene sequences and multilocus sequence typing. So far, the only high-resolution, phylogenomic studies of the genus Aeromonas included one A. sobria strain (CECT 4245 / Popoff 208), making it impossible to robustly conclude on the phylogenetic intra-species diversity and the positioning among other Aeromonas species. To further understand the biodiversity and the spectrum of host-microbe interactions in A. sobria as well as its potential genomic diversity, we assessed the genomic and phenotypic heterogeneity among five A. sobria strains: two clinical isolates recovered from infected fish (JF2635 and CECT 4245), one from an infected amphibian (08005) and two recently isolated brook charr probionts (TM12 and TM18) which inhibit in vitro growth of A. salmonicida subsp. salmonicida (a salmonid fish pathogen). A phylogenomic assessment including 2,154 softcore genes corresponding to 946,687 variable sites from 33 Aeromonas genomes confirms the status of A. sobria as a distinct species divided in two subclades, with 100% bootstrap support. The phylogenomic split of A. sobria in two subclades is corroborated by a deep dichotomy between all five A. sobria strains in terms of inhibitory effect against A. salmonicida subsp. salmonicida, gene contents and codon usage. Finally, the antagonistic effect of A. sobria strains TM12 and TM18 suggests novel control methods against A. salmonicida subsp. salmonicida.