Biological control of tomato bacterial wilt by oxydifficidin and difficidin-producing Bacillus methylotrophicus DR-08

Screening, identification and evaluation of an acidophilic strain of Bacillus velezensis B4-7 for the biocontrol of tobacco bacterial wilt

Tobacco (Nicotiana tabacum L.) bacterial wilt, caused by Ralstonia solanacearum, is indeed a highly destructive plant disease, leading to substantial damage in tobacco production. While biological control is considered an effective measure for managing bacterial wilt, related research in this area has been relatively limited compared to other control methods. In order to discover new potential antagonistic bacteria with high biocontrol efficacy against tobacco bacterial wilt, we conducted an analysis of the microbial composition differences between disease-suppressive and disease-conducive soils using Illumina sequencing. As a result, we successfully isolated six strains from the disease-suppressive soil that exhibited antibacterial activity against Ralstonia solanacearum. Among these strains, B4-7 showed the strongest antibacterial activity, even at acidic conditions with a pH of 4.0. Based on genome analysis using Average Nucleotide Identity (ANI), B4-7 was identified as Bacillus velezensis. In greenhouse and field trials, strain B4-7 significantly reduced the disease index of tobacco bacterial wilt, with control efficiencies reaching 74.03% and 46.88% respectively. Additionally, B4-7 exhibited plant-promoting abilities that led to a 35.27% increase in tobacco production in field conditions. Quantitative real-time (qPCR) analysis demonstrated that strain B4-7 effectively reduced the abundance of R. solanacearum in the rhizosphere. Genome sequencing and Liquid Chromatography-Mass Spectrometry (LC-MS) analysis revealed that strain B4-7 potentially produces various lipopeptide metabolites, such as microlactin, bacillaene, difficidin, bacilysin, and surfactin. Furthermore, B4-7 influenced the structure of the rhizosphere soil microbial community, increasing bacterial abundance and fungal diversity, while also promoting the growth of different beneficial microorganisms. In addition, B4-7 enhanced tobacco's resistance to R. solanacearum by increasing the activities of defense enzymes, including superoxide dismutase (SOD), phenylalanine ammonia-lyase (PAL), peroxidase (POD), catalase (CAT), and polyphenol oxidase (PPO). Collectively, these findings suggest that B. velezensis B4-7 holds significant biocontrol potential and can be considered a promising candidate strain for eco-friendly management of tobacco bacterial wilt.

Characterization of Bacillus velezensis 32a metabolites and their synergistic bioactivity against crown gall disease

Crown gall disease caused by Agrobacterium tumefaciens is considered to be the main bacterial threat of stone fruit plants in Mediterranean countries. In a previous study, Bacillus velezensis strain 32a was isolated from Tunisian rhizosphere soil and revealed high antagonistic potential against A. tumefaciens strains. In order to better characterize the antagonistic activity of this strain against this important plant pathogen, the production of secondary metabolites was analyzed using liquid chromatography coupled with mass spectrometry. The results revealed the production of different compounds identified as surfactins, fengycins, iturins and bacillibactin belonging to the lipopeptide group, three polyketides (macrolactins, oxydifficidin and bacillaenes), bacilysin and its chlorinated derivative; chlorotetaine. The involvement of lipopeptides in this antagonistic activity was ruled out by performing agar and broth dilution tests with pure molecules. Thus, the construction of B. velezensis 32a mutants defective in polyketides and bacilysin biosynthesis and their antagonistic activity was performed and compared to a set of derivative mutants of a comparable strain, B. velezensis GA1. The defective difficidin mutants (△dfnA and △dfnD) were unable to inhibit the growth of A. tumefaciens, indicating the high-level contribution of difficidin in the antagonism process. While the macrolactin deficient mutant (∆mlnA) slightly decreased the activity, suggesting a synergetic effect with difficidin. Remarkably, the mutant △dhbC only deficient in bacillibactin production showed significant reduction in its capacity to inhibit the growth of Agrobacterium.Taken collectively, our results showed the strong synergetic effect of difficidin and macrolactins and the significant implication of siderophore to manage crown gall disease.

Analysis of the Genomic Sequences and Metabolites of Bacillus velezensis YA215

Discovering more novel antimicrobial compounds has become a keen research problem. In this study, YA215 genome was sequenced by the Illumina HiSeq + PacBio sequencing platform. Genome assembly was performed by Unicycler software and the gene clusters responsible for secondary metabolite biosynthesis were predicted by antiSMASH. The genome comprised 3976514 bp and had a 46.56% G + C content. 3809 coding DNA sequences, 27 rRNAs, 86 tRNAs genes, and 79 sRNA were predicted. Strain YA215 was re-identified as Bacillus velezensis based on ANI and OrthoANI analysis. In the COG database, 23 functional groups from 3090 annotations were predicted. In the GO database, 2654 annotations were predicted. 2486 KEGG annotations linked 41 metabolic pathways. Glycosyl transferases, polysaccharide lyases, auxiliary activities, glycoside hydrolases, carbohydrate esterases, and carbohydrate-binding modules were predicted among the 127 annotations in the CAZy database. AntiSMASH analysis predicted that B. velezensis YA215 boasted 13 gene clusters involved in synthesis of antimicrobial secondary metabolites including surfactin, fengycin, macrolactin H, bacillaene, difficidin, bacillibactin, bacilysin, and plantazolicin. Three of the gene clusters (gene cluster 5, gene cluster 9, and gene cluster 10) have the potential to synthesize unknown compounds. The research underscore the considerable potential of secondary metabolites, identified in the genomic composition of B. velezensis YA215, as versatile antibacterial agents with a broad spectrum of activity against pathogenic bacteria.

Bacillus-based biocontrol beyond chemical control in central Africa: the challenge of turning myth into reality

Agricultural productivity in the Great Lakes Countries of Central Africa, including Burundi, Rwanda, and the Democratic Republic of Congo, is affected by a wide range of diseases and pests which are mainly controlled by chemical pesticides. However, more than 30% of the pesticides used in the region are banned in European Union due to their high toxicity. Globally available safe and eco-friendly biological alternatives to chemicals are virtually non-existent in the region. Bacillus PGPR-based biocontrol products are the most dominant in the market and have proven their efficacy in controlling major plant diseases reported in the region. With this review, we present the current situation of disease and pest management and urge the need to utilize Bacillus-based control as a possible sustainable alternative to chemical pesticides. A repertoire of strains from the Bacillus subtilis group that have shown great potential to antagonize local pathogens is provided, and efforts to promote their use, as well as the search for indigenous and more adapted Bacillus strains to local agro-ecological conditions, should be undertaken to make sustainable agriculture a reality in the region.

Isolation, Antimicrobial Effect and Metabolite Analysis of Bacillus amyloliquefaciens ZJLMBA1908 against Citrus Canker Caused by Xanthomonas citri subsp. citri

Citrus canker caused by Xanthomonas citri subsp. citri is a devastating bacterial disease with severe implications for the citrus industry. Microorganisms possessing biocontrol capabilities against X. citri subsp. citri offer a highly promising strategy for healthy citrus management. In the present study, a broad-spectrum antagonist strain ZJLMBA1908 with potent antibacterial activity against X. citri subsp. citri was isolated from symptomatic lemon leaves, and identified as Bacillus amyloliquefaciens. Cell-free supernatant (CFS) of strain ZJLMBA1908 also exhibited remarkable antimicrobial activity, especially suppressing the growth of X. citri subsp. citri and Nigrospora oryzae, with inhibition rates of 27.71% and 63.75%, respectively. The antibacterial crude extract (CE) derived from the CFS displayed effective activity against X. citri subsp. citri. A preventive treatment using the CE significantly reduced the severity and incidence of citrus canker in a highly susceptible citrus host. Additionally, the CE maintained activity in the presence of protease and under a wide range of temperature and pH treatments. Applying high-performance liquid chromatography (HPLC) to separate and purify the CE resulted in the discovery of one highly potent anti-X. citri subsp. citri subfraction, namely CE3, which could completely inhibit the growth of X. citri subsp. citri. Liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS) analysis revealed that CE3 mainly consisted of palmitic acid, surfactin C15, phytosphingosine and dihydrosphingosine. Taken together, the results contribute to the possible biocontrol mechanisms of B. amyloliquefaciens ZJLMBA1908, as well as providing a promising new candidate strain as a biological control agent for controlling citrus canker.

Characterization and genomic insight of surfactin-producing Bacillus velezensis and its biocontrol potential against pathogenic contamination in lettuce hydroponics

Due to food borne pathogen, maintaining the viability of fresh fruits and vegetable is a great concern. Several strategies including microbial and plant-based formulations to reduce their infection and maintain quality of the fresh food are in practice. Currently, Bacillus has gained significant traction as a biocontrol agent for regulating diseases affecting a variety of agricultural and horticultural crops. Food-grade citric acid and plant growth-promoting rhizobacteria (PGPR) were used as antimicrobial agent, MIC results showed that PGPR (14.87 mm) and CA (20.25 mm) exhibited notable antimicrobial activity against E. coli. Lettuce treated with PGPR showed reduction in E. coli contamination, E. coli was detected at 3.30, 3.68 in control, and 2.7 log CFU/g in random root injury lettuce inoculated with PGPR KACC 21110 respectively. Random root injury showed a trend toward increasing E. coli internalization. The strains exhibited resistance to multiple antibiotics, including Imipenem, tetracycline, ampicillin, cefotaxime, cefoxitin, and ceftriaxone. Comprehensive data analysis revealed the presence of ten putative bacteriocin or bacteriocin-like gene clusters. The structure of lipopeptide homologs was characterized by using QTOF-MS/MS. The mass ion peaks attributed to surfactin homologs, surfactin A ion at m/z 1008.66, surfactin B, C at m/z 1022.67 and 1036.69. In addition to surfactin, a polyketide oxydifficidin and lipopeptide NO were extracted and detected from the extract of B. velezensis. Both isolates are key biocontrol agents and have significant potential in combating foodborne pathogens and can be utilized to explore novel antibacterial products for preventing pathogens in fresh produce.

Combined application of microbial inoculant and kelp-soaking wastewater promotes wheat seedlings growth and improves structural diversity of rhizosphere microbial community

Industrial processing of kelp generates large amounts of kelp-soaking wastewater (KSW), which contains a large amount of nutrient-containing substances. The plant growth-promoting effect might be further improved by combined application of growth-promoting bacteria and the nutrient-containing KSW. Here, a greenhouse experiment was conducted to determine the effect of the mixture of KSW and Bacillus methylotrophicus M4-1 (MS) vs. KSW alone (SE) on wheat seedlings, soil properties and the microbial community structure in wheat rhizosphere soil. The available potassium, available nitrogen, organic matter content and urease activity of MS soil as well as the available potassium of the SE soil were significantly different (p < 0.05) from those of the CK with water only added, increased by 39.51%, 36.25%, 41.61%, 80.56% and 32.99%, respectively. The dry and fresh weight of wheat seedlings from MS plants increased by 166.17% and 50.62%, respectively, while plant height increased by 16.99%, compared with CK. Moreover, the abundance and diversity of fungi in the wheat rhizosphere soil were significantly increased (p < 0.05), the relative abundance of Ascomycetes and Fusarium spp. decreased, while the relative abundance of Bacillus and Mortierella increased. Collectively, the combination of KSW and the plant growth-promoting strain M4-1 can promote wheat seedlings growth and improve the microecology of rhizosphere microorganisms, thereby solving the problems of resource waste and environmental pollution, ultimately turning waste into economic gain.

Pectic homogalacturonan sensed by Bacillus acts as host associated cue to promote establishment and persistence in the rhizosphere

Bacillus velezensis isolates are among the most promising plant-associated beneficial bacteria used as biocontrol agents. However, various aspects of the chemical communication between the plant and these beneficials, determining root colonization ability, remain poorly described. Here we investigated the molecular basis of such interkingdom interaction occurring upon contact between Bacillus velezensis and its host via the sensing of pectin backbone homogalacturonan (HG). We showed that B. velezensis stimulates key developmental traits via a dynamic process involving two conserved pectinolytic enzymes. This response integrates transcriptional changes leading to the switch from planktonic to sessile cells, a strong increase in biofilm formation, and an accelerated sporulation dynamics while conserving the potential to efficiently produce specialized secondary metabolites. As a whole, we anticipate that this response of Bacillus to cell wall-derived host cues contributes to its establishment and persistence in the competitive rhizosphere niche and ipso facto to its activity as biocontrol agent.

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.

Analysis of changes in bacterial diversity in healthy and bacterial wilt mulberry samples using metagenomic sequencing and culture-dependent approaches

Introduction

Mulberry bacterial wilt is a serious destructive soil-borne disease caused by a complex and diverse group of pathogenic bacteria. Given that the bacterial wilt has been reported to cause a serious damage to the yield and quality of mulberry, therefore, elucidation of its main pathogenic groups is essential in improving our understanding of this disease and for the development of its potential control measures.

Methods

In this study, combined metagenomic sequencing and culture-dependent approaches were used to investigate the microbiome of healthy and bacterial wilt mulberry samples.

Results

The results showed that the healthy samples had higher bacterial diversity compared to the diseased samples. Meanwhile, the proportion of opportunistic pathogenic and drug-resistant bacterial flora represented by Acinetobacter in the diseased samples was increased, while the proportion of beneficial bacterial flora represented by Proteobacteria was decreased. Ralstonia solanacearum species complex (RSSC), Enterobacter cloacae complex (ECC), Klebsiella pneumoniae, K. quasipneumoniae, K. michiganensis, K. oxytoca, and P. ananatis emerged as the main pathogens of the mulberry bacterial wilt.

Discussion

In conclusion, this study provides a valuable reference for further focused research on the bacterial wilt of mulberry and other plants.

Transcription Factor Spo0A Regulates the Biosynthesis of Difficidin in Bacillus amyloliquefaciens

Bacillus amyloliquefaciens WH1 produces multiple antibiotics with antimicrobial activity and can control bacterial wilt disease caused by Ralstonia solanacearum. Antibacterial substances produced by WH1 and the regulation mechanism are unknown. In this study, it was found that difficidin, and to a minor extent bacillibactin, exhibited antibacterial activity against R. solanacearum. Lipopeptides, macrolactin, bacillaene, and bacilysin had no antibacterial activity. Ferric iron uptake transcriptional regulator Fur bound the promoter region of the dhb gene cluster of bacillibactin biosynthesis. Mutant Δfur showed a higher bacillibactin production and its antibacterial activity increased by 27% than wild-type WH1. Difficidin inhibited R. solanacearum growth and disrupted the integrity of the cells. Lack of transcription factor Spo0A abolished difficidin biosynthesis. Spo0A bound the promoter region of the dfn gene cluster of difficidin biosynthesis. Changing phosphorylation levels of Spo0A via deletion of phosphatase gene spo0E and histidine kinases genes kinA and kinD affected the biosynthesis of difficidin. Deletion of spo0E increased the phosphorylation level of Spo0A and consequently improved the difficidin production. The antibacterial activity of mutant Δspo0E and ΔkinA increased by 12% and 19%. The antibacterial activity of mutant ΔkinD decreased by 28%. Collectively, WH1 produced difficidin to disrupt the cell of R. solanacearum and secreted siderophore bacillibactin to compete for ferric iron. Spo0A regulated difficidin biosynthesis. Spo0A regulates quorum-sensing responses and controls the biosynthesis of secondary metabolites in B. amyloliquefaciens. This study has important findings in the regulation mechanism of antibiotic synthesis and helps to improve antibiotic yield in Bacillus. IMPORTANCE Pathogen R. solanacearum causes bacterial wilt disease in many crops. There is no chemical bactericide that can control bacterial wilt disease. It is vital to find antagonistic microorganisms and antibacterial substances that can efficiently control bacterial wilt disease. B. amyloliquefaciens WH1 could inhibit the growth of R. solanacearum. Via genetic mutation, it was found that difficidin and to a minor extent bacillibactin produced by WH1 acted efficiently against R. solanacearum. The transcription factor Spo0A regulated the synthesis of difficidin. Phosphorylation of Spo0A affected the production of difficidin. Increasing the phosphorylation level of Spo0A improved the difficidin production and antibacterial activity. In-depth analysis of the regulation mechanism of antibiotic difficidin is meaningful for enhancing the control efficiency of WH1. B. amyloliquefaciens WH1 and the antibacterial substances have vast application potential in controlling bacterial wilt disease.

In vitro and in vivo antimicrobial activity of the fungal metabolite toluquinol against phytopathogenic bacteria

Introduction

Bacterial plant diseases cause tremendous economic losses worldwide. However, a few effective and sustainable control methods are currently available. To discover novel and effective management approaches, we screened marine fungi for their antibacterial activity against phytopathogenic bacteria in vitro and in vivo.

Methods

We screened the culture broth of 55 fungal strains isolated from various marine sources (seawater, algae, and sediment) for their in vitro antibacterial activity using the broth microdilution method. Then, only the fungal strain (designated UL-Ce9) with higher antibacterial activity in vitro was tested in an in vivo experiment against tomato bacterial wilt. The active compounds of UL-Ce9 were extracted using ethyl acetate, purified by a series of chromatography, and the structure was elucidated by nuclear magnetic resonance spectroscopy. Pesticide formulations of toluquinol were prepared as soluble concentrates and wettable powder. The disease control efficacy of toluquinol formulations was evaluated against blight of rice and the bacterial wilt of tomato.

Results and discussion

The culture broth of UL-Ce9 showed high antibacterial activity against Agrobacterium tumefaciens, Ralstonia solanacearum, and Xanthomonas arboricola pv. pruni in vitro, and we selected UL-Ce9 for the in vivo test. The UL-Ce9 culture broth completely suppressed the bacterial wilt of tomato at a dilution of 1:5. The phylogenetic analysis identified UL-Ce9 as Penicillium griseofulvum, and the antibacterial metabolites were revealed as patulin, gentisyl alcohol, and toluquinol, all of which were associated with the biosynthetic pathway of the mycotoxin patulin. Patulin exhibited the highest antibacterial activity against 16 phytopathogenic bacteria in vitro, followed by toluquinol and gentisyl alcohol. As patulin is toxic, we selected toluquinol to investigate its potential use as a pesticide against bacterial plant diseases. Compared with the chemicals currently being applied in agriculture (streptomycin and oxytetracycline), toluquinol formulations exhibited similar and higher control efficacies against bacterial leaf blight of rice and bacterial wilt of tomato, respectively. To the best of our knowledge, this is the first report of the antibacterial activity of toluquinol against phytopathogenic bacteria. Our results suggest that toluquinol is a potential candidate for the development of novel and effective pesticides for the management of bacterial plant diseases.

Bacillus velezensis RC116 Inhibits the Pathogens of Bacterial Wilt and Fusarium Wilt in Tomato with Multiple Biocontrol Traits

Soil-borne plant diseases seriously threaten the tomato industry worldwide. Currently, eco-friendly biocontrol strategies have been increasingly considered as effective approaches to control the incidence of disease. In this study, we identified bacteria that could be used as biocontrol agents to mitigate the growth and spread of the pathogens causing economically significant diseases of tomato plants, such as tomato bacterial wilt and tomato Fusarium wilt. Specifically, we isolated a strain of Bacillus velezensis (RC116) from tomato rhizosphere soil in Guangdong Province, China, with high biocontrol potential and confirmed its identity using both morphological and molecular approaches. RC116 not only produced protease, amylase, lipase, and siderophores but also secreted indoleacetic acid, and dissolved organophosphorus in vivo. Moreover, 12 Bacillus biocontrol maker genes associated with antibiotics biosynthesis could be amplified in the RC116 genome. Extracellular secreted proteins of RC116 also exhibited strong lytic activity against Ralstonia solanacearum and Fusarium oxysporum f. sp. Lycopersici. Pot experiments showed that the biocontrol efficacy of RC116 against tomato bacteria wilt was 81%, and consequently, RC116 significantly promoted the growth of tomato plantlets. Based on these multiple biocontrol traits, RC116 is expected to be developed into a broad-spectrum biocontrol agent. Although several previous studies have examined the utility of B. velezensis for the control of fungal diseases, few studies to date have evaluated the utility of B. velezensis for the control of bacterial diseases. Our study fills this research gap. Collectively, our findings provide new insights that will aid the control of soil-borne diseases, as well as future studies of B. velezensis strains.

Whole-genome sequencing of a biocontrol Myxococcus xanthus R31 isolate and comparative genomic analysis

Tomato bacterial wilt (TBW) caused by Ralstonia solanacearum is one of the most destructive soil-borne diseases. Myxococcus xanthus R31, isolated from healthy tomato rhizosphere soil using the R. solanacearum baiting method, exhibiting good biocontrol efficacy against TBW. However, the genomic information and evolutionary features of R31 are largely unclear. Here, the high-quality genome assembly of R31 was presented. Using Nanopore sequencing technology, we assembled the 9.25 Mb complete genome of R31 and identified several extracellular enzyme proteins, including carbohydrate-active enzymes (CAZymes) and peptidases. We also performed a comparative genome analysis of R31 and 17 other strains of M. xanthus with genome sequences in the NCBI database to gain insights into myxobacteria predation and genome size expansion. Average nucleotide identity and digital DNA-DNA hybridization calculation and phylogenetic analysis indicated that R31 was closely related to the species M. xanthus. Further comparative genomics analysis suggested that, in addition to characteristics of predatory microorganisms, R31 contains many strain-specific genes, which may provide a genetic basis for its proficient predatory ability. This study provides new insights into R31 and other closely related species and facilitates studies using genetic approaches to further elucidate the predation mechanism of myxobacteria.

Composition and activity of antifungal lipopeptides produced by Bacillus spp. in daqu fermentation

Daqu is a solid-state fermentation and saccharification starter for the Chinese liquor baijou. During the daqu stage, amylolytic and proteolytic enzymes are produced by Bacillus and fungi. Bacillus spp. also produce lipopeptides with a broad spectrum of antimicrobial activities but direct evidence for their impact on community assembly in daqu is lacking. This study aimed to study the interaction between Bacillus spp. and fungi in daqu models. The antifungal activity of surfactin, fengycin, and iturin A was initially assessed in vitro. Iturin A displayed the strongest antifungal activity (MIC = 10-50 mg/L). In situ antifungal activity of B. amyloliquefaciens and B. velezensis against molds was observed in a simple daqu model inoculated with single strains of Bacillus species. Formation of lipopeptides in situ was supported by quantification of mRNA encoding for enzymes for surfactin, fengycin, and iturin A biosynthesis. In situ antifungal activity of Bacillus species was also observed in a complex daqu model that was inoculated with 8 bacterial or fungal strains plus one of the three strains of Bacillus. A relationship of lipopeptides to in situ antifungal activity was further supported by detection of the lipopeptides by liquid chromatography coupled to mass spectrometry. Both results indicated that B velezensis FUA2155 had higher antifungal activity in the daqu model, and was the only strain that produced multiple iturin A congeners in situ. Taken together, this study provides evidence that production of lipopeptides by Bacillus species in daqu may impact community assembly and hence product quality.

Ralstonia solanacearum - A soil borne hidden enemy of plants: Research development in management strategies, their action mechanism and challenges

Plant pathogens present in soil cause severe losses to plants every year. Among them, Ralstonia solanacearum, because of its destructive nature, is the world's second most damaging bacterial phytopathogen. Over 310 species of plants belonging to 42 plant families are infected by this deadly pathogen. Around the world, the bacterial wilt (BW) disease causes yield losses that range from 20 to 100%. Control measures for managing this pathogen comprises several diverse approaches. Regardless of whether several control methods are developed to manage the BW disease, efficient management strategies with eco-friendly effects and the desired level of effective control is still awaited and there is need to developed effective management methods to eliminate this fetal disease in several crops under field conditions. An analysis of development in the management strategies will provide an effective way to search and develop control methods with desirable level of effectiveness. In this review, we discussed and analyzed the information reported on the development of various management strategies for the management of R. solanacearum along with the comprehensive presentation on action mechanism of these management strategies. We have also made an effort to summarize the challenges that make hurdle in the effective management of this deadly pathogen. The analysis of the information in this review article will assist in future implications of management strategies and help in developing effective control measures with more efficacy.

Bacillus lipopeptide-mediated biocontrol of peanut stem rot caused by Athelia rolfsii

INTRODUCTION

Peanut (Arachis hypogaea L.) is a widespread oilseed crop of high agricultural importance in tropical and subtropical areas. It plays a major role in the food supply in the Democratic Republic of Congo (DRC). However, one major constraint in the production of this plant is the stem rot (white mold or southern blight) disease caused by Athelia rolfsii which is so far controlled mainly using chemicals. Considering the harmful effect of chemical pesticides, the implementation of eco-friendly alternatives such as biological control is required for disease management in a more sustainable agriculture in the DRC as in the other developing countries concerned. Bacillus velezensis is among the rhizobacteria best described for its plant protective effect notably due to the production of a wide range of bioactive secondary metabolites. In this work, we wanted to evaluate the potential of B. velezensis strain GA1 at reducing A. rolfsii infection and to unravel the molecular basis of the protective effect.

RESULTS AND DISCUSSION

Upon growth under the nutritional conditions dictated by peanut root exudation, the bacterium efficiently produces the three types of lipopeptides surfactin, iturin and fengycin known for their antagonistic activities against a wide range of fungal phytopathogens. By testing a range of GA1 mutants specifically repressed in the production of those metabolites, we point out an important role for iturin and another unidentified compound in the antagonistic activity against the pathogen. Biocontrol experiments performed in greenhouse further revealed the efficacy of B. velezensis to reduce peanut disease caused by A. rolfsii both via direct antagonism against the fungus and by stimulating systemic resistance in the host plant. As treatment with pure surfactin yielded a similar level of protection, we postulate that this lipopeptide acts as main elicitor of peanut resistance against A. rolfsii infection.

Management of take-all disease caused by Gaeumannomyces graminis var. tritici in wheat through Bacillus subtilis strains

Wheat (Triticum aestivum) is the second largest grain crop worldwide, and one of the three major grain crops produced in China. Take-all disease, caused by Gaeumannomyces graminis var. tritici (Ggt) infection, is a widespread and devastating soil-borne disease that harms wheat production. At present, the prevention and control of wheat take-all depend largely on the application of chemical pesticides. Chemical pesticides, however, not only lead to increased drug resistance of pathogens but also leave significant residues in the soil, causing serious environmental pollution. In this study, we investigated the application of Bacillus subtilis to achieve take-all disease control in wheat while reducing pesticide application. Antagonistic bacteria were screened by plate test, species identification of strains was performed by Gram staining and sequencing of 16s rDNA, secondary metabolite activity of strains was detected by clear circle method, strain compatibility and effect of compounding on Ggt were detected by plate, and the application prospects of specific strains were analyzed by greenhouse and field experiments. We found that five B. subtilis strains, JY122, JY214, ZY133, NW03, Z-14, had significant antagonistic effects against Ggt, and could secrete antimicrobial proteins including amylase, protease, and cellulase. Furthermore, Z-14 and JY214 cultures have also been shown to change the morphology of Ggt mycelium. These results also showed that Z-14, JY214, and their combination can control take-all disease in wheat at a reduced level of pesticide use. In summary, we screened two Bacillus spp. strains, Z-14 and JY214, that could act as antagonists that contribute to the biological control of wheat take-all disease. These findings provide resources and ideas for controlling crop diseases in an environmentally friendly manner.

Bacillales: From Taxonomy to Biotechnological and Industrial Perspectives

For a long time, the genus Bacillus has been known and considered among the most applicable genera in several fields. Recent taxonomical developments resulted in the identification of more species in Bacillus-related genera, particularly in the order Bacillales (earlier heterotypic synonym: Caryophanales), with potential application for biotechnological and industrial purposes such as biofuels, bioactive agents, biopolymers, and enzymes. Therefore, a thorough understanding of the taxonomy, growth requirements and physiology, genomics, and metabolic pathways in the highly diverse bacterial order, Bacillales, will facilitate a more robust designing and sustainable production of strain lines relevant to a circular economy. This paper is focused principally on less-known genera and their potential in the order Bacillales for promising applications in the industry and addresses the taxonomical complexities of this order. Moreover, it emphasizes the biotechnological usage of some engineered strains of the order Bacillales. The elucidation of novel taxa, their metabolic pathways, and growth conditions would make it possible to drive industrial processes toward an upgraded functionality based on the microbial nature.

Metabolites derived from fungi and bacteria suppress in vitro growth of Gnomoniopsis smithogilvyi, a major threat to the global chestnut industry

INTRODUCTION

Chestnut rot caused by the fungus Gnomoniopsis smithogilvyi is a disease present in the world's major chestnut growing regions. The disease is considered a significant threat to the global production of nuts from the sweet chestnut (Castanea sativa). Conventional fungicides provide some control, but little is known about the potential of biological control agents (BCAs) as alternatives to manage the disease.

OBJECTIVE

Evaluate whether formulated BCAs and their secreted metabolites inhibit the in vitro growth of G. smithogilvyi.

METHODS

The antifungal potential of BCAs was assessed against the pathogen through an inverted plate assay for volatile compounds (VOCs), a diffusion assay for non-volatile compounds (nVOCs) and in dual culture. Methanolic extracts of nVOCs from the solid medium were further evaluated for their effect on conidia germination and were screened through an LC-MS-based approach for antifungal metabolites.

RESULTS

Isolates of Trichoderma spp., derived from the BCAs, significantly suppressed the pathogen through the production of VOCs and nVOCs. The BCA from which Bacillus subtilis was isolated was more effective in growth inhibition through the production of nVOCs. The LC-MS based metabolomics on the nVOCs derived from the BCAs showed the presence of several antifungal compounds.

CONCLUSION

The results show that G. smithogilvyi can be effectively controlled by the BCAs tested and that their use may provide a more ecological alternative for managing chestnut rot. The in vitro analysis should now be expanded to the field to assess the effectiveness of these alternatives for chestnut rot management.

Comprehensive genome sequence analysis of the devastating tobacco bacterial phytopathogen Ralstonia solanacearum strain FJ1003

Due to its high genetic diversity and broad host range, Ralstonia solanacearum, the causative phytopathogen of the bacterial wilt (BW) disease, is considered a "species complex". The R. solanacearum strain FJ1003 belonged to phylotype I, and was isolated from the Fuzhou City in Fujian Province of China. The pathogen show host specificity and infects tobacco, especially in the tropical and subtropical regions. To elucidate the pathogenic mechanisms of FJ1003 infecting tobacco, a complete genome sequencing of FJ1003 using single-molecule real-time (SMRT) sequencing technology was performed. The full genome size of FJ1003 was 5.90 Mb (GC%, 67%), containing the chromosome (3.7 Mb), megaplasmid (2.0 Mb), and small plasmid (0.2 Mb). A total of 5133 coding genes (3446 and 1687 genes for chromosome and megaplasmid, respectively) were predicted. A comparative genomic analysis with other strains having the same and different hosts showed that the FJ1003 strain had 90 specific genes, possibly related to the host range of R. solanacearum. Horizontal gene transfer (HGT) was widespread in the genome. A type Ⅲ effector protein (Rs_T3E_Hyp14) was present on both the prophage and genetic island (GI), suggesting that this gene might have been acquired from other bacteria via HGT. The Rs_T3E_Hyp14 was proved to be a virulence factor in the pathogenic process of R. solanacearum through gene knockout strategy, which affects the pathogenicity and colonization ability of R. solanacearum in the host. Therefore, this study will improve our understanding of the virulence of R. solanacearum and provide a theoretical basis for tobacco disease resistance breeding.

Plant-Microbe Interaction: Mining the Impact of Native Bacillus amyloliquefaciens WS-10 on Tobacco Bacterial Wilt Disease and Rhizosphere Microbial Communities

Ralstonia solanacearum, the causative agent of bacterial wilt disease, has been a major threat to tobacco production globally. Several control methods have failed. Thus, it is imperative to find effective management for this disease. The biocontrol agent Bacillus amyloliquefaciens WS-10 displayed a significant control effect due to biofilm formation, and secretion of hydrolytic enzymes and exopolysaccharides. In addition, strain WS-10 can produce antimicrobial compounds, which was confirmed by the presence of genes encoding antimicrobial lipopeptides (fengycin, iturin, surfactin, and bacillomycinD) and polyketides (difficidin, bacilysin, bacillibactin, and bacillaene). Strain WS-10 successfully colonized tobacco plant roots and rhizosphere soil and suppressed the incidence of bacterial wilt disease up to 72.02% by reducing the R. solanacearum population dynamic in rhizosphere soil. Plant-microbe interaction was considered a key driver of disease outcome. To further explore the impact of strain WS-10 on rhizosphere microbial communities, V3-V4 and ITS1 variable regions of 16S and ITS rRNA were amplified, respectively. Results revealed that strain WS-10 influences the rhizosphere microbial communities and dramatically changed the diversity and composition of rhizosphere microbial communities. Interestingly, the relative abundance of genus Ralstonia significantly decreased when treated with strain WS-10. A complex microbial co-occurrence network was present in a diseased state, and the introduction of strain WS-10 significantly changed the structure of rhizosphere microbiota. This study suggests that strain WS-10 can be used as a novel biocontrol agent to attain sustainability in disease management due to its intense antibacterial activity, efficient colonization in the host plant, and ability to transform the microbial community structure toward a healthy state.

IMPORTANCE The plant rhizosphere acts as the first line of defense against the invasion of pathogens. The perturbation in the rhizosphere microbiome is directly related to plant health and disease development. The introduction of beneficial microorganisms in the soil shifted the rhizosphere microbiome, induced resistance in plants, and suppressed the incidence of soilborne disease. Bacillus sp. is widely used as a biocontrol agent against soilborne diseases due to its ability to produce broad-spectrum antimicrobial compounds and colonization with the host plant. In our study, we found that the application of native Bacillus amyloliquefaciens WS-10 significantly suppressed the incidence of tobacco bacterial wilt disease by shifting the rhizosphere microbiome and reducing the interaction between rhizosphere microorganisms and bacterial wilt pathogen.

RNA-Seq Provides Insights into the Mechanisms Underlying Ilyonectria robusta Responding to Secondary Metabolites of Bacillus methylotrophicus NJ13

(1) Background: Ilyonectria robusta can cause ginseng to suffer from rusty root rot. Secondary metabolites (SMs) produced by Bacillus methylotrophicus NJ13 can inhibit the mycelial growth of I. robusta. However, the molecular mechanism of the inhibition and response remains unclear. (2) Methods: Through an in vitro trial, the effect of B. methylotrophicus NJ13’s SMs on the hyphae and conidia of I. robusta was determined. The change in the physiological function of I. robusta was evaluated in response to NJ13’s SMs by measuring the electrical conductivity, malondialdehyde (MDA) content, and glucose content. The molecular interaction mechanism of I. robusta’s response to NJ13’s SMs was analyzed by using transcriptome sequencing. (3) Results: NJ13’s SMs exhibited antifungal activity against I. robusta: namely, the hyphae swelled and branched abnormally, and their inclusions leaked out due to changes in the cell membrane permeability and the peroxidation level; the EC50 value was 1.21% (v/v). In transcripts at 4 dpi and 7 dpi, the number of differentially expressed genes (DEGs) (|log2(fold change)| > 1, p adj ≤ 0.05) was 1960 and 354, respectively. NJ13’s SMs affected the glucose metabolism pathway, and the sugar-transporter-related genes were downregulated, which are utilized by I. robusta for energy production. The cell wall structure of I. robusta was disrupted, and chitin-synthase-related genes were downregulated. (4) Conclusions: A new dataset of functional responses of the ginseng pathogenic fungus I. robusta was obtained. The results will benefit the development of targeted biological fungicides for I. robusta and the study of the molecular mechanisms of interaction between biocontrol bacteria and phytopathogenic fungi.

Identification, Characterization, and Efficacy Evaluation of Bacillus velezensis for Shot-Hole Disease Biocontrol in Flowering Cherry

Though information exists regarding the pathogenesis of the shot-hole disease (SH) in flowering cherry (FC), there has been a lack of research focusing on SH management. Therefore, here, we investigated the inhibitory activities of antagonistic bacteria against SH pathogens both in vitro and in vivo as well as their biochemical characteristics and bioactive compounds. Two biosurfactant-producing bacterial antagonists, identified as Bacillus velezensis strains JCK-1618 and JCK-1696, exhibited the best effects against the growth of both bacterial and fungal SH pathogens in vitro through their cell-free culture filtrates (CFCFs). These two strains also strongly inhibited the growth of the pathogens via the action of their antimicrobial diffusible compounds and antimicrobial volatile organic compounds (VOCs). Crude enzymes, solvent extracts, and biosurfactants of the two strains exhibited antimicrobial activities. Liquid chromatography/electrospray ionization time-of-flight mass spectrometric analysis of the partially purified active fractions revealed that the two antagonists produced three cyclic lipopeptides, including iturin A, fengycin A, and surfactin, and a polyketide, oxydifficidin. In a detached leaf assay, pre-treatment and co-treatment of FC leaves with the CFCFs led to a large reduction in the severity of the leaf spots caused by Epicoccum tobaicum and Bukholderia contaminans, respectively. In addition, the two antagonists produced indole-3-acetic acid, siderophore, and a series of hydrolytic enzymes, along with the formation of a substantial biofilm. To our knowledge, this is the first report of the antimicrobial activities of the diffusible compounds and VOCs of B. velezensis against the SH pathogens and their efficiency in the biocontrol of SH.

Plant growth-promoting and antibacterial activities of cultivable bacteria alive in tobacco field against Ralstonia solanacearum

Bacterial wilt disease caused by Ralstonia solanacearum leads to decrease of crops yield. Investigation of cultivable bacteria diversity provides more microbial species for screening antagonistic bacteria. In the present study, a variety of cultivation methods were used to investigate the diversity of cultivable bacteria alive in tobacco field. A total of 441 bacterial strains were obtained that belonged to four phyla, 49 genera and 146 species. Actinobacteria and Proteobacteria were the dominant phyla. Agrobacterium, Arthrobacter, Bacillus, Klebsiella, Paenarthrobacter, Pseudomonas and Pseudarthrobacter were the dominant genera. Some rare genera were discovered including Bosea, Cedecea, Delftia and Dyella. Diversity, species and abundances of bacteria altered under different cultivation conditions. One hundred three bacterial strains showed plant growth-promoting attributes. Twenty Bacillus strains showed high antibacterial activity against R. solanacearum. In field experiments, individual strain and consortia of Bacillus subtilis, B. siamensis and B. vallismortis effectively inhibited bacterial wilt. The core genes that controlled synthesis of secondary metabolites were knocked out in B. vallismortis SSB-10. Difficidin, which was synthesized by dif operon and controlled by sfp gene, was the antibacterial substance produced by SSB-10. Difficidin destroyed cell wall and cell membrane of R. solanacearum and inhibited its motility, production of extracellular polysaccharides and cellulase activity.

Myxococcus xanthus R31 Suppresses Tomato Bacterial Wilt by Inhibiting the Pathogen Ralstonia solanacearum With Secreted Proteins

The pathogenic bacterium Ralstonia solanacearum caused tomato bacterial wilt (TBW), a destructive soil-borne disease worldwide. There is an urgent need to develop effective control methods. Myxobacteria are microbial predators and are widely distributed in the soil. Compared with other biocontrol bacteria that produce antibacterial substances, the myxobacteria have great potential for biocontrol. This study reports a strain of Myxococcus xanthus R31 that exhibits high antagonistic activity to R. solanacearum. Plate test indicated that the strain R31 efficiently predated R. solanacearum. Pot experiments showed that the biocontrol efficacy of strain R31 against TBW was 81.9%. Further study found that the secreted protein precipitated by ammonium sulfate had significant lytic activity against R. solanacearum cells, whereas the ethyl acetate extract of strain R31 had no inhibitory activity against R. solanacearum. Substrate spectroscopy assay and liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis of secreted proteins showed that some peptidases, lipases, and glycoside hydrolases might play important roles and could be potential biocontrol factors involved in predation. The present study reveals for the first time that the use of strain M. xanthus R31 as a potential biocontrol agent could efficiently control TBW by predation and secreting extracellular lyase proteins.

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.

Genomic Analysis and Secondary Metabolites Production of the Endophytic Bacillus velezensis Bvel1: A Biocontrol Agent against Botrytis cinerea Causing Bunch Rot in Post-Harvest Table Grapes

Botrytis bunch rot caused by Botrytis cinerea is one of the most economically significant post-harvest diseases of grapes. In the present study, we showed that the bacterial strain Bvel1 is phylogenetically affiliated to Bacillus velezensis species. The strain Bvel1 and its secreted metabolites exerted an antifungal activity, under in vitro conditions, against B. cinerea. UHPLC-HRMS chemical analysis revealed that iturin A2, surfactin-C13 and -C15, oxydifficidin, bacillibactin, L-dihydroanticapsin, and azelaic acid were among the metabolites secreted by Bvel1. Treatment of wounded grape berries with Bacillus sp. Bvel1 cell culture was effective for controlling grey mold ingress and expansion in vivo. The effectiveness of this biological control agent was a function of the cell culture concentration of the antagonist applied, while preventive treatment proved to be more effective compared to curative. The strain Bvel1 exhibited an adequate colonization efficiency in wounded grapes. The whole-genome phylogeny, combined with ANI and dDDH analyses, provided compelling evidence that the strain Bvel1 should be taxonomically classified as Bacillus velezensis. Genome mining approaches showed that the strain Bvel1 harbors 13 antimicrobial biosynthetic gene clusters, including iturin A, fengycin, surfactin, bacilysin, difficidin, bacillaene, and bacillibactin. The results provide new insights into the understanding of the endophytic Bacillus velezensis Bvel1 biocontrol mechanism against post-harvest fungal pathogens, including bunch rot disease in grape berries.

The mutated Bacillus amyloliquefaciens strain shows high resistance to Aeromonas hydrophila and Aeromonas veronii in grass carp

Bacillus amyloliquefaciens X030 (BaX030) has broad-spectrum antibacterial activity against the fish pathogens Aeromonas hydrophila and Aeromonas veronii. To improve its antibacterial effect, BaX030 was subjected to compound mutagenesis of atmospheric and room temperature plasma (ARTP) and nitrosoguanidine (NTG). The results showed that, compared with the original strain, the production of macrolactin A and oxydifficidin in mutated strain N-11 increased to 39 % and 268 %, respectively. The re-sequencing analysis suggested that there were SNPs and InDels in the gene clusters focused on the sucrose utilization pathway, glycolysis pathway and fatty acid synthesis pathway. Scanning electron microscopy revealed that strain N-11 became thin and long. The qRT-PCR results indicated that the expression of immune factors in the liver or kidney tissue of grass carp increased after feeding with N-11. H&E staining and protection experiments also showed that the mortality and surface symptoms of grass carp infected by the two pathogens were significantly reduced. The study identified a probiotic strain with potential application value in aquaculture production and provided a new strategy for the discovery of new strains with higher antibacterial biological activity.

Characterization of the biocontrol activity of three bacterial isolates against the phytopathogen Erwinia amylovora

Antibiotics are sprayed on apple and pear orchards to control, among other pathogens, the bacterium Erwinia amylovora, the causative agent of fire blight. As with many other pathogens, we observe the emergence of antibiotic-resistant strains of E. amylovora. Consequently, growers are looking for alternative solutions to combat fire blight. To find alternatives to antibiotics against this pathogen, we have previously isolated three bacterial strains with antagonistic and extracellular activity against E. amylovora, both in vitro and in planta, corresponding to three different bacterial genera: Here, we identified the inhibitory mode of action of each of the three isolates against E. amylovora. Isolate Bacillus amyloliquefaciens subsp. plantarum (now B. velezensis) FL50S produces several secondary metabolites including surfactins, iturins, and fengycins. Specifically, we identified oxydifficidin as the most active against E. amylovora S435. Pseudomonas poae FL10F produces an active extracellular compound against E. amylovora S435 that can be attributed to white-line-inducing principle (WLIP), a cyclic lipopeptide belonging to the viscosin subfamily (massetolide E, F, L, or viscosin). Pantoea agglomerans NY60 has a direct cell-to-cell antagonistic effect against E. amylovora S435. By screening mutants of this strain generated by random transposon insertion with decreased antagonist activity against strain S435, we identified several defective transposants. Of particular interest was a mutant in a gene coding for a Major Facilitator Superfamily (MFS) transporter corresponding to a transmembrane protein predicted to be involved in the extracytoplasmic localization of griseoluteic acid, an intermediate in the biosynthesis of the broad-spectrum phenazine antibiotic D-alanylgriseoluteic acid.

Antimicrobial Activities of Lipopeptides and Polyketides of Bacillus velezensis for Agricultural Applications

Since the discovery of penicillin, bacteria are known to be major sources of secondary metabolites that can function as drugs or pesticides. Scientists worldwide attempted to isolate novel compounds from microorganisms; however, only less than 1% of all existing microorganisms have been successfully identified or characterized till now. Despite the limitations and gaps in knowledge, in recent years, many Bacillus velezensis isolates were identified to harbor a large number of biosynthetic gene clusters encoding gene products for the production of secondary metabolites. These chemically diverse bioactive metabolites could serve as a repository for novel drug discovery. More specifically, current projects on whole-genome sequencing of B. velezensis identified a large number of biosynthetic gene clusters that encode enzymes for the synthesis of numerous antimicrobial compounds, including lipopeptides and polyketides; nevertheless, their biological applications are yet to be identified or established. In this review, we discuss the recent research on synthesis of bioactive compounds by B. velezensis and related Bacillus species, their chemical structures, bioactive gene clusters of interest, as well as their biological applications for effective plant disease management.