Screening of Bacillus velezensis E2 and the Inhibitory Effect of Its Antifungal Substances on Aspergillus flavus

Antifungal lipopeptides from the marine Bacillus amyloliquefaciens HY2-1: A potential biocontrol agent exhibiting in vitro and in vivo antagonistic activities against Penicillium digitatum

This study aimed to clarify the antifungal activity and action mechanism of the lipopeptides from a marine Bacillus amyloliquefaciens HY2-1 against Penicillium digitatum both in vitro and in vivo. Results showed that HY2-1 lipopeptides exerted obvious inhibitions on spore germination and mycelium growth of P. digitatum. Furthermore, HY2-1 lipopeptides caused the aggravated lipid peroxidation, the decreased ergosterol biosynthesis, and the increased nucleic acid and protein leakage in P. digitatum, suggesting the damaged membrane integrity and permeability of P. digitatum. In addition, HY2-1 lipopeptides induced ROS burst and reduced the activities of antioxidant superoxide dismutase and peroxidase in P. digitatum, indicating that the intracellular redox homeostasis of P. digitatum was disturbed. In vivo biocontrol experiments showed that HY2-1 lipopeptides could effectively reduce the occurrence of green mold disease in citrus fruits artificially inoculated with P. digitatum spores. Most significantly, 4.8 mg/mL of HY2-1 lipopeptides achieved 100 % of biocontrol efficacy after 30 days of storage and increased significantly contents of total phenols and flavonoids in citrus peels, demonstrating that HY2-1 lipopeptides inhibited green mold disease by exerting their antifungal activities and triggering fruit defense responses. This study deepens the understanding of marine Bacillus lipopeptides in the biological control of postharvest diseases.

Antifungal efficacy of Bacillus amyloliquefaciens ZK-9 against Fusarium graminearum and analysis of the potential mechanism of its lipopeptides

Fusarium graminearum is a destructive fungal pathogen that seriously threatens wheat production and quality. In the management of fungal infections, biological control is an environmentally friendly and sustainable approach. Here, the antagonistic strain ZK-9 with a broad antifungal activity was identified as Bacillus amyloliquefaciens. ZK-9 could produce extracellular enzymes such as pectinase, protease, cellulase, and amylase, as well as plant growth-promoting substances including IAA and siderophore. Lipopeptides extracted from strain ZK-9 had the high inhibitory effects on the mycelia of F. graminearum with the minimum inhibitory concentration (MIC) of 0.8 mg/mL. Investigation on the action mechanism of lipopeptides showed they could change the morphology of mycelia, damage the cell membrane, lower the content of ergosterol and increase the relative conductivity of membrane, cause nucleic acid and proteins leaking out from the cells, and disrupt the cell membrane permeability. Furthermore, metabolomic analysis of F. graminearum revealed the significant differences in the expression of 100 metabolites between the lipopeptides treatment group and the control group, which were associated with various metabolic pathways, mainly including amino acid biosynthesis, pentose, glucuronate and glycerophospholipid metabolism. In addition, strain ZK-9 inhibited Fusarium crown rot (FCR) with a biocontrol efficacy of 82.14 % and increased the plant height and root length by 24.23 % and 93.25 %, respectively. Moreover, the field control efficacy of strain ZK-9 on Fusarium head blight (FHB) was 71.76 %, and the DON content in wheat grains was significantly reduced by 69.9 %. This study puts valuable insights into the antifungal mechanism of lipopeptides against F. graminearum, and provides a promising biocontrol agent for controlling F. graminearum.

Bacillus siamensis 3BS12-4 Extracellular Compounds as a Potential Biological Control Agent against Aspergillus flavus

Aspergillus flavus, the primary mold that causes food spoilage, poses significant health and economic problems worldwide. Eliminating A. flavus growth is essential to ensure the safety of agricultural products, and extracellular compounds (ECCs) produced by Bacillus spp. have been demonstrated to inhibit the growth of this pathogen. In this study, we aimed to identify microorganisms efficient at inhibiting A. flavus growth and degrading aflatoxin B1. We isolated microorganisms from soil samples using a culture medium containing coumarin (CM medium) as the sole carbon source. Of the 498 isolates grown on CM medium, only 132 bacterial strains were capable of inhibiting A. flavus growth. Isolate 3BS12-4, identified as Bacillus siamensis, exhibited the highest antifungal activity with an inhibition ratio of 43.10%, and was therefore selected for further studies. The inhibition of A. flavus by isolate 3BS12-4 was predominantly attributed to ECCs, with a minimum inhibitory concentration and minimum fungicidal concentration of 0.512 g/ml. SEM analysis revealed that the ECCs disrupted the mycelium of A. flavus. The hydrolytic enzyme activity of the ECCs was assessed by protease, β-1,3-glucanase, and chitinase activity. Our results demonstrate a remarkable 96.11% aflatoxin B1 degradation mediated by ECCs produced by isolate 3BS12-4. Furthermore, treatment with these compounds resulted in a significant 97.93% inhibition of A. flavus growth on peanut seeds. These findings collectively present B. siamensis 3BS12-4 as a promising tool for developing environmentally friendly products to manage aflatoxin-producing fungi and contribute to the enhancement of agricultural product safety and food security.

Mechanism of oxalate decarboxylase Oxd_S12 from Bacillus velezensis BvZ45-1 in defence against cotton verticillium wilt

Verticillium wilt, a soilborne vascular disease caused by Verticillium dahliae, strongly affects cotton yield and quality. In this study, an isolated rhizosphere bacterium, designated Bacillus velezensis BvZ45-1, exhibited >46% biocontrol efficacy against cotton verticillium wilt under greenhouse and field conditions. Moreover, through crude protein extraction and mass spectrometry analyses, we found many antifungal compounds present in the crude protein extract of BvZ45-1. The purified oxalate decarboxylase Odx_S12 from BvZ45-1 inhibited the growth of V. dahliae Vd080 by reducing the spore yield, causing mycelia to rupture, spore morphology changes, cell membrane rupture, and cell death. Subsequently, overexpression of Odx_S12 in Arabidopsis significantly improved plant resistance to V. dahliae. Through studies of the resistance mechanism of Odx_S12, V. dahliae was shown to produce oxalic acid (OA), which has a toxic effect on Arabidopsis leaves. Odx_S12 overexpression reduced Arabidopsis OA content, enhanced tolerance to OA, and improved resistance to verticillium wilt. Transcriptomics and quantitative real-time PCR analysis revealed that Odx_S12 promoted a reactive oxygen species burst and a salicylic acid- and abscisic acid-mediated defence response in Arabidopsis. In summary, this study not only identified B. velezensis BvZ45-1 as an efficient biological control agent, but also identified the resistance gene Odx_S12 as a candidate for cotton breeding against verticillium wilt.

Microbial Biofungicides as a Substitute for Chemical Fungicides in the Control of Phytopathogens: Current Perspectives and Research Directions

These days, two important issues are causing concern in the global community: the alarmingly growing trend of the human population and the issue of food security. To this end, people around the world have been searching for solutions that could feed the needy in a sustainable way. In response to this urgent call, scientists from around the world started working on increasing crop production and productivity by controlling crop pathogens that could harm the productivity of crops. Synthetic fungicides have been in use for controlling crop diseases for several decades, but later, due to the evidenced side effects of the fungicides, there have been attempts to shift towards a less cost-effective and eco-friendly method of controlling crop diseases, and so far, many remarkable results have been achieved. However, due to the less effective and shorter shelf life of microbial biofungicides, as well as the less accessibility of these microbial biofungicides to growers around the world, it became difficult to remove the fungicides totally from the market. To minimize this problem, researchers suggested an integrated approach: the combination of microbial biofungicides with a reduced dose of synthetic fungicides. Hence, this review explored the status as well as the merits and demerits of microbial biofungicides as compared to synthetic fungicides.

Isolation and Characterization of Paenibacillus polymyxa B7 and Inhibition of Aspergillus tubingensis A1 by Its Antifungal Substances

Screening of Bacillus with antagonistic effects on paddy mold pathogens to provide strain resources for biological control of mold in Oryza sativa L. screening of Bacillus isolates antagonistic towards Aspergillus tubingensis from rhizosphere soil of healthy paddy; classification and identification of antagonistic strains by biological characteristics and 16S rDNA sequence analysis; transcriptome sequencing after RNA extraction from Bacillus-treated Aspergillus tubingensis; and extraction of inhibitory crude proteins of Bacillus by ammonium sulfate precipitation; inhibitory crude protein and Bacillus spp. were treated separately for A. tubingensis and observed by scanning electron microscopy (SEM). An antagonistic strain of Bacillus, named B7, was identified as Paenibacillus polymyxa by 16S rDNA identification and phylogenetic evolutionary tree comparison analysis. Analysis of the transcriptome results showed that genes related to secondary metabolite biosynthesis such as antifungal protein were significantly downregulated. SEM results showed that the mycelium of A. tubingensis underwent severe rupture after treatment with P. polymyxa and antifungal proteins, respectively. In addition, the sporocarp changed less after treatment with P. polymyxa, and the sporangium stalks had obvious folds. P. polymyxa B7 has a good antagonistic effect against A. tubingensis and has potential for biocontrol applications of paddy mold pathogens.

Heat stability of Trichoderma asperelloides SKRU-01 culture filtrates: Potential applications for controlling fungal spoilage and AFB1 production in peanuts

This study aimed to examine the heat stability of culture filtrates of Trichoderma asperelloides SKRU-01 (culture filtrates SKRU-01) over a temperatures range (40-121 °C) and the effects on the antifungal activity against two aflatoxin-producing strains (Aspergillus parasiticus TISTR 3276 and A. flavus PSRDC-4), aflatoxin B1 (AFB1) degradation, and the role in mycotoxin control in peanuts. The impact of SKRU-01 culture age (2-12 day-old) on both pathogenic strains revealed that the culture age of 6-12 day-old cultures exhibited no significant difference (p > 0.05) of growth inhibition for strain TISTR 3276 (81.89-82.28 %) and 4-12 day-old cultures for strain PSRDC-4 (74.87-79.06 %). The heat-treated temperatures from 40 °C to 121 °C caused no significant (p > 0.05) reduction of mycelial growth for strain TISTR 3276 (82.61 % to 79.13 %) but significant (p < 0.05) deduction for strain PSRDC-4 (75.15 % to 59.17 %). Heat treatment of the culture filtrates SKRU-01 at 60-121 °C caused the reduction on spore germination inhibition (from about 68 % to 58.16 % for strain TISTR 3276 and 51.11 % for strain PSRDC-4). These results indicate that strain TISTR 3276 exhibited greater susceptibility to culture filtrates SKRU-01 compared to strain PSRDC-4. Furthermore, the culture filtrates SKRU-01 exhibited remarkable thermal stability at 121 °C, degrading AFB1 to 63.91 %. Application of heat-stable culture filtrates SKRU-01 in peanuts demonstrated that the reduction in fungal population and AFB1 production of both pathogenic strains depended significantly (p < 0.05) on the level of heat treatment. The non-treated and 40 °C treated culture filtrates SKRU-01 could reduce AFB1 production to lower than the Standard Aflatoxin Limitation (<20 μg/kg), ensuring food safety and mitigating the health risks associated with aflatoxin exposure.

Antifungal Peptide P852 Effectively Controls Fusarium oxysporum, a Wilt-Causing Fungus, by Affecting the Glucose Metabolism and Amino Acid Metabolism as well as Damaging Mitochondrial Function

Fusarium oxysporum causes wilt disease, which causes huge economic losses to a wide range of agricultural cash crops. Antifungal peptide P852 is an effective biocide. However, the mechanism of direct inhibition of pathogenic fungus needs to be explored. The proteomics and transcriptomics results showed that P852 mainly affected intracellular pathways such as glucose metabolism, amino acid metabolism, and oxidoreductase activity in F. oxysporum. P852 disrupts the intracellular oxidative equilibrium in F. oxysporum, and transmission electron microscopy observed mitochondrial swelling, disruption of membrane structure, and leakage of contents. Decreased mitochondrial membrane potential, mitochondrial cytochrome c leakage, and reduced ATP production were also detected. These results suggest that P852 is able to simultaneously inhibit intracellular metabolism and disrupt the mitochondrial function of F. oxysporum, exerting its inhibitory effects in multiple pathways together. The present study provides some insights into the multitargeted mechanism of fungus inhibition of antifungal lipopeptide substances produced by Bacillus spp.

Effect of Streptomyces roseolus Cell-Free Supernatants on the Fungal Development, Transcriptome, and Aflatoxin B1 Production of Aspergillus flavus

Crop contamination by aflatoxin B1 (AFB1), an Aspergillus-flavus-produced toxin, is frequently observed in tropical and subtropical regions. This phenomenon is emerging in Europe, most likely as a result of climate change. Alternative methods, such as biocontrol agents (BCAs), are currently being developed to reduce the use of chemicals in the prevention of mycotoxin contamination. Actinobacteria are known to produce many bioactive compounds, and some of them can reduce in vitro AFB1 concentration. In this context, the present study aims to analyze the effect of a cell-free supernatant (CFS) from Streptomyces roseolus culture on the development of A. flavus, as well as on its transcriptome profile using microarray assay and its impact on AFB1 concentration. Results demonstrated that in vitro, the S. roseolus CFS reduced the dry weight and conidiation of A. flavus from 77% and 43%, respectively, and was therefore associated with a reduction in AFB1 concentration reduction to levels under the limit of quantification. The transcriptomic data analysis revealed that 5198 genes were differentially expressed in response to the CFS exposure and among them 5169 were downregulated including most of the genes involved in biosynthetic gene clusters. The aflatoxins' gene cluster was the most downregulated. Other gene clusters, such as the aspergillic acid, aspirochlorine, and ustiloxin B gene clusters, were also downregulated and associated with a variation in their concentration, confirmed by LC-HRMS.

Isolation of a potential probiotic strain Bacillus amyloliquefaciensLPB-18 and identification of antimicrobial compounds responsible for inhibition of food-borne pathogens

This study was carried out to screen a potential probiotic microbe with broad-spectrum antagonistic activity against food-borne pathogens and identify the antimicrobial compounds. Based on morphological and molecular analysis, a new Bacillus strain with the ability to produce effective antimicrobial agents was isolated from the breeding soil of earthworms and identified as having a close evolutionary footprint to Bacillus amyloliquefaciens. The antimicrobial substances produced by B. amyloliquefaciens show effective inhibition of Aspergillus flavus and Fusarium oxysporum in an agar diffusion assay. Antimicrobial agents were identified as a series of fengycin and its isoforms (fengycin A and fengycin B) after being submitted to RT-HPLC and MALDI-TOF MS analyses. To evaluate the probiotic activity of the B. amyloliquefaciens, antibiotic safety and viability of the isolated strain in a simulated gastrointestinal environment were carried out. The safety test result revealed that strain LPB-18 is susceptible to multiple common antibiotics. Moreover, acidic condition and bile salts assay were carried out, and the results revealed that it couble be a potential probiotic microbe B. amyloliquefaciens LPB-18 is good choice for biological strains in agricultural commodities and animal feedstuffs.

Biocontrol Capabilities of Bacillus subtilis E11 against Aspergillus flavus In Vitro and for Dried Red Chili (Capsicum annuum L.)

As a condiment with extensive nutritional value, chili is easy to be contaminated by Aspergillus flavus (A. flavus) during field, transportation, and storage. This study aimed to solve the contamination of dried red chili caused by A. flavus by inhibiting the growth of A. flavus and detoxifying aflatoxin B₁ (AFB₁). In this study, Bacillus subtilis E11 (B. subtilis) screened from 63 candidate antagonistic bacteria exhibited the strongest antifungal ability, which could not only inhibit 64.27% of A. flavus but could also remove 81.34% of AFB₁ at 24 h. Notably, scanning electron microscopy (SEM) showed that B. subtilis E11 cells could resist a higher concentration of AFB₁, and the fermentation supernatant of B. subtilis E11 could deform the mycelia of A. flavus. After 10 days of coculture with B. subtilis E11 on dried red chili inoculated with A. flavus, the mycelia of A. flavus were almost completely inhibited, and the yield of AFB₁ was significantly reduced. Our study first concentrated on the use of B. subtilis as a biocontrol agent for dried red chili, which could not only enrich the resources of microbial strains for controlling A. flavus but also could provide theoretical guidance to prolong the shelf life of dried red chili.

The Exploitation of Microbial Antagonists against Postharvest Plant Pathogens

Postharvest disease management is vital to increase the quality and productivity of crops. As part of crop disease protection, people used different agrochemicals and agricultural practices to manage postharvest diseases. However, the widespread use of agrochemicals in pest and disease control has detrimental effects on consumer health, the environment, and fruit quality. To date, different approaches are being used to manage postharvest diseases. The use of microorganisms to control postharvest disease is becoming an eco-friendly and environmentally sounds approach. There are many known and reported biocontrol agents, including bacteria, fungi, and actinomycetes. Nevertheless, despite the abundance of publications on biocontrol agents, the use of biocontrol in sustainable agriculture requires substantial research, effective adoption, and comprehension of the interactions between plants, pathogens, and the environment. To accomplish this, this review made an effort to locate and summarize earlier publications on the function of microbial biocontrol agents against postharvest crop diseases. Additionally, this review aims to investigate biocontrol mechanisms, their modes of operation, potential future applications for bioagents, as well as difficulties encountered during the commercialization process.

Insights into Antifungal Mechanisms of Bacillus velezensis S141 against Cercospora Leaf Spot in Mungbean (V. radiata)

Cercospora leaf spot (CLS) is caused by Cercospora canescens and is one of the most important diseases of mungbean (Vigna radiata). Cercospora leaf spot may result in economic loss in production areas. The present study investigated the potential of Bacillus velezensis S141 as a biocontrol agent for C. canescens PAK1 growth on culture plates. Cell-free secretions from a dual culture of S141+PAK1 inhibited fungal growth more than those from a single culture of S141. The biocontrol efficiency of S141 against Cercospora leaf spot on mungbean was then evaluated by spraying. The disease severity of Cercospora leaf spot was significantly reduced in plants treated with S141, with a control efficiency of 83% after 2 days of infection. Comparative transcriptomics and qRT-PCR ana-lyses of S141 during C. canescens inhibition were performed to elucidate the antifungal mechanisms underlying its antifungal activity against Cercospora leaf spot. According to the differentially expressed genes, most up-regulated genes involved in the biosynthetic genes encoding enzymatic hydrolases, including protease, β-glucanase, and N-acyl glucosaminase, were detected in strain S141 following its interaction. Moreover, genes related to secondary metabolites (surfactin, bacilysin, and bacillomycin D) were up-regulated. Collectively, these results suggest that S141 exhibited strong antifungal activity against C. canescens due to multiple enzymatic hydrolases and secondary metabolites. Therefore, the present study provides insights into the biological network responsible for the antifungal activity of B. velezensis S141 against C. canescens.

Differential Expression of Genes Related to Growth and Aflatoxin Synthesis in Aspergillus flavus When Inhibited by Bacillus velezensis Strain B2

Aspergillus flavus is a saprophytic soil fungus that infects and contaminates seed crops with the highly carcinogenic aflatoxin, which brings health hazards to animals and humans. In this study, bacterial strains B1 and B2 isolated from the rhizosphere soil of camellia sinensis had significant antagonistic activities against A. flavus. Based on the phylogenetic analysis of 16SrDNA gene sequence, bacterial strains B1 and B2 were identified as Bacillus tequilensis and Bacillus velezensis, respectively. In addition, the transcriptome analysis showed that some genes related to A. flavus growth and aflatoxin synthesis were differential expressed and 16 genes in the aflatoxin synthesis gene cluster showed down-regulation trends when inhibited by Bacillus velezensis strain B2. We guessed that the Bacillus velezensis strain B2 may secrete some secondary metabolites, which regulate the related gene transcription of A. flavus to inhibit growth and aflatoxin production. In summary, this work provided the foundation for the more effective biocontrol of A. flavus infection and aflatoxin contamination by the determination of differential expression of genes related to growth and aflatoxin synthesis in A. flavus when inhibited by B. velezensis strain B2.

Characterization of Bacillus velezensis E2 with abilities to degrade ochratoxin A and biocontrol against Aspergillus westerdijkiae fc-1

Ochratoxin A (OTA), primarily produced by the fungi belonging to the species of Aspergillus and Penicillium, is one of the most common mycotoxins found in cereals and fruits. In addition to resulting in huge economic losses, OTA contamination also poses considerable threat to human and livestock health. Microbial degradation of mycotoxins has been considered with great potential in mycotoxins decontamination. In a previous study, Bacillus velezensis E2 was isolated by our laboratory and showed appreciable inhibitory effect on Aspergillus flavus growth and aflatoxin production in rice grains. In this study, B. velezensis E2 was investigated for its ability to remove OTA and biocontrol against the ochratoxigenic Aspergillus westerdijkiae fc-1. The results revealed that B. velezensis E2 has considerable inhibitory effect on A. westerdijkiae fc-1 both on PDA medium and pear fruits, with inhibitory rate of 51.7% and 73.9%, respectively. In addition, its ability to remove OTA was evaluated in liquid medium and the results showed that more than 96.1% of OTA with an initial concentration of 2.5 μg/mL could be removed by B. velezensis E2 in 48 h. Further experiments revealed that enzymatic transformation and alkaline hydrolysis might be the main mechanisms related to OTA degradation by B. velezensis E2, with ring open ochratoxin α (OP-OTα) as a possible degradation product. Our study indicated that the B. velezensis E2 strain could be a potential bacterial candidate in biodegradation of OTA and biocontrol against A. westerdijkiae fc-1.