Termination of the structural confusion between plipastatin A1 and fengycin IX

Genomic and biological control of Sclerotinia sclerotiorum using an extracellular extract from Bacillus velezensis 20507

Introduction

Sclerotinia sclerotiorum is a known pathogen that harms crops and vegetables. Unfortunately, there is a lack of effective biological control measures for this pathogen. Bacillus velezensis 20507 has a strong antagonistic effect on S. Sclerotiorum; however, the biological basis of its antifungal effect is not fully understood.

Methods

In this study, the broad-spectrum antagonistic microorganisms of B. velezensis 20507 were investigated, and the active antifungal ingredients in this strain were isolated, purified, identified and thermal stability experiments were carried out to explore its antifungal mechanism.

Results

The B. velezensis 20507 genome comprised one circular chromosome with a length of 4,043,341 bp, including 3,879 genes, 185 tandem repeats, 87 tRNAs, and 27 rRNAs. Comparative genomic analysis revealed that our sequenced strain had the closest genetic relationship with Bacillus velezensis (GenBank ID: NC 009725.2); however, there were significant differences in the positions of genes within the two genomes. It is predicted that B. velezensis 20507 encode 12 secondary metabolites, including difficidin, macrolactin H, fengycin, surfactin, bacillibactin, bacillothiazole A-N, butirosin a/b, and bacillaene. Results showed that B. velezensis 20507 produced various antagonistic effects on six plant pathogen strains: Exserohilum turcicum, Pyricularia oryzae, Fusarium graminearum, Sclerotinia sclerotiorum, Fusarium oxysporum, and Fusarium verticillioides. Acid precipitation followed by 80% methanol leaching is an effective method for isolating the antifungal component ME80 in B. velezensis 20507, which can damage the membranes of S. sclerotiorum hyphae and has good heat resistance. Using high-performance liquid chromatography, and Mass Spectrometry analysis, it is believed that fengycin C72H110N12O20 is the main active antifungal substance.

Discussion

This study provides new resources for the biological control of S. Sclerotiorum in soybeans and a theoretical basis for further clarification of the mechanism of action of B. velezensis 20507.

Discovery of Cyclic Lipopeptides Ralstopeptins A and B from Ralstonia solanacearum Species Complex and Analysis of Biosynthetic Gene Evolution

Ralstonia solanacearum species complex (RSSC) strains are plant pathogens that produce lipopeptides (ralstonins and ralstoamides) by the polyketide synthase-nonribosomal peptide synthetase (PKS-NRPS) enzyme hybrid. Recently, ralstonins were found to be key molecules in the parasitism of RSSC to other hosts, Aspergillus and Fusarium fungi. The PKS-NRPS genes of RSSC strains in the GenBank database suggest the production of additional lipopeptides, although it has not been confirmed to date. Here, we report the genome-driven and mass-spectrometry-guided discovery, isolation, and structural elucidation of ralstopeptins A and B from strain MAFF 211519. Ralstopeptins were found to be cyclic lipopeptides with two amino acid residues less than ralstonins. The partial deletion of the gene encoding PKS-NRPS obliterated the production of ralstopeptins in MAFF 211519. Bioinformatic analyses suggested possible evolutionary events of the biosynthetic genes of RSSC lipopeptides, where intragenomic recombination may have occurred within the PKS-NRPS genes, reducing the gene size. The chlamydospore-inducing activities of ralstopeptins A and B, ralstonins A and B, and ralstoamide A in the fungus Fusarium oxysporum indicated a structural preference for ralstonins. Altogether, we propose a model for the evolutionary processes that contribute to the chemical diversity of RSSC lipopeptides and its relation to the endoparasitism of RSSC in fungi.

Biosurfactant from Bacillus sp. A5F Reduces Disease Incidence of Sclerotinia sclerotiorum in Soybean Crop

The present study was conducted to assess the biocontrol activity of biosurfactants obtained from Bacillus species A5F. The variables significantly influencing the production of biosurfactants under in vitro conditions were further optimized using response surface methodology. Optimal values of selected culture variables, i.e., glucose, soybean oil, and incubation time were 3.5 g l-1, 3.5 ml l-1, and 78 h, respectively, resulting in 2.14-fold enhancement in biosurfactant levels in 5 l fermentor. Identified biosurfactants had a significant effect on chlorophyll content, shoot biomass, number of pods, and seed weight. Biosurfactants also reduced the disease incidence in S. sclerotiorum infected soybean plants and showed antagonistic action against major phytopathogens by disrupting the hyphal cell wall. 16% reduction in ITS gene copy number was observed as compared to control with less non-target effect upon biosurfactant spray on foliar parts of soybean. Thus, the study confirms that biosurfactants from strain A5F can be used as a potent biocontrol agent to control sclerotium wilt on soybean plants.

Fengycin A Analogues with Enhanced Chemical Stability and Antifungal Properties

Fengycins are cyclic lipo-depsipeptides produced by Bacillus spp. that display potent antifungal properties but are chemically unstable. This instability has meant that no total synthesis of any fengycin has been published. Here we report the synthesis of fengycin A analogues that display enhanced antifungal properties and chemical stability under both basic and acidic conditions. The analogues prepared also demonstrate that the fengycin core structure can be modified and simplified without the loss of antifungal activity.

Mining New Plipastatins and Increasing the Total Yield Using CRISPR/Cas9 in Genome-Modified Bacillus subtilis 1A751

CRISPR/Cas9 is one of the robust and effective gene manipulation tools which has been widely applied in various organisms. In this study, the plipastatin gene cluster was successfully expressed in genome-modified Bacillus subtilis 1A751 by disrupting the surfactin operon (srf) through CRISPR/Cas9 technology. The presumed plipastatin biosynthetic pathway was proposed based on the analysis of its biosynthetic gene cluster. Two new plipastatins were identified by a combination of ultra-high performance liquid chromatography-coupled electron spray ionization-tandem mass spectrometry and gas chromatography-mass spectrometry analyses, together with nine known plipastatins or their derivatives. The yield of plipastatin was as high as 1600 mg/L which is the highest reported to date. Antimicrobial experiments revealed that its methanolic extracts exhibited powerful inhibitory effects on the growth of the tested pathogens and fungi. The results from this investigation highlight the remarkable utility of CRISPR/Cas9 in mining new plipastatins and increasing the total plipastatin yield, providing a new pipeline for the industrial application of plipastatin.

Total Solid-Phase Synthesis of Dehydroxy Fengycin Derivatives

A rapid and efficient solid-phase strategy for the synthesis of dehydroxy fengycins derivatives is described. This synthetic approach involved the linkage of a Tyr to a Wang resin via a Mitsunobu reaction and the elongation of the peptide sequence followed by subsequent acylation of the N-terminus of the resulting linear peptidyl resin, esterification of the phenol group of a Tyr with an Ile, and final macrolactamization. The amino acid composition as well as the presence of the N-terminal acyl group significantly influenced the stability of the macrolactone. Cyclic lipodepsipeptides with a l-Tyr³/d-Tyr⁹ configuration were more stable than those containing the Tyr residues with an opposite configuration. This work constitutes the first approach on the total solid-phase synthesis of dehydroxy fengycin derivatives.

Secondary metabolite production and the safety of industrially important members of the Bacillus subtilis group

Members of the 'Bacillus subtilis group' include some of the most commercially important bacteria, used for the production of a wide range of industrial enzymes and fine biochemicals. Increasingly, group members have been developed for use as animal feed enhancers and antifungal biocontrol agents. The group has long been recognised to produce a range of secondary metabolites and, despite their long history of safe usage, this has resulted in an increased focus on their safety. Traditional methods used to detect the production of secondary metabolites and other potentially harmful compounds have relied on phenotypic tests. Such approaches are time consuming and, in some cases, lack specificity. Nowadays, accessibility to genome data and associated bioinformatical tools provides a powerful means for identifying gene clusters associated with the synthesis of secondary metabolites. This review focuses primarily on well-characterised strains of B. subtilis and B. licheniformis and their synthesis of non-ribosomally synthesised peptides and polyketides. Where known, the activities and toxicities of their secondary metabolites are discussed, together with the limitations of assays currently used to assess their toxicity. Finally, the regulatory framework under which such strains are authorised for use in the production of food and feed enzymes is also reviewed.

Draft Genome Sequence of Bacillus velezensis B6, a Rhizobacterium That Can Control Plant Diseases

The draft genome of Bacillus velezensis strain B6, a rhizobacterium with good biocontrol performance isolated from soil in China, was sequenced. The assembly comprises 32 scaffolds with a total size of 3.88 Mb. Gene clusters coding either ribosomally encoded bacteriocins or nonribosomally encoded antimicrobial polyketides and lipopeptides in the genome may contribute to plant disease control.

Plipastatin A1 produced by a marine sediment-derived Bacillus amyloliquefaciens SH-B74 contributes to the control of gray mold disease in tomato

Certain Bacillus species have the capacity to produce cyclic lipopeptides and these lipopeptides are promising determinants contributing to the biocontrol of plant diseases. In the current study, a cyclic lipopeptide plipastatin A1 was isolated from the fermentation broth of a marine sediment-derived Bacillus amyloliquefaciens SH-B74 by the combination of solid-phase extraction and reversed-phase high-performance liquid chromatography, and its structure was identified by tandem mass spectrometry, high-resolution electro-spray ionization mass spectrometry, and gas chromatography-mass spectrometry together with nuclear magnetic resonance analysis. Moreover, data from activity evaluation revealed that plipastatin A1 has excellent in vitro activity on the suppression of the conidia germination of B. cinerea, the causal agent of gray mold disease in tomato. Furthermore, plipastatin A1 can successfully decrease the incidence of gray mold disease on tomato leaves at 50 µM concentration. This study indicates that B. amyloliquefaciens SH-B74 appears to be a potentially sustainable pesticide to control gray mold disease in tomato plants, and its cyclic lipopeptide plipastatin A1 plays an important role in the in vitro and in planta biocontrol of B. cinerea.

Fengycin produced by Bacillus subtilis 9407 plays a major role in the biocontrol of apple ring rot disease

Apple ring rot, caused by Botryosphaeria dothidea, is a serious apple disease in China. Bacillus subtilis 9407 was isolated from healthy apples and showed strong antifungal activity against B. dothidea. To identify the primary antifungal compound of B. subtilis 9407 and determine its role in controlling apple ring rot, a transposon mutant library was constructed using TnYLB-1, and a mutant completely defective in antifungal activity was obtained. The gene inactivated in the antifungal activity mutant had 98.5% similarity to ppsB in B. subtilis subsp. subtilis str. 168, which encodes one of the five synthetases responsible for synthesizing fengycin. A markerless ppsB deletion mutant was constructed. Compared with the wild-type strain, lipopeptide crude extracts from ΔppsB showed almost no inhibition of B. dothidea mycelial growth. Furthermore, fengycin-like lipopeptides (retention factor 0.1-0.2) that exhibited antifungal activity against B. dothidea were observed in the wild-type strain by thin-layer chromatography (TLC)-bioautography analysis, but not in ΔppsB. Semipreparative reverse-phase high performance liquid chromatography (RP-HPLC) detection revealed that ΔppsB lost the ability to synthesize fengycin. These results suggest that ppsB is responsible for synthesizing fengycin and that fengycin is the major antifungal compound produced by B. subtilis 9407 against B. dothidea. Moreover, a biocontrol assay showed that the control efficacy of ΔppsB was reduced by half compared with the wild-type strain, indicating that fengycin plays a major role in controlling apple ring rot disease. This is the first report on the use of a B. subtilis strain as a potential biological control agent to control apple ring rot disease by the production of fengycin.

Octaminomycins A and B, Cyclic Octadepsipeptides Active against Plasmodium falciparum

Two new cyclic octadepsipeptides, octaminomycins A (1) and B (2), were isolated from a microbial metabolite fraction library of Streptomyces sp. RK85-270 based on Natural Products Plot screening. Their structures were elucidated on the basis of HRESIMS, 1D and 2D NMR spectroscopic data, and MS/MS experiments for sequence analysis. The absolute configurations of the constituent amino acid residues were determined by a combination of single-crystal X-ray diffraction and Marfey's methodology. Notably, octaminomycins A (1) and B (2) showed good in vitro antiplasmodial activity against chloroquine-sensitive as well as chloroquine-resistant strains with no cytotoxicity up to 30 μM.

Biocombinatorial Synthesis of Novel Lipopeptides by COM Domain-Mediated Reprogramming of the Plipastatin NRPS Complex

Both donors and acceptors of communication-mediating (COM) domains are essential for coordinating intermolecular communication within nonribosomal peptides synthetases (NRPSs) complexes. Different sets of COM domains provide selectivity, allowing NRPSs to utilize different natural biosynthetic templates. In this study, novel lipopeptides were synthesized by reprogramming the plipastatin biosynthetic machinery. A Thr-to-Asp point mutation was sufficient to shift the selectivity of the donor COM domain of ppsB toward that of ppsD. Deletion and/or interchangeability established donor and acceptor function. Variations in acceptor COM domain did not result in novel product formation in the presence of its partner donor, whereas plipastatin formation was completely abrogated by altering donor modules. Five novel lipopeptides (cyclic pentapeptide, linear hexapeptide, nonapeptide, heptapeptide, and cyclic octapeptide) were identified and verified by high-resolution LC-ESI-MS/MS. In addition, we demonstrated the potential to generate novel strains with the antimicrobial activity by selecting compatible COM domains, and the novel lipopeptides exhibited antimicrobial activity against five of the fungal species at a contention of 31.25–125 μg/ml.

Solid-Phase Synthesis of Cyclic Depsipeptides Containing a Tyrosine Phenyl Ester Bond

The first solid-phase strategy for the synthesis of cyclic depsipeptides containing a phenyl ester linkage in their structure is described. The key steps of the synthesis were the formation of the phenyl ester bond and the on-resin head-to-side-chain cyclization. The amino acid configuration significantly influenced the formation and the stability of the cyclic depsipeptides. The presence of a l-Tyr(1) and a d-Tyr(7) led to the most stable sequences.

Structural Analysis of the Lipopeptide Produced by the Bacillus subtilis Mutant R2-104 with Mutagenesis

The lipopeptide and its homologues are a kind of the five major biosurfactants with prominent interfacial and biological activities. A suite of mutagenesis method was adopted to expose a wild lipopeptide-producing strain Bacillus subtilis HSO121 to improve lipopeptide yield, and a stable mutant named R2-104 with a 2.0-fold production of lipopeptide was obtained. Compared to that of the wild strain HSO121, the lipopeptide produced by R2-104 showed a similar surface activity, but the course profiles of lipopeptide production during cultivation were different, with the peak yield of 500 mg at about 9 h by R2-104, and 400 mg at about 5 h by HSO121. The constituent abundance of the lipopeptide homologues produced by R2-104 was also different from that by HSO121. Combined methods of ESI-MS, GC-MS and MS-MS were applied for structural characterization of lipopeptide homologues, and it showed that the lipopeptides produced by R2-104 and HSO121 were attributed to a surfactin family with different constituents. The dominant constituent of the surfactin family produced by R2-104 was anteiso C15-surfactin with a relative content of 43.8 %, while the dominant one produced by HSO121was iso C14-surfactin with a relative content of 33.1 %.

Antagonistic mechanism of iturin A and plipastatin A from Bacillus amyloliquefaciens S76-3 from wheat spikes against Fusarium graminearum

Controlling toxigenic Fusarium graminearum (FG) is challenging. A bacterial strain (S76-3, identified as Bacillus amyloliquefaciens) that was isolated from diseased wheat spikes in the field displayed strong antifungal activity against FG. Reverse-phase high performance liquid chromatography and electrospray ionization mass spectrometry analyses revealed that S76-3 produced three classes of cyclic lipopeptides including iturin, plipastatin and surfactin. Each class consisted of several different molecules. The iturin and plipastatin fractions strongly inhibited FG; the surfactin fractions did not. The most abundant compound that had antagonistic activity from the iturin fraction was iturin A (m/z 1043.35); the most abundant active compound from the plipastatin fraction was plipastatin A (m/z 1463.90). These compounds were analyzed with collision-induced dissociation mass spectrometry. The two purified compounds displayed strong fungicidal activity, completely killing conidial spores at the minimal inhibitory concentration range of 50 µg/ml (iturin A) and 100 µg/ml (plipastatin A). Optical and fluorescence microscopy analyses revealed severe morphological changes in conidia and substantial distortions in FG hyphae treated with iturin A or plipastatin A. Iturin A caused leakage and/or inactivation of FG cellular contents and plipastatin A caused vacuolation. Time-lapse imaging of dynamic antagonistic processes illustrated that iturin A caused distortion and conglobation along hyphae and inhibited branch formation and growth, while plipastatin A caused conglobation in young hyphae and branch tips. Transmission electron microscopy analyses demonstrated that the cell walls of conidia and hyphae of iturin A and plipastatin A treated FG had large gaps and that their plasma membranes were severely damaged and separated from cell walls.

Interactions between fengycin and model bilayers quantified by coarse-grained molecular dynamics

Bacteria, particularly of the genus Bacillus, produce a wide variety of antifungal compounds. They act by affecting the lipid bilayers of fungal membranes, causing curvature-induced strain and eventual permeabilization. One class of these, known as fengycins, has been commercialized for treating agricultural infections and shows some promise as a possible antifungal pharmaceutical. Understanding the mechanism by which fengycins damage lipid bilayers could prove useful to the future development of related antifungal treatments. In this work, we present multi-microsecond-long simulations of fengycin interacting with different lipid bilayer systems. We see fengycin aggregation and uncover a clear aggregation pattern that is partially influenced by bilayer composition. We also quantify some local bilayer perturbations caused by fengycin binding, including curvature of the lipid bilayer and local electrostatic-driven reorganization.

Cyclic lipodepsipeptides verlamelin A and B, isolated from entomopathogenic fungus Lecanicillium sp

Verlamelin and its new derivative (verlamelin B) were isolated from fermentation broth of entomopathogenic fungus Lecanicillium sp. HF627. As the structural elucidation of verlamelin so far was only preliminary, we studied and determined the absolute structure of these two compounds to be cyclo(5S-hydroxytetradecanoic acid-D-alloThr/Ser-D-Ala-L-Pro-L-Gln-D-Tyr-L-Val). This is the first study that precisely analyzed the structure of verlamelin.

Analysis of calcium-induced effects on the conformation of fengycin

Fengycin is a natural lipopeptide with antifungal and eliciting properties and able to inhibit the activity of phospholipase A2. A combination of CD, FT-IR, NMR and fluorescence spectroscopic techniques was applied to elucidate its conformation in a membrane-mimicking environment and to investigate the effect of calcium ions on it. We mainly observed that fengycin adopts a turn conformation. Our results showed that calcium ions are bound by the two charged glutamates. The calcium binding has an influence on the fengycin conformation and more particularly, on the environment of the tyrosine residues. The modulation of the fengycin conformation by the environmental conditions may influence its biological properties.