Production of a novel mixture of mycosubtilins by mutants of Bacillus subtilis

Deciphering the mechanisms involved in reduced sensitivity to azoles and fengycin lipopeptide in Venturia inaequalis

Apple scab, caused by the hemibiotrophic fungus Venturia inaequalis, is currently the most common and damaging disease in apple orchards. Two strains of V. inaequalis (S755 and Rs552) with different sensitivities to azole fungicides and the bacterial metabolite fengycin were compared to determine the mechanisms responsible for these differences. Antifungal activity tests showed that Rs552 had reduced sensitivity to tebuconazole and tetraconazole, as well as to fengycin alone or in a binary mixture with other lipopeptides (iturin A, pumilacidin, lichenysin). S755 was highly sensitive to fengycin, whose activity was close to that of tebuconazole. Unlike fengycin, lipopeptides from the iturin family (mycosubtilin, iturin A) had similar activity on both strains, while those from the surfactin family (lichenysin, pumilacidin) were not active, except in binary mixtures with fengycin. The activity of lipopeptides varies according to their family and structure. Analyses to determine the difference in sensitivity to azoles (which target the CYP51 enzyme involved in the ergosterol biosynthesis pathway) showed that the reduced sensitivity in Rs552 is linked to (i) a constitutive increased expression of the Cyp51A gene caused by insertions in the upstream region and (ii) greater efflux by membrane pumps with the involvement of ABC transporters. Microscopic observations revealed that fengycin, known to interact with plasma membranes, induced morphological and cytological changes in cells from both strains. Sterol and phospholipid analyses showed a higher level of ergosta-7,22-dien-3-ol and a lower level of PI(C16:0/C18:1) in Rs552 compared with S755. These differences could therefore influence the composition of the plasma membrane and explain the differential sensitivity of the strains to fengycin. However, the similar antifungal activities of mycosubtilin and iturin A in the two strains indirectly indicate that sterols are probably not involved in the fengycin resistance mechanism. This leads to the conclusion that different mechanisms are responsible for the difference in susceptibility to azoles or fengycin in the strains studied.

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.

Broad-Spectrum Antifungal, Biosurfactants and Bioemulsifier Activity of Bacillus subtilis subsp. spizizenii-A Potential Biocontrol and Bioremediation Agent in Agriculture

In this study, the antifungal, biosurfactant and bioemulsifying activity of the lipopeptides produced by the marine bacterium Bacillus subtilis subsp. spizizenii MC6B-22 is presented. The kinetics showed that at 84 h, the highest yield of lipopeptides (556 mg/mL) with antifungal, biosurfactant, bioemulsifying and hemolytic activity was detected, finding a relationship with the sporulation of the bacteria. Based on the hemolytic activity, bio-guided purification methods were used to obtain the lipopeptide. By TLC, HPLC and MALDI-TOF, the mycosubtilin was identified as the main lipopeptide, and it was further confirmed by NRPS gene clusters prediction based on the strain's genome sequence, in addition to other genes related to antimicrobial activity. The lipopeptide showed a broad-spectrum activity against ten phytopathogens of tropical crops at a minimum inhibitory concentration of 400 to 25 μg/mL and with a fungicidal mode of action. In addition, it exhibited that biosurfactant and bioemulsifying activities remain stable over a wide range of salinity and pH and it can emulsify different hydrophobic substrates. These results demonstrate the potential of the MC6B-22 strain as a biocontrol agent for agriculture and its application in bioremediation and other biotechnological fields.

Isolation and characterization of a mycosubtilin homologue antagonizing Verticillium dahliae produced by Bacillus subtilis strain Z15

Bacillus subtilis strain Z15 (BS-Z15) was isolated from the cotton field of Xinjiang, China, and characterized as an effective biocontrol agent antagonizing plant pathogen Verticillium dahliae 991 (VD-991). However, the chemical substance produced by BS-Z15 for resistance to VD-991 remains elusive. Here, a serial purification methods including HCl precipitation, organic solvent extraction, and separation by semi-preparative High-Performance Liquid Chromatography were performed to obtain a single compound about 3.5 mg/L from the fermentation broth of BS-Z15, which has an antifungal activity against VD-991. Moreover, Fourier Transform Infrared spectrum, Nuclear Magnetic Resonance Spectroscopy, and Tandem Mass Spectrometry analyses were carried out to finally confirm that the active compound from BS-Z15 is a mycosubtilin homologue with C17 fatty acid chain. Genomic sequence prediction and PCR verification further showed that the BS-Z15 genome contains the whole mycosubtilin operon comprising four ORFs: fenF, mycA, mycB, and mycC, and the expression levels of mycA-N, mycB-Y and mycC-N reached a peak at 32-h fermentation. Although mycosubtilin homologue at 1 μg/mL promoted the germination of cotton seed, that with high concentration at 10 μg/mL had no significant effect on seed germination, plant height and dry weight. Furthermore, mycosubtilin homologue sprayed at 10 μg/mL on two-week-old cotton leaves promotes the expression of pathogen-associated genes and gossypol accumulation, and greatly decreases VD-991 infection in cotton with disease index statistics. This study provides an efficient purification strategy for mycosubtilin homologue from BS-Z15, which can potentially be used as a biocontrol agent for controlling verticillium wilt in cotton.

Bioinformatics Modelling and Metabolic Engineering of the Branched Chain Amino Acid Pathway for Specific Production of Mycosubtilin Isoforms in Bacillus subtilis

Mycosubtilin belongs to the family of lipopeptides. Different isoforms with various antifungal activities can be obtained according to the length and the isomery of the fatty acid. In this work, the activities of the mycosubtilin isoforms were first studied against the pathogen Aspergillus niger, revealing the high activity of the anteiso-C17 isoform. Modification of the mycosubtilin isoform patterns during cultures of the natural strain Bacillus subtilis ATCC 6633 was then investigated through amino acid feeding experiments. In parallel, single-gene knockouts and single-gene overexpression, leading to the overproduction of the anteiso-C15 fatty acid chains, were predicted using informatics tools which provide logical reasoning with formal models of reaction networks. In this way, it was in silico predicted that the single overexpression of the ilvA gene as well as the single knockout of the codY gene may lead to the overproduction of anteiso-C15 fatty acid chains. For the first time, it has been demonstrated that overexpression of ilvA helps to enhance the furniture of odd anteiso fatty acids leading to a favored mycosubtilin anteiso-C17 production pattern (+41%). Alternatively, a knock-out codY mutant led to a higher furniture of even iso fatty acids, leading to a favored mycosubtilin iso-C16 production pattern (+180%). These results showed that increased selective synthesis of particular isoforms of mycosubtilin through metabolic engineering is feasible, disclosing the interest of these approaches for future development of lipopeptide-producing strains.

Impact of the Purification Process on the Spray-Drying Performances of the Three Families of Lipopeptide Biosurfactant Produced by Bacillus subtilis

Lipopeptides produced by Bacillus subtilis display many activities (surfactant, antimicrobial, and antitumoral), which make them interesting compounds with a wide range of applications. During the past years, several processes have been developed to enable their production and purification with suitable yield and purity. The already implemented processes mainly end with a critical drying step, which is currently achieved by freeze-drying. In this study, the possibility to replace this freeze-drying step with a spray-drying one, more suited to industrial applications, was analyzed. After evaluating their thermal resistance, we have developed a spray-drying methodology applicable for the three lipopeptides families produced by B. subtilis, i.e., surfactin, mycosubtilin (iturin family), and plipastatin (fengycin family). For each lipopeptide, the spray-drying procedure was applied at three steps of the purification process by ultrafiltration (supernatant, diafiltered solution, and pre-purified fraction). The analysis of the activities of each spray-dried lipopeptide showed that this drying method is not decreasing its antimicrobial and biosurfactant properties. The methodology developed in this study enabled for the first time the spray-drying of surfactin, without adjuvants’ addition and regardless of the purification step considered. In the case of fengycin and mycosubtilin, only diafiltered solution and purified fraction could be successfully spray-dried without the addition of adjuvant. Maltodextrin addition was also investigated as the solution for the direct drying of supernatant. As expected, the performances of the spray-drying step and the purity of the powder obtained are highly related to the purification step at which the product was dried. Interestingly, the impact of mycosubtilin concentration on spray-drying yield was also evidenced.

Effects of Bacillus lipopeptides on the survival and behavior of the rosy apple aphid Dysaphis plantaginea

Within the framework of biocontrol development, several natural lipopeptides produced by Bacillus subtilis show well-documented anti-microbial properties, especially in orchards. However, the number of studies on their putative insecticidal effects remain low despite the growing interest to develop new strategies of orchards pests' control. The rosy apple aphid Dysaphis plantaginea is the major aphid pest causing great leaf damage to apple trees. In this study, we submitted young adult aphids to topical application of three different families of lipopeptides, Plipastatin (Fengycin), Mycosubtilin (Iturin), and Surfactin, either separately or as a ternary mixture. Their aphicidal effects were investigated at 1, 2.5 and 5 g/L, both at 1 h and 24 h after exposure, and their effects on aphid behavior were studied at the 2.5 g/L concentration at 24 h after exposure. When delivered alone, lipopeptides displayed contrasted effects varying from no aphicidal activity for Mycosubtilin to a mortality induced even at low concentrations by Surfactin. Surprisingly, locomotor activity of the surviving aphids was only affected by the two least lethal treatments, Mycosubtilin and the ternary mix. Their feeding behavior was only impacted by Surfactin, the most lethal treatment, that unexpectedly increased phloem sap ingestion. The results are discussed in the context of lipopeptides applicability for integrated pest management.

New Continuous Process for the Production of Lipopeptide Biosurfactants in Foam Overflowing Bioreactor

In this work, an original culture process in bioreactor named overflowing continuous culture (O-CC) was developed to produce and recover continuously mycosubtilin, a lipopeptide antifungal biosurfactant of major interest. The lipopeptide production was first investigated in shake conical flasks in different culture media [ammonium citrate sucrose (ACS), Difco sporulation medium (DSM), and Landy], followed by a pH condition optimization using 3-(N-morpholino)propanesulfonic acid (MOPS) and 2-(N-morpholino)ethanesulfonic acid (MES) buffered media. A simple theoretical modeling of the biomass evolution combined with an experimental setup was then proposed for O-CC processed in stirred tank reactor at laboratory scale. Seven O-CC experiments were done in modified Landy medium at the optimized pH 6.5 by applying dilution rates comprised between 0.05 and 0.1 h^-1. The O-CC allowed the continuous recovery of the mycosubtilin contained in the foam overflowing out of the reactor, achieving a remarkable in situ product removal superior to 99%. The biomass concentration in the overflowing foam was found to be twofold lower than the biomass concentration in the reactor, relating advantageously this process to a continuous one with biomass feedback. To evaluate its performances regarding the type of lipopeptide produced, the O-CC process was tested with strain BBG116, a mycosubtilin constitutive overproducing strain that also produces surfactin, and strain BBG125, its derivative strain obtained by deleting surfactin synthetase operon. At a dilution rate of 0.1 h^-1, specific productivity of 1.18 mg of mycosubtilin⋅g^-1(DW)⋅h^-1 was reached. Compared with other previously described bioprocesses using almost similar culture conditions and strains, the O-CC one allowed an increase of the mycosubtilin production rate by 2.06-fold.

Development and Genetic Engineering of Hyper-Producing Microbial Strains for Improved Synthesis of Biosurfactants

Current research energies are fixated on the synthesis of environmentally friendly and non-hazardous products, which include finding and recognizing biosurfactants that can substitute synthetic surfactants. Microbial biosurfactants are surface-active compounds synthesized intracellularly or extracellularly. To use biosurfactants in various industries, it is essential to understand scientific engagements that demonstrate its potentials as real advancement in the 21st century. Other than applying a substantial effect on the world economic market, engineered hyper-producing microbial strains in combination with optimized cultivation parameters have made it probable for many industrial companies to receive the profits of 'green' biosurfactant innovation. There needs to be an emphasis on the worldwide state of biosurfactant synthesis, expression of biosurfactant genes in expressive host systems, the recent developments, and prospects in this line of research. Thus, molecular dynamics with respect to genetic engineering of biosynthetic genes are proposed as new biotechnological tools for development, improved synthesis, and applications of biosurfactants. For example, mutant and hyper-producing recombinants have been designed efficaciously to advance the nature, quantity, and quality of biosurfactants. The fastidious and deliberate investigation will prompt a comprehension of the molecular dynamics and phenomena in new microorganisms. Throughout the decade, valuable data on the molecular genetics of biosurfactant have been produced, and this solid foundation would encourage application-oriented yields of the biosurfactant production industry and expand its utilization in diverse fields. Therefore, the conversations among different interdisciplinary experts from various scientific interests such as microbiology, biochemistry, molecular biology, and genetics are indispensable and significant to accomplish these objectives.

Antifungal Activities of Bacillus subtilis Lipopeptides to Two Venturia inaequalis Strains Possessing Different Tebuconazole Sensitivity

Within the framework of biocontrol development, three natural substances produced by Bacillus subtilis, called lipopeptides, have been studied: fengycin (F), surfactin (S), and mycosubtilin (M). Their antifungal properties were tested in vitro, in liquid medium, on two strains of Venturia inaequalis, ascomycete fungi causing apple scab. These two strains were, respectively sensitive and less sensitive to tebuconazole, an active substance of the triazole family. These three molecules were tested on their own, in binary (FS, FM, SM) and ternary mixtures (FSM). The antifungal activities of lipopeptides were estimated by calculating an IC₅₀, compared to tebuconazole chemical substance. In tests involving the sensitive strain, all lipopeptide modalities exhibited antifungal activity. However, modalities involving fengycin and its mixtures exhibited the best antifungal activities; the activity of fengycin alone being very similar to that of tebuconazole. Interestingly, regarding the strain with reduced sensitivity to tebuconazole, surfactin and fengycin alone were not efficient while mycosubtilin and the different mixtures showed interesting antifungal activities. Specifically, the antifungal activity of FS and FSM mixture were equivalent to that of tebuconazole. For both fungal strains, microscopic observations revealed important morphological modifications in the presence of fengycin and in a less important proportion in the presence of surfactin but not in the presence of mycosubtilin. Overall, this study highlights the diversity in mode of action of lipopeptides on apple scab strains.

Rational strain improvement for surfactin production: enhancing the yield and generating novel structures

Surfactin, one of the most powerful microbial surfactants, is a lipopeptide-type biosurfactant which combines interesting physicochemical properties and biological activities. However, the high cost caused by its low productivity largely limits the commercial application of surfactin. Hence, many engineered bacterium have also been used to enhance surfactin biosynthesis. This review briefly summarizes the mechanism of surfactin biosynthesis, highlighting the synthesis pathway of N-terminally attached fatty acids, and outlines the main genetic engineering strategies for improving the yield and generating novel structures of surfactin, including promoter engineering, enhancing efflux systems, modifying the transcriptional regulatory genes of surfactin synthase (srfA), genomics and transcriptomics analysis, non ribosomal peptide synthetase (NRPS) domain and combinatorial biosynthesis. Finally, we discuss the future prospects of the research on surfactin.

Lipopeptides produced by Bacillus subtilis as new biocontrol products against fusariosis in ornamental plants

In this study, we have investigated the effects of three lipopeptides (fengycin, surfactin and mycosubtilin) produced by different strains of Bacillus subtilis against the phytopathogenic fungi Fusarium oxysporum f. sp. iridacearum, which affects the ornamental bulb plant populations of Iris sp. The antifungal effects were tested using minimum inhibitory concentration assay, determination of mycelium growth and spore germination inhibition rates. Also, in vivo tests on infected rhizomes and scanning electron microscopy were employed. Mycosubtilin alone and in combination with fengycin or/and surfactin showed potent inhibitory activity at concentrations as low as 5 μg ml^-1 which is 100 times lower compared to Topsin M, a common chemical fungicide frequently used against fusariosis in ornamental plants. An enhancement of mycosubtilin antifungal activity was observed when it was used in combination with surfactin due to a synergistic effect. At a concentration of 20 μg ml^-1, mycosubtilin inhibited the growth of the mycelium up to 49% and the spore germination ability up to 26% in comparison to control. In addition, significant changes on the macro- and micro-morphology have been observed. The antifungal activity is related to the inhibition of spore germination and the irreversible damage of the hyphae cell wall. To the best of our knowledge, this is the first attempt to propose the lipopeptides as biopesticides against the fusariosis of ornamental plants.

Polynucleotide phosphorylase is involved in the control of lipopeptide fengycin production in Bacillus subtilis

Bacillus subtilis is a wealth source of lipopeptide molecules such as iturins, surfactins and fengycins or plipastatins endowed with a range of biological activities. These molecules, designated secondary metabolites, are synthesized via non-ribosomal peptides synthesis (NRPS) machinery and are most often subjected to a complex regulation with involvement of several regulatory factors. To gain novel insights on mechanism regulating fengycin production, we investigated the effect of the fascinating polynucleotide phosphorylase (PNPase), as well as the effect of lipopeptide surfactin. Compared to the wild type, the production of fengycin in the mutant strains B. subtilis BBG235 and BBG236 altered for PNPase has not only decreased to about 70 and 40%, respectively, but also hampered its antifungal activity towards the plant pathogen Botrytis cinerea. On the other hand, mutant strains BBG231 (srfAA^-) and BBG232 (srfAC^-) displayed different levels of fengycin production. BBG231 had registered an important decrease in fengycin production, comparable to that observed for BBG235 or BBG236. This study permitted to establish that the products of pnpA gene (PNPase), and srfAA^- (surfactin synthetase) are involved in fengycin production.

Review lipopeptides biosurfactants: Mean classes and new insights for industrial, biomedical, and environmental applications

Lipopeptides are microbial surface active compounds produced by a wide variety of bacteria, fungi, and yeast. They are characterized by high structural diversity and have the ability to decrease the surface and interfacial tension at the surface and interface, respectively. Surfactin, iturin, and fengycin of Bacillus subtilis are among the most popular lipopeptides. Lipopepetides can be applied in diverse domains as food and cosmetic industries for their emulsification/de-emulsification capacity, dispersing, foaming, moisturizing, and dispersing properties. Also, they are qualified as viscosity reducers, hydrocarbon solubilizing and mobilizing agents, and metal sequestering candidates for application in environment and bioremediation. Moreover, their ability to form pores and destabilize biological membrane permits their use as antimicrobial, hemolytic, antiviral, antitumor, and insecticide agents. Furthermore, lipopeptides can act at the surface and can modulate enzymes activity permitting the enhancement of the activity of certain enzymes ameliorating microbial process or the inhibition of certain other enzymes permitting their use as antifungal agents. This article will present a detailed classification of lipopeptides biosurfactant along with their producing strain and biological activities and will discuss their functional properties and related applications.

Identification of a highly efficient stationary phase promoter in Bacillus subtilis

A promoter that enabled high-level expression of the target gene during the stationary phase in the absence of an inducer would facilitate the efficient production of heterogeneous proteins at a low cost. In this study, a genome-scale microarray-based approach was employed to identify promoters that induced high-level expression of the target genes in Bacillus subtilis from the late log phase to the stationary phase without an inducer. Eleven candidate promoters were selected based on B. subtilis microarray data and the quantitative PCR analysis. Among the selected promoters, Pylb exhibited the highest activity with the reporter bgaB during the stationary phase. Compared with P43 (a commonly used constitutive promoter), promoter Pylb could express two reporter genes (egfp and mApple), and the expression levels of EGFP and RFP were 7.8- and 11.3-fold higher than that of P43, respectively. This finding was verified by overexpression of the genes encoding pullulanase and organophosphorus hydrolase, the activities of which were 7.4- and 2.3-fold higher, respectively, when driven by Pylb compared with P43. Therefore, our results suggest that the Pylb promoter could be used to overexpress target genes without an inducer; this method could facilitate the identification and evaluation of attractive promoters in the genome.

Lipopeptide surfactants: Production, recovery and pore forming capacity

Lipopeptides are microbial surface active compounds produced by a wide variety of bacteria, fungi and yeast. They are characterized by highly structural diversity and have the ability to decrease the surface and interfacial tension at the surface and interface, respectively. Surfactin, iturin and fengycin of Bacillus subtilis are among the most studied lipopeptides. This review will present the main factors encountering lipopeptides production along with the techniques developed for their extraction and purification. Moreover, we will discuss their ability to form pores and destabilize biological membrane permitting their use as antimicrobial, hemolytic and antitumor agents. These open great potential applications in biomediacal, pharmaceutic and agriculture fields.

Cyclic lipopeptides from Bacillus subtilis activate distinct patterns of defence responses in grapevine

Non-self-recognition of microorganisms partly relies on the perception of microbe-associated molecular patterns (MAMPs) and leads to the activation of an innate immune response. Bacillus subtilis produces three main families of cyclic lipopeptides (LPs), namely surfactins, iturins and fengycins. Although LPs are involved in induced systemic resistance (ISR) activation, little is known about defence responses induced by these molecules and their involvement in local resistance to fungi. Here, we showed that purified surfactin, mycosubtilin (iturin family) and plipastatin (fengycin family) are perceived by grapevine plant cells. Although surfactin and mycosubtilin stimulated grapevine innate immune responses, they differentially activated early signalling pathways and defence gene expression. By contrast, plipastatin perception by grapevine cells only resulted in early signalling activation. Gene expression analysis suggested that mycosubtilin activated salicylic acid (SA) and jasmonic acid (JA) signalling pathways, whereas surfactin mainly induced an SA-regulated response. Although mycosubtilin and plipastatin displayed direct antifungal activity, only surfactin and mycosubtilin treatments resulted in a local long-lasting enhanced tolerance to the necrotrophic fungus Botrytis cinerea in grapevine leaves. Moreover, challenge with specific strains overproducing surfactin and mycosubtilin led to a slightly enhanced stimulation of the defence response compared with the LP-non-producing strain of B. subtilis. Altogether, our results provide the first comprehensive view of the involvement of LPs from B. subtilis in grapevine plant defence and local resistance against the necrotrophic pathogen Bo. cinerea. Moreover, this work is the first to highlight the ability of mycosubtilin to trigger an immune response in plants.

Mycosubtilin and surfactin are efficient, low ecotoxicity molecules for the biocontrol of lettuce downy mildew

The use of surfactin and mycosubtilin as an eco-friendly alternative to control lettuce downy mildew caused by the obligate pathogen Bremia lactucae was investigated. Preliminary ecotoxicity evaluations obtained from three different tests revealed the rather low toxicity of these lipopeptides separately or in combination. The EC50 (concentration estimated to cause a 50 % response by the exposed test organisms) was about 100 mg L(-1) in Microtox assays and 6 mg L(-1) in Daphnia magna immobilization tests for mycosubtilin and 125 mg L(-1) and 25 mg L(-1) for surfactin, respectively. The toxicity of the mixture mycosubtilin/surfactin (1:1, w/w) was close to that obtained with mycosubtilin alone. In addition, the very low phytotoxic effect of these lipopeptides has been observed on germination and root growth of garden cress Lepidium sativum L. While a surfactin treatment did not influence the development of B. lactucae on lettuce plantlets, treatment with 100 mg L(-1) of mycosubtilin produced about seven times more healthy plantlets than the control samples, indicating that mycosubtilin strongly reduced the development of B. lactucae. The mixture mycosubtilin/surfactin (50:50 mg L(-1)) gave the same result on B. lactucae development as 100 mg L(-1) of mycosubtilin. The results of ecotoxicity as well as those obtained in biocontrol experiments indicated that the presence of surfactin enhances the biological activities of mycosubtilin. Mycosubtilin and surfactin were thus found to be efficient compounds against lettuce downy mildew, with low toxicity compared to the toxicity values of chemical pesticides. This is the first time that Bacillus lipopeptides have been tested in vivo against an obligate pathogen and that ecotoxic values have been given for surfactin and mycosubtilin.

Self-assembly of three bacterially-derived bioactive lipopeptides

The self-assembly in aqueous solution of three lipopeptides obtained from Bacillus subtilis has been investigated. The lipopeptides surfactin, plipastatin and mycosubtilin contain distinct cyclic peptide headgroups as well as differences in alkyl chain length, branching and chain length distribution. Cryogenic transmission electron microscopy and X-ray scattering reveal that surfactin and plipastatin aggregate into 2 nm-radius spherical micelles, whereas in complete contrast mycosubtilin self-assembles into extended nanotapes based on bilayer ordering of the lipopeptides. Circular dichroism and FTIR spectroscopy indicate the presence of turn structures in the cyclic peptide headgroup. The unexpected distinct mode of self-assembly of mycosubtilin compared to the other two lipopeptides is ascribed to differences in the surfactant packing parameter. This in turn is due to specific features of the conformation of the peptide headgroup and alkyl chain branching.