Further aspects on the hemolytic activity of the antibiotic lipopeptide iturin A

The bacterial lipopeptide iturin A is able to cause hemolysis of human erythrocytes in a dose-dependent manner. Hemolysis takes place at iturin concentrations below its critical micellar concentration. Relative kinetics determinations clearly show that K⁺ leakage occurs prior to hemoglobin release. Furthermore, hemolysis can be prevented by addition to the outer solution of osmotic protectants of appropriate size. Altogether these results indicate that iturin A-induced hemolysis follows a colloid-osmotic mechanism, with the formation of a membrane pore of average diameter 32 Å. Iturin A is capable of inducing leakage of an aqueous fluorescent probe trapped in human erythrocyte ghosts, but not in large unilamellar liposomes made of various lipid compositions. The different permeabilizing effects of iturin A on model and biological membranes are discussed on the light of the presented results.     

Pore-forming properties of iturin A, a lipopeptide antibiotic

The addition of iturin A, a lipopeptide antibiotic extracted from Bacillus subtilis, to a bimolecular lipid membrane (BLM) increases dramatically its electrical conductance. For very low concentration of iturin A, discrete conductance steps are observed which are assigned to the formation of conducting pores. The characteristics of these pores depend on the lipid content of the BLM and they change with time. Cholesterol considerably increases the lifetimes of open states. The pores are slightly anion versus cation selective. These first observations unable us to briefly discuss the pore-forming properties of lipopeptides.     

A biophysical study of the interaction of the lipopeptide antibiotic iturin A with aqueous phospholipid bilayers

Iturin A is a lipopeptide extracted from the culture media of Bacillus subtilis which shows a strong antifungal action. The interaction of iturin A with multilamellar vesicles of dimyristoylphosphatidylcholine (DMPC) induced structures which did not sediment during centrifugation. Electron microscopy after negative staining showed that, at 30 mol%, iturin A/DMPC vesicles were visible but smaller than those formed by pure DMPC. Thermograms of DMPC/iturinA obtained after differential scanning calorimetry, at low concentrations of iturin A, were interpreted as indicating the presence of two laterally separated phases, one formed by pure phospholipid and the other by lipopeptide-phospholipid complexes, these two separated phases being already detected even at low concentrations such as 2 mol%. Fluorescence quenching experiments showed that the D-Tyr residue of the lipopeptide was fully accessible to the aqueous medium, indicating that the polar part of iturin A is located outside of the membrane hydrophobic palisade. It was concluded that the membrane barrier properties are likely to be damaged in the area where the lipid complexes are accumulated, due to structural fluctuations, and this may be one of the bases of its biological activity. Iturin-A was also able to greatly destabilize dielaidoylphosphatidylethanolamine (DEPE) membranes in the fluid form, producing a new structure which had a poor correlation in X-ray diffraction, and in 31P NMR spectroscopy gave rise to a spectrum containing a double isotropic signal. Iturin A was shown to induce DEPE to adopt phases other than H(II) inverted hexagonal, underlining that this lipopeptide is capable of modifying the curvature of the membrane, which may also be important in explaining the tendency of iturin A to create small vesicles and which may be another of the bases of its biological activity.     

Effect of the lipopeptide antibiotic, iturin A, on morphology and membrane ultrastructure of yeast cells

Abstract The effects of iturin A, at fungicidal concentrations, on yeast cells were studied by scanning electron microscopy and by transmission electron microscopy. A depression, observed in each iturin A-treated cell, was the consequence of the release of electrolytes and other cytoplasmic components. Iturin A passes through the cell wall and disrupts the plasma membrane with the formation of small vesicles and the aggregation of intramembranous particles. Moreover, iturin A passes through the plasma membrane and interacts with the nuclear membrane and probably with membranes of other cytoplasmic organelles.     

Production of lipopeptide antibiotic iturin A using soybean curd residue cultivated with Bacillus subtilis in solid-state fermentation

Bacillus subtilis RB14-CS, which suppresses the growth of various plant pathogens in vitro by producing the lipopeptide antibiotic iturin A, was cultured using soybean curd residue, okara, a by-product of tofu manufacture in solid-state fermentation. After 4 days incubation, iturin A production reached 3,300 mg/kg wet solid material (14 g/kg dry solid material), which is approximately tenfold higher than that in submerged fermentation. When the okara product cultured with RB14-CS was introduced into soil infested with Rhizoctonia solani, which is a causal agent of damping-off of tomato, the disease occurrence was significantly suppressed. After 14 days, the number of RB14-CS cells remained in soil at the initial level, whereas almost no iturin A was detected in soil. As the okara cultured with RB14-CS exhibited functions of both plant disease suppression and nutritional effect on tomato seedlings, this product is expected to contribute to the recycling of the soybean curd residue.     

Aggregational behavior of aqueous dispersions of the antifungal lipopeptide iturin A

Iturin A, a lipopeptide isolated from Bacillus subtilis, possesses a strong antifungal activity, and has been devoted to a great deal of attention. Since iturin is an amphiphilic compound with a great propensity to self-associate in solution as well as inside the membrane, the question arises to whether its aggregational behavior is dependent on the concentration of the lipopeptide. In order to test this, the ability of iturin suspensions to encapsulate water-soluble molecules has been examined. Iturin was dispersed at different concentrations above its critical micellar concentration, in a buffer containing the water-soluble dye 5,6-carboxyfluorescein. For iturin A micelles, a Stokes radius of 1.3 nm and an aggregational number of 7 was obtained. The results shown in this work clearly demonstrate that iturin dispersions in water, at concentrations of 0.7, 1.4 and 3 mM, i.e. far above the critical micellar concentration (40 microM), are capable of encapsulating carboxyfluorescein, probably by adopting a type of aggregate different from the micelle. Negative-staining electron microscopy shows the presence of vesicles with an average size of 150 nm. By using (14)C-iturin, it is shown that, at 3 mM concentration, 40 % of the iturin molecules adopt this vesicular state. It is proposed that iturin molecules form a fully interdigitated bilayer, where each hydrocarbon tail span the entire hydrocarbon width of the bilayer, resulting in multilamellar vesicles capable of encapsulating an aqueous compartment. The possible implications of these results to the membrane destabilizing effect of iturin A, are discussed according to the dynamic cone-shape of the iturin molecule.     

Interaction of iturin A, a lipopeptide antibiotic, with Saccharomyces cerevisiae cells: influence of the sterol membrane composition

The binding of the membrane-active lipopeptide antibiotic iturin A to yeast cells was studied using radioactive iturin A. Saccharomyces cerevisiae had a maximum binding capacity of 5.6 x 10(9) molecules per single cell. The Scatchard plot of binding showed a biphasic profile, with a lower dissociation constant for small concentrations of iturin A. The break of slope at 30 microM iturin A corresponds to the micellization of antibiotic in solution. The binding is also dependent on the nature of the sterol present in the membrane. A mutant yeast strain with a membrane containing cholesterol instead of ergosterol showed the highest affinity for iturin A and the highest sensitivity to this antibiotic, as measured by K+ ion release. In contrast the presence of stigmasterol increased the resistance of the cells to iturin A.     

Interactions of the lipopeptide antifungal iturin A with lipids in mixed monolayers

The miscibility and the interactions of the antifungal lipopeptide iturin A with lipids, DMPC and cholesterol, are studied in monolayers at the air/water interface and a comparison of the respective behaviour of iturin A and the biologically inactive methylated derivative MeTyr-iturin A is made. Each lipopeptide is miscible with anyone of the lipids. This behaviour is revealed by the dependence of the transition pressure upon composition and by deviations from the additivity rule of the mean molecular area. The thermodynamic properties of the mixed systems are studied by the method of Goodrich. The mixed monolayers are always more stable than the two separate components, subsequently there are interactions between the components. However, the excess free energy of mixing delta Gexm is positive for the iturin A/DMPC system which is an indication that the interactions between lipopeptide and lipid molecules are weaker than the interactions between the pure components themselves. This is compatible with the presence of self-associated lipopeptide molecules. However, delta Gexm is highly negative for the iturin A/cholesterol system giving evidence of the formation of a specific complex between iturin A and cholesterol which is not the case with the methylated derivative. These data are analysed in connection with previous results concerning the pore-forming properties of these lipopeptides and the lack of biological activity of MeTyr-iturin A.     

Methylation of the antifungal lipopeptide iturin A modifies its interaction with lipids

Iturin A, extracted from the culture media of Bacillus subtilis, is an antifungal lipopeptide, the peptide cycle of which includes a D-Tyr residue in position 2. The antibiotic strength of iturin A is related to a change in the permeability of the membrane cells which leads to a leakage of K+ from the intracellular medium. Methylation of the D-Tyr residue dramatically decreases the biological activity of iturin A. Using the intrinsic fluorescence of D-Tyr we have shown that both iturin A and O-methyl-tyrosine iturin A enter the lipid membranes. When dimyristoylphosphatidylcholine vesicles contain iturin A we observe a change in the order degree of the lipid phase and an increase in the transition temperature. The methylated derivative has no effect. Two model membranes have been used to study the permeability changes induced by iturin A and O-methyltyrosine iturin A. Studying ionic permeability we have found that the conductance of a planar lipid membrane increases very much less when the lipopeptide is methylated. On the other hand, the release of carboxyfluorescein trapped in lipid vesicles is less upon addition of O-methyltyrosine-iturin A. We conclude that the Tyr residue of the peptide cycle plays a role in determining the interactions of iturin A with lipid membrane.     

Studies on the biosynthesis of iturin, an antibiotic of Bacillus subtilis, and a lipopeptide containing beta-hydroxy fatty acids

The biosynthesis of iturin, an antibiotic containing a beta-amino fatty acid, was studied by incubating Bacillus subtilis in the presence of various 14C-labelled precursors. Sodium acetate or palmitic acid were incorporated into the beta-amino acids of iturin. Among the alpha-amino acids (asparagine, glutamine, serine, proline and tyrosine) in the peptidic part of iturin, asparagine appears to be the best precursor. In the presence of sodium [14C]acetate or [14C]asparagine, there was a synthesis of radioactive compound (compound X) before the synthesis of radioactive iturin. Compound X contained asparagine and/or aspartic acid, glutamine and/or glutamic acid and beta-hydroxy fatty acids.     

产表面活性素和伊枯草菌素A菌株的筛选及其脂肽类产物的特性

采用血琼脂平板法, 从菜园土壤中分离到8株代谢表面活性剂的菌株, 比较各菌株的排油性、抑菌性, 根据合成脂肽类物质表面活性素(Surfactin)和伊枯草菌素A (Iturin A)必需的sfp、ituD和lpa-14基因设计引物, 结合PCR的方法筛选到一株具广谱抗菌性且含有sfp、ituD和lpa-14 3个关键基因的细菌XZ-173。经过生理生化试验测定和16S rDNA序列系统发育学分析, 将其鉴定为解淀粉芽孢杆菌(Bacillus amyloliquefaciens)。通过红外光谱(FT-IR)分析该菌株代谢产物, 初步鉴定为脂肽类物质, 并对照高效液相色谱(HPLC)与标准品比对结果, 确定含有Surfactin和Iturin A组分。该菌株产生的脂肽粗品能使纯水的表面张力降低至26.6 mN/m, 临界胶束浓度(CMC)为500 mg/L, 具有很好的乳化性能, 对立枯丝核菌和青枯菌表现出很好的拮抗活性。因此, 产脂肽细菌XZ-173是一株应用前景广阔的功能菌。     

Quantification of the antifungal lipopeptide iturin A by high performance liquid chromatography coupled with aqueous two-phase extraction

Iturin A, a powerful antifungal surfactant, is a kind of bacterial lipopeptide produced by Bacillus strains. This study addresses the use of an aqueous two-phase system (ATPS) using ethanol/ammonium sulfate to extract iturin A from Bacillus amyloliquefaciens NJN-6 fermentation broth and the quantification of iturin A by HPLC. Baseline separation of iturin A homologues was performed using an RP-C(18) column with a mixture of water and acetonitrile. The results showed that the correlation coefficient between integral area and concentration was 0.9961 within the range of 20-140 mg/l. The RSD of the retention time and the peak area were 1.29% and 1.45%, respectively. The effects of some operating parameters in ATPS, e.g., pH, temperature and centrifugation time, were also studied. This method can be successfully used for the rapid quantification of iturin A.     

CRAMP analog having potent antibiotic activity without hemolytic activity

CRAMP-18 is an 18-residue functional region, corresponding to residues 16-33 of a mouse-derived antibiotic peptide CRAMP. To develop novel antibiotic peptides possessing strong antibiotic activity against bacterial, fungal and tumor cells without hemolytic activity, three analogs of CRAMP-18 were synthesized containing either Leu- or Lys-substitution. Leu-substitution ([L(1, 8)]-CRAMP-18) in the hydrophobic helix face of CRAMP-18 induced a dramatic increase in antibiotic activity without a significant increase in hemolytic activity. Lys-substitution ([K(2, 13)]-CRAMP-18 or [K(9, 16)]-CRAMP-18) in the hydrophilic helix face produced a smaller response. Therefore, [L(1, 8)]-CRAMP-18 may be an attractive candidate for developing novel peptide antibiotics.     

The interaction of analogues of the antimicrobial lipopeptide, iturin A2, with alkali metal ions

Electrospray mass spectrometry was employed as a tool in this first study on the molecular interaction between the alkali metal ions and antifungal lipopeptide iturin A, and some analogues. Cationisation by sodium and signal intensity of lipopeptide species depended on sodium concentration, but was independent of sample solvent, carrier solvent polarity and sample pH between 4 and 11. 8-Beta, a linear analogue of iturin A2 (8-Beta; beta-aminotetradecanoyl-NYNQPNS), and its shorter linear lipopeptide analogues, associated either one or two alkali metal cations, while the N-->C cyclic peptides associated with only one cation. The chirality of the beta-NC14 residue had a limited influence on the cationisation. It was observed that 8-Beta contained at least four interaction sites for a cation of which two, the C-terminal carboxylate and the side-chain of tyrosine, can take part in ionic interaction with a cation. It is proposed that the remaining two interaction centres of alkali metal ions are within the two type II beta-turns found in conformation of natural iturin A. This was corroborated by the diminished capacity of the shorter peptides, in which one of the beta-turns was eliminated to bind a second larger cation. All the lipopeptides showed the same order of alkali metal ion selectivity: Na+ > K+ > Rb+. These results indicated a size limitation in the interaction cavity or cavities. The absence of, or observation of only low abundance, di-cationised complexes of cyclic peptides the indicated association of the cation in the interior of the peptide ring. It is thus hypothesised that alkali metal ions can bind in one of the two beta-turns in the natural iturin A molecule.     

Structure activity relationships of stendomycin, a lipopeptide antibiotic from Streptomyces.

A strain of Streptomyces which produced stendomycin, a lipopeptide antibiotic, was grown in culture media containing various amino acids as nitrogen substrates. The nature of the fatty acid component of stendomycin was dependent on the nature of the amino acid present in the medium, but this did not affect antibiotic activity. Modifications in the peptide moiety resulted in a loss of antifungal activity.     

The role of acyl chain character and other determinants on the bilayer activity of A21978C an acidic lipopeptide antibiotic

An acidic lipopeptide A21978C has previously been shown to have a powerful antibiotic activity against Gram-positive organisms. Due to its ability to increase the K+ permeability of bacterial cells and its specific calcium requirement, which is similar to a previously described ionophore CDA, its effect on planar bilayer membranes has been studied. Although it produces significant increases in the conductivity of lipid bilayers it is shown that this alone cannot account for its in vivo activity. Similarly, unlike the in vivo results, the Ca2+-induced increases in bilayer conductivity can be mimicked by Mg2+ and charged lipids. Results from a series of homologues differing in the length of the acyl moiety show a close similarity between bilayer conductance and LD50 trends from in vivo studies. A complex activity is proposed which depends upon incorporation in, rather than disruption of, the bilayer membrane.     

Use of soybean curd residue, okara, for the solid state substrate in the production of a lipopeptide antibiotic, iturin A, by Bacillus subtilis NB22

The possibility of the utilization of soybean curd residue, okara, for the production of a lipopeptide antibiotic, iturin A, in solid state fermentation (SSF) by Bacillus subtilis NB22 was investigated. Okara is a by-product of the tofu manufacturing process, now treated as an industrial waste and disposed of mostly by incineration. Dehydrated okara, with improved transportability and preservability, could be used as effectively as the fresh, intact okara for SSF by B. subtilis NB22 for the production of iturin A.