Isolation of anteiso-C17, iso-C17, iso-C16, and iso-C15 bacillomycin D from Bacillus amyloliquefaciens SD-32 and their antifungal activities against plant pathogens

Two new cyclic lipopeptides (3 and 4) were isolated from the culture filtrate of Bacillus amyloliquefaciens strain SD-32, together with two known metabolites, iso-C15 and iso-C16 bacillomycin D (1 and 2). Spectroscopic and chemical analyses identified the structures of the new compounds 3 and 4 as anteiso-C17 bacillomycin D, cyclic (l-Asn-d-Tyr-d-Asn-l-Pro-l-Glu-d-Ser-l-Thr-3-amino-14-methylhexadecanoic acid) and iso-C17 bacillomycin D, cyclic (l-Asn-d-Tyr-d-Asn-l-Pro-l-Glu-d-Ser-l-Thr-3-amino-15-methylhexadecanoic acid), respectively. The absolute configuration of C-3 in the β-amino fatty acid was determined to be R on the basis of the CD spectrum of its dinitrophenyl-p-methoxyaniline derivative. The activities of compounds 1-4 were evaluated against 13 plant pathogens: the activities of anteiso- and iso-C17 bacillomycin D (3 and 4) were almost the same and stronger than those of iso-C15 and iso-C16 bacillomycin D (1 and 2); iso-C15 bacillomycin D (1) was weakest. Compounds 2-4 inhibited the growth of all fungi tested; however, Pythium aphanidermatum was not inhibited at all by any of the compounds. Furthermore, compounds 1-4 at concentrations of 80, 40, 30, and 30 μM, respectively, inhibited completely the Botrytis cinerea infection in cucumber leaf.

Conformational analyses of bacillomycin D, a natural antimicrobial lipopeptide, alone or in interaction with lipid monolayers at the air-water interface

Bacillomycin D is a natural antimicrobial lipopeptide belonging to the iturin family. It is produced by Bacillus subtilis strains. Bacillomycin D is characterized by its strong antifungal and hemolytic properties, due to its interaction with the plasma membrane of sensitive cells. Until now, only few limited analyses were conducted to understand the biological activities of bacillomycin D at the molecular level. Our purpose was to analyze the conformation of bacillomycin D using IR spectroscopy and to model its interactions with cytoplasmic membranes using Langmuir interfacial monolayers. Our findings indicate that bacillomycin D contains turns and allow to model its three-dimensional structure. Bacillomycin D formed a monolayer film at the air-water interface and kept its turn conformation, as shown by polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). To identify the membrane lipid target of bacillomycin D, its interactions with pure lipid monolayers were analyzed and an original behavior of the lipopeptide toward cholesterol-containing monolayers was shown. This original behavior was lost when bacillomycin D was interacting with pure cholesteryl acetate monolayers, suggesting the involvement of the alcohol group of cholesterol in the lipopeptide-cholesterol interaction.

Biochemical characterization of an anti-Candida factor produced by Enterococcus faecalis

Background

Because Candida albicans is resistant to several antifungal antibiotics, there is a need to identify other less toxic natural products, particularly antimicrobial proteins, peptides or bacteriocin like inhibitory substances. An attempt has been made to purify and characterise an anti-Candida compound produced by Enterococcus faecalis.

Results

An anti-Candida protein (ACP) produced by E. faecalis active against 8 C. albicans strains was characterised and partially purified. The ACP showed a broad-spectrum activity against multidrug resistant C. albicans MTCC 183, MTCC 7315, MTCC 3958, NCIM 3557, NCIM 3471 and DI. It was completely inactivated by treatment with proteinase K and partially by pronase E.

The ACP retained biological stability after heat-treatment at 90°C for 20 min, maintained activity over a pH range 6–10, and remained active after treatment with α-amylase, lipase, organic solvents, and detergents. The antimicrobial activity of the E. faecalis strain was found exclusively in the extracellular filtrate produced in the late logarithmic growth phase. The highest activity (1600 AU mL^-1) against C. albicans MTCC 183 was recorded at 48 h of incubation, and activity decreased thereafter. The peptide showed very low haemagglutination and haemolytic activities against human red blood cells. The antimicrobial substance was purified by salt-fractionation and chromatography.

Partially purified ACP had a molecular weight of approximately 43 KDa in Tricine-PAGE analysis. The 12 amino acid N terminal sequence was obtained by Edman degradation. The peptide was de novo sequenced by ESI-MS, and the deduced combined sequence when compared to other bacteriocins and antimicrobial peptide had no significant sequence similarity.

Conclusions

The inhibitory activity of the test strain is due to the synthesis of an antimicrobial protein. To our knowledge, this is the first report on the isolation of a promising non-haemolytic anti-Candida protein from E. faecalis that might be used to treat candidiasis especially in immunocompromised patients.

Cyclic lipopeptide profile of the plant-beneficial endophytic bacterium Bacillus subtilis HC8

In a previous study (Malfanova et al. in Microbial Biotech 4:523-532, 2011), we described the isolation and partial characterization of the biocontrol endophytic bacterium B. subtilis HC8. Using thin-layer chromatography, we have detected several bioactive antifungal compounds in the methanolic extract from the acid-precipitated supernatant of HC8. In the present study, we have further analyzed this methanolic extract using liquid chromatography-mass spectrometry. Based on the comparison of retention times and molecular masses with those of known antifungal compounds, we identified three families of lipopeptide antibiotics. These include four iturins A having fatty acyl chain lengths of C14 to C17, eight fengycins A (from C14 to C18 and from C15 to C17 containing a double bond in the acyl chain), four fengycins B (C15 to C18), and five surfactins (C12 to C16). Evaluation of the antifungal activity of the isolated lipopeptides showed that fengycins are the most active ones. To our knowledge, this is the first report of an endophytic Bacillus subtilis producing all three major families of lipopeptide antibiotics containing a very heterogeneous mixture of homologues. The questions remain open which of these lipopeptides (1) are being produced during interaction with the plant and (2) are contributing to the biocontrol activity of HC8.