Purification and structural characterization of fengycin homologues produced by Bacillus subtilis from poplar wood bark

Abstract

Poplar fungi canker is a disease that constitutes a serious threat to the poplar plant, a model tree in forest research. The control of this disease by use of chemical fumigants is associated with unwanted side effects such as environmental pollution and drug resistance. However, poplar canker can be effectively controlled with fengycin, an environmentally friendly bio-surfactant of microbial origin. Fengycin homologues are a series of lipopeptides with variations in both the length and branching of the β-hydroxy fatty acid. This study was aimed at purifying and characterizing fengycin homologues produced by Bacillus subtilis from poplar wood bark. A total of 13 fengycin homologues were obtained through 70% ammonium sulfate precipitation, methanol extraction, ion exchange chromatography, gel filtration chromatography, and semi-preparative high-performance liquid chromatography (HPLC). Electro-spray ionization mass spectrometry (ESI-MS) and electro-spray ionization tandem mass spectrometry (ESI-MS/MS) were employed to elucidate the chemical structures of these compounds, and the structures were further characterized by amino acid analysis. Thirteen (13) of the active compounds were fengycin A or B homologues. All the 13 homologues exhibited antifungal activity against the indicator strain Botryosphaeria dothidea. This finding is considered significant, in that it is the first report on the production of so many fengycin homologues and isomers by Bacillus subtilis. Thus, B. subtilis from poplar wood bark is a rich source of environment-friendly, anti-microbial agents for control of poplar canker.