Induction of Chlamydospore Formation in Fusarium by Cyclic Lipopeptide Antibiotics from Bacillus subtilis C2

Abstract

The culture filtrate of Bacillus subtilis strain C2 showed strong activity against the pathogenic fungus Fusarium solani f. sp. radicicola. A partially purified fraction (PPF) from the extract induced chlamydospore formation in Fusarium. Reverse-phase high performance liquid chromatography yielded 8 different fractions, six of which had chlamydospore-inducing activity. Mass spectrometry and nuclear magnetic resonance analyses identified the main active constituent as C(17) fengycin A (FA17), a cyclic lipopeptide. The effect of FA17 on morphology and physiology of two Fusarium species was dependent on the lipopeptide concentration. When challenged with FA17 at concentrations (0.5, 8, 64 μg ml(-1)) below the minimum inhibitory concentration (MIC) (128 μg ml(-1)), two species of Fusarium formed chlamydospores from hyphae, germ tubes, or inside the conidia within 2 days. At concentrations close to the MIC, FA17 caused Fusarium to form sparse and swollen hyphae or lysed conidia. The other five fractions were identified as fengycin A homologues. The homologues could also induce chlamydospore-like structures in 17 species of filamentous fungi including some specimens that do not normally produce chlamydospores, according to their taxonomic descriptions. Like other chlamydospores, these structures contained nuclei and lipid bodies as revealed by DAPI and Nile Red staining, and could germinate. This is the first study to demonstrate that under laboratory conditions fengycin, an antifungal lipopeptide produced by B. subtilis, can induce chlamydospore formation in Fusarium and chlamydospore-like structures in many filamentous fungi.