All-or-none membrane permeabilization by fengycin-type lipopeptides from Bacillus subtilis QST713

The fungicidal activity of Bacillus subtilis QST713 has been utilized for the highly effective and environmentally safe protection of crops against a variety of pathogens. It is based mainly on the production of cyclic lipopeptides of the fengycin (FEs), surfactin, and iturin families. The mixed population of native FEs forms micelles which solubilize individual FEs such as agrastatin 1 (AS1) that are otherwise rather insoluble on their own. Fluorescence lifetime-based calcein efflux measurements and cryo transmission electron microscopy show that these FEs show a unique scenario of membrane permeabilization. Poor miscibility of FEs with lipid probably promotes the formation of pores in 10% of the vesicles at only ≈ 1 μM free FE and in 15% of the vesicles at 10 μM. We explain why this limited, all-or-none leakage could nevertheless account for the killing of virtually all fungi whereas the same extent of graded vesicle leakage may be biologically irrelevant. Then, crystallization of AS1 and micellization of plipastatins cause a cut-off in leakage at 15% that might regulate the biological activity of FEs, protecting Bacillus and plant membranes. The fact that FE micelles solubilize only about 10 mol-% fluid lipid resembles the behavior of detergent resistance.

Research highlights
► Fungicidal Bacillus subtilis fengycin lipopeptides cause all‐or‐none membrane leakage.
► Leakage curve shows plateau due to parallel formation of micelles and crystals.
► Very active yet little soluble lipopeptide agrastatin was solubilized by others in native mix.
► Detailed leakage and partitioning data were obtained from new, fluorescence lifetime‐based assay.
► We hypothesize that all‐or‐none mechanism causes larger efflux than graded in vivo.