Phospholipid membrane permeabilization and leakage of cell content by surfactin-C15 from novel B. subtilis B-11 strain: a computational and experimental analysis.

Surfactin-C₁₅ produced by novel Bacillus subtilis B-11 strain has the potential to inhibit phytopathogens by permeabilizing their phospholipid cell membranes at the water/bilayer interface. This permeabilization leads to the disintegration of cell membranes, thus inhibiting growth, replication, and pathogenicity of phytopathogens. Model dipalmitoyl phosphocholine (DPPC) vesicles for pathogenic membranes were prepared by liposomal assays and used as representatives for phospholipid bilayer cell membrane. Results show that the hydrophobic fatty acid tail of surfactin-C₁₅ binds with the hydrophobic acyl chains of the DPPC bilayer membrane rather than with their hydrophilic head groups which tilt these acyl chains, causing the lipid headgroups to reorient forming pores in the membrane. AFM results show that structural disorderness increases at the nanoscale, specifically within the range of 0 to 3 nm. The fluorescence intensity of the encapsulated carboxyfluorescein probe increases in a concentration-dependent manner with surfactin-C₁₅ at 25 µM, 50 µM, and 75 µM, measured at a constant DPPC concentration of 10 µM, showing an emission increase from 200 to 800 nm. Heat flow decreases from DPPC: surfactin-C₁₅ (100:0) with a pretransition temperature of T_m 42.2 ± 0.1 (T_onset 40.9 ± 0.1) to DPPC: surfactin-C₁₅ (10:90) with a pretransition temperature of T_m 39.2 ± 0.1 (T_onset 36.9 ± 0.1). An increase in cholesterol concentration causes the size of DPPC vesicles to increase from 240 to 285 nm. These results confirm that larger vesicles exhibit higher interfacial activity compared to smaller vesicles, due to their greater surface area exposed to surfactin-C₁₅ at the membrane-water interface. This increase in vesicle size with cholesterol content is likely due to cholesterol's ability to modulate membrane fluidity and packing, resulting in altered vesicle morphology. The larger vesicles provide a more extensive contact area for surfactin-C₁₅ molecules at the membrane-water interface, facilitating stronger interactions that disrupt membrane integrity and enhance antimicrobial efficacy. This study suggests that surfactin-C₁₅ could be exploited for developing major biocontrol strategies in agriculture field.