Surfactin is a lipopetide produced by Bacillus substilis strains which contains a cyclic heptapetide and a beta-fatty acid chain. It has biological properties such as antiviral, antibacterial and hemolytic. Although the biological properties are suggested to be the consequence of its interaction the membrane of the target cell, the detailed molecular mechanism of action is not clearly understood. There is a substantial interest in understanding the molecular organization and nanomechanical properties of mixed surfactin/lipid bilayers system. Neutron reflection and small angle neutron scattering (SANS) are able to give a direct view of how surfactin interact with phospholipid bilayers.

The results obtained on different contrasts of DPPC bilayers built on the silicon-D2O interface verify the idea of surfactin penetrating into bilayers below the critical micelle concentration (CMC). Two mechanisms may be predicted, (a). at low concentrations, surfactin penetrates into the phospholipid membrane by interacting via its fatty acid chain, (b). near the CMC, surfactin self-associates to form micelles involving membrane phospholipids and leading to membrane rupture.

Although experimental results have proved strong haemolytic activity of surfactin with blood cells, the mechanism is not known. By using SANS technique for a series of DPPC/surfactin ratios, strong evidence of its haemolytic activity has been found. There is insertion of surfactin into the lipid bilayer and self-aggregation of surfactin within the membrane which results in pore formation. The results show a large surfactin aggregation number about 150 (diameter: 25 Å) and with a d-spacing of about 200 Å between the aggregates in the membrane.