“Seeing is believing”: An AFM study of target cells lysis by Antimicrobial Peptides

Marina Rautenbach & Dale Holroyd

Department of Biochemistry, University of Stellenbosch, South Africa

Atomic force microscopy, AFM, has advanced microscopy of biological samples to nanoscale level. The greatest advantage of AFM is the possibility to image the surface of bio-macromolecules in situ, if not in vivo. The objective of this study was to use AFM as a new technique in unravelling the mechanism of action antimicrobial peptides (AMPs). Using the simplest AFM imaging technique, we were able to analyse the influence of two peptides, the haemolytic bee-venom melittin and the antibacterial magainin from Xenopus laevis, on different target membranes at nanometre resolution, without using fixing agents. The AFM-images of the target cells, before and after peptide treatment, showed distinct changes as a consequence of peptide action in membrane surface topology. We observed enlarged cells (due to osmotic pressure increase) with long grooves and lesions that eventually led to membrane collapse and vesicle formation. The AFM results allowed us to construct a new hypothesis on the mechanism of action of melittin, magainin and other linear cationic AMPs. The model that we propose resembles a "zipper", in which the peptides assemble in rows rather than circular pores as with the "wormhole" and "barrel-stave" models. Our "zipper" model is currently under further investigation.