The impact of antibacterial peptides on bacterial lipid membranes depends on stage of growth.

The impact of maculatin 1.1 (Mac1) on the mechanical properties of supported lipid membranes derived from exponential growth phase (EGP) and stationary growth phase (SGP) E. coli lipid extracts was analysed by surface plasmon resonance and atomic force microscopy. Each membrane was analysed by quantitative nanomechanical mapping to derive measurements of the modulus, adhesion and deformation in addition to bilayer height. Image analysis revealed the presence of two domains in the EGP membrane differing in height by 0.4 nm. Three distinct domains were observed in the SGP membrane corresponding to 4.2, 4.7 and 5.4 nm in height. Using surface plasmon resonance, Mac1 was observed to bind strongly to both membranes and then disrupt the membranes as evidenced by a sharp drop in baseline. Atomic force microscopy (AFM) topographic analysis revealed the formation of domains of different height and confirmed that membrane destruction was much faster for the SGP derived bilayer. Moreover, Mac1 selectively disrupted the domain with the lowest thickness, which may correspond to a liquid ordered domain. Overall, the results provide insight into the role of lipid domains in the response of bacteria to antimicrobial peptides.