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In addition, while the outer leaflet of bacterial cell is a bilayer, purified LPS self-aggregates, to give complex structures as micelles or large lamellar structures, whose size and shape depend on several factors including LPS concentration and osmotic pressure 19 and therefore does not always reproduce the properties of the outer leaflet of Gram negative bacteria. This picture of the bacterial membrane envelopes suggests that the lipid mixtures or LPS employed in studies aimed at determining the structure assumed by antimicrobial peptides on bacterial cells or their mechanism of action are a simplified model system. Membrane proteins are also embedded into the inner leaflet of Gram positive cell membrane. The LPS is absent on the Gram positive bacteria outer membrane, which possess instead a very thick peptidoglycan layer to which wall teichoic acids are covalently linked lipoteichoic acids are deeply inserted into the peptidoglycan and are attached to the head group of membrane lipids. On the outer membrane reside also some enzymes, for example the E.coli membrane contains a phospholipase (PldA), a protease (Omp T) and a LPS modifying enzyme (PagP). In Gram negative bacteria it is an asymmetric bilayer having the inner leaflet composed of phospholipids, as phosphatidylethanolamine, phosphatidylglycerol and cardiolipin, a peptidoglycan cell wall and the outer leaflet composed mainly of LPS and proteins as lipoproteins and beta barrel proteins as porins 17, 18. The envelope of bacteria is a very complex system 17. Solid state NMR studies carried out with membranes of different composition demonstrated that the structure, dynamics and orientation of the peptides on the membrane is strictly related to the membrane composition 16. It is not clear yet how closely the results of these studies reproduce what really happens when antimicrobial peptides meet bacterial cells. A different perspective is actually offered by solid state NMR studies, which give interesting insights on the changes occurring to the membranes after interaction with the antimicrobial peptides, other than on the membrane bound structure of peptides 13, 14, 15. Nuclear Magnetic Resonance (NMR) and Circular Dichroism (CD) studies carried out in the presence of either detergents, as mimic of the bacterial outer leaflet, or LPS, which is the main component of the Gram negative bacteria outer membrane, allowed the determination of the three-dimensional and secondary structure of antimicrobial peptides in cell-like environments 8, 9, 10, 11, 12. According to those mechanisms, antimicrobial peptides interact with bacterial outer membranes, perturb their integrity causing their disgregation. Biophysical studies carried out using molecules which mimic the outer leaflet of bacterial cells, as lipid mixtures or purified lipopolysaccharides (LPS), leaded to hypothesize three different mechanisms by which peptides kill bacterial cells, namely the barrel stave, toroidal pore and carpet mechanism 4, 5, 6, 7. It has been demonstrated that antimicrobial peptides impair cell viability by mechanisms which likely depend on their sequence and structure 3. The emergence of bacterial strains resistant to common antibiotics has strongly encouraged studies on antimicrobial peptides (AMPs) from natural sources and on their mechanism of action 1, 2.