Gene-encoded antimicrobial peptides (AMPs) represent attractive candidates for the development of a new generation of anti-infective agents.There is compelling evidence that unlike conventional antibiotics, most AMPs do act by altering the membrane permeability of the target cell.However, before reaching it, they need to cross the microbial cell wall that, in Gram-negative bacteria, is surrounded by the lipopolysaccharide (LPS)-outer membrane, which forms an efficient barrier against a variety of molecules.In Nature, the frog skin temporins are among the shortest (10 to 14 residues) AMPs, with up to ten isoforms within the same specimen [1]. We have shown that some of them (temporin-1Tb, TB, and temporin-1Tl, TL) have a synergistic effect in killing Gram-negative bacteria [2]. This suggests an important strategy to overcome bacterial resistance due to the LPS layer. More precisely, we have found that LPS induces oligomerization of TB. This would prevent its translocation across the outer membrane making the peptide inactive on Gram-negative bacteria. Differently, TL is highly active on these bacteria, presumably due to the lack of such self-association. To investigate the interactions of TL and TB within LPS, we studied their structures and interactions in LPS micelles using NMR spectroscopy. Interestingly, TL assumes an anti-parallel dimeric helical structure stabilized by intimate packing between aromatic-aromatic and aromatic-aliphatic residues. In contrast, TB shows populations of helical and aggregated conformations. Note that the LPS-induced aggregated states of TB are largely destabilized in the presence of TL. Importantly, these results provide the first structural insights into the mechanism of action and synergism of AMPs at the level of the LPS-outer membrane.1 Mangoni ML (2006) CMLS 63: 1060-692 Mangoni ML et al. (2008) JBC 283:22907-17