LPS is a Key Molecule in the Synergistic Effect of Temporins on Gram-Negative Bacteria

The growing emergence of multidrug-resistant microbes requires the discovery of new antibiotics with new modes of action. Naturally occurring antimicrobial peptides (AMPs), which are produced by almost all forms of life, represent promising candidates [1]. There is compelling evidence that unlike conventional antibiotics, most AMPs interact and increase the permeability of the bacterial membrane as part of their killing mechanism. However, before reaching it, they need to cross the cell wall that, in Gram-negative bacteria, is surrounded by the lipopolysaccharide (LPS)-outer membrane, which forms a very efficient barrier against a variety of hydrophilic and hydrophobic molecules [2, 3]. In Amphibia, temporins are among the shortest (10 to 16 residues) AMPs, with up to ten isoforms within the same specimen [4]. However, the biological significance of the coexistence of so many isoforms in a single living organism is not clear. We addressed this question using temporins A, B and L isolated from Rana temporaria skin secretion. In this study, we show that temporins A and B, which are only weakly active on Gram-negative bacteria, can synergize, when combined each with temporin L, to overcome the bacterial resistance imposed by the LPS protective layer. Furthermore, this effect is highly dependent on the type of LPS. To understand the underlying mechanism, we investigated the effect of two types of LPS (from E. coli O111:B4 and O26:B6) on the organization of temporins, alone and when mixed one with each other. Our data indicate that the synergism between temporins is related to the ability of temporin L to prevent the oligomerization of A and B when in contact with LPS O111:B4, thus allowing their traslocation across the bacterial cell wall into the target cytoplasmic membrane [5]. Overall, such studies should contribute to the development of new peptide-based anti-infective therapeutic strategies, urgently needed because of the increasing microbial resistance to the available antibiotics. References 1. Zasloff, M. 2002. Antimicrobial peptides of multicellular organisms. Nature. 415: 389-395. 2. Nikaido, H. 1994. Prevention of drug access to bacterial targets: permeability barriers and active efflux. Science. 264: 382-8. 3. Papo, N. and Y. Shai. 2005.A molecular mechanism for lipopolysaccharide protection of Gram-negative bacteria from antimicrobial peptides. J Biol Chem. 280: 10378-87. 4. Mangoni, M.L. 2006.Temporins, anti-infective peptides with expanding properties. Cell Mol Life Sci. 5. Rosenfeld, Y., D. Barra, M. Simmaco, Y. Shai and M.L. Mangoni. 2006. A synergism between temporins toward Gram-negative bacteria overcomes resistance imposed by the lipopolysaccharide protective layer. J Biol Chem. 281: 28565-74.