How Many AMP Molecules Kill a Bacterium? Spectroscopic Determination of PMAP-23 Binding to E. Coli

Antimicrobial peptides (AMPs) kill bacteria mainly through the permeabilization of the plasma membrane. Experiments on these molecules generally focus either on their biophysical characterization in model membranes, or on their activity on bacterial cells, but studies demonstrating a correlation between biological activity and behaviour in liposomes are still lacking.One unanswered issue is the minimal amount of bound peptide that is necessary to kill a bacterial cell. Different attempts to assess this quantity [1, 2] reached different conclusions, probably because the fraction of peptide bound to bacteria is usually extrapolated based on binding experiments performed on liposomes.Trying to fill the hiatus between biological and biophysical studies, we determined by fluorescence measurements the affinity of a dansyl-labeled analogue of the PMAP-23 AMP [3] for both liposomes and E. coli cells. Experiments were performed in the peptide concentration range that displays bactericidal activity.These results will provide a direct determination of the minimal number of peptide molecules which are necessary to kill a bacterial cell.[1] ACS Chem. Biol. 2010, 10: 905.[2] Nat. Rev. Microbiol. 2009, 7: 245.[3] Biochim. Biophys. Acta2009, 1788: 1523.