Breaching the Membrane Barrier with Antimicrobial Agents that Cluster Anionic Lipids

Abstract

The bacterial membrane plays an important role in the action of antimicrobial agents. The presence of a larger exposed fraction of negatively charged lipids in bacterial membranes contributes to the higher toxicity against bacteria. The mechanism of action of many antimicrobial agents is thought to be by damaging the bacterial membrane. Several mechanisms exist that result in such damage. The membrane also must be breached in order for the agent to reach an intracellular target. One recently recognized contribution to membrane damage by certain antimicrobial agents is their ability to cluster anionic lipids from zwitterionic lipids. This results in the formation of membrane domains enriched in the antimicrobial agent and the anionic lipid. Such lipid clustering has been demonstrated by DSC, FTIR, 31P-MAS/NMR, 2H-NMR, freeze fracture transmission electron microscopy and AFM combined with polarized fluorescence microscopy. In cases where this is the principal mechanism of membrane damage, it predicts that those species of bacteria whose membrane is composed largely of anionic lipids are more resistant to these agents, while other bacterial species that contain both anionic and zwitterionic lipids in their membrane exhibit greater susceptibility. The smallest active antimicrobial fragment of LL-37 (KRIVQRIKDFLR) is capable of inducing clustering of anionic lipids and is toxic against E. coli that has a high PE content but not against S. aureus that is composed largely of anionic lipids. The loss of both lipid clustering ability and antimicrobial action that occurs on removal of two cationic residues to make RI-10, gives further support to the role of lipid clustering in the antimicrobial activity. These predictions also hold well for certain antimicrobial oligo-acyl-lysines and also for the peptide PFWRIRIRR-amide and its analogs against several Gram positive bacterial strains having different membrane compositions.