Due to increasing reports of antibiotic resistance, antimicrobial peptides have been studied over the past few decades as possible substitutes for traditional antibiotic drugs. Their mechanism of action, however, is not fully understood and this hinders efforts to design new peptides for use in human therapies. We previously performed over 150ns of simulation of a protegrin-1 (PG-1) pore in a lipid bilayer composed of plamitoyloleoylphosphatidylethanolamine (POPE) and palmitoyloleoyl phosphatidylglycerol (POPG) lipids meant to mimic the inner membrane of a bacterial cell [1]. The simulation improves on a model of an octameric pore proposed from NMR experiments [2]. Additional experimental data was made available to us that describes the movement of potassium ions from E. coli treated with PG-1. In the experiments, potassium ion leakage, membrane potential, and membrane permeability were monitored with respect to time for samples treated with various PG-1 concentrations for cells in two growth phases. Quick, significant loss of potassium from the cells combined with a decrease in cell volume is observed and for samples treated with higher concentrations of PG-1, this is followed by an increase in the volume and eventual cell lysis. For a more direct comparison to these experimental data, a simulation of the pore was carried out with potassium ions investigate the movement of this ion type through the pore. From the results of the experiments and simulations, we propose a timeline of the events that result in bacterial cell death: after pores form in the membrane potassium leaves the cell due to differences in its intracellular and extracellular concentrations. Other species such as sodium and chloride ions will move into the cell due to concentration gradients and to balance the charge distribution. As this happens, the transmembrane potential is disrupted, and the bacteria are no longer able to control their cellular processes. Water will enter the cell, at rates determined by the number of pores on the cell. For bacteria treated with a high concentration of protegrin, this will result in osmotic lysis of the cell.

This work was supported by a grant from NIH (GM 070989). Computational support from the Minnesota Supercomputing Institute is gratefully acknowledged. This work was also partially supported by National Computational Science Alliance.

1. Langham, A.A., A. Sayyed Ahmad, and Y.N. Kaznessis, On the nature of antimicrobial activity: a model for Protegrin-1. J Am Chem Soc, 2008. 130(13): p. 4338-46.

2. Mani, R., et al., Membrane-bound dimer structure of a beta-hairpin antimicrobial peptide from rotational-echo double-resonance solid-state NMR. Biochemistry, 2006. 45(27): p. 8341-9.