High Concentrations of the Antimicrobial Peptide Magainin 2 Induce Distinct Biomechanical Changes in Escherichia coli

Antimicrobial peptides (AMPs) are found widely as part of nonspecific immune defenses. One class of AMPs forms stable pores in membranes, including the two chemically distinct membranes found in the Gram-negative cell envelope. As the Gram-negative cell envelope is a significant barrier to drug development, some have hypothesized that these AMPs could be used clinically, either alone or in combination with other drugs that cannot cross the Gram-negative cell envelope on their own. Here, we use atomic force microscopy (AFM), fluorescence spectroscopy, and fluorescence microscopy to elucidate the biomechanical changes that occur in Escherichia coli treated with various concentrations of the pore-forming AMP magainin 2 (MAG2). We find that near the minimum inhibitory concentration, MAG2 induces a loss of cell stiffness and a decrease in cell height consistent with pore formation and cellular leakage. Surprisingly, treatment with high concentrations of MAG2 leads to cells becoming stiffer and increasing in height. We confirmed that MAG2 forms pores at high concentrations using a standard propidium iodide (PI) uptake assay, in which PI is added to a cell suspension and is detected only after pores form in the cell membrane. However, when PI was added after 30 min of treatment with high concentrations of MAG2, less PI fluorescence was observed than in the standard PI uptake assay, indicating that movement across the cell membrane was restricted at the end of our experiments. We also observed that the modulus of the cell envelope increased with increasing MAG2 concentration, consistent with greater packing of MAG2 into the cell envelope membranes. Finally, our AFM images in air revealed that cells formed blebs when treated with high concentrations of MAG2. These data suggest that MAG2 initially forms pores at high concentrations, but as membrane packing increases, movement across the cell envelope becomes restricted. Understanding the concentration-dependent restriction of movement across the cell envelope could be important if AMPs are to be used clinically.