Bacterial selectivity and plausible mode of antibacterial action of designed Pro-rich short model antimicrobial peptides.

To develop novel Pro-rich model AMPs with shorter length and higher bacterial selectivity/therapeutic index (TI) than natural AMP, indolicidin, we synthesized a series of undodecapeptides derived from the sequence XXPXXPWXPXX-NH2 (X indicates Leu or Lys) with different ratios of Lys and Leu residues. Several Pro-rich model peptides (K7 WP3, K6 WL1 P3, K5 WL2 P3-1, K5 WL2 P3-2, and K4 WL3 P3) had approximate 8- to 11-fold higher bacterial selectivity/TI compared to indolicidin. These peptides selectively bind to negatively charged liposomes (EYPG/EYPG; 7:3, w/w) mimicking bacterial membranes. Their high selectivity to negatively charged phospholipids corresponds well with their high bacterial selectivity. Indolicidin showed almost complete depolarization of the cytoplasmic membrane of Staphylococcus aureus and dye-leakage from negatively charged liposomes at 10 microM, whereas all of Pro-rich model peptides had very little activity in these assays even at 80 microM, as observed in buforin 2. These results suggest that the ultimate target of our designed Pro-rich model peptides is probably the intracellular components (e.g. protein, DNA or RNA) rather than the cytoplasmic membranes. Collectively, our designed Pro-rich short model peptides appear to be excellent candidates for future development as a novel antimicrobial agent.