Scientists tweaked a small piece of the human antimicrobial peptide LL-37 (called KR-12) by swapping some building blocks for alanine or lysine. They found that certain changes, especially swapping a glutamine at position 5 for lysine and an aspartic acid at position 9 for alanine or lysine, made the peptide up to eight times better at killing common germs like Staph, Pseudomonas, and Candida, without harming human cells. This shows the peptide can be fine‑tuned for stronger, safe antimicrobial action.

The human host defence peptide LL-37 is a broad-spectrum antibiotic with immunomodulatory functions. Residues 18-29 in LL-37 have previously been identified as a minimal peptide (KR-12) that retains antibacterial activity with decreased cytotoxicity. In this study, analogues of KR-12 were generated by Ala and Lys scans to identify key elements for activity. These were tested against a panel of human pathogens and for membrane permeabilisation on liposomes. Replacements of hydrophobic and cationic residues with Ala were detrimental for antibiotic potency. Substitutions by Lys increased activity, as long as the increase in cationic density did not disrupt the amphiphilic disposition of the helical structure. Importantly, substitutions showed differential effects against different organisms. Replacement of Gln5 with Lys and Asp9 with Ala or Lys improved the broad-spectrum activity most, each resulting in up to an eightfold increase in potency against Staphylococcus aureus, Pseudomonas aeruginosa and Candida albicans. The improved analogues displayed no significant toxicity against human cells, and thus, KR-12 is a tuneable template for antibiotic development.