Methods: Antibacterial activities of the peptides (0.022 ~ 4.4 M corresponding to 0.1 ~ 10 g/ml) were assessed by alamarBlue^TM assay using Staphylococcus aureus, Streptococcus pneumoniae, Streptococcus pyogenes, Escherichia coli and Pseudomonas aeruginosa as target organisms. Furthermore, the membrane-permeabilization activities of the peptides were examined by using E. coli ML-35p as a target.

Results: By substituting E¹⁶ and K²⁵ with two L residues, the hydrophobicity of the peptide (18-mer LL) was increased, and by further substituting Q²², D²⁶ and N³⁰ with three K residues, the cationicity of the peptide (18-mer LLKKK) was enhanced. Among peptide derivatives, 18-mer LLKKK exhibited the most potent antibacterial actions against S. aureus (methicillin-resistant and -sensitive), S. pneumoniae, S. pyogenes, E. coli and P. aeruginosa, and possessed the most powerful membrane-permeabilizing activities against E. coli ML-35p at the effective concentrations (p < 0.05,="" 18-mer="" llkkk="">vs. 18-mer LL, 18-mer K¹⁵-V³² and LL-37).

Conclusions: Bactericidal activities of the amphipathic human CAP18/LL-37-derived 18-mer peptide can be augmented by modifying its hydrophobicity and cationicity, and 18-mer LLKKK is the most potent among peptide derivatives with therapeutic potential for Gram-positive and Gram-negative bacterial infections.