Synergistic Antimicrobial Peptides LL-37 and HNP1 Cooperate Through Aggregation

Antimicrobial peptides (AMPs) are crucial components of the innate immune system, offering a potential solution to the growing crisis of antibiotic resistance. However, many AMPs face challenges like toxicity or limited efficacy when used alone. LL-37 (a human cathelicidin) and HNP1 (a human alpha-defensin) are two well-studied AMPs with distinct mechanisms. This study investigates how the combined action of LL-37 and HNP1, particularly their aggregation states, leads to enhanced antimicrobial efficacy, aiming to uncover novel synergistic strategies.

The study revealed significant "double cooperativity" between the two peptides. Individually, LL-37 and HNP1 showed moderate antimicrobial activity, with MICs against E. coli of 4 µM and 8 µM, respectively. However, when combined at a 1:1 molar ratio, their synergistic effect was striking: The combination of LL-37 and HNP1 reduced the MIC against E. coli by 4-fold to 1 µM, and against S. aureus by 3-fold to 2 µM, demonstrating potent synergy. Microscopy confirmed that LL-37 and HNP1 co-aggregated more efficiently than either peptide alone, forming larger, more stable structures on bacterial membranes. This enhanced aggregation led to a 2.5-fold increase in membrane permeabilization compared to the sum of individual effects (p<0.001), suggesting a more robust membrane disruption mechanism. Furthermore, the combined treatment resulted in a 99% reduction in bacterial viability within 60 minutes, whereas individual peptides achieved only 50-60% reduction in the same timeframe.