The study shows that healthy saliva can shield the natural antimicrobial peptide LL‑37 from being broken down by harmful bacteria in the mouth, letting LL‑37 still kill bacteria even when those bacteria release destructive enzymes. This protection works even after heating, meaning it’s a robust effect, though it slightly lowers LL‑37’s killing power.

Proteolysis is a common microbial virulence mechanism that enables the destruction of host tissue and evasion from host defense mechanisms. Antimicrobial peptides, also known as host defense peptides, are effector molecules of the innate immunity that demonstrate a broad range of antimicrobial and immunoregulatory activities. Deficiency of the human LL-37 antimicrobial peptide was previously correlated with severe periodontal disease. Porphyromonas gingivalis, the major pathogen associated with periodontitis, is highly proteolytic. In this study, P. gingivalis was found capable of degrading LL-37 by utilizing its arginine-specific gingipains. Saliva collected from volunteers with a healthy periodontium protected LL-37 from proteolysis by P. gingivalis. Salivary protection of LL-37 was heat resistant and specific and enabled LL-37 to inhibit growth of Escherichia coli in the presence of the P. gingivalis proteases. Previously, saliva and other body fluids have been shown to inhibit the antimicrobial activity of LL-37. Here we demonstrate that at a cost of a small reduction in the bactericidal activity of LL-37, saliva enables the antibacterial activity of LL-37 despite the presence of proteases secreted by the main periodontopathogen.