Biofilm disruption vs. rapid resistance: Evaluating the role of Calliphora vicina defensin in Staphylococcus aureus clearance.

Defensins are evolutionarily conserved antimicrobial peptides (AMPs) considered a cornerstone of innate immunity. However, the rapid emergence of resistance in pathogens like Staphylococcus aureus challenges our understanding of defensin efficacy in vivo. We used the blowfly Calliphora vicina-an insect adapted to pathogen-rich environments-to investigate S. aureus resistance dynamics. During an active immune response, defensin concentrations in larval hemolymph reach 11-16 μM. At these levels, purified defensin effectively eradicates both planktonic cells and pre-formed S. aureus biofilms (MBEC90 = 50 μg/mL). Crucially, S. aureus develops high-level genetic resistance to the defensin within only nine passages, and the total larval AMP complex fails to prevent this adaptation. Since defensin accounts for over 90% of the hemolymph's anti-Gram-positive activity, this rapid resistance renders the entire systemic humoral response ineffective from an evolutionary perspective. Our findings challenge the traditional view of defensins as a primary "first line of defense." Instead, we propose a "Final Clearance" hypothesis, suggesting that systemic AMPs act as a secondary precision tool to eliminate pathogens already decimated by cellular and enzymatic responses. This shift is vital for understanding innate immunity stability and developing sustainable antimicrobial strategies.