Mechanistic Insights into Human Antimicrobial Peptide-Induced Activation of a Broadly Conserved Bacterial Signaling System

Abstract

Antimicrobial peptides (AMPs) represent a promising class of therapeutics against bacterial pathogens. While their direct bactericidal mechanisms are well-characterized, how bacteria sense and respond to these peptides at sublethal concentrations remains poorly understood. Here, we investigate the activation of the Escherichia coli PhoQ-PhoP signaling system by the human antimicrobial peptide LL-37 and its derivatives (KR-12 and RI-10). We demonstrate that these peptides exhibit variable antimicrobial potency but surprisingly similar abilities to activate the PhoQ-PhoP pathway, indicating that signaling function is separable from bactericidal activity. Notably, sublethal concentrations of these peptides induce significant cell elongation, a phenotype dependent on PhoQ and mediated by the upregulation of QueE, which interferes with bacterial cell division. Contrary to the previous model suggesting peptides activate PhoQ passively by displacing its inhibitor MgrB, we observed enhanced cell elongation in{Delta} mgrB strains across all tested peptides, including RI-10, lacking antibacterial activity. Our findings suggest peptides actively stimulate PhoQ through a mechanism independent of MgrB dissociation, providing a more refined understanding of the peptide signaling through the PhoQ-PhoP system. These insights into bacterial adaptation mechanisms against host-derived peptides may guide the development of peptide therapeutics with enhanced efficacy against drug-resistant pathogens. Significance statementAntimicrobial peptides (AMPs) are promising alternatives to conventional antibiotics, yet how bacteria detect and respond to these host defense molecules remains poorly understood. This study investigates the interaction between bacterial sensing systems and AMPs at the genetic and molecular levels. Unlike other eukaryotes with multiple cathelicidin peptides, humans have only one cathelicidin that produces various active fragments through processing. Rather than creating multiple detectors, E. coli deploys an elegant solution, the PhoQ-PhoP signaling system that recognizes the conserved antimicrobial region shared by all LL-37 fragments. We demonstrate that peptides directly stimulate PhoQ independent of their bactericidal activity, even in the absence of inhibitor MgrB, inducing cell elongation. These insights may inform the development of effective peptide-based therapeutics against drug-resistant pathogens.