Structural Basis of Self-Promoted Uptake of Cationic Antimicrobial Peptides across the Lipopolysaccharide-Rich Outer Membrane of Gram-Negative Bacteria

Lipopolysaccharide, a specialized multiacylated glycolipid, constitutes the outer leaflet of the outer membrane (OM) of Gram-negative bacteria. Densely packed anionic LPS molecules, stabilized by divalent cations such as Mg2+ and Ca2+, form a formidable permeability barrier that restricts the entry of diverse molecules, including antibiotics, antibacterial proteins, and toxins. Pioneering studies from Bob Hancock’s laboratory demonstrated that treatment of Gram-negative bacteria with polycationic antibiotics markedly enhances OM permeability to large proteins and hydrophobic compounds. This discovery led to the concept of self-promoted uptake pathway, a mechanism by which cationic antimicrobial agents gain access to bacterial cells through their own disruption of the LPS barrier. Cationic antimicrobial peptides (AMPs), also known as host-defense peptides, exhibit broad-spectrum antimicrobial activity and have emerged as promising candidates for combating the global crisis of antibiotic resistance. A critical component of their antibacterial mechanism is the permeabilization of the LPS-rich outer membrane through the self-promoted uptake pathway. This process involves a series of coordinated molecular events, including initial electrostatic recognition of LPS, peptide-induced structural reorganization of the outer membrane, perturbation of LPS acyl-chain packing, membrane-assisted peptide folding and oligomerization, and dynamic AMP–LPS interactions that collectively facilitate peptide translocation across the outer membrane. To commemorate Bob Hancock’s seminal contributions to this field, we review recent advances in understanding AMP-mediated self-promoted uptake by integrating insights from atomic-resolution structural studies by NMR, biophysical investigations of AMP–LPS interactions, and molecular simulations of AMP–LPS membrane systems. Together, these studies provide a comprehensive structural framework for understanding how cationic AMPs overcome the LPS permeability barrier and permeabilize the outer membrane of Gram-negative bacteria.