How do antimicrobial peptides interact with the outer membrane of Gram-negative bacteria? Role of lipopolysaccharides in the peptide binding, anchoring and penetration
Gram-negative bacteria possess a complex structural cell envelope that constitutes a barrier for antimicrobial peptides which neutralize the microbes by disrupting their cell membranes. Computational and experimental approaches were used to study a model outer membrane interaction with an antimicrobial peptide, melittin. The investigated membrane included di[3-deoxy-D-manno-octulosonyl]-lipid A (KLA) in the outer leaflet and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) in the inner leaflet. Molecular dynamics simulations revealed, that the positively charged helical C-terminus of melittin anchors rapidly into the hydrophilic head-group region of KLA, while the flexible N-terminus makes contacts with the phosphate groups of KLA moving melittin into the boundary between the hydrophilic and hydrophobic regions of the lipids. Electrochemical techniques confirmed binding of melittin to the model membrane. To probe the peptide conformation and orientation during interaction with the membrane, polarization modulation infrared reflection absorption spectroscopy was used. The measurements revealed conformational changes in the peptide accompanied by reorientation and translocation of the peptide at the membrane surface. The study suggests that melittin insertion into the outer membrane affects its permeability and capacitance, but does not disturb the membranes integrity, indicating a distinct mechanism of the peptide action on the outer membrane of Gram-negative bacteria.