(537d) Insights Into the Lytic Mechanism of Antimicrobial Piscidin 1 and 3 Using QCM-D

Antimicrobial peptides (AMPs) are naturally occurring antibiotics which induce cell death in bacteria by destabilizing their membranes.  Unlike traditional antibiotics, AMPs do not induce antibiotic resistance in pathogens and have additional immunomodulatory and metabolic functions, making them good candidates as novel agents in antibacterial therapies.

In this work, we focused on piscidin 1 (P1) and 3 (P3), which are twenty-two-residue-long amphipathic, cationic AMPs isolated from hybrid striped bass.  Extensive work has been done to characterize the atomic-level structure of both P1 and P3 in magnetically and mechanically aligned bilayers using solid-state NMR.  Here we investigated the mechanism of action of both P1 and P3 in supported lipid bilayers (SLB), using a quartz crystal microbalance with dissipation monitoring (QCM-D).  In order to mimic bacterial cell membranes, we selected a 3:1 ratio of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) : 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG).  Liposomes were deposited onto a QCM-D silicon oxide crystal until SLB formation, then P1 and P3 were introduced into the system and the interaction between the peptide and the lipid system was monitored by changes in frequency (Δf) and energy dissipation (ΔD).  Different concentrations (2-20 μM) of the peptide and buffer conditions (pH 6.0 and 7.4) were tested.  In particular we observed a Δf increase (~ mass removal) as we increased the concentration of P1 at pH 7.4, suggesting lipid removal, membrane thinning, or pore formation; while at pH 6.0 the Δf decrease (~ mass addition) was probably due to the electrostatic interaction between the more cationic peptide and the anionic POPG.  P1 and P3 actions were also compared.  In all cases, ΔD values were a clear indication of the destabilization of the SLB.  Overall, this study, which shows how P1 and P3 disrupt a bacterial membrane mimic, helps us better understand the mode of action of piscidin and related AMPs.