Influence of the lipid composition on the membrane affinity, and the membrane-perturbing ability of cetylpyridinium chloride

Cetylpyridinium chloride (CPC) is used for dental plaque control and has been shown to interact with Streptococcus mutans biofilms. Part of this effect may be related to interactions with bacterial plasmic membranes. We investigated the affinity of CPC for model bacteria membranes to examine the hypothesis that negatively charged membranes could favour the bactericide power of CPC. First, we determined the association constant (K) of CPC with model lipid membranes from isothermal titration calorimetry, and the influence of lipid composition on this affinity. Second, we tested the CPC influence on membrane permeability, by measuring the release of encapsulated fluorescent calcein in liposomes. We investigated four model membranes: one with only zwitterionic 1-palmitoyl-2-oleoyl-sn-glycerol-3-phosphocholine, two containing anionic phospholipids (1-palmitoyl-2-oleoyl-sn-glycerol-3-phosphoglycerol and cardiolipin), and one containing cholesterol. Our results revealed a limited influence of electrostatic interactions on K, indicating that the hydrophobic factor was predominant in the association of this surfactant with phospholipid membranes. CPC could induce release from the three model membranes composed only of phospholipids, with a kinetic of release more complex for negatively charged membranes. We suggest that the release process implies the passage of the detergent from the outer sheet of the bilayer to the inner one, and the process would be slowed down by the interaction of the CPC head group with anionic phospholipids. The cholesterol-containing membranes showed a smaller K and remain impermeable in the presence of CPC. The stiffness of the cholesterol-containing membrane may restrict the insertion of CPC in the membranes and limits its translocation across the membrane. The thermodynamic model of the partitioning of CPC micelle and lipid bilayer has also been demonstrated.