Interaction of colistin with lipids in liposomes and monolayers

Abstract

Colistin is an antibiotic member of the polymixin family, showing a high amphipathic character. Its mechanism of action has been related to its ability to disrupt phospholipid bilayers of the bacterial membrane. Due to its hydrophobic properties colistin can interact both with the polar heads and the alkyl chains of the phospholipids. Moreover, it has a polycationic structure, so its interactions with phospholipids should be highly dependent on the electric charge of the lipid and the ionisation state of polymixin molecule. In the present paper we report on physicochemical studies to define the type and characteristics of these interactions. The surface activity of colistin has been shown to be highly dependent on the pH of the medium, the less protonated forms being more stable at the air–water interface. Interaction with phospholipid monolayers shows the same tendency. Colistin is also able to form mixed monolayers with phospholipids with small deviations from ideality. Physicochemical studies carried out with fluorescent probes indicate the presence of pure colistin aggregates that can coexist with mixed micelles composed of colistin/phospholipids. In no case did the interaction cause significative changes in the transition temperature of phospholipids.