The antibiotic activity of cationic linear amphipathic peptides:: lessons from the action of leucine/lysine copolymers on bacteria of the class Mollicutes

Peptides composed of leucyl and lysyl residues ('LK peptides') with different compositions and sequences were compared for their antibacterial activities using cell wall-less bacteria of the class Mollicutes (acholeplasmas, mycoplasmas and spiroplasmas) as targets. The antibacterial activity of the amphipathic alpha-helical peptides varied with their size, 15 residues being the optimal length, independent of the membrane hydrophobic core thickness and the amount of cholesterol. The 15-residue ideally amphipathic alpha helix with a +5 positive net charge (KLLKLLLKLLLKLLK) had the strongest antibacterial activity, similar to that of melittin. In contrast, scrambled peptides devoid of amphipathy and the less hydrophobic beta-sheeted peptides [(LK) (n) K], even those 15-residue long, were far less potent than the helical ones. Furthermore, the growth inhibitory activity of the peptides was correlated with their ability to abolish membrane potential. These data are fully consistent with a predominantly flat orientation of LK peptides at the lipid/water interface and strongly supports that these peptides and probably the linear polycationic amphipathic defence peptides act on bacterial membranes in four main steps according to the 'carpet' model: (a) interfacial partitioning with accumulation of monomers on the target membrane (limiting step); (b) peptide structural changes (conformation, aggregation, and orientation) induced by interactions with the lipid bilayer (as already shown with liposomes and erythrocytes); (c) plasma membrane permeabilization/depolarization via a detergent-like effect; and (d) rapid bacterial cell death if the extent of depolarization is maintained above a critical threshold.