Inhibitory effect of short cationic homopeptides against Gram-negative bacteria

Previous work demonstrated that Lys homopeptides with an odd number of residues (9, 11 and 13) were capable of inhibiting the growth of Gram-positive bacteria in a broader spectrum and more efficiently than those with an even number of Lys residues or Arg homopeptides of the same size. Indeed, all Gram-positive bacteria tested were totally inhibited by 11-residue Lys homopeptides. In the present work, a wide variety of Gram-negative bacteria were used to evaluate the inhibitory activity of chemically synthesized homopeptides of l-Lys and l-Arg ranging from 7 to 14 residues. Gram-negative bacteria were comparatively more resistant than Gram-positive bacteria to Lys homopeptides with an odd number of residues, but exhibited a similar inhibition pattern than on Gram-positive bacteria. CD spectra for the odd-numbered Lys homopeptides in anionic lipid dimyristoylphosphatidylglycerol, and Escherichia coli membrane extract increased polyproline II content, as compared to those measured in phosphate buffer solution. Lys and Arg homopeptides were covalently linked to rhodamine to visualize the peptide interactions with E. coli cells using confocal laser scanning microscopy. Analysis of Z-stack images showed that Arg homopeptides indeed appear to be localized intracellularly, while the Lys homopeptide is localized exclusively on the plasma membrane. Moreover, these Lys homopeptides induced membrane disruption since the Sytox fluorophore was able to bind to the DNA in E. coli cultures.