Analyses of dose–response curves to compare the antimicrobial activity of model cationic α-helical peptides highlights the necessity for a minimum of two activity parameters

To assess and compare different model Leu-Lys-containing cationic α-helical peptides, their antimicrobial activities were tested against Escherichia coli as target organism over a broad peptide concentration range. The natural cationic α-helical peptides magainin 2 and PGLa and the cyclic cationic peptide gramicidin S were also tested between comparison. The dose–response curves differed widely for these peptides, making it difficult to rank them into an activity order over the whole concentration range. We therefore compared five different inhibition parameters from dose–response curves: ICmin (lowest concentration leading to growth inhibition), IC50 (concentration that gives 50% growth inhibition), ICmax (related to minimum inhibition concentration and minimum bactericidal concentration), inhibition concentration factor (ICF; describing the increase in concentration of the peptide between minimum and maximum inhibition), and activity slope (AS; related to the Hill coefficient). We found that these parameters were covariant: two of them sufficed to characterize the dose dependence and hence the activity of the peptides. This was corroborated by showing that the dose dependences followed the Hill equation, with a small, constant aberration. We propose that the activity of antimicrobial peptides can readily be characterized by both IC50 and ICF (or AS) rather than by a single parameter and discuss how this may relate to investigations into their mechanisms of action.