Bacterial susceptibility and resistance to modelin-5

Abstract

Modelin-5 (M5-NH₂) killed Pseudomonas aeruginosa with a minimum lethal concentration (MLC) of 5.86 μM and strongly bound its cytoplasmic membrane (CM) with a K_d of 23.5 μM. The peptide adopted high levels of amphiphilic α-helical structure (75.0%) and penetrated the CM hydrophobic core (8.0 mN m^-1). This insertion destabilised CM structure via increased lipid packing and decreased fluidity (ΔG_mix < 0), which promoted high levels of lysis (84.1%) and P. aeruginosa cell death. M5-NH₂ showed a very strong affinity (K_d = 3.5 μM) and very high levels of amphiphilic α-helical structure with cardiolipin membranes (96.0%,) which primarily drove the peptide's membranolytic action against P. aeruginosa. In contrast, M5-NH₂ killed Staphylococcus aureus with an MLC of 147.6 μM and weakly bound its CM with a K_d of 117.6 μM, The peptide adopted low levels of amphiphilic α-helical structure (35.0%) and only penetrated the upper regions of the CM (3.3 mN m^-1). This insertion stabilised CM structure via decreased lipid packing and increased fluidity (ΔG_mix > 0) and promoted only low levels of lysis (24.3%). The insertion and lysis of the S. aureus CM by M5-NH₂ showed a strong negative correlation with its lysyl phosphatidylglycerol (Lys-PG) content (R² > 0.98). In combination, these data suggested that Lys-PG mediated mechanisms inhibited the membranolytic action of M5-NH₂ against S. aureus, thereby rendering the organism resistant to the peptide. These results are discussed in relation to structure/function relationships of M5-NH₂ and CM lipids that underpin bacterial susceptibility and resistance to the peptide.