Bacterial susceptibility and resistance to modelin-5.

Modelin-5 (M5-NH 2 ) 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 (Δ G mix < 0), which promoted high levels of lysis (84.1%) and P. aeruginosa cell death. M5-NH 2 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-NH 2 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 (Δ G mix > 0) and promoted only low levels of lysis (24.3%). The insertion and lysis of the S. aureus CM by M5-NH 2 showed a strong negative correlation with its lysyl phosphatidylglycerol (Lys-PG) content ( R 2 > 0.98). In combination, these data suggested that Lys-PG mediated mechanisms inhibited the membranolytic action of M5-NH 2 against S. aureus , thereby rendering the organism resistant to the peptide. These results are discussed in relation to structure/function relationships of M5-NH 2 and CM lipids that underpin bacterial susceptibility and resistance to the peptide.