Bacterial susceptibility and resistance to modelin-5.

Abstract

Modelin-5 (M5-NH₂) killed <i>Pseudomonas aeruginosa</i> with a minimum lethal concentration (MLC) of 5.86 μM and strongly bound its cytoplasmic membrane (CM) with a <i>K</i>_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 <i>via</i> increased lipid packing and decreased fluidity (Δ<i>G</i>_mix < 0), which promoted high levels of lysis (84.1%) and <i>P. aeruginosa</i> cell death. M5-NH₂ showed a very strong affinity (<i>K</i>_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 <i>P. aeruginosa</i>. In contrast, M5-NH₂ killed <i>Staphylococcus aureus</i> with an MLC of 147.6 μM and weakly bound its CM with a <i>K</i>_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 <i>via</i> decreased lipid packing and increased fluidity (Δ<i>G</i>_mix > 0) and promoted only low levels of lysis (24.3%). The insertion and lysis of the <i>S. aureus</i> CM by M5-NH₂ showed a strong negative correlation with its lysyl phosphatidylglycerol (Lys-PG) content (<i>R</i>² > 0.98). In combination, these data suggested that Lys-PG mediated mechanisms inhibited the membranolytic action of M5-NH₂ against <i>S. aureus</i>, 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.