Bacteria may cope differently to similar membrane damage caused by the Australian tree frog antimicrobial peptide maculatin 1.1* [Lipids]

June 22nd, 2015 by Sani, M.-A., Henriques, S. T., Weber, D., Separovic, F.

Maculatin 1.1. (Mac1) is an antimicrobial peptide from the skin of Australian tree frogs and is known to possess selectivity towards Gram-positive bacteria. Although Mac1 has membrane-disrupting activity, it is not known how Mac1 selectively targets Gram-positive over Gram-negative bacteria. The interaction of Mac1 with E. coli, S. aureus and human red blood cells (hRBC), and with their mimetic model membranes, is here reported. The peptide showed a 16-fold greater growth inhibition activity against S. aureus (4 μM) than against E. coli (64 μM) and an intermediate cytotoxicity against hRBC (30 μM). Surprisingly, Sytox Green uptake monitored by flow cytometry showed that Mac1 compromised both bacterial membranes with similar efficiency at about 20-fold lower concentration than the reported minimum inhibition concentration (MIC) against S. aureus. Mac1 also reduced the negative potential of S. aureus and E. coli membrane with similar efficacy. Furthermore, liposomes mimicking the cell membrane of S. aureus (POPG/TOCL) and E. coli (POPE/POPG) were lysed at similar concentrations while hRBC-like vesicles (POPC/SM/Chol) remained mostly intact in the presence of Mac1. Remarkably, when POPG/TOCL and POPE/POPG liposomes were co-incubated, Mac1 did not induce leakage from POPE/POPG liposomes, suggesting a preference towards POPG/TOCL membranes that was supported by surface plasma resonance assays. Interestingly, circular dichroism spectroscopy showed a similar helical conformation in the presence of the anionic liposomes but not the hRBC mimics. Overall, the study showed that Mac1 disrupts bacterial membranes in a similar fashion before cell death events and would preferentially target S. aureus over E. coli or hRBC membranes.