2H Solid-State Nuclear Magnetic Resonance Investigation of Whole Escherichia coli Interacting with Antimicrobial Peptide MSI-78

A key aspect of the activity of antimicrobial peptides (AMPs) is their interaction with membranes. Efforts to elucidate their detailed mechanisms have focused on applying biophysical methods, including nuclear magnetic resonance (NMR), to AMPs in model lipid systems. However, these highly simplified systems fail to capture many of the features of the much more complex cell envelopes with which AMPs interact in vivo. To address this issue, we have designed a procedure to incorporate high levels of 2H NMR labels specifically into the cell membrane of Escherichia coli and used this approach to study the interactions between the AMP MSI-78 and the membranes of intact bacteria. The 2H NMR spectra of these membrane-deuterated bacteria can be reproduced in the absence and presence of MSI-78. Because the 2H NMR data provide a quantitative measure of lipid disorder, they directly report on the lipid bilayer disruption central to the function of AMPs, in the context of intact bacteria. Addition of MSI-78 to the bacteria leads to decreases in the order of the lipid acyl chains. The molar peptide:lipid ratios required to observe the effects of MSI-78 on acyl chain order are approximately 30 times greater than the ratios needed to observe effects in model lipid systems and approximately 100 times less than the ratios required to observe inhibition of cell growth in biological assays. The observations thus suggest that MSI-78 disrupts the bilayer even at sublethal AMP levels and that a large fraction of the peptide does not actually reach the inner membrane.