Phospholipid composition of the outer membrane of Escherichia coli influences its susceptibility against antimicrobial peptide apidaecin 1b

Proline-rich antimicrobial peptides (PrAMPs) kill bacteria in a multimodal mechanism by inhibiting the 70S ribosome (i.e., protein translation) as dominant lethal mechanism besides inhibition of several other proteins, such as chaperone DnaK. PrAMPs pass the outer membrane of Gram-negative bacteria, probably by a self-promoted uptake followed by a transporter-mediated uptake from the periplasm. Mutation of transporter protein SbmA is a well-studied resistance mechanism observed in vitro by resistance induction with PrAMPs. Here, we compared the membrane compositions of Escherichia coli BL21AI and BL21AI Apir, which was obtained by resistance induction with PrAMP apidaecin 1b. Lipid A was partially modified by phosphatidylethanolamine, 4-aminoarabinose, or both groups, but the relative contents of these and further unidentified species did not differ much between wild-type and resistant strains, indicating that resistance was not related to lipid A modifications. The same was true for 20 glycerophospholipids identified, i.e., 11 phosphatidylethanolamines and 9 phosphatidylglycerols. However, glycerophospholipids in BL21AI Apir contained much lower levels of cyclopropane-modified acyl groups, which probably alter the biophysical properties of the inner membrane and the inner leaflet of the outer membrane. Indeed, when cyclopropane-fatty-acyl-phospholipid synthase was knocked out in E. coli BW25113, the resulting BW25113 Δcfa was less susceptible against apidaecin 1b.