Resistance to the antimicrobial peptide polymyxin requires myristoylation of Escherichia coli and Salmonella typhimurium lipid A

Attachment of positively charged, amine-containing residues such as 4-amino-4-deoxy-L-arabinose (L-Ara4N) and phosphoethanolamine (pEtN) to Escherichia coli and Salmonella typhimurium lipid A is required for resistance to the cationic antimicrobial peptide, polymyxin. In an attempt to discover additional lipid A modifications important for polymyxin resistance, we generated polymyxin-sensitive mutants of an E. coli pm-rAC strain, WD101. A subset of polymyxin-sensitive mutants produced a lipid A that lacked both the 3'-acyloxyacyllinked myristate (C-14) and L-Ara4N, even though the necessary enzymatic machinery required to synthesize L-Ara4N-modified lipid A was present. Inactivation of lpxM in both E. coli and S. typhimurium resulted in the loss of L-Ara4N addition, as well as, increased sensitivity to polymyxin. However, decoration of the lipid A phosphate groups with pEtN residues was not effected in lpxM mutants. In summary, we demonstrate that attachment of L-Ara4N to the phosphate groups of lipid A and the subsequent resistance to polymyxin is dependent upon the presence of the secondary linked myristoyl group.