Cationic antimicrobial peptides (CAMPs), such as polymyxins, are used as a last-line defense in treatment of many bacterial infections. However, some bacteria have developed resistance mechanisms to survive these compounds. Current pandemic O1 biotype El Tor is resistant to polymyxins, whereas a previous pandemic strain of the biotype Classical is polymyxin-sensitive. The operon found in El Tor confers >100-fold resistance to antimicrobial peptides through aminoacylation of lipopolysaccharide (LPS), expected to decrease the negatively charged surface of the outer membrane. This Gram-negative system bears striking resemblance to a related Gram-positive cell-wall remodeling strategy that also promotes CAMP resistance. Mutants defective in AlmEF-dependent LPS modification exhibit reduced fitness Here, we present investigation of AlmG, the hitherto uncharacterized member of the AlmEFG pathway. Evidence for AlmG glycyl to lipid substrate transferase activity is demonstrated by heterologous expression of pathway enzymes in a specially engineered strain. Development of a minimal keto-deoxyoctulosonate (Kdo)-lipid A domain in was necessary to facilitate chemical structure analysis and to produce a mimetic Kdo-lipid A domain AlmG substrate to that synthesized by Our biochemical studies support a uniquely nuanced pathway of Gram-negative CAMPs resistance and provide a more detailed description of an enzyme of the pharmacologically relevant lysophosphospholipid acyltransferase (LPLAT) superfamily.