Lipopolysaccharides of polymyxin B‐resistant mutants of Escherichia coii are extensively substituted by 2‐aminoethyl pyrophosphate and contain aminoarabinose in lipid A

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Abstract
Lipopolysaccharides (LPS) of two polymyxin-resistant (pmr) mutants and the corresponding parent strain of Escherichia Coli were chemically analysed for composition and subjected to 31P-NMR (nuclear magnetic resonance) for assessment of phosphate substitution. Whereas the saccharide portions, fatty acids, and phosphate contents were similar in wild-type and pmr LPS, the latter contained two- to threefold higher amounts of 2-aminoethanol. The pmr LPS also contained 4-amino-4-deoxy-l-arabinopyranose (l-Arap4N), which is normally not a component of E. coli LPS. This aminopentose has been assigned to be linked to the 4′-phosphate of lipid A. Comparative 31P-NMR analysis of the de-O-acylated LPS of the wild-type and pmr strains revealed that phosphate groups of the pmr LPS were mainly (71-79%) diphosphate diesters, which accounted for only 20% in the wild-type LPS. Diphosphate monoesters were virtually nonexistent in the pmr LPS, whereas they accounted for 42% of all phosphates in wild-type LPS. In the lipid A of the pmr strains, the 4′-phosphate was to a significant degree (35%) substituted by l-Arap4N, whereas in the wild-type LPS the l-ArapN was absent. In the pmr lipid A1 2-aminoethanol was completely substituting the glycosidic pyrophosphate but not the glycosidic monophosphate, forming a diphosphate diester linkage at this position in 40% of lipid A molecules. In the wild-type LPS the glycosidic position of lipid A carried mostly unsubstituted monophosphate and pyrophosphate. Thus the polymyxin resistance was shown to be associated, along with the esterification of the lipid A 4′-monophosphate by aminoarabinose, with extensive esterification of diphosphates in LPS by 2-aminoethanol.