This study shows that lipid A of
Escherichia coliO157 : H7 differs from that of E. coliK-12 in that it has a phosphoform at the C-1 position, which is distinctively modified by a phosphoethanolamine (PEtN) moiety, in addition to the diphosphoryl form. The pmrCgene responsible for the addition of PEtN to the lipid A of E. coliO157 : H7 was inactivated and the changes in lipid A profiles were assessed. The pmrCnull mutant still produced PEtN-modified lipid A species, albeit in a reduced amount, indicating that PmrC was not the only enzyme that could be used to add PEtN to lipid A. Natural PEtN substitution was shown to be present in the lipid A of other serotypes of enterohaemorrhagic E. coliand absent from the lipid A of E. coliK-12. However, the cloned pmrCO157gene in a high-copy-number plasmid generated a large amount of PEtN-substituted lipid A species in E. coliK-12. The occurrence of PEtN-substituted lipid A species was associated with a slight increase in the MICs of cationic peptide antibiotics, suggesting that the lipid A modification with PEtN would be beneficial for survival of E. coliO157 : H7 in certain environmental niches. However, PEtN substitution in the lipid A profiles was not detected when putative inner-membrane proteins (YhbX/YbiP/YijP/Ecf3) that show significant similarity with PmrC in amino acid sequence were expressed from high-copy-number plasmids in E. coliK-12. This suggests that these potential homologues are not responsible for the addition of PEtN to lipid A in the pmrCmutant of E. coliO157 : H7. When cells were treated with EDTA, the amount of palmitoylated lipid A from the cells carrying a high-copy-number plasmid clone of pmrCO157that resulted in significant increase of PEtN substitution was unchanged compared with cells without PEtN substitution, suggesting that the PEtN moiety substituted in lipid A does not compensate for the loss of divalent cations required for bridging neighbouring lipid A molecules.