Characterization of an Acinetobacter baumannii lptD

Lipid A on the Gram-negative outer membrane (OM) is synthesized in the cytoplasm by the Lpx pathway and translocated to the OM by the Lpt pathway. Some Acinetobacter baumannii strains can tolerate complete loss of lipopolysaccharide (LPS) resulting from inactivation of early LPS pathway genes such as lpxC. Here, we characterized a mutant deleted for lptD, which encodes an OM protein that mediates the final translocation of fully synthesized LPS to the OM. Cells lacking lptD had a growth defect comparable to that of an lpxC::KmR mutant under the growth conditions tested, but were more sensitive to hydrophobic antibiotics, revealing a more significant impact on cell permeability from impaired LPS translocation than from loss of LPS synthesis. Consistent with this, adenosine triphosphate (ATP) leakage and N-phenyl-1-naphthylamine (NPN) fluorescence assays indicated a more severe impact of lptD deletion than lpxC deletion on inner and outer membrane permeability, respectively. Targeted liquid chromatography–mass spectrometry (LCMS) analysis of LPS intermediates from UDP-3-O-R-3-hydroxylauroyl-N-acetyl-α-D-glucosamine through lipid IVA, showed that loss of LptD caused an accumulation of lipid IVA. This suggested that pathway intermediate accumulation or mislocalization caused by blockage of later LPS pathway steps impact envelope integrity. Supporting this notion, chemical inhibition of lipid A precursor enzymes including LpxC and Fab B/F in the lptD::KmR strain partially rescued growth and permeability defects