Structural lipid modifications associated with the persister state in Acinetobacter baumannii: LC-TIMS-TOF for collision cross section determination and focus on uncommon wax esters.

The bacterial membrane is the first barrier against environmental variations and is mainly composed of lipids which have a key role in bacterial growth. Studying them is important for a better understanding of bacterial cell physiology. Notably for persister cells, which constitute a bacterial subpopulation able to survive high concentrations of antibiotics. This work complements a preliminary study of Acinetobacter baumannii persister cell lipids. Here, the coupling of liquid chromatography, ion mobility spectrometry, and tandem mass spectrometry (LC-IMS-MS/MS) allowed a more complete and comprehensive characterization of all lipids in A. baumannii antibiotic persister cells. Moreover, experimental collision cross section (CCS) values for all these lipid species were determined, which should help future annotations, in particular for uncommon lipids such as wax esters (WEs) and 1-phosphatidyl-2-acyl-glycerol-3-phosphoethanolamine (PAGPE), which are reported for the first time in this work. We showed that hydroxylation of fatty acyl chains significantly increased CCS values. Finally, we showed that ammonium and ethylamine adducts can be formed for WEs and that precautions must be taken in WE identification. To this end, this work proposes an analytical methodology and associated precautions for lipid annotation in persister cells using three identification criteria: m/z, retention time, and CCS.