Abstract
Bacteria inhabit a vast range of biological niches and have evolved diverse mechanisms to cope with environmental stressors. The genus Acinetobacter comprises a complex group of Gram-negative bacteria. Some of these bacteria such as A. baumannii are nosocomial pathogens. They are often resistant to multiple antibiotics and are associated with epidemic outbreaks. A. radioresistens is generally considered to be a commensal bacterium on human skin or an opportunistic pathogen. Interestingly, this species has exceptional resistance to a range of environmental challenges which contributes to its persistence in clinical environment and on human skin. We studied changes in its lipid composition induced by the onset of stationary phase. This strain produced triglycerides (TG) as well as four common phospholipids: phosphatidylethanolamine (PE), phosphatidylglycerol (PG), cardiolipin (CL) and lysocardiolipin (LCL). It also produced small amounts of acyl-phosphatidylglycerol (APG). As the bacterial growth entered the stationary phase, the lipidome switched from one dominated by PE and PG to another dominated by CL and LCL. Surprisingly, bacteria in the stationary phase produced N-acyl-phosphatidylethanolamine (NAPE) and another rare lipid we tentatively name as 1-phosphatidyl-2-acyl-glycero-3-phosphoethanolamine (PAGPE) based on tandem mass spectrometry. It is possible these tri-acylated lipids play an important role in coping with nutrient depletion.
Highlights
Bacteria thrive in every corner of the biosphere
Since the lipidome of this bacterium was observed to transition from a PE and PG-dominated state in the exponential growth phase to a CL and LCL-dominated state in the stationary phase, we postulate that phosphatidyl-2-acyl-glycero-3 -phophoethanolamine (PAGPE) with a compatible shape to LCL and CL may serve as a better charge modulator than the much smaller PE in the bacterial membrane during stationary phase
We were able to carry out a thorough survey of fatty acid compositions by tandem mass spectrometry in the most abundant PG anions (Supplementary Table S2)
Summary
Bacteria thrive in every corner of the biosphere They have evolved a diverse array of survival adaptations to cope with environmental stressors such as starvation, desiccation, change in temperature, pH and salinity. Cardiolipin synthase activity has been observed to increase approximately 10-fold in the Gram-negative model organism Escherichia coli, suggesting CL is an essential membrane component to cope with starvation[4]. By searching for precursors of m/z 140 deprotonated phosphoethanolamine head group fragment in lipids extracted from Gram-negative bacteria Acinetobacter radioresistens, we observed a lipid species similar in size to lysocardiolipin (LCL), and tentatively assigned its chemical structure as 1-phosphatidyl-2-acyl-glycero-3 -phophoethanolamine (PAGPE). The lipid extracts contained large amounts of triglycerides (TG) which likely serves as an fatty acid reserve Biosynthesis of these tri-acylated species appear to be an evolved adaptation in A. radioresistens
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