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Updated: Aug 23, 2025
Isolation and Chemical Characterization of Lipid A from Gram-negative Bacteria
Published on: September 16, 2013
A Sensitive GC-MS Method for Quantitation of Lipid A Backbone Components and Terminal Phosphate Modifications
Matthew E Sherman1, Richard D Smith1, Francesca M Gardner1
1Department of Microbial Pathogenesis, University of Maryland─Baltimore, Baltimore, Maryland 21201, United States.
Abstract:
Lipid A, the hydrophobic anchor of lipopolysaccharide (LPS) present in the outer membrane of Gram-negative bacteria, serves as a target for cationic antimicrobial peptides, such as polymyxins. Membrane stress from polymyxins results in activation of two-component regulatory systems that produce lipid A modifying enzymes. These enzymes add neutral moieties, such as aminoarabinose (AraN) and ethanolamine (EtN) to lipid A terminal phosphates that mask the phosphate's negative charge and inhibit electrostatic interaction with the cationic polymyxins. Currently, these modifications may be detected by MALDI-TOF MS; however, this analysis is only semiquantitative. Herein we describe a GC-MS method to quantitate lipid A backbone components, glucosamine (GlcN) and inorganic phosphate (Pi), along with terminal phosphate modifications AraN and EtN. In this assay, lipid A is isolated from Gram-negative bacterial samples, hydrolyzed into its individual moieties, and derivatized via methoximation followed by silylation prior to analysis via GC-MS. Changes in AraN and EtN quantity were characterized using a variety of regulatory mutants of Salmonella, revealing differences that were not detected using MALDI-TOF MS analysis. Additionally, an increase in the abundance of AraN and EtN modifications were observed when resistant Enterobacter and Escherichia coli strains were grown in the presence of colistin (polymyxin E). Lastly, increased levels of Pi were found in bisphosphorylated lipid A compared to monophosphorylated lipid A samples. Because lipid A modifications serve as indicators of polymyxin resistance in Gram-negative bacteria, this method provides the capacity to monitor polymyxin resistance by quantification of lipid A modification using GC-MS.
Insights
A new GC-MS method quantifies lipid A modifications in Gram-negative bacteria, aiding polymyxin resistance monitoring. This technique reveals differences missed by MALDI-TOF MS and detects increased modifications in resistant strains and colistin exposure.
Area of Science:
- Microbiology
- Analytical Chemistry
- Biochemistry
Background:
- Lipid A anchors lipopolysaccharide (LPS) in Gram-negative bacteria and is targeted by polymyxin antibiotics.
- Polymyxin-induced membrane stress activates regulatory systems producing lipid A modifying enzymes.
- These enzymes add neutral moieties (aminoarabinose and ethanolamine) to lipid A, hindering polymyxin binding and conferring resistance.
Purpose of the Study:
- To develop and validate a quantitative gas chromatography-mass spectrometry (GC-MS) method for analyzing lipid A modifications.
- To compare the sensitivity of the new GC-MS method with MALDI-TOF MS for detecting lipid A modifications.
- To investigate the role of lipid A modifications in polymyxin resistance in various Gram-negative bacteria.
Main Methods:
- Lipid A was isolated from bacterial samples and hydrolyzed into constituent components.
- Components were derivatized using methoximation and silylation.
- Analysis was performed using GC-MS to quantify lipid A backbone (glucosamine, inorganic phosphate) and modification (aminoarabinose, ethanolamine) moieties.
Main Results:
- The GC-MS method successfully quantified lipid A backbone components and terminal phosphate modifications.
- Significant differences in aminoarabinose and ethanolamine levels were detected in *Salmonella* mutants, surpassing MALDI-TOF MS sensitivity.
- Increased lipid A modifications were observed in colistin-resistant *Enterobacter* and *Escherichia coli* strains and in bisphosphorylated lipid A.
Conclusions:
- The developed GC-MS method provides a quantitative approach to analyze lipid A modifications.
- This method enhances the detection of polymyxin resistance mechanisms in Gram-negative bacteria.
- Quantification of lipid A modifications via GC-MS can serve as a valuable tool for monitoring antibiotic resistance.
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