Isolation of Lipoprotein Particles from Chicken Egg Yolk for the Study of Bacterial Pathogen Fatty Acid Incorporation into Membrane Phospholipids

Summary

This method provides a framework for studying incorporation of exogenous fatty acids from complex host sources into bacterial membranes, particularly Staphylococcus aureus. To achieve this, protocols for the enrichment of lipoprotein particles from chicken egg yolk and subsequent fatty acid profiling of bacterial phospholipids utilizing mass spectrometry are described.

Abstract

Staphylococcus aureus and other Gram-positive pathogens incorporate fatty acids from the environment into membrane phospholipids. During infection, the majority of exogenous fatty acids are present within host lipoprotein particles. Uncertainty remains as to the reservoirs of host fatty acids and the mechanisms by which bacteria extract fatty acids from the lipoprotein particles. In this work, we describe protocols for enrichment of low-density lipoprotein (LDL) particles from chicken egg yolk and determining whether LDLs serve as fatty acid reservoirs for S. aureus. This method exploits unbiased lipidomic analysis and chicken LDLs, an effective and economical model for the exploration of interactions between LDLs and bacteria. The analysis of S. aureus integration of exogenous fatty acids from LDLs is performed using high-resolution/accurate mass spectrometry and tandem mass spectrometry, enabling the characterization of the fatty acid composition of the bacterial membrane and unbiased identification of novel combinations of fatty acids that arise in bacterial membrane lipids upon exposure to LDLs. These advanced mass spectrometry techniques offer an unparalleled perspective of fatty acid incorporation by revealing the specific exogenous fatty acids incorporated into the phospholipids. The methods outlined here are adaptable to the study of other bacterial pathogens and alternative sources of complex fatty acids.

Introduction

Methicillin-resistant S. aureus (MRSA) is the leading cause of healthcare-associated infection and the associated antibiotic resistance is a considerable clinical challenge^1,2,3. Therefore, the development of novel therapeutic strategies is a high priority. A promising treatment strategy for Gram-positive pathogens is inhibiting fatty acid synthesis, a requirement for membrane phospholipid production that, in S. aureus, includes phosphatidylglycerol (PG), lysyl-PG, and cardiolipin⁴. In bacteria, fatty acid production occurs via the fa....

Protocol

NOTE: The following protocol for enrichment of LDL particles from chicken egg yolk is derived from Moussa et al. 2002³³.

1. Preparation of chicken egg yolk for enrichment of LDL particles

1. Sanitize two large chicken eggs by washing the shells with 70% ethanol solution and allow to air dry.
2. Sanitize the egg separator using 70% ethanol solution and allow to air dry. Attach the egg separator onto the lip of a medium sized beaker.
3. Crack each egg individually into the egg separator and allow the albumen to flow into the beaker. The intact egg yolk will be retained by the separator.

Results

The protocol for the enrichment of LDL from chicken egg yolk is illustrated in Figure 1. This process begins by diluting whole egg yolk with saline and separating the egg yolk solids referred to as granules from the soluble or plasma fraction containing the LDLs (Figure 1)³³. The LDL content of the plasma fraction is further enriched by precipitation of the ~ 30-40 kDa β-livetins (

Discussion

S. aureus incorporates exogenous fatty acids into its membrane phospholipids^27,32,43. Phospholipid synthesis using exogenous fatty acids bypasses FASII inhibition but also alters the biophysical properties of the membrane^27,32,44. While incorporation of exogenous fatty acids into phospholipids of Gram-positive pathogens is well.......

Materials

Name	Company	Catalog Number	Comments
Ammonium sulfate	Fisher	BP212R-1	≥99.5% pure
Cell culture incubator	Thermo	MaxQ 6000
Centrafuge	Thermo	75-217-420	Sorvall Legen XTR, rotor F14-6x250 LE
Costar assay plate	Corning	3788	96 well
Filter paper	Schleicher & Schuell	597
Large chicken egg	N/A	N/A	Common store bought egg
Microplate spectrophotometer	BioTek	Epoch 2
NaCl	Sigma	S9625
S. aureus strain AH1263	N/A	N/A	Provided by Alex Horswill of the University of Colorado
Dialysis tubing	Pierce	68700	7,000 MWCO
Tryptone	Becton, Dickison and Company	211705
0.5 mm zirconium oxide beads	Next Advance	ZROB05
Bullet Blender	Next Advance	BBX24B
Methanol (LC-MS grade)	Fisher	A4561
Chloroform (reagent grade)	Fisher	MCX10559
Isopropanol (LC-MS grade)	Fisher	A4611
Dimyristoyl phosphatidylcholine	Avanti Polar Lipids	850345C-25mg
Ammonium bicarbonate	Sigma	9830	≥99.5% pure
Ammonium formate	Sigma	70221-25G-F
Xcalibur software	Thermo Scientific	OPTON-30801
LTQ-Orbitrap Velos mass spectrometer	Thermo Scientific		high resolution/accurate mass MS
Agilent 1260 capillary HPLC	Agilent
SpeedVac Vacuum Concentrators	Thermo Scientific