The MPLEx Protocol for Multi-omic Analyses of Soil Samples

Summary

A protocol is presented for simultaneously extracting metabolites, proteins, and lipids from a single soil sample, allowing reduced sample preparation times and enabling multi-omic mass spectrometry analyses of samples with limited quantities.

Abstract

Mass spectrometry (MS)-based integrated metaproteomic, metabolomic, and lipidomic (multi-omic) studies are transforming our ability to understand and characterize microbial communities in environmental and biological systems. These measurements are even enabling enhanced analyses of complex soil microbial communities, which are the most complex microbial systems known to date. Multi-omic analyses, however, do have sample preparation challenges, since separate extractions are typically needed for each omic study, thereby greatly amplifying the preparation time and amount of sample required. To address this limitation, a 3-in-1 method for the simultaneous extraction of metabolites, proteins, and lipids (MPLEx) from the same soil sample was created by adapting a solvent-based approach. This MPLEx protocol has proven to be both simple and robust for many sample types, even when utilized for limited quantities of complex soil samples. The MPLEx method also greatly enabled the rapid multi-omic measurements needed to gain a better understanding of the members of each microbial community, while evaluating the changes taking place upon biological and environmental perturbations.

Introduction

Evaluating soil microbial communities has important implications for understanding carbon cycling and climate change. Recent studies have however highlighted difficulties, such as the lack of sequenced genomes for microbiota in various soil types and the unknown function of many of the proteins detected. These challenges result due to soil being the most complex microbial community known to date^1,2,3. Multi-omic analyses, which combine results from metagenomic, metatranscriptomic, metaproteomic, metabolomic, and lipidomic studies, have recently been implemented in numerous so....

Protocol

NOTE: Very wet soils can be lyophilized prior to extraction without detriment to the effectiveness of the extraction. Wet soil can also be used but should be considered when adding reagents at specific ratios.

NOTE: It is recommended to use 20 g of dry soil weight per extraction, which must be split between two 50 mL tubes (maximum of 10 g soil per 50 mL tube). Extractions can be scaled up or down dependent upon available sample.

NOTE: Dry soil samples can be sieved through a 3 mm screen in order to homogenize and remove small roots and rocks. Do not sieve wet soil samples, as the sample will get stuck in the scr....

Results

When the MPLEx protocol was used to extract molecules from Kansas native prairie soil (a Mollisol soil), the triplicate analyses provided results for 3376 peptides, 105 lipids, and 102 polar metabolites (all unique identifications). While the MPLEx protocol has been well-established for general extraction of lipids and metabolites^12,13,14,15,

Discussion

It is important to note that not all laboratories will have the same available equipment so certain methods, for example the lysis step, can be adapted. Here we use vortexing and sonicating, however the use of a large 50 mL bead beater would work. If a lyophilizer with a collector temperature capable of -105 °C is not available, then samples can be dried under a nitrogen stream. Also soil types vary greatly and can include sand, silt, clay, peat, and loam (etc.), and they can also vary based on pH, salinity.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Chloroform	Sigma-Aldrich	650498	Stored at -20°C !Caution chloroform has acute potential health effects, skin irritation and possible chemical burns, irritation to the respiratory system, may affect the kidneys, liver, heart. Wear suitable protective glasses, clothing and gloves, work in a fume hood.
Methanol	Sigma-Aldrich	34860	Stored at -20°C !Caution Methanol may cause respiratory tract, skin and eye irritation, may damage the nerves, kidneys and liver. Wear suitable protective glasses, clothing and gloves, work in a fume hood.
Purified water from Millipore	Milli-Q		Water purification system.
Sodium dodecyl sulfate	Sigma-Aldrich	L6026	!Caution SDS causes acute toxicity and is flammable. It is a skin, eye and airway irritant. Wear gloves and safety glasses.
Soil protein extraction kit	MoBio, NoviPure Soil Protein Extraction Kit, Qiagen	30000-20
DL-dithiothreitol	Sigma-Aldrich	43815
1M Trizma HCL	Sigma-Aldrich	T2694
Trichloroacetic acid	Sigma-Aldrich	T0699	!Caution TCA is caustic, toxic and may cause skin burns. Wear gloves and safety glasses.
Acetone	Sigma-Aldrich	650501	Stored at -20°C !Caution Acetone may cause respiratory tract and skin and eye irritation. Flammable liquid and vapor. Wear safety glasses gloves and a lab coat, work in a fume hood.
Urea	Sigma-Aldrich	208884	!Caution Urea is an eye and skin irritant, use gloves and safety glasses
Ammonium bicarbonate	Fluka	09830
Trypsin	Promega	V528A	20µg vials
Bicinchoninic acid protein assay kit	Pierce	23227
Ammonium Formate	Sigma-Aldrich	09735
Acetonitrile	Sigma-Aldrich	34998	!Caution Acetonitrile is a skin and eye irritant. Highly flammable. Wear gloves and safety glasses. Work in a fume hood.
Trifluoroacetic acid	Sigma-Aldrich	T6508	!Caution TFA is extremely hazardous in case of skin contact, eye contact, ingestion and inhalation. May produce tissue damage particularly on mucous membranes of eyes, mouth and respiratory tract. Skin contact may produce burns. Wear gloves, lab coat, safety glasses and work in a fume hood.
Methoxyamine hydrochloride	Sigma-Aldrich	226904	!Caution Methoxyamine hydrochloride causes severe burns and serious damage to eyes, may cause sensitization by skin contact. Wear safety glasses, gloves and lab coat, work in a fume hood.
Pyridine	Sigma-Aldrich	270970	!Caution Pyridine can cause skin and eye irritation, central nervous system depression. Vapor may cause flash fire. Wear safety glasses, gloves and lab coat, work in a fume hood.
N-Methyl-N-(trimethylsilyl)trifluoroacetamide with 1% trimethylchlorosilane	Sigma-Aldrich	69478	!Caution MSTFA + 1% TMCS can cause skin corrosion, serious eye damage and specific target organ toxicity. Flammable liquid and vapor. Wear safety glasses, gloves and lab coat, work in a fume hood.
Potassium chloride	Sigma-Aldrich	P9541
Milli-Q water purification system	Millipore		model MPGP04001
Vortex	Scientific Industries	SI-0236	Vortex Genie 2
Probe sonicator	FisherBrand		model FB505
Refrigerated centrifuge	Eppendorf		model 5810R
50mL tube swinging bucket rotor	Eppendorf	A-4-44
50mL fixed angle rotor	Eppendorf	FA-45-6-30
Balance	OHAUS		model V22PWE150IT
Serological pipette controller	Eppendorf	12-654-100
10mL, 25mL glass serological pipettes	FisherBrand	13-678-27F, 13-678-36D
Thermomixer with Thermotop	Eppendorf	5382000015, 5308000003
0.9 – 2.0 mm blend stainless steel beads	NextAdvance	SSB14B
0.15 mm garnet beads	MoBio	13122-500
Magnetic stir plate	FisherBrand	11-100-16SH
Magnetic stir bar	FisherBrand	14512130
pH paper strips, pH range 0–14	FisherBrand	M95903
15mL, 50mL conical polypropylene centrifuge tube	Genesee Scientific	21-103 21-108	chloroform compatible
50mL vortex attachment	MoBio	13000-V1-50
Ice bucket	FisherBrand	02-591-44
27.25x70mm glass vials	FisherBrand	03-339-22K
Breathe Easier plate membranes	Midwest Scientific	BERM-2000
Alcohol wipes	Diversified Biotech	BPWP-1000
Heater shaker incubator	Benchmark, Incu-Shaker Mini
Analog rotisserie tube rotator	SoCal BioMed, LLC	82422001
Filter-Aided-Sample-Prep kit	FASP; Expedeon	44250
Microplate reader	Biotek, EPOCH
-20 Degree Celsius Freezer	Fisher	13986149
-80 Degree Celsius Freezer	Stirling Ultracold	SU78OUE
Q-Exactive ion trap mass spectrometer	Thermo Scientific
Agilent 7890A gas chromatograph coupled with a single quadrupole 5975C mass spectrometer	Agilent Technologies, Inc.
LTQ-Orbitrap Velo	Thermo Scientific
Waters NanoEquityTM UPLC system	Millford, MA
250mL media bottle	FisherBrand	1395-250
Waters vial	Waters	186002805
Glass MS sample vial and inserts	MicroSolv	9502S-WCV, 9502S-02ND
Glass HPLC vial and snap caps	MicroSolv	9512C-0DCV, 9502C-10C-B
HPLC 96-well plate	Agilent	5042-6454
Large glass vial 27.25x70mm	FisherBrand	03-339-22K
Lyophilizer	Labconco	7934021
Polished stainless steel flat head spatula	Spoonula; FisherBrand	14-375-10
Kim wipes	Kimberly-Clark	34721
XBridge C18, 250x4.6 mm, 5 μM with 4.6x20 mm guard column	Waters	186003117, 186003064
Agilent 1100 series HPLC system	Agilent Technologies	G1380-90000
1.7mL centrifuge tube	Sorenson	11700
Hamilton Glass Syringes, 5mL, 50µL and 250µL	Hamilton	81517, 80975, 81175
Pasteur Pipettes	FisherBrand	13-678-20A
Pasteur Pipette Bulbs	Sigma-Aldrich	Z111597
Bath Sonicator	Branson 1800 Ultrasonic Cleaner
Vacuum Centrifuge	Labconco Centrivap Acid-Resistant Concentrator System
MicroSpin Columns, C18 Silica	The Nest Group	SEM SS18V