This work was funded in part by an NIH Pathway to Independence Award (grant GM 099893) and faculty startup funds to SRB, as well as a Research Project Grant (grant GM 042219). The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.

1. Preparation of Buffer Solutions
<ol>
	<li>Prepare phosphate-buffered saline (PBS; pH 7.4) by diluting a stock solution of 10x PBS to a final concentration of 1x with ultrapure (distilled and deionized) water.</li>
	<li>Prepare quenching solution by combining 2 mL of acetonitrile, 2 mL of methanol, 1 mL of ultrapure H2O, and 19 &#181;L (0.1 mM final concentration) of formic acid.</li>
	<li>Prepare LC-MS solvent A by adding formic acid (0.2% [v/v] final concentration) to ultrapure water.</li>
	<li>Prepare L...

The authors declare that they have no competing financial interests.

All small-molecule metabolites are connected to one another through their common origins in central metabolic pathways. During exponential growth, bacterial cells are at biological and metabolic steady state, providing a snapshot of the physiological state under specific conditions. CodY monitors nutrient sufficiency by responding to ILV and GTP. As ILV and GTP pools drop, CodY activity is likely progressively reduced, adjusting the expression of its target genes to adapt to increasing nutrient depletion<sup class="xref"...

Bacterial pathogens must contend with many challenges within the host environment. In addition to direct attack by immune cells, the host also sequesters nutrients essential for bacterial survival and replication, generating nutritional immunity1,2. To survive these hostile environments, bacterial pathogens deploy virulence factors. Some of these factors allow the bacteria to evade the immune response; other factors include secreted digestive enzymes, such as hyaluronidase, thermonuclease, and lipase, which may enable the bacteria to replenish missing nutrients by consuming tissue-derived constituents3,4,5. Indeed, bacteria have evolved regulatory systems that tie the physiological state of the cell to the production of virulence factors6,7,8,9,10.
A growing body of evidence points to CodY as a critical regulator linking metabolism and virulence. Although first discovered in Bacillus subtilis as a repressor of the dipeptide permease (dpp) gene11, CodY is now known to be produced by nearly all the low G+C Gram-positive bacteria12,13 and regulates dozens of genes involved in carbon and nitrogen metabolism14,15,16,17,18,19. In pathogenic species, CodY also controls the expression of some of the most important virulence genes20,21,22,23,24,25,26,27. CodY is activated as a DNA-binding protein by two classes of ligands: branched-chain amino acids (BCAAs; isoleucine, leucine, and valine [ILV]) and GTP. When these nutrients are abundant, CodY represses (or in some cases, stimulates) transcription. As these nutrients become limited, CodY activity is progressively reduced, resulting in a graded transcriptional response that re-routes precursors through various metabolic pathways connected to central metabolism28,29,30. 
Tandem liquid chromatography coupled to mass spectrometry (LC-MS) is a powerful technique that can accurately identify and quantify small-molecule intracellular metabolites31. When paired with transcriptome analysis (e.g., RNA-Seq), this analytical workflow can provide insight into the physiological changes that occur in response to environmental or nutritional stress. Here, we present a method for metabolite extraction from Staphylococcus aureus cells and subsequent analysis via LC-MS. This approach has been used to demonstrate the pleiotropic effects of CodY on S. aureus physiology.

<table border="0" cellpadding="0" cellspacing="0" width="933">
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			<td height="22" style="height:22px;width:363px;">Material/Equipmenta</td>
			<td style="width:167px;"></td>
			<td style="width:163px;"></td>
			<td style="width:241px;"></td>
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		<tr height="19">
			<td height="19" style="height:19px;width:363px;">DeLong Culture Flask (250 ml)</td>
			<td style="width:167px;">Belco</td>
			<td>2510-00250</td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:363px;">Sidearm Flask, 500 ml</td>
			<td style="width:167px;">Pyrex</td>
			<td style="width:163px;">5340</td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:363px;">3-hole Rubber Stopper, #7</td>
			<td style="width:167px;">Fisher</td>
			<td style="width:163px;">14-131E</td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:363px;">Stainless Steel Filter holder/frit</td>
			<td style="width:167px;">VWR</td>
			<td>89428-936</td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:363px;">Petri Dish, 35 mm</td>
			<td style="width:167px;">Corning</td>
			<td style="width:163px;">430588</td>
			<td>Not tissue culture treated</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:363px;">Mixed cellulose ester membrane, 0.22 &#956;m pore size</td>
			<td style="width:167px;">Millipore</td>
			<td>GSWP02500</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:363px;">Impact-resistant tubes, 2 ml</td>
			<td style="width:167px;">USA Scientific</td>
			<td style="width:163px;">1420-9600</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:363px;">Silica Beads, 0.1 mm</td>
			<td style="width:167px;">Biospec Products Inc</td>
			<td>11079101Z</td>
			<td></td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:363px;">Precellys 24 homogenizer</td>
			<td style="width:167px;">Bertin Instruments</td>
			<td style="width:163px;">EQ03119-200-RD000.0</td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:363px;">Micro BCA Protein Assay Kit</td>
			<td style="width:167px;">Pierce (Thermo Scientific)</td>
			<td style="width:163px;">23235</td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:363px;">Cogent Diamond hydride type C column</td>
			<td style="width:167px;">Agilent</td>
			<td style="width:163px;">70000-15P-2</td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:363px;">Accurate-Mass Time-of-Flight (TOF) LC-MS, 6200 Series</td>
			<td style="width:167px;">Agilent</td>
			<td style="width:163px;">G6230B</td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:363px;">Quat Pump, 1290 Series</td>
			<td style="width:167px;">Agilent</td>
			<td style="width:163px;">G4204A&#160;</td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:363px;">Bin Pump, 1290 Series</td>
			<td style="width:167px;">Agilent</td>
			<td style="width:163px;">G4220A&#160;</td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:363px;">Valve Drive, 1290 Series</td>
			<td style="width:167px;">Agilent</td>
			<td style="width:163px;">G1107A&#160;</td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:363px;">Isocratic Pump, 1290 Series</td>
			<td style="width:167px;">Agilent</td>
			<td style="width:163px;">G1310B&#160;</td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:363px;">TCC, 1290 Series</td>
			<td style="width:167px;">Agilent</td>
			<td style="width:163px;">G1316C&#160;</td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:363px;">Sampler, 1290 Series</td>
			<td style="width:167px;">Agilent</td>
			<td style="width:163px;">G4226A&#160;</td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:363px;">Thermostat, 1290 Series</td>
			<td style="width:167px;">Agilent</td>
			<td style="width:163px;">G1330B&#160;</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Name</td>
			<td style="width:167px;">Company</td>
			<td style="width:163px;">Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:363px;">Chemical</td>
			<td style="width:167px;"></td>
			<td style="width:163px;"></td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:363px;">Tryptic Soy Broth</td>
			<td style="width:167px;">Becton Dickinson</td>
			<td style="width:163px;">211825</td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:363px;">Difco Agar, Granulated</td>
			<td style="width:167px;">Becton Dickinson</td>
			<td style="width:163px;">214530</td>
			<td>Solid media contains 1.5% [w/v] agar</td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:363px;">Phosphate-buffered saline (pH 7.4) 10X</td>
			<td style="width:167px;">Ambion</td>
			<td style="width:163px;">AM9624</td>
			<td>Dilute fresh to 1X with ultra-pure water</td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:363px;">Acetonitrile</td>
			<td style="width:167px;">Fisher Scientific</td>
			<td style="width:163px;">A955-500</td>
			<td>Optima LC-MS</td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:363px;">Methanol</td>
			<td style="width:167px;">Fisher Scientific</td>
			<td style="width:163px;">A456-500</td>
			<td>Optima LC-MS; toxic</td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:363px;">Formic Acid</td>
			<td style="width:167px;">Sigma Aldrich</td>
			<td style="width:163px;">94318</td>
			<td>For mass spectrometry, 98%</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Name</td>
			<td style="width:167px;">Company</td>
			<td style="width:163px;">Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:363px;">Software</td>
			<td style="width:167px;"></td>
			<td style="width:163px;"></td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:363px;">MassHunter</td>
			<td style="width:167px;">Agilent</td>
			<td style="width:163px;">G3337AA</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:363px;">Bacterial Strain</td>
			<td style="width:167px;">Species</td>
			<td style="width:163px;">Strain</td>
			<td>Genotype</td>
		</tr>
		<tr height="38">
			<td height="38" style="height:38px;width:363px;">SRB 337</td>
			<td style="width:167px;">Staphylococcus aureus</td>
			<td style="width:163px;">USA200 MSSA UAMS-1</td>
			<td>wild type</td>
		</tr>
		<tr height="39">
			<td height="39" style="height:39px;width:363px;">SRB 372</td>
			<td style="width:167px;">Staphylococcus aureus</td>
			<td style="width:163px;">USA200 MSSA UAMS-1</td>
			<td>&#916;codY::erm</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:933px;">aChemicals and materials listed are specific to the method described and do not include standard laboratory chemicals or supplies.</td>
		</tr>
	</tbody>
</table>

a tandem liquid chromatography&#8211;mass spectrometry-based approach for metabolite analysis of staphylococcus aureus

In an effort to thwart bacterial pathogens, hosts often limit the availability of nutrients at the site of infection. This limitation can alter the abundances of key metabolites to which regulatory factors respond, adjusting cellular metabolism. In recent years, a number of proteins and RNA have emerged as important regulators of virulence gene expression. For example, the CodY protein responds to levels of branched-chain amino acids and GTP and is widely conserved in low G+C Gram-positive bacteria. As a global regulator in Staphylococcus aureus, CodY controls the expression of dozens of virulence and metabolic genes. We hypothesize that S. aureus uses CodY, in part, to alter its metabolic state in an effort to adapt to nutrient-limiting conditions potentially encountered in the host environment. This manuscript describes a method for extracting and analyzing metabolites from S. aureus using liquid chromatography coupled with mass spectrometry, a protocol that was developed to test this hypothesis. The method also highlights best practices that will ensure rigor and reproducibility, such as maintaining biological steady state and constant aeration without the use of continuous chemostat cultures. Relative to the USA200 methicillin-susceptible S. aureus isolate UAMS-1 parental strain, the isogenic codY mutant exhibited significant increases in amino acids derived from aspartate (e.g., threonine and isoleucine) and decreases in their precursors (e.g., aspartate and O-acetylhomoserine). These findings correlate well with transcriptional data obtained with RNA-seq analysis: genes in these pathways were up-regulated between 10- and 800-fold in the codY null mutant. Coupling global analyses of the transcriptome and the metabolome can reveal how bacteria alter their metabolism when faced with environmental or nutritional stress, providing potential insight into the physiological changes associated with nutrient depletion experienced during infection. Such discoveries may pave the way for the development of novel anti-infectives and therapeutics.

We have analyzed intracellular metabolite pools in S. aureus during in vitro growth in a rich, complex medium. As proof of principle, we compared metabolite profiles between the methicillin-susceptible S. aureus osteomyelitis isolate UAMS-1 (wild-type [WT]) and an isogenic strain lacking the global transcriptional regulator CodY (&#916;codY)26. Steady-state, exponential cultures of the WT and codY strains were establishe...

Watch this Scientific Journal Video about A Tandem Liquid Chromatographyâ€“Mass Spectrometry-based Approach for Metabolite Analysis of Staphylococcus aureus at JoVE.com

A Tandem Liquid Chromatography&amp;#8211;Mass Spectrometry-based Approach for Metabolite Analysis of &lt;em&gt;Staphylococcus aureus&lt;/em&gt;

Here we describe a protocol for the extraction of metabolites from Staphylococcus aureus and their subsequent analysis via liquid chromatography and mass spectrometry.

A Tandem Liquid Chromatography&#8211;Mass Spectrometry-based Approach for Metabolite Analysis of Staphylococcus aureus

In an effort to thwart bacterial pathogens, hosts often limit the availability of nutrients at the site of infection. This limitation can alter the ...

The overall goal of this sampling method is to harvest metabolite samples from Staphyococcus aureus for subsequent quantification by Tandem Liquid Chromatography-Mass Spectrometry. Our lab is interested in studying the intersection of metabolism and pathogenesis in bacteria relevant to human health. Currently, we're using the human opportunistic pathogen Staphylococcus aureus as a model organism.One key question that we wish to answer is how bacteria adjust their physiology and pathogenicity when faced with nutrient depletion during host infection. This technique is advantageous because it provides a snapshot into the distribution of specific metabolites amongst the many metabolic pathways in actively growing cells, with minimal perturbation of their physiological state. To begin, streak the S.aureus strains of interest for isolation on tryptic soy agar from the frozen glycerol stock.Incubate the cells at 37 degrees Celsius for 16-24 hours. The next day, inoculate 4 mLs of tryptic soy broth, or TSB, in sterile glass incubation tubes with single colonies of each strain. Incubate the cultures, inclined with rotation, at 37 degrees Celsius for 16-20 hours.Use a spectrophotometer to measure the optical density of the cultures at 600 nanometers, abbreviated as OD600. Using this measurement, dilute the cells to an OD600 of 0.05 in 50 mLs of sterile TSB medium, in 250 mL Delong flasks. Incubate the cultures at 37 degrees Celsius in a warm bath with shaking at 280 rpm.Every 30 minutes, take OD600 measurements. As the optical densities increase, it may become necessary to dilute the cultures with TSB so that they remain within the linear absorbance range of the spectrophotometer. While the cells are growing, freshly prepare the extraction buffer solutions using the highest purity reagents available as described in the text protocol.When the cultures achieve an OD600 between 0.8 and 1.0, subculture them into 50 mLs of fresh pre-warmed TSB to an OD600 of 0.01 to 0.05, and repeat the incubation and measurement steps. Prepare a bed of crushed dry ice in an appropriate vessel, such as a glass dish, ice bucket, or cooler. As the optical densities of the cultures approach the desired harvest point, add 1 mL of quenching solution to a 35 mm untreated petri dish, and pre-cool on dry ice for at least 5 minutes.For our purposes, the desired harvest point is an optical density of about 0.4 to 0.5. But this can be varied, depending on experimental goals. Next, place a stainless steel filter frit in a rubber stopper, and place it atop a vacuum flask attached to a house vacuum, or vacuum pump.Apply the vacuum, and place a mixed cellulose ester membrane on top. Use a filter with the same diameter as the frit to ensure that all cells are collected. It's critical to collect the cells and halt metabolic activity as quickly as possible, so as to provide a snapshot of metabolic abundances under a given condition.Even small delays can affect the steady-state physiology of the cells. At an OD600 of between 0.4 and 0.5, use a serological pipette to remove 13 mLs of culture from the flask. Immediately apply the sample to the filter.After the entire sample has been filtered, immediately wash the filter with no more than 5 mL of ice cold PBS to wash away medium-associated metabolites. Then disconnect the vacuum and use a pair of sterile tweezers to remove the filter from the frit. Invert the filter into the pre-chilled quench solution.Incubate the filter in quench solution on dry ice for at least 20 minutes. Using the sterile tweezers, invert the filter in the petri dish and use a micropipette to rinse the cells off of the membrane into the quench solution. Resuspend the cells in quench solution, and then transfer the cell suspension to a sterile 2 mL impact-resistant tube containing approximately 100 microliters of 0.1 mm silica beads.Store this on dry ice or at 80 degrees Celsius. To begin the metabolite extraction, first thaw the samples on wet ice. Then disrupt the cells in a homogenizer with four 30-second bursts at 6000 rpm, with two minute cooling periods on dry ice between cycles.Clarify the lysates for 15 minutes in a pre-chilled refrigerated microcentrifuge tube at maximum speed. Following centrifugation, transfer the supernatant to a clean microcentrifuge tube. Using a micropipette, transfer a small portion of the sample to a microcentrifuge tube for quantification of residual peptide content as described in the text protocol.Store the remainder of the sample at 80 degrees Celsius. Refer to the text protocol for analysis of the metabolites via liquid chromatography and mass spectrometry. The CodY null mutant was used to investigate branched-chain amino acid depletion as the CodY protein adjusts the expression of dozens of genes in response to the drop in concentration of these nutrients.The codY mutation has little effect on the growth rate of S.aureus cells. The drop in optical density at the 3-hour mark indicates the back-dilution step, which increases the number of cell doublings before harvest, and dilutes out various molecules that accumulate during overnight growth. An RNA sequence analysis of the transcriptomes of the two strains show significant changes in the expression of genes involved in the synthesis of the aspartate family of amino acids.Here, the numbers below gene names indicate the fold change in expression in a codY null mutant, when compared to the wild type strain. Following LC-MS quantification of the extracted metabolites, the abundances between the two strains were compared. Focusing on some compounds related to the aspartate family of amino acids, the abundances of precursors, such as aspartate, and O-acetylhomoserine, are decreased in the codY mutant.Conversely, end products like threonine and isoleucine are increased. Here, the dashed line indicates the cutoff for changes in abundance for this experiment. After watching this video, you should have a good understanding of how to harvest metabolites from Staphylococcus aureus for quantification via liquid chromatography mass spectrometry.This protocol is also suitable with modification for metabolite harvest from other bacterial species. Because bacteria respond to dynamic environments, it's important to perform the filtering and wash steps quickly to minimize potential physiological changes induced by sampling. Changes can also be minimized by keeping the frit and PBS cold during sampling.

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9.9K vistas.  Georgetown University. The overall goal of this sampling method is to harvest metabolite samples from Staphyococcus aureus for subsequent quantification by Tandem Liquid Chromatography-Mass Spectrometry. Our lab is interested in studying the intersection of metabolism and pathogenesis in bacteria relevant to human health. Currently, we're using the human opportunistic pathogen Staphylococcus aureus as a model organism.One key question that we wish to answer is how bacteria adjust their physiology and pathoge...