Using NMR Metabolomics to Investigate Tricarboxylic AcidCycle-dependent Signal Transduction in Staphylococcusepidermidis*□S

Received for publication, June 8, 2010, and in revised form, September 21, 2010 Published, JBC Papers in Press, September 22, 2010, DOI 10.1074/jbc.M110.152843

Marat R. Sadykov‡, Bo Zhang§, Steven Halouska§, Jennifer L. Nelson‡1, Lauren W. Kreimer¶, Yefei Zhu‡,Robert Powers§2, and Greg A. Somerville‡3

From the ‡School of Veterinary Medicine and Biomedical Sciences, University of Nebraska, Lincoln, Nebraska 68583-0905, the§Department of Chemistry, University of Nebraska, Lincoln, Nebraska 68588-0304, and the ¶Department of Biochemistry,University of Nebraska, Lincoln, Nebraska 68588-0664

Staphylococcus epidermidis is a skin-resident bacteriumand amajor cause of biomaterial-associated infections. The transitionfrom residing on the skin to residing on an implanted biomate-rial is accompanied by regulatory changes that facilitate bacte-rial survival in the new environment. These regulatory changesare dependent upon the ability of bacteria to “sense” environ-mental changes. In S. epidermidis, disparate environmental sig-nals can affect synthesis of the biofilm matrix polysaccharideintercellular adhesin (PIA). Previously, we demonstrated thatPIA biosynthesis is regulated by tricarboxylic acid (TCA) cycleactivity. The observations that very different environmental sig-nals result in a commonphenotype (i.e. increasedPIA synthesis)and that TCA cycle activity regulates PIA biosynthesis led us tohypothesize that S. epidermidis is “sensing” disparate environ-mental signals through themodulation of TCA cycle activity. Inthis study, we used NMR metabolomics to demonstrate thatdivergent environmental signals are transduced into commonmetabolomic changes that are “sensed” by metabolite-respon-sive regulators, such as CcpA, to affect PIA biosynthesis. Thesedata clarify one mechanism by which very different environ-mental signals cause common phenotypic changes. In addition,due to the frequency of the TCA cycle in diverse genera of bac-teria and the intrinsic properties of TCA cycle enzymes, it islikely the TCA cycle acts as a signal transduction pathway inmany bacteria.

Staphylococcus epidermidis is a skin-resident, opportunisticpathogen that is the leading cause of hospital-associated infec-tions (1). Although the type and severity of diseases producedby this bacterium varies, its most common infectious manifes-tation is associated with implanted biomaterials. The dramaticenvironmental changes that occur during the transition from

being skin-resident to residing on implanted biomaterialsnecessitates the need for changes in the expression of genescoding for enzymes required for growth in the new environ-ment. This environmental adaptation often includes activatingtranscription of virulence genes; hence, most virulence genesare regulated by environmental and nutritional signals (2).Accordingly, a major area of interest in microbiology is deter-mining how bacteria “sense” and respond to environmental sig-nals. Given the tremendous diversity of microbial life, it is notsurprising that the mechanisms bacteria employ are equallydiverse. Thesemechanisms include two-component regulatorysystems, alternative � factors, mechanosensors, small RNAs,riboswitches, and many others. Although remarkable advanceshave been made in identifying the response regulators, ourknowledge of signaling mechanisms has lagged behind, theexception being cell-density signaling.The tricarboxylic acid (TCA) cycle has been implicated as

regulating or affecting staphylococcal virulence and/or viru-lence determinant biosynthesis (3–9). The TCA cycle has threeprimary functions: (i) to provide biosynthetic intermediates, (ii)to generate reducing potential, and (iii) to directly produce asmall amount of ATP. The availability of biosynthetic interme-diates affects the availability of amino acids and nucleic acids.Increasing the reducing potential alters the bacterial redox bal-ance, necessitating oxidation reactions via the electron transferchain. The small amount of ATP produced directly by the TCAcycle is amplified many times when the ATP generated by oxi-dative phosphorylation is considered. In short, the TCA cyclehas a central function in maintaining the bacterial metabolicstatus. Importantly, the activity of TCA cycle enzymes isaffected by the availability of nutrients and a variety of stress-inducing stimuli (9–12); thus, the availability of biosyntheticintermediates, the redox status, and the energy status can bealtered by nutritional and environmental stimuli. These obser-vations led us to propose a fourth function for the TCA cycle,the transduction of external signals into intracellular metabolicsignals that can be “sensed” by metabolite-responsive regula-tory proteins (2). Fundamental to this hypothesis are the pre-dictions that disparate environmental stimuli will cause com-mon metabolic changes and that these metabolic changes willprecede regulatory changes.Two of the more extensively studied environmental stimuli

that influence S. epidermidis virulence determinant biosynthe-

* This manuscript is a contribution of the University of Nebraska AgriculturalResearch Division, supported in part by funds provided through the HatchAct and from National Institutes of Health Grant AI087668 (to G. A. S.) andthe American Heart Association Grant 0860033Z (to R. P.).

□S The on-line version of this article (available at http://www.jbc.org) containssupplemental Fig. S1.

1 Present address: Dept. of Microbiology, Cornell University, Ithaca, NY 14853.2 To whom correspondence may be addressed: 722 Hamilton Hall, Lincoln,

NE 68588-0304. Fax: 402-472-9402; E-mail: rpowers3@unl.edu.3 To whom correspondence may be addressed: 155 VBS, Fair St. and East

Campus Loop, Lincoln, NE 68583-0905. Fax: 402-472-9690; E-mail:gsomerville3@unl.edu.

THE JOURNAL OF BIOLOGICAL CHEMISTRY VOL. 285, NO. 47, pp. 36616 –36624, November 19, 2010© 2010 by The American Society for Biochemistry and Molecular Biology, Inc. Printed in the U.S.A.

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sis are iron limitation (13, 14) and ethanol stress (15, 16). Theeffect of iron limitation on bacterial growth is primarilythrough preventing the activity of enzymes that require iron asa cofactor and altering transcription of iron-regulated genes (2).As S. epidermidis has many iron-requiring enzymes (e.g. acon-itase, serine dehydratase, peptide deformylase, iron-containingalcohol dehydrogenase, nitrate reductase, etc.), it is reasonableto expect that the metabolic effects of iron-limited growth arediverse and not restricted to the TCA cycle. Ethanol denaturesproteins in the cytoplasmic membrane, causing changes inmembrane permeability, which can lead to the loss of mem-brane integrity (17). With the exception of the succinate dehy-drogenase complex, most TCA cycle enzymes are not mem-brane-associated; hence, it is reasonable to predict that thedeleterious effects of ethanol stress are largely independentof the TCA cycle. Taken together, these observations suggestthat disparate environmental conditions will cause divergentmetabolomic changes. In contrast to this suggestion, our cen-tral hypothesis predicts that different stresses will cause com-mon metabolomic changes that are dependent on the TCAcycle. To test our central hypothesis, we chose to induce envi-ronmental stress by growing bacteria in an iron-limitedmedium or in a medium containing ethanol and assessing themetabolic changes using NMR metabolomics.

EXPERIMENTAL PROCEDURES

Bacterial Strains, Media, and Growth Conditions—S. epider-midis wild-type strain 1457 (18) and the isogenic aconitasemutant strain 1457-acnA::tetM (tetM cassette inserted intoposition 856 of the 2,702-bp acnA gene) and �B mutant strain1457-sigB::dhfr (7, 19) have been described. Strains 1457-codY,1457-ccpA, 1457-acnA-codY, and 1457-acnA-ccpA were con-structed using the gene splicing by overlap extension (geneSOEing) technique (20) to replace the gene of interest with anantibiotic resistance marker (i.e. ermB or tetM). Primers weredesigned to amplify �1-kb regions upstream and downstreamof the gene of interest based on the genome sequence of S. epi-dermidis strain RP62A. Gene knockouts were confirmed byPCR and Southern blot hybridization. In addition, strains con-taining mutations in the single S. epidermidis aconitase genewere assayed to ensure that no aconitase activity was detected(data not shown). All strains were grown in tryptic soy brothwithout dextrose (TSB;4 BD Biosciences) supplemented with0.25% glucose (Sigma) or 0.25% 13C6-glucose (Cambridge Iso-tope Laboratories). Deferrated TSB (DTSB) was prepared byadding 50 g of Chelex 100 (Sigma) to �1 liter of TSB and stir-ring at 4 °C for 20 h. After 20 h, the Chelex resin was removed,1mMMgSO4was added, the volumewas adjusted to 1 liter, andthe medium was filter-sterilized. To induce ethanol stress andminimize growth defects, ethanol or deuterated ethanol (Iso-tec) was added to the medium at a final concentration of 4%(v/v). All cultures were inoculated 1:200 from overnight cul-

tures (normalized for growth) into glucose supplemented TSB,incubated at 37 °C, and aerated at 225 rpm with a flask-to-me-dium ratio of 7:1. Bacterial growth was assessed by measuringthe optical density at 600 nm (A600). Antibiotics, when used,were purchased from Fisher Scientific or Sigma and used at thefollowing concentrations: chloramphenicol (8 �g/ml), tri-methoprim (10 �g/ml), and erythromycin (8 �g/ml).Aconitase Activity Assay—Cell-free lysates of S. epidermidis

were prepared as follows. Aliquots (3 ml) were harvested bycentrifugation (1 min at 20,800 � g) at the indicated times,suspended in 1.5 ml of lysis buffer containing 90 mM Tris (pH8.0) and 100 �M fluorocitrate. The samples were lysed in 2-mlscrew cap tubes containing lysing matrix B using a FastPrepinstrument (MP Biomedicals). The lysate was centrifuged for 5min at 20,800 � g at 4 °C. Aconitase activity in the resultingcell-free lysate was assayed by the method described byKennedy et al. (21). One unit of aconitase activity is defined asthe amount of enzyme necessary to give a �A240 min�1 of0.0033 (22). Protein concentrations were determined by theLowry method (23).Northern Blot Analysis—Northern blot analysis of transcripts

was performed as described (7). Oligonucleotide primers usedin making DNA probes were designed using the S. epidermidisRP62A genome sequence. Probes for Northern blotting weregenerated by PCR amplification of unique internal regions ofRNAIII and glnA (femC) genes using the following primers:femC, forward, 5�-GATGTTTGATGGTTCATCTATTGAA-GGTTTCG-3�; femC, reverse, 5�-GCAGTATCAGTCAATT-GTAAATCACCTTCAG-3�; RNAIII, forward, 5�-TGAAAAA-TTTGCTTAATCTAGTCGAGTG-3�; and RNAIII, reverse,5�-CATGATAAATTGAATGTTGTTTACGATAGC-3�.DNA probes were labeled using the North2South random

prime labeling kit (Pierce). Electrophoresis, transfer of the RNAto the Nytran SPC nylon membrane (Whatman), and hybrid-izationwere done using theNorthernMax kit (Ambion). Detec-tion was performed using the chemiluminescent nucleic aciddetection module (Pierce).PIA Immunoblot Assay—PIA accumulation was determined

as described (24).NMR Sample Preparation—NMR samples for one-dimen-

sional 1H spectra were prepared from 10 independent, 25-mlS. epidermidis cultures. Two-dimensional 1H-13C HSQC (25,26) and two-dimensional 1H-1H TOCSY (27) spectra were pre-pared from three independent 50-ml cultures. The TSBmedium used in the two-dimensional 1H-13C HSQC analysiscontained 0.25% 13C6-glucose (Cambridge Isotope Laborato-ries). For two-dimensional 1H-13CHSQCand two-dimensional1H-1H TOCSY involving ethanol stress, deuterated ethanol(Isotec) was used to minimize the contribution of exogenousethanol to the NMR spectra. For one-dimensional 1H NMRexperiments, 2.74A600 units were harvested at each time point,and for the two-dimensional 1H-13C HSQC and two-dimen-sional 1H-1H TOCSY experiments, 5.48 A600 units were col-lected. Bacteriawere harvested by centrifugation (4,000 rpm for5 min), suspended in 50 mM phosphate buffer in 100% D2O atpH 7.2 (uncorrected), and lysed using lysingmatrix B tubes anda FastPrep instrument. The lysates were centrifuged to removecell debris and glass beads and then frozen in liquid nitrogen.

4 The abbreviations used are: TSB, tryptic soy broth; DTSB, deferrated TSB; PIA,polysaccharide intercellular adhesin; HSQC, heteronuclear single quantumcoherence; TOCSY, total correlation spectroscopy; PCA, principal compo-nent analysis; HPr, histidine-containing protein; CcpA, catabolite controlprotein A; PC, principal component.

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All samples were kept at �80 °C until ready for analysis. At thetime of use, a 600-�l aliquot of the cell-free lysate was trans-ferred to each NMR tube.NMR Analysis—The NMR spectra were collected on a

Bruker 500-MHz Avance spectrometer equipped with a triple-resonance, z axis gradient cryoprobe. A BACS-120 samplechanger with Bruker Icon software was used to automate theNMR data collection. The one-dimensional 1H NMR spectracollection and principal component analysis (PCA) were per-formed as describedwithminormodifications (28–30). Briefly,each multidimensional NMR spectrum (chemical shifts andpeak intensities) was converted to a single point in a multidi-mensional Cartesian space. Conceptually, each axis corre-sponds to a specific chemical shift, where the peak intensity isthe value along the axis. PCA identifies a principal componentvector (

3P1) corresponding to the largest variation in the data set

within this multidimensional space. The second vector (3P2) is

orthogonal to the first and represents the next largest variationin the data set. Each successive vector describes a diminishingamount of the variability of the data set, where most of thevariability is described by the first two principal components.The PC1 and PC2 scores (unitless values) are effectively theindividual fit of eachNMR spectrum to

3P1 and

3P2. The PC1 and

PC2 scores are usually presented in a two-dimensional plot,where similar NMR spectra cluster together.Solvent presaturation used excitation sculpting to efficiently

remove the solvent andmaintain a flat baseline, eliminating anyneed for baseline collection that may induce artifacts in thetwo-dimensional scores plot (31). Each NMR spectrum wascenter-averaged for PCA to minimize any experimental varia-tions between cultures (32).Two-dimensional 1H-13C HSQC spectra were collected and

processed as described previously (7). Two-dimensional 1H-1HTOCSY spectra were collected with WATERGATE solventpresaturation (33) and a relaxation delay of 2 s. A total of 1,024data points with a sweep width of 5,000 Hz and 256 data pointswith a sweep width of 5,001.324 Hz were collected in the directand indirect 1H dimensions, respectively. A total of 16 dummyscans and 8 acquisition scans were used to obtain each of thetwo-dimensional 1H-1H TOCSY NMR spectra. The two-di-mensional 1H-1HTOCSYNMR spectra were processed similarto the two-dimensional 1H-13C HSQC spectra, and both spec-tra were analyzed using NMRView (One Moon Scientific (34))and Sparky (71) to identify chemical shifts and assign peakintensities.The observed NMR peaks in the two-dimensional 1H-13C

HSQC and 1H-1H TOCSY spectra were assigned to specificmetabolites using 1H and 13C chemical shift tolerances of 0.05and 0.50 ppm, respectively, and the Madison MetabolomicsConsortiumDatabase (MMCD) (35), the BioMagResBank (36),and the Human Metabolome Database (37). The presence ofmetabolites and metabolic pathways were verified with theKyoto Encyclopedia of Genes and Genomes (KEGG) (38) andthe MetaCyc (39) databases.Peak intensities were normalized for each two-dimensional

NMR spectrum by dividing by the average peak intensity. Thetriplicate data sets were then used to calculate average intensi-ties for each peak observed in the two-dimensional spectra for

strain 1457, 1457-acnA, ethanol stress, and iron limitation. Apercentage of error was calculated for each peak by dividing thestandard deviation by the average peak intensity. The averagepeak intensities were then used to calculate a percentage ofdifference relative to the wild-type bacteria in TSB medium.Peaks with calculated percentage of differences greater thanfive times the average percentage of error were considered tohave either decreased or increased concentrations relative tothewild-type strain 1457. Peakswith less than a 5-fold deviationwere considered similar. Secondary peaks assigned to the samemetabolite were required to have the same relative change inintensity to be classified as a metabolite with an increase ordecrease in concentration.Metabolomic Dendrogram—The relative clustering patterns

in the PCA two-dimensional scores plots were quantitativelyanalyzed using a tree diagram and bootstrapping technique(40). The PC1 and PC2 scores for each set of 10 duplicate NMRspectra representing a specific metabolic state (iron limitation,ethanol treatment, etc.) were used to calculate an average PCscore and standard deviation. Any PC scores outside 2 standarddeviations were removed, and a new average was calculated.The average PC scores represent the center of a cluster of NMRspectra (metabolic state) in the two-dimensional scores plot.The process is repeated for each set of 10 duplicate NMR spec-tra. Distances between the average PC positions for each met-abolic state are then calculated using the standard equation fora Euclidean distance to create a distance matrix.To assess the significance of the similarity (overlap) or differ-

ence (separation) observed between pairs of clusters in the two-dimensional scores plot, standard bootstrappingmethods werealso applied (41, 42). Briefly, the average PC scores were recal-culated by randomly selecting points from the data set. Dis-tances were recalculated between the clusters using the newaverage PC scores to create a new distance matrix. The processwas repeated until 100 different distancematrices were createdand transferred to version 3.68 of the PHYLIP (43) suite ofsoftware programs. PHYLIP calculates a tree for each distancematrix and then determines a consensus tree. The programcalculates a bootstrap value for each node, which is simply thenumber of times the node appears in all 100 trees. Bootstrapvalues below 50% imply a statistically insignificant separation.Conversely, as the bootstrap number increases above 50%, theconfidence in the tree branch or separation increases.

RESULTS

Disparate Environmental Stresses Create a Metabolic Blockin the TCA Cycle—To determine whether ethanol stress andiron limitation alter TCA cycle activity, the specific activity ofaconitase in S. epidermidis strain 1457 at 2 (exponentialgrowth) and 6 h (post-exponential growth) after inoculationwas assessed (Fig. 1). As expected, iron-limited growth and eth-anol stress prevented the post-exponential growth phase in-crease in the specific activity of the iron-requiring enzymeaconitase, creating a metabolic block in the TCA cycle (Fig. 1).Although ethanol-stressed bacteria are in the post-exponentialgrowth phase at 6 h after inoculation, their growth is slower,which slows the consumption of glucose, and excess glucosecan repress transcription of TCA cycle genes. Irrespective of

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the mechanism by which ethanol repressed aconitase-specificactivity, the normal post-exponential growth phase increase inTCA cycle activity did not occur. These data demonstrate thatenvironmental stresses whose deleterious effects are substan-tially different from one another have a similar effect on TCAcycle function.Environmental Stimuli Elicit TCA Cycle-dependent Meta-

bolic Changes—The TCA cycle provides biosynthetic interme-diates, ATP, and reducing potential; therefore, alteration ofTCAcycle activitywill alter themetabolic status of a bacterium.To determine the metabolic changes associated with iron limi-tation, ethanol stress, and TCA cycle inactivation, NMRmetabolomic analysis (28, 29) was used to assess the stressedand non-stressed metabolomes of strains 1457 and the TCAcycle inactive strain 1457-acnA. Specifically, S. epidermidisstrains 1457 and 1457-acnAwere grown for 2 or 6 h inTSB,TSBwith 4% ethanol, or DTSB. Following acquisition of the NMRspectra, the table of integrals was used for PCA (Fig. 2A). Asexpected, during the exponential growth phase, PCA revealedthat the effects of ethanol stress and iron limitation on themetabolome were largely independent of the TCA cycle (sup-plemental Fig. S1). This was expected due to the normal repres-sion of TCA cycle activity during nutrient-rich growth (Fig. 1)(8, 44). Despite the TCA cycle being repressed during the expo-nential growth phase, the different stresses induced commonmetabolomic changes (Table 1). In contrast to the exponentialgrowth phase, PCA of post-exponential growth phase metabo-lomes revealed that ethanol stress and iron limitation inducedmetabolomic changes very similar to TCA cycle inactivation(Fig. 2A and Table 2). In addition, these data highlight the rel-ative insensitivity of the metabolome of strain 1457-acnA toethanol stress and iron-limited growth, confirming that themajor effect of these stressors is dependent upon the TCAcycle. That being said, the more diffuse clustering of ethanol-stressed metabolomes of both the wild-type and the aconitasemutant strains suggest that ethanol stress had TCA cycle-inde-pendent metabolomic effects (Fig. 2A). The TCA cycle-inde-pendent effects are likely due to the denaturation of membraneproteins not related to electron transport or the TCA cycle.

Taken together, these data demonstrate that diverse environ-mental stimuli elicit common metabolic changes that requirethe TCA cycle.

FIGURE 1. The temporal induction of aconitase-specific activity is inhib-ited by dissimilar stressors. Aconitase activity was assessed during theexponential (2 and 4 h) and post-exponential (6 h) growth phases duringgrowth in DTSB or TSB containing 4% ethanol. The data are presented as themean and S.E. of two independent experiments each determined in triplicate.

FIGURE 2. Environmental stressors cause metabolomic perturbationssimilar to TCA cycle inactivation. A, PCA two-dimensional scores plot com-paring non-stressed, ethanol-stressed, or iron-limited cultures of strains 1457,1457-acnA, and 1457-sigB::dhfr grown for 6 h. Symbols and colors are definedin the figure. The ovals are manually drawn to identify clusters of relatedsamples and to guide the reader. They are not statistically relevant. The rela-tive contribution of each principal component is indicated in the parentheses.B, metabolic tree generated using the PCA scores plot data demonstratingthe relationship between stresses and strains. Bootstrap values are indicatedon the dendrogram branches. Bootstrap values below 50% imply a statisti-cally insignificant separation; conversely, as the bootstrap number increasesabove 50%, the confidence in the tree branch or separation increases.

TABLE 1Metabolites that have increased or decreased concentrationsrelative to the wild-type strain 1457 at 2 h after inoculationMetabolites whose concentrations were changed under all stress conditions relativeto the wild-type strain grown in TSB are shaded in gray.

a The intracellular concentration was considered to be increased or decreasedwhen the percentage of difference in the NMR peak intensities was 5-foldgreater than the percentage of error observed in the peak intensities betweentriplicate NMR spectra.

b Due to peak overlap, we are unable to determine whether the metabolite is acetyl-glutamine or acetyl-glutamate; however, we note that acetyl-glutamine is uncom-mon in prokaryotes.

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The commonmetabolomic response to environmental stim-uli can bemore easily observed by a recently developedmethodto visualize PCA data (40). By calculating an average positionfor each data set, such that each PC value (PC1, PC2, etc.) istreated as an axis in a Cartesian coordinate system, a distancematrix can be generated. Correspondingly, methods developedfor representing genetic distances in phylogenetic trees can beused to create a metabolomic dendrogram (43) (Fig. 2B). Usingthis approach, it becomes clear that stress-induced metabolo-mic responses are very similar to the metabolome of the acon-itase-deficient strain 1457-acnA. As with the two-dimensionalscores plot (Fig. 2A), the higher bootstrap values in the dendro-

gram for ethanol-stressed cultures also indicate that ethanolhas TCA cycle-independent effects on the metabolome.S. epidermidis grown in TSB under aerobic conditions have

two distinct metabolic states: the nutrient-rich exponentialphase and the nutrient-limited post-exponential phase. Thetransition from nutrient-rich conditions to nutrient-limitedgrowth coincides with the transition from generating ATP bysubstrate-level phosphorylation to using oxidative phosphory-lation. The reduced dinucleotides that drive oxidative phos-phorylation are primarily derived from the TCA cycle; thus,inhibiting TCA cycle activity (Fig. 1) hinders the transition tooxidative phosphorylation and the post-exponential growthphase (8, 44). Iron-limited growth of strain 1457 or aconitaseinactivation did not significantly alter the growth rate, althoughaconitase inactivation did increase the lag phase (data notshown). Both aconitase inactivation and iron-limited growthcaused an early entry into the stationary phase; as such, thegrowth yield was decreased. As stated, the addition of 4% etha-nol decreased the growth rate; therefore, it slowed the con-sumption of glucose. Based on these observations, it was rea-sonable to hypothesize that post-exponential growth phase (6h)-stressed metabolomes will be more similar to an unstressedexponential growth phase (2 h) metabolome than to the un-stressed metabolome of post-exponential growth phase of cul-tures. As expected, PCAs of unstressed strain 1457 cultures,grown for 2 or 6 h, form separate subsets in a three-dimensionalscores plot (Fig. 3). Consistent with our hypothesis, PCAs ofpost-exponential growth phase-stressed and acnAmutant cul-tures were more closely associated with the unstressed expo-nential growth phase metabolome of strain 1457 than with the

TABLE 2Metabolites that have increased or decreased concentrationsrelative to the wild-type strain 1457 at 6 h after inoculationMetabolites whose concentrations were changed under all stress conditions relativeto the wild-type strain grown inTSB are shaded in gray.

a The intracellular concentration was considered to be increased or decreasedwhen the percentage of difference in the NMR peak intensities was 5-foldgreater than the percentage of error observed in the peak intensities betweentriplicate NMR spectra.

b Due to peak overlap, we are unable to determine whether the metabolite is acetyl-glutamine or acetyl-glutamate; however, we note that acetyl-glutamine is uncom-mon in prokaryotes.

c The percentage of difference in the NMR peak intensities of these metabolites felljust below the 5-fold cutoff in the percentage of error observed in the peak inten-sities between the triplicate NMR spectra.

FIGURE 3. PCA three-dimensional scores plot comparing non-stressed,ethanol-stressed, or iron-limited cultures of strains 1457 grown for 6 hwith that of strain 1457 grown for 2 h. Symbols and colors are defined in thefigure. The ovals are manually drawn to identify clusters of related samplesand to guide the reader. They are not statistically relevant. The relative con-tribution of each principal component is indicated within the parentheses.

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strain 1457 post-exponential growth phase metabolome (Fig.3). These data suggest that any stress that interferes with TCAcycle function results in a metabolome similar to an unstressedexponential phase culture.Metabolomic Changes Are Largely Independent of the

�B-mediated General Stress Response—In staphylococci, �B

controls the general stress response and as such is activatedduring stress conditions, growth phase transitions, and mor-phological changes (45, 46). As stated previously, the regulationof many virulence determinants is affected by environmentalstresses; therefore, the stress-dependent activation of �B hasbeen an important area of research into the environmental reg-ulation of staphylococcal virulence determinants (47–49).Interestingly, �B does not directly respond to most environ-mental signals, suggesting another path to transduce stress sig-nals that is independent of �B. To test this possibility, an S. epi-dermidis sigB mutant strain (1457-sigB::dhfr) (19) was grownfor 2 or 6 h in TSB, TSB with 4% ethanol, or DTSB and themetabolomes were analyzed by NMR spectroscopy. The PCAscores plot demonstrates that the majority of metabolomicchanges associated with iron limitation and ethanol stressoccur largely independent of �B (Fig. 2A). Additionally, themetabolomic dendrogram confirms that the stressor-inducedmetabolic changes observed in strain 1457-sigB::dhfr are mostclosely associated with those in the TCA cycle mutant strain1457-acnA (Fig. 2B). Taken together, these data demonstratethat environmental stresses can alter the staphylococcalmetabolome by a largely �B-independent mechanism thatrequires the TCA cycle.Metabolomic Changes Precede Genetic Changes—Metabolo-

mic data demonstrate that ethanol stress, iron limitation, andTCAcycle inactivation decrease the intracellular concentrationof Gln relative to the wild-type strain grown in TSB medium(Tables 1 and 2). The two more likely explanations for thedecreased intracellular concentration of Gln are: (i) the stres-sors alter enzymatic activity, causing a decrease in the concen-tration of Gln; or (ii) the stressors decrease transcription ofgenes involved in the biosynthesis of Gln, resulting in adecreased concentration ofGln. If the first possibility is correct,then stressors will cause an increase in the transcription of Glnbiosynthetic genes as bacteria attempt to compensate for thedecreased availability of Gln. If the second possibility is correct,then stressors will cause a decrease in the transcription of Glnbiosynthetic genes. To determine which of these two possibili-ties was correct, we performed Northern blot analysis on glu-tamine synthetase (femC; also known as glnA) (Fig. 4). The datasuggest that the first possibility is the more correct one; specif-ically, bacteria are responding to metabolomic changes byincreasing transcription of genes necessary to counterbalancethose changes. Interestingly, in untreated wild-type cultures,the post-exponential growth phase concentration of Glu andGln increased between two and five times that of the exponen-tial growth phase concentration (data not shown), and thisincrease correlated with a post-exponential growth phasedecrease in glnA mRNA levels (Fig. 4). Similarly, Gln and Gluwere not detected in the NMR spectra of the aconitase mutantstrain, and this correlated with a high level of glnA mRNA inboth the exponential and the post-exponential growth phases.

The correlation between Gln and Glu concentrations and glnAmRNA levels is consistent with aGlnR-dependent regulation ofglnA transcription (2). This correlation was maintained forGln/Glu-sufficient or -insufficient conditions; however, theintermediate concentrations of Gln and Glu found during eth-anol stress and iron-limited growth (data not shown) producemixed glnA mRNA levels (Fig. 4). These data suggest that forthe concentrations of Gln and Glu to affect glnA transcription,the stress-induced concentration change must be sufficientlylarge.CcpA Responds to TCA Cycle-associated Metabolomic

Changes—Ethanol stress, iron limitation, and TCA cycleinactivation increased the post-exponential growth phaseconcentration of glucose-6-phosphate (Table 2). Glycolyticintermediates such as glucose-6-phosphate and fructose-1,6-bisphosphate increase the ATP-dependent phosphory-lation of the histidine-containing protein (HPr) by enhanc-ing the activity of the HPr kinase (50). The increase inphosphorylated HPr enhances its interaction with the catab-olite control protein A (CcpA) (51–53). CcpA primarilyfunctions as a repressor; however, it also activates transcrip-tion of genes involved in fermentation and overflow metab-olism (52, 54). In addition to the concentration of glucose-6-phosphate being increased by TCA cycle stress, theconcentrations of several fermentation products or interme-diates (i.e. lactate, acetate, and acetaldehyde) and the smallphosphodonor acetyl phosphate (an indicator of overflowmetabolism) were also increased (Table 1), consistent with achange in CcpA-mediated regulation. The repressor CodYalso contributes to the regulation of overflow metabolism(54); however, CodY responds to the intracellular concentra-tions of branched chain amino acids (55). TCA cycle stress didnot alter the concentrations of branched chain amino acidsbeyond the 5-fold threshold (Table 1), suggesting that theincrease in overflow metabolism was independent of CodY.In Staphylococcus aureus, CcpA enhances biofilm formation

and PIA biosynthesis, whereas CodY represses PIA synthesis

FIGURE 4. Northern blot analysis of RNAIII and glnA mRNA levels in theexponential (2 h) and post-exponential (6 h) phases of growth. To ensurethat equivalent quantities of RNA were loaded in the gel, 23 S and 16 S rRNAwere visualized by ethidium bromide staining and used as loading controls(bottom panel). The results are representative of at least two independentexperiments.

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(56, 57). In S. epidermidis, ethanol stress, iron limitation, andTCA cycle inactivation enhance biofilm formation and PIAsynthesis (7, 13, 14, 16, 24, 58). Based on the metabolomic dataand published observations, it was reasonable to hypothesizethat PIA biosynthesis was regulated in response to TCA cycle-associated metabolomic changes by a CcpA-dependent andCodY-independent mechanism. To test this hypothesis, ccpAand codY deletion mutants were constructed in strains 1457and 1457-acnA, and the amount of cell-associated PIA wasdetermined after 6 h of growth (Fig. 5). Consistent with previ-ous observations (7), TCAcycle inactivation (strain 1457-acnA)dramatically increased the accumulation of PIA, whereas nei-ther CodY nor CcpA had a dramatic effect on the post-expo-nential growth phase amount of PIA.When the codYmutationwas introduced into an aconitase mutant background, PIAaccumulation resembled the response in strain 1457-acnA, sug-

gesting that TCA cycle-associated changes in PIA biosynthesisare independent of CodY. In contrast to the codY-acnA doublemutant, the ccpA-acnA double mutant failed to produce PIA,strongly suggesting that some TCA cycle-associatedmetabolo-mic changes (i.e. glucose-6-phosphate) are sensed by CcpA,which in turn activates PIA biosynthesis.TCA Cycle Stress Decreases RNAIII Transcription—In S. au-

reus, inactivation of the TCA cycle increases the transcriptionor stability of the riboregulator RNAIII of the accessory generegulator (Agr) system (8, 9). Conversely, increasing TCA cycleactivity decreases the transcription or stability of RNAIII (59).The causal relationship betweenTCAcycle activity andRNAIIItranscript levels in S. aureus led us to examine whether dispar-ate environmental conditions would similarly affect RNAIIItranscription or message stability in S. epidermidis. In contrastto S. aureus, TCA cycle inactivation decreased RNAIII tran-scription or stability during the exponential and post-exponen-tial growth phases in S. epidermidis (Fig. 4). Importantly, etha-nol stress and iron limitation decreased RNAIII transcriptionor stability in a similar manner to TCA cycle inactivation (Fig.4). In total, these data suggest that environmental stresses actthrough the TCA cycle to elicit transcriptional changes to atleast two of the major staphylococcal virulence regulators (i.e.CcpA and RNAIII).

DISCUSSION

In the life cycle of S. epidermidis, the transition from a skin-resident, commensal state to adhering on implanted biomate-rials represents a dramatic environmental change. In mostpathogenic bacteria, environmental changes are accompaniedby changes in the transcription of virulence genes; thus, en-vironmental signals (e.g. nutrient replete, iron-limiting, or

oxygen-limiting growth conditions)commonly regulate virulence genetranscription (2, 60–64). AlthoughS. epidermidis has relatively few vir-ulence determinants, one of its pri-mary pathogenic effectors is theexopolysaccharide PIA (65–68).Previously, we demonstrated thatPIA biosynthesis is regulated byTCA cycle activity; specifically, re-pression of TCA cycle activity dra-matically enhances transcription ofPIA biosynthetic genes (icaADBC)and PIA accumulation (7, 24, 59). Inthis study, we demonstrate that dis-similar environmental signals de-crease TCA cycle activity (Fig. 1),resulting in common metabolomicchanges (Fig. 2 and Tables 1 and 2;summarized in Fig. 6) that alter theactivity of metabolite-responsive reg-ulators such as CcpA (Fig. 5). Thesedata lead us to propose that it is theTCA cycle itself that is “sensing” theenvironmental transition and trans-ducing this information into meta-

FIGURE 5. CcpA is required for PIA synthesis during TCA cycle stress. PIAimmunoblot assay of strain 1457 and isogenic mutants of acnA, codY, ccpA,codY/acnA, and ccpA/acnA grown for 6 h in TSB. The results are representativeof three independent experiments.

FIGURE 6. Summary of post-exponential growth phase metabolic changes associated with TCA cyclestress. Metabolites in green represent an increased concentration relative to the wild-type strain. Metab-olites in red represent a decreased concentration relative to the wild-type strain. Metabolites and path-ways in black are inferred from the data, but they are inconclusive. Glucose-6-P, glucose-6-phosphate;GlcN-6-P, GlcN-6-phosphate.

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bolic signals that activate or repress the activity of metabolite-responsive regulators to modulate the expression of PIA andother virulence determinants.As S. epidermidis transitions from residing on the skin to

being implanted in a host, it enters into an environment wherefree iron is present at a concentration of 10�18 M (61), a condi-tion antagonistic to TCA cycle activity (Fig. 1) (9, 11). Similarly,this transition dramatically decreases the availability of freeoxygen (the partial pressure of atmosphericO2 is 159mmHg atsea level, and this decreases to an estimated 3–5 mm Hg at thehost cell level), a condition that is also antagonistic to TCAcycle activity. Taken together, this type of environmental tran-sition is accompanied by conditions that are inhibitory to TCAcycle activity and stimulatory to PIA biosynthesis (7, 24). Inother words, the transition from an external environment to aninternal environment represses TCA cycle activity and primesS. epidermidis for PIA synthesis, which enhances biofilm for-mation and increases the likelihood of establishing a biomate-rial-associated infection.The significance of the data presented here is 5-fold. First, it

establishes amechanism by which well established regulators (e.g.CcpA) participate in responding to environmental stresses. Sec-ond, these data suggest how disparate environmental stimuli cancause common phenotypic changes (e.g. iron limitation and etha-nol stress both increase PIA synthesis and biofilm formation (13,69)). Third, these data suggest that a difficulty in attributing theeffects of an environmental stimulus, such as iron limitation, to aspecific regulator, such as the ferric uptake regulator (Fur) (70), isthat many of the effects are due to metabolite-responsive regula-tors reacting to changes in themetabolome.Fourth,S. epidermidishas a second general stress response system that is largely inde-pendent of the �B-controlled general stress response (Fig. 2).Finally, in bacteria, threemetabolic pathways (Embden-Meyer-hof-Parnas, pentose phosphate, and TCA cycle) produce the 13biosynthetic intermediates needed to synthesize all macromol-ecules in a bacterial cell. Therefore, by linking virulence factorsynthesis to the TCA cycle, bacteria are connecting virulence tothe availability of biosynthetic intermediates needed to synthe-size virulence determinants.

CONCLUSION

TCA cycle stress alters the intracellular concentrations ofmetabolites (Tables 1 and 2) relative to those of the wild-typestrain 1457. If the change in the concentration of ametabolite issufficiently large, then the activity of a regulator that canrespond to one of those metabolites may be altered. Therefore,these data present an exceptional opportunity to identify regu-lators that coordinatemetabolism and virulence in S. epidermi-dis. Although a considerable amount of research needs to bedone to determine which metabolite-responsive regulators areinvolved in responding to TCA cycle-associated metabolomicchanges, the work presented here sheds light on how environ-mental signals alter the bacterial metabolic status to regulateadaptation to a new environment.

Acknowledgments—We thank the reviewers for their helpful sugges-tions for improving the manuscript.

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Kreimer, Yefei Zhu, Robert Powers and Greg A. SomervilleMarat R. Sadykov, Bo Zhang, Steven Halouska, Jennifer L. Nelson, Lauren W.

Staphylococcus epidermidisSignal Transduction in Using NMR Metabolomics to Investigate Tricarboxylic Acid Cycle-dependent

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