PASTA kinase-dependent control of peptidoglycan synthesis via ReoM is required for cell wall stress responses, cytosolic survival, and virulence in Listeria monocytogenes

C-di-AMP accumulation disrupts glutathione metabolism and inhibits virulence program expression in Listeria monocytogenes

C-di-AMP is an essential second messenger in many bacteria but its levels must be regulated. Unregulated c-di-AMP accumulation attenuates the virulence of many bacterial pathogens, including those that do not require c-di-AMP for growth. However, the mechanisms by which c-di-AMP regulates bacterial pathogenesis remain poorly understood. In Listeria monocytogenes , a mutant lacking both c-di-AMP phosphodiesterases, denoted as the ΔPDE mutant, accumulates a high c-di-AMP level and is significantly attenuated in the mouse model of systemic infection. All key L. monocytogenes virulence genes are transcriptionally upregulated by the master transcription factor PrfA, which is activated by reduced glutathione (GSH) during infection. Our transcriptomic analysis revealed that the ΔPDE mutant is significantly impaired for the expression of virulence genes within the PrfA core regulon. Subsequent quantitative gene expression analyses validated this phenotype both at the basal level and upon PrfA activation by GSH. A constitutively active PrfA * variant, PrfA G145S, which mimics the GSH-bound conformation, restores virulence gene expression in ΔPDE but only partially rescues virulence defect. Through GSH quantification and uptake assays, we found that the ΔPDE strain is significantly depleted for GSH, and that c-di-AMP inhibits GSH uptake. Constitutive expression of gshF (encoding a GSH synthetase) does not restore GSH levels in the ΔPDE strain, suggesting that c-di-AMP inhibits GSH synthesis activity or promotes GSH catabolism. Taken together, our data reveals GSH metabolism as another pathway that is regulated by c-di-AMP. C-di-AMP accumulation depletes cytoplasmic GSH levels within L. monocytogenes that leads to impaired virulence program expression.

IMPORTANCE

C-di-AMP regulates both bacterial pathogenesis and interactions with the host. Although c-di-AMP is essential in many bacteria, its accumulation also attenuates the virulence of many bacterial pathogens. Therefore, disrupting c-di-AMP homeostasis is a promising antibacterial treatment strategy, and has inspired several studies that screened for chemical inhibitors of c-di-AMP phosphodiesterases. However, the mechanisms by which c-di-AMP accumulation diminishes bacterial pathogenesis are poorly understood. Such understanding will reveal the molecular function of c-di-AMP, and inform therapeutic development strategies. Here, we identify GSH metabolism as a pathway regulated by c-di-AMP that is pertinent to L. monocytogenes replication in the host. Given the role of GSH as a virulence signal, nutrient, and antioxidant, GSH depletion impairs virulence program expression and likely diminishes host adaptation.

Experimental measurement and computational prediction of bacterial Hanks-type Ser/Thr signaling system regulatory targets

Bacteria possess diverse classes of signaling systems that they use to sense and respond to their environments and execute properly timed developmental transitions. One widespread and evolutionarily ancient class of signaling systems are the Hanks-type Ser/Thr kinases, also sometimes termed "eukaryotic-like" due to their homology with eukaryotic kinases. In diverse bacterial species, these signaling systems function as critical regulators of general cellular processes such as metabolism, growth and division, developmental transitions such as sporulation, biofilm formation, and virulence, as well as antibiotic tolerance. This multifaceted regulation is due to the ability of a single Hanks-type Ser/Thr kinase to post-translationally modify the activity of multiple proteins, resulting in the coordinated regulation of diverse cellular pathways. However, in part due to their deep integration with cellular physiology, to date, we have a relatively limited understanding of the timing, regulatory hierarchy, the complete list of targets of a given kinase, as well as the potential regulatory overlap between the often multiple kinases present in a single organism. In this review, we discuss experimental methods and curated datasets aimed at elucidating the targets of these signaling pathways and approaches for using these datasets to develop computational models for quantitative predictions of target motifs. We emphasize novel approaches and opportunities for collecting data suitable for the creation of new predictive computational models applicable to diverse species.

Comparative genomics reveals the correlations of stress response genes and bacteriophages in developing antibiotic resistance of Staphylococcus saprophyticus

IMPORTANCE

Staphylococcus saprophyticus is the second most common bacteria associated with urinary tract infections (UTIs) in women. The antimicrobial treatment regimen for uncomplicated UTI is normally nitrofurantoin, trimethoprim-sulfamethoxazole (TMP-SMX), or a fluoroquinolone without routine susceptibility testing of S. saprophyticus recovered from urine specimens. However, TMP-SMX-resistant S. saprophyticus has been detected recently in UTI patients, as well as in our cohort. Herein, we investigated the understudied resistance patterns of this pathogenic species by linking genomic antibiotic resistance gene (ARG) content to susceptibility phenotypes. We describe ARG associations with known and novel SCCmec configurations as well as phage elements in S. saprophyticus, which may serve as intervention or diagnostic targets to limit resistance transmission. Our analyses yielded a comprehensive database of phenotypic data associated with the ARG sequence in clinical S. saprophyticus isolates, which will be crucial for resistance surveillance and prediction to enable precise diagnosis and effective treatment of S. saprophyticus UTIs.

PASTA-kinase-mediated signaling drives accumulation of the peptidoglycan synthesis protein MurAA to promote cephalosporin resistance in Enterococcus faecalis

The bacterial PASTA kinase, IreK, is required for intrinsic cephalosporin resistance in the Gram-positive opportunistic pathogen, Enterococcus faecalis. IreK activity is enhanced in response to cell wall stress, such as cephalosporin exposure. The downstream consequences of IreK activation are not well understood in E. faecalis, but recent work in other low-GC Gram-positive bacteria demonstrated PASTA kinase-dependent regulation of MurAA, an enzyme that performs the first committed step in the peptidoglycan synthesis pathway. Here, we used genetic suppressor selections to identify MurAA as a downstream target of IreK signaling in E. faecalis. Using complementary genetic and biochemical approaches, we demonstrated that MurAA abundance is regulated by IreK signaling in response to physiologically relevant cell wall stress to modulate substrate flux through the peptidoglycan synthesis pathway. Specifically, the IreK substrate, IreB, promotes proteolysis of MurAA through a direct physical interaction in a manner responsive to phosphorylation by IreK. MurAB, a homolog of MurAA, also promotes MurAA proteolysis and interacts directly with IreB. Our results therefore establish a connection between the cell wall stress sensor IreK and one critical physiological output to modulate peptidoglycan synthesis and drive cephalosporin resistance.

Secrets of getting started: Regulation of the first committed step of peptidoglycan synthesis by protein phosphorylation in Enterococcus and other Gram-positive bacteria

Regulation of the first committed step of peptidoglycan precursor synthesis by MurA-enzyme homologs has recently taken center stage in many different bacteria. In different low-GC Gram-positive bacteria, regulation of this step has been shown to be regulated by phosphorylation of homologs of the IreB/ReoM regulatory protein by PASTA-domain Ser/Thr-protein kinases. In this issue, Mascari, Little, and Kristich determine this regulatory pathway and its links to resistance to cephalosporin β-lactam antibiotics in the major human pathogen, Enterococcus faecalis (Efa). Unbiased genetic selections identified MurAA (MurA-family homolog) as the downstream target of IreB regulation in the absence of the IreK Ser/Thr-protein kinase. Physiological and biochemical approaches, including determination of MICs to ceftriaxone, Western blotting of MurAA cellular amounts, isotope incorporation into peptidoglycan sacculi, and thermal-shift binding assays of purified proteins, demonstrated that unphosphorylated IreB, together with proteins MurAB (MurZ-family homolog), and ReoY(Efa) negatively regulate MurAA stability and cellular amount by the ClpCP protease. Importantly, this paper supports the idea that ceftriaxone stimulates phosphorylation of IreB, which leads to increased cellular MurAA amount and precursor pathway flux required for E. faecalis cephalosporin resistance. Overall, findings in this paper significantly contribute to understanding variations of this central regulatory pathway in other low-GC Gram-positive bacteria.

Comparative secretomic and proteomic analysis reveal multiple defensive strategies developed by Vibrio cholerae against the heavy metal (Cd2+, Ni2+, Pb2+, and Zn2+) stresses

Vibrio cholerae is a common waterborne pathogen that can cause pandemic cholera in humans. The bacterium with heavy metal-tolerant phenotypes is frequently isolated from aquatic products, however, its tolerance mechanisms remain unclear. In this study, we investigated for the first time the response of such V. cholerae isolates (n = 3) toward the heavy metal (Cd2+, Ni2+, Pb2+, and Zn2+) stresses by comparative secretomic and proteomic analyses. The results showed that sublethal concentrations of the Pb2+ (200 μg/mL), Cd2+ (12.5 μg/mL), and Zn2+ (50 μg/mL) stresses for 2 h significantly decreased the bacterial cell membrane fluidity, but increased cell surface hydrophobicity and inner membrane permeability, whereas the Ni2+ (50 μg/mL) stress increased cell membrane fluidity (p < 0.05). The comparative secretomic and proteomic analysis revealed differentially expressed extracellular and intracellular proteins involved in common metabolic pathways in the V. cholerae isolates to reduce cytotoxicity of the heavy metal stresses, such as biosorption, transportation and effluxing, extracellular sequestration, and intracellular antioxidative defense. Meanwhile, different defensive strategies were also found in the V. cholerae isolates to cope with different heavy metal damage. Remarkably, a number of putative virulence and resistance-associated proteins were produced and/or secreted by the V. cholerae isolates under the heavy metal stresses, suggesting an increased health risk in the aquatic products.

Negative regulation of MurZ and MurA underlies the essentiality of GpsB- and StkP-mediated protein phosphorylation in Streptococcus pneumoniae D39

GpsB links peptidoglycan synthases to other proteins that determine the shape of the respiratory pathogen Streptococcus pneumoniae (pneumococcus; Spn) and other low-GC Gram-positive bacteria. GpsB is also required for phosphorylation of proteins by the essential StkP(Spn) Ser/Thr protein kinase. Here we report three classes of frequently arising chromosomal duplications (≈21-176 genes) containing murZ (MurZ-family homolog of MurA) or murA that suppress ΔgpsB or ΔstkP. These duplications arose from three different repeated sequences and demonstrate the facility of pneumococcus to modulate gene dosage of numerous genes. Overproduction of MurZ or MurA alone or overproduction of MurZ caused by ΔkhpAB mutations suppressed ΔgpsB or ΔstkP phenotypes to varying extents. ΔgpsB and ΔstkP were also suppressed by MurZ amino-acid changes distant from the active site, including one in commonly studied laboratory strains, and by truncation or deletion of the homolog of IreB(ReoM). Unlike in other Gram-positive bacteria, MurZ is predominant to MurA in pneumococcal cells. However, ΔgpsB and ΔstkP were not suppressed by ΔclpCP, which did not alter MurZ or MurA amounts. These results support a model in which regulation of MurZ and MurA activity, likely by IreB(Spn), is the only essential requirement for StkP-mediated protein phosphorylation in exponentially growing D39 pneumococcal cells.

A link between STK signalling and capsular polysaccharide synthesis in Streptococcus suis

Synthesis of capsular polysaccharide (CPS), an important virulence factor of pathogenic bacteria, is modulated by the CpsBCD phosphoregulatory system in Streptococcus. Serine/threonine kinases (STKs, e.g. Stk1) can also regulate CPS synthesis, but the underlying mechanisms are unclear. Here, we identify a protein (CcpS) that is phosphorylated by Stk1 and modulates the activity of phosphatase CpsB in Streptococcus suis, thus linking Stk1 to CPS synthesis. The crystal structure of CcpS shows an intrinsically disordered region at its N-terminus, including two threonine residues that are phosphorylated by Stk1. The activity of phosphatase CpsB is inhibited when bound to non-phosphorylated CcpS. Thus, CcpS modulates the activity of phosphatase CpsB thereby altering CpsD phosphorylation, which in turn modulates the expression of the Wzx-Wzy pathway and thus CPS production.

Listeria monocytogenes GlmR Is an Accessory Uridyltransferase Essential for Cytosolic Survival and Virulence

The cytosol of eukaryotic host cells is an intrinsically hostile environment for bacteria. Understanding how cytosolic pathogens adapt to and survive in the cytosol is critical to developing novel therapeutic interventions against these pathogens. The cytosolic pathogen Listeria monocytogenes requires glmR (previously known as yvcK), a gene of unknown function, for resistance to cell-wall stress, cytosolic survival, inflammasome avoidance, and, ultimately, virulence in vivo. In this study, a genetic suppressor screen revealed that blocking utilization of UDP N-acetylglucosamine (UDP-GlcNAc) by a nonessential wall teichoic acid decoration pathway restored resistance to lysozyme and partially restored virulence of ΔglmR mutants. In parallel, metabolomic analysis revealed that ΔglmR mutants are impaired in the production of UDP-GlcNAc, an essential peptidoglycan and wall teichoic acid (WTA) precursor. We next demonstrated that purified GlmR can directly catalyze the synthesis of UDP-GlcNAc from GlcNAc-1P and UTP, suggesting that it is an accessory uridyltransferase. Biochemical analysis of GlmR orthologues suggests that uridyltransferase activity is conserved. Finally, mutational analysis resulting in a GlmR mutant with impaired catalytic activity demonstrated that uridyltransferase activity was essential to facilitate cell-wall stress responses and virulence in vivo. Taken together, these studies indicate that GlmR is an evolutionary conserved accessory uridyltransferase required for cytosolic survival and virulence of L. monocytogenes. IMPORTANCE Bacterial pathogens must adapt to their host environment in order to cause disease. The cytosolic bacterial pathogen Listeria monocytogenes requires a highly conserved protein of unknown function, GlmR (previously known as YvcK), to survive in the host cytosol. GlmR is important for resistance to some cell-wall stresses and is essential for virulence. The ΔglmR mutant is deficient in production of an essential cell-wall metabolite, UDP-GlcNAc, and suppressors that increase metabolite levels also restore virulence. Purified GlmR can directly catalyze the synthesis of UDP-GlcNAc, and this enzymatic activity is conserved in both Bacillus subtilis and Staphylococcus aureus. These results highlight the importance of accessory cell wall metabolism enzymes in responding to cell-wall stress in a variety of Gram-positive bacteria.

Chromosomal Duplications of MurZ (MurA2) or MurA (MurA1), Amino Acid Substitutions in MurZ (MurA2), and Absence of KhpAB Obviate the Requirement for Protein Phosphorylation in Streptococcus pneumoniae D39

GpsB links peptidoglycan synthases to other proteins that determine the shape of the respiratory pathogen Streptococcus pneumoniae (pneumococcus; Spn ) and other low-GC Gram-positive bacteria. GpsB is also required for phosphorylation of proteins by the essential StkP( Spn ) Ser/Thr protein kinase. Here we report three classes of frequently arising chromosomal duplications (≈21-176 genes) containing murZ (MurZ-family homolog of MurA) or murA that suppress Δ gpsB or Δ stkP . These duplications arose from three different repeated sequences and demonstrate the facility of pneumococcus to modulate gene dosage of numerous genes. Overproduction of MurZ or MurA alone or overexpression of MurZ caused by Δ khpAB mutations suppressed Δ gpsB or Δ stkP phenotypes to varying extents. Δ gpsB and Δ stkP were also suppressed by MurZ amino-acid changes distant from the active site, including one in commonly studied laboratory strains, and by truncation or deletion of the homolog of IreB(ReoM). Unlike in other Gram-positive bacteria, MurZ is predominant to MurA in pneumococcal cells. However, Δ gpsB and Δ stkP were not suppressed by Δ clpCP , which did not alter MurZ or MurA amounts. These results support a model in which regulation of MurZ and MurA activity, likely by IreB( Spn ), is the only essential requirement for protein phosphorylation in exponentially growing D39 pneumococcal cells.

GpsB Promotes PASTA Kinase Signaling and Cephalosporin Resistance in Enterococcus faecalis

Enterococci are opportunistic pathogens that can cause severe bacterial infections. Treatment of these infections is challenging because enterococci possess intrinsic and acquired mechanisms of resistance to commonly used antibiotics, including cephalosporins. The transmembrane serine/threonine PASTA kinase, IreK, is an important determinant of enterococcal cephalosporin resistance. Upon exposure to cephalosporins, IreK becomes autophosphorylated, which stimulates its kinase activity to phosphorylate downstream substrates and drive cephalosporin resistance. However, the molecular mechanisms that modulate IreK autophosphorylation in response to cell wall stress, such as that induced by cephalosporins, remain unknown. A cytoplasmic protein, GpsB, promotes signaling by PASTA kinase homologs in other bacterial species, but the function of enterococcal GpsB has not been previously investigated. We used in vitro and in vivo approaches to test the hypothesis that enterococcal GpsB promotes IreK signaling in response to cephalosporins to drive cephalosporin resistance. We found that GpsB promotes IreK activity both in vivo and in vitro. This effect is required for cephalosporins to trigger IreK autophosphorylation and activation of an IreK-dependent signaling pathway, and thereby is also required for enterococcal intrinsic cephalosporin resistance. Moreover, analyses of GpsB mutants and a ΔireK gpsB double mutant suggest that GpsB has an additional function, beyond regulation of IreK activity, which is required for optimal growth and full cephalosporin resistance. Collectively, our data provide new insights into the mechanism of signal transduction by the PASTA kinase IreK and the mechanism of enterococcal intrinsic cephalosporin resistance. IMPORTANCE Enterococci are opportunistic pathogens that can cause severe bacterial infections. Treatment of these infections is challenging because enterococci possess intrinsic and acquired resistance to commonly used antibiotics. In particular, enterococci are intrinsically resistant to cephalosporin antibiotics, a trait that requires the activity of a transmembrane serine/threonine kinase, IreK, which belongs to the bacterial PASTA kinase family. The mechanisms by which PASTA kinases are regulated in cells are poorly understood. Here, we report that the cytoplasmic protein GpsB directly promotes IreK signaling in enterococci to drive cephalosporin resistance. Thus, we provide new insights into PASTA kinase regulation and control of enterococcal cephalosporin resistance, and suggest that GpsB could be a promising target for new therapeutics to disable cephalosporin resistance.

In-Depth Characterization of the Clostridioides difficile Phosphoproteome to Identify Ser/Thr Kinase Substrates

Clostridioides difficile is the leading cause of postantibiotic diarrhea in adults. During infection, the bacterium must rapidly adapt to the host environment by using survival strategies. Protein phosphorylation is a reversible post-translational modification employed ubiquitously for signal transduction and cellular regulation. Hanks-type serine/threonine kinases (STKs) and serine/threonine phosphatases have emerged as important players in bacterial cell signaling and pathogenicity. C. difficile encodes two STKs (PrkC and CD2148) and one phosphatase. We optimized a titanium dioxide phosphopeptide enrichment approach to determine the phosphoproteome of C. difficile. We identified and quantified 2500 proteins representing 63% of the theoretical proteome. To identify STK and serine/threonine phosphatase targets, we then performed comparative large-scale phosphoproteomics of the WT strain and isogenic ΔprkC, CD2148, Δstp, and prkC CD2148 mutants. We detected 635 proteins containing phosphorylated peptides. We showed that PrkC is phosphorylated on multiple sites in vivo and autophosphorylates in vitro. We were unable to detect a phosphorylation for CD2148 in vivo, whereas this kinase was phosphorylated in vitro only in the presence of PrkC. Forty-one phosphoproteins were identified as phosphorylated under the control of CD2148, whereas 114 proteins were phosphorylated under the control of PrkC including 27 phosphoproteins more phosphorylated in the ∆stp mutant. We also observed enrichment for phosphothreonine among the phosphopeptides more phosphorylated in the Δstp mutant. Both kinases targeted pathways required for metabolism, translation, and stress response, whereas cell division and peptidoglycan metabolism were more specifically controlled by PrkC-dependent phosphorylation in agreement with the phenotypes of the ΔprkC mutant. Using a combination of approaches, we confirmed that FtsK was phosphorylated in vivo under the control of PrkC and that Spo0A was a substrate of PrkC in vitro. This study provides a detailed mapping of kinase-substrate relationships in C. difficile, paving the way for the identification of new biomarkers and therapeutic targets.

Listeria monocytogenes genes supporting growth under standard laboratory cultivation conditions and during macrophage infection

The Gram-positive bacterium Listeria monocytogenes occurs widespread in the environment and infects humans when ingested along with contaminated food. Such infections are particularly dangerous for risk group patients, for whom they represent a life-threatening disease. To invent novel strategies to control contamination and disease, it is important to identify those cellular processes that maintain pathogen growth inside and outside the host. Here, we have applied transposon insertion sequencing (Tn-Seq) to L. monocytogenes for the identification of such processes on a genome-wide scale. Our approach identified 394 open reading frames that are required for growth under standard laboratory conditions and 42 further genes, which become necessary during intracellular growth in macrophages. Most of these genes encode components of the translation machinery and act in chromosome-related processes, cell division, and biosynthesis of the cellular envelope. Several cofactor biosynthesis pathways and 29 genes with unknown functions are also required for growth, suggesting novel options for the development of antilisterial drugs. Among the genes specifically required during intracellular growth are known virulence factors, genes compensating intracellular auxotrophies, and several cell division genes. Our experiments also highlight the importance of PASTA kinase signaling for general viability and of glycine metabolism and chromosome segregation for efficient intracellular growth of L. monocytogenes.

MurA escape mutations uncouple peptidoglycan biosynthesis from PrkA signaling

Gram-positive bacteria are protected by a thick mesh of peptidoglycan (PG) completely engulfing their cells. This PG network is the main component of the bacterial cell wall, it provides rigidity and acts as foundation for the attachment of other surface molecules. Biosynthesis of PG consumes a high amount of cellular resources and therefore requires careful adjustments to environmental conditions. An important switch in the control of PG biosynthesis of Listeria monocytogenes, a Gram-positive pathogen with a high infection fatality rate, is the serine/threonine protein kinase PrkA. A key substrate of this kinase is the small cytosolic protein ReoM. We have shown previously that ReoM phosphorylation regulates PG formation through control of MurA stability. MurA catalyzes the first step in PG biosynthesis and the current model suggests that phosphorylated ReoM prevents MurA degradation by the ClpCP protease. In contrast, conditions leading to ReoM dephosphorylation stimulate MurA degradation. How ReoM controls degradation of MurA and potential other substrates is not understood. Also, the individual contribution of the ~20 other known PrkA targets to PG biosynthesis regulation is unknown. We here present murA mutants which escape proteolytic degradation. The release of MurA from ClpCP-dependent proteolysis was able to activate PG biosynthesis and further enhanced the intrinsic cephalosporin resistance of L. monocytogenes. This latter effect required the RodA3/PBP B3 transglycosylase/transpeptidase pair. One murA escape mutation not only fully rescued an otherwise non-viable prkA mutant during growth in batch culture and inside macrophages but also overcompensated cephalosporin hypersensitivity. Our data collectively indicate that the main purpose of PrkA-mediated signaling in L. monocytogenes is control of MurA stability during standard laboratory growth conditions and intracellular growth in macrophages. These findings have important implications for the understanding of PG biosynthesis regulation and β-lactam resistance of L. monocytogenes and related Gram-positive bacteria.

Peptidoglycan (PG) is the main component of the bacterial cell wall and many of the PG synthesizing enzymes are antibiotic targets. We previously have discovered a new signaling route controlling PG production in the human pathogen Listeria monocytogenes. This route also determines the intrinsic resistance of L. monocytogenes against cephalosporins, a group of β-lactam antibiotics. Signaling involves PrkA, a membrane-embedded protein kinase, that is activated during cell wall stress to phosphorylate its target ReoM. Depending on its phosphorylation, ReoM activates or inactivates PG production by controlling the proteolytic stability of MurA, which catalyzes the first step in PG biosynthesis. MurA degradation depends on the ClpCP protease and we here have isolated murA mutations that escape this degradation. Using these mutants, we could show that regulation of PG biosynthesis through control of MurA stability is an important purpose of PrkA-mediated signaling in L. monocytogenes. Further experiments identified the transglycosylase RodA and the transpeptidase PBP B3 as additional downstream factors. Our results suggest that both proteins act together to translate the signals received by PrkA into adjustment of PG biosynthesis. These findings shed new light on the regulation of PG biosynthesis in Gram-positive bacteria with intrinsic β-lactam resistance.

Use of an Interspecies Chimeric Receptor for Inducible Gene Expression Reveals that Metabolic Flux through the Peptidoglycan Biosynthesis Pathway is an Important Driver of Cephalosporin Resistance in Enterococcus faecalis

Cephalosporins are commonly prescribed antibiotics that impair cross-linking of the bacterial cell wall. The Gram-positive opportunistic pathogen, Enterococcus faecalis, is intrinsically resistant to these antibiotics and proliferates substantially during cephalosporin therapy. As a result, the usage of cephalosporins has the potential to lead to life-threatening enterococcal infections. Yet, the molecular mechanisms that drive cephalosporin resistance (CR) are incompletely understood. Previously, we demonstrated that MurAA, an enzyme that catalyzes the first committed step in peptidoglycan (PG) synthesis, is required for CR. However, the mechanism by which MurAA contributes to CR remained unknown. Here, we tested the hypothesis that MurAA drives CR by controlling metabolic flux through the PG synthesis pathway. To do so, we developed and exploited an inducible gene expression system for E. faecalis based on an interspecies chimeric receptor that responds to exogenous nitrate for control of expression from a NisR-regulated promoter (P_nisA). We used this tool to demonstrate synthetic lethality of MurAA with its homolog MurAB, to titrate expression of MurAA, and to conditionally deplete multiple PG synthesis enzymes downstream of MurAA that are predicted to be essential. These genetic manipulations, in addition to pharmacological inhibition of the PG synthesis pathway, all led to reductions in PG synthesis that correlated with reductions in CR. Our findings are consistent with a model in which control of metabolic flux through the PG synthesis pathway is a major driver of CR. IMPORTANCE Enterococci are dangerous opportunistic pathogens with the potential to cause life-threatening infections due in part to their intrinsic resistance to cephalosporin antibiotics. Elucidating the molecular mechanisms that provide this resistance is critical for the development of strategies to both prevent and treat enterococcal infections. Here, we report that the cell wall synthesis enzyme, MurAA, drives cephalosporin resistance at least in part by controlling metabolic flux through the peptidoglycan synthesis pathway. To demonstrate this, we designed and validated an inducible gene expression system based on a chimeric receptor that is functional in multiple lineages of E. faecalis. In doing so, we provided a new tool for inducible gene expression with broad applications beyond our studies, including studies of essential genes.