Roles of DNA sequence and sigma A factor in transcription of the vraSR operon

Effects of simvastatin on the mevalonate pathway and cell wall integrity of Staphylococcus aureus

AIMS

To investigate the effects of simvastatin as an antimicrobial, considering its influence on the mevalonate pathway and the bacterial cell wall of S. aureus.

METHODS AND RESULTS

S. aureus ATCC 29213 and 33591 were exposed to simvastatin in the presence of exogenous mevalonate to determine whether mevalonate could reverse the inhibition. S. aureus was also treated with simvastatin and gene expression analysis assays were performed to evaluate genes associated with the mevalonate pathway (mvaA, mvaS, mvaK1, and mvaK2), peptidoglycan synthesis (uppS, uppP, and murG), and cell wall stress (vraX, sgtB, and tcaA). Transmission electron microscopy was used to identify the presence of morphological changes. The data were compared using two-way ANOVA and Bonferroni post-test, or the Mann-Whitney test. Addition of exogenous mevalonate was able to partially or completely reverse the inhibition caused by simvastatin. A significant increase of the vraX gene and a reduction of the mvaA gene were observed, together with changes in bacterial morphology.

CONCLUSION

Simvastatin can exert its antimicrobial effect by means of changes in the cell wall associated with the mevalonate pathway.

Molecular physiological characterization of the dynamics of persister formation in Staphylococcus aureus

Bacteria possess the ability to enter a growth-arrested state known as persistence in order to survive antibiotic exposure. Clinically, persisters are regarded as the main causative agents for chronic and recurrent infectious diseases. To combat this antibiotic-tolerant population, a better understanding of the molecular physiology of persisters is required. In this study, we collected samples at different stages of the biphasic kill curve to reveal the dynamics of the cellular molecular changes that occur in the process of persister formation. After exposure to antibiotics with different modes of action, namely, vancomycin and enrofloxacin, similar persister levels were obtained. Both shared and distinct stress responses were enriched for the respective persister populations. However, the dynamics of the presence of proteins linked to the persister phenotype throughout the biphasic kill curve and the molecular profiles in a stable persistent population did show large differences, depending on the antibiotic used. This suggests that persisters at the molecular level are highly stress specific, emphasizing the importance of characterizing persisters generated under different stress conditions. Additionally, although generated persisters exhibited cross-tolerance toward tested antibiotics, combined therapies were demonstrated to be a promising approach to reduce persister levels. In conclusion, this investigation sheds light on the stress-specific nature of persisters, highlighting the necessity of tailored treatment approaches and the potential of combined therapy.

Revisiting the Role of VraTSR in Staphylococcus aureus Response to Cell Wall-Targeting Antibiotics

Exposure of Staphylococcus aureus to cell wall inhibitors leads to the activation of the VraTSR three-component sensory regulatory system. This system is composed of VraS, a membrane histidine kinase; VraR, its cognate response regulator, and VraT, a protein required for the full activity of VraTSR. The exact function of VraT remains mostly uncharacterized, although it has been proposed to detect the unknown stimulus sensed by the VraTSR system. Here, we elucidate the topology of VraT, showing that its C-terminal domain is extracellular. We also demonstrate that the signal sensed by VraTSR is not an intermediate in the peptidoglycan synthesis pathway, as previously suggested. Instead, the specific inhibition of the penicillin-binding protein (PBP)2 leads to strong activation of the system.

IMPORTANCE The Gram-positive bacterial pathogen Staphylococcus aureus is currently the second most frequent cause of global deaths associated with antibiotic resistance. Its response to cell wall-targeting antibiotics requires the VraTSR three-component system, which senses cell wall damage. Here, we show that the signal sensed by VraTSR is not an intermediate in the peptidoglycan synthesis pathway, as previously suggested. Instead, the specific inhibition of the penicillin-binding protein (PBP)2, the major peptidoglycan synthase in S. aureus, leads to strong activation of the system. Identifying the exact cell wall damage signal is key to fully understanding the response of S. aureus to cell wall-targeting antibiotics.

The dimerization interface in VraR is essential for induction of the cell wall stress response in Staphylococcus aureus: a potential druggable target

Background

Staphylococcus aureus remains a medical challenge in the treatment of bacterial infections. It has acquired resistance to commonly used antibiotics, and to those considered to be the last weapons in treating staphylococcal infections, such as vancomycin. Studies have revealed that S. aureus is capable of mounting a rapid response to antibiotics that target cell wall peptidoglycan biosynthesis, such as β-lactams and vancomycin. The two-component system VraSR has been linked to the coordination of this response. VraS is a histidine kinase that undergoes autophosphorylation in the presence of signals elicited upon cell wall damage and it then transfers its phosphoryl group to VraR. VraR is a response regulator protein that functions as a transcription factor. Phosphorylation of VraR leads to its dimerization, which is required for optimum binding to its target promoters. Two-component systems have been targeted for the development of antibacterial agents. Deletion of the vraS or vraR gene has been shown to re-sensitize S. aureus to β-lactams and vancomycin.

Results

In this study, we explored perturbation of the VraR phosphorylation-induced activation as a means to inhibit the VraSR-mediated signal transduction pathway. We show that dimerization of VraR is essential for the phosphorylation-induced activation of VraR. A single point mutation in the dimerization interface of VraR, in which Met13 was replaced by Ala, led to the inability of VraR to dimerize and to bind optimally to the target promoter. The consequences of these in vitro molecular deficiencies are equally dramatic in vivo. Complementation of a vraR deletion S. aureus strain with the vraRM13Ala mutant gene failed to induce the cell wall stress response.

Conclusions

This study highlights the potential of targeting the phosphorylation-induced dimerization of VraR to disrupt the S. aureus cell wall stress response and in turn to re-sensitize S. aureus to β-lactams and vancomycin.

Electronic supplementary material

The online version of this article (10.1186/s12866-019-1529-0) contains supplementary material, which is available to authorized users.

The Staphylococcal Cell Wall

Dating back to the 1960s, initial studies on the staphylococcal cell wall were driven by the need to clarify the mode of action of the first antibiotics and the resistance mechanisms developed by the bacteria. During the following decades, the elucidation of the biosynthetic path and primary composition of staphylococcal cell walls was propelled by advances in microbial cell biology, specifically, the introduction of high-resolution analytical techniques and molecular genetic approaches. The field of staphylococcal cell wall gradually gained its own significance as the complexity of its chemical structure and involvement in numerous cellular processes became evident, namely its versatile role in host interactions, coordination of cell division and environmental stress signaling.This chapter includes an updated description of the anatomy of staphylococcal cell walls, paying particular attention to information from the last decade, under four headings: high-resolution analysis of the Staphylococcus aureus peptidoglycan; variations in peptidoglycan composition; genetic determinants and enzymes in cell wall synthesis; and complex functions of cell walls. The latest contributions to a more precise picture of the staphylococcal cell envelope were possible due to recently developed state-of-the-art microscopy and spectroscopy techniques and to a wide combination of -omics approaches, that are allowing to obtain a more integrative view of this highly dynamic structure.

An Adaptive Mutation in Enterococcus faecium LiaR Associated with Antimicrobial Peptide Resistance Mimics Phosphorylation and Stabilizes LiaR in an Activated State

The cyclic antimicrobial lipopeptide daptomycin (DAP) triggers the LiaFSR membrane stress response pathway in enterococci and many other Gram-positive organisms. LiaR is the response regulator that, upon phosphorylation, binds in a sequence-specific manner to DNA to regulate transcription in response to membrane stress. In clinical settings, non-susceptibility to DAP by Enterococcus faecium is correlated frequently with a mutation in LiaR of Trp73 to Cys (LiaR^W73C). We have determined the structure of the activated E. faecium LiaR protein at 3.2Å resolution and, in combination with solution studies, show that the activation of LiaR induces the formation of a LiaR dimer that increases LiaR affinity at least 40-fold for the extended regulatory regions upstream of the liaFSR and liaXYZ operons. In vitro, LiaR^W73C induces phosphorylation-independent dimerization of LiaR and provides a biochemical basis for non-susceptibility to DAP by the upregulation of the LiaFSR regulon. A comparison of the E. faecalis LiaR, E. faecium LiaR, and the LiaR homolog from Staphylococcus aureus (VraR) and the mutations associated with DAP resistance suggests that physicochemical properties such as oligomerization state and DNA specificity, although tuned to the biology of each organism, share some features that could be targeted for new antimicrobials.

Insights into the regulation of small RNA expression: SarA represses the expression of two sRNAs in Staphylococcus aureus

The opportunistic pathogen Staphylococcus aureus expresses transcription factors (TFs) and regulatory small RNAs (sRNAs) which are essential for bacterial adaptation and infectivity. Until recently, the study of S. aureus sRNA gene expression regulation was under investigated, but it is now an expanding field. Here we address the regulation of Srn_3610_SprC sRNA, an attenuator of S. aureus virulence. We demonstrate that SarA TF represses srn_3610_sprC transcription. DNase I footprinting and deletion analyses show that the SarA binding site on srn_3610_sprC belongs to an essential 22 bp DNA region. Comparative analysis also revealed another possible site, this time in the srn_9340 promoter. SarA specifically binds these two sRNA promoters with high affinity in vitro and also represses their transcription in vivo. Chromatin immunoprecipitation (ChIP) assays confirmed SarA attachment to both promoters. ChIP and electrophoretic mobility shift assays targeting σ^A RNA polymerase subunit or using bacterial RNA polymerase holoenzyme suggested that SarA and the σ^A bind srn_3610_sprC and srn_9340 promoters in a mutually exclusive way. Beyond the mechanistic study of SarA repression of these two sRNAs, this work also suggests that some S. aureus sRNAs belong to the same regulon and act jointly in responding to environmental changes.

Regulatory mechanism of the three-component system HptRSA in glucose-6-phosphate uptake in Staphylococcus aureus

Glucose-6-phosphate (G6P) is a common alternative carbon source for various bacteria, and its uptake usually relies on the hexose phosphate antiporter UhpT. In the human pathogenic bacterium Staphylococcus aureus, the ability to utilize different nutrients, particularly alternative carbon source uptake in glucose-limiting conditions, is essential for its fitness in the host environment during the infectious process. It has been reported that G6P uptake in S. aureus is regulated by the three-component system HptRSA. When G6P is provided as the only carbon source, HptRSA could sense extracellular G6P and activate uhpT expression to facilitate G6P utilization. However, the regulatory mechanism of HptRSA is still unclear. In this study, we further investigated the HptRSA system in S. aureus. First, we confirmed that HptRSA is necessary for the normal growth of this pathogen in chemically defined medium with G6P supplementation, and we discovered that HptRSA could exclusively sense extracellular G6P compared to the other organophosphates we tested. Next, using isothermal titration calorimetry, we found that HptA could bind to G6P, suggesting that it may be the G6P sensor. After that experiment, using an electrophoresis mobility shift assay, we verified that the response regulator HptR could directly bind to the uhpT promoter and identified a putative binding site from -67 to -96-bp. Subsequently, we created different point mutations in the putative binding site and revealed that the entire 30-bp sequence is essential for HptR regulation. In summary, we unveiled the regulatory mechanism of the HptRSA system in S. aureus, HptA most likely functions as the G6P sensor, and HptR could implement its regulatory function by directly binding to a conserved, approximately 30-bp sequence in the uhpT promoter.

A variable DNA recognition site organization establishes the LiaR-mediated cell envelope stress response of enterococci to daptomycin

LiaR is a ‘master regulator’ of the cell envelope stress response in enterococci and many other Gram-positive organisms. Mutations to liaR can lead to antibiotic resistance to a variety of antibiotics including the cyclic lipopeptide daptomycin. LiaR is phosphorylated in response to membrane stress to regulate downstream target operons. Using DNA footprinting of the regions upstream of the liaXYZ and liaFSR operons we show that LiaR binds an extended stretch of DNA that extends beyond the proposed canonical consensus sequence suggesting a more complex level of regulatory control of target operons. We go on to determine the biochemical and structural basis for increased resistance to daptomycin by the adaptive mutation to LiaR (D191N) first identified from the pathogen Enterococcus faecalis S613. LiaR^D191N increases oligomerization of LiaR to form a constitutively activated tetramer that has high affinity for DNA even in the absence of phosphorylation leading to increased resistance. Crystal structures of the LiaR DNA binding domain complexed to the putative consensus sequence as well as an adjoining secondary sequence show that upon binding, LiaR induces DNA bending that is consistent with increased recruitment of RNA polymerase to the transcription start site and upregulation of target operons.

Modulation of cell wall synthesis and susceptibility to vancomycin by the two-component system AirSR in Staphylococcus aureus NCTC8325

Background

Vancomycin has been the medication of last resort to cure infections caused by Staphylococcus aureus since the increase in the prevalence of methicillin-resistant Staphylococcus aureus (MRSA). Some strains have developed vancomycin-intermediate resistance, which is generally associated with altered expression of or mutations in some part of the two-component system (TCS), such as GraSR, VraSR, and WalKR.

Results

We deleted the AirSR TCS in S. aureus NCTC8325 and compared the resultant transcript levels with those of its parent strain using microarray analysis. The results indicated that more than 20 genes that are related to cell wall metabolism were down-regulated in the airSR mutant. The airSR mutant exhibited reduced autolysis rates and reduced viability in the presence of vancomycin. Real-time reverse transcription PCR and DNA mobility shift assays verified that AirR can directly bind to and regulate genes that function in cell wall metabolism (cap, pbp1, and ddl) and autolysis (lytM).

Conclusions

AirSR acts as a positive regulator in cell wall biosynthesis and turnover in Staphylococcus aureus NCTC8325.

Two unique phosphorylation-driven signaling pathways crosstalk in Staphylococcus aureus to modulate the cell-wall charge: Stk1/Stp1 meets GraSR

The Stk1/Stp1 and GraSR signal-transduction pathways are two distinct pathways in Staphylococcus aureus that rely on a reversible phosphorylation process in transducing external stimuli intracellularly. Stk1/Stp1 is an eukaryote-like Ser/Thr kinase phosphatase pair involved in purine biosynthesis, cell-wall metabolism, and autolysis. GraSR is a two-component system involved in resistance to cationic antimicrobial peptides. Both systems are implicated in S. aureus virulence and resistance to cell-wall inhibitors. Our study shows that the response regulator protein GraR undergoes phosphorylation by Stk1 at three threonine residues in the DNA-binding domain. Phosphorylation by Stk1 depends on the structural integrity of GraR as well as the amino acid sequences flanking the phosphorylation sites. Its homologue in Bacillus subtilis , BceR, which harbors two of the three phosphorylation sites in GraR, does not undergo Stk1-dependent phosphorylation. GraR is involved in regulation of the dltABCD operon, the gene products of which add the d-Ala on wall teichoic acid (WTA). Investigation of WTA isolated from the S. aureus RN6390 ΔgraR strain by NMR spectroscopy showed a clear negative effect that graR deletion has on the d-Ala content of WTA. Moreover, complementation of ΔgraR mutant with graR lacking the Stk1 phosphorylation sites mirrors this effect. These findings provide evidence that GraR is a target of Stk1 in vivo and suggest that modification of WTA by d-Ala is modulated by Stk1. The crosstalk between these two otherwise independent signaling pathways may facilitate S. aureus interaction with its environment to modulate processes such as cell growth and division and virulence.

Phosphorylation-dependent conformational changes and domain rearrangements in Staphylococcus aureus VraR activation

Staphylococcus aureus VraR, a vancomycin-resistance-associated response regulator, activates a cell-wall-stress stimulon in response to antibiotics that inhibit cell wall formation. X-ray crystal structures of VraR in both unphosphorylated and beryllofluoride-activated states have been determined, revealing a mechanism of phosphorylation-induced dimerization that features a deep hydrophobic pocket at the center of the receiver domain interface. Unphosphorylated VraR exists in a closed conformation that inhibits dimer formation. Phosphorylation at the active site promotes conformational changes that are propagated throughout the receiver domain, promoting the opening of a hydrophobic pocket that is essential for homodimer formation and enhanced DNA-binding activity. This prominent feature in the VraR dimer can potentially be exploited for the development of novel therapeutics to counteract antibiotic resistance in this important pathogen.

VraT/YvqF is required for methicillin resistance and activation of the VraSR regulon in Staphylococcus aureus

Staphylococcus aureus infections caused by strains that are resistant to all forms of penicillin, so-called methicillin-resistant S. aureus (MRSA) strains, have become common. One strategy to counter MRSA infections is to use compounds that resensitize MRSA to methicillin. S. aureus responds to diverse classes of cell wall-inhibitory antibiotics, like methicillin, using the two-component regulatory system VraSR (vra) to up- or downregulate a set of genes (the cell wall stimulon) that presumably facilitates resistance to these antibiotics. Accordingly, VraS and VraR mutations decrease resistance to methicillin, vancomycin, and daptomycin cell wall antimicrobials. vraS and vraR are encoded together on a transcript downstream of two other genes, which we call vraU and vraT (previously called yvqF). By producing nonpolar deletions in vraU and vraT in a USA300 MRSA clinical isolate, we demonstrate that vraT is essential for optimal expression of methicillin resistance in vitro, whereas vraU is not required for this phenotype. The deletion of vraT also improved the outcomes of oxacillin therapy in mouse models of lung and skin infection. Since vraT expressed in trans did not complement a vra operon deletion, we conclude that VraT does not inactivate the antimicrobial. Genome-wide transcriptional microarray experiments reveal that VraT facilitates resistance by playing a necessary regulatory role in the VraSR-mediated cell wall stimulon. Our data prove that VraTSR comprise a novel three-component regulatory system required to facilitate resistance to cell wall agents in S. aureus. We also provide the first in vivo proof of principle for using VraT as a sole target to resensitize MRSA to β-lactams.

Staphylococcus aureus methicillin-resistance factor fmtA is regulated by the global regulator SarA

fmtA encodes a low-affinity penicillin binding protein in Staphylococcus aureus. It is part of the core cell wall stimulon and is involved in methicillin resistance in S. aureus. Here, we report that the transcription factor, SarA, a pleiotropic regulator of virulence genes in S. aureus, regulates the expression of fmtA. In vitro binding studies with purified SarA revealed that it binds to specific sites within the 541-bp promoter region of fmtA. Mutation of a key residue of the regulatory activity of SarA (Arg90) abolished binding of SarA to the fmtA promoter, suggesting that SarA binds specifically to the fmtA promoter region. In vivo analysis of the fmtA promoter using a lux operon reporter fusion show high level expression following oxacillin induction, which was abrogated in a sarA mutant strain. These data suggest that SarA is essential for the induction of fmtA expression by cell wall-specific antibiotics. Further, in vitro transcription studies show that SarA enhances fmtA transcription and suggest that regulation of fmtA could be via a SigA-dependent mechanism. Overall, our results show that SarA plays a direct role in the regulation of fmtA expression via binding to the fmtA promoter.

Deletion of hypothetical wall teichoic acid ligases in Staphylococcus aureus activates the cell wall stress response

The Staphylococcus aureus cell wall stress stimulon (CWSS) is activated by cell envelope-targeting antibiotics or depletion of essential cell wall biosynthesis enzymes. The functionally uncharacterized S. aureus LytR-CpsA-Psr (LCP) proteins, MsrR, SA0908 and SA2103, all belong to the CWSS. Although not essential, deletion of all three LCP proteins severely impairs cell division. We show here that VraSR-dependent CWSS expression was up to 250-fold higher in single, double and triple LCP mutants than in wild type S. aureus in the absence of external stress. The LCP triple mutant was virtually depleted of wall teichoic acids (WTA), which could be restored to different degrees by any of the single LCP proteins. Subinhibitory concentrations of tunicamycin, which inhibits the first WTA synthesis enzyme TarO (TagO), could partially complement the severe growth defect of the LCP triple mutant. Both of the latter findings support a role for S. aureus LCP proteins in late WTA synthesis, as in Bacillus subtilis where LCP proteins were recently proposed to transfer WTA from lipid carriers to the cell wall peptidoglycan. Intrinsic activation of the CWSS upon LCP deletion and the fact that LCP proteins were essential for WTA-loading of the cell wall, highlight their important role(s) in S. aureus cell envelope biogenesis.

The posttranslocational chaperone lipoprotein PrsA is involved in both glycopeptide and oxacillin resistance in Staphylococcus aureus

Understanding in detail the factors which permit Staphylococcus aureus to counteract cell wall-active antibiotics is a prerequisite to elaborating effective strategies to prolong the usefulness of these drugs and define new targets for pharmacological intervention. Methicillin-resistant S. aureus (MRSA) strains are major pathogens of hospital-acquired and community-acquired infections and are most often treated with glycopeptides (vancomycin and teicoplanin) because of their resistance to most penicillins and a limited arsenal of clinically proven alternatives. In this study, we examined PrsA, a lipid-anchored protein of the parvulin PPIase family (peptidyl-prolyl cis/trans isomerase) found ubiquitously in all Gram-positive species, in which it assists posttranslocational folding at the outer surface of the cytoplasmic membrane. We show by both genetic and biochemical assays that prsA is directly regulated by the VraRS two-component sentinel system of cell wall stress. Disruption of prsA is tolerated by S. aureus, and its loss results in no detectable overt macroscopic changes in cell wall architecture or growth rate under nonstressed growth conditions. Disruption of prsA leads, however, to notable alterations in the sensitivity to glycopeptides and dramatically decreases the resistance of COL (MRSA) to oxacillin. Quantitative transcriptional analysis reveals that prsA and vraR are coordinately upregulated in a panel of stable laboratory and clinical glycopeptide-intermediate S. aureus (GISA) strains compared to their susceptible parents. Collectively, our results point to a role for prsA as a facultative facilitator of protein secretion or extracellular folding and provide a framework for understanding why prsA is a key element of the VraRS-mediated cell wall stress response.