Expression of SarX, a negative regulator of agr and exoprotein synthesis, is activated by MgrA in Staphylococcus aureus

Interactions between Helcococcus kunzii and Staphylococcus aureus: How a commensal bacterium modulates the virulence and metabolism of a pathogen in a chronic wound in vitro model

BACKGROUND

Staphylococcus aureus is the predominant pathogen isolated in diabetic foot infections. Recently, the skin commensal bacterium, Helcococcus kunzii, was found to modulate the virulence of this pathogen in an in vivo model. This study aims to elucidate the molecular mechanisms underlying the interaction between these two bacterial species, using a proteomic approach.

RESULTS

Our results reveal that H. kunzii can coexist and proliferate alongside S. aureus in a Chronic Wound Media (CWM), thereby mimicking an in vitro chronic wound environment. We noted that the secreted proteome of H. kunzii induced a transcriptional effect on S. aureus virulence, resulting in a decrease in the expression level of agrA, a gene involved in quorum sensing. The observed effect could be ascribed to specific proteins secreted by H. kunzii including polysaccharide deacetylase, peptidoglycan DD-metalloendopeptidase, glyceraldehyde-3-phosphate dehydrogenase, trypsin-like peptidase, and an extracellular solute-binding protein. These proteins potentially interact with the agr system, influencing S. aureus virulence. Additionally, the virulence of S. aureus was notably affected by modifications in iron-related pathways and components of cell wall architecture in the presence of H. kunzii. Furthermore, the overall metabolism of S. aureus was reduced when cocultured with H. kunzii.

CONCLUSION

Future research will focus on elucidating the role of these excreted factors in modulating virulence.

Presence and mRNA Expression of the sar Family Genes in Clinical and Non-clinical (Healthy Conjunctiva and Healthy Skin) Isolates of Staphylococcus epidermidis

Staphylococcus aureus possesses sar family genes, including sarA, S, R, T, U, V, X, Y, Z, and rot, which are transcription factors involved in biofilm formation and quorum sensing. In contrast, Staphylococcus epidermidis has sarA, R, V, X, Y, Z, and rot genes; specifically, SarA, Z, and X are involved in biofilm formation. The expression of the sar family members in S. epidermidis isolated from clinical and non-clinical environments is unknown. This study aimed to establish if clinical and non-clinical isolates of S. epidermidis express the sar family members. We genotyped isolates from clinical ocular infections (n = 52), or non-clinical healthy conjunctiva (n = 40), and healthy skin (n = 50), using multilocus sequence typing (MLST) and the staphylococcal chromosomal cassette mec (SCCmec). We selected strains with different genotypes and representatives of each source of isolation, and the presence of the sar family genes was detected using PCR and RT-qPCR to determine their expression. The sar family genes were present in all selected strains, with no observed differences. The relative expression of the sar family showed that all selected strains expressed each gene weakly, with no significant differences observed between them or between different sources of isolation. In conclusion, the presence and relative expression of the sar family genes are very similar among strains, with no differences based on their origin of isolation and genotype.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12088-024-01339-x.

Targeting the Holy Triangle of Quorum Sensing, Biofilm Formation, and Antibiotic Resistance in Pathogenic Bacteria

Chronic and recurrent bacterial infections are frequently associated with the formation of biofilms on biotic or abiotic materials that are composed of mono- or multi-species cultures of bacteria/fungi embedded in an extracellular matrix produced by the microorganisms. Biofilm formation is, among others, regulated by quorum sensing (QS) which is an interbacterial communication system usually composed of two-component systems (TCSs) of secreted autoinducer compounds that activate signal transduction pathways through interaction with their respective receptors. Embedded in the biofilms, the bacteria are protected from environmental stress stimuli, and they often show reduced responses to antibiotics, making it difficult to eradicate the bacterial infection. Besides reduced penetration of antibiotics through the intricate structure of the biofilms, the sessile biofilm-embedded bacteria show reduced metabolic activity making them intrinsically less sensitive to antibiotics. Moreover, they frequently express elevated levels of efflux pumps that extrude antibiotics, thereby reducing their intracellular levels. Some efflux pumps are involved in the secretion of QS compounds and biofilm-related materials, besides being important for removing toxic substances from the bacteria. Some efflux pump inhibitors (EPIs) have been shown to both prevent biofilm formation and sensitize the bacteria to antibiotics, suggesting a relationship between these processes. Additionally, QS inhibitors or quenchers may affect antibiotic susceptibility. Thus, targeting elements that regulate QS and biofilm formation might be a promising approach to combat antibiotic-resistant biofilm-related bacterial infections.

Accessory Gene Regulator (agr) Allelic Variants in Cognate Staphylococcus aureus Strain Display Similar Phenotypes

The accessory gene regulator (agr) quorum-sensing system is an important global regulatory system of Staphylococcus aureus and contributes to its pathogenicity. The S. aureus agr system is divided into four agr groups based on the amino acid polymorphisms of AgrB, AgrD, and AgrC. The agr activation is group-specific, resulting in variations in agr activity and pathogenicity among the four agr groups. Strains with divergent agr system always have different phenotypes. In the present report, we, respectively, exchanged the agr system of a certain S. aureus with other three agr alleles and assessed the corresponding phenotypes of these congenic strains. Replacement of the agr system led to significant variations in hemolytic activity, protein expression, and virulence gene expression comparing with that of the parental strain. Interestingly, we found that the biological characteristics of these agr congenic strains in the same strain background were highly similar to each other, and the allele-dependent differences of the agr systems were weakened. These findings indicate that the allele-dependent agr predilections of S. aureus are determined by some factors in addition to the polymorphisms of AgrB, AgrD, and AgrC. Future studies may reveal the novel mechanism to improve our understanding of the agr network.

Deletion of SarX Decreases Biofilm Formation of Staphylococcus aureus in a Polysaccharide Intercellular Adhesin (PIA)-Dependent Manner by Downregulating spa

Introduction

Biofilm formation by Staphylococcus aureus is an important virulence determinant mediated by the polysaccharide intercellular adhesin (PIA) encoded by the ica operon or mediated by surface and extracellular proteins. SarX is a 250-residue two-domain SarA homolog that activates spa transcription. Previous studies demonstrated that Staphylococcus epidermidis SarX protein regulated the transcriptional activity of the agr and ica loci and controlled the biofilm phenotype, primarily by regulating icaADBC transcription and PIA production.

Results

In this study, biofilm formation and detachment of the clinical isolate S. aureus SA75 were significantly decreased in the sarX mutant strain. The effect of sarX mutation on S. aureus biofilm formation was related to the production of PIA and not to that of eDNA. Deletion of sarX was associated with a 1.8-fold reduction in spa transcription as determined by RT-PCR analysis, and this reduction could be restored by chromosomal complementation of sarX. Expression of Spa protein was also decreased in the S. aureus sarX mutant.

Conclusion

sarX promoted biofilm production of S. aureus that may primarily be mediated through increasing ica operon expression and PIA production. Furthermore, deletion of sarX reduced S. aureus biofilm formation by downregulating spa.

The structural mechanism for the nucleoside tri- and diphosphate hydrolysis activity of Ntdp from Staphylococcus aureus

Staphylococcus aureus is a well-known clinical pathogenic bacterium. In recent years, due to the emergence of multiple drug-resistant strains of S. aureus in clinical practice, S. aureus infections have become an increasingly severe clinical problem. Ntdp (nucleoside tri- and diphosphatase, also known as Sa1684) is a nucleotide phosphatase that has a significant effect on the proliferation of S. aureus colonies and the killing ability of the host. Here, we identified the nucleoside tri- and diphosphate hydrolysis activity of Ntdp and obtained the three-dimensional structures of apo-Ntdp and three substrate analog (ATP_γ S, GDP_β S, and GTP_γ S) complexes of Ntdp. Through structural analysis and biochemical verification, we illustrated the structural basis for the divalent cation selectivity, substrate recognition model, and catalytic mechanism of Ntdp. We also revealed a possible basal functional pattern of the DUF402 domain and hypothesized the potential pathways by which the protein regulates the expression of the two-component regulatory factor agr and the downstream virulence factors. Overall, the above findings provide crucial insights into our understanding of the Ntdp functional mechanism in the infection process.

Nonsteroidal antiinflammatory drugs alter antibiotic susceptibility and expression of virulence-related genes and protein A of Staphylococcus aureus

Background/aim

Nonsteroidal antiinflammatory drugs (NSAIDs) including diclofenac, naproxen, ibuprofen, acetylsalicylic acid, and acetaminophen have been shown to have antimicrobial effects on various microorganisms. The aim of this study was to investigate the antibacterial effects of NSAIDs on Staphylococcus aureus .

Materials and methods

Susceptibilities of S. aureus strains to NSAIDs with or without antimicrobials (moxifloxacin, vancomycin, ciprofloxacin, clindamycin, and gentamicin) were determined using the microdilution method and disk diffusion test. Expression levels of genes in the presence of drugs were investigated by real-time quantitative RT-PCR (qRT-PCR), and immunoblotting analysis was performed for staphylococcal protein A (SpA).

Results

Our results showed that all NSAIDs were active against S. aureus strains with MIC values ranging from 195 µg/mL to 6250 µg/mL. NSAIDs increased the antibiotic susceptibility of the strains, and diclofenac was found to be more effective than the other drugs. Drugs showed different effects on expression levels of virulence factor and/or regulatory genes. Immunoblotting analysis of SpA protein was mostly in accordance with qRT-PCR results.

Conclusion

The regulatory/virulence factor genes and proteins of S. aureus investigated in this study may be reasonable targets for these drugs, and we suggest that the data may contribute to the field of infection control and antimicrobial resistance.

Genomic Epidemiology of CC30 Methicillin-Resistant Staphylococcus aureus Strains from Argentina Reveals Four Major Clades with Distinctive Genetic Features

Staphylococcus aureus clonal complex 30 (CC30) has given rise to epidemics worldwide and is one of the most prevalent lineages in Argentina, represented by sequence type 30 methicillin-resistant S. aureus SCCmec type IV (ST30-MRSA-IV). ST30-MRSA-IV has displaced previous prevalent clones in the country and demonstrated increased virulence. Despite the burden of infections caused by ST30-MRSA-IV both in hospitals and in communities in Argentina, no detailed genome-based characterization of this clone is available to date. In this study, we used whole-genome sequencing (WGS) to evaluate the genetic diversity, population structure, and genomic characteristics of 190 CC30-MRSA strains circulating in Argentina between 2004 and 2015. Phylogenetic analysis revealed the existence of 4 major clades: ARG-1 (CC30-MRSA-IVc-spa t012), ARG-2 (ST30-MRSA-IVc-spa t021 related), ARG-3 (ST30-MRSA-IVh/j-spa t021 and related), and ARG-4 (CC30-MRSA-IVc-spa t019 and related). The clades were characterized by different distributions of antimicrobial resistance determinants, virulence genes, and mobile genetic elements (MGEs). While ARG-1 and ARG-4 were related to global epidemic MRSA-16 (EMRSA-16) and South West Pacific (SWP) clones, respectively, ARG-3 was phylogenetically distinct from previously defined CC30 epidemic clones. ARG-4, the most prevalent and geographically disseminated in the collection (N = 164), was characterized by specific MGEs and chromosomal mutations that might have contributed to its virulence and success. To our knowledge, this is the first genomic epidemiology study of CC30-MRSA in Argentina, which will serve as baseline genomic data going forward to inform public health measures for infection prevention and control.IMPORTANCE The rise in prevalence of community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) is of public health concern. In Argentina, several studies documented a shift in the epidemiology of CA-MRSA since 2009, with clonal complex 30 (CC30) and, in particular, sequence type 30 MRSA SCCmec type IV (ST30-MRSA-IV) replacing other clones both in communities and in hospitals and possibly displaying increased virulence. By sequencing the whole genomes of 190 CC30 MRSA isolates recovered from Argentina between 2005 and 2015, we showed that they represented a diverse population composed of 4 major clades. The predominant clade evolved from the South West Pacific clone but has acquired a distinct repertoire of mobile genetic elements, virulence genes, and chromosomal mutations that might play a role in its success. Our work is the first extensive genomic study of CC30 S. aureus in Argentina and will contribute not only to the development of genomic surveillance in the region but also to our understanding of the global epidemiology of this pathogen.

A Functional ClpXP Protease is Required for Induction of the Accessory Toxin Genes, tst, sed, and sec

Staphylococcal toxic shock syndrome is a potentially lethal illness attributed to superantigens produced by Staphylococcus aureus, in particular toxic shock syndrome toxin 1 (TSST-1), but staphylococcal enterotoxins (SEs) are also implicated. The genes encoding these important toxins are carried on mobile genetic elements, and the regulatory networks controlling expression of these toxins remain relatively unexplored. We show here that the highly conserved ClpXP protease stimulates transcription of tst (TSST-1), sec (SEC), and sed (SED) genes in the prototypical strains, SA564 and RN4282. In the wild-type cells, the post-exponential upregulation of toxin gene transcription was proposed to occur via RNAIII-mediated downregulation of the Rot repressor. Contradictive to this model, we showed that the post-exponential induction of tst, sed, and sec transcription did not occur in cells devoid of ClpXP activity, despite the Rot level being diminished. To identify transcriptional regulators with a changed expression in cells devoid of ClpXP activity, RNA sequencing was performed. The RNAseq analysis revealed a number of global virulence regulators that might act downstream of ClpXP, to control expression of tst and other virulence genes. Collectively, the results extend our understanding of the complex transcriptional regulation of the tst, sed, and sec genes.

Mapping the Global Network of Extracellular Protease Regulation in Staphylococcus aureus

The complex regulatory role of the proteases necessitates very tight coordination and control of their expression. While this process has been well studied, a major oversight has been the consideration of proteases as a single entity rather than as 10 enzymes produced from four different promoters. As such, in this study, we comprehensively characterized the regulation of each protease promoter, discovering vast differences in the way each protease operon is controlled. Additionally, we broaden the picture of protease regulation using a global screen to identify novel loci controlling protease activity, uncovering a cadre of new effectors of protease expression. The impact of these elements on the activity of proteases and known regulators was characterized by producing a comprehensive regulatory circuit that emphasizes the complexity of protease regulation in Staphylococcus aureus.

A primary function of the extracellular proteases of Staphylococcus aureus is to control the progression of infection by selectively modulating the stability of virulence factors. Consequently, a regulatory network exists to titrate protease abundance/activity to influence the accumulation, or lack thereof, of individual virulence factors. Herein, we comprehensively map this system, exploring the regulation of the four protease loci by known and novel factors. In so doing, we determined that seven major elements (SarS, SarR, Rot, MgrA, CodY, SaeR, and SarA) form the primary network of control, with the latter three being the most powerful. We note that expression of aureolysin is largely repressed by these factors, while the spl operon is subject to the strongest upregulation of any protease loci, particularly by SarR and SaeR. Furthermore, when exploring scpA expression, we find it to be profoundly influenced in opposing fashions by SarA (repressor) and SarR (activator). We also present the screening of >100 regulator mutants of S. aureus, identifying 7 additional factors (ArgR2, AtlR, MntR, Rex, XdrA, Rbf, and SarU) that form a secondary circuit of protease control. Primarily, these elements serve as activators, although we reveal XdrA as a new repressor of protease expression. With the exception or ArgR2, each of the new effectors appears to work through the primary network of regulation to influence protease production. Collectively, we present a comprehensive regulatory circuit that emphasizes the complexity of protease regulation and suggest that its existence speaks to the importance of these enzymes to S. aureus physiology and pathogenic potential.

IMPORTANCE The complex regulatory role of the proteases necessitates very tight coordination and control of their expression. While this process has been well studied, a major oversight has been the consideration of proteases as a single entity rather than as 10 enzymes produced from four different promoters. As such, in this study, we comprehensively characterized the regulation of each protease promoter, discovering vast differences in the way each protease operon is controlled. Additionally, we broaden the picture of protease regulation using a global screen to identify novel loci controlling protease activity, uncovering a cadre of new effectors of protease expression. The impact of these elements on the activity of proteases and known regulators was characterized by producing a comprehensive regulatory circuit that emphasizes the complexity of protease regulation in Staphylococcus aureus.

Regulation of Staphylococcus aureus Virulence

Staphylococcus aureus is a Gram-positive opportunistic pathogen that has evolved a complex regulatory network to control virulence. One of the main functions of this interconnected network is to sense various environmental cues and respond by altering the production of virulence factors necessary for survival in the host, including cell surface adhesins and extracellular enzymes and toxins. Of these S. aureus regulatory systems, one of the best studied is the accessory gene regulator (agr), which is a quorum-sensing system that senses the local concentration of a cyclic peptide signaling molecule. This system allows S. aureus to sense its own population density and translate this information into a specific gene expression pattern. Besides agr, this pathogen uses other two-component systems to sense specific cues and coordinates responses with cytoplasmic regulators of the SarA protein family and alternative sigma factors. These divergent regulatory systems integrate the various environmental and host-derived signals into a network that ensures optimal pathogen response to the changing conditions. This article gives an overview of the most important and best-studied S. aureus regulatory systems and summarizes the functions of these regulators during host interactions. The regulatory systems discussed include the agr quorum-sensing system; the SaeRS, SrrAB, and ArlRS two-component systems, the cytoplasmic SarA-family regulators (SarA, Rot, and MgrA); and the alternative sigma factors (SigB and SigH).

Proteomic analysis of bacterial response to a 4-hydroxybenzylidene indolinone compound, which re-sensitizes bacteria to traditional antibiotics

Halogenated 4-hydroxybenzylidene indolinones have been shown to re-sensitize methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus faecalis (VRE) to methicillin and vancomycin respectively. The mechanism of antibiotic re-sensitization was however not previously studied. Here, we probe the scope of antibiotic re-sensitization and present the global proteomics analysis of S. aureus treated with GW5074, a 4-hydroxybenzylidene indolinone compound. With a minimum inhibitory concentration (MIC) of 8 μg/mL against S. aureus, GW5074 synergized with beta-lactam antibiotics like ampicillin, carbenicillin and cloxacillin, the DNA synthesis inhibitor, ciprofloxacin, the protein synthesis inhibitor, gentamicin and the folate acid synthesis inhibitor, trimethoprim. Global proteomics analysis revealed that GW5074 treatment resulted in significant downregulation of enzymes involved in the purine biosynthesis. S. aureus proteins involved in amino acid metabolism and peptide transport were also observed to be downregulated. Interestingly, anti-virulence targets such as AgrC (a quorum sensing-related histidine kinase), AgrA (a quorum sensing-related response regulator) as well as downstream targets, such as hemolysins, lipases and proteases in S. aureus were also downregulated by GW5074. We observed that the peptidoglycan hydrolase, SceD was significantly upregulated. The activity of GW5074 on S. aureus suggests that the compound primes bacteria for the antibacterial action of ineffective antibiotics. SIGNIFICANCE: Antibiotic resistance continues to present significant challenges to the treatment of bacterial infections. Given that antibiotic resistance is a natural phenomenon and that it has become increasingly difficult to discover novel antibiotics, efforts to improve the activity of existing agents are worth pursuing. A few small molecules that re-sensitize resistant bacteria to traditional antibiotics have been described but the molecular details that underpin how these compounds work to re-sensitize bacteria remain largely unknown. In this report, global label-free quantitative proteomics was used to identify changes in the proteome that occurs when GW5074, a compound that re-sensitize MRSA to methicillin, is administered to S. aureus. The identification of pathways that are impacted by GW5074 could help identify novel targets for antibiotic re-sensitization.

MroQ Is a Novel Abi-Domain Protein That Influences Virulence Gene Expression in Staphylococcus aureus via Modulation of Agr Activity

Numerous factors have, to date, been identified as playing a role in the regulation of Agr activity in Staphylococcus aureus, including transcription factors, antisense RNAs, and host elements. Herein we investigated the product of SAUSA300_1984 (termed MroQ), a transmembrane Abi-domain/M79 protease-family protein, as a novel effector of this system. Using a USA300 mroQ mutant, we observed a drastic reduction in proteolysis, hemolysis, and pigmentation that was fully complementable. This appears to result from diminished agr activity, as transcriptional analysis revealed significant decreases in expression of both RNAII and RNAIII in the mroQ mutant. Such effects appear to be direct, rather than indirect, as known agr effectors demonstrated limited alterations in their activity upon mroQ disruption. A comparison of RNA sequencing data sets for both mroQ and agr mutants revealed a profound overlap in their regulomes, with the majority of factors affected being known virulence determinants. Importantly, the preponderance of alterations in expression were more striking in the agr mutant, indicating that MroQ is necessary, but not sufficient, for Agr function. Mechanism profiling revealed that putative residues for metalloprotease activity within MroQ are required for its Agr-controlling effect; however, this was not wielded at the level of AgrD processing. Virulence assessment demonstrated that both mroQ and agr mutants exhibited increased formation of renal abscesses but decreased skin abscess formation alongside diminished dermonecrosis. Collectively, we present the characterization of a novel agr effector in S. aureus which would appear to be a direct regulator, potentially functioning via interaction with the AgrC histidine kinase.

Small RNA-Based Regulation of Bacterial Quorum Sensing and Biofilm Formation

Quorum sensing is a vital property of bacteria that enables community-wide coordination of collective behaviors. A key example of such a behavior is biofilm formation, in which groups of bacteria invest in synthesizing a protective, joint extracellular matrix. Quorum sensing involves the production, release, and subsequent detection of extracellular signaling molecules called autoinducers. The architecture of quorum-sensing signal transduction pathways is highly variable among different species of bacteria, but frequently involves posttranscriptional regulation carried out by small regulatory RNA molecules. This review illustrates the diverse roles small trans-acting regulatory RNAs can play, from constituting a network's core to auxiliary roles in adjusting the rate of autoinducer synthesis, mediating cross talk among different parts of a network, or integrating different regulatory inputs to trigger appropriate changes in gene expression. The emphasis is on describing how the study of small RNA-based regulation in quorum sensing and biofilm formation has uncovered new general properties or expanded our understanding of bacterial riboregulation.

MgrA Negatively Regulates Biofilm Formation and Detachment by Repressing the Expression of psm Operons in Staphylococcus aureus

Phenol-soluble modulins (PSMs) are amphipathic peptides that are produced by staphylococci and play important roles in Staphylococcus aureus biofilm formation and dissemination. Although the multiple functions of PSMs have been recognized, the regulatory mechanisms controlling the expression of psm operons remain largely unknown. In this study, we identified MgrA in a DNA pulldown assay and further demonstrated, by electrophoretic mobility shift assays and DNase I footprinting assays, that MgrA could bind specifically to the promoter regions of psm operons. We then constructed an isogenic mgrA deletion strain and compared biofilm formation and detachment in the wild-type and isogenic mgrA deletion strains. Our results indicated that biofilm formation and detachment were significantly increased in the mgrA mutant strain. Real-time quantitative reverse transcription-PCR data indicated that MgrA repressed the transcription of psm operons in cultures and biofilms, suggesting that MgrA is a negative regulator of psm expression. Furthermore, we analyzed biofilm formation by the psm mutant strains, and we found that PSMs promoted biofilm structuring and development in the mgrA mutant strain. These findings reveal that MgrA negatively regulates biofilm formation and detachment by repressing the expression of psm operons through direct binding to the psm promoter regions.IMPORTANCEStaphylococcus aureus is a human and animal pathogen that can cause biofilm-associated infections. PSMs have multiple functions in biofilm development and virulence in staphylococcal pathogenesis. This study has revealed that MgrA can negatively regulate psm expression by binding directly to the promoter regions of psm operons. Furthermore, our results show that MgrA can modulate biofilm structuring and development by repressing the production of PSMs in S. aureus Our findings provide novel insights into the regulatory mechanisms of S. aureus psm gene expression, biofilm development, and pathogenesis.

MgrA Negatively Regulates Biofilm Formation and Detachment by Repressing the Expression of psm Operons in Staphylococcus aureus

Staphylococcus aureus is a human and animal pathogen that can cause biofilm-associated infections. PSMs have multiple functions in biofilm development and virulence in staphylococcal pathogenesis. This study has revealed that MgrA can negatively regulate psm expression by binding directly to the promoter regions of psm operons. Furthermore, our results show that MgrA can modulate biofilm structuring and development by repressing the production of PSMs in S. aureus. Our findings provide novel insights into the regulatory mechanisms of S. aureus psm gene expression, biofilm development, and pathogenesis.

Phenol-soluble modulins (PSMs) are amphipathic peptides that are produced by staphylococci and play important roles in Staphylococcus aureus biofilm formation and dissemination. Although the multiple functions of PSMs have been recognized, the regulatory mechanisms controlling the expression of psm operons remain largely unknown. In this study, we identified MgrA in a DNA pulldown assay and further demonstrated, by electrophoretic mobility shift assays and DNase I footprinting assays, that MgrA could bind specifically to the promoter regions of psm operons. We then constructed an isogenic mgrA deletion strain and compared biofilm formation and detachment in the wild-type and isogenic mgrA deletion strains. Our results indicated that biofilm formation and detachment were significantly increased in the mgrA mutant strain. Real-time quantitative reverse transcription-PCR data indicated that MgrA repressed the transcription of psm operons in cultures and biofilms, suggesting that MgrA is a negative regulator of psm expression. Furthermore, we analyzed biofilm formation by the psm mutant strains, and we found that PSMs promoted biofilm structuring and development in the mgrA mutant strain. These findings reveal that MgrA negatively regulates biofilm formation and detachment by repressing the expression of psm operons through direct binding to the psm promoter regions.

IMPORTANCEStaphylococcus aureus is a human and animal pathogen that can cause biofilm-associated infections. PSMs have multiple functions in biofilm development and virulence in staphylococcal pathogenesis. This study has revealed that MgrA can negatively regulate psm expression by binding directly to the promoter regions of psm operons. Furthermore, our results show that MgrA can modulate biofilm structuring and development by repressing the production of PSMs in S. aureus. Our findings provide novel insights into the regulatory mechanisms of S. aureus psm gene expression, biofilm development, and pathogenesis.

Distinct virulent network between healthcare- and community-associated Staphylococcus aureus based on proteomic analysis

Background

Staphylococcus aureus (S. aureus or SA) is a leading cause of healthcare-associated (HA-) and community-associated (CA) infection. HA-SA isolates usually cause nosocomial pneumonia, bloodstream infections, catheter-related urinary tract infections, etc. On the other hand, CA-SA isolates usually cause highly fatal diseases, such as SSTIs as well as post influenza necrotic hemorrhagic pneumonia. The differences of the infection types are partially due to the unique characteristics between HA-SA and CA-SA isolates. For example, HA-SA isolates showed strong adherence to host epithelial cells, while CA-SA isolates displayed higher virulence due to the increased activity of the important quorum-sensing system accessory gene regulator (agr). Thus, the aim of this study was to characterize the proteomic difference between HA-SA and CA-SA lineage.

Methods

In this study, the extracted peptides from those representative strains were analyzed by LC-MS/MS. The protein-protein interaction network was constructed by bioinformatics and their expressions were verified by RT-PCR and Western blot.

Results

We demonstrated that Agr system (AgrA and AgrC) and its interactive factors (PhoP, SrrB, YycG, SarX, SigB and ClpP) based on the protein–protein interaction network were expressed significantly higher in the epidemic Chinese CA-SA lineage ST398 compared to HA-SA lineage ST239 by LC-MS/MS. We further verified the increased transcription of all these genes in ST398 by RT-PCR, suggesting that the higher expression of these genes/proteins probably play role in the acute infection of CA-SA. Moreover, surface-related proteins (FnbpA, SpA, Atl, ClfA, IsaA, IsaB, LtaS, SsaA and Cna) that are repressed by the Agr system have significantly higher expression in the epidemic Chinese HA-SA clone ST239 in comparison to CA-SA lineage ST398 by LC-MS/MS. Furthermore, we confirmed the significantly increased expression of two important adhesive proteins (Atl and ClfA) in ST239 by Western blot, which may contribute to the durative infection of HA-SA.

Conclusion

The results suggest that the different proteomic profile, at least partially, contribute to the pathogenic differences between HA-SA and CA-SA.

Electronic supplementary material

The online version of this article (10.1186/s12014-017-9178-5) contains supplementary material, which is available to authorized users.

The ATP-Dependent Protease ClpP Inhibits Biofilm Formation by Regulating Agr and Cell Wall Hydrolase Sle1 in Staphylococcus aureus

Biofilm causes hospital-associated infections on indwelling medical devices. In Staphylococcus aureus, Biofilm formation is controlled by intricately coordinated network of regulating systems, of which the ATP-dependent protease ClpP shows an inhibitory effect. Here, we demonstrate that the inhibitory effect of ClpP on biofilm formation is through Agr and the cell wall hydrolase Sle1. Biofilm formed by clpP mutant consists of proteins and extracellular DNA (eDNA). The increase of the protein was, at least in part, due to the reduced protease activity of the mutant, which was caused by the decreased activity of agr. On the other hand, the increase of eDNA was due to increased cell lysis caused by the higher level of Sle1. Indeed, as compared with wild type, the clpP mutant excreted an increased level of eDNA, and showed higher sensitivity to Triton-induced autolysis. The deletion of sle1 in the clpP mutant decreased the biofilm formation, the level of eDNA, and the Triton-induced autolysis to wild-type levels. Despite the increased biofilm formation capability, however, the clpP mutant showed significantly reduced virulence in a murine model of subcutaneous foreign body infection, indicating that the increased biofilm formation capability cannot compensate for the intrinsic functions of ClpP during infection.

Novel Nucleoside Diphosphatase Contributes to Staphylococcus aureus Virulence

We identified SA1684 as a Staphylococcus aureus virulence gene using a silkworm infection model. The SA1684 gene product carried the DUF402 domain, which is found in RNA-binding proteins, and had amino acid sequence similarity with a nucleoside diphosphatase, Streptomyces coelicolor SC4828 protein. The SA1684-deletion mutant exhibited drastically decreased virulence, in which the LD50 against silkworms was more than 10 times that of the parent strain. The SA1684-deletion mutant also exhibited decreased exotoxin production and colony-spreading ability. Purified SA1684 protein had Mn(2+)- or Co(2+)-dependent hydrolyzing activity against nucleoside diphosphates. Alanine substitutions of Tyr-88, Asp-106, and Asp-123/Glu-124, which are conserved between SA1684 and SC4828, diminished the nucleoside diphosphatase activity. Introduction of the wild-type SA1684 gene restored the hemolysin production of the SA1684-deletion mutant, whereas none of the alanine-substituted SA1684 mutant genes restored the hemolysin production. RNA sequence analysis revealed that SA1684 is required for the expression of the virulence regulatory genes agr, sarZ, and sarX, as well as metabolic genes involved in glycolysis and fermentation pathways. These findings suggest that the novel nucleoside diphosphatase SA1684 links metabolic pathways and virulence gene expression and plays an important role in S. aureus virulence.

Structure of Rot, a global regulator of virulence genes in Staphylococcus aureus

Staphylococcus aureus is a highly versatile pathogen that can infect human tissue by producing a large arsenal of virulence factors that are tightly regulated by a complex regulatory network. Rot, which shares sequence similarity with SarA homologues, is a global regulator that regulates numerous virulence genes. However, the recognition model of Rot for the promoter region of target genes and the putative regulation mechanism remain elusive. In this study, the 1.77 Å resolution X-ray crystal structure of Rot is reported. The structure reveals that two Rot molecules form a compact homodimer, each of which contains a typical helix-turn-helix module and a β-hairpin motif connected by a flexible loop. Fluorescence polarization results indicate that Rot preferentially recognizes AT-rich dsDNA with ~30-base-pair nucleotides and that the conserved positively charged residues on the winged-helix motif are vital for binding to the AT-rich dsDNA. It is proposed that the DNA-recognition model of Rot may be similar to that of SarA, SarR and SarS, in which the helix-turn-helix motifs of each monomer interact with the major grooves of target dsDNA and the winged motifs contact the minor grooves. Interestingly, the structure shows that Rot adopts a novel dimerization model that differs from that of other SarA homologues. As expected, perturbation of the dimer interface abolishes the dsDNA-binding ability of Rot, suggesting that Rot functions as a dimer. In addition, the results have been further confirmed in vivo by measuring the transcriptional regulation of α-toxin, a major virulence factor produced by most S. aureus strains.

CodY-mediated regulation of the Staphylococcus aureus Agr system integrates nutritional and population density signals

The Staphylococcus aureus Agr system regulates virulence gene expression by responding to cell population density (quorum sensing). When an extracellular peptide signal (AIP-III in strain UAMS-1, used for these experiments) reaches a concentration threshold, the AgrC-AgrA two-component regulatory system is activated through a cascade of phosphorylation events, leading to induction of the divergently transcribed agrBDCA operon and the RNAIII gene. RNAIII is a posttranscriptional regulator of numerous metabolic and pathogenesis genes. CodY, a global regulatory protein, is known to repress agrBDCA and RNAIII transcription during exponential growth in rich medium, but the mechanism of this regulation has remained elusive. Here we report that phosphorylation of AgrA by the AgrC protein kinase is required for the overexpression of the agrBDCA operon and the RNAIII gene in a codY mutant during the exponential-growth phase, suggesting that the quorum-sensing system, which normally controls AgrC activation, is active even in exponential-phase cells in the absence of CodY. In part, such premature expression of RNAIII was attributable to higher-than-normal accumulation of AIP-III in a codY mutant strain, as determined using ultrahigh-performance liquid chromatography coupled to mass spectrometry. Although CodY is a strong repressor of the agr locus, CodY bound only weakly to the agrBDCA-RNAIII promoter region, suggesting that direct regulation by CodY is unlikely to be the principal mechanism by which CodY regulates agr and RNAIII expression. Taken together, these results strongly suggest that cell population density signals inducing virulence gene expression can be overridden by nutrient availability, a condition monitored by CodY.

Subinhibitory concentrations of pinocembrin exert anti-Staphylococcus aureus activity by reducing α-toxin expression

AIMS

Natural products have been used as potentially important sources of novel antibacterials in combating pathogenic Staphylococcus aureus isolates, a major problem around the world. In this study, we aimed to investigate the antibacterial effects of pinocembrin (PNCB) against Staph. aureus pneumonia in a murine model and its influence on the production of Staph. aureus α-haemolysin (Hla).

METHODS AND RESULTS

The in vitro activities of PNCB on α-haemolysin production were determined using haemolysis, Western blot and real-time RT-PCR assays. The viability and cytotoxicity assays were performed to evaluate the influence of PNCB on α-toxin-mediated injury of human alveolar epithelial cells. Moreover, through histopathologic analysis, we further determined the in vivo effects of PNCB on Staph. aureus pneumonia in a mouse model. In vitro, PNCB at low concentrations exhibited inhibitory activity against α-haemolysin production and attenuated α-haemolysin-mediated cell injury. Furthermore, the in vivo findings demonstrated that PNCB protected mice from Staph. aureus pneumonia.

CONCLUSIONS

We have provided new evidence of the effects of PNCB, which suggest that PNCB attenuated α-haemolysin-mediated cell injury and protected mice from Staph. aureus pneumonia.

SIGNIFICANCE AND IMPACT OF THE STUDY

The findings indicate that PNCB may be used as a basis for anti-Staphylococcus agent.

Targeting agr- and agr-Like quorum sensing systems for development of common therapeutics to treat multiple gram-positive bacterial infections

Invasive infection by the Gram-positive pathogen Staphylococcus aureus is controlled by a four gene operon, agr that encodes a quorum sensing system for the regulation of virulence. While agr has been well studied in S. aureus, the contribution of agr homologues and analogues in other Gram-positive pathogens is just beginning to be understood. Intriguingly, other significant human pathogens, including Clostridium perfringens, Listeria monocytogenes, and Enterococcus faecalis contain agr or analogues linked to virulence. Moreover, other significant human Gram-positive pathogens use peptide based quorum sensing systems to establish or maintain infection. The potential for commonality in aspects of these signaling systems across different species raises the prospect of identifying therapeutics that could target multiple pathogens. Here, we review the status of research into these agr homologues, analogues, and other peptide based quorum sensing systems in Gram-positive pathogens as well as the potential for identifying common pathways and signaling mechanisms for therapeutic discovery.

Activation of sarX by Rbf is required for biofilm formation and icaADBC expression in Staphylococcus aureus

A major constituent of many Staphylococcus aureus biofilms is a polysaccharide known as the polysaccharide intercellular adhesin, or poly N-acetylglucosamine (PIA/PNAG). PIA/PNAG is synthesized by the 4 gene products of the icaADBC operon, which is negatively regulated by the divergently transcribed icaR gene. We previously reported the identification of a gene, rbf, involved in the positive transcriptional regulation of icaADBC transcription by repressing icaR in S. aureus strain 8325-4. However, we were unable to show binding of Rbf to DNA upstream of icaR or icaA, suggesting that Rbf may control expression of an unknown factor(s) that, in turn, regulates ica expression. Here we report that the unknown factor is SarX protein. Results from epistasis assays and genetic complementation analyses suggest that Rbf upregulates SarX, which then downregulates IcaR, thereby activating icaADBC. Electrophoretic mobility shift assays revealed that SarX protein bound to a sequence upstream of icaR within the icaA coding region. Cross-linking and immunoprecipitation experiments further suggested that Rbf binds to the sarX promoter in S. aureus. These results demonstrate that Rbf and SarX represent a regulatory cascade that promotes PIA-dependent biofilm formation in S. aureus.

Attenuating Staphylococcus aureus virulence gene regulation: a medicinal chemistry perspective

Virulence gene expression in Staphylococcus aureus is tightly regulated by intricate networks of transcriptional regulators and two-component signal transduction systems. There is now an emerging body of evidence to suggest that the blockade of S. aureus virulence gene expression significantly attenuates infection in experimental models. In this Perspective, we will provide insights into medicinal chemistry strategies for the development of chemical reagents that have the capacity to inhibit staphylococcal virulence expression. These reagents can be broadly grouped into four categories: (1) competitive inhibitors of the accessory gene regulator (agr) quorum sensing system, (2) inhibitors of AgrA–DNA interactions, (3) RNAIII transcription inhibitors, and (4) inhibitors of the SarA family of transcriptional regulators. We discuss the potential of specific examples of antivirulence agents for the management and treatment of staphylococcal infections.

From genotype to phenotype: can systems biology be used to predict Staphylococcus aureus virulence?

With the advent of high-throughput whole-genome sequencing, it is now possible to sequence a bacterial genome in a matter of hours. However, although the presence or absence of a particular gene can be determined, we do not yet have the tools to extract information about the true virulence potential of an organism from sequence data alone. Here, we focus on the important human pathogen Staphylococcus aureus and present a framework for the construction of a broad systems biology-based tool that could be used to predict virulence phenotypes from S. aureus genomic sequences using existing technology.

Bacterial quorum sensing: its role in virulence and possibilities for its control

Quorum sensing is a process of cell-cell communication that allows bacteria to share information about cell density and adjust gene expression accordingly. This process enables bacteria to express energetically expensive processes as a collective only when the impact of those processes on the environment or on a host will be maximized. Among the many traits controlled by quorum sensing is the expression of virulence factors by pathogenic bacteria. Here we review the quorum-sensing circuits of Staphylococcus aureus, Bacillus cereus, Pseudomonas aeruginosa, and Vibrio cholerae. We outline these canonical quorum-sensing mechanisms and how each uniquely controls virulence factor production. Additionally, we examine recent efforts to inhibit quorum sensing in these pathogens with the goal of designing novel antimicrobial therapeutics.

Staphylococcus aureus NorD, a putative efflux pump coregulated with the Opp1 oligopeptide permease, contributes selectively to fitness in vivo

Staphylococcus aureus readily infects humans, causing infections from mild superficial skin infections to lethal bacteremia and endocarditis. Transporters produced by S. aureus allow the pathogen to adapt to a variety of settings, including survival at sites of infection and in the presence of antibiotics. The native functions of many transporters are unknown, but their potential dual contribution to fitness and antimicrobial resistance highlights their importance in staphylococcal infections. Here, we show that S. aureus NorD, a newly recognized efflux pump of the major facilitator superfamily, contributes to fitness in a murine subcutaneous abscess model. In community-associated methicillin-resistant S. aureus (CA-MRSA) strain MW2, norD was selectively upregulated 36-fold at the infection site relative to growth in vitro, and the norD mutant demonstrated significant fitness impairment in abscesses, with fitness 20- to 40-fold lower than that of the parent MW2 strain. Plasmid-encoded NorD could complement the fitness defect of the MW2 norD mutant. Chromosomal norD expression is polycistronic with the upstream oligopeptide permease genes (opp1ABCDF), which encode an ABC oligopeptide transporter. Both norD and opp1 were upregulated in abscesses and iron-restricted culture medium and negatively regulated by Fur, but only NorD contributed to fitness in the murine abscess model.

Transcribing virulence in Staphylococcus aureus

Staphylococcus aureus (S. aureus) is an important human pathogen capable of causing a diverse range of infections. Once regarded as an opportunistic pathogen causing primarily nosocomial infections, recent years have seen the emergence of S. aureus strains capable of causing serious infection even in otherwise healthy human hosts. There has been much debate about whether this transition is a function of unique genotypic characteristics or differences in the expression of conserved virulence factors, but irrespective of this debate it is clear that the ability of S. aureus to cause infection in all of its diverse forms is heavily influenced by its ability to modulate gene expression in response to changing conditions within the human host. Indeed, the S. aureus genome encodes more than 100 transcriptional regulators that modulate the production of virulence factors either directly via interactions with cis elements associated with genes encoding virulence factors or indirectly through their complex interactions with each other. The goal of this review is to summarize recent work describing these regulators and their contribution to defining S. aureus as a human pathogen.

Genetic regulation of the intercellular adhesion locus in staphylococci

The formation of biofilms by Staphylococcus aureus and Staphylococcus epidermidis is an important aspect of many staphylococcal infections, most notably endocarditis, osteomyelitis and infections associated with indwelling medical devices. The major constituents of staphylococcal biofilms are polysaccharides, such as poly N-acetyl glucosamine (PIA/PNAG), cell surface and secreted bacterial proteins, and extracellular DNA. The exact composition of biofilms often varies considerably between different strains of staphylococci and between different sites of infection by the same strain. PIA/PNAG is synthesized by the products of four genes, icaADBC, that are encoded in a single operon. A fifth gene, icaR, is a negative regulator of icaADBC. Expression of icaADBC is tightly regulated, but can often be induced in vitro by growing staphylococci in the presence of high salt, high glucose, or ethanol. Regulation of icaADBC is complex and numerous regulatory factors have been implicated in control of icaADBC. Many of these are well known global transcriptional regulatory factors like SarA and sigmaB, whereas other regulators, such as IcaR, seem to affect expression of relatively few genes. Here, we will summarize how various regulatory factors affect the production of PIA/PNAG in staphylococci.

A novel role for SarX in Staphylococcus epidermidis biofilm regulation

Biofilm production by staphylococci is an important virulence determinant mediated by the icaADBC-encoded polysaccharide intercellular adhesin (PIA) or by surface and extracellular proteins. Deletion of the Staphylococcus accessory regulator sarX significantly reduced biofilm-forming capacity in Staphylococcus epidermidis CSF41498, whereas multicopy sarX complemented the sarX mutant and increased wild-type biofilm production. In Staphylococcus aureus, SarX negatively regulates the accessory gene regulator (Agr) system, which in turn has strain-specific effects on biofilm regulation. Here we found that purified S. epidermidis SarX protein bound specifically to the agr P3 promoter. However RT-PCR analysis revealed that both mutation of sarX and multicopy sarX activated RNAIII transcription, making it difficult to correlate sarX-mediated biofilm regulation with altered agr activity. In contrast, RT-PCR and immunoblot analysis revealed that icaA transcription and PIA expression were decreased in the sarX mutant, whereas multicopy sarX increased ica and PIA expression. Furthermore, multicopy sarX did not promote biofilms in an icaC mutant. Finally, purified SarX protein bound specifically to the ica operon promoter. Taken together, these data reveal that the S. epidermidis SarX protein regulates the transcriptional activity of the agr and ica loci and controls the biofilm phenotype, primarily by regulating icaADBC transcription and PIA production.

Staphylococcus aureus adaptation to the host and persistence: role of loss of capsular polysaccharide expression

A vast array of virulence factors enable Staphylococcus aureus to readily adapt to different environmental niches in diverse hosts. The cap gene cluster is present in almost all relevant clinical S. aureus isolates and capsular polysaccharide expression is apparent in isolates from patients with acute infection. The number of S. aureus isolates from patients with chronic infections that do not express capsular polysaccharide, however, is significantly high, indicating that loss of capsular polysaccharide expression may be a key S. aureus feature associated with persistence. The role of the loss of capsular polysaccharide expression as well as the emergence of other defined phenotypes and their relevance to persistence of S. aureus and chronicity of the infection is discussed in this article.

The staphylococcus-specific gene rsr represses agr and virulence in Staphylococcus aureus

The expression of virulence factors in Staphylococcus aureus is tightly coordinated by a vast network of regulatory molecules. In this report, we characterize a genetic locus unique to staphylococci called rsr that has a role in repressing two key virulence regulators, sarR and agr. Using strain SH1000, we showed that the transcription of virulence effectors, such as hla, sspA, and spa, is altered in an rsr mutant in a way consistent with agr upregulation. Analysis of RNAIII expression of the agr locus in rsr and rsr-sarR mutants indicated that rsr likely contributes to agr expression independently of SarR. We also provide evidence using a murine model of S. aureus skin infection that the effects mediated by rsr reduce disease progression.

Transcriptional profiling of XdrA, a new regulator of spa transcription in Staphylococcus aureus

Transcription of spa, encoding the virulence factor protein A in Staphylococcus aureus, is tightly controlled by a complex regulatory network, ensuring its temporal expression over growth and at appropriate stages of the infection process. Transcriptomic profiling of XdrA, a DNA-binding protein that is conserved in all S. aureus genomes and shares similarity with the XRE family of helix-turn-helix, antitoxin-like proteins, revealed it to be a previously unidentified activator of spa transcription. To assess how XdrA fits into the complex web of spa regulation, a series of regulatory mutants were constructed; consisting of single, double, triple, and quadruple mutants lacking XdrA and/or the three key regulators previously shown to influence spa transcription directly (SarS, SarA, and RNAIII). A series of lacZ reporter gene fusions containing nested deletions of the spa promoter identified regions influenced by XdrA and the other three regulators. XdrA had almost as strong an activating effect on spa as SarS and acted on the same spa operator regions as SarS, or closely overlapping regions. All data from microarrays, Northern and Western blot analyses, and reporter gene fusion experiments indicated that XdrA is a major activator of spa expression that appears to act directly on the spa promoter and not through previously characterized regulators.

Control of thioredoxin reductase gene (trxB) transcription by SarA in Staphylococcus aureus

Thioredoxin reductase (encoded by trxB) protects Staphylococcus aureus against oxygen or disulfide stress and is indispensable for growth. Among the different sarA family mutants analyzed, transcription of trxB was markedly elevated in the sarA mutant under conditions of aerobic as well as microaerophilic growth, indicating that SarA acts as a negative regulator of trxB expression. Gel shift analysis showed that purified SarA protein binds directly to the trxB promoter region DNA in vitro. DNA binding of SarA was essential for repression of trxB transcription in vivo in S. aureus. Northern blot analysis and DNA binding studies of the purified wild-type SarA and the mutant SarAC9G with oxidizing agents indicated that oxidation of Cys-9 reduced the binding of SarA to the trxB promoter DNA. Oxidizing agents, in particular diamide, could further enhance transcription of the trxB gene in the sarA mutant, suggesting the presence of a SarA-independent mode of trxB induction. Analysis of two oxidative stress-responsive sarA regulatory target genes, trxB and sodM, with various mutant sarA constructs showed a differential ability of the SarA to regulate expression of the two above-mentioned genes in vivo. The overall data demonstrate the important role played by SarA in modulating expression of genes involved in oxidative stress resistance in S. aureus.

Rbf promotes biofilm formation by Staphylococcus aureus via repression of icaR, a negative regulator of icaADBC

We previously reported the identification of a gene, rbf, involved in the regulation of biofilm formation by Staphylococcus aureus 8325-4. In an effort to study the mechanism of regulation, microarrays were used to compare the transcription profiles of the wild-type strain with an rbf mutant and an rbf overexpression strain of the clinical isolate UAMS-1. Among the genes affected by rbf overexpression are those of the intercellular adhesion (ica) locus; however, expression of these genes was not affected by an rbf deletion in the chromosome. The icaADBC genes are responsible for production of poly-N-acetylglucosamine (PNAG), a major constituent of biofilm. The icaR gene encodes a negative regulator of icaADBC. In UAMS-1 carrying an Rbf-encoding plasmid, Rbf was found to repress icaR transcription with a concomitant increase in icaADBC expression and increased PNAG and biofilm production relative to isogenic strains lacking the plasmid. Sequencing of the rbf gene from UAMS-1 showed that there was a 2-bp insertion affecting the 50th codon of the rbf open reading frame, suggesting that rbf is a pseudogene in UAMS-1. This finding explains why deletion of rbf had no effect on biofilm formation in UAMS-1. To further characterize the Rbf regulation on biofilm we compared biofilm formation, icaA and icaR transcription, and PNAG production in 8325-4 and its isogenic rbf and icaR single mutants and an rbf icaR double mutant. Our results are consistent with a model wherein rbf represses synthesis of icaR, which in turn results in derepression of icaADBC and increased PNAG production. Furthermore, purified rbf did not bind to the icaR or icaA promoter region, suggesting that rbf controls expression of an unknown factor(s) that represses icaR. The role of rbf in controlling the S. aureus biofilm phenotype was further demonstrated in a clinical strain, MW2.

Transcriptional variation of regulatory and virulence genes due to different media in Staphylococcus aureus

The pathogenesis of staphylococcal infections is a multifactorial process that depends on expression of different virulence factors. Expression of these factors is controlled by multiple regulatory systems in conjunction with environmental signals. Most of the genetic studies in Staphylococcus aureus have been performed using different growth media, therefore, we examined the effects of different growth media on transcription of the selective target (e.g., hla, hlb, spa, sspA) and regulatory (e.g., agr, sarA family) genes. The results from this study suggest that different growth media have substantial effect on transcription of various genes being analyzed. Interestingly, when compared with the wild-type, the isogenic sarA mutant showed a media-dependent distinct regulatory effect on expression of the target genes.

Expression of the sarA family of genes in different strains of Staphylococcus aureus

Expression of genes involved in the pathogenesis of Staphylococcus aureus is controlled by global regulatory loci, including two-component regulatory systems and transcriptional regulators. The staphylococcal-specific SarA family of transcription regulators control large numbers of target genes involved in virulence, autolysis, biofilm formation, stress responses and metabolic processes, and are recognized as potential therapeutic targets. Expression of some of these important regulators has been examined, mostly in laboratory strains, while the pattern of expression of these genes in other strains, especially clinical isolates, is largely unknown. In this report, a comparative analysis of 10 sarA-family genes was conducted in six different S. aureus strains, including two laboratory (RN6390, SH1000) and four clinical (MW2, Newman, COL and UAMS-1) strains, by Northern and Western blot analyses. Transcription of most of the sarA-family genes showed a strong growth phase-dependence in all strains tested. Among these genes, no difference was observed in expression of the sarA, sarV, sarT and sarU genes, while a major difference was observed in expression of the sarX gene only in strain RN6390. Expression of mgrA, rot, sarZ, sarR and sarS was observed in all strains, but the level of expression varied from strain to strain.

sarA inactivation reduces vancomycin-intermediate and ciprofloxacin resistance expression by Staphylococcus aureus

It is known that multiple genome-wide transcriptional changes often accompany the development of antimicrobial resistance and occur in response to challenge with antimicrobial agents. We now show that inactivation of the staphylococcal accessory gene regulator sarA, which controls at least tens of genes in Staphylococcus aureus, leads to dramatic reductions in vancomycin and ciprofloxacin resistance in vancomycin-intermediate and ciprofloxacin-resistant strains of S. aureus. This is particularly evident when judged by antimicrobial-gradient plate analysis or population analysis profiles. Whilst the intact sarA cistron is required for full vancomycin resistance expression by vancomycin-intermediate S. aureus (VISA), sarA expression as determined by quantitative real-time polymerase chain reaction was found to be VISA strain-dependent. Reductions in vancomycin resistance expression levels following sarA inactivation do not necessarily include an alteration in autolysis. Expression of sarR, the negative regulator of sarA, was downregulated in two VISA mutants, and transcription of the alternative sigma factor sigB was downregulated in one VISA strain. This study contributes to a growing body of evidence demonstrating the importance of loci previously identified to control virulence in the regulation of clinically relevant antibiotic resistance mechanisms.

Regulation of superoxide dismutase (sod) genes by SarA in Staphylococcus aureus

The scavenging of reactive oxygen species (ROS) within cells is regulated by several interacting factors, including transcriptional regulators. Involvement of sarA family genes in the regulation of proteins involved in the scavenging of ROS is largely unknown. In this report, we show that under aerobic conditions, the levels of sodM and sodA transcription, in particular the sodM transcript, are markedly enhanced in the sarA mutant among the tested sarA family mutants. Increased levels of sod expression returned to near the parental level in a single-copy sarA complemented strain. Under microaerophilc conditions, transcription of both sodM and sodA was considerably enhanced in the sarA mutant compared to the wild-type strain. Various genotypic, phenotypic, and DNA binding studies confirmed the involvement of SarA in the regulation of sod transcripts in different strains of Staphylococcus aureus. The sodA mutant was sensitive to an oxidative stress-inducing agent, methyl viologen, but the sarA sodA double mutant was more resistant to the same stressor than the single sodA mutant. These results suggest that overexpression of SodM, which occurs in the sarA background, can rescue the methyl viologen-sensitive phenotype observed in the absence of the sodA gene. Analysis with various oxidative stress-inducing agents indicates that SarA may play a greater role in modulating oxidative stress resistance in S. aureus. This is the first report that demonstrates the direct involvement of a regulatory protein (SarA) in control of sod expression in S. aureus.

Quorum sensing in staphylococci

The staphylococcal agr locus encodes a quorum sensing (QS) system that controls the expression of virulence and other accessory genes by a classical two-component signaling module. Like QS modalities in other Gram-positive bacteria, agr encodes an autoactivating peptide (AIP) that is the inducing ligand for AgrC, the agr signal receptor. Unlike other such systems, agr variants have arisen that show strong cross-inhibition in heterologous combinations, with important evolutionary implications. Also unlike other systems, the effector of global gene regulation in the agr system is a major regulatory RNA, RNAIII. In this review, we describe the functions of the agr system's elements, show how they interact to bring about the regulatory response, and discuss the role of QS in staphylococcal pathobiology. We conclude with the suggestion that agr autoactivation, unlike classical enzyme induction, can occur under suboptimal conditions and can distinguish self from non-self by inducing an exclusive and coordinated population wide response.

Overexpression of MazFsa in Staphylococcus aureus induces bacteriostasis by selectively targeting mRNAs for cleavage

The role of chromosomally encoded toxin-antitoxin (TA) loci in bacterial physiology has been under debate, with the toxin proposed as either an inducer of bacteriostasis or a mediator of programmed cell death (PCD). We report here that ectopic expression of MazF(Sa), a toxin of the TA module from Staphylococcus aureus, led to a rapid decrease in CFU counts but most cells remained viable as determined by differential Syto 9 and propidium iodide staining after MazF(Sa) induction. This finding suggested that the toxin MazF(Sa) induced cell stasis rather than cell death. We also showed that MazF(Sa) selectively cleaves cellular mRNAs in vivo, avoiding "important" transcripts such as recA, gyrB, and sarA mRNAs in MazF(Sa)-induced cells, while these three mRNAs can be cleaved in vitro. The results of Northwestern blotting showed that both sarA and recA mRNAs bind strongly to a putative RNA-binding protein. These data suggest that S. aureus likely undergoes stasis by protecting selective mRNA with RNA-binding proteins upon the expression of MazF(Sa) in vivo.

sarZ, a sarA family gene, is transcriptionally activated by MgrA and is involved in the regulation of genes encoding exoproteins in Staphylococcus aureus

The expression of genes involved in the pathogenesis of Staphylococcus aureus is controlled by global regulatory loci, including two-component regulatory systems and transcriptional regulators (e.g., sar family genes). Most members of the SarA family have been partially characterized and shown to regulate a large numbers of target genes. Here, we describe the characterization of sarZ, a sarA paralog from S. aureus, and its regulatory relationship with other members of its family. Expression of sarZ was growth phase dependent with maximal expression in the early exponential phase of growth. Transcription of sarZ was reduced in an mgrA mutant and returned to a normal level in a complemented mgrA mutant strain, which suggests that mgrA acts as an activator of sarZ transcription. Purified MgrA protein bound to the sarZ promoter region, as determined by gel shift assays. Among the sarA family of genes analyzed, inactivation of sarZ increased sarS transcription, while it decreased agr transcription. The expression of potential target genes involved in virulence was evaluated in single and double mutants of sarZ with mgrA, sarX, and agr. Northern and zymogram analyses indicated that the sarZ gene product played a role in regulating several virulence genes, particularly those encoding exoproteins. Gel shift assays demonstrated nonspecific binding of purified SarZ protein to the promoter regions of the sarZ-regulated target genes. These results demonstrate the important role played by SarZ in controlling regulatory and virulence gene expression in S. aureus.

Regulation and characterization of rot transcription in Staphylococcus aureus

The pathogenesis of Staphylococcus aureus infections is dependent upon expression of various virulence factors, which are under the control of multiple regulatory systems, including two-component regulatory systems and transcriptional regulators such as the SarA family of proteins. As a part of a continuing effort to understand the regulatory mechanisms that involve the SarA protein family, the regulation and physical characterization of rot transcription is described here. The rot gene, a member of the sarA family of genes, was previously characterized and has been shown to regulate a large number of genes. The rot locus is composed of multiple overlapping transcripts as determined by primer extension and was proposed to encode an open reading frame of 133 residues. Transcription of rot was significantly increased in the sarA mutant. Gel shift and transcriptional studies revealed that SarA could bind to the rot promoter region, probably acting as a repressor for rot transcription. The data indicate that the expression of rot transcription is significantly repressed only by SarA among the sarA family of mutants tested at the post-exponential phase of growth.