The Staphylococcal accessory regulator (sar) represses transcription of the Staphylococcus aureus collagen adhesin gene (cna) in an agr-independent manner

Virulence attributes of successful methicillin-resistant Staphylococcus aureus lineages

Methicillin-resistant Staphylococcus aureus (MRSA) is a leading cause of severe and often fatal infections. MRSA epidemics have occurred in waves, whereby a previously successful lineage has been replaced by a more fit and better adapted lineage. Selection pressures in both hospital and community settings are not uniform across the globe, which has resulted in geographically distinct epidemiology. This review focuses on the mechanisms that trigger the establishment and maintenance of current, dominant MRSA lineages across the globe. While the important role of antibiotic resistance will be mentioned throughout, factors which influence the capacity of S. aureus to colonize and cause disease within a host will be the primary focus of this review. We show that while MRSA possesses a diverse arsenal of toxins including alpha-toxin, the success of a lineage involves more than just producing toxins that damage the host. Success is often attributed to the acquisition or loss of genetic elements involved in colonization and niche adaptation such as the arginine catabolic mobile element, as well as the activity of regulatory systems, and shift metabolism accordingly (e.g., the accessory genome regulator, agr). Understanding exactly how specific MRSA clones cause prolonged epidemics may reveal targets for therapies, whereby both core (e.g., the alpha toxin) and acquired virulence factors (e.g., the Panton-Valentine leukocidin) may be nullified using anti-virulence strategies.

CodY: An Essential Transcriptional Regulator Involved in Environmental Stress Tolerance in Foodborne Staphylococcus aureus RMSA24

Staphylococcus aureus (S. aureus), as the main pathogen in milk and dairy products, usually causes intoxication with vomiting and various kinds of inflammation after entering the human body. CodY, an important transcriptional regulator in S. aureus, plays an important role in regulating metabolism, growth, and virulence. However, little is known about the role of CodY on environmental stress tolerance. In this research, we revealed the role of CodY in environmental stress tolerance in foodborne S. aureus RMSA24. codY mutation significantly reduced the tolerance of S. aureus to desiccation and oxidative, salt, and high-temperature stresses. However, S. aureus was more tolerant to low temperature stress due to mutation of codY. We found that the expressions of two important heat shock proteins-GroEL and DanJ-were significantly down-regulated in the mutant codY. This suggests that CodY may indirectly regulate the high- and low-temperature tolerance of S. aureus by regulating the expressions of groEL and danJ. This study reveals a new mechanism of environmental stress tolerance in S. aureus and provides new insights into controlling the contamination and harm caused by S. aureus in the food industry.

Study of SarA by DNA Affinity Capture Assay (DACA) Employing Three Promoters of Key Virulence and Resistance Genes in Methicillin-Resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA), one of the most well-known human pathogens, houses many virulence factors and regulatory proteins that confer resistance to diverse antibiotics. Although they have been investigated intensively, the correlations among virulence factors, regulatory proteins and antibiotic resistance are still elusive. We aimed to identify the most significant global MRSA regulator by concurrently analyzing protein-binding and several promoters under same conditions and at the same time point. DNA affinity capture assay (DACA) was performed with the promoters of mecA, sarA, and sarR, all of which significantly impact survival of MRSA. Here, we show that SarA protein binds to all three promoters. Consistent with the previous reports, ΔsarA mutant exhibited weakened antibiotic resistance to oxacillin and reduced biofilm formation. Additionally, production and activity of many virulence factors such as phenol-soluble modulins (PSM), α-hemolysin, motility, staphyloxanthin, and other related proteins were decreased. Comparing the sequence of SarA with that of clinical strains of various lineages showed that all sequences were highly conserved, in contrast to that observed for AgrA, another major regulator of virulence and resistance in MRSA. We have demonstrated that SarA regulates antibiotic resistance and the expression of various virulence factors. Our results warrant that SarA could be a leading target for developing therapeutic agents against MRSA infections.

Expanding the Staphylococcus aureus SarA Regulon to Small RNAs

SarA, a transcriptional regulator of Staphylococcus aureus, is a major global regulatory system that coordinates the expression of target genes involved in its pathogenicity. Various studies have identified a large number of SarA target genes, but an in-depth characterization of the sarA regulon, including small regulatory RNAs (sRNAs), has not yet been done. In this study, we utilized transcriptome sequencing (RNA-Seq) and chromatin immunoprecipitation sequencing (ChIP-Seq) to determine a comprehensive list of SarA-regulated targets, including both mRNAs and sRNAs. RNA-Seq analysis indicated 390 mRNAs and 51 sRNAs differentially expressed in a ΔsarA mutant, while ChIP-Seq revealed 354 mRNAs and 55 sRNA targets in the S. aureus genome. We confirmed the authenticity of several novel SarA targets by Northern blotting and electrophoretic mobility shift assays. Among them, we characterized repression of sprG2, a gene that encodes the toxin of a type I toxin-antitoxin system, indicating a multilayer lockdown of toxin expression by both SarA and its cognate antitoxin, SprF2. Finally, a novel SarA consensus DNA binding sequence was generated using the upstream promoter sequences of 15 novel SarA-regulated sRNA targets. A genome-wide scan with a deduced SarA motif enabled the discovery of new potential SarA target genes which were not identified in our RNA-Seq and ChIP-Seq analyses. The strength of this new consensus was confirmed with one predicted sRNA target. The RNA-Seq and ChIP-Seq combinatory analysis gives a snapshot of the regulation, whereas bioinformatic analysis reveals a permanent view of targets based on sequence. Altogether these experimental and in silico methodologies are effective to characterize transcriptional factor (TF) regulons and functions.

IMPORTANCEStaphylococcus aureus, a commensal and opportunist pathogen, is responsible for a large number of human and animal infections, from benign to severe. Gene expression adaptation during infection requires a complex network of regulators, including transcriptional factors (TF) and sRNAs. TF SarA influences virulence, metabolism, biofilm formation, and resistance to some antibiotics. SarA directly regulates expression of around 20 mRNAs and a few sRNAs. Here, we combined high-throughput expression screening methods combined with binding assays and bioinformatics for an in-depth investigation of the SarA regulon. This combinatory approach allowed the identification of 85 unprecedented mRNAs and sRNAs targets, with at least 14 being primary. Among novel SarA direct targets, we characterized repression of sprG2, a gene that encodes the toxin of a toxin-antitoxin system, indicating a multilayer lockdown of toxin expression by both SarA and its cognate antitoxin, SprF2.

Comparative Proteomic Profiling of Methicillin-Susceptible and Resistant Staphylococcus aureus

Staphylococcus aureus is a highly successful human pathogen responsible for a wide range of infections. This study provides insights into the virulence, pathogenicity, and antimicrobial resistance determinants of methicillin-susceptible and methicillin-resistant S. aureus (MSSA; MRSA) recovered from non-healthcare environments. Three environmental MSSA and three environmental MRSA are selected for proteomic profiling using isobaric tag for relative and absolute quantitation tandem mass spectrometry (iTRAQ MS/MS). Gene Ontology annotation and Kyoto Encyclopedia of Genes and Genomes pathway annotation are applied to interpret the functions of the proteins detected. 792 proteins are identified in MSSA and MRSA. Comparative analysis of MRSA and MSSA reveals that 8 of out 792 proteins are upregulated and 156 are downregulated. Proteins that have differences in abundance are predominantly involved in catalytic and binding activity. Among 164 differently abundant proteins, 29 are involved in pathogenesis, antimicrobial resistance, stress response, mismatch repair, and cell wall synthesis. Twenty-two proteins associated with pathogenicity including SPA, SBI, CLFA, and DLT are upregulated in MRSA. Moreover, the upregulated pathogenic protein ENTC2 in MSSA is determined to be a super antigen, potentially capable of triggering toxic shock syndrome in the host. Enhanced pathogenicity, antimicrobial resistance, and stress response are observed in MRSA compared to MSSA.

The Impacts of msaABCR on sarA-Associated Phenotypes Are Different in Divergent Clinical Isolates of Staphylococcus aureus

The staphylococcal accessory regulator (sarA) plays an important role in Staphylococcus aureus infections, including osteomyelitis, and the msaABCR operon has been implicated as an important factor in modulating expression of sarA Thus, we investigated the contribution of msaABCR to sarA-associated phenotypes in the S. aureus clinical isolates LAC and UAMS-1. Mutation of msaABCR resulted in reduced production of SarA and a reduced capacity to form a biofilm in both strains. Biofilm formation was enhanced in a LAC msa mutant by restoring the production of SarA, but this was not true in a UAMS-1 msa mutant. Similarly, extracellular protease production was increased in a LAC msa mutant but not a UAMS-1 msa mutant. This difference was reflected in the accumulation and distribution of secreted virulence factors and in the impact of extracellular proteases on biofilm formation in a LAC msa mutant. Most importantly, it was reflected in the relative impact of mutating msa as assessed in a murine osteomyelitis model, which had a significant impact in LAC but not in UAMS-1. In contrast, mutation of sarA had a greater impact on all of these in vitro and in vivo phenotypes than mutation of msaABCR, and it did so in both LAC and UAMS-1. These results suggest that, at least in osteomyelitis, it would be therapeutically preferable to target sarA rather than msaABCR to achieve the desired clinical result, particularly in the context of divergent clinical isolates of S. aureus.

Impact of Staphylococcus aureus regulatory mutations that modulate biofilm formation in the USA300 strain LAC on virulence in a murine bacteremia model

Staphylococcus aureus causes acute and chronic forms of infection, the latter often associated with formation of a biofilm. It has previously been demonstrated that mutation of atl, codY, rot, sarA, and sigB limits biofilm formation in the USA300 strain LAC while mutation of agr, fur, and mgrA has the opposite effect. Here we used a murine sepsis model to assess the impact of these same loci in acute infection. Mutation of agr, atl, and fur had no impact on virulence, while mutation of mgrA and rot increased virulence. In contrast, mutation of codY, sarA, and sigB significantly attenuated virulence. Mutation of sigB resulted in reduced accumulation of AgrA and SarA, while mutation of sarA resulted in reduced accumulation of AgrA, but this cannot account for the reduced virulence of sarA or sigB mutants because the isogenic agr mutant was not attenuated. Indeed, as assessed by accumulation of alpha toxin and protein A, all of the mutants we examined exhibited unique phenotypes by comparison to an agr mutant and to each other. Attenuation of the sarA, sigB and codY mutants was correlated with increased production of extracellular proteases and global changes in extracellular protein profiles. These results suggest that the inability to repress the production of extracellular proteases plays a key role in attenuating the virulence of S. aureus in acute as well as chronic, biofilm-associated infections, thus opening up the possibility that strategies aimed at the de-repression of protease production could be used to broad therapeutic advantage. They also suggest that the impact of codY, sarA, and sigB on protease production occurs via an agr-independent mechanism.

The Staphylococcal Biofilm: Adhesins, Regulation, and Host Response

The staphylococci comprise a diverse genus of Gram-positive, nonmotile commensal organisms that inhabit the skin and mucous membranes of humans and other mammals. In general, staphylococci are benign members of the natural flora, but many species have the capacity to be opportunistic pathogens, mainly infecting individuals who have medical device implants or are otherwise immunocompromised. Staphylococcus aureus and Staphylococcus epidermidis are major sources of hospital-acquired infections and are the most common causes of surgical site infections and medical device-associated bloodstream infections. The ability of staphylococci to form biofilms in vivo makes them highly resistant to chemotherapeutics and leads to chronic diseases. These biofilm infections include osteomyelitis, endocarditis, medical device infections, and persistence in the cystic fibrosis lung. Here, we provide a comprehensive analysis of our current understanding of staphylococcal biofilm formation, with an emphasis on adhesins and regulation, while also addressing how staphylococcal biofilms interact with the immune system. On the whole, this review will provide a thorough picture of biofilm formation of the staphylococcus genus and how this mode of growth impacts the host.

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.

RpiRc Is a Pleiotropic Effector of Virulence Determinant Synthesis and Attenuates Pathogenicity in Staphylococcus aureus

In Staphylococcus aureus, metabolism is intimately linked with virulence determinant biosynthesis, and several metabolite-responsive regulators have been reported to mediate this linkage. S. aureus possesses at least three members of the RpiR family of transcriptional regulators. Of the three RpiR homologs, RpiRc is a potential regulator of the pentose phosphate pathway, which also regulates RNAIII levels. RNAIII is the regulatory RNA of the agr quorum-sensing system that controls virulence determinant synthesis. The effect of RpiRc on RNAIII likely involves other regulators, as the regulators that bind the RNAIII promoter have been intensely studied. To determine which regulators might bridge the gap between RpiRc and RNAIII, sarA, sigB, mgrA, and acnA mutations were introduced into an rpiRc mutant background, and the effects on RNAIII were determined. Additionally, phenotypic and genotypic differences were examined in the single and double mutant strains, and the virulence of select strains was examined using two different murine infection models. The data suggest that RpiRc affects RNAIII transcription and the synthesis of virulence determinants in concert with σ(B), SarA, and the bacterial metabolic status to negatively affect virulence.

Sigma Factor SigB Is Crucial to Mediate Staphylococcus aureus Adaptation during Chronic Infections

Staphylococcus aureus is a major human pathogen that causes a range of infections from acute invasive to chronic and difficult-to-treat. Infection strategies associated with persisting S. aureus infections are bacterial host cell invasion and the bacterial ability to dynamically change phenotypes from the aggressive wild-type to small colony variants (SCVs), which are adapted for intracellular long-term persistence. The underlying mechanisms of the bacterial switching and adaptation mechanisms appear to be very dynamic, but are largely unknown. Here, we analyzed the role and the crosstalk of the global S. aureus regulators agr, sarA and SigB by generating single, double and triple mutants, and testing them with proteome analysis and in different in vitro and in vivo infection models. We were able to demonstrate that SigB is the crucial factor for adaptation in chronic infections. During acute infection, the bacteria require the simultaneous action of the agr and sarA loci to defend against invading immune cells by causing inflammation and cytotoxicity and to escape from phagosomes in their host cells that enable them to settle an infection at high bacterial density. To persist intracellularly the bacteria subsequently need to silence agr and sarA. Indeed agr and sarA deletion mutants expressed a much lower number of virulence factors and could persist at high numbers intracellularly. SigB plays a crucial function to promote bacterial intracellular persistence. In fact, ΔsigB-mutants did not generate SCVs and were completely cleared by the host cells within a few days. In this study we identified SigB as an essential factor that enables the bacteria to switch from the highly aggressive phenotype that settles an acute infection to a silent SCV-phenotype that allows for long-term intracellular persistence. Consequently, the SigB-operon represents a possible target to develop preventive and therapeutic strategies against chronic and therapy-refractory infections.

Comparative impact of diverse regulatory loci on Staphylococcus aureus biofilm formation

The relative impact of 23 mutations on biofilm formation was evaluated in the USA300, methicillin-resistant strain LAC. Mutation of sarA, atl, codY, rsbU, and sigB limited biofilm formation in comparison to the parent strain, but the limitation imposed by mutation of sarA was greater than that imposed by mutation of any of these other genes. The reduced biofilm formation of all mutants other than the atl mutant was correlated with increased levels of extracellular proteases. Mutation of fur- and mgrA-enhanced biofilm formation but in LAC had no impact on protease activity, nuclease activity, or accumulation of the polysaccharide intercellular adhesin (PIA). The increased capacity of these mutants to form a biofilm was reversed by mutation of sarA, and this was correlated with increased protease production. Mutation of sarA, mgrA, and sigB had the same phenotypic effect in the methicillin-sensitive strain UAMS-1, but mutation of codY increased rather than decreased biofilm formation. As with the UAMS-1 mgrA mutant, this was correlated with increased production of PIA. Examination of four additional clinical isolates suggests that the differential impact of codY on biofilm formation may be a conserved characteristic of methicillin-resistant versus methicillin-sensitive strains.

Impact of the functional status of saeRS on in vivo phenotypes of Staphylococcus aureus sarA mutants

We investigated the in vivo relevance of the impact of sarA and saeRS on protease production using derivatives of the USA300 strain LAC. The results confirmed that mutation of saeRS or sarA reduces virulence in a bacteremia model to a comparable degree. However, while eliminating protease production restored virulence in the sarA mutant, it had little impact in the saeRS mutant. Additionally, constitutive activation of saeRS (saeRS(C)) enhanced the virulence of LAC and largely restored virulence in the isogenic sarA mutant. Based on these results, together with our analysis of the representative virulence factors alpha toxin, protein A (Spa), and extracellular nucleases, we propose a model in which the attenuation of saeRS mutants is defined primarily by decreased production of such factors, while constitutive activation of saeRS increases virulence, and reverses the attenuation of sarA mutants, because it results in both increased production and decreased protease-mediated degradation of these same factors. This regulatory balance was also apparent in a murine model of catheter-associated infection, with the results suggesting that the impact of saeRS on nuclease production plays an important role during the early stages of these infections that is partially offset by increased protease production in sarA mutants.

Staphylococcus aureus sarA regulates inflammation and colonization during central nervous system biofilm formation

Infection is a frequent and serious complication following the treatment of hydrocephalus with CSF shunts, with limited therapeutic options because of biofilm formation along the catheter surface. Here we evaluated the possibility that the sarA regulatory locus engenders S. aureus more resistant to immune recognition in the central nervous system (CNS) based on its reported ability to regulate biofilm formation. We utilized our established model of CNS catheter-associated infection, similar to CSF shunt infections seen in humans, to compare the kinetics of bacterial titers, cytokine production and inflammatory cell influx elicited by wild type S. aureus versus an isogenic sarA mutant. The sarA mutant was more rapidly cleared from infected catheters compared to its isogenic wild type strain. Consistent with this finding, several pro-inflammatory cytokines and chemokines, including IL-17, CXCL1, and IL-1β were significantly increased in the brain following infection with the sarA mutant versus wild type S. aureus, in agreement with the fact that the sarA mutant displayed impaired biofilm growth and favored a planktonic state. Neutrophil influx into the infected hemisphere was also increased in the animals infected with the sarA mutant compared to wild type bacteria. These changes were not attributable to extracellular protease activity, which is increased in the context of SarA mutation, since similar responses were observed between sarA and a sarA/protease mutant. Overall, these results demonstrate that sarA plays an important role in attenuating the inflammatory response during staphylococcal biofilm infection in the CNS via a mechanism that remains to be determined.

Quantitative PCR analysis of genes expressed during biofilm development of methicillin resistant Staphylococcus aureus (MRSA)

Staphylococcus aureus biofilm associated infections remains a major clinical concern in patients with indwelling devices. Quantitative real-time PCR (qPCR) can be used to investigate the pathogenic role of such biofilms. We describe qPCRs for 12 adhesion and biofilm-related genes of four S. aureus isolates which were applied during in vitro biofilm development. An endogenous control (16S rRNA) was used for signal normalization. We compared the qPCR results with structural analysis using scanning electron microscopy (SEM). The SEM studies showed different cellular products surrounding the aggregated cells at different times of biofilm formation. Using qPCR, we found that expression levels of the gene encoding fibronectin binding protein A and B and clumping factor B (fnbA/B and clfB), which involves in primary adherence of S. aureus, were significantly increased at 24h and decreased slightly and variably at 48 h when all 4 isolates were considered. The elastin binding protein (ebps) RNA expression level was significantly enhanced more than 6-fold at 24 and 48 h compared to 12h. Similar results were obtained for the intercellular adhesion biofilm required genes type C (icaC). In addition, qPCR revealed a fluctuation in expression levels at different time points of biofilm growth of other genes, indicating that different parameter modes of growth processes are operating at different times.

sarA-mediated repression of protease production plays a key role in the pathogenesis of Staphylococcus aureus USA300 isolates

Mutation of staphylococcal accessory regulator (sarA) results in increased production of extracellular proteases in Staphylococcus aureus, which has been correlated with decreased biofilm formation and decreased accumulation of extracellular toxins. We used murine models of implant-associated biofilm infection and S. aureus bacteraemia (SAB) to compare virulence of USA300 strain LAC, its isogenic sarA mutant, and derivatives of each of these strains with mutations in all 10 of the genes encoding recognized extracellular proteases. The sarA mutant was attenuated in both models, and this was reversed by eliminating production of extracellular proteases. To examine the mechanistic basis, we identified proteins impacted by sarA in a protease-dependent manner. We identified 253 proteins where accumulation was reduced in the sarA mutant compared with the parent strain, and was restored in the sarA/protease mutant. Additionally, in SAB, the LAC protease mutant exhibited a hypervirulent phenotype by comparison with the isogenic parent strain, demonstrating that sarA also positively regulates production of virulence factors, some of which are subject to protease-mediated degradation. We propose a model in which attenuation of sarA mutants is defined by their inability to produce critical factors and simultaneously repress production of extracellular proteases that would otherwise limit accumulation of virulence factors.

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.

saeRS and sarA act synergistically to repress protease production and promote biofilm formation in Staphylococcus aureus

Mutation of the staphylococcal accessory regulator (sarA) limits biofilm formation in diverse strains of Staphylococcus aureus, but there are exceptions. One of these is the commonly studied strain Newman. This strain has two defects of potential relevance, the first being mutations that preclude anchoring of the fibronectin-binding proteins FnbA and FnbB to the cell wall, and the second being a point mutation in saeS that results in constitutive activation of the saePQRS regulatory system. We repaired these defects to determine whether either plays a role in biofilm formation and, if so, whether this could account for the reduced impact of sarA in Newman. Restoration of surface-anchored FnbA enhanced biofilm formation, but mutation of sarA in this fnbA-positive strain increased rather than decreased biofilm formation. Mutation of sarA in an saeS-repaired derivative of Newman (P18L) or a Newman saeRS mutant (ΔsaeRS) resulted in a biofilm-deficient phenotype like that observed in clinical isolates, even in the absence of surface-anchored FnbA. These phenotypes were correlated with increased production of extracellular proteases and decreased accumulation of FnbA and/or Spa in the P18L and ΔsaeRS sarA mutants by comparison to the Newman sarA mutant. The reduced accumulation of Spa was reversed by mutation of the gene encoding aureolysin, while the reduced accumulation of FnbA was reversed by mutation of the sspABC operon. These results demonstrate that saeRS and sarA act synergistically to repress the production of extracellular proteases that would otherwise limit accumulation of critical proteins that contribute to biofilm formation, with constitutive activation of saeRS limiting protease production, even in a sarA mutant, to a degree that can be correlated with increased enhanced capacity to form a biofilm. Although it remains unclear whether these effects are mediated directly or indirectly, studies done with an sspA::lux reporter suggest they are mediated at a transcriptional level.

The staphylococcal accessory regulator, SarA, is an RNA-binding protein that modulates the mRNA turnover properties of late-exponential and stationary phase Staphylococcus aureus cells

The modulation of mRNA turnover is gaining recognition as a mechanism by which Staphylococcus aureus regulates gene expression, but the factors that orchestrate alterations in transcript degradation are poorly understood. In that regard, we previously found that 138 mRNA species, including transcripts coding for the virulence factors protein A (spa) and collagen-binding protein (cna), are stabilized in a sarA-dependent manner during exponential phase growth, suggesting that SarA directly or indirectly affects the RNA turnover properties of these transcripts. Herein, we expanded our characterization of the effects of sarA on mRNA turnover during late-exponential and stationary phases of growth. Results revealed that the locus affects the RNA degradation properties of cells during both growth phases. Further, using gel mobility shift assays and RIP-Chip, it was found that SarA protein is capable of binding mRNA species that it stabilizes both in vitro and within bacterial cells. Taken together, these results suggest that SarA post-transcriptionally regulates S. aureus gene expression in a manner that involves binding to and consequently altering the mRNA turnover properties of target transcripts.

Transcriptional profiling analysis of the global regulator NorG, a GntR-like protein of Staphylococcus aureus

The GntR-like protein NorG has been shown to affect Staphylococcus aureus genes involved in resistance to quinolones and β-lactams, such as those encoding the NorB and AbcA transporters. To identify the target genes regulated by NorG, we carried out transcriptional-profiling assays using S. aureus RN6390 and its isogenic norG::cat mutant. Our data showed that NorG positively affected the transcription of global regulators mgrA, arlS, and sarZ. The three putative drug efflux pump genes most positively affected by NorG were the NorB efflux pump (5.1-fold), the MmpL-like protein SACOL2566 (5.2-fold), and the BcrA-like drug transporter SACOL2525 (5.7-fold) genes. The S. aureus predicted MmpL protein showed 53% homology with the MmpL lipid transporter of Mycobacterium tuberculosis, and the putative SACOL2525 protein showed 87% homology with the bacitracin drug transporter BcrA of Staphylococcus hominis. Two pump genes most negatively affected by NorG were the NorC (4-fold) and AbcA (6-fold) genes. Other categories of genes, such as those participating in amino acid, inorganic ion, or nucleotide transporters and metabolism, were also affected by NorG. Real-time reverse transcription (RT)-PCR assays for mgrA, arlS, sarZ, norB, norC, abcA, mmpL, and bcrA-like were carried out to verify microarray data and showed the same level of up- or downregulation by NorG. The norG mutant showed a 2-fold increase in resistance to norfloxacin and rhodamine, both substrates of the NorC transporter, which is consistent with the resistance phenotype conferred by overexpression of norC on a plasmid. These data indicate that NorG has broad regulatory function in S. aureus.

Defining the strain-dependent impact of the Staphylococcal accessory regulator (sarA) on the alpha-toxin phenotype of Staphylococcus aureus

We demonstrate that mutation of the staphylococcal accessory regulator (sarA) limits the accumulation of alpha-toxin and phenol-soluble modulins (PSMs) in Staphylococcus aureus isolates of the USA300 clonal lineage. Degradation assays and experiments done with protease inhibitors suggested that this was due to the increased production of extracellular proteases rather than differences associated with the impact of sarA on transcription of the target gene (hla) or the accessory gene regulator (agr). This was confirmed by demonstrating that concomitant mutation of the gene encoding aureolysin (aur) reversed the alpha-toxin and PSM-deficient phenotypes of a USA300 sarA mutant. Mutation of sarA had little impact on the alpha-toxin or PSM phenotypes of the commonly studied strain Newman, which is known to have a mutation in saeS that results in constitutive activation of the saeRS regulatory system, and we also demonstrate that repair of this defect resulted in the increased production of extracellular proteases and reversed both the alpha-toxin and PSM-positive phenotypes of a Newman sarA mutant.

Control of the Staphylococcus aureus toxic shock tst promoter by the global regulator SarA

The Staphylococcus aureus SarA global regulator controls the expression of numerous virulence genes, often in conjunction with the agr quorum-sensing system and its effector RNA, RNAIII. In the present study, we have examined the role of both SarA and RNAIII on the regulation of the promoter of tst, encoding staphylococcal superantigen toxic shock syndrome toxin 1 (TSST-1). In vitro DNA-protein interaction studies with purified SarA using gel shift and DNase I protection assays revealed one strong SarA binding site and evidence for a weaker site nearby within the minimal 400-bp promoter region upstream of tst. In vivo analysis of tst promoter activation using a p(tst)-luxAB reporter inserted in the chromosome revealed partial but not complete loss of tst expression in a Δhld-RNAIII strain. In contrast, disruption of sarA abrogated tst expression. No significant tst expression was found for the double Δhld-RNAIII-ΔsarA mutant. Introduction of a plasmid containing cloned hld-RNAIII driven by a non-agr-dependent promoter, p(HU), into isogenic parental wild-type or ΔsarA strains showed comparable levels of RNAIII detected by quantitative reverse transcription-PCR (qRT-PCR) but a two-log(10) reduction in p(tst)-luxAB reporter expression in the ΔsarA strain, arguing that RNAIII levels alone are not strictly determinant for tst expression. Collectively, our results indicate that SarA binds directly to the tst promoter and that SarA plays a significant and direct role in the expression of tst.

Phosphorylation of the virulence regulator SarA modulates its ability to bind DNA in Staphylococcus aureus

Staphylococcus aureus is one of the main bacterial species of clinical importance. Its virulence is considered multifactorial and is attributed to the combined action of a variety of molecular determinants including the virulence regulator SarA. Phosphorylation of SarA was observed to occur in vivo. From this finding, SarA was overproduced and purified to homogeneity. In an in vitro assay, it was found to be unable to autophosphorylate, but was effectively modified at threonine and serine residues by each of the two Ser/Thr kinases of S. aureus, Stk1 (PknB) and SA0077, respectively. In addition, phosphorylation of SarA was shown to modify its ability to bind DNA. Together, these data support the concept that protein phosphorylation directly participates, at the transcription level, in the control of bacterial pathogenicity.

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.

Structural and functional role of Staphylococcus aureus surface components recognizing adhesive matrix molecules of the host

Staphylococcus aureus is a versatile and harmful human pathogen in both hospital- and community-acquired infections. S. aureus can initiate host infection by adhering to components of the extracellular matrix. Adherence is mediated by a variety of protein adhesins of the microbial surface component recognizing adhesive matrix molecule (MSCRAMM) family. In this article, we describe these MSCRAMMs in terms of structural organization and ligand-binding capacity and discuss their role as a possible target for immunotherapy.

Staphylococcus aureus SarA is a regulatory protein responsive to redox and pH that can support bacteriophage lambda integrase-mediated excision/recombination

Staphylococcus aureus produces a wide array of virulence factors and causes a correspondingly diverse array of infections. Production of these virulence factors is under the control of a complex network of global regulatory elements, one of which is sarA. sarA encodes a DNA binding protein that is considered to function as a transcription factor capable of acting as either a repressor or an activator. Using competitive ELISA assays, we demonstrate that SarA is present at approximately 50 000 copies per cell, which is not characteristic of classical transcription factors. We also demonstrate that SarA is present at all stages of growth in vitro and is capable of binding DNA with high affinity but that its binding affinity and pattern of shifted complexes in electrophoretic mobility shift assays is responsive to the redox state. We also show that SarA binds to the bacteriophage lambda (lambda) attachment site, attL, producing SarA-DNA complexes similar to intasomes, which consist of bacteriophage lambda integrase, Escherichia coli integration host factor and attL DNA. In addition, SarA stimulates intramolecular excision recombination in the absence of lambda excisionase, a DNA binding accessory protein. Taken together, these data suggest that SarA may function as an architectural accessory protein.

Tricarboxylic acid cycle-dependent attenuation of Staphylococcus aureus in vivo virulence by selective inhibition of amino acid transport

Staphylococci are the leading causes of endovascular infections worldwide. Commonly, these infections involve the formation of biofilms on the surface of biomaterials. Biofilms are a complex aggregation of bacteria commonly encapsulated by an adhesive exopolysaccharide matrix. In staphylococci, this exopolysaccharide matrix is composed of polysaccharide intercellular adhesin (PIA). PIA is synthesized when the tricarboxylic acid (TCA) cycle is repressed. The inverse correlation between PIA synthesis and TCA cycle activity led us to hypothesize that increasing TCA cycle activity would decrease PIA synthesis and biofilm formation and reduce virulence in a rabbit catheter-induced model of biofilm infection. TCA cycle activity can be induced by preventing staphylococci from exogenously acquiring a TCA cycle-derived amino acid necessary for growth. To determine if TCA cycle induction would decrease PIA synthesis in Staphylococcus aureus, the glutamine permease gene (glnP) was inactivated and TCA cycle activity, PIA accumulation, biofilm forming ability, and virulence in an experimental catheter-induced endovascular biofilm (endocarditis) model were determined. Inactivation of this major glutamine transporter increased TCA cycle activity, transiently decreased PIA synthesis, and significantly reduced in vivo virulence in the endocarditis model in terms of achievable bacterial densities in biofilm-associated cardiac vegetations, kidneys, and spleen. These data confirm the close linkage of TCA cycle activity and virulence factor production and establish that this metabolic linkage can be manipulated to alter infectious outcomes.

Impact of sarA on daptomycin susceptibility of Staphylococcus aureus biofilms in vivo

We used a murine model of catheter-associated biofilm formation to determine whether the mutation of the staphylococcal accessory regulator (sarA) has an impact on the susceptibility of established Staphylococcus aureus biofilms to treatment with daptomycin in vivo. The experiments were done with two clinical isolates, one of which (UAMS-1) was obtained from the bone of a patient suffering from osteomyelitis, while the other (UAMS-1625) is an isolate of the USA300 clonal lineage of community-acquired methicillin (meticillin)-resistant S. aureus. UAMS-1625 had a reduced capacity to form a biofilm in vivo compared to that of UAMS-1 (P = 0.0015), but in both cases the mutation of sarA limited biofilm formation compared to that of the corresponding parent strain (P < or = 0.001). The mutation of sarA did not affect the daptomycin MIC for either strain, but it did result in increased susceptibility in vivo in the context of an established biofilm. Specifically, daptomycin treatment resulted in the clearance of detectable bacteria from <10% of the catheters colonized with the parent strains, while treatment with an equivalent daptomycin concentration resulted in the clearance of 46.4% of the catheters colonized with the UAMS-1 sarA mutant and 69.1% of the catheters colonized with the UAMS-1625 sarA mutant. In the absence of daptomycin treatment, mice with catheters colonized with the UAMS-1625 parent strain also developed skin lesions in the region adjacent to the implanted catheter. No such lesions were observed in any other experimental group, including untreated mice containing catheters colonized with the UAMS-1625 sarA mutant.

Transcriptomic and metabolic responses of Staphylococcus aureus exposed to supra-physiological temperatures

BACKGROUND

Previous evaluation by different molecular and physiological assays of Staphylococcus aureus (S. aureus) responses to heat shock exposure yielded a still fragmentary view of the mechanisms determining bacterial survival or death at supra-physiological temperatures. This study analyzed diverse facets of S. aureus heat-shock adjustment by recording global transcriptomic and metabolic responses of bacterial cultures shifted for 10 min from 37 degrees C to a sub-lethal (43 degrees C) or eventually lethal (48 degrees C) temperature. A relevant metabolic model of the combined action of specific stress response mechanisms with more general, energy-regulating metabolic pathways in heat-shocked S. aureus is presented.

RESULTS

While S. aureus cultures shifted to 43 degrees C or left at 37 degrees C showed marginal differences in growth and survival rates, bacterial cultures exposed to 48 degrees C showed a rapid growth arrest followed by a subsequent decline in viable counts. The most substantial heat shock-induced changes at both 43 degrees C and 48 degrees C occurred in transcript levels of HrcA- and CtsR-regulated genes, encoding classical chaperones DnaK and GroESL, and some Hsp100/Clp ATPases components, respectively. Other metabolic pathways up-regulated by S. aureus exposure at 48 degrees C included genes encoding several enzymes coping with oxidative stress, and DNA damage, or/and impaired osmotic balance. Some major components of the pentose phosphate cycle and gluconeogenesis were also up-regulated, which reflected depletion of free glucose by bacterial cultures grown in Mueller-Hinton broth prior to heat shock. In contrast, most purine- and pyrimidine-synthesis pathway components and amino acyl-tRNA synthetases were down-regulated at 48 degrees C, as well as arginine deiminase and major fermentative pathway components, such as alcohol, lactate and formate dehydrogenases. Despite the heat-induced, increased requirements for ATP-dependent macromolecular repair mechanisms combined with declining energy sources, intracellular ATP levels remained remarkably constant during heat shock.

CONCLUSION

The sequential loss of replication and viability at 48 degrees C cannot be explained by significant reductions in intracellular ATP levels, but may reflect ATP rerouting for macromolecular repair mechanisms and cell survival. Our metabolic model also suggests that heat-stressed S. aureus should down-regulate the production of potential, DNA-damaging reactive oxygen species that might result from electron transport-generated ATP, involving excessive levels of free heavy metals, in particular iron.

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.

Variations in amount of TSST-1 produced by clinical methicillin resistant Staphylococcus aureus (MRSA) isolates and allelic variation in accessory gene regulator (agr) locus

Background

Staphylococcus aureus (S. aureus) is an important pathogen associated with both nosocomial and community-acquired infections and its pathogenicity is attributed to its potential to produce virulence factors. Since the amount of toxin produced is related to virulence, evaluating toxin production should be useful for controlling S. aureus infection. We previously found that some strains produce relatively large amounts of TSST-1; however, no reports have described the amount of TSST-1 produced by clinical isolates.

Methods

Amounts of TSST-1 produced by clinical methicillin resistant S. aureus (MRSA) isolates were measured by Western blotting. We determined their accessory gene regulator (agr) class by PCR and investigated whether TSST-1 production correlates with variations in the class and structure of the agr.

Results

We found that 75% of surveyed MRSA isolates (n = 152) possessed the tst gene and that 96.7% belonged to agr class 2. The concentrations of TSST-1 secreted into culture supernatants by 34 strains measured by Western blotting differed 170-fold. Sequencing the entire agr locus (n = 9) revealed that some had allelic variations regardless of the amount of TSST-1 produced whereas sequencing the sar, sigma factor B and the tst promoter region revealed no significant changes.

Conclusion

The amounts of TSST-1 produced by clinical MRSA isolates varied. The present results suggest that TSST-1 production is not directly associated with the agr structure, but is instead controlled by unknown transcriptional/translational regulatory systems, or synthesized by multiple regulatory mechanisms that are interlinked in a complex manner.

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.

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.

Point mutations within the streptococcal regulator of virulence (Srv) alter protein-DNA interactions and Srv function

Group A Streptococcus (GAS) possesses a complex regulatory system enabling the organism to colonize a range of physiologically distinct host sites. Within this network of regulators is the streptococcal regulator of virulence (Srv). Srv is a member of the CRP/FNR family of transcriptional regulators and is most similar to pleiotropic regulatory factor A (PrfA), a positive regulator of virulence in Listeria monocytogenes. Members of this family possess a characteristic C-terminal helix-turn-helix motif (HTH) that facilitates binding to DNA targets. Genome scanning identified four targets in GAS that were similar to the consensus DNA target recognized by PrfA. Furthermore, previous amino acid sequence alignments identified conserved residues within the Srv HTH which are necessary for function in PrfA and CRP. Here we investigated the ability of Srv to interact with DNA and evaluated the role of the HTH in this interaction. Purified recombinant Srv (rSrv) was found to co-purify with an untagged form of Srv. Glutaraldehyde cross-linking and gel-filtration chromatography indicated that this co-purification is likely due to the ability of Srv to oligomerize. Electrophoretic mobility shift assays (EMSAs) demonstrated that rSrv retarded the mobility of DNA targets and a supershift analysis confirmed the observation was rSrv-dependent. Competition EMSA indicated that rSrv had a higher relative affinity for the DNA targets studied than non-specific DNA. Site-directed mutagenesis of residues predicted to be in or near the HTH resulted in a decrease or abrogation of DNA binding. Complementation of MGAS5005Deltasrv with one of these site-directed mutants failed to restore wild-type SpeB activity. Taken together, these data suggest that the Srv HTH is necessary for DNA binding and Srv function.

Staphylococcus aureus CodY negatively regulates virulence gene expression

CodY is a global regulatory protein that was first discovered in Bacillus subtilis, where it couples gene expression to changes in the pools of critical metabolites through its activation by GTP and branched-chain amino acids. Homologs of CodY can be found encoded in the genomes of nearly all low-G+C gram-positive bacteria, including Staphylococcus aureus. The introduction of a codY-null mutation into two S. aureus clinical isolates, SA564 and UAMS-1, through allelic replacement, resulted in the overexpression of several virulence genes. The mutant strains had higher levels of hemolytic activity toward rabbit erythrocytes in their culture fluid, produced more polysaccharide intercellular adhesin (PIA), and formed more robust biofilms than did their isogenic parent strains. These phenotypes were associated with derepressed levels of RNA for the hemolytic alpha-toxin (hla), the accessory gene regulator (agr) (RNAII and RNAIII/hld), and the operon responsible for the production of PIA (icaADBC). These data suggest that CodY represses, either directly or indirectly, the synthesis of a number of virulence factors of 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.

Transcriptional regulation of the agr locus and the identification of DNA binding residues of the global regulatory protein SarR in Staphylococcus aureus

Many members of the transcriptional regulator SarA protein family are winged-helix proteins that are involved in gene regulation essential to Staphylococcus aureus pathogenesis. Investigation of the mechanism by which this family of genes acts at the molecular level will likely contribute to the understanding of the pathogenesis process and enhance the potential for the development of inhibitors capable of controlling S. aureus infections. Our previously published studies demonstrate that SarR is a repressor of sarA expression. Here, we show that SarR (a member of the SarA protein family) in part regulates agr expression by direct binding to the agr intergenic promoter region as determined by gel shift and DNase I footprinting assays. Analysis of the double sarA/sarR mutant in the early phase of growth reveals its significant role in regulating agr expression as compared with single mutants. Based on the previously reported crystal structure of SarR, we conducted site-specific mutagenesis and demonstrate that K52 residues within helix-turn-helix (HTH), K80, R82 and R88 (in the wing) and L105 (in the alpha5 helix) are important for DNA binding. Interestingly, SarR and SarA binding sites on the agr promoter are confined within the same region of DNA. Additional gel shift studies with SarR and SarA suggest that these two proteins may bind the same region of the agr promoter.

Erosion from Staphylococcus aureus biofilms grown under physiologically relevant fluid shear forces yields bacterial cells with reduced avidity to collagen

An estimated 65% of infective diseases are associated with the presence of bacterial biofilms. Biofilm-issued planktonic cells promote blood-borne, secondary sites of infection by the inoculation of the infected sites with bacteria from the intravascular space. To investigate the potential role of early detachment events in initiating secondary infections, we studied the phenotypic attributes of Staphylococcus aureus planktonic cells eroding from biofilms with respect to expression of the collagen adhesin, CNA. The collagen-binding abilities of S. aureus have been correlated to the development of osteomyelitis and septic arthritis. In this study, we focused on the impact of CNA expression on S. aureus adhesion to immobilized collagen in vitro under physiologically relevant shear forces. In contrast to the growth phase-dependent adhesion properties characteristic of S. aureus cells grown in suspension, eroding planktonic cells expressed invariant and lower effective adhesion rates regardless of the age of the biofilm from which they originated. These results correlated directly with the surface expression level of CNA. However, subsequent analysis revealed no qualitative differences between biofilms initiated with suspension cells and secondary biofilms initiated with biofilm-shed planktonic cells. Taken together, our findings suggest that, despite their low levels of CNA expression, S. aureus planktonic cells shed from biofilms retain the capacity for metastatic spread and the initiation of secondary infection. These findings demonstrate the need for a better understanding of the phenotypic properties of eroding planktonic cells, which could lead to new therapeutic strategies to target secondary infections.

SarA is a repressor of hla (alpha-hemolysin) transcription in Staphylococcus aureus: its apparent role as an activator of hla in the prototype strain NCTC 8325 depends on reduced expression of sarS

In most Staphylococcus aureus strains, inactivation of sarA increases hla transcription, indicating that sarA is a repressor. However, in S. aureus NCTC 8325 and its derivatives, used for most studies of hla regulation, inactivation of sarA resulted in decreased hla transcription. The disparate phenotype of strain NCTC 8325 seems to be associated with its rsbU mutation, which leads to sigma(B) deficiency. This has now been verified by the demonstration that sarA repressed hla transcription in an rsbU+ derivative of strain 8325-4 (SH1000). That sarA could act as a repressor of hla in an 8325-4 background was confirmed by the observation that inactivation of sarA in an agr sarS rot triple mutant dramatically increased hla transcription to wild-type levels. However, the apparent role of sarA as an activator of hla in 8325-4 was not a result of the rsbU mutation alone, as inactivation of sarA in another rsbU mutant, strain V8, led to increased hla transcription. Northern blot analysis revealed much higher levels of sarS mRNA in strain V8 than in 8325-4, which was likely due to the mutation in the sarS activator, tcaR, in 8325-4, which was not found in strain V8. On the other hand, the relative increase in sarS transcription upon the inactivation of sarA was 15-fold higher in 8325-4 than in strain V8. Because of this, inactivation of sarA in 8325-4 means a net increase in repressor activity, whereas in strain V8, inactivation of sarA means a net decrease in repressor activity and, therefore, enhanced hla transcription.

Identification and characterization of msa (SA1233), a gene involved in expression of SarA and several virulence factors in Staphylococcus aureus

The staphylococcal accessory regulator (sarA) plays a central role in the regulation of virulence in Staphylococcus aureus. To date, studies involving sarA have focused on its activity as a global regulator that modulates transcription of a wide variety of genes (>100) and its role in virulence. However, there is also evidence to suggest the existence of accessory elements that modulate SarA production and/or function. A reporter system was developed to identify such elements, and a new gene, msa (SA1233), mutation of which results in reduced expression of SarA, was identified and characterized. Additionally, it was shown that mutation of msa resulted in altered transcription of the accessory gene regulator (agr) and the genes encoding several virulence factors including alpha toxin (hla) and protein A (spa). However, the impact of mutating msa was different in the laboratory strain RN6390 and the clinical isolate UAMS-1. For instance, mutation of msa caused a decrease in spa and hla transcription in RN6390 but had a different effect in UAMS-1. The strain-dependent effects of the msa mutation were similar to those observed previously, which suggests that msa may modulate the production of specific virulence factors through its impact on sarA. Interestingly, sequence analysis of Msa suggests that it is a putative membrane protein with three membrane-spanning regions, indicating that Msa might interact with the environment. The findings show that msa is involved in the expression of SarA and several virulence factors.

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

The expression of genes involved in the pathogenesis of Staphylococcus aureus is known to be controlled by global regulatory loci, including agr, sarA, saeRS, arlRS, and sarA-like genes. As part of our continuing efforts to understand the regulatory mechanisms that involve sarA-like genes, we describe here the characterization of a novel transcriptional regulator called SarX, a member of the SarA protein family. The transcription of sarX was growth phase dependent and was expressed maximally during the stationary phase of growth, which was significantly decreased in the mgrA mutant. MgrA acted as an activator of sarX expression as confirmed by transcriptional fusion and Northern blot analyses. Purified MgrA protein bound to the upstream region of the sarX promoter as demonstrated by gel shift assay. The expression levels of various potential target genes involved in virulence and regulation, specifically those affected by sarA and mgrA, were analyzed with isogenic sarX mutant strains. Our data indicated that SarX acted as a repressor of the agr locus and consequently target genes regulated by the agr system. We propose that SarX is an important regulator in the SarA protein family and may be part of the common pathway by which agr and members of the sarA gene family control virulence in S. aureus.

Characterizing the effect of the Staphylococcus aureus virulence factor regulator, SarA, on log-phase mRNA half-lives

Bacterial pathogens regulate virulence factor expression at both the level of transcription initiation and mRNA processing/turnover. Within Staphylococcus aureus, virulence factor transcript synthesis is regulated by a number of two-component regulatory systems, the DNA binding protein SarA, and the SarA family of homologues. However, little is known about the factors that modulate mRNA stability or influence transcript degradation within the organism. As our entree to characterizing these processes, S. aureus GeneChips were used to simultaneously determine the mRNA half-lives of all transcripts produced during log-phase growth. It was found that the majority of log-phase transcripts (90%) have a short half-life (<5 min), whereas others are more stable, suggesting that cis- and/or trans-acting factors influence S. aureus mRNA stability. In support of this, it was found that two virulence factor transcripts, cna and spa, were stabilized in a sarA-dependent manner. These results were validated by complementation and real-time PCR and suggest that SarA may regulate target gene expression in a previously unrecognized manner by posttranscriptionally modulating mRNA turnover. Additionally, it was found that S. aureus produces a set of stable RNA molecules with no predicted open reading frame. Based on the importance of the S. aureus agr RNA molecule, RNAIII, and small stable RNA molecules within other pathogens, it is possible that these RNA molecules influence biological processes within the organism.

sae is essential for expression of the staphylococcal adhesins Eap and Emp

Eap and Emp are two Staphylococcus aureus adhesins initially described as extracellular matrix binding proteins. Eap has since emerged as being important in adherence to and invasion of eukaryotic cells, as well as being described as an immunomodulator and virulence factor in chronic infections. This paper describes the mapping of the transcription start point of the eap and emp promoters. Moreover, using reporter-gene assays and real-time PCR in defined regulatory mutants, environmental conditions and global regulators affecting expression of eap and emp were investigated. Marked differences were found in expression of eap and emp between strain Newman and the 8325 derivatives SH1000 and 8325-4. Moreover, both genes were repressed in the presence of glucose. Analysis of expression of both genes in various regulatory mutants revealed that sarA and agr were involved in their regulation, but the data suggested that there were additional regulators of both genes. In a sae mutant, expression of both genes was severely repressed. sae expression was also reduced in the presence of glucose, suggesting that repression of eap and emp in glucose-containing medium may, in part, be a consequence of a decrease in expression of sae.

Plant models for animal pathogenesis

Several bacteria that are pathogenic to animals also infect plants. Mechanistic studies have proven that some human/animal pathogenic bacteria employ a similar subset of virulence determinants to elicit disease in animals, invertebrates and plants. Therefore, the results of plant infection studies are relevant to animal pathogenesis. This discovery has resulted in the development of convenient, cost-effective, and reliable plant infection models to study the molecular basis of infection by animal pathogens. Plant infection models provide a number of advantages in the study of animal pathogenesis. Using a plant model, mutations in animal pathogenic bacteria can easily be screened for putative virulence factors, a process which if done using existing animal infection models would be time-consuming and tedious. High-throughput screening of plants also provides the potential for unravelling the mechanisms by which plants resist animal pathogenic bacteria, and provides a means to discover novel therapeutic agents such as antibiotics and anti-infective compounds. In this review, we describe the developing technique of using plants as a model system to study Pseudomonas aeruginosa, Enterococcus faecalis and Staphylococcus aureus pathogenesis, and discuss ways to use this new technology against disease warfare and other types of bioterrorism.

SarA positively controls bap-dependent biofilm formation in Staphylococcus aureus

The biofilm-associated protein Bap is a staphylococcal surface protein involved in biofilm formation. We investigated the influence of the global regulatory locus sarA on bap expression and Bap-dependent biofilm formation in three unrelated Staphylococcus aureus strains. The results showed that Bap-dependent biofilm formation was diminished in the sarA mutants by an agr-independent mechanism. Complementation studies using a sarA clone confirmed that the defect in biofilm formation was due to the sarA mutation. As expected, the diminished capacity to form biofilms in the sarA mutants correlated with the decreased presence of Bap in the bacterial surface. Using transcriptional fusion and Northern analysis data, we demonstrated that the sarA gene product acts as an activator of bap expression. Finally, the bap promoter was characterized and the transcriptional start point was mapped by the rapid amplification of cDNA ends technique. As expected, we showed that purified SarA protein binds specifically to the bap promoter, as determined by gel shift and DNase I footprinting assays. Based on the previous studies of others as well as our work demonstrating the role for SarA in icaADBC and bap expression, we propose that SarA is an essential regulator controlling biofilm formation in S. aureus.

Induction of fibronectin adhesins in quinolone-resistant Staphylococcus aureus by subinhibitory levels of ciprofloxacin or by sigma B transcription factor activity is mediated by two separate pathways

We recently reported on the involvement of a RecA-LexA-dependent pathway in the ciprofloxacin-triggered upregulation of fibronectin-binding proteins (FnBPs) by fluoroquinolone-resistant Staphylococcus aureus. The potential additional contribution of the transcription factor sigma B (SigB) to the ciprofloxacin-triggered upregulation of FnBPs was studied in isogenic mutants of fluoroquinolone-resistant strain RA1 (a topoisomerase IV gyrase double mutant of S. aureus NCTC strain 8325), which exhibited widely different levels of SigB activity, as assessed by quantitative reverse transcription-PCR of their respective sigB and SigB-dependent asp23 transcript levels. These mutants were Tn551 insertion sigB strain TE1 and rsbU(+) complemented strain TE2, which exhibited a wild-type SigB operon. Levels of FnBP surface display and fibronectin-mediated adhesion were lower in sigB mutant TE1 or higher in the rsbU(+)-restored strain TE2 compared to their sigB(+) but rsbU parent, strain RA1, exhibiting low levels of SigB activity. Steady-state fnbA and fnbB transcripts levels were similar in strains TE1 and RA1 but increased by 4- and 12-fold, respectively, in strain TE2 compared to those in strain RA1. In contrast, fibronectin-mediated adhesion of strains TE1, RA1, and TE2 was similarly enhanced by growth in the presence of one-eighth the MIC of ciprofloxacin, which led to a significantly higher increase in their fnbB transcript levels compared to the increase in their fnbA transcript levels. Increased SigB levels led to a significant reduction in agr RNAIII; in contrast, it led to a slight increase in sarA transcript levels. In conclusion, upregulation of FnBPs by increased SigB levels and ciprofloxacin exposure in fluoroquinolone-resistant S. aureus occurs via independent pathways whose concerted actions may significantly promote bacterial adhesion and colonization.

Staphylococcus aureus pathogenicity on Arabidopsis thaliana is mediated either by a direct effect of salicylic acid on the pathogen or by SA-dependent, NPR1-independent host responses

Staphylococcus aureus is a ubiquitous gram-positive bacterium that can cause superficial to serious systemic infections in animals and humans. Here we report the development of a plant infection model to study the pathogenesis of this bacterium. Three global regulatory mutants, RN6911 (agr-), ALC 488 (sarA-) ALC 842 (sarA-/agr-) and an alpha-toxin mutant defective in biofilm formation (DU1090) which are attenuated in animal pathogenesis, were also attenuated in their ability to infect plants, suggesting that these regulators that mediate synthesis of virulence factors essential for animal pathogenesis are also required for plant pathogenesis. Further, using Arabidopsis plants altered in defense responses such as the transgenic lines NahG [defective in salicylic acid (SA) accumulation], and 35S-LOX2- (defective in jasmonic acid production and hyper-accumulator of SA), and mutants ics1 (depleted in SA accumulation), and npr1-1 (non-expressor of pathogenesis-related protein) we show that resistance of Arabidopsis to typical plant pathogens and the animal pathogen S. aureus is conserved and is mediated by SA. The data presented here suggest that Arabidopsis thaliana resistance to S. aureus is mediated either by a direct effect of SA on the pathogen, specifically one that affects the attachment/aggregate formation on the root surface and reduces the pathogen's virulence, or by SA-dependent, NPR1-independent host responses.

Staphylococcus aureus AgrA binding to the RNAIII-agr regulatory region

The control of virulence gene expression in the human pathogen Staphylococcus aureus is under the partial control of the two-component quorum-sensing system encoded by genes of the agr locus. The product of the agrA gene has been shown by amino acid sequence similarity to be the putative response regulator; however, binding of AgrA to promoters under its control has not yet been demonstrated. In this study, we isolated and purified soluble AgrA by expression under osmotic shock conditions and ion-exchange chromatography. Purified AgrA showed high-affinity binding to the RNAIII-agr intergenic region by electrophoretic mobility shift assays. Binding was localized by DNase I protection assays to a pair of direct repeats in the P2 and P3 promoter regions of the agr locus. We found that this binding was enhanced by the addition of the small phosphoryl donor, acetyl phosphate. The difference in binding affinity between these two promoters was found to result from a 2-bp difference between the downstream direct repeats of the P2 and P3 sites. Mutation of these base pairs in the P3 site to match those found in the P2 site increased the affinity of AgrA for the P3 site relative to that for the P2 site. These results are consistent with the function of AgrA as a response regulator with recognition sites in the promoter regions of RNAIII and the agr locus.

Global gene expression in Staphylococcus aureus biofilms

We previously demonstrated that mutation of the staphylococcal accessory regulator (sarA) in a clinical isolate of Staphylococcus aureus (UAMS-1) results in an impaired capacity to form a biofilm in vitro (K. E. Beenken, J. S. Blevins, and M. S. Smeltzer, Infect. Immun. 71:4206-4211, 2003). In this report, we used a murine model of catheter-based biofilm formation to demonstrate that a UAMS-1 sarA mutant also has a reduced capacity to form a biofilm in vivo. Surprisingly, mutation of the UAMS-1 ica locus had little impact on biofilm formation in vitro or in vivo. In an effort to identify additional loci that might be relevant to biofilm formation and/or the adaptive response required for persistence of S. aureus within a biofilm, we isolated total cellular RNA from UAMS-1 harvested from a biofilm grown in a flow cell and compared the transcriptional profile of this RNA to RNA isolated from both exponential- and stationary-phase planktonic cultures. Comparisons were done using a custom-made Affymetrix GeneChip representing the genomic complement of six strains of S. aureus (COL, N315, Mu50, NCTC 8325, EMRSA-16 [strain 252], and MSSA-476). The results confirm that the sessile lifestyle associated with persistence within a biofilm is distinct by comparison to the lifestyles of both the exponential and postexponential phases of planktonic culture. Indeed, we identified 48 genes in which expression was induced at least twofold in biofilms over expression under both planktonic conditions. Similarly, we identified 84 genes in which expression was repressed by a factor of at least 2 compared to expression under both planktonic conditions. A primary theme that emerged from the analysis of these genes is that persistence within a biofilm requires an adaptive response that limits the deleterious effects of the reduced pH associated with anaerobic growth conditions.