Role of upstream sequences in the expression of the staphylococcal enterotoxin B gene

GraS Sensory Activity in Staphylococcus epidermidis Is Modulated by the "Guard Loop" of VraG and the ATPase Activity of VraF

Antimicrobial peptides (AMPs) are one of the key immune responses that can eliminate pathogenic bacteria through membrane perturbation. As a successful skin commensal, Staphylococcus epidermidis can sense and respond to AMPs through the GraXRS two-component system and an efflux system comprising the VraG permease and VraF ATPase. GraS is a membrane sensor known to function in AMP resistance through a negatively charged, 9-residue extracellular loop, which is predicted to be linear without any secondary structure. An important question is how GraS can impart effective sensing of AMPs through such a small unstructured sequence. In this study, we verified the role of graS and vraG in AMP sensing in S. epidermidis, as demonstrated by the failure of the ΔgraS or ΔvraG mutants to sense. Deletion of the extracellular loop of VraG did not affect sensing but reduced survival with polymyxin B. Importantly, a specific region within the extracellular loop, termed the guard loop (GL), has inhibitory activity since sensing of polymyxin B was enhanced in the ΔGL mutant, indicating that the GL may act as a gatekeeper for sensing. Bacterial two-hybrid analysis demonstrated that the extracellular regions of GraS and VraG interact, but interaction appears dispensable to sensing activity. Mutation of the extracellular loop of VraG, the GL, and the active site of VraF suggested that an active detoxification function of VraG is necessary for AMP resistance. Altogether, we provide evidence for a unique sensory scheme that relies on the function of a permease to impart effective information processing. IMPORTANCE Staphylococcus epidermidis has become an important opportunistic pathogen that is responsible for nosocomial and device-related infections that account for considerable morbidity worldwide. A thorough understanding of the mechanisms that enable S. epidermidis to colonize human skin successfully is essential for the development of alternative treatment strategies and prophylaxis. Here, we demonstrate the importance of an AMP response system in a clinically relevant S. epidermidis strain. Furthermore, we provide evidence for a unique sensory scheme that would rely on the detoxification function of a permease to effect information processing.

Highly Sensitive and Specific Detection of Staphylococcal Enterotoxins SEA, SEG, SEH, and SEI by Immunoassay

Staphylococcal food poisoning (SFP) is one of the most common foodborne diseases worldwide, resulting from the ingestion of staphylococcal enterotoxins (SEs), primarily SE type A (SEA), which is produced in food by enterotoxigenic strains of staphylococci, mainly S. aureus. Since newly identified SEs have been shown to have emetic properties and the genes encoding them have been found in food involved in poisoning outbreaks, it is necessary to have reliable tools to prove the presence of the toxins themselves, to clarify the role played by these non-classical SEs, and to precisely document SFP outbreaks. We have produced and characterized monoclonal antibodies directed specifically against SE type G, H or I (SEG, SEH or SEI respectively) or SEA. With these antibodies, we have developed, for each of these four targets, highly sensitive, specific, and reliable 3-h sandwich enzyme immunoassays that we evaluated for their suitability for SE detection in different matrices (bacterial cultures of S. aureus, contaminated food, human samples) for different purposes (strain characterization, food safety, biological threat detection, diagnosis). We also initiated and described for the first time the development of monoplex and quintuplex (SEA, SE type B (SEB), SEG, SEH, and SEI) lateral flow immunoassays for these new staphylococcal enterotoxins. The detection limits in buffer were under 10 pg/mL (0.4 pM) by enzyme immunoassays and at least 300 pg/mL (11 pM) by immunochromatography for all target toxins with no cross-reactivity observed. Spiking studies and/or bacterial supernatant analysis demonstrated the applicability of the developed methods, which could become reliable detection tools for the routine investigation of SEG, SEH, and SEI.

Bypassing the Restriction System To Improve Transformation of Staphylococcus epidermidis

Staphylococcus epidermidis is the leading cause of infections on indwelling medical devices worldwide. Intrinsic antibiotic resistance and vigorous biofilm production have rendered these infections difficult to treat and, in some cases, require the removal of the offending medical prosthesis. With the exception of two widely passaged isolates, RP62A and 1457, the pathogenesis of infections caused by clinical S. epidermidis strains is poorly understood due to the strong genetic barrier that precludes the efficient transformation of foreign DNA into clinical isolates. The difficulty in transforming clinical S. epidermidis isolates is primarily due to the type I and IV restriction-modification systems, which act as genetic barriers. Here, we show that efficient plasmid transformation of clinical S. epidermidis isolates from clonal complexes 2, 10, and 89 can be realized by employing a plasmid artificial modification (PAM) in Escherichia coli DC10B containing a Δdcm mutation. This transformative technique should facilitate our ability to genetically modify clinical isolates of S. epidermidis and hence improve our understanding of their pathogenesis in human infections.IMPORTANCE Staphylococcus epidermidis is a source of considerable morbidity worldwide. The underlying mechanisms contributing to the commensal and pathogenic lifestyles of S. epidermidis are poorly understood. Genetic manipulations of clinically relevant strains of S. epidermidis are largely prohibited due to the presence of a strong restriction barrier. With the introductions of the tools presented here, genetic manipulation of clinically relevant S. epidermidis isolates has now become possible, thus improving our understanding of S. epidermidis as a pathogen.

The SaeRS Two-Component System Is a Direct and Dominant Transcriptional Activator of Toxic Shock Syndrome Toxin 1 in Staphylococcus aureus

Toxic shock syndrome toxin 1 (TSST-1) is a Staphylococcus aureus superantigen that has been implicated in both menstrual and nonmenstrual toxic shock syndrome (TSS). Despite the important role of TSST-1 in severe human disease, a comprehensive understanding of staphylococcal regulatory factors that control TSST-1 expression remains incomplete. The S. aureus exotoxin expression (Sae) operon contains a well-characterized two-component system that regulates a number of important exotoxins in S. aureus, although regulation of TSST-1 by the Sae system has not been investigated. We generated a defined deletion mutant of the Sae histidine kinase sensor (saeS) in the prototypic menstrual TSS strain S. aureus MN8. Mutation of saeS resulted in a complete loss of TSST-1 expression. Using both luciferase reporter experiments and quantitative real-time PCR, we demonstrate that the Sae system is an important transcriptional activator of TSST-1 expression. Recombinant SaeR was able to bind directly to the tst promoter to a region containing two SaeR consensus binding sites. Although the stand-alone SarA transcriptional regulator has been shown to be both a positive and a negative regulator of TSST-1, deletion of sarA in S. aureus MN8 resulted in a dramatic overexpression of TSST-1. As expected, mutation of agr also reduced TSST-1 expression, but this phenotype appeared to be independent of Sae. A double mutation of saeS and sarA resulted in the loss of TSST-1 expression. This work indicates that the Sae system is a dominant and direct transcriptional activator that is required for expression of TSST-1.

IMPORTANCE

The TSST-1 superantigen is an exotoxin, produced by some strains of S. aureus, that has a clear role in both menstrual and nonmenstrual TSS. Although the well-characterized agr quorum sensing system is a known positive regulator of TSST-1, the molecular mechanisms that directly control TSST-1 expression are only partially understood. Our studies demonstrate that the Sae two-component regulatory system is a positive transcriptional regulator that binds directly to the TSST-1 promoter, and furthermore, our data suggest that Sae is required for expression of TSST-1. This work highlights how major regulatory circuits can converge to fine-tune exotoxin expression and suggests that the Sae regulatory system may be an important target for antivirulence strategies.

Sequence Variability in Staphylococcal Enterotoxin Genes seb, sec, and sed

Ingestion of staphylococcal enterotoxins preformed by Staphylococcus aureus in food leads to staphylococcal food poisoning, the most prevalent foodborne intoxication worldwide. There are five major staphylococcal enterotoxins: SEA, SEB, SEC, SED, and SEE. While variants of these toxins have been described and were linked to specific hosts or levels or enterotoxin production, data on sequence variation is still limited. In this study, we aim to extend the knowledge on promoter and gene variants of the major enterotoxins SEB, SEC, and SED. To this end, we determined seb, sec, and sed promoter and gene sequences of a well-characterized set of enterotoxigenic Staphylococcus aureus strains originating from foodborne outbreaks, human infections, human nasal colonization, rabbits, and cattle. New nucleotide sequence variants were detected for all three enterotoxins and a novel amino acid sequence variant of SED was detected in a strain associated with human nasal colonization. While the seb promoter and gene sequences exhibited a high degree of variability, the sec and sed promoter and gene were more conserved. Interestingly, a truncated variant of sed was detected in all tested sed harboring rabbit strains. The generated data represents a further step towards improved understanding of strain-specific differences in enterotoxin expression and host-specific variation in enterotoxin sequences.

Improving transformation of Staphylococcus aureus belonging to the CC1, CC5 and CC8 clonal complexes

Methicillin resistant Staphylococcus aureus (MRSA) is an opportunistic pathogen found in hospital and community environments that can cause serious infections. A major barrier to genetic manipulations of clinical isolates has been the considerable difficulty in transforming these strains with foreign plasmids, such as those from E. coli, in part due to the type I and IV Restriction Modification (R-M) barriers. Here we combine a Plasmid Artificial Modification (PAM) system with DC10B E. coli cells (dcm mutants) to bypass the barriers of both type I and IV R-M of S. aureus, thus allowing E. coli plasmid DNA to be transformed directly into clinical MRSA strains MW2, N315 and LAC, representing three of the most common clonal complexes. Successful transformation of clinical S. aureus isolates with E. coli-derived plasmids should greatly increase the ability to genetically modify relevant S. aureus strains and advance our understanding of S. aureus pathogenesis.

Addressing bioterrorism concerns: options for investigating the mechanism of action of Staphylococcus aureus enterotoxin B

Staphylococcal enterotoxin B (SEB) is of concern to military and civilian populations as a bioterrorism threat agent. It is a highly potent toxin produced by Staphylococcus aureus and is stable in storage and under aerosolisation; it is able to produce prolonged highly incapacitating illness at very low-inhaled doses and death at elevated doses. Concerns regarding SEB are compounded by the lack of effective medical countermeasures for mass treatment of affected populations. This article considers the mechanism of action of SEB, the availability of appropriate experimental models for evaluating the efficacy of candidate medical countermeasures with particular reference to the need to realistically model SEB responses in man and the availability of candidate countermeasures (with an emphasis on commercial off-the-shelf options). The proposed in vitro approaches would be in keeping with Dstl’s commitment to reduction, refinement and replacement of animal models in biomedical research, particularly in relation to identifying valid alternatives to the use of nonhuman primates in experimental studies.

Lysis-deficient phages as novel therapeutic agents for controlling bacterial infection

BACKGROUND

Interest in phage therapy has grown over the past decade due to the rapid emergence of antibiotic resistance in bacterial pathogens. However, the use of bacteriophages for therapeutic purposes has raised concerns over the potential for immune response, rapid toxin release by the lytic action of phages, and difficulty in dose determination in clinical situations. A phage that kills the target cell but is incapable of host cell lysis would alleviate these concerns without compromising efficacy.

RESULTS

We developed a recombinant lysis-deficient Staphylococcus aureus phage P954, in which the endolysin gene was rendered nonfunctional by insertional inactivation. P954, a temperate phage, was lysogenized in S. aureus strain RN4220. The native endolysin gene on the prophage was replaced with an endolysin gene disrupted by the chloramphenicol acetyl transferase (cat) gene through homologous recombination using a plasmid construct. Lysogens carrying the recombinant phage were detected by growth in presence of chloramphenicol. Induction of the recombinant prophage did not result in host cell lysis, and the phage progeny were released by cell lysis with glass beads. The recombinant phage retained the endolysin-deficient genotype and formed plaques only when endolysin was supplemented. The host range of the recombinant phage was the same as that of the parent phage. To test the in vivo efficacy of the recombinant endolysin-deficient phage, immunocompromised mice were challenged with pathogenic S. aureus at a dose that results in 80% mortality (LD80). Treatment with the endolysin-deficient phage rescued mice from the fatal S. aureus infection.

CONCLUSIONS

A recombinant endolysin-deficient staphylococcal phage has been developed that is lethal to methicillin-resistant S. aureus without causing bacterial cell lysis. The phage was able to multiply in lytic mode utilizing a heterologous endolysin expressed from a plasmid in the propagation host. The recombinant phage effectively rescued mice from fatal S. aureus infection. To our knowledge this is the first report of a lysis-deficient staphylococcal phage.

Direct targets of CodY in Staphylococcus aureus

More than 200 direct CodY target genes in Staphylococcus aureus were identified by genome-wide analysis of in vitro DNA binding. This analysis, which was confirmed for some genes by DNase I footprinting assays, revealed that CodY is a direct regulator of numerous transcription units associated with amino acid biosynthesis, transport of macromolecules, and virulence. The virulence genes regulated by CodY fell into three groups. One group was dependent on the Agr system for its expression; these genes were indirectly regulated by CodY through its repression of the agr locus. A second group was regulated directly by CodY. The third group, which includes genes for alpha-toxin and capsule synthesis, was regulated by CodY in two ways, i.e., by direct repression and by repression of the agr locus. Since S. aureus CodY was activated in vitro by the branched chain amino acids and GTP, CodY appears to link changes in intracellular metabolite pools with the induction of numerous adaptive responses, including virulence.

Development of shuttle vectors for evaluation of essential regulator regulated gene expression in Staphylococcus aureus

We describe the construction of a series of shuttle vectors for Staphylococcus aureus. In order to determine transcriptional regulation by essential regulators, we constructed promoterless luxABCDE reporter system using a TetR-regulated antisense RNA expression vector, pJYJ909, which is composed of S. aureus plasmid pE194, the Gram(-) plasmid pUC18, a TetR regulatory cassette, and Pxyl/teto-driven yhcS antisense expression construct. The reformed shuttle vector was utilized to construct an opuCA promoter-luxABCDE fusion and simultaneously examine transcriptional regulation by measuring bioluminescence intensity during down-regulating yhcSR. In addition, we utilized the same plasmid, pJYJ909, and constructed a Pspac-driven constant expression system, which allows us to determine the complementary effect of overexpression of opuCA operon modulated by yhcSR. These plasmids provide important tools for elucidating regulatory mechanisms for genes that are essential for bacterial growth in S. aureus.

Regulation of Rot expression in Staphylococcus aureus

Repressor of toxins (Rot) is known to be a global regulator of virulence gene expression in Staphylococcus aureus. The function of Rot, but not the transcription of rot, is regulated by the staphylococcal accessory gene regulator (Agr) quorum-sensing system. In addition, the alternative sigma factor (sigma(B)) has a repressive effect on rot expression during the postexponential phase of growth. The transcriptional profiles of Rot in sigma(B)-positive and sigma(B)-negative strains in the postexponential and stationary phases of growth were compared. An upregulation of rot expression was observed during the stationary phase of growth, and this upregulation occurred in a sigma(B)-dependent manner. The effects of other staphylococcal transcriptional factors were also investigated. Electrophoretic mobility shift assays revealed that proteins present in staphylococcal lysates retarded the mobility of the rot promoter fragment and that the effect was reduced, but not eliminated, with lysates from strains lacking a functional SarS protein. A modest upregulation of rot expression was also observed in sarS-negative strains. Affinity purification of proteins binding to the rot promoter fragment, followed by N-terminal protein sequencing, identified the SarA and SarR proteins. Primer extension analysis of the rot promoter revealed a number of discreet products. However, these RNA species were not associated with identifiable promoter activity and likely represented RNA breakdown products. Loss of Rot function during the postexponential phase of growth likely involves degradation of the rot mRNA but not the inhibition of rot transcription.

Salicylic acid activates sigma factor B by rsbU-dependent and -independent mechanisms

Salicylic acid (SAL) may impact Staphylococcus aureus virulence by activating the sigB operon (rsbU-V-W-sigB), thus leading to reductions in alpha-toxin production and decreased fibronectin binding (L. I. Kupferwasser et al., J. Clin. Investig. 112:222-233, 2003). As these prior studies were performed in strain RN6390 (an rsbU mutant) and its rsbU-repaired variant, SH1000, the current investigation was designed to determine if the SAL effect occurs via rsbU- and/or rsbV-dependent pathways in an rsbU-intact S. aureus strain (FDA486). We thus quantified the transcription from two sigB-dependent promoters (asp23 and sarA P3) in FDA486 in response to SAL exposure in vitro, using isogenic single-knockout constructs of rsbU, rsbV, or rsbW and a green fluorescent protein reporter system. SAL induced sarA P3 and asp23 promoter activities in a dose-dependent manner in the parental strain. In contrast, sigB activation by SAL was progressively more mitigated in the rsbU and rsbV mutants. As predicted, SAL caused significant reductions in both alpha-toxin production and fibrinogen and fibronectin binding in the parental strain. The extent of these reductions, compared with the parent, was reduced in the rsb mutants (rsbV > rsbU), especially at low SAL concentrations. Since generation of the free SigB protein usually requires a sequential rsbU-V-W-sigB activation cascade, the present phenotypic and genotypic data suggest key roles for both rsbU and rsbV in SAL-mediated activation of sigB in strains with a fully intact sigB operon.

Rot repression of enterotoxin B expression in Staphylococcus aureus

The accessory gene regulator (Agr) system is a quorum-sensing system of Staphylococcus aureus responsible for upregulation of certain exoprotein genes and downregulation of certain cell-wall associated proteins during the post-exponential phase of growth. The enterotoxin B (seb) determinant is upregulated by the Agr system. Agr-regulated cis elements within the seb promoter region were examined by deletion analyses of the seb promoter by a hybrid promoter approach utilizing the staphylococcal lac operon promoter. To identify the regulatory pathway for enterotoxin B expression, the seb promoter fused to the chloramphenicol acetyltransferase reporter gene was introduced into mutants of S. aureus lacking agr or different members of the Sar family of transcriptional regulators. Agr control of seb promoter activity was found to be dependent upon the presence of a functional Rot protein, and Rot was shown to be able to bind to the seb promoter. Therefore, the Agr-mediated post-exponential-phase increase in seb transcription results from the Agr system's inactivation of Rot repressor activity.

Influences of sigmaB and agr on expression of staphylococcal enterotoxin B (seb) in Staphylococcus aureus

In Staphylococcus aureus, enterotoxin B (SEB) is a superantigen that activates host interleukins and induces adverse responses, ranging from food poisoning to toxic shock. The alternate sigma factor, sigmaB (SigmaB), and agr are two known regulators of S. aureus. Northern blots of strain COL, a sigB-positive strain, showed an inverse correlation between sigmaB expression and seb message. seb expression was also measured as a function of a seb promoter linked to green fluorescent protein (GFP) expression in RN6390, COL, and Newman. In sigB mutants of RN6390, SH1000, COL, and Newman, seb promoter activities, as measured by GFP expression, increased relative to the respective parental types but at differing levels, suggesting alternate strain-specific regulation. In agr mutants of RN6390 and Newman, seb promoter activities were intermediate between the high level seen for the sigB mutant and the low level in the sigB active strains. A sigB agr double mutant of RN6390 displayed lower GFP expression than the agr mutant. These results suggest that while sigmaB and agr regulate seb expression in a divergent manner, other activator(s) of seb that depend on sigB expression may be present in S. aureus.

Accessory gene regulator control of staphyloccoccal enterotoxin d gene expression

The quorum-sensing system of Staphylococcus aureus, the accessory gene regulator (Agr) system, is responsible for increased transcription of certain exoprotein genes and decreased transcription of certain cell wall-associated proteins during the postexponential phase of growth. This regulation is important for virulence, as evidenced by a reduction in virulence associated with a loss of the Agr system. The enterotoxin D (sed) determinant is upregulated by the Agr system. To define the Agr-regulated cis element(s) within the sed promoter region, we utilized promoters not regulated by Agr to create hybrid promoters. Hybrid promoters were created by using sed sequences combined with the enterotoxin A (sea) promoter or the S. aureus lac operon promoter sequences. The results obtained indicated that the Agr control element of the sed promoter resides within the -35 promoter element and at the Pribnow box to the +1 site of the promoter. At these positions of the sed promoter, a directly repeated 6-bp sequence was found. This repeat is important for overall promoter activity, and maximal regulation of the promoter activity requires both repeat elements. Furthermore, Agr control of sed promoter activity was found to be dependent upon the presence of a functional Rot protein. Therefore, the postexponential increase in sed transcription results from the Agr-mediated reduction in Rot activity rather than as a direct effect of the Agr system.

Inactivation of mprF affects vancomycin susceptibility in Staphylococcus aureus

A chemically generated mutant of Staphylococcus aureus RN4220, GC6668, was isolated that had a fourfold increase in resistance to vancomycin. This phenotype reverted back to susceptibility by insertional mutagenesis with Tn917. In a selected set of revertants, Tn917 insertion was mapped to a unique chromosomal region upstream of mprF, a recently described gene that determines staphylococcal resistance to several host defense peptides. The genetic linkage between the vancomycin susceptibility and Tn917 insertion was then confirmed by transduction backcrosses into both GC6668 and GISA isolates, MER-S12 and HT2002 0127. Northern blot analysis, insertional inactivation and complementation experiments showed that mprF mediates vancomycin susceptibility in S. aureus. The inactivation of mprF by Tn917 insertion in HT2002 0127 caused a significant increase in the binding of vancomycin to the cell membranes. This observation serves as a likely mechanism of the increased vancomycin susceptibility associated with mprF inactivation.

Transcriptional induction of the penicillin-binding protein 2 gene in Staphylococcus aureus by cell wall-active antibiotics oxacillin and vancomycin

We found an increased abundance of pbpB-specific transcripts in vancomycin intermediate-resistant Staphylococcus aureus (VISA) isolates compared with that found in paired, genetically identical, susceptible isolates. This difference in expression cannot be explained by differences in the pbpB promoter sequence. Since the factors controlling pbpB gene expression have remained largely unexplored, various conditions that might affect pbpB transcript abundance were examined. In both vancomycin-susceptible and VISA strains, pbpB expression varied with the growth phase, with the highest abundance of pbpB-specific transcripts detected during mid-log phase. Interestingly, both vancomycin and oxacillin were able to induce pbpB transcription above a constitutive level. When vancomycin was absent, one of the three pbpB-specific transcripts that were usually faintly detected in non-VISA strains was more readily detected in VISA strains during mid-log but not stationary phase. This transcript was enhanced in non-VISA strains by vancomycin induction. Gel shift assays indicated that an increased amount of the putative transcription factor that binds to both P1 and P1' promoter regions is present in the cytosol of vancomycin-induced cells. Neither the SigB sigma factor nor the quorum-sensing agr locus was required for growth phase-variable pbpB expression or transcriptional induction of pbpB by vancomycin or oxacillin. Also, MecI, MecR1, BlaI, and BlaR1, regulatory proteins that mediate beta-lactam-inducible expression of mecA and beta-lactamase, were not required for antibiotic induction of pbpB transcription. These data support the idea that pbpB expression is modulated by a trans-acting factor in response to the presence of the cell wall-active antibiotics vancomycin and oxacillin.

Global repression of exotoxin synthesis by staphylococcal superantigens

Virulent Staphylococcus aureus strains typically produce and secrete large quantities of many extracellular proteins involved in pathogenesis. Such strains cause the classical staphylococcal lesion--local tissue destruction and aggressive inflammation accompanied by the massive influx of polymorphonuclear leukocytes, leading to the formation of pus. Most strains causing toxic shock syndrome, however, produce and secrete very small quantities of most exoproteins although they elaborate high levels of toxic shock syndrome toxin-1 (TSST-1). These strains cause local infections that are remarkably apurulent although potentially fatal owing to the superantigen. We have analyzed this disparity and have found that TSST-1 itself is a negative global regulator of exoprotein gene transcription. TSST-1 not only represses most exoprotein genes but determines its own high expression level by autorepression. We report also that a second superantigen, enterotoxin B, has similar regulatory properties.

Characterization of sarR, a modulator of sar expression in Staphylococcus aureus

The expression of virulence determinants in Staphylococcus aureus is controlled by global regulatory loci (e.g., sar and agr). The sar locus is composed of three overlapping transcripts (sar P1, P3, and P2 transcripts from P1, P3, and P2 promoters, respectively), all encoding the 372-bp sarA gene. The level of SarA, the major regulatory protein, is partially controlled by the differential activation of sar promoters. We previously partially purified a approximately 12 kDa protein with a DNA-specific column containing a sar P2 promoter fragment. In this study, the putative gene, designated sarR, was identified and found to encode a 13.6-kDa protein with homology to SarA. Transcriptional and immunoblot studies revealed the sarR gene to be expressed in other staphylococcal strains. Recombinant SarR protein bound sar P1, P2, and P3 promoter fragments in gel shift and footprinting assays. A sarR mutant expressed a higher level of P1 transcript than the parent, as confirmed by promoter green fluorescent protein fusion assays. As the P1 transcript is the predominant sar transcript, we confirmed that the sarR mutant expressed more SarA than the parental strain. We thus proposed that SarR is a regulatory protein that binds to the sar promoters to down-regulate P1 transcription and the ensuing SarA protein expression.

Characterization of the promoter elements for the staphylococcal enterotoxin D gene

Deletion analysis of the promoter for the Staphylococcus aureus enterotoxin D determinant indicated that a 52-bp sequence, from -34 to +18, was sufficient for sed promoter function and agr regulation. A consensus -10 Pribnow box sequence, a less conserved -35 sequence, and a TG dinucleotide motif were present. Transcribed sequences (+1 to +18) are essential for promoter activity.

Agr-independent regulation of fibronectin-binding protein(s) by the regulatory locus sar in Staphylococcus aureus

Fibronectin-binding proteins (FnBPs) are thought to be important for the attachment of Staphylococcus aureus during infection. The regulation of the genes fnbA and fnbB by the global regulatory loci sar and agr was examined using site-specific regulatory mutants of S. aureus strain Newman. The results from binding assays using both aqueous and solid-phase fibronectin as well as ligand blotting with biotinylated fibronectin showed that the expression of FnBPA is enhanced in the agr mutant but inhibited in the sar mutant and the sar-agr double mutant. The same regulatory pattern was observed in Northern blot analysis using fnbA-specific probes. The introduction of sar on a multicopy plasmid increased the already enhanced fnbA transcription of the agr mutant. FnBPB was not detectable by ligand blotting and the fnbB promoter activity in promoter fusion assays was not affected by either sar or agr. The sequence encompassing ORF3 located upstream of sarA was found to be essential for the activation of fnbA transcription. We hypothesize that this sequence may modulate SarA expression and/or activity on the post-transcriptional level. Gel shift assays demonstrated that SarA binds to the fnbA promoter fragments, probably as a dimer. DNase I footprinting assays with SarA revealed a protected area of 102 bp upstream of fnbA.

Involvement of enterotoxins G and I in staphylococcal toxic shock syndrome and staphylococcal scarlet fever

We investigated the involvement of the recently described staphylococcal enterotoxins G and I in toxic shock syndrome. We reexamined Staphylococcus aureus strains isolated from patients with menstrual and nonmenstrual toxic shock syndrome (nine cases) or staphylococcal scarlet fever (three cases). These strains were selected because they produced none of the toxins known to be involved in these syndromes (toxic shock syndrome toxin 1 and enterotoxins A, B, C, and D), enterotoxin E or H, or exfoliative toxin A or B, despite the fact that superantigenic toxins were detected in a CD69-specific flow cytometry assay measuring T-cell activation. Sets of primers specific to the enterotoxin G and I genes (seg and sei, respectively) were designed and used for PCR amplification. All of the strains were positive for seg and sei. Sequence analysis confirmed that the PCR products, corresponded to the target genes. We suggest that staphylococcal enterotoxins G and I may be capable of causing human staphylococcal toxic shock syndrome and staphylococcal scarlet fever.

Promoter analysis of the cap8 operon, involved in type 8 capsular polysaccharide production in Staphylococcus aureus

The production of type 8 capsular polysaccharide (CP8) in Staphylococcus aureus is regulated in response to a variety of environmental factors. The cap8 genes required for the CP8 production in strain Becker are transcribed as a single large transcript by a primary promoter located within a 0.45-kb region upstream of the first gene of the cap8 gene cluster. In this study, we analyzed the primary cap8 promoter region in detail. We determined the transcription initiation site of the primary transcript by primer extension and identified the potential promoter sequences. We found several inverted and direct repeats upstream of the promoter. Deletion analysis and site-directed mutagenesis showed that a 10-bp inverted repeat of one of the repeats was required for promoter activity. We showed that the distance but not the specific sequences between the inverted repeat and the promoter was critical to the promoter activity. However, insertion of a DNA sequence with two or four helix turns in this intervening region had a slight effect on promoter activity. To demonstrate the biological significance of the 10-bp inverted repeat, we constructed a strain with a mutation in the repeat in the S. aureus Becker chromosome and showed that the repeat affected CP8 production mostly at the transcriptional level. By gel mobility shift assay, we demonstrated that strain Becker produced at least one protein capable of specific binding to the 10-bp inverted repeat, indicating that the repeat serves as a positive regulatory protein binding site. In addition, reporter gene fusion analysis showed that the cap8 promoter activity was influenced by various growth media and affected most by yeast extract. Our results suggest that yeast extract may exert its profound inhibitory effect on cap8 gene expression through the 10-bp inverted repeat element.

SarA level is a determinant of agr activation in Staphylococcus aureus

The control of virulence determinant expression in Staphylococcus aureus is a complex process involving global regulatory loci such as sar and agr. The sar locus consists of a 372 bp sarA open reading frame (ORF) preceded by a triple promoter region interspersed with two putative smaller ORFs (ORF3 and ORF4). The triple promoter system yields three overlapping sar transcripts (sarA, sarC and sarB of 0.56, 0.8 and 1.2 kb respectively). We have recently shown that the SarA protein binds to the agr promoter region to stimulate the transcription of RNAII and RNAIII, two major transcripts encoded within the agr locus. To assess the role of the region upstream of sarA in agr expression, we evaluated the contribution of ORF3 and ORF4 to SarA protein expression and to agr activation by introducing nonsense mutations into the respective ORFs. Northern analysis of sar mutant clones containing these mutations carried on a shuttle plasmid revealed that all three sar-related transcripts are present. Using anti-SarA monoclonal antibodies with defined epitopes in a competitive ELISA to determine the SarA protein level, we found that the introduction of a stop codon in ORF3 on a shuttle plasmid carrying the intact sar locus in a sar mutant led to a significant decrease in SarA protein level compared with the non-mutated control. The effect of a nonsense mutation in ORF4 on SarA levels is much less. Likewise, an analogous sar mutant clone with a deletion in ORF3 also displayed a lower SarA level than its intact counterpart. The reduction in SarA expression correlated with a lower level of agr activation in the corresponding sar mutant clone. These data suggest that ORF3, and to a lesser degree ORF4, may affect agr expression by modulating SarA protein expression.

Selective activation of sar promoters with the use of green fluorescent protein transcriptional fusions as the detection system in the rabbit endocarditis model

The global regulatory locus sar is composed of three overlapping transcripts initiated from a triple-promoter system (designated P1, P3, and P2). To explore if the individual sar promoters are differentially expressed in vitro and in vivo, we constructed a shuttle plasmid (pALC1434) containing a promoterless gfpUV gene (a gfp derivative [Clontech]) preceded by a polylinker region. Recombinant shuttle vectors containing individual sar promoters upstream of the gfpUV reporter gene were then introduced into Staphylococcus aureus RN6390. Northern and immunoblot analysis revealed that P1 is stronger than the P2 and P3 promoters in vitro. Additionally, the levels of the gfpUV transcript driven by individual sar promoters also correlated with the growth cycle dependency of these promoters in liquid cultures, thus suggesting the utility of pALC1434 as a vehicle for reporter fusion. Using the rabbit endocarditis model, we examined the expression of these three GFPUV fusions in vivo by fluorescence microscopy of infected cardiac vegetations 24 h after initial intravenous challenge. Similar to the in vitro findings, P1 was activated both in the center and on the surface of the vegetations. In contrast, the P3 promoter was silent both in vivo and in vitro as determined by fluorescence microscopy. Remarkably, P2 was silent in vitro but became highly activated in vivo. In particular, the sar P2 promoter was activated on the surface of the vegetation but not in the center of the lesion. These data imply that in vivo promoter activation of sar differed from that observed in vitro. Moreover, the individual sar promoters may be differentially expressed in different areas within the same anatomic niche, presumably reflecting the microbial physiological response to distinct host microenvironments. As the sar locus controls the synthesis of both extracellular and cell wall virulence determinants, these promoter-gfpUV constructs should be useful to characterize many aspects of S. aureus gene regulation in vivo.

Role of SarA in virulence determinant production and environmental signal transduction in Staphylococcus aureus

The staphylococcal accessory regulator (encoded by sarA) is an important global regulator of virulence factor biosynthesis in Staphylococcus aureus. To further characterize its role in virulence determinant production, an sarA knockout mutant was created by insertion of a kanamycin antibiotic resistance cassette into the sarA gene. N-terminal sequencing of exoproteins down-regulated by sarA identified several putative proteases, including a V8 serine protease and a novel metalloprotease, as the major extracellular proteins repressed by sarA. In kinetic studies, the sarA mutation delays the onset of alpha-hemolysin (encoded by hla) expression and reduces levels of hla to approximately 40% of the parent strain level. Furthermore, SarA plays a role in signal transduction in response to microaerobic growth since levels of hla were much lower in a microaerobic environment than after aerobic growth in the sarA mutant. An exoprotein exhibiting hemolysin activity on sheep blood, and up-regulated by sarA independently of the accessory gene regulator (encoded by agr), was specifically induced microaerobically. Transcriptional gene fusion and Western analysis revealed that sarA up-regulates both toxic shock syndrome toxin 1 gene (tst) expression and staphylococcal enterotoxin B production, respectively. This study demonstrates the role of sarA as a signal transduction regulatory component in response to aeration stimuli and suggests that sarA functions as a major repressor of protease activity. The possible role of proteases as regulators of virulence determinant stability is discussed.

Characterization of a sar homolog of Staphylococcus epidermidis

Coagulase-negative staphylococci are common nosocomial pathogens. A regulatory element, designated sar, partially controls exoprotein synthesis in coagulase-positive Staphylococcus aureus by modulating the expression of another regulatory locus, called agr. We report here the cloning of a sar homolog in S. epidermidis. The major open reading frame within sar in S. epidermidis is highly homologous (84%) to the S. aureus SarA protein. Primer extension studies revealed three sar transcripts (0.64, 0.76, and 0.85 kb) initiated from three distinct promoters. The interpromoter region in S. epidermidis differs from its S. aureus counterpart, possibly suggesting target gene differences and a disparate pattern for sar activation. Remarkably, the S. epidermidis sar homolog interacts with an agr promoter fragment of S. aureus in gel shift assays. Additionally, S. epidermidis sar fragments could restore hemolysin production in an S. aureus sar mutant. As typical virulence determinants controlled by sar in S. aureus are not present in S. epidermidis, an examination of functional and structural similarities and divergence of sar in staphylococci will be of major interest.

Detection of staphylococcal superantigenic toxins by a CD69-specific cytofluorimetric assay measuring T-cell activation

The presence of staphylococcal superantigenic toxins in the supernatants of liquid cultures was detected by an easy and rapid method assessing the activation of T lymphocytes by cytofluorimetric measurement of CD69 expression. Staphylococcus aureus cells were grown in Eagle's minimum essential medium supplemented with 5% heat-inactivated fetal calf serum. Supernatant fluids from all S. aureus strains producing superantigen-related toxins, including enterotoxins A to E, toxic shock syndrome toxin, and exfoliative toxins A and B, induced CD69 expression in a significantly higher number of T cells than a cutoff of 2%. This CD69 assay might be used for initial detection of superantigens from S. aureus strains isolated in the context of staphylococcal toxemia or related chronic human diseases such as atopic dermatitis or Kawasaki syndrome.

sar Genetic determinants necessary for transcription of RNAII and RNAIII in the agr locus of Staphylococcus aureus

The temporal expression of most virulence factors in Staphylococcus aureus is regulated by pleiotropic loci such as agr and sar. We have previously shown that the sar locus affects hemolysin production because it is required for agr transcription. To delineate the sar genetic determinant required for agr transcription, single copies of fragments from the sar locus, encompassing the individual sar transcripts (sarA, sarC, and sarB), were introduced into a sar mutant via the integration vector pCL84. Although a DNA fragment encompassing the sarA transcript plus a 189-bp upstream region was sufficient for agr expression, complementation analysis revealed that the sarB transcript was the most effective in augmenting agr transcription as determined by RNAII and RNAIII transcription and gel retardation assays with the P2 and P3 promoters of agr. As the region upstream of the sarA transcript encodes a 39-amino-acid open reading frame, ORF3, it is possible that posttranslational cooperation between the sarA gene product and ORF3 may be necessary for optimal agr expression. Deletion studies demonstrated that an intact sarA gene is essential for agr transcription. However, mutagenesis and in vitro translation studies revealed that unlike the agr locus, the required element is the SarA protein and not the RNA molecule. Taken together, these results indicate that the sarA-encoded protein, possibly in conjunction with peptides encoded in the upstream region, regulates hemolysin production by controlling agr P2 and P3 transcription.

Characterization of the sar locus and its interaction with agr in Staphylococcus aureus

The expression of cell wall and extracellular proteins in Staphylococcus aureus is controlled by global regulatory systems, including sar and agr. We have previously shown that a transposon insertion into the 372-bp sarA gene within the sar locus resulted in decreased expression of several extracellular and cell wall proteins (A. L. Cheung and S. J. Projan, J. Bacteriol. 176:4168-4172, 1994). In this study, Northern (RNA blot) analysis with a 732-bp sarA probe indicated that two major transcripts (0.56 and 1.2 kb) were absent in the sar mutant compared with the parental strain RN6390. Additional transcriptional studies revealed that the sarA gene is encoded within the 0.56-kg transcript. Notably, a plasmid carrying the sarA gene together with a 1.2-kb upstream fragment (1.7 kb total) was able to reestablish the 1.2-kb transcript in the mutant. Although reconstitution of the parental phenotype by the sarA gene was incomplete, the introduction of a plasmid carrying the 1.7-kb fragment to the mutant restored the parental phenotype. Transcription of RNAII and RNAIII, which encode the structural and regulatory genes of agr, respectively, was diminished in the mutant but restored to wild-type levels by complementation with the 1.7-kb fragment. In gel shift assays, cell extracts of this clone were able to retard the mobility of a labeled RNAII promoter probe but not an RNAIII promoter element. These data suggest that sarA and the adjacent upstream DNA are essential to the expression of a DNA-binding protein(s) with specificity for the RNAII promoter, thereby controlling agr-related transcription.

Osmotic and growth-phase dependent regulation of the eta gene of Staphylococcus aureus: a role for DNA supercoiling

Transcriptional fusions were constructed between the promoter for the epidermolytic toxin A (eta) gene of Staphylococcus aureus and the luxAB and xylE reporter gene systems. The expression of the fusion products was found to be dependent upon the accessory gene regulator (agr) locus and was observed to increase significantly during the transition from the exponential to the stationary phase of growth. Furthermore the expression of the eta gene promoter was found to be osmotically regulated, with the expression levels of the eta fusions being inversely related to the osmolyte levels. The ability of environmental factors to influence DNA topology (and thence gene expression) was investigated. High osmolarity (0.7 M NaCl) resulted in an increase in the degree of negative supercoiling of plasmid DNA in the S. aureus strain 8325-4 (Agr+) but not in strain ISP546 (Agr-). Furthermore the eta promoter was strongly induced in S. aureus cultures grown in the presence of sub-inhibitory concentrations of novobiocin, a DNA gyrase inhibitor. However this induction was independent of agr, suggesting that the eta promoter is subject to both agr-dependent (osmolarity, growth phase) and-independent (DNA topology) regulatory processes.

A temporal signal, independent of agr, is required for hla but not spa transcription in Staphylococcus aureus

Staphylococcus aureus exoprotein expression is controlled by a global regulon known as agr. This system activates transcription of some target genes and represses transcription of others. Target genes expressed postexponentially such as alpha-hemolysin (hla) are activated by agr; target genes expressed during exponential phase such as protein A (spa) are repressed by agr. A unique feature of the agr system is that this transcriptional regulation is mediated by a 517-nucleotide transcript, RNAIII. While it is clear that agr differentially regulates the expression of exponential and postexponential exoproteins, the precise role of agr in the temporal control of these events has not yet been explored. In this report, we examine the effects of expressing RNAIII, the agr regulator, under the control of the inducible beta-lactamase (bla) promoter at different times in the growth cycle. We confirm previous results showing that agr is required for postexponential-phase expression of hla and further show that a separate postexponential-phase signal independent of agr function is also needed for activation of hla transcription. We also show that in an agr mutant transcription of spa occurs throughout the growth cycle, is inhibited immediately upon induction of RNAIII, and is thus indifferent to the postexponential signal required for hla activation.