The extracellular protein regulator (xpr) affects exoprotein and agr mRNA levels in Staphylococcus aureus

A novel comprehensive analysis method for Staphylococcus aureus pathogenicity islands

Staphylococcus aureus pathogenicity islands (SaPIs) form a growing family of mobile genetic elements (MGEs) in Staphylococci. Horizontal genetic transfer by MGEs plays an important role in the evolution of S. aureus. Several SaPIs carry staphylococcal enterotoxin and SE-like toxin genes. To comprehensively investigate the diversity of SaPIs, a series of primers corresponding to sequences flanking six SaPI insertion sites in S. aureus genome were designed and a long and accurate (LA)-PCR analysis method established. LA-PCR products of 13-17 kbp were observed in strains with seb, selk or selq genes. Restriction fragment length polymorphism (RFLP) analysis showed that the products have different RFLP characteristics than do previously described SaPIs; they were therefore predicted to include new SaPIs. Nucleotide sequencing analysis revealed seven novel SaPIs: seb-harboring SaPIivm10, SaPishikawa11, SaPIivm60, SaPIno10 and SaPIhirosaki4, selk and selq-harboring SaPIj11 and non-superantigen-harboring SaPIhhms2. These SaPIs have mosaic structures containing components of known SaPIs and other unknown genes. Strains carrying different SaPIs were found to have significantly different production of superantigen toxins. The present results show that the LA-PCR approach can comprehensively identify SaPI diversity and is useful for investigating the evolution of S. aureus pathogenicity.

Hyaluronidase expression and biofilm involvement in Staphylococcus aureus UAMS-1 and its sarA, agr and sarA agr regulatory mutants

In a previous study, two proteins identified as hyaluronidases were detected in spent media by MS and found to be in greater quantity in the sarA and sarA agr mutant strains when compared with the parent and agr mutant strains of Staphylococcus aureus UAMS-1. In the present study, spent media and total RNA were isolated from UAMS-1 and its regulatory mutants and analysed for hyaluronidase activity and steady-state hyaluronidase (hysA) RNA message levels. Hyaluronidase activity was observed throughout all time points examined regardless of the regulatory effects of sarA and agr but activity was always substantially higher in the sarA and sarA agr mutant strains than in the UAMS-1 parent and agr mutant strains. Northern analysis did not detect hysA message for either the UAMS-1 parent or the agr mutant strains at any time point examined, while steady-state hysA message levels were detected throughout growth for the sarA mutant strain, but only at exponential and early post-exponential growth for the sarA agr mutant strain. An in vitro biofilm plate assay, pre-coated with human plasma as a source of hyaluronic acid, demonstrated no significant increase in biofilm for a sarA mutant strain of S. aureus UAMS-1 defective in hyaluronidase activity when compared with the sarA mutant strain. These data indicate that, while hysA message levels and hyaluronidase activity are elevated in the sarA mutant strains of S. aureus UAMS-1, the increase in activity did not contribute to the biofilm-negative phenotype observed in the sarA mutant strain of S. aureus UAMS-1.

Staphylococcus aureus ArcR controls expression of the arginine deiminase operon

We identified a single open reading frame that is strongly similar to ArcR, a member of the Crp/Fnr family of bacterial transcriptional regulators, in all sequenced Staphylococcus aureus genomes. The arcR gene encoding ArcR forms an operon with the arginine deiminase (ADI) pathway genes arcABDC that enable the utilization of arginine as a source of energy for growth under anaerobic conditions. In this report, we show that under anaerobic conditions, S. aureus growth is subject to glucose catabolic repression and is enhanced by arginine. Likewise, glucose and arginine have reciprocal effects on the transcription of the arcABDCR genes. Furthermore, we show using a mutant deleted for arcR that the transcription of the arc operon under anaerobic conditions depends strictly on a functional ArcR. These findings are supported by proteome analyses, which showed that under anaerobic conditions the expression of the ADI catabolic proteins depends on ArcR. Bioinformatic analysis of S. aureus ArcR predicts an N-terminal nucleotide binding domain and a C-terminal helix-turn-helix DNA binding motif. ArcR binds to a conserved Crp-like sequence motif, TGTGA-N(6)-TCACA, present in the arc promoter region and thereby activates the expression of the ADI pathway genes. Crp-like sequence motifs were also found in the regulatory regions of some 30 other S. aureus genes mostly encoding anaerobic enzymatic systems, virulence factors, and regulatory systems. ArcR was tested and found to bind to the regulatory regions of four such genes, adh1, lctE, srrAB, and lukM. In one case, for lctE, encoding l-lactate dehydrogenase, ArcR was able to bind only in the presence of cyclic AMP. These observations suggest that ArcR is likely to play an important role in the expression of numerous genes required for anaerobic growth.

Regulatory elements of the Staphylococcus aureus protein A (Spa) promoter

Staphylococcal protein A (Spa) is an important virulence factor of Staphylococcus aureus. Transcription of the spa determinant occurs during the exponential growth phase and is repressed when the cells enter the postexponential growth phase. Regulation of spa expression has been found to be complicated, with regulation involving multiple factors, including Agr, SarA, SarS, SarT, Rot, and MgrA. Our understanding of how these factors work on the spa promoter to regulate spa expression is incomplete. To identify regulatory sites within the spa promoter, analysis of deletion derivatives of the promoter in host strains deficient in one or more of the regulatory factors was undertaken, and several critical features of spa regulation were revealed. The transcriptional start sites of spa were determined by primer extension. The spa promoter sequences were subcloned in front of a promoterless chloramphenicol acetyltransferase reporter gene. Various lengths of spa truncations with the same 3' end were constructed, and the resultant plasmids were transduced into strains with different regulatory genetic backgrounds. Our results identified upstream promoter sequences necessary for Agr system regulation of spa expression. The cis elements for SarS activity, an activator of spa expression, and for SarA activity, a repressor of spa expression, were identified. The well-characterized SarA consensus sequence on the spa promoter was found to be insufficient for SarA repression of the spa promoter. Full repression required the presence of a second consensus site adjacent to the SarS binding site. Sequences directly upstream of the core promoter sequence were found to stimulate transcription.

Transcriptional regulation of the Staphylococcus aureus thioredoxin and thioredoxin reductase genes in response to oxygen and disulfide stress

In this report we describe the cloning, organization, and promoter analysis of the Staphylococcus aureus thioredoxin (trxA) and thioredoxin reductase (trxB) genes and their transcription in response to changes in oxygen concentration and to oxidative stress compounds. Northern analysis showed that the S. aureus trxA and trxB genes were transcribed equally well in aerobic and anaerobic conditions. Several oxidative stress compounds were found to rapidly induce transcription of the trxA and trxB genes. The most pronounced effects were seen with diamide, a thiol-specific oxidant that promotes disulfide bond formation; menadione, a redox cycling agent; and tau-butyl hydroperoxide, an organic peroxide. In each case the induction was independent of the general stress sigma factor sigma(B). These studies show that the S. aureus trxA and trxB genes are upregulated following exposure to these oxidative stress agents, resulting in increased disulfide bond formation. In contrast, no effect of hydrogen peroxide on induction of the trxA and trxB genes was seen. We also show that the S. aureus thioredoxin reductase appears to be essential for growth. This observation, coupled with structural differences between the bacterial and mammalian thioredoxin reductases, suggests that it may serve as a target for the development of new antimicrobials.

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.

Identification of Staphylococcus aureus exotoxins by combined sodium dodecyl sulfate gel electrophoresis and matrix-assisted laser desorption/ ionization-time of flight mass spectrometry

Staphylococcus aureus is an important human pathogen whose pathogenesis involves the synthesis of cell wall associated virulence factors and secreted toxins with damaging effects on the host cells. Most of these pathogenic factors are synthesized in a growth-phase dependent manner as a response to environmental stress like heat, lack of nutrients or other deleterious conditions. Conventional identification of these pathogenic factors is based on Western blot analysis or enzyme-linked immunosorbent assay (ELISA) and is limited by the commercial availability of antibodies against these toxins. We report here the use of matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry for monitoring the pathogenic factors of S. aureus. For the identification of pathogenic factors, a methicillin sensitive strain of S. aureus, ATCC-29213, was grown at 37 degrees C or 42 degrees C in brain-heart infusion broth and harvested during the early stationary phase of growth. Secreted proteins were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, enzymatically digested with trypsin and analyzed by MALDI-TOF mass spectrometry. When grown at 42 degrees C, alpha- and beta-hemolysins were found to accumulate in S. aureus supernatants while the concentration of protein A was slightly decreased. The identity of some of these toxins was confirmed by Western-blot analysis. MALDI-TOF mass spectrometry combined with sodium dodecyl sulfate gel electrophoresis represents a rapid and simple approach to characterize the virulence of S. aureus strains which seems to be particularly valuable for the identification of S. aureus exotoxins for which ELISA is not established.

The superoxide dismutase gene sodM is unique to Staphylococcus aureus: absence of sodM in coagulase-negative staphylococci

Superoxide dismutase (SOD) profiles of clinical isolates of Staphylococcus aureus and coagulase-negative staphylococci (CoNS) were determined by using whole-cell lysates and activity gels. All S. aureus clinical isolates exhibited three closely migrating bands of activity as previously determined for laboratory strains of S. aureus: SodM, SodA, and a hybrid composed of SodM and SodA (M. W. Valderas and M. E. Hart, J. Bacteriol. 183:3399-3407, 2001). In contrast, the CoNS produced only one SOD activity, which migrated similarly to SodA of S. aureus. Southern analysis of eight CoNS species identified only a single sod gene in each case. A full-length sod gene was cloned from Staphylococcus epidermidis and determined to be more similar to sodA than to sodM of S. aureus. Therefore, this gene was designated sodA. The deduced amino acid sequence of the S. epidermidis sodA was 92 and 76% identical to that of the SodA and SodM proteins of S. aureus, respectively. The S. epidermidis sodA gene expressed from a plasmid complemented a sodA mutation in S. aureus, and the protein formed a hybrid with SodM of S. aureus. Both hybrid SOD forms as well as the SodM and SodA proteins of S. aureus and the S. epidermidis SodA protein exist as dimers. These data indicate that sodM is found only in S. aureus and not in the CoNS, suggesting an important divergence in the evolution of this genus and a unique role for SodM in S. aureus.

Analysis of transcription of the Staphylococcus aureus aerobic class Ib and anaerobic class III ribonucleotide reductase genes in response to oxygen

Staphylococcus aureus is a gram-positive facultative aerobe that can grow in the absence of oxygen by fermentation or by using an alternative electron acceptor. To investigate the mechanism by which S. aureus is able to adapt to changes in oxygen concentration, we analyzed the transcriptional regulation of genes that encode the aerobic class Ib and anaerobic class III ribonucleotide reductase (RNR) systems that are responsible for the synthesis of deoxyribonucleotides needed for DNA synthesis. The S. aureus class Ib RNR nrdIEF and class III RNR nrdDG genes and their regulatory regions were cloned and sequenced. Inactivation of the nrdDG genes showed that the class III RNR is essential for anaerobic growth. Inhibition of aerobic growth by hydroxyurea showed that the class Ib RNR is an oxygen-dependent enzyme. Northern blot analysis and primer extension analysis demonstrated that transcription of class III nrdDG genes is regulated by oxygen concentration and was at least 10-fold higher under anaerobic than under aerobic conditions. In contrast, no significant effect of oxygen concentration was found on the transcription of class Ib nrdIEF genes. Disruption or deletion of S. aureus nrdDG genes caused up to a fivefold increase in nrdDG and nrdIEF transcription under anaerobic conditions but not under aerobic conditions. Similarly, hydroxyurea, an inhibitor of the class I RNRs, resulted in increased transcription of class Ib and class III RNR genes under aerobic conditions. These findings establish that transcription of class Ib and class III RNR genes is upregulated under conditions that cause the depletion of deoxyribonucleotide. Promoter analysis of class Ib and class III RNR operons identified several inverted-repeat elements that may account for the transcriptional response of the nrdIEF and nrdDG genes to oxygen.

Identification and characterization of a second superoxide dismutase gene (sodM) from Staphylococcus aureus

A gene encoding superoxide dismutase (SOD), sodM, from S. aureus was cloned and characterized. The deduced amino acid sequence specifies a 187-amino-acid protein with 75% identity to the S. aureus SodA protein. Amino acid sequence comparisons with known SODs and relative insensitivity to hydrogen peroxide and potassium cyanide indicate that SodM most likely uses manganese (Mn) as a cofactor. The sodM gene expressed from a plasmid rescued an Escherichia coli double mutant (sodA sodB) under conditions that are otherwise lethal. SOD activity gels of S. aureus RN6390 whole-cell lysates revealed three closely migrating bands of activity. The two upper bands were absent in a sodM mutant, while the two lower bands were absent in a sodA mutant. Thus, the middle band of activity most likely represents a SodM-SodA hybrid protein. All three bands of activity increased as highly aerated cultures entered the late exponential phase of growth, SodM more so than SodA. Viability of the sodA and sodM sodA mutants but not the sodM mutant was drastically reduced under oxidative stress conditions generated by methyl viologen (MV) added during the early exponential phase of growth. However, only the viability of the sodM sodA mutant was reduced when MV was added during the late exponential and stationary phases of growth. These data indicate that while SodA may be the major SOD activity in S. aureus throughout all stages of growth, SodM, under oxidative stress, becomes a major source of activity during the late exponential and stationary phases of growth such that viability and growth of an S. aureus sodA mutant are maintained.

The signal transducer (BlaRI) and the repressor (BlaI) of the Staphylococcus aureus beta-lactamase operon are inducible

The precise start points for transcription of the blaZ and of the blaRI/blaI genes of the Staphylococcus aureus beta-lactamase operon have been determined by primer extension analysis. Consequently the overlapping promoter sequences were deduced. Northern blots showed that the synthesis of the 2100 nt mRNA from blaRI is inducible and that a blaI probe hybridized to the same mRNA as the blaRI probe. The gene cat, encoding chloramphenicol acetyltransferase, was fused separately to the blaZ and blaRI/blaI promoters, and used to compare their strengths. The promoter for blaZ is about six times stronger than that for blaRI/blaI and the synthesis of chloramphenicol acetyltransferase from both promoters is inducible, supporting the results from the Northern blots.

Analysis of genetic elements controlling Staphylococcus aureus lrgAB expression: potential role of DNA topology in SarA regulation

Penicillin-induced killing and murein hydrolase activity in Staphylococcus aureus are dependent on a variety of regulatory elements, including the LytSR two-component regulatory system and the virulence factor regulators Agr and Sar. The LytSR effects on these processes can be explained, in part, by the recent finding that a LytSR-regulated operon, designated lrgAB, affects murein hydrolase activity and penicillin tolerance. To examine the regulation of lrgAB expression in greater detail, we performed Northern blot and promoter fusion analyses. Both methods revealed that Agr and Sar, like LytSR, positively regulate lrgAB expression. A mutation in the agr locus reduced lrgAB expression approximately sixfold, while the sar mutation reduced lrgAB expression to undetectable levels. cis-acting regulatory elements involved in lrgAB expression were identified by fusing various fragments of the lrgAB promoter region to the xylE reporter gene and integrating these constructs into the chromosome. Catechol 2,3-dioxygenase assays identified DNA sequences, including an inverted repeat and intrinsic bend sites, that contribute to maximal lrgAB expression. Confirmation of the importance of the inverted repeat was achieved by demonstrating that multiple copies of the inverted repeat reduced lrgAB promoter activity, presumably by titrating out a positive regulatory factor. The results of this study demonstrate that lrgAB expression responds to a variety of positive regulatory factors and suggest that specific DNA topology requirements are important for optimal 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.

The Staphylococcus aureus lrgAB operon modulates murein hydrolase activity and penicillin tolerance

Previous studies in our laboratory have shown that the Staphylococcus aureus LytSR two-component regulatory system affects murein hydrolase activity and autolysis. A LytSR-regulated dicistronic operon has also been identified and shown to encode two potential membrane-associated proteins, designated LrgA and LrgB, hypothesized to be involved in the control of murein hydrolase activity. In the present study, a lrgAB mutant strain was generated and analyzed to test this hypothesis. Zymographic and quantitative analysis of murein hydrolase activity revealed that the lrgAB mutant produced increased extracellular murein hydrolase activity compared to that of the wild-type strain. Complementation of the lrgAB defect by providing the lrgAB genes in trans restored the wild-type phenotype, indicating that these genes confer negative control on extracellular murein hydrolase activity. In addition to these effects, the influence of the lrgAB mutation on penicillin-induced lysis and killing was examined. These studies demonstrated that the lrgAB mutation enhanced penicillin-induced killing of cells approaching the stationary phase of growth, the time at which the lrgAB operon was shown to be maximally expressed. This effect of the lrgAB mutation on penicillin-induced killing was shown to be independent of cell lysis. In contrast, the lrgAB mutation did not affect penicillin-induced killing of cells growing in early-exponential phase, a time in which lrgAB expression was shown to be minimal. However, expression of the lrgAB operon in early-exponential-phase cells inhibited penicillin-induced killing, again independent of cell lysis. The data generated by this study suggest that penicillin-induced killing of S. aureus involves a novel regulator of murein hydrolase activity.

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.

Expression analysis of the autolysin gene (atl) of Staphylococcus aureus

The bifunctional autolysin gene (atl) of Staphylococcus aureus was transcribed into a 4.1-kb transcript. The transcription initiation site was located at an adenine residue 33-nt upstream from the putative atl start codon. Analysis using a promoter-reporter plasmid showed that promoter activity increased during the exponential growth phase. The Tn551 insertion site of the autolysis-deficient mutant S. aureus RUSAL2 was located in the putative catalytic region of the glucosaminidase domain.

The role of environmental factors in the regulation of virulence-determinant expression in Staphylococcus aureus 8325-4

Staphylococcus aureus is a major human pathogen, which produces a variety of virulence determinants. To study environmental regulation of virulence-determinant production, several transcriptional reporter gene fusions were constructed. Chromosomal fusions were made with the staphylococcal accessory regulator (sarA), alpha-haemolysin (hla), surface protein A (spa) and toxic-shock syndrome toxin-1 (tst) genes. The effect of many different environmental conditions on the expression of the fusions was examined. Expression of hla, tst and spa was strongly repressed in the presence of sodium chloride (1 M) or sucrose (20 mM), but sarA was relatively unaffected. The global regulator of expression of virulence-determinant genes, agr (accessory gene regulator) was not involved in the salt or sucrose repression. Novobiocin, a DNA gyrase inhibitor, did not significantly increase the expression of tst in wild-type or agr backgrounds and failed to relieve the salt suppression. Expression of tst was strongly stimulated in several low-metal environments, independently of agr, whilst spa levels were significantly reduced by EGTA. The complex, interactive role of environmental factors in the control of expression of the virulence determinants is discussed.

Transcriptional analysis of different promoters in the sar locus in Staphylococcus aureus

The expression of extracellular virulence determinants in Staphylococcus aureus is controlled by a 510-nucleotide RNA molecule (RNAIII) which is a part of the agr system. The agr operon, which encodes a multicomponent signal transduction system, is partially under the influence of an unlinked regulatory locus called sar. The sar locus is composed of three overlapping transcripts, designated sarA (0.56 kb), sarC (0.8 kb), and sarB (1.2 kb), originating from the P1, P3, and P2 promoters, respectively. In this study, we analyzed the differential expression of these promoters by using transcriptional fusion with the xylE reporter gene to study the activation of the sar locus. The data confirm the existence of three independent promoters with different promoter activities. Maximal promoter activity was observed with the combined fusion of P2-P3-P1 promoters. Expression studies with a sigB mutant revealed that the P3 promoter is SigB dependent. Analysis of these transcriptional fusions in a sarA mutant and in complemented strains with each of the sar transcriptional units revealed that the sar locus is autoregulatory, with SarA acting as a positive regulator. From various transcriptional fusion studies of the upstream region of the P1 promoter, we have localized a 34-bp sequence which seems to play a role in down-modulating P1 transcription. Using heparin-Sepharose and DNA-specific columns, we partially purified a 12-kDa protein, possibly a repressor, which binds to the promoter regions upstream of P2 and P1 and which also binds to the 34-bp sequence. These data indicated that the regulation of the sar locus is complex and may involve the sar gene product(s) and other regulatory protein(s).

Factors affecting the collagen binding capacity of Staphylococcus aureus

To determine whether the ability of Staphylococcus aureus to bind collagen involves an adhesin other than the collagen adhesin encoded by cna, we examined the collagen binding capacity (CBC) of 32 strains of S. aureus. With only two exceptions, a high CBC corresponded with the presence of cna. Both exceptions involved cna-positive strains with a low CBC. The first was a single strain (ACH5) that encoded but did not express cna. The second were the mucoid strains Smith diffuse and M, both of which encoded and expressed cna but bound only minimal amounts of collagen. Analysis of capsule mutants suggests that the reduced CBC observed in the mucoid strains was due to masking of the collagen adhesin on the cell surface and that this masking effect is restricted to heavily encapsulated strains. Differences in the CBC of the remaining cna-positive strains were correlated to variations in the level of cna transcription and were independent of the number of B domain repeats in the cna gene. In all cna-positive strains other than ACH5, cna transcription was temporally regulated, with cna mRNA levels being highest in cells taken from exponentially growing cultures and falling to almost undetectable levels as cultures entered the post-exponential growth phase. The CBC was also highest with cells taken from exponentially growing cultures. Mutation of agr resulted in a slight increase in cna transcription and a corresponding increase in CBC during the exponential growth phase but did not affect the temporal pattern of cna transcription. Mutation of sar resulted in a more dramatic increase in CBC and a delay in the post-exponential-phase repression of cna transcription. Mutation of both sar and agr had an additive effect on both CBC and cna transcription. We conclude that (i) cna encodes the primary collagen-binding adhesin in S. aureus, (ii) sar is the primary regulatory element controlling expression of cna, and (iii) the regulatory effects of sar and agr on cna transcription are independent of the interaction between sar and agr.

Genetic instability of the global regulator agr explains the phenotype of the xpr mutation in Staphylococcus aureus KSI9051

Staphylococcus aureus KSI9051 has a complex mutation that was associated with the aberrant expression of cell surface and extracellular proteins (M. S. Smeltzer, M. E. Hart, and J. J. Iandolo, J. Bacteriol. 61:919-925, 1993). This mutation was named xpr, although no specific gene was identified. Here this mutation is referred to as Delta1058::Tn551. In this study, we show that in strain KSI9051, the Delta1058::Tn551 mutation occurred coincidentally with a frameshift in agrC that is expected to truncate the sensor component of the known staphylococcal global regulatory locus agr. Remarkably, pleiotropic mutations affecting cell surface and extracellular proteins are generated at frequencies approaching 50% upon the transduction of erythromycin resistance (Emr) encoded by Delta1058::Tn551 from S. aureus KSI905 back to its parental strain, S6C. Three independent isolates created in the manner of KSI9051 contained mutations within agrC. Each isolate had different mutations, suggesting that the transduction of Emr encoded by Delta1058::Tn551 affects the stability of agrC in S6C. In similar experiments with strains from an S. aureus 8325 genetic background, a mutant AgrC phenotype could not be isolated, implying that strain S6 has aberrant genetic behavior. A comparison of the nucleotide sequences of AgrC from several strains revealed seven errors in the GenBank entry for agr (X52543); these data were confirmed with plasmid pRN6650, the original wild-type clone of agr.

The genome of Staphylococcus aureus: a review

The genome of Staphylococcus aureus consists of a single circular chromosome (2.7-2.8 mbp) plus an assortment of extrachromosomal accessory genetic elements: conjugative and nonconjugative plasmids, mobile elements (IS, Tn, Hi), prophages and other variable elements. Plasmids (1-60 kbp) are classified into 4 classes and there are 15 known incompatibility groups. Mobile elements of the genome (0.8-18 kbp) appear in the chromosome or in plasmids of classes II and III. Prophages (45-60 kbp) are integrated in the bacterial chromosome, and they are UV- or mitomycin-inducible. Temperate bacteriophages of S. aureus are members of the Siphoviridae and the serological groups A, B and F occur most frequently. In the paper presented, the characteristics of chromosome, plasmids, transposons and other genetic elements of S. aureus genome are given and an alphabetical list of known genes of this species is included.

bldA-dependent expression of the Streptomyces exfoliatus M11 lipase gene (lipA) is mediated by the product of a contiguous gene, lipR, encoding a putative transcriptional activator

Extracellular lipase synthesis by Streptomyces lividans 66 carrying the cloned lipase gene (lipA) from Streptomyces exfoliatus M11 was found to be growth phase dependent, since lipase was secreted into the medium mainly during the stationary phase; S1 nuclease protection experiments revealed abundant lipA transcripts in RNA preparations obtained during the stationary phase but not in those obtained during exponential growth. Transcription from the lipA promoter was dependent on the presence of lipR, a contiguous downstream gene with a very high guanine-plus-cytosine content (80.2%). The deduced lipR product consists of a protein of 934 amino acids that shows similarity to known transcriptional activators and has a strong helix-turn-helix motif at its C terminus; this motif is part of a domain homologous to DNA-binding domains of bacterial regulators of the UhpA/LuxR superfamily. The lipR sequence revealed the presence of a leucine residue, encoded by the rare TTA codon, which caused bldA dependence of lipA transcription in Streptomyces coelicolor A3(2); replacement of the TTA codon by the alternate CTC leucine codon alleviated bidA dependence but not the apparent growth phase-dependent regulation of lipA transcription. When lipR expression was induced in a controlled fashion during the exponential growth phase, by placing it under the inducible tipA promoter, lipase synthesis was shifted to the exponential growth phase, indicating that the timing of lipR expression, and not its bldA dependence, is the main cause for stationary-phase transcription of lipA.

Cell-cell communication in gram-positive bacteria

In gram-positive bacteria, many important processes are controlled by cell-to-cell communication, which is mediated by extracellular signal molecules produced by the bacteria. Most of these signaling molecules are peptides or modified peptides. Signal processing, in most cases, involves either transduction across the cytoplasmic membrane or import of the signal and subsequent interaction with intracellular effectors. Concentrations of signal in the nanomolar range or below are frequently sufficient for biological activity. The microbial processes controlled by extracellular signaling include the expression of virulence factors, the expression of gene transfer functions, and the production of antibiotics.

Prevalence and chromosomal map location of Staphylococcus aureus adhesin genes

Using genomic DNA from 25 unrelated strains and probes specific for each gene, we assessed the prevalence of the Staphylococcus aureus (Sa) adhesion genes cna, fnbA, fnbB, fib, clfA, fbpA, ebpS and map. All 25 strains encoded fib, clfA, ebpS, map and at least one of the fnb genes. fbpA and coa appeared to be allelic variants of the same gene with the fbpA variant being present in only four of 25 isolates. cna was present in 10 of 25 strains. Using Southern blot analysis of SmaI-digested genomic DNA resolved by pulsed-field gel electrophoresis, the adhesion genes were mapped to SmaI fragments A (ebpS), B (fib and clfA), C (fnbA/fnbB), E (fbpA), F (map) and G (cna). Despite variations in SmaI restriction profiles, co-localization of adhesin genes with genes known to map to specific SmaI fragments in the Sa 8325-4 chromosome strains suggests that the chromosomal location of each adhesin gene is conserved.

The Staphylococcus aureus scdA gene: a novel locus that affects cell division and morphogenesis

A new Staphylococcus aureus gene termed scdA was found upstream of the autolysis regulatory genes, lytS and lytR, and was shown to potentially encode a hydrophilic 25 kDa protein. Analysis of scdA transcription revealed that it is transcribed as a monocistronic message and is lytSR-independent. A role in cell wall metabolism was indicated by examination of the scdA mutant S. aureus KB323, which had a grossly aberrant cellular morphology and formed large cell clusters when grown in liquid culture medium. Furthermore, KB323 exhibited a reduced rate of autolysis and had increased peptidoglycan cross-linking compared to the parental strain, NCTC 8325-4. These data suggest that scdA plays an important role in staphylococcal cell division.

Modification of the Staphylococcus aureus fibronectin binding phenotype by V8 protease

The amount of cell surface fibronectin (Fn)-binding protein (FnBP) adhesin expressed by Staphylococcus aureus is maximal during exponential growth but disappears rapidly as the culture progresses into stationary phase. To identify factors responsible for the loss of cell surface FnBP, a culture of S. aureus L170, which shows high levels of Fn binding, was supplemented at the time of inoculation with concentrated stationary-phase supernatant from S. aureus L530, a strain which binds Fn poorly. The resulting exponential-phase cells were devoid of FnBP. The factor responsible for this activity was purified from the culture supernatant and identified as V8 protease. When cultured with 375 ng of exogenous V8 protease ml(-1), exponential-phase cells of S. aureus L170 were devoid of cell surface FnBP, and concentrations as low as 23 ng x ml(-1) resulted in reduced amounts of FnBP. Addition of the protease inhibitor alpha2-macroglobulin to the culture medium prevented the growth-phase-dependent loss of cell surface FnBP, whereas growth with exogenous V8 protease resulted in reduced adherence to the solid-phase N-terminal fragment of Fn and to the extracellular matrix synthesized by fetal rabbit lung fibroblasts. Although FnBP was extremely sensitive to V8 protease, exogenous protease did not exert a significant influence on the amount of cell surface protein A. However, a limited number of other high-molecular-weight cell surface proteins were also sensitive to V8 protease. Therefore, both the adhesive phenotype and cell surface protein profile of S. aureus can be modified by V8 protease activity.

Increased expression of fibronectin-binding proteins by fluoroquinolone-resistant Staphylococcus aureus exposed to subinhibitory levels of ciprofloxacin

Bacterial adhesion, which plays an important role in Staphylococcus aureus colonization and infection, may be altered by the presence of antibiotics or/and antibiotic resistance determinants. This study evaluated the effect of fluoroquinolone resistance determinants on S. aureus adhesion to solid-phase fibronectin, which is specifically mediated by two surface-located fibronectin-binding proteins. Five isogenic mutants, derived from strain NCTC 8325 and expressing various levels of quinolone resistance, were tested in an in vitro bacterial adhesion assay with polymethylmethacrylate coverslips coated with increasing amounts of fibronectin. These strains contained single or combined mutations in the three major loci contributing to fluoroquinolone resistance, namely, grlA, gyrA, and flqB, which code for altered topoisomerase IV, DNA gyrase, and increased norA-mediated efflux of fluoroquinolones, respectively. Adhesion characteristics of the different quinolone-resistant mutants grown in the absence of fluoroquinolone showed only minor differences from those of parental strains. However, more important changes in adhesion were exhibited by mutants highly resistant to quinolones following their exponential growth in the presence of one-quarter MIC of ciprofloxacin. Increased bacterial adhesion of the highly quinolone-resistant mutants, which contained combined mutations in grlA and gyrA, was associated with and explained by the overexpression of their fibronectin-binding proteins as assessed by Western ligand affinity blotting. These findings contradict the notion that subinhibitory concentrations of antibiotics generally decrease the expression of virulence factors by S. aureus. Perhaps the increased adhesion of S. aureus strains highly resistant to fluoroquinolones contributes in part to that emergence in clinical settings.

Transcription analysis of the Staphylococcus aureus gene encoding penicillin-binding protein 4

The high level of cross-linking found in Staphylococcus aureus peptidoglycan is dependent on the low-molecular-weight penicillin-binding protein PBP4. Recently, the PBP4 gene, pbpD, was cloned and shown to be adjacent to and divergently transcribed relative to the putative ABC-type transporter gene, abcA. Disruption of abcA (in strain KB400) was previously shown to result in heightened resistance to several antibiotics known to interact with PBP4, suggesting that the regulation of pbpD is affected by abcA. In this report, this hypothesis was confirmed by use of a Northern (RNA) blot analysis which revealed increased accumulation of pbpD-specific transcripts in KB400 compared to that in the wild-type strain, 8325-4. By using reverse-phase high-performance liquid chromatography to examine the structure of the peptidoglycan, it was demonstrated that the increased expression of pbpD resulted in an increased level of peptidoglycan cross-linking in the staphylococcal cell wall. Promoter fusion studies demonstrated that the abcA mutation caused approximately 7-fold and 100-fold increases in pbpD and abcA promoter activities, respectively. Primer extension experiments revealed that these genes have long, untranslated leader sequences that result in a transcriptional overlap of 80 bp. Interestingly, deletion of a 26-bp region containing an inverted repeat sequence resulted in the loss of expression from both the abcA and the pbpD promoters. These data provide evidence that abcA and pbpD are under the control of a common regulatory mechanism that may involve the transport function of the abcA gene product.

Transcriptional regulation of the Staphylococcus aureus collagen adhesion gene, cna

We demonstrate that transcription of the Staphylococcus aureus collagen adhesin gene (cna) is temporally regulated, with expression being highest in exponentially growing cultures and falling to almost undetectable levels as cultures enter the post-exponential-growth phase. The temporal regulation of cna transcription was not affected by mutation of agr. We also demonstrate that the collagen adhesin is expressed by both agr+ and agr-negative S. aureus cells growing in bone.

Host/parasite interactions in bacterial endophthalmitis

Bacterial infections within the eye arise as complications of intraocular surgery, penetrating injury, or hematogenous spread from distant anatomical sites. Because: 1) the interior surfaces of the eye are lined with sensitive, nonregenerating tissues, 2) the inner chambers of the eye are relatively sequestered from circulating immunological components, 3) the integrity of blood-ocular barriers provides poor penetration of systemically administered antibiotics, and 4) aqueous and vitreous humor represent rich, relatively acellular culture media; endophthalmitis often progresses rapidly and total loss of vision frequently results. Years of clinical experience have shown that current therapies for endophthalmitis, including antimicrobials, antiinflammatory agents, and vitrectomy, are frequently unsuccessful in ameliorating destruction of intraocular tissues. While bacterial and host factors were thought to play key roles in the course and severity of endophthalmitis, it is only recently that their contributions have been experimentally defined. Molecular-based techniques are gaining increased use in the study of infectious eye diseases. Current findings regarding the host/parasite interactions within the eye are reviewed, and a resulting integrative model of the natural course of endophthalmitis proposed. A molecular-level understanding of the roles of both bacterial and host factors during endophthalmitis will likely reveal potential targets for therapeutic intervention aimed at salvaging vision.

Identification of LytSR-regulated genes from Staphylococcus aureus

In this report, the characterization of a Staphylococcus aureus operon containing two LytSR-regulated genes, lrgA and lrgB, is described. Sequence and mutagenesis studies of these genes suggest that lrgA encodes a murein hydrolase exporter similar to bacteriophage holin proteins while lrgB may encode a protein having murein hydrolase activity.

Characterization of the primary sigma factor of Staphylococcus aureus

RNA polymerase (RNAP) isolated from Staphylococcus aureus is deficient in sigma factor and is poorly active in transcription assays. Based on amino acid sequence homology of the Bacillus subtilis vegetative sigma factor sigmaA and the predicted product of the chromosomally located plaC gene of S. aureus, it was hypothesized that plaC could encode the vegetative sigma factor. We cloned plaC under a T7 promoter and overexpressed it in Escherichia coli strain BL21(DE3)pLysE. The overproduced protein, present in inclusion bodies, was solubilized with guanidine hydrochloride, renatured, and purified by DEAE-Sephacel and Sephadex G-75 chromatography. The purified protein, designated sigmaSA, cross-reacted with the B. subtilis anti-sigmaA antibody. E. coli core RNAP, reconstituted with sigmaSA, initiated promoter-specific transcription from the S. aureus promoters hla, sea, and sec and from the E. coli promoters rpoH P1, rpoH P4, and ColE1 RNA-1, which are recognized by the E. coli sigma70. sigmaSA, when added to the purified RNAP from S. aureus, stimulated transcriptional activity of the RNAP up to 72-fold. As determined by primer extension studies, the 5'-ends of the sigmaSA-initiated mRNAs synthesized in vitro from the agr P2 and sea promoters are in general agreement with the 5'-ends of the cellular RNAs. Disruption of the plaC gene on the S. aureus chromosome was lethal. We conclude that plaC encodes the primary sigma factor in S. aureus.

Identification and molecular characterization of a putative regulatory locus that affects autolysis in Staphylococcus aureus

Previously in our laboratory, a PCR-based strategy was used to isolate potential sensor gene fragments from the Staphyloccus aureus genome. One DNA fragment was isolated that shared strong sequence similarity to genes encoding bacterial sensor proteins, indicating that it originated from within a potential staphylococcal sensor protein gene. In this study, the DNA surrounding the PCR product origin was cloned and sequenced. This analysis revealed the presence of two genes, termed lytS and lytR, whose deduced amino acid sequences were similar to those of members of the two-component regulatory system family of proteins. S. aureus cells containing an insertional disruption of lytS exhibited a marked propensity to form aggregates in liquid culture, suggesting that alterations in cell surface components exist in this strain. Transmission electron microscopic examination of these cells revealed that the cell surface was rough and diffuse and that a large proportion of the cell population had lysed. The lytS mutant also exhibited increased autolysis and an altered level of murein hydrolase activity produced compared with the parental strain, NCTC 8325-4. These data suggest that the lytS and lytR gene products control the rate of autolysis in S. aureus by affecting the intrinsic murein hydrolase activity associated with the cell.

Role of the accessory gene regulator (agr) in pathogenesis of staphylococcal osteomyelitis

To examine the role of the accessory gene regulator (agr) in staphylococcal osteomyelitis, we compared a Staphylococcus aureus osteomyelitis isolate (UAMS-1) with a derivative of the same strain (UAMS-4) carrying an inactivated agr locus. Virulence was assessed with a rabbit model of acute, exogenous osteomyelitis. Bacteria were delivered by microinjection into the midradial region of the forelimb. After 4 weeks, UAMS-1 was identified in the bone of 12 of 13 rabbits infected with > or = 2 x 10(6) CFU and 5 of 6 infected with < or = 2 x 10(5) CFU. In contrast, UAMS-4 was found in 6 of 13 infected with the higher dose and 1 of 6 infected with the lower dose. Additionally, on the basis of a five-point scale assessing radiographic evidence of disease, rabbits infected with UAMS-1 had average scores of 2.64 +/- 0.30 (high dose) and 1.43 +/- 0.39 (low dose) while rabbits infected with UAMS-4 had average scores of 0.95 +/- 0.23 (high dose) and 0.63 +/- 0.20 (low dose). Uninfected controls had an average score of 0.53 +/- 0.08. The results obtained with UAMS-1 were significantly different from those obtained with UAMS-4 at both doses (P < or = 0.047). The results obtained with UAMS-4 were not significantly different from those obtained with the controls at either dose of UAMS-4 (P > or = 0.150). On the basis of a similar five-point scale assessing histopathological evidence of disease, rabbits infected with UAMS-1 had average scores of 2.31 +/- 0.22 (high dose) and 1.96 +/- 0.36 (low dose) while rabbits infected with UAMS-4 had average scores of 1.58 +/- 0.29 (high dose) and 0.83 +/- 0.32 (low dose). Controls had an average score of 0.33 +/- 0.05. The results obtained with UAMS-1 were significantly different from those obtained with UAMS-4 at both doses (P < or = 0.040). However, the results obtained with UAMS-4 were significantly different from the controls only at the high dose of UAMS-4 (P = 0.025). We conclude that mutation of agr reduces the incidence and severity of disease but does not eliminate the ability to colonize bone and cause histopathological evidence of osteomyelitis.

Insertional inactivation of a chromosomal locus that modulates expression of potential virulence determinants in Staphylococcus aureus

A single insertion of transposon Tn551 into a unique chromosomal locus of Staphylococcus aureus ISP479C has resulted in a pleiotropic effect on the expression of both extracellular and cell wall proteins. In particular, the expression of cell wall protein A and clumping activity with fibrinogen were rendered undetectable in the mutant 1E3 compared with the parent. The secretion of alpha-hemolysin in mutant 1E3 was modestly increased. Southern blot and phenotypic analyses indicated that this locus is distinct from agr, xpr, and sar, three previously described global regulatory loci. Transduction experiments demonstrated that the genotype associated with mutant 1E3 could be transferred back into the parental strain ISP479C. The transductant 1E3-2 displayed a phenotypic profile similar to that of the original mutant. Northern (RNA) blot studies showed that this locus may be involved in modulating target genes at the mRNA level. In the rabbit endocarditis model, there was a significant decrease in both the infectivity rate and intravegetation bacterial density with mutant 1E3 compared with the parent at an inoculum of 10(3) CFU. Since protein A and the fibrinogen-binding protein(s) are major surface proteins that may mediate bacterial adhesion to host tissues, this locus may be an important genetic element involved in the expression of virulence determinants in S. aureus.

In vitro transcription of pathogenesis-related genes by purified RNA polymerase from Staphylococcus aureus

The RNA polymerase (RNAP) holoenzyme of Staphylococcus aureus was purified by DNA affinity, gel filtration, and ion-exchange chromatography. This RNAP contained four major subunits with apparent molecular masses of 165, 130, 60, and 47 kDa. All four subunits of the RNAP were serologically related to the subunits of Escherichia coli E sigma 70 holoenzyme by Western immunoblot analysis. The 60-kDa subunit was subsequently isolated and found to react with a monoclonal antibody specific to the E. coli sigma 70 subunit. This sigma 70-related protein allowed E. coli core RNAP promoter-specific initiation and increased transcription by S. aureus RNAP that is unsaturated with sigma. We therefore suggest that this 60-kDa protein is a sigma factor. Purified S. aureus RNAP transcribed from the promoters of several important S. aureus virulence genes (sea, sec, hla, and agr P2) in vitro. The in vitro transcription start sites of the sea, sec, and agr P2 promoters, mapped by primer extension, were similar to those identified in vivo. The putative promoter hexamers of these three genes showed strong sequence similarity to the E. coli sigma 70 consensus promoter, and transcription by E sigma 70 from some of these promoters has been observed. Conversely, S. aureus RNAP does not transcribe from all E. coli sigma 70-dependent promoters. Taken together, our results indicate that the promoter sequences recognized by purified S. aureus RNAP are similar but not identical to those recognized by E. coli E sigma 70.

Bacterial lipases

Many different bacterial species produce lipases which hydrolyze esters of glycerol with preferably long-chain fatty acids. They act at the interface generated by a hydrophobic lipid substrate in a hydrophilic aqueous medium. A characteristic property of lipases is called interfacial activation, meaning a sharp increase in lipase activity observed when the substrate starts to form an emulsion, thereby presenting to the enzyme an interfacial area. As a consequence, the kinetics of a lipase reaction do not follow the classical Michaelis-Menten model. With only a few exceptions, bacterial lipases are able to completely hydrolyze a triacylglycerol substrate although a certain preference for primary ester bonds has been observed. Numerous lipase assay methods are available using coloured or fluorescent substrates which allow spectroscopic and fluorimetric detection of lipase activity. Another important assay is based on titration of fatty acids released from the substrate. Newly developed methods allow to exactly determine lipase activity via controlled surface pressure or by means of a computer-controlled oil drop tensiometer. The synthesis and secretion of lipases by bacteria is influenced by a variety of environmental factors like ions, carbon sources, or presence of non-metabolizable polysaccharides. The secretion pathway is known for Pseudomonas lipases with P. aeruginosa lipase using a two-step mechanism and P. fluorescens lipase using a one-step mechanism. Additionally, some Pseudomonas lipases need specific chaperone-like proteins assisting their correct folding in the periplasm. These lipase-specific foldases (Lif-proteins) which show a high degree of amino acid sequence homology among different Pseudomonas species are coded for by genes located immediately downstream the lipase structural genes. A comparison of different bacterial lipases on the basis of primary structure revealed only very limited sequence homology. However, determination of the three-dimensional structure of the P. glumae lipase indicated that at least some of the bacterial lipases will presumably reveal a conserved folding pattern called the alpha/beta-hydrolase fold, which has been described for other microbial and human lipases. The catalytic site of lipases is buried inside the protein and contains a serine-protease-like catalytic triad consisting of the amino acids serine, histidine, and aspartate (or glutamate). The Ser-residue is located in a strictly conserved beta-epsilon Ser-alpha motif. The active site is covered by a lid-like alpha-helical structure which moves away upon contact of the lipase with its substrate, thereby exposing hydrophobic residues at the protein's surface mediating the contact between protein and substrate.(ABSTRACT TRUNCATED AT 400 WORDS)

Cloning and sequencing of sarA of Staphylococcus aureus, a gene required for the expression of agr

To evaluate the effect of a sar mutation on the agr locus, Northern (RNA) blotting was performed to determine the levels of RNAIII, the agr regulatory molecule, in two isogenic pairs of Staphylococcus aureus strains. Our results demonstrated that RNAIII was either significantly diminished or absent in both sar mutants compared with the parents. The RNAIII level was partially restored in sar mutants complemented with an intact sar gene (designated sarA). Additionally, we were able to complement selected sar phenotypes with a plasmid carrying RNAIII (pRN6735). These studies suggest that the sarA gene is necessary for the optimal expression of agr. The sarA gene of strain RN450 was subsequently cloned and sequenced. Sequence analysis revealed an open reading frame of 372 bp with a predicted molecular size of 14,718 Da and a deduced pI of 8.52. The deduced protein sequence has a predominance of charged residues (33%) and shares sequence similarity with the virF gene of Shigella flexneri.