Molecular cloning of staphylococcal enterotoxin B gene in Escherichia coli and Staphylococcus aureus

Gene cloning and characterization of EfmA, a multidrug efflux pump, from Enterococcus faecium

A DNA fragment responsible for resistance to antimicrobial agents was cloned from chromosomal DNA of Enterococcus faecium FN-1, a clinically isolated strain. Escherichia coli KAM32, a drug-hypersusceptible mutant, was used as a host for gene cloning. Cells of E. coli KAM32 harboring a recombinant plasmid (pTFM8) carrying the DNA fragment became resistant to fluoroquinolones, macrolides, ethidium bromide, 4',6-diamidino-2-phenylindole (DAPI) and tetraphenylphosphonium chloride (TPPCl). Three complete open reading frames (ORFs) were found in the DNA insert of pTFM8, and the deduced amino acid sequences of one of the ORFs showed high similarity to Mdt(A) from Lactococcus lactis. Mdt(A) is a multidrug efflux pump belonging to a major facilitator superfamily. We designated the ORF efmA. E. coli KAM32 cells harboring the efmA showed energy-dependent efflux of DAPI and TPP(+). We also observed norfloxacin/H(+) antiport due to EfmA. The mRNA expression of efmA was observed in E. faecium FN-1 grown without any exogenously added antimicrobial agents. Thus, we conclude that efmA is constitutively expressed under laboratory growth conditions and would contribute to intrinsic resistance against multiple antimicrobial agents in E. faecium FN-1.

Streptococcus sp. and Staphylococcus aureus isolates from patients with psoriasis possess genes that code for toxins (superantigens): clinical and therapeutic implications

Superantigens are powerful T lymphocyte-stimulating agents that are believed to contribute to the pathogenesis of certain diseases such as psoriasis. Toxins produced by Streptococcus pyogenes and Staphylococcus aureus are superantigens. The aim of this study was to detect genes that code for superantigens in Streptococcus and Staphylococcus aureus isolates from psoriatic patients. Primers to amplify streptococcal pyrogenic exotoxin A, B, and C and streptolysin O genes and staphylococcal enterotoxin A, B, C, and D genes were used. Streptococcal exotoxin B was detected in five streptococcal isolates. Staphyloccocus aureus enterotoxin A and/or C genes were detected in nine S. aureus isolates. Isolates from 13 of 22 patients possesed gene(s) that code for toxin(s) (superantigens). These results might support the role of superantigens in the exacerbation of psoriasis.

Transcriptional profiling of a Staphylococcus aureus clinical isolate and its isogenic agr and sarA mutants reveals global differences in comparison to the laboratory strain RN6390

The production of Staphylococcus aureus virulence factors is under the control of complex regulatory circuits. Most studies aimed at defining these regulatory networks have focused on derivatives of the strain NCTC 8325, most notably RN6390. However, all NCTC 8325 derivatives, including RN6390, possess an 11 bp deletion in rsbU. This deletion renders NCTC 8325 derivatives naturally sigma-factor-B deficient. Recent studies have shown that RN6390 is also deficient, in comparison to clinical isolates, with respect to biofilm formation, a process which is important for both pathogenesis and antimicrobial resistance. Based on these considerations, the authors carried out genome-scale transcriptional profiling, comparing RN6390 with the virulent rsbU-positive clinical isolate UAMS-1. The results revealed significant genome-wide differences in expression patterns between RN6390 and UAMS-1, and suggested that the overall transcriptional profile of UAMS-1 is geared toward expression of factors that promote colonization and biofilm formation. In contrast, the transcriptional profile of RN6390 was heavily influenced by RNAIII expression, resulting in a phenotype characterized by increased production of exoproteins, and decreased capacity to form a biofilm. The greater influence of agr in RN6390 relative to UAMS-1 was also evident when the transcriptional profile of UAMS-1 was compared with that of its isogenic sarA and agr mutants. Specifically, the results indicate that, in contrast to NCTC 8325 derivatives, agr plays a limited role in overall regulation of gene expression in UAMS-1, when compared with sarA. Furthermore, by defining the sarA regulon in a biofilm-positive clinical isolate, and comparing the results with transcriptional profiling experiments defining biofilm-associated gene expression patterns in the same strain, the authors identified a sarA-regulated operon (alsSD) that is also induced in biofilms, and demonstrated that mutation of alsSD results in reduced capacity to form a biofilm.

Development of a routine laboratory direct detection system of staphylococcal enterotoxin genes

A novel direct detection system has been developed for eight staphylococcal enterotoxin (SE)-encoding genes (sea, seb, sec, sed, see, seg, seh and sei) in milk. Specific detection by real-time PCR was successful for all SE-encoding genes in the reference strains. Furthermore, a novel DNA-preparation method with good reproducibility [coefficients of variation 0.31, 0.99 and 1.21 % at 10(6), 10(4) and 10(2) c.f.u. (ml milk sample)(-1), respectively] was developed to overcome PCR inhibition in the milk samples. The combination of this DNA-preparation method and real-time PCR resulted in high sensitivity [between 1.1 x 10(2) and 1.0 x 10(4) c.f.u. (ml milk sample)(-1)] and allowed the completion of the entire procedure within 4 h. Results of an evaluation of this method for the detection of SE-encoding genes using known outbreak milk samples produced results showing good correspondence with the reversed passive latex agglutination assay. In addition, this newly developed system can be applied to clinical samples such as faeces and vomit. Consequently, the system should be useful in the routine direct detection of SE-encoding genes in food-borne-poisoning samples.

Toxicity of the staphylococcal enterotoxin B mutants with histidine-to-tyrosine substitutions

In this study we made a series of site-directed mutants of staphylococcal enterotoxin B (SEB), in which histidine residues in the molecule were replaced by tyrosine. The mutant genes were cloned and expressed, and the corresponding proteins were purified. These mutant proteins were tested for binding to human HLA-DR4 and for mitogenetic activity in mouse splenocyte culture. Toxicity of the proteins in vivo was evaluated in the actinomycin D-primed C3H/HeJ mouse model. We found that SEB mutant proteins with fewer than four histidine-to-tyrosine (his-to-tyr) substitutions retained toxic properties similar to wild-type SEB. However, studies showed that his-to-tyr substitution of four consecutive histidine residues eliminated SEB toxicity. Our results clearly show that this genetically modified SEB protein is non-toxic and justifies its further development as a component of a new, safer vaccine to prevent SEB intoxication.

Stable low-copy-number Staphylococcus aureus shuttle vectors

A series of Staphylococcus aureus-Escherichia coli shuttle vectors were constructed which contained the replication and maintenance functions of the S. aureus theta-mode multiresistance plasmid pSK1. The utility of the newly constructed vectors was demonstrated by the successful cloning and expression of several genes that had previously proven difficult to express in S. aureus. Additional vectors which permit the generation of transcriptional and translational fusions to an S. aureus blaZ reporter gene were also produced and subsequently employed to determine the relative strengths in S. aureus of a number of promoters. By utilizing the theta-mode replication functions of pSK1, the shuttle vectors described largely avoided the segregational and structural stability problems frequently encountered with Gram-positive rolling-circle-based vectors. In addition, these plasmids represent vectors which are suitable for the analysis of genes in S. aureus at low copy number.

Rapid screening for putative exported proteins from Staphylococcus aureus using alkaline phosphatase as a reporter molecule

Staphylococcus aureus causes a wide range of infections in humans, ranging from superficial skin infections to the more serious toxin-mediated diseases such as toxic shock syndrome. Owing to the increasing resistance of this bacterium to a wide range of antibiotics, the need to determine the virulence factors involved in infection is becoming more important as these molecules are potential therapeutic targets. In this study, we have screened for putative exported proteins from S. aureus on the basis that these proteins are likely to be the first point of contact between the bacterium and host during infection. We have constructed gene fusions between S. aureus DNA and a truncated version of the Escherichia coli phoA gene, and we report on the characterization of the recombinants exhibiting alkaline phosphatase activity. As well as known S. aureus proteins, we have identified a number of putative open reading frames that encode proteins similar to those from nonstaphylococcal species and also unique proteins that do not have any homologues on the current databases.

Expression of the multidrug resistance transporter NorA from Staphylococcus aureus is modified by a two-component regulatory system

To dissect genetically the regulation of NorA, a multidrug transporter of Staphylococcus aureus, we analyzed the differential expression of the norA promoter using a transcriptional fusion with a beta-lactamase reporter gene. Expression studies with an arlS mutant revealed that the norA promoter is ArlS dependent. The arlR-arlS locus was shown to code for a two-component regulatory system. The protein ArlR has strong similarity to response regulators, and ArlS has strong similarity to protein histidine kinases. We have also analyzed the 350-bp region upstream of the Shine-Dalgarno sequence of norA by gel mobility shift experiments. It was shown that only the 115-bp region upstream of the promoter was necessary for multiple binding of an 18-kDa protein. From transcriptional fusions, we have localized four different putative boxes of 6 bp, which appear to play a role in the binding of the 18-kDa protein and in the up-regulation of norA expression in the presence of the arlS mutation. Furthermore, the gel mobility shift of the 18-kDa protein was modified in the presence of the arlS mutation, and the arlS mutation altered the growth-phase regulation of NorA. These results indicate that expression of norA is modified by a two-component regulatory system.

Molecular characterization of the putative T-cell receptor cavity of the superantigen staphylococcal enterotoxin B

A number of investigators have utilized a variety of methods to identify the structural basis for the interaction of superantigens with the T-cell receptor beta-chain. The previous studies strongly suggest that a region of the toxin near residues N23, Y61, Y91 and D209 is important for this binding activity. Examination of crystal structure data shows that these residues line the rim of one side of a shallow cavity in the toxin. In an attempt further to define the face of the staphylococcal enterotoxin B (SEB) molecule involved in the interaction with the beta-chain, we have employed a polymerase chain reaction (PCR)-based, site-specific mutagenesis method to generate amino acid substitutions of residues on the opposite side of this putative T-cell receptor interaction cavity. Our results show that Y175 and N179 appear to be involved in the function of this superantigen, since each of several substitutions at this position exhibits a significantly reduced ability to induce T-cell proliferation. At the same time, mutation of the proximal Y186 does not alter the superantigen activity of SEB. Binding analysis of these mutants shows that class II binding activity is not significantly altered. Analysis of the responding T cells shows that the mutant toxins maintain T-cell receptor V beta selectivity. However, responses of T cells bearing the V beta 8.1 allele appear to be particularly diminished. When viewed in the context of other results reported in the literature, our results suggest that the T-cell receptor interaction site involves SEB residues which ring both the Y175/N179-side and the N23-side of a cavity on one side of the toxin molecule.

Identification of novel staphylococcal virulence genes by in vivo expression technology

We have applied in vivo expression technology (IVET) to the study of staphylococcal virulence. Using a promoter trap that relies on genetic recombination as a reporter of gene expression, we identified 45 staphylococcal genes that are induced during infection in a murine renal abscess model. Of these, only six were known previously; 11 others have homology to known non-staphylococcal genes. The known staphylococcal genes include agrA, part of a key locus regulating numerous virulence products, and a glycerol ester hydrolase, which may enhance staphylococcal survival in abscesses. We constructed 11 strains containing mutations in previously unknown ivi genes. Of these strains, seven were significantly attenuated in virulence compared with the wild-type parent. The mutagenized ivi genes may encode novel staphylococcal virulence factors.

Structural Dichotomy of Staphylococcal Enterotoxin C Superantigens Leading to MHC Class II-Independent Activation of T Lymphocytes

We have recently characterized an MHC class II-deficient human cell line, SW480, that supports the proliferation of purified human T cells in the presence of the staphylococcal enterotoxin and superantigen SEC1, but not the closely related isotypes SEC2 or SEC3. We now investigate the structural basis of this dichotomy and explore possible mechanisms that may account for it. Differences in activity between SEC1 and SEC2 were not attributable to differences in biochemical modification, to differences in Vβ specificity, or to the potential to induce anergy. SEC2 inhibited SEC1-mediated T cell activation in the presence of SW480 cells, suggesting that SEC2 could compete with SEC1 for binding to the TCR but was unable to productively signal through the TCR. Utilizing a panel of hybrid enterotoxins we identified specific amino acids near the NH2-terminus of SEC1 that abrogated MHC class II-independent T cell activation, yet did not alter potency in the presence of class II+ APC. These residues mapped to the putative TCR binding domain of SEC1, and suggest that subtle differences in TCR binding affinity or the topology of the SEC1-TCR interaction can compensate for the lack of MHC class II and hence promote T cell proliferation.

Mutations affecting the superantigen activity of staphylococcal enterotoxin B

As a superantigen, staphylococcal enterotoxin B (SEB) possesses the ability to bind to major histocompatibility complex class II molecules and be recognized by T cells bearing certain T-cell receptor (TCR) V beta alleles. Other investigators have utilized site-specific mutagenesis to generate amino acid substitutions to identify residues that may be involved in the interaction with the TCR beta-chain. In an attempt further to define the face of the SEB molecule involved in the interaction with the beta-chain, we have employed a polymerase chain reaction (PCR)-based, site-specific mutagenesis method to generate amino acid substitutions with altered superantigen activity. Our results show that valine at position 169 appears to be involved in the function of this superantigen, since each of several substitutions at this position exhibit a significantly reduced ability to induce T-cell proliferation. Analysis of the responding T cells to the residue 169 substitution shows that the mutant toxins maintain TCR V beta selectivity. At the same time, mutation of the proximal histidine at position 166 does not alter the superantigen activity of SEB. Radiolabelled binding analysis of these H166 and V169 mutants shows that class II-binding activity is not significantly altered. When viewed in the context of other results reported in the literature, combined with the crystal structure of the toxin, our results suggest that the interaction with the TCR probably involves SEB residues which ring a cavity along one side of the toxin molecule.

Mitogenic activities of amino acid substitution mutants of staphylococcal enterotoxin B in human and mouse lymphocyte cultures

Site-directed mutagenesis has been used to introduce amino acid substitutions at specific residues of the staphylococcal enterotoxin B (SEB) gene cloned from Staphylococcus aureus 10-275. The mitogenic activities of these derivatives were determined in two assay systems: (i) mouse spleen cells and (ii) a mixture of human peripheral blood mononuclear cells and lymphocytes. Substitution of either His-12, His-32, His-121, His-166, Lys-152, or Gly-205 did not significantly alter the mitogenic activity from that of the wild-type toxin in either proliferation assay. Substitution of either residue Asn-23, Phe-44, or Cys-93 reduced the mitogenicity of SEB by a degree that depended upon the assay system used. Similar to the results reported by others measuring toxin activation of mouse lymphoid cells, we found that substitutions of these three residues of SEB caused at least 800-fold reductions of mitogenic activity from that of the wild-type toxin. When tested for toxicity in vivo in D-galactosamine-treated mice, the reduced activities of these mutant toxins, however, were not as pronounced. In contrast, when tested in the human cell mitogenicity assay, these mutant toxins were active. Small alterations in activity (two- to fivefold reduction) were observable only at low concentrations. Our findings reveal the importance of using human lymphocytes in addition to the traditional mouse spleen cell assay when assessing biological activities of staphylococcal enterotoxins.

Identification and characterization of the origin of conjugative transfer (oriT) and a gene (nes) encoding a single-stranded endonuclease on the staphylococcal plasmid pGO1

The genes mediating the conjugative transfer of the 52-kb staphylococcal plasmid pGO1 are within a 14.4-kb gene cluster designated trs. However, a clone containing trs alone cannot transfer independently and no candidate oriT has been found within or contiguous to trs. In this study, we identified a 1,987-bp open reading frame (ORF) 24 kb 3' and 13 kb 5' to trs that was essential for conjugative transfer: transposon insertions into the ORF abolished transfer and a plasmid containing the ORF could complement these transposon-inactivated pGO1 mutants for transfer. Analysis of the nucleotide sequence of this ORF revealed significant homology between the amino terminus of its predicted protein and those of several single-stranded endonucleases. In addition, a 12-bp DNA sequence located 100 bp 5' to the ORF's translational start site was identical to the oriT sequences of the conjugative or mobilizable plasmids RSF1010, pTF1, R1162, pSC101, and pIP501. The ability of the ORF, designated nes (for nicking enzyme of staphylococci), to generate a single-stranded nick at the oriT was demonstrated in Escherichia coli by alkaline gel and DNA sequence analysis of open circular plasmid DNA. Plasmids that could be converted to the open circular form by the presence of oriT and nes could also be mobilized at high frequency into Staphylococcus aureus recipients with a second plasmid containing only trs. We propose that the 14.4 kb of trs and the approximately 2.2 kb of the oriT-nes region, coupled with an origin of replication, make up the minimal staphylococcal conjugative replicon.

Cell wall sorting of lipoproteins in Staphylococcus aureus

Many surface proteins are thought to be anchored to the cell wall of gram-positive organisms via their C termini, while the N-terminal domains of these molecules are displayed on the bacterial surface. Cell wall anchoring of surface proteins in Staphylococcus aureus requires both an N-terminal leader peptide and a C-terminal cell wall sorting signal. By fusing the cell wall sorting of protein A to the C terminus of staphylococcal beta-lactamase, we demonstrate here that lipoproteins can also be anchored to the cell wall of S. aureus. The topology of cell wall-anchored beta-lactamase is reminiscent of that described for Braun's murein lipoprotein in that the N terminus of the polypeptide chain is membrane anchored whereas the C-terminal end is tethered to the bacterial cell wall.

The agr P2 operon: an autocatalytic sensory transduction system in Staphylococcus aureus

The synthesis of virulence factors and other exoproteins in Staphylococcus aureus is controlled by the global regulator, agr. Expression of secreted proteins is up-regulated in the postexponential growth phase, whereas expression of surface proteins is down-regulated by agr. The agr locus consists of two divergent operons, transcribed from neighboring but non-overlapping promoters, P2 and P3. The P2 operon sequence, reported here, contains 4 open reading frames, agrA, C, D, and B, of which A and C appear to encode proteins of a classical 2-component signal transduction pathway. The P3 operon specifies a 0.5 kb transcript, RNA III, which is the actual effector of the agr response, and, incidentally, encodes the agr-regulated peptide delta-hemolysin. Transcriptional fusions have shown that both P2 and P3 are agr sensitive (function in an agr+ but not in an agr- background) and deletion analysis has shown that all four of the P2 ORFs are involved; agrA and agrC seem to be absolutely required for the transcriptional activation of the agr locus, whereas agrB and agrD seem to be partially required. Since transcription of P2 requires P2 operon products, the P2 operon is autocatalytic, and is thus admirably suited to the need for rapid production of exoproteins at a time when overall growth is coming to a halt.

Characterization of a conjugative staphylococcal mupirocin resistance plasmid

We studied conjugative plasmids encoding high-level mupirocin resistance. These plasmids were found in Staphylococcus aureus isolates from two geographic locations in the United States. Transfer genes on three mupirocin resistance plasmids with different restriction endonuclease profiles were indistinguishable by DNA hybridization from those on pG01, a conjugative aminoglycoside resistance plasmid representative of similar plasmids that are prevalent in the United States. One mupirocin resistance plasmid, pG0400 (34 kb), was smaller than pG01 (52 kb) because of the absence from pG0400 of DNA, found on pG01, that contained genes encoding resistance to aminoglycosides, trimethoprim, and quaternary ammonium compounds flanked by directly repeated copies of the insertion sequence (IS)-like element IS431-IS257. The plasmids pG0400 and pG01 were otherwise indistinguishable except for the presence in pG0400 of a 4.5-kb HinDIII fragment encoding mupirocin resistance. The added mupirocin resistance gene was flanked by two directly repeated copies of IS431/257. The nucleotide sequence of DNA contiguous to the outside of the IS elements, as well as those of the elements themselves, was identical in both pG01 and pG0400, and there were no target site duplications flanking either copy of the element. We conclude that the mupirocin resistance gene was added to an existing conjugative plasmid in conjunction with the deletion of other resistance genes by recombination at IS elements. The construction of conjugative plasmids carrying a mupirocin resistance gene may be a model for the mobility of other resistance genes newly acquired by staphylococci.

Lack of complete correlation between emetic and T-cell-stimulatory activities of staphylococcal enterotoxins

This study examined the emetic activity of several staphylococcal enterotoxin type A and B (SEA and SEB, respectively) mutants that had either one or two amino acid residue substitutions. New sea gene mutations were constructed by site-directed mutagenesis; gene products were obtained with glycine residues at position 25, 47, 48, 81, 85, or 86 of mature SEA. Culture supernatants from Staphylococcus aureus RN4220, or derivatives containing either sea or a sea mutation, were analyzed for the ability to stimulate proliferation of murine splenocytes, as determined by incorporation of [3H]thymidine. Culture supernatants containing SEA-N25G (a SEA mutant with a substitution of glycine for the asparagine residue at position 25), SEA-F47G, or SEA-L48G did not stimulate T-cell proliferation, unlike supernatants containing the other substitution mutants. Purified preparations of SEA-N25G had weak activity and those of SEA-F47G and SEA-L48G had essentially no activity in the T-cell proliferation assay. All mutants except SEA-V85G, which was degraded by monkey stomach lavage fluid in vitro, were tested for emetic activity. SEA-C106A and two SEB mutants, SEB-D9N/N23D and SEB-F44S (previously referred to as BR-257 and BR-358, respectively), whose construction and altered immunological properties have been reported previously, were also tested in the emetic assay. Each mutant was initially administered intragastrically at doses of 75 to 100 micrograms per animal; if none of the animals responded, the dose was increased four-to fivefold. SEA-F47G, SEA-C106A, and SEB-D9N/N23D were the only mutants that did not induce vomiting at either dose tested; these three mutants had reduced immunological activity. However, there was not a perfect correlation between immunological and emetic activities; SEA-L48G and SEB-F44S retained emetic activity, although they had essentially no T-cell-stimulatory activity. These studies suggest that these two activities can be dissociated.

DNA sequence and units of transcription of the conjugative transfer gene complex (trs) of Staphylococcus aureus plasmid pGO1

The conjugative transfer genes of 52-kb staphylococcal R plasmid pGO1 were localized to a single BglII restriction fragment and cloned in Escherichia coli. Sequence analysis of the 13,612-base transfer region, designated trs, identified 14 intact open reading frames (ORFs), 13 of which were transcribed in the same direction. Each ORF identified was preceded by a typical staphylococcal ribosomal binding sequence, and 10 of the 14 proteins predicted to be encoded by these ORFs were seen when an E. coli in vitro transcription-translation system was used. Functional transcription units were identified in a Staphylococcus aureus host by complementation of Tn917 inserts that abolished transfer and by Northern (RNA) blot analysis of pGO1 mRNA transcripts. These studies identified three complementation groups (trsA through trsC, trsD through trsK, and trsL-trsM) and four mRNA transcripts (trsA through trsC [1.8 kb], trsA-trsB [1.3 kb], trsL-trsM [1.5 kb], and trsN [400 bases]). No definite mRNA transcript was seen for the largest complementation group, trsD through trsK (10 kb). Comparison of predicted trs-encoded amino acid sequences to those in the data base showed 20% identity of trsK to three related genes necessary for conjugative transfer of plasmids in gram-negative species and 32% identity of trsC to a gene required for conjugative mobilization of plasmid pC221 from staphylococci.

Staphylococcal enterotoxin type A internal deletion mutants: serological activity and induction of T-cell proliferation

Previous findings indicate that the N-terminal region of staphylococcal enterotoxin type A (SEA) is required for its ability to induce T-cell proliferation. To better localize internal peptides of SEA that are important for induction of murine T-cell proliferation, SEA mutants that had internal deletions in their N-terminal third were constructed. A series of unique restriction enzyme sites were first engineered into sea; only one of these changes resulted in an amino acid substitution (the aspartic acid residue at position 60 of mature SEA was changed to a glycine [D60G]). Because the D60G substitution had no discernible effect on serological or biological activity, the sea allele encoding this mutant SEA was used to construct a panel of mutant SEAs lacking residues 3 to 17, 19 to 23, 24 to 28, 29 to 49, 50 to 55, 56 to 59, 61 to 73, 68 to 74, or 74 to 85. Recombinant plasmids with the desired mutations were constructed in Escherichia coli and transferred to Staphylococcus aureus. Staphylococcal culture supernatants containing the mutant SEAs were examined. Western immunoblot analysis with polyclonal anti-SEA antiserum revealed that each of the recombinant S. aureus strains produced a mutant SEA of the predicted size. All the mutant SEAs exhibited increased sensitivity to monkey stomach lavage fluid in vitro, which is consistent with these mutants having conformations unlike that of wild-type SEA or the SEA D60G mutant. In general, deletion of internal peptides had a deleterious effect on the ability to induce T-cell proliferation; only SEA mutants lacking either residues 3 to 17 or 56 to 59 consistently produced a statistically significant increase in the incorporation of [3H]thymidine. In the course of this work, two monoclonal antibodies that had different requirements for binding to SEA in Western blots were identified. The epitope for one monoclonal antibody was contained within residues 108 to 230 of mature SEA. Binding of the other monoclonal antibody to SEA appeared to be dependent on the conformation of SEA.

Mutations defining functional regions of the superantigen staphylococcal enterotoxin B

Staphylococcal enterotoxin B (SEB) is both a superantigen and toxin. As a superantigen, SEB can bind to major histocompatibility complex (MHC) class II molecules to form a ligand for alpha/beta T cell receptors bearing particular V beta elements. As a toxin, SEB causes rapid weight loss in mice sometimes leading to death. We show here that both of these functions map to the NH2-terminal portion of the toxin. Three regions were identified: one important in MHC class II binding, one in T cell recognition, and one in both functions. These results support the conclusion that the toxicity of SEB is related to massive T cell stimulation and release of cytokine mediators and show that the residues interacting with MHC and the T cell receptor are intertwined.

The carboxyl-terminal region of staphylococcal enterotoxin type A is required for a fully active molecule

Staphylococcal enterotoxin type A (SEA) gene (sea+) mutations were constructed by exonuclease III digestion or cassette mutagenesis. Five different sea mutations that had 1, 3, 7, 39, and 65 codons deleted from the 3' end of sea+ were identified and confirmed by restriction enzyme and nucleotide sequence analyses. Each of these sea mutations was constructed in Escherichia coli and transferred to Staphylococcus aureus by using the plasmid vector pC194. Culture supernatants from the parent S. aureus strain that lacked an enterotoxin gene (negative controls) and from derivatives that contained either sea+ (positive control) or a sea mutation were examined for in vitro sensitivity to degradation by monkey stomach lavage fluid, the ability to cause emesis when administered by an intragastric route to rhesus monkeys, and the ability to induce T-cell proliferation and by Western immunoblot analysis and a gel double-diffusion assay with polyclonal antibodies prepared against SEA. Altered SEAs corresponding to the predicted sizes were visualized by Western blot analysis of culture supernatants for each of the staphylococcal derivatives that contained a sea mutation. The altered SEA that lacked the C-terminal amino acid residue behaved like SEA in all of the assays performed. The altered SEA that lacked the three C-terminal residues of SEA caused T-cell proliferation but was not emetic; this altered SEA was degraded in vitro by monkey stomach lavage fluid and did not reach in the gel double diffusion assay. Altered SEAs that lacked 7, 39, or 65 carboxyl-terminal residues were degraded by stomach lavage fluid in vitro, did not produce an emetic response, and did not induce T-cell proliferation or form a visible reaction in the gel double-diffusion assay.

Role of host factors in the regulation of the enterotoxin B gene

The levels of staphylococcal enterotoxin B (SEB) produced by various naturally occurring toxinogenic strains of Staphylococcus aureus are highly variable. The SEB gene (seb) from a high-producer strain, S6, has previously been cloned and characterized. Cloning and nucleotide sequence analysis of the upstream region of the seb gene from DU4916 and COL (medium- and low-level toxin-producer strains, respectively) showed that their sequence was identical to that of the seb gene from strain S6. Strains carrying the cloned seb gene from DU4916 and COL produced similar levels of SEB protein and mRNA to those produced by strains carrying the cloned seb gene from strain S6. An RNA encoded by the delta-lysin gene (hld) has been shown to regulate the genes for a number of extracellular proteins, including SEB. Northern (RNA) blot analysis showed that variable levels of hld RNA were present in various SEB-producer strains, with the order being S6 greater than DU4916 greater than COL. Our results suggest that differences in host factor(s), including the hld RNA, are responsible for the production of different amounts of SEB by many naturally occurring strains.

Staphylococcal and streptococcal pyrogenic toxins involved in toxic shock syndrome and related illnesses

Toxic-shock syndrome (TSS) is an acute onset, multiorgan illness which resembles severe scarlet fever. The illness is caused by Staphylococcus aureus strains that express TSS toxin-1 (TSST-1), enterotoxin B, or enterotoxin C. TSST-1 is associated with menstrual TSS and approximately one-half of nonmenstrual cases; the other two toxins cause nonmenstrual cases, 47% and 3%, respectively. The three toxins are expressed in culture media under similar environmental conditions. These conditions may explain the association of certain tampons with menstrual TSS. Biochemically, the toxins are all relatively low molecular weight and fairly heat and protease stable. Enterotoxins B and C, share nearly 50% sequence homology with streptococcal scarlet fever toxin A; they share no homology with TSST-1 despite sharing numerous biological properties. Numerous animal models for development of TSS have suggested mechanisms of toxin action, though the exact molecular action is not known. The toxins are all potent pyrogens, induce T lymphocyte proliferation, requiring interleukin 1 release from macrophages, suppress immunoglobulin production, enhance endotoxin shock, and enhance hypersensitivity skin reactions. The genetic control of the toxins has been studied and suggests the exotoxins are variable traits. Some additional properties of TSS S. aureus which facilitate disease causation have been clarified.

Production of enterotoxin A by supposedly nonenterotoxigenic Staphylococcus aureus strains

The production of staphylococcal enterotoxins A (SEA) and B (SEB) was studied by inoculating six well-defined staphylococcal collection strains into cow's, goat's, or sheep's milk (individually or as a 50% mixture of cow's + goat's or cow's + sheep's), into brain heart infusion, and into a medium generally used to enhance the synthesis of enterotoxins (3+3 medium). Four of the strains used are considered to be SEB producers, another is considered an SEA producer, and the remaining strain is nonenterotoxigenic but produces large quantities of staphylococcal protein A. Staphylococcal protein A masked the results in most cases. Only one strain secreted exclusively SEB, while the other three SEB producers synthesized SEA in different amounts. We conclude that enterotoxin production depends on the natural substrate and may differ from the results obtained when the strain is grown on cellophane over agar to determine its toxigenicity.

Rapid purification of staphylococcal enterotoxin B by high-pressure liquid chromatography

The Staphylococcus aureus enterotoxins represent a group of proteins that cause emesis and diarrhea in humans and other primates. We have developed a rapid two-step high-pressure liquid chromatography (HPLC) procedure for purification of staphylococcal enterotoxin B (SEB). Sterile filtrates (2.5 liters) of strain 10-275 were adsorbed directly onto a reversed-phase column (50 mm by 30 cm Delta Pak; 300 A [30 nm], 15 microns, C18). SEB was obtained by using a unique sequential gradient system. First, an aqueous ammonium acetate to acetonitrile gradient followed by an aqueous trifluoroacetic acid (TFA) wash was used to remove contaminants. A subsequent TFA to acetonitrile-TFA gradient eluted the bound SEB. Further purification was obtained by rechromatography on a cation-exchange column. From 35 to 45% of the SEB in starting filtrates was recovered. Analysis by immunoblotting of samples separated on sodium dodecyl sulfate-polyacrylamide gels indicated that HPLC-purified SEB exhibited immunological and biochemical properties similar to those of the SEB standard. Induction of an emetic response in rhesus monkeys showed that the HPLC-purified toxin also retained biological activity.

Biological and immunological properties of the carboxyl terminus of staphylococcal enterotoxin C1

Comparisons of recently published primary sequences of staphylococcal and streptococcal pyrogenic toxins prompted an evaluation of biological and immunological properties of the C terminus of staphylococcal enterotoxin C1. The 59 N-terminal amino acids were deleted from the toxin by digestion with trypsin. The resulting fragment (Mr, 20,659) contained the remaining 180 C-terminal residues. This fragment (Trp F1) consisted of two polypeptide chains (Trp F1a and Trp F1b) linked by cysteine residues. Trp F1 was mitogenic, pyrogenic, and enhanced susceptibility of rabbits to lethal endotoxin shock. In addition, this fragment contained at least one antigenic epitope that cross-reacted with enterotoxin B.

Construction, transfer and properties of a novel temperature-sensitive integrable plasmid for genomic analysis of Staphylococcus aureus

As an alternative approach to genetic transfer and analysis, a novel integrable plasmid system was developed that should prove useful for mapping and cloning various genes in Staphylococcus aureus and other Gram-positive bacteria. The use of a restriction-deficient recipient strain and an improved protocol for protoplast plasmid transformation facilitated direct cloning of a recombinant plasmid (pPQ126) in S. aureus NCTC 8325-4. Plasmid pPQ126 (13.6 kb) is a novel, temperature-sensitive integrable plasmid containing genes encoding resistance to erythromycin and chloramphenicol (from plasmid pTV1ts), and resistance to gentamicin (from transposon Tn4001). When introduced into an appropriate recipient strain at the permissive temperature (30 degrees C), pPQ126 replicates autonomously. Integration of pPQ126 is directed into homologous chromosomal target sequences (chromosomal insertions of Tn551 or Tn4001) by growing a population of cells containing autonomous pPQ126 in the presence of gentamicin, erythromycin, and chloramphenicol at 39 degrees C (nonpermissive temperature). Elevated temperature both selects for and maintains pPQ126 as an integrated replicon. Integration of pPQ126 occurs at significantly reduced frequency in a recombination-deficient host, and does not occur in the absence of host chromosomal homology. Integrated pPQ126 excises from the chromosome under permissive conditions (30 degrees C), and excision results in derivatives of pPQ126 that harbour DNA of chromosomal origin.

Secretion and processing of the Bacillus subtilis endo-beta-1,3-1,4-glucanase in Escherichia coli

The endo-beta-1,3-1,4-glucanase enzyme of Bacillus subtilis C120, when synthesized in Escherichia coli, is located mainly in the cytoplasm, but enzyme activity is also detected in the periplasmic space and in the extracellular medium. The proportion recovered in the extracellular medium is not altered by changes in the levels of synthesis of the enzyme. Lysis of E. coli cells is ruled out as the cause of the secretion by the normal localization of beta-galactosidase, an intracellular protein. However, beta-lactamase, which is normally found in the periplasmic space, is detected in the extracellular medium of E. coli transformants containing beta-glucanase plasmids, suggesting that the presence of beta-glucanase in the cell alters the permeability of the outer membrane. The beta-glucanase proteins found in the extracellular medium, the periplasmic space and the cytoplasm have the same electrophoretic mobilities as the secreted enzyme of B. subtilis. Amino-terminal sequencing has shown that the beta-glucanase enzyme in the intracellular fraction of E. coli is processed at a site two amino acids distant from the processing site used in B. subtilis.

Staphylococcal enterotoxin B gene is associated with a discrete genetic element

The chromosomal location of the enterotoxin B gene in Staphylococcus aureus is unknown. Southern hybridization analysis of the chromosomal DNA from several enterotoxin B (SEB)-producing strains has shown that at least 26.8 kilobases (kb) of DNA is associated with the enterotoxin B gene (entB). We have found that one end of the entB element is located approximately 1.5 kb downstream of the entB gene. The chromosomal region adjacent to this end of the entB element was found to be homologous in several SEB-producing (SEB+) and SEB-nonproducing (SEB-) S. aureus strains. The chromosomes of all the SEB+ strains studied were homologous for at least 24 kb upstream of the entB gene. Some naturally occurring SEB- strains lacked the entire entB element, while others showed variable homology to the region upstream of the entB gene. These data suggest that the entB gene is part of a discrete genetic element that is at least 26.8 kb in size.

Nucleotide sequence of streptococcal pyrogenic exotoxin type C

The nucleotide sequence of the gene speC, encoding streptococcal pyrogenic exotoxin type C (SPE C), was determined. The gene encoded a mature protein of 208 amino acids, with a calculated molecular weight of 24,354. The mature amino acid sequence of SPE C was analyzed for homology with the amino acid sequences of streptococcal pyrogenic exotoxin type A, the staphylococcal enterotoxins, and toxic shock syndrome toxin-1. Of these, SPE C shared the greatest amount of homology with streptococcal exotoxin type A.

Cloning of the gene, speB, for streptococcal pyrogenic exotoxin type B in Escherichia coli

The structural gene encoding streptococcal pyrogenic exotoxin type B, designated speB, was cloned in Escherichia coli and localized onto a 4.5-kilobase BamHI-BglII DNA fragment. Streptococcal pyrogenic exotoxin type B, partially purified from E. coli clones, was immunologically related to streptococcus-derived toxin. Also, toxin derived from either E. coli or Streptococcus pyogenes had similar lymphocyte mitogenic activity and molecular weight (29,300) and displayed comparable microheterogeneity when evaluated by isoelectric focusing.

Nucleotide sequence of the type A staphylococcal enterotoxin gene

We determined the nucleotide sequence of the gene encoding staphylococcal enterotoxin A (entA). The gene, composed of 771 base pairs, encodes an enterotoxin A precursor of 257 amino acid residues. A 24-residue N-terminal hydrophobic leader sequence is apparently processed, yielding the mature form of staphylococcal enterotoxin A (Mr, 27,100). Mature enterotoxin A has 82, 72, 74, and 34 amino acid residues in common with staphylococcal enterotoxins B and C1, type A streptococcal exotoxin, and toxic shock syndrome toxin 1, respectively. This level of homology was determined to be significant based on the results of computer analysis and biological considerations. DNA sequence homology between the entA gene and genes encoding other types of staphylococcal enterotoxins was examined by DNA-DNA hybridization analysis with probes derived from the entA gene. A 624-base-pair DNA probe that represented an internal fragment of the entA gene hybridized well to DNA isolated from EntE+ strains and some EntA+ strains. In contrast, a 17-base oligonucleotide probe that encoded a peptide conserved among staphylococcal enterotoxins A, B, and C1 hybridized well to DNA isolated from EntA+, EntB+, EntC1+, and EntD+ strains. These hybridization results indicate that considerable sequence divergence has occurred within this family of exotoxins.

Location of the coagulase gene in Staphylococcus aureus

The localization of the coagulase gene in Staphylococcus aureus JM 750 strain was investigated. S. aureus JM 750 strain in contrast to other S. aureus strains shows instability of coagulase production. This strain contains the 23.5 kb penicillinase plasmid, determining beta-lactamase synthesis and resistance to heavy metal salts, and a larger plasmid greater than 100 kb in size, which was detectable only in coagulase positive variants. The results suggest the unparalleled, extrachromosomal localization of the coagulase gene in S. aureus JM 750 strain.

Sequence determination and comparison of the exfoliative toxin A and toxin B genes from Staphylococcus aureus

The DNA encoding the exfoliative toxin A gene (eta) of Staphylococcus aureus was cloned into bacteriophage lambda gt11 and subsequently into plasmid pLI50 on a 1,391-base-pair DNA fragment of the chromosome. Exfoliative toxin A is expressed in the Escherichia coli genetic background, is similar in length to the toxin purified from culture medium, and is biologically active in an animal assay. The nucleotide sequence of the DNA fragment containing the gene was determined. The protein deduced from the nucleotide sequence is a polypeptide of 280 amino acids. The mature protein is 242 amino acids. The DNA sequence of the exfoliative toxin B gene was also determined. Corrections indicate that the amino acid sequence of exfoliative toxin B is in accord with chemical sequence data.

Expression of staphylococcal enterotoxin C1 in Escherichia coli

The structural gene encoding staphylococcal enterotoxin C1 was cloned into Escherichia coli and localized on a 1.5-kilobase HindIII-ClaI DNA fragment by subcloning. The toxin was partially purified from E. coli clones and shown to be immunologically identical to enterotoxin C1 from Staphylococcus aureus. The cloned toxin also had the same molecular weight (26,000) and charge heterogeneity as staphylococcus-derived enterotoxin. Toxins from both sources were equally biologically active.

Sequence of the exfoliative toxin B gene of Staphylococcus aureus

We sequenced the Staphylococcus aureus exfoliative toxin B gene contained on a 1.7-kilobase HindIII fragment of plasmid pRW001. The gene was located by comparison of the amino acid sequences of open reading frames with the amino-terminal sequence of exfoliative toxin B and the total amino acid composition of the protein (A.D. Johnson, L. Spero, J.S. Cades, and B.T. De Cicco, Infect. Immun. 24:679-684, 1979). The primary translation product consists of 274 amino acids and contains a 31-amino-acid N-terminal peptide presumably necessary for transport.

Lysogenic conversion of staphylococcal lipase is caused by insertion of the bacteriophage L54a genome into the lipase structural gene

Staphylococcus aureus PS54 manifests no lipase (geh) activity. This is due to the insertion of bacteriophage L54a DNA into the geh structural gene. The nucleotide sequence of this 2,968-base-pair DNA fragment was determined. Lipase deduced from the nucleotide sequence is a polypeptide of 690 amino acids which extends from nucleotide 706 to 2776.

Cloning and expression of the exfoliative toxin B gene from Staphylococcus aureus

Exfoliative toxin type B is produced by bacteriophage group II strains of Staphylococcus aureus and is a causative agent of staphylococcal scalded-skin syndrome. In addition to exfoliative toxin B, most isolates also produce a bacteriocin and are immune to the action of the bacteriocin. These phenotypes, as well as resistance to cadmium, were lost after elimination of a 37.5-kilobase plasmid, pRW001, from S. aureus UT0007. Transduction and transformation showed that pRW001 carries the structural genes for four phenotypic characteristics of S. aureus UT0007: (i) exfoliative toxin B production, (ii) bacteriocin production, (iii) bacteriocin immunity, and (iv) resistance to Cd(NO3)2. The exfoliative toxin B structural gene (etb), which is located on a 1.7-kilobase HindIII fragment of pRW001, was cloned in the plasmid pDH5060 and transformed into phage group III S. aureus RN4220. Transformant clones produced extracellular exfoliative toxin B that was biologically active in the neonatal mouse assay. In the Escherichia coli genetic background, the exfoliative toxin B gene was expressed only after being cloned into the positive selection-expression vector pSCC31. The structural gene for cadmium resistance was also isolated on an HindIII fragment of pRW001 cloned in pDH5060. The loci for the exfoliative toxin B gene and the cadmium resistance gene(s) were identified on a restriction map of plasmid pRW001.

Nucleotide sequence of the enterotoxin B gene from Staphylococcus aureus

The complete nucleotide sequence of the enterotoxin B gene from Staphylococcus aureus, as well as the 5' and 3' flanking regions, was determined. Starting from an ATG initiator codon, an open reading frame encoded the enterotoxin B precursor that consisted of 266 amino acids (Mr, 31,400). The 5' terminal portion of the gene encodes a signal peptide 27 amino acids long. The deduced amino acid sequence matched, with a few exceptions, the published amino acid sequence of enterotoxin B. The structural gene was flanked on the 5' side by a promoter-like sequence and on the 3' side by a palindromic structure followed by a thymine-rich region that resembled a transcription termination signal. Downstream from the entB gene were two overlapping open reading frames corresponding to 134 and 185 amino acids in the opposite orientation. The signal sequence of the enterotoxin B precursor resembled that of other secreted proteins found in other bacteria.