Inactivation of the alpha-haemolysin gene of Staphylococcus aureus 8325-4 by site-directed mutagenesis and studies on the expression of its haemolysins

Genetic evidence that bound coagulase of Staphylococcus aureus is not clumping factor

Staphylococcus aureus Newman cells carry a surface receptor for fibrinogen called clumping factor. The bacteria also express coagulase, an extracellular protein that binds to prothrombin to form a complex with thrombinlike activity which coverts fibrinogen to fibrin. We have confirmed a recent report (M. K. Bodén and J.-I. Flock, Infect. Immun. 57:2358-2363, 1989) that coagulase can bind to fibrinogen as well as to prothrombin and also that a fraction of coagulase is firmly attached to the cell. A mutant with a deletion in the chromosomal coa gene was isolated by allelic replacement. Allelic replacement either was directly selected by electrotransformation of S. aureus R3N4220 with a nonreplicating suicide plasmid, pCOA18, carrying the delta coa::Tcr mutation or occurred after transduction of the integrated pCOA18 plasmid. The coa mutant was completely devoid of coagulase activity but interacted both with soluble fibrinogen and with solid-phase fibrinogen with the same avidity as the parental strain. This strongly suggests that the bound form of coagulase is not clumping factor and is not responsible for the adherence of S. aureus Newman to solid-phase fibrinogen. The fibrinogen binding determinant of coagulase was located in the C terminus of the protein, by analyzing truncated fusion proteins, in contrast to the prothrombin-binding region which was located in the N terminus.

Alpha-toxin of Staphylococcus aureus

Alpha-toxin, the major cytotoxic agent elaborated by Staphylococcus aureus, was the first bacterial exotoxin to be identified as a pore former. The protein is secreted as a single-chain, water-soluble molecule of Mr 33,000. At low concentrations (less than 100 nM), the toxin binds to as yet unidentified, high-affinity acceptor sites that have been detected on a variety of cells including rabbit erythrocytes, human platelets, monocytes and endothelial cells. At high concentrations, the toxin additionally binds via nonspecific absorption to lipid bilayers; it can thus damage both cells lacking significant numbers of the acceptor and protein-free artificial lipid bilayers. Membrane damage occurs in both cases after membrane-bound toxin molecules collide via lateral diffusion to form ring-structured hexamers. The latter insert spontaneously into the lipid bilayer to form discrete transmembrane pores of effective diameter 1 to 2 nm. A hypothetical model is advanced in which the pore is lined by amphiphilic beta-sheets, one surface of which interacts with lipids whereas the other repels apolar membrane constitutents to force open an aqueous passage. The detrimental effects of alpha-toxin are due not only to the death of susceptible targets, but also to the presence of secondary cellular reactions that can be triggered via Ca2+ influx through the pores. Well-studied phenomena include the stimulation of arachidonic acid metabolism, triggering of granule exocytosis, and contractile dysfunction. Such processes cause profound long-range disturbances such as development of pulmonary edema and promotion of blood coagulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Cryptic alpha-toxin gene in toxic shock syndrome and septicaemia strains of Staphylococcus aureus

The majority of clinical isolates of Staphylococcus aureus that produce toxic shock syndrome toxin-1 (TSST-1) fail to express alpha-toxin, despite having a copy of the hla gene in the chromosome. The hla gene was cloned from an Hla- TSST-1+ strain, Todd 555, which had been isolated from a case of toxic shock syndrome in the USA. Of the 630 bases of the Todd 555 gene sequenced, 46 differed from the hla gene sequence of strain Wood 46. The defect in alpha-toxin expression was shown to be due to a nonsense mutation which converted a CAG glutamine codon in the equivalent position in the functional Wood 46 sequence to a TAG stop codon. The same mutation was present in the hla gene cloned from a human septicaemia strain (V37) isolated in Dublin. The nonsense mutation of Todd 555 was suppressed by the supE44 mutation in Escherichia coli resulting in haemolytic activity in cell lysates. Hybrid hla genes were formed by splicing fragments of hla from Todd 555 and Wood 46. Expression of one such chimaeric hla gene in S. aureus demonstrated that the Todd 555 hla gene has a functional agr-regulated promoter. The silent hla gene may be a cryptic gene in S. aureus.

The coagulase of Staphylococcus aureus 8325-4. Sequence analysis and virulence of site-specific coagulase-deficient mutants

The sequence of the coagulase gene (coa) from Staphylococcus aureus strain 8325-4 is reported. The deduced amino acid sequence of the coagulase protein is compared with previously reported sequences of coagulases from strains 213 and BB. The secreted mature forms of coagulase proteins are composed of three distinct segments: (i) the N-terminal 150-270 residues, which are c. 50% identical, (ii) a central region with high (greater than 90%) residue identities, and (iii) a C-terminal region composed of repeated 27-amino-acid residue sequences. The variable N-terminal sequences are probably responsible for antigenic differences among coagulases of different serotype. The region of coagulase which binds to prothrombin and activates it to form staphylothrombin is also located in the N-terminal half of the protein. A site-specific substitution mutation in the coa gene, which abolished plasma clotting activity, was isolated by recombinational allele-replacement in strains 8325-4 and M60. The Coa- mutants did not show diminished virulence in subcutaneous and intramammary infections of mice. No evidence for a role for coagulase in virulence of toxigenic or nontoxigenic strains was obtained. This contradicts findings of several groups using Coa- mutants generated by chemical mutagenesis and suggests that the earlier results were obtained with strains that had suffered additional mutations in virulence-related genes.

Reaction of staphylococcal alpha-toxin with peptide-induced antibodies

Two peptides representing separate 13-amino-acid sequences of staphylococcal alpha-toxin have been synthesized and acrylamide gel-purified alpha-toxin monomer and hexamer forms have been prepared and used to produce antisera in rabbits. We report here that each synthetic peptide, P-I and P-II, induces the formation of a specific precipitating antiserum. Moreover, these sera also react with the toxin monomer and sometimes with the hexamer, indicating that each peptide has more than one epitope. The purified toxin monomer can induce antibodies to fragments of toxin but is significantly less potent than the hexamer in inducing antibodies to the toxin monomer and almost not effective in inducing a response to the toxin hexamer. The purified toxin hexamer induces responses that are almost the reciprocals of the monomers, with the antihexamer and -monomer responses dominating and almost no responses to fragments of toxin being induced. These responses are interpreted in terms of the stability of the toxin hexamer to proteolytic degradation, compared with the relative sensitivity of the monomer to proteases. In assays of toxin-neutralization activity, only those sera containing antihexamer antibodies can block toxin hemolytic activity. This is true for both peptide- and toxin-induced antisera. The basis for this apparent association between toxin-neutralizing potency and antihexamer reactivity is being studied. Peptide P-I contains the uniquely reactive tyrosine residue and may be involved in monomer-to-monomer associations required to form hexamers. Peptide P-II is near the carboxyl terminus of alpha-toxin and may be involved in the binding of toxin to membranes. In a study of the ability of each peptide to inhibit the rate of hexamer formation induced by membrane lipoprotein, peptide P-I (as expected) proves to be more efficient than peptide P-II. Finally, one rabbit immunized with the toxin hexamer produces antibodies to peptides P-I and P-II. This finding suggests that the two synthetic peptides selected for study are relevant to the in vivo immunoprocessing of staphylococcal alpha-toxin.

Membrane transport and disease

Four situations in which membrane transport is altered by disease are discussed: (a) non-specific leaks induced by poreforming agents; (b) glucose transport and cellular stress; (c) Ca2+-ATPase and hypertension; (d) Na+ channels and HSV infection.

Release of interleukin-1 beta associated with potent cytocidal action of staphylococcal alpha-toxin on human monocytes

The pathogenetic relevance of Staphylococcus aureus alpha-toxin in humans has been debated because human cells have been thought to display a natural resistance toward the cytotoxic action of this cytolysin. Following our previous demonstration that human platelets represent sensitive targets for toxin attack, we have now identified monocytes as a second, highly vulnerable human cell species that succumb to attack by low doses (20 ng/ml) of alpha-toxin. The cytotoxic action of alpha-toxin is reflected in a rapid depletion of cellular ATP that is essentially complete within 30 min. The presence of human plasma proteins affords some protection of monocytes against the action of the toxin. In 10% autologous serum, ATP depletion commences at 80 to 300 ng of toxin per ml. Subcytolytic doses stimulate the release of tumor necrosis factor alpha, a process that is slightly accentuated in the presence of 50% serum. Cytocidal toxin doses unfailingly cause the release of large amounts of interleukin-1 beta from cultured cells, with levels of this monokine generally exceeding 10 ng/ml in the cell supernatants 60 min after application of toxin. Initial evidence suggests that this is due to processing of intracellular interleukin-1 rather than to de novo synthesis of the cytokine. All noted effects are abrogated in the presence of a neutralizing monoclonal antibody against alpha-toxin. Through its capacity to provoke cytokine release from monocytes and its attack on platelets, alpha-toxin may initiate cellular events that are relevant to the pathogenesis of staphylococcal infection.

Human hyperimmune globulin protects against the cytotoxic action of staphylococcal alpha-toxin in vitro and in vivo

Alpha-toxin, the major cytolysin of Staphylococcus aureus, preferentially attacks human platelets and cultured monocytes, thereby promoting coagulation and the release of interleukin-1 and tumor necrosis factor. Titers of naturally occurring antibodies in human blood are not high enough to substantially inhibit these pathological reactions. In the present study, F(ab')2 fragment preparations from hyperimmune globulin obtained from immunized volunteers were tested for their capacity to inhibit the cytotoxic action of alpha-toxin in vitro and in vivo. These antibody preparations exhibited neutralizing anti-alpha-toxin titers of 80 to 120 IU/ml, whereas titers in commercial immunoglobulin preparations were 1 to 4 IU/ml. In vitro, the presence of 2 to 4 mg of hyperimmune globulin per ml protected human platelets against the action of 1 to 2 micrograms of alpha-toxin per ml. Similarly, these antibodies fully protected human monocytes against the ATP-depleting and cytokine-liberating effects of 0.1 to 1 microgram of alpha-toxin per ml. Intravenous application of 0.5 mg (85 to 120 micrograms/kg of body weight) of alpha-toxin in cynomolgus monkeys elicited acute pathophysiological reactions which were heralded by a selective drop in blood platelet counts. Toxin doses of 1 to 2 mg (170 to 425 micrograms/kg) had a rapid lethal effect, the animals presenting with signs of cardiovascular collapse and pulmonary edema. Prior intravenous application of 4 ml of hyperimmune globulins per kg inhibited the systemic toxic and lethal effects of 1 mg (200 micrograms/kg) of alpha-toxin. In contrast, normal human immunoglobulins exhibited no substantial protective efficacy in vitro and only marginal effects in vivo. It is concluded that high-titered anti-alpha-toxin antibodies effectively protect against the cytotoxic actions of alpha-toxin.

Genetic and molecular analyses of the gene encoding staphylococcal enterotoxin D

The gene (entD) encoding staphylococcal enterotoxin D (SED) has been located on a 27.6-kilobase penicillinase plasmid designated pIB485. This plasmid was present in all SED-producing strains tested. The entD gene was cloned on a 2.0-kilobase DNA fragment and was expressed in Escherichia coli. Sequence analysis of this fragment revealed an open reading frame that encoded a 258-amino-acid protein that possessed a 30-amino-acid signal peptide. The 228-amino-acid mature polypeptide had a molecular weight of 26,360 and contained a high degree of sequence similarity to the other staphylococcal enterotoxins. S1 nuclease mapping showed that transcription of entD was initiated 266 nucleotides upstream from the translation start codon. The entD gene was also shown to be activated by the staphylococcal regulatory element known as agr.

Roles of alpha-toxin and beta-toxin in virulence of Staphylococcus aureus for the mouse mammary gland

Mutants of Staphylococcus aureus which fail to express alpha-toxin (Hly), beta-toxin (Hlb), or both have been constructed by site-specific mutagenesis. The virulence of the mutants was compared with that of wild-type toxigenic strains by intramammary inoculation of lactating mice. A bovine strain, M60, and a laboratory strain, 8325-4, caused acute mastitis and death within 48 h for 60% of the mice inoculated. Animals inoculated with Hly mutants also developed acute mastitis, but no deaths occurred. Comparisons of Hly- or Hlb-positive strains with the double mutation Hly Hlb showed that both toxins led to a significantly higher recovery of S. aureus from the gland 48 h postinfection. Histopathological examination of mammary glands showed that phagocytosis of bacteria occurred irrespective of toxigenicity, but toxigenic strains, particularly those which were Hly+, continued to multiply, invaded the interalveolar tissues, and produced severe lesions. Stimulation of an inflammatory response by inoculation of the mammary gland with endotoxin prior to challenge with S. aureus reduced recovery of the bacteria 10- to 100-fold and, under these conditions, neither alpha-toxin nor beta-toxin contributed significantly to growth and survival.

Cloning, characterization, and sequencing of an accessory gene regulator (agr) in Staphylococcus aureus

We have previously identified a gene in Staphylococcus aureus, agr, whose activity is required for high-level post-exponential-phase expression of a series of secreted proteins. In this paper, we describe the cloning of this gene in Escherichia coli by using an inserted transposon (Tn551) as a cloning probe. The cloned gene, consisting of a 241-codon open reading frame containing the site of the transposon insertion, was recloned to an S. aureus vector, pSK265, and shown to be functional in S. aureus. Activity was evaluated by determinations of alpha-hemolysin, beta-hemolysin, and toxic shock syndrome toxin-1 production in early-stationary-phase cultures. The cloned gene showed considerable variation with respect to different exoproteins and different host strains compared with the chromosomal agr determinant; this variation could not be attributed to the higher copy number of the cloned gene and probably reflects inapparent subtleties of the regulatory system.

Genetic analysis of gentamicin resistance in methicillin- and gentamicin-resistant strains of Staphylococcus aureus isolated in Dublin hospitals

Methicillin- and gentamicin-resistant strains of Staphylococcus aureus isolated in Dublin hospitals have been classified into groups I, II, and III based on resistance to antimicrobial agents and plasmid profiles. Each group expresses a characteristic level of resistance to gentamicin, tobramycin, and sisomicin. Enzyme assays showed that resistant strains expressed 2"-aminoglycoside phosphotransferase-6'-aminoglycoside transferase activity by a determinant which is known to be chromosomally located. The gentamicin resistance (Gmr) determinants were transferred from group I, II, or III strains by transduction into a laboratory strain where each expressed the same low level of resistance. This finding suggests that high-level resistance in some clinical strains is due to a second, unlinked resistance mechanism. No evidence was obtained by hybridization experiments that clinical isolates or spontaneous mutants expressing high-level Gmr carried more than one copy of the Gmr determinant, thus eliminating the possibility that a gene dosage effect was responsible for high-level resistance. Hybridization experiments with transductants and wild strains suggested that the Gmr determinant was located at homologous sites in wild strains from different groups, although restriction site differences were observed in flanking sequences. Electron microscope analysis of a cloned Gmr determinant and genetic evidence suggested that a Dublin clinical isolate harbored a transposon very similar to Tn4001.

Staphylococcal alpha toxin promotes blood coagulation via attack on human platelets

Staphylococcus aureus plays a major role as a bacterial pathogen in human medicine, causing diseases that range from superficial skin and wound to systemic nosocomial infections . The majority of S. aureus strains produces a toxin, a proteinaceous exotoxin whose hemolytic, dermonecrotic, and lethal properties have long been known (1-6). The toxin is secreted as a single- chained, nonglycosylated polypeptide with a M(r) of 3.4 x 10(4) (7, 8). The protein spontaneously binds to lipid monolayers and bilayers (9-14), producing functional transmembrane pores that have been sized to 1.5-2.0-nm diameters (15-18). The majority of pores formed at high toxin concentrations (20 mug/ml) is visible in the electron microscope as circularized rings with central pores of approximately 2 nm in diameter. The rings have been isolated, and molecular weight determinations indicate that they represent hexamers of the native toxin (7). We have proposed that transmembrane leakiness is due to embedment of these ring structures in the bilayer, with molecular flux occurring through the central channels (15, 19). Pore formation is dissectable into two steps (20, 21). Toxin monomers first bind to the bilayer without invoking bilayer leakiness . Membrane-bound monomers then laterally diffuse and associate to form non-covalently bonded oligomers that generate the pores. When toxin pores form in membranes of nucleated cells, they may elicit detrimental secondary effects by serving as nonphysiologic calcium channels, influx of this cation triggering diverse reactions, including release of potent lipid mediators originating from the arachidonate cascade (22-24). That alpha toxin represents an important factor of staphylococcal pathogenicity has been clearly established in several models of animal infections through the use of genetically engineered bacterial strains deleted of an active alpha toxin gene (25-27). Whether the toxin is pathogenetically relevant in human disease, however, is a matter of continuing debate. Doubts surrounding this issue originate from two main findings. First, whereas 60 percent hemolysis of washed rabbit erythrocytes is effected by approximately 75 ng/ml alpha toxin, approximately 100-fold concentrations are required to effect similar lysis of human cells (4-6, 13). The general consensus is that human cells display a natural resistance towards toxin attack. The reason for the wide inter-species variations in susceptibility towards alpha toxin is unknown but does not seem to be due to the presence or absence of high-affinity binding sites on the respective target cells (20, 21). Second, low-density lipoprotein (28) and neutralizing antibodies present in plasma of all healthy human individuals inactivate a substantial fraction of alpha toxin in vitro. These inactivating mechanisms presumably further raise the concentration threshold required for effective toxin attack, and it is most unlikely that such high toxin levels will ever be encountered during infections in the human organism. The aforegoing arguments rest on the validity of two general assumptions. First, the noted natural resistance of human erythrocytes to alpha toxin must be exhibited by other human cells. Second, toxin neutralization by plasma components, usually tested and quantified after their preincubation with toxin in vitro, must be similarly effective under natural conditions, and protection afforded by these components must not be restricted to specific cell species.

Cytotoxic effects of ingested Staphylococcus aureus on bovine endothelial cells: role of S. aureus alpha-hemolysin

Previously we observed that Staphylococcus aureus phagocytized by cultured bovine endothelial cells do not proliferate intracellularly, but are cytotoxic to bovine endothelial cells. To investigate S. aureus virulence factors which may be produced intracellularly and cause lysis of endothelial cells, we tested S. aureus mutants defective in production of one or more potential virulence factors and corresponding parent strains for cytotoxicity to endothelial cell monolayers subsequent to being ingested. Following incubation of endothelial cell monolayers with S. aureus for 3.5 h, cultures were supplemented with lysostaphin to destroy extracellular but not intracellular S. aureus. At subsequent times, viability of endothelial cells was assayed by retention of 3H-adenine and the number of intracellular S. aureus was measured. The cytotoxic activity of S. aureus culture supernatants was also characterized. The results indicate that S. aureus alpha-hemolysin is cytotoxic to bovine endothelial cells and plays an important role in the damage suffered by bovine endothelial cell monolayers following ingestion of S. aureus. Ingestion of alpha-hemolysin-producing S. aureus by endothelial cells in vivo might be expected to result in destruction of endothelium followed by development of platelet-fibrin vegetations. This possible sequence of events is compatible with the frequently fulminant course of S. aureus endocarditis.

Production of gamma-hemolysin and lack of production of alpha-hemolysin by Staphylococcus aureus strains associated with toxic shock syndrome

The hemolytic activity of toxic shock syndrome isolates of Staphylococcus aureus is enhanced when agarose is substituted for agar in blood plates or when strains are grown in liquid culture in the presence of 20% (vol/vol) CO2 in air. Hemolytic activity of a representative panel of toxic shock syndrome isolates was rigorously assessed both on blood agar and in liquid culture to unequivocally identify the predominant hemolysins produced. As determined by isoelectric focusing and Western immunoblotting, 15 of 15 TSS isolates produced gamma-lysin and 10 of 15 produced delta-lysin. None produced beta-lysin, and only 2 of 15 produced alpha-lysin. The low rate of alpha-lysin production was a most striking characteristic, since all strains were found to have the alpha-lysin gene by Southern blot hybridization.

Oligomerization of 3H-labelled staphylococcal alpha-toxin and fragments on adrenocortical Y1 tumour cells

Staphylococcus aureus alpha-toxin has previously been shown to bind to erythrocyte membranes and the isolated membranes contain the toxin in both monomeric and hexameric form. The hexamers are believed to form the ring-shaped structures observed by electron microscopy on toxin-treated erythrocytes. It has not previously been shown that hexamers are formed also on nucleated mammalian cells although it has been assumed that hexamers in both systems create transmembrane channels, responsible for the toxin-induced membrane damage. Here we demonstrate by autoradiography that 3H-alpha-toxin bound to and formed high molecular weight complexes-presumably hexamers-on cultured adrenocortical Y1 tumour cells. The binding kinetics suggested a non-specific association of alpha-toxin with the membrane, rather than specific receptor-binding. The pH during toxin binding did not influence the subsequently induced membrane damage. Non-membrane damaging alpha-toxin fragment preparations also bound firmly to the cell membranes. Upon contact with Y1 cells the fragments formed complexes of the same apparent molecular size as those generated from intact alpha-toxin. Two interpretations are possible: either the fragment oligomers are somehow defective i.e. not able to form transmembrane structures or the functional relevance of toxin oligomerization for alpha-toxin-induced membrane damage must be questioned.

Production and characterization of monoclonal antibodies against Staphylococcus aureus alpha-toxin

Murine monoclonal antibodies against staphylococcal alpha-toxin were produced using a well-characterized alpha-toxin fragment preparation as immunizing agent. Three monoclonal antibodies were selected for anti-alpha-toxin activity in an ELISA using alpha-toxin as antigen. The monoclonal antibodies (MAbs) belonged to different immunoglobulin classes/subclasses and showed different abilities to neutralize the hemolytic, cell-membrane-damaging, dermonecrotizing and lethal action of alpha-toxin. One MAb was superior to mouse polyclonal antiserum in all test systems except for hemolysis, whereas another MAb neutralized essentially as the polyclonal serum. The third MAb did not neutralize the hemolytic or dermonecrotic effect but still inhibited the lethal and membrane-damaging effect of alpha-toxin. These results indicate that the three MAbs recognize different epitopes on the toxin molecule and that different biological activities might correspond to these epitopes.

Staphylococcal alpha toxin--recent advances

The elucidation of the amino acid sequence of alpha toxin in 1984 has greatly promoted our understanding of the basic biochemistry and interaction of this toxin with membranes. These aspects are discussed and the concept of alpha toxin as a channel forming protein is critically evaluated. The lethal action of alpha toxin has not yet been clarified, but the previously postulated action as a neurotoxin is not supported by recent observations.

Virulence of protein A-deficient and alpha-toxin-deficient mutants of Staphylococcus aureus isolated by allele replacement

The gene coding for protein A (spa) of Staphylococcus aureus 8325-4 has been inactivated by substituting part of the spa coding sequence for a DNA fragment specifying resistance to ethidium bromide. The in vitro-constructed spa::EtBrr substitution mutation was introduced into the S. aureus chromosome by recombinational allele replacement. Southern blot hybridization showed that the in vitro-constructed mutation was present in the chromosomal spa locus. We have previously reported the inactivation of the alpha-toxin gene (hly) by allele replacement with an in vitro-constructed hly::Emr (erythromycin resistance) mutation (M. O'Reilly, J.C.S. de Azavedo, S. Kennedy, and T.J. Foster, Microb. Pathogen. 1:125-138, 1986). A double Spa- Hly- mutant was constructed by transduction. The virulence of Spa- and Hly- mutants was tested by experimental infection of mice. When subcutaneous injections were given, Hly- mutants formed a flat, darkened lesion, whereas Hly+ strains caused a raised, cream lesion. Alpha-toxin was shown to be a major factor in forming subcutaneous lesions and in causing the death of mice injected intraperitoneally. Spa- mutants were slightly less virulent than their Spa+ counterparts, which suggests that protein A is also a virulence factor of S. aureus.

Quantitation of monomeric and oligomeric forms of membrane-bound staphylococcal alpha-toxin by enzyme-linked immunosorbent assay with a neutralizing monoclonal antibody

A murine monoclonal antibody generated against staphylococcal alpha-toxin was shown to react only with the monomeric (native), 3S form of the toxin. A sensitive sandwich enzyme-linked immunosorbent assay (ELISA) constructed with this antibody permitted detection of 0.25 to 0.5 ng of native toxin per ml. Toxin oligomers formed either by heat aggregation in solution, on target erythrocyte membranes, or on phosphatidylcholine-cholesterol liposomes were unreactive in the ELISA when membranes were solubilized with the nondenaturing detergent Triton X-100. After dissociation of the oligomers by boiling in sodium dodecyl sulfate, however, the ELISA reactivity of the liberated 3S toxin was fully restored. Parallel determinations of membrane-bound toxin with sodium dodecyl sulfate and Triton X-100 solubilization thus permitted direct quantitation of total and monomeric toxin, respectively; the difference between these two values was represented by toxin oligomers. The detection limits for membrane-bound oligomeric and monomeric toxin on erythrocyte membranes are in the order of 100 molecules and 1 molecule per cell, respectively. Using this ELISA, we show that over 90% of alpha-toxin molecules bound to target membranes at 37 degrees C are in oligomeric form. Evidence is given that the monoclonal antibody neutralizes alpha-toxin by inhibiting its binding to both rabbit and human erythrocytes. This ELISA is the first assay that quantitatively discriminates between mono- and oligomeric forms of a pore-forming protein on target cell membranes.

Molecular cloning and expression of the epidermolytic toxin A gene of Staphylococcus aureus

The gene coding for serotype A of epidermolytic (exfoliative) toxin has been cloned from Staphylococcus aureus in Escherichia coli phage lambda and plasmid vectors. The coding sequence for eta was localised by subcloning and transposon Tn5 mutagenesis experiments. The eta gene was probably expressed from its natural promoter in E. coli. The protein synthesised in E. coli was located predominantly in the periplasm. It was immunochemically indistinguishable from the toxin purified from S. aureus culture supernatants and had the same molecular weight. Furthermore, subcutaneous injection of this material caused epidermal splitting (the Nikolsky reaction) showing that it was biologically active. An eta shuttle plasmid was transformed into protoplasts of S. aureus. The level of expression of toxin in strain 8325-4 was shown to be dependent on the integrity of the agr gene which is known to be required for the expression of several exoproteins.

Chromosomal map location of the alpha-hemolysin structural gene in Staphylococcus aureus NCTC 8325

The alpha-hemolysin structural gene, hly+, previously cloned, insertionally inactivated, and introduced into the chromosome by allele replacement (M.O. O'Reilly, J.C.S. De Azavedo, S. Kennedy, and T.J. Foster, Microb. Pathog. 1:125-138, 1986), was shown by protoplast fusion and transformation to be in the gene order purC-hly-uraB-omega[chr::Tn916]1101-thrB on the chromosome of Staphylococcus aureus NCTC 8325. This location is clearly distinct from that of the agr determinant, a regulatory gene affecting several extracellular proteins, including alpha-hemolysin, located between tmn and ilv.