Nucleotide sequence of the streptolysin O (SLO) gene: structural homologies between SLO and other membrane-damaging, thiol-activated toxins

Virulence factors of Streptococcus anginosus - a molecular perspective

Streptococcus anginosus together with S. constellatus and S. intermedius constitute the Streptococcus anginosus group (SAG), until recently considered to be benign commensals of the human mucosa isolated predominantly from oral cavity, but also from upper respiratory, intestinal, and urogenital tracts. For years the virulence potential of SAG was underestimated, mainly due to complications in correct species identification and their assignment to the physiological microbiota. Still, SAG representatives have been associated with purulent infections at oral and non-oral sites resulting in abscesses formation and empyema. Also, life threatening blood infections caused by SAG have been reported. However, the understanding of SAG as potential pathogen is only fragmentary, albeit certain aspects of SAG infection seem sufficiently well described to deserve a systematic overview. In this review we summarize the current state of knowledge of the S. anginosus pathogenicity factors and their mechanisms of action.

Streptococcus pyogenes NAD+-Glycohydrolase Reduces Skeletal Muscle βNAD+ Levels Independently of Streptolysin O

Necrotizing soft tissue infections caused by Streptococcus pyogenes (group A streptococcus [GAS]) are characterized by rapid and extensive necrosis of fascia and muscle. Molecular epidemiological studies have demonstrated a positive correlation between GAS isolates that cause invasive infections and the production of S. pyogenes NAD+-glycohydrolase (SPN), an NADase secreted by GAS, but the effect of SPN on muscle cells has not been described. Thus, using standard βNAD+ and ATP quantification assays, we investigated the effects of SPN on cultured human skeletal muscle cell (SkMC) βNAD+ and ATP with and without streptolysin O (SLO)–a secreted cholesterol-dependent cytolysin known to act synergistically with SPN. We found that culture supernatants from GAS strains producing SLO and SPN depleted intracellular βNAD+ and ATP, while exotoxins from a GAS strain producing SLO and an enzymatically-inactive form of SPN had no effect on βNAD+ or ATP. Addition of purified, enzymatically-active SPN to NADase-negative culture supernatants or sterile media reconstituted βNAD+ depletion but had no effect ATP levels. Further, SPN-mediated βNAD+ depletion could be augmented by SLO or the homologous cholesterol-dependent cytolysin, perfringolysin O (PFO). Remarkably, SPN-mediated βNAD+ depletion was SkMC-specific, as purified SPN had minimal effect on epithelial cell βNAD+. Taken together, this study identifies a previously unrecognized role for SPN as a major disruptor of skeletal muscle βNAD+. Such activity could contribute to the rapid and widespread myonecrosis characteristic of severe GAS soft tissue infections.

Enrichment of Antigen-Specific Class-Switched B Cells from Individuals Naturally Immunized by Infection with Group A Streptococcus

The low frequency of circulating antigen-specific memory B cells is a considerable obstacle in the discovery and development of human monoclonal antibodies for therapeutic application. Here, we evaluate two solid-phase isolation methods to enrich the number of antigen-specific B cells from individuals naturally immunized against streptolysin O (SLO), a key virulence factor and known immunogen of group A streptococcus (GAS). Class-switched B cells obtained from individuals with a history of GAS infection were separated from peripheral blood mononuclear cells (PBMCs) by immunomagnetic methods. SLO-specific B cells were further enriched directly by binding to SLO monomers and captured by streptavidin-coated magnetic microbeads or indirectly by binding a fluorescently labeled SLO-streptavidin tetramer and captured by anti-fluorophore immunomagnetic microbeads. SLO-bound B cells were quantitated by flow cytometry and/or expanded in batch culture to determine IgG specificity. From individuals who have suffered a GAS infection ≥2 years prior, only the direct method enriched SLO-specific B cells, as determined by flow cytometry. Likewise, in batch culture, B cells isolated by the direct method resulted in an average of 375-fold enrichment in anti-SLO IgG, while no enrichment was observed for B cells isolated by the indirect method. The direct method established here provides a simple approach to increase low-frequency antigen-specific B cell populations supporting many downstream applications, such as immortalization of B cells, cloning of immunoglobulin genes, or purification of antibodies from supernatant for future study. Overall, this process is efficient, is inexpensive, and can be applied to many naturally immunogenic antigens.IMPORTANCE Bacteria called group A streptococci can cause a variety of skin and soft tissue infections ranging from mild pharyngitis ("strep throat") to deadly necrotizing fasciitis (sometimes called "flesh-eating" disease). In each case, the development of disease and the degree of tissue damage are mediated by toxins released from the bacteria during infection. Consequently, novel therapies aimed at clearing bacterial toxins are greatly needed. One promising new treatment is the utilization of monoclonal antibodies delivered as an immunotherapeutic for toxin neutralization. However, current methods of antibody development are laborious and costly. Here, we report a method to enrich and increase the detection of highly desirable antigen-specific memory B cells from individuals previously exposed to GAS using a cost-effective and less-time-intensive strategy. We envision that this method will be incorporated into many applications supporting the development of immunotherapeutics.

Toxins and Superantigens of Group A Streptococci

Streptococcus pyogenes (i.e., the group A Streptococcus) is a human-restricted and versatile bacterial pathogen that produces an impressive arsenal of both surface-expressed and secreted virulence factors. Although surface-expressed virulence factors are clearly vital for colonization, establishing infection, and the development of disease, the secreted virulence factors are likely the major mediators of tissue damage and toxicity seen during active infection. The collective exotoxin arsenal of S. pyogenes is rivaled by few bacterial pathogens and includes extracellular enzymes, membrane active proteins, and a variety of toxins that specifically target both the innate and adaptive arms of the immune system, including the superantigens; however, despite their role in S. pyogenes disease, each of these virulence factors has likely evolved with humans in the context of asymptomatic colonization and transmission. In this article, we focus on the biology of the true secreted exotoxins of the group A Streptococcus, as well as their roles in the pathogenesis of human disease.

The Unique Molecular Choreography of Giant Pore Formation by the Cholesterol-Dependent Cytolysins of Gram-Positive Bacteria

The mechanism by which the cholesterol-dependent cytolysins (CDCs) assemble their giant β-barrel pore in cholesterol-rich membranes has been the subject of intense study in the past two decades. A combination of structural, biophysical, and biochemical analyses has revealed deep insights into the series of complex and highly choreographed secondary and tertiary structural transitions that the CDCs undergo to assemble their β-barrel pore in eukaryotic membranes. Our knowledge of the molecular details of these dramatic structural changes in CDCs has transformed our understanding of how giant pore complexes are assembled and has been critical to our understanding of the mechanisms of other important classes of pore-forming toxins and proteins across the kingdoms of life. Finally, there are tantalizing hints that the CDC pore-forming mechanism is more sophisticated than previously imagined and that some CDCs are employed in pore-independent processes.

Structural studies of Streptococcus pyogenes streptolysin O provide insights into the early steps of membrane penetration

Cholesterol-dependent cytolysins (CDCs) are a large family of bacterial toxins that exhibit a dependence on the presence of membrane cholesterol in forming large pores in cell membranes. Significant changes in the three-dimensional structure of these toxins are necessary to convert the soluble monomeric protein into a membrane pore. We have determined the crystal structure of the archetypical member of the CDC family, streptolysin O (SLO), a virulence factor from Streptococcus pyogenes. The overall fold is similar to previously reported CDC structures, although the C-terminal domain is in a different orientation with respect to the rest of the molecule. Surprisingly, a signature stretch of CDC sequence called the undecapeptide motif, a key region involved in membrane recognition, adopts a very different structure in SLO to that of the well-characterized CDC perfringolysin O (PFO), although the sequences in this region are identical. An analysis reveals that, in PFO, there are complementary interactions between the motif and the rest of domain 4 that are lost in SLO. Molecular dynamics simulations suggest that the loss of a salt bridge in SLO and a cation-pi interaction are determining factors in the extended conformation of the motif, which in turn appears to result in a greater flexibility of the neighboring L1 loop that houses a cholesterol-sensing motif. These differences may explain the differing abilities of SLO and PFO to efficiently penetrate target cell membranes in the first step of toxin insertion into the membrane.

Intermedilysin Is Essential for the Invasion of Hepatoma HepG2 Cells byStreptococcus intermedius

Streptococcus intermedius causes endogenous infections leading to abscesses. This species produces intermedilysin (ILY), a human-specific cytolysin. Because of the significant correlation between higher ILY production levels by S. intermedius and deep-seated abscesses, we constructed ily knockout mutant UNS38 B3 and complementation strain UNS38 B3R1 in order to investigate the role of ILY in deep-seated infections. Strain UNS38 reduced the viability of human liver cell line HepG2 at infection but not of rat liver cell line BRL3A. Isogenic mutant strain UNS38 B3 was not cytotoxic in either cell line. Quantification of S. intermedius revealed that in infected HepG2 cells UNS38 but not UNS38 B3 increased intracellularly concomitantly with increasing cell damage. This difference between UNS38 and UNS38 B3 was not observed with UNS38 B3R1. Invasion and proliferation in BRL3A cells was not observed. Masking UNS38 or UNS38 B3R1 with ILY antibody drastically decreased adherence and invasion of HepG2. Moreover, coating strain UNS38 B3 with ILY partially restored adherence to HepG2 but without subsequent bacterial growth. At 1 day post-infection, many intact UNS38 were detected in the damaged phagosomes of HepG2 with bacterial proliferation observed in the cytoplasm of dead HepG2 after an additional 2 day incubation. These results indicate that surface-bound ILY on S. intermedius is an important factor for invasion of human cells by this bacterium and that secretion of ILY within host cells is essential for subsequent host cell death. These data strongly implicate ILY as an important factor in the pathogenesis of abscesses in vivo by this streptococcus.

Analysis of tryptophan-rich region in Clostridium septicum alpha-toxin involved with binding to glycosylphosphatidylinositol-anchored proteins

Clostridium septicum alpha-toxin has a unique tryptophan-rich region ((302)NGYSEWDWKWV(312)) that consists of 11 amino acid residues near the C-terminus. Using mutant toxins, the contribution of individual amino acids in the tryptophan-rich region to cytotoxicity and binding to glycosylphosphatidylinositol (GPI)-anchored proteins was examined. For retention of maximum cytotoxic activity, W307 and W311 are essential residues and residue 309 has to be hydrophobic and possess an aromatic side chain, such as tryptophan or phenylalanine. When residue 308, which lies between tryptophans (W307 and W309) is changed from an acidic to a basic amino acid, the cytotoxic activity of the mutant is reduced to less than that of the wild type. It was shown by a toxin overlay assay that the cytotoxic activity of each mutant toxin correlates closely with affinity to GPI-anchored proteins. These findings indicate that the WDW_W sequence in the tryptophan-rich region plays an important role in the cytotoxic mechanism of alpha-toxin, especially in the binding to GPI-anchored proteins as cell receptors.

Membrane assembly of the cholesterol-dependent cytolysin pore complex

The cholesterol-dependent cytolysins (CDCs) are a large family of pore-forming toxins that are produced, secreted and contribute to the pathogenesis of many species of Gram-positive bacteria. The assembly of the CDC pore-forming complex has been under intense study for the past 20 years. These studies have revealed a molecular mechanism of pore formation that exhibits many novel features. The CDCs form large β-barrel pore complexes that are assembled from 35 to 40 soluble CDC monomers. Pore formation is dependent on the presence of membrane cholesterol, which functions as the receptor for most CDCs. Cholesterol binding initiates significant secondary and tertiary structural changes in the monomers, which lead to the assembly of a large membrane embedded β-barrel pore complex. This review will focus on the molecular mechanism of assembly of the CDC membrane pore complex and how these studies have led to insights into the mechanism of pore formation for other pore-forming proteins. This article is part of a Special Issue entitled: Protein Folding in Membranes.

Embryonic stem cell extracts: use in differentiation and reprogramming

Stem cells have been studied extensively for decades and they have the inherent capacity to self-renew as well as to generate one or more types of specialized cells. The current focus of research on stem cells, particularly on embryonic stem cells, is on directed differentiation of these cells into specific cell types for future regenerative medicine. For the past few years, the process of reprogramming, which mediates convertion of somatic cells to their pluripotent state, has been given much attention, as it provides a possible source of autologous stem cells. In addition, understanding the molecular mechanism of differentiation and reprogramming has long been a subject of interest. In this article, we have briefly introduced stem cells and discussed the use of embryonic stem cells in reprogramming of somatic cells and differentiation to different lineages. The application of embryonic stem cells extracts in inducing reprogramming and transdifferentiation has also been described and discussed. Should this approach be successful, patient-specific cells will be produced safely and the likelihood of rejection will be decreased when used in cell therapy for many debilitating human diseases for which there is no cure such as Parkinson’s disease, Alzheimer’s disease, diabetes and others.

Inhibition of streptolysin O by allicin - an active component of garlic

Streptolysin O (SLO) is a potent cytolytic toxin produced by almost all strains of group A streptococci and is considered an important virulence factor for this organism. In this study we investigated the effect of allicin and aqueous garlic extracts on the haemolytic activity of SLO. All tested materials potentially inhibited the SLO haemolytic activity. Allicin neutralized SLO in a dose- and time-dependent manner. A 15 min incubation of SLO with 35 microg allicin totally inhibited the haemolytic activity of SLO [IC(50) (concentration necessary to reach half maximum inhibition)=5.97 microg]. The inhibitory activity of an old extract of garlic was equipotent to pure allicin (IC(50)=6.27 microg; P<0.05). In contrast, fresh extract of garlic inhibited the SLO haemolytic activity at lower concentrations (IC(50)=1.59 microl; 1.9 microg allicin). The inhibitory effect of the allicin was restored by addition of reducing agent DTT at 2 mM, suggesting that allicin likely inhibits the SLO by binding to the cysteine residue in the binding site. These results indicate a new activity for allicin and allicin may be a potential alternative drug against streptococcal diseases.

Chemokine degradation by the Group A streptococcal serine proteinase ScpC can be reconstituted in vitro and requires two separate domains

Streptococcus pyogenes is one of the most common human pathogens and possesses diverse mechanisms to evade the human immune defence. One example of its immune evasion is the degradation of the chemokine IL (interleukin)-8 by ScpC, a serine proteinase that prevents the recruitment of neutrophils to an infection site. By applying the ANTIGENome technology and using human serum antibodies, we identified Spy0416, annotated as ScpC, as a prominent antigen that induces protective immune responses in animals. We demonstrate here for the first time that the recombinant form of Spy0416 is capable of IL-8 degradation in vitro in a concentration- and time-dependent manner. Mutations in the conserved amino acid residues of the catalytic triad of Spy0416 completely abolished in vitro activity. However, the isolated predicted proteinase domain does not exhibit IL-8-degrading activity, but is dependent on the presence of the C-terminal region of Spy0416. Binding to IL-8 is mainly mediated by the catalytic domain. However, the C-terminal region modulates substrate binding, indicating that the proteolytic activity is amenable to regulation via the non-catalytic regions. The specificity for human substrates is not restricted to IL-8, since we also detected in vitro protease activity for another CXC chemokine GRO-alpha (growth-related oncogene alpha), but not for NAP-2 (neutrophil-activating protein 2), SDF (stromal-cell-derived factor)-1alpha, PF-4 (platelet factor 4), I-TAC (interferon-gamma-inducible T-cell alpha-chemoattractant), IP-10 (interferon-gamma-inducible protein 10) and MCP-1 (monocyte chemoattractant protein 1). The degradation of two human CXC chemokines in vitro, the high sequence conservation, the immunogenicity of the protein in humans and the shown protection in animal studies suggest that Spy0416 is a promising vaccine candidate for the prevention of infections by S. pyogenes.

Irreversible loss of membrane-binding activity of Listeria-derived cytolysins in non-acidic conditions: a distinct difference from allied cytolysins produced by other Gram-positive bacteria

Listeriolysin O (LLO), a member of the cholesterol-dependent cytolysin (CDC) family, is a major virulence factor of Listeria monocytogenes and contributes to bacterial escape from intracellular killing of macrophages. LLO is activated under weakly acidic conditions; however, the molecular mechanism of this pH-dependent expression of cytolytic activity of LLO is poorly understood. In this study, CDCs including LLO, ivanolysin O (ILO), seeligeriolysin O (LSO), pneumolysin (PLY), streptolysin O (SLO) and perfringolysin O (PFO) were prepared as recombinant proteins and examined for their functional changes after treatment under various pH conditions. Haemolytic and membrane cholesterol-binding activities were not affected in PLY, SLO and PFO at any pH examined. By contrast, all the Listeria-derived cytolysins, LLO, ILO and LSO, were active only at an acidic pH and rapidly inactivated under neutral or alkaline conditions. Once inactivated, LLO could not be reactivated even by a downward pH shift. The hydrophobicity of LLO treated at neutral or alkaline pH was increased. These data suggested that the pH-dependent loss of cytolytic activity appeared to be due to irreversible structural changes of domain 4 that resulted in the loss of target membrane cholesterol binding.

Ultrastructural analysis of the membrane insertion of domain 3 of streptolysin O

Streptolysin O (SLO) is a membrane-damaging toxic protein produced by group A streptococci. We performed an ultrastructural analysis of pore formation and the mechanism of hemolysis by SLO, using a mutant form of SLO [SLO(C/A)-SS] and native SLO. SLO(C/A)-SS was unable to penetrate the erythrocyte membrane as a consequence of immobilization that was due to a disulfide bond between domains. The SLO(C/A)-SS molecules that bound to membranes formed numerous single-layered ring-shaped structures that did not result in pores on the membranes. These structures were similar to the structures formed by native SLO at 0 degrees C. After treatment with dithiothreitol, SLO(C/A)-SS that had bound to membranes formed double-layered rings with pores on the membranes, as does native SLO at room temperature. Our morphological evidence demonstrates that an increase in temperature is necessary for the occurrence of conformational changes and for the formation of double-layered rings after the insertion of domain 3 into the host cell membrane. On the basis of a model of the oligomeric structure of SLO, we propose some new details of the mechanism of hemolysis by SLO.

Exploring Unique Structures: Flexibility is a Significant Factor in Biological Activity

The effect of molecular flexibility on biological activity was described for soft (e.g. hGH peptides) and hard molecules (e.g. biscoclaurine-type alkaloids). These molecules had a macrocyclic structure during molecular mechanics analysis, and the minimum essential unit, which affects insulin-involved fatty acid synthesis, was observed. The flexibility of the molecular center is concerning with biological activity through the diversification of structural feature, and compared with two types of molecules which have a rigid (haloacetylcarbamoyl-2-nitroimidazole analogs: chiral-TXs) or flexible (bis-quaternary ammonium compounds: bis-QACs) molecular center. Center flexibility reflected the conformation occurrence in TXs and bis-QACs. A parameter (solvation-free energy: dGW), which reflects structural hydrophobicity, was shown, and applied to the molecular design of brefeldin A analog. This hydrophobic index was very useful, and was used for conformational analysis of chiral-TXs and bis-QACs. In molecular dynamics analysis of cholesterol-dependent cytolysin (e.g. streptolysin O) and -independent cytolysin (e.g. intermedilysin), whole molecules moved like a bow and different conformations were shown in every moment. In such situations, the membrane-associated 11mer region in these cytolysins were flexible and could always interact with extramolecular factors (e.g. membrane constitution).

Expression of Recombinant Streptolysin O and Specific Antibody Production

Streptolysin O (SLO), an oxygen-labile cytolysin, is the cholesterol-binding exotoxin of hemolytic streptococci. Besides microbiological and pathological interests, this cytolysin has been used as a tool for permeabilization of biomembranes. SLO serves as a diagnostic reagent for determination of anti-SLO antibody titer in streptococcal infection. Availability of highly purified SLO, however, has been limited by low yield in streptococcal culture and purification process. Present subcloning of mature-type full-length SLO gene into an expression vector having strictly controllable araBAD promoter enabled efficient production of the cytolysin. Further, anti-SLO antibody with high specificity was obtained by immunizing with purified SLO protein.

Molecular characterization of NADase-streptolysin O operon of hemolytic streptococci

Whether slo, the gene encoding streptolysin O (SLO), a streptococcal cytolysin, has its own promoter or not is unsettled as yet. Present analyses demonstrate that slo is a member of an operon covering the upper-stream nusG and nga (NADase) genes, from which transcription of slo proceeds polycistronically, and major transcript is produced by readthrough from nga promoter. Mutational conversion of the sixth nucleotide T at the putative -10 region of chromosomal nga gene into C caused a drastic decrease in both NADase and SLO activities and the disappearance of the two corresponding mRNA bands from the Northern blot profile. The initiation site of the transcription was determined at 56 bp upstream (NusG gene) and 25 bp upstream (NADase gene) of each initiation codon. Although the promoter region of slo gene is highly conserved between group A and C streptococci, the proper slo promoter is nonfunctional in group C strain H46A. Moreover, commonly conserved arrangement was limited to the nusG-nga-orf1-slo region. These results indicate an intimate relationship between NADase and SLO in the regulation of their biosynthesis. Additional results suggest that NADase, synthesized as precursor with feeble activity, is activated by removing the carboxyl terminal region during or after secretion into culture medium.

Novel genomic rearrangement that affects expression of the Streptococcus pyogenes streptolysin O (slo) gene

A RecA-independent chromosomal rearrangement in the upstream region of the streptolysin O (slo) gene of Streptococcus pyogenes which affects slo expression was identified. PCR analysis was used to demonstrate that this kind of rearrangement was found in several strains of different lineages. Chromosomal loci involved in the recombination were found to be 746 kb apart on the 1.85-Mb-long chromosome. The primary structure of the splicing region, the reproducibility of the rearrangement, and the fact that reconstructed recombinant molecules fused to erm and lacZ reporter genes affected their expression indicate that this event is not accidental but may play a role in the expression of the slo gene. In addition, the product of the recombining DNAs, including the splicing site, does not follow any example of a known recombination mechanism. The implications of this rearrangement for slo expression are discussed.

Chemokine production by rat macrophages stimulated with streptolysin O from Streptococcus pyogenes

The contribution of streptolysin O (SLO) from Streptococcus pyogenes to neutrophil infiltration in inflammatory lesions was determined by production of cytokine-induced neutrophil chemoattractant (CINC)-1, -2 and -3, and macrophage inflammatory protein (MIP)-1alpha by rat macrophages stimulated with SLO in culture. Active SLO induced the production of CINCs and MIP-1alpha in dose- and time-dependent manners. These inductions were ascertained by chemokine mRNA expression in macrophages. Streptolysin S was without effect. The SLO-cholesterol complex induced the chemokine production in proportion to the residual hemolytic activity of the complex. In addition, the effects of SLO on the chemokine production were confirmed by the injection of active SLO into the preformed air pouch on the back of rats. The infiltration of neutrophils into the pouch fluid (exudate) increased steadily with a lag phase of about 2 hr. The major chemokine found in exudates was MIP-1alpha but not CINCs. In this study, it became clear that active SLO, but not the inactive one, contributed to the production of MIP-1alpha and CINCs in the conditioned medium and in exudates.

The role of clostridial toxins in the pathogenesis of gas gangrene

Clostridium perfringens gas gangrene is, without a doubt, the most fulminant necrotizing infection that affects humans. In victims of traumatic injury, the infection can become well established in as little as 6-8 h, and the destruction of adjacent healthy muscle can progress several inches per hour despite appropriate antibiotic coverage. Shock and organ failure are present in 50% of patients and, among these, 40% die. Despite modern medical advances and intensive-care regimens, radical amputation remains the single best life-saving treatment. Over the past century, much has been learned about the pathogenesis of this disease, and novel therapies are on the horizon for patients with this devastating infection.

Autonomous expression of the slo gene of the bicistronic nga-slo operon of Streptococcus pyogenes

A recent model for cytolysin-mediated translocation in Streptococcus pyogenes proposes that NAD-glycohydrolase is translocated through streptolysin O-generated pores into a host cell (J. Madden, N. Ruiz, and M. Caparon, Cell 104:143-152, 2001). This model also assumes that the NAD-glycohydrolase (nga) and streptolysin O (slo) genes that code for these products are organized in an operon-like structure expressed from a single promoter only (nga). We expand this model by showing that slo possesses its own autonomous promoter, which is located 155 bp upstream of the slo gene. Under experimental conditions in which S. pyogenes is grown in THY medium, the strength of the slo promoter, as measured by the activity of a lacZ reporter gene, resulted in low but highly reproducible values. Finally, we demonstrated that sloR, a S. pyogenes gene that closely resembles the Clostridium perfringens pfoR gene, exerts a negative effect on the expression of the slo gene.

Antibodies against a synthetic peptide of SagA neutralize the cytolytic activity of streptolysin S from group A streptococci

Virtually all group A streptococci (GAS) produce streptolysin S (SLS), a cytolytic toxin that is responsible for the beta-hemolysis surrounding colonies of the organisms grown on blood agar. SLS is an important virulence determinant of GAS, and recent studies have identified a nine-gene locus that is responsible for synthesis and transport of the toxin. SLS is not immunogenic; thus, no neutralizing antibodies are evoked during the course of natural infection. In the present study, we show that a synthetic peptide containing amino acid residues 10 to 30 of the putative SLS (SagA) propeptide [SLS(10-30)] coupled to keyhole limpet hemocyanin evoked antibodies in rabbits that completely neutralized the hemolytic activity of the toxin in vitro. Inhibition of hemolysis was reversed by preincubation of the immune serum with soluble, unconjugated peptide, indicating the specificity of the antibodies. In addition, antibodies that were affinity purified over an SLS(10-30) peptide column completely inhibited SLS-mediated hemolysis. The SLS(10-30) antisera did not opsonize group A streptococci; however, when combined with type-specific M protein antisera, the SLS antibodies significantly enhanced phagocytosis mediated by M protein antibodies. Thus, we have shown for the first time that it is possible to raise neutralizing antibodies against one of the most potent bacterial cytolytic toxins known. Our data also provide convincing evidence that the sagA gene actually encodes the SLS peptide of GAS. The synthetic peptide may prove to be an important component of vaccines designed to prevent GAS infections.

NAD+-glycohydrolase acts as an intracellular toxin to enhance the extracellular survival of group A streptococci

Group A streptococci (GAS) produce several secreted products that are thought to enhance pathogenicity by facilitating spread of the organisms through host tissues. Two such products, streptolysin O (SLO) and NAD+-glycohydrolase, appear to be functionally linked, in that SLO is required for transfer of NAD+-glycohydrolase into epithelial cells. However, the effects of NAD+-glycohydrolase on host cells are largely unexplored. We now report that SLO-mediated delivery of NAD+-glycohydrolase to the cytoplasm of human keratinocytes results in major changes in host cell biology that enhance GAS pathogenicity. We derived isogenic mutant strains deficient in the expression of SLO, NAD+-glycohydrolase or both proteins in the background of a virulent, M-type 3 strain of GAS. All three mutant strains were internalized by human keratinocytes more rapidly and in higher numbers than were organisms from the wild-type strain. Association of the mutant strains with keratinocytes also resulted in reduced cytotoxicity and reduced keratinocyte apoptosis compared with wild-type GAS. These results support a model in which NAD+-glycohydrolase contributes to GAS pathogenesis by modulating host cell signalling pathways to inhibit GAS internalization, to augment SLO-mediated cytotoxicity and to induce keratinocyte apoptosis. We conclude that NAD+-glycohydrolase is a novel type of bacterial toxin that acts intracellularly in the infected host to enhance the survival and proliferation of an extracellular pathogen.

The family of thiol-activated, cholesterol-binding cytolysins

Several species of both pathogenic and non-pathogenic grampositive bacteria within the genera Streptococcus, Clostridium and Bacillus secrete cytolytic proteins that belong to a single, highly homologous family. The most widely known members of this family are streptolysin O, listeriolysin, perfringolysin, and pneumolysin. These toxins specifically require membrane cholesterol but, apparently, do not depend on any other specific cell surface receptor, so that they are able to lyse the cytoplasmic membranes of virtually any animal cell. Upon binding as monomers, they oligomerize to form large pores with up to 30 nm internal diameter. These are the largest pores known, permitting permeation not only of ions and small metabolites but also of macromolecules. The latter property renders these toxins useful tools in cell biology. While several of these cytolysins have been shown to be determinants of bacterial pathogenicity, their biological roles may vary, as do the lifestyles of the bacteria secreting them. A unique function is surely fulfilled by listeriolysin O, which helps the intracellular pathogen Listeria monocytogenes escape from phagolysosomes and then spread to adjacent host cells.

The cholesterol-dependent cytolysins

In view of the recent studies on the CDCs, a reasonable schematic of the stages leading to membrane insertion of the CDCs can be assembled. As shown in Fig. 3, we propose that the CDC first binds to the membrane as a monomer. These monomers then diffuse laterally on the membrane surface to encounter other monomers or incomplete oligomeric complexes. Presumably, once the requisite oligomer size is reached, the prepore complex is converted into the pore complex and a large membrane channel is formed. During the conversion of the prepore complex to the pore complex, we predict that the TMHs of the subunits in the prepore complex insert into the bilayer in a concerted fashion to form the large transmembrane beta-barrel, although this still remains to be confirmed experimentally. Many intriguing problems concerning the cytolytic mechanism of the CDCs remain unsolved. The nature of the initial interaction of the CDC monomer with the membrane is currently one of the most controversial questions concerning the CDC mechanism. Is cholesterol involved in this interaction, as previously assumed, or do specific receptors exist for these toxins that remain to be discovered? Also, the trigger for membrane insertion and the regions of these toxins that facilitate the [figure: see text] interaction of the monomers during prepore complex formation are unknown. In addition, the temporal sequence of the multiple structural changes that accompany the conversion of the soluble CDC monomer into a membrane-inserted oligomer have yet to be defined or characterized kinetically.

Virulence of group A streptococci in fertile hens eggs is mainly effected by M protein and streptolysin O

In this study we have investigated whether streptolysin O contributes to the virulence of group A streptococci. For this purpose we generated M-negative and SLO-negative mutants by insertion mutagenesis into the chromosome of an M type 1 strain. The inactivation of M1 protein expression was achieved by the construction of the integrative plasmid pSFABS, which contains the internal fragment abs of the emm1 gene. Integration of pSFABS by homologous recombination into the chromosome of strain 38 541 resulted in the generation of mutant EMM1. Inactivation of slo with plasmid pFWSLOD resulted in two different mutant forms. The homologous recombination with plasmid pFWSLOD carrying the two slo fragments slo1 (899 base pairs in the 5' region) and slo2 (709 base pairs in the downstream part) resulted in mutants SLO3, SLO4 and SLO17. In SLO17 a double crossover event took place with insertion of the spectinomycin resistance gene aad9 between the slo fragments 1 and 2. In mutants SLO3 and SLO4 the homologous recombination with the same plasmid led to the integration of the whole plasmid construct into the chromosome of strain 38 541. Both forms of mutation failed to express SLO. In mutant SLO4 additionally M1 protein expression was significantly decreased. The mutants EMM1 (M-, SLO+) and SLO4 (M decreased, SLO-) showed a reduced binding to collagen-coated surfaces. In contrast the mutants SLO3 and SLO17 (both M+, SLO-) and the wild-type strain 38 541 (M+, SLO+) showed an affinity to collagen similar to purified M1 protein. All mutants were less virulent for chicken embryos compared to the wild-type strain after infection by intravenous injection as well as by application onto the chorioallantoic membrane. The results show that besides M protein SLO can also influence virulence of group A streptococci. Moreover, it became obvious that streptococci need more than one tool to fully develop their infectious potential.

Hemolytic and hemoxidative activities in Mycoplasma penetrans

Mycoplasma penetrans is a newly isolated Mollicute from the urine of patients infected with human immunodeficiency virus that demonstrates the capacity to adhere to and invade human cells. A previous report, based on assays with mouse red blood cells (RBCs), indicated that M. penetrans lacked hemolytic activity. In our studies, we incubated different isolates of M. penetrans with various RBC species and observed hemolytic zones surrounding individual mycoplasma colonies. All M. penetrans strains displayed hemolysis after 2 to 3 days of incubation. Hemolytic activity diffused from single colonies, eventually causing complete lysis. Hemolysis was most pronounced with sheep RBCs, followed by horse, chicken, and human cells. Furthermore, hemolytic activity was demonstrable in both intact mycoplasma cell preparations and spent culture supernatant. However, unlike intact mycoplasmas, the hemolytic activity in the supernatant was dependent on the reducing agent, cysteine. In addition to hemolysis, a brown precipitate was closely associated with mycoplasma colonies, suggesting oxidation of hemoglobin. Absorption spectra indicated that hemoglobin was oxidized to methemoglobin, and the addition of catalase demonstrated H(2)O(2)-mediated hemoxidation. Other experiments suggested that hemoxidation enhanced total hemolysis, providing the first evidence of both hemolytic and hemoxidative activities in M. penetrans.

Cholesterol-binding cytolytic protein toxins

Cholesterol-binding cytolysins (CBCs) are a large family of 50- to 60-kDa single-chain proteins produced by 23 taxonomically different species of Gram-positive bacteria from the genera Streptococcus, Bacillus, Clostridium, Listeria and Arcanobacterium. Apart pneumolysin, which is an intracytoplasmic toxin, all the other toxins are secreted in the extracellular medium. Among the species producing CBCs, only L. monocytogenes and L. ivanovii are intracellular pathogens which grow and release their toxins in the phagocytic cells of the host. CBCs are lethal to animals and highly lytic toward eukaryotic cells, including erythrocytes. Their lytic and lethal properties are suppressed by sulfhydryl-group-blocking agents and reversibly restored by thiols or other reducing agents. These properties are irreversibly abrogated by very low concentrations of cholesterol and other 3beta-hydroxysterols. Membrane cholesterol is thought to be the toxin-binding site at the surface of eukaryotic cells. Toxins molecules bind as monomers to the membrane surface with subsequent oligomerization into arc-and ring-shaped structures surrounding large pores generated by this process. Thirteen structural genes of the toxins (all chromosomal) have been cloned and sequenced to date. The deduced primary structure of the proteins shows obvious sequence homology particularly in the C-terminal part and a characteristic common consensus sequence containing a unique Cys residue (ECTGLAWEWWR) near the C-terminus of the molecules (except pyolysin and intermedilysin). However, another Cys residue outside this undecapeptide and closer to the C-terminus occurs in ivanolysin. Genetic replacement of the Cys residue in the consensus undecapeptide by certain amino acids demonstrated that this residue was not essential for toxin function. Other residues in the undecapeptide have been mutagenized, particularly the Trp residues. One of these Trp appeared critical for lytic activity. The recent elucidation of the 3-D structure of perfringolysin O provided interesting information on the structure-activity relationship. The molecule was divided into four domains. Three domains are arranged in a row, giving an elongated shape. Domain 3 is covalently connected to the N-terminal domain 1 and packed laterally against domain 2. Membrane interaction of the monomer appears to be mediated by domain 4, while, oligomerization involves several sites scattered throughout the sequence. The Trp-rich region around the conserved Cys residue within domain 4 is assumed to conformationally adapt to cholesterol, and domain 3 is envisaged to move across the "hinge" by which it is connected to domain 1.

The pathogenesis of clostridial myonecrosis

These pieces of evidence can be assimilated into a molecular and cellular model of pathogenesis which is initiated by direct toxin effects upon venous capillary endothelial cell function, leading to expression of pro-inflammatory mediators and adhesion molecules, and initiation of platelet aggregation. Toxin-induced hyperadhesion of leukocytes (see above section) with enhanced respiratory burst activity (due to toxins directly or to toxin-induced IL-8 or PAF synthesis by host cells) and toxin-induced chemotaxis deficits could result in neutrophil-mediated vascular injury. Direct toxin-induced cytopathic effects on EC may also contribute to vascular abnormalities associated with gas gangrene. Over prolonged incubation periods, PLC at sublytic concentrations causes EC to undergo profound shape changes similar to those described following prolonged TNF or interferon gamma exposure. In vivo, conversion of EC to this fibroblastoid morphology could contribute to the localized vascular leakage and massive swelling observed clinically with this infection. Similarly, the direct cytotoxicity of PFO could disrupt endothelial integrity and contribute to progressive edema both locally and systemically. Thus, via the mechanisms outlined above, both PLC and PFO may cause local, regional and systemic vascular dysfunction. For instance, local absorption of exotoxins within the capillary beds could affect the physiological function of the endothelium lining the postcapillary venules, resulting in impairment of phagocyte delivery at the site of infection. Toxin-induced endothelial dysfunction and microvascular injury could also cause loss of albumin, electrolytes, and water into the interstitial space resulting in marked localized edema. These events, combined with intravascular platelet aggregation and leukostasis, would increase venous pressures and favor further loss of fluid and protein in the distal capillary bed. Ultimately, a reduced arteriolar flow would impair oxygen delivery thereby attenuating phagocyte oxidative killing and facilitating anaerobic glycolysis of muscle tissue. The resultant drop in tissue pH, together with reduced oxygen tension, might further decrease the redox potential of viable tissues to a point suitable for growth of this anaerobic bacillus. As infection progresses and additional toxin is absorbed, larger venous channels would become affected, causing regional vascular compromise, increased compartment pressures and rapid anoxic necrosis of large muscle groups. When toxins reach arterial circulation, systemic shock and multiorgan failure rapidly ensue, and death is common.

Presence of the Streptococcus suis suilysin gene and expression of MRP and EF correlates with high virulence in Streptococcus suis type 2 isolates

Nineteen Streptococccus suis type 2 isolates that had been analyzed previously for hemolysin production, ribotype, and virulence in pigs were examined for presence of the gene coding for suilysin by PCR amplification, and southern blot and hybridization techniques. Based on southern blot and hybridization analysis, all isolates tested contained at least a portion of the suilysin gene. PCR amplification of the entire gene resulted in gene fragments from five of the seven highly virulent isolates and none of the moderately virulent or avirulent isolates. Additional PCR analysis showed that mutation or deletions at the 5' end of the suilysin gene in the less virulent isolates prevented amplification of the sly gene fragment from those isolates. The MRP+ (muramidase-released protein) EF+ (extracellular protein) phenotype was also expressed by the same five highly virulent/sly+ isolates.

C-terminal amino acid residues are required for the folding and cholesterol binding property of perfringolysin O, a pore-forming cytolysin

Perfringolysin O (theta-toxin) is a pore-forming cytolysin whose activity is triggered by binding to cholesterol in the plasma membrane. The cholesterol binding activity is predominantly localized in the beta-sheet-rich C-terminal half. In order to determine the roles of the C-terminal amino acids in theta-toxin conformation and activity, mutants were constructed by truncation of the C terminus. While the mutant with a two-amino acid C-terminal truncation retains full activity and has similar structural features to native theta-toxin, truncation of three amino acids causes a 40% decrease in hemolytic activity due to the reduction in cholesterol binding activity with a slight change in its higher order structure. Furthermore, both mutants were found to be poor at in vitro refolding after denaturation in 6 M guanidine hydrochloride, resulting in a dramatic reduction in cholesterol binding and hemolytic activities. These activity losses were accompanied by a slight decrease in beta-sheet content. A mutant toxin with a five-amino acid truncation expressed in Escherichia coli is recovered as a further truncated form lacking the C-terminal 21 amino residues. The product retains neither cholesterol binding nor hemolytic activities and shows a highly disordered structure as detected by alterations in the circular dichroism and tryptophan fluorescence spectra. These results show that the C-terminal region of theta-toxin has two distinct roles; the last 21 amino acids are involved to maintain an ordered overall structure, and in addition, the last two amino acids at the C-terminal end are needed for protein folding in vitro, in order to produce the necessary conformation for optimal cholesterol binding and hemolytic activities.

Listeria monocytogenes stimulates mucus exocytosis in cultured human polarized mucosecreting intestinal cells through action of listeriolysin O

When the intracellular pathogen Listeria monocytogenes infects cultured human mucosecreting polarized HT29-MTX cells apically, it induces the stimulation of mucus exocytosis without cell entry. Using a set of isogenic mutants and purified listeriolysin O (LLO), we identified the L. monocytogenes thiol-activated exotoxin LLO as the agonist of mucus secretion. We demonstrated that the LLO-induced mucus exocytosis did not result from the LLO membrane-damaging activity. We found that LLO-induced mucus exocytosis is an event requiring the binding of LLO to a brush border-associated receptor and membrane oligomerization of the exotoxin. By a pharmacological approach, we demonstrated that no regulatory system or intracellular transducing signal known to be involved in control of mucin exocytosis was activated by LLO. Based on the present data, the stimulatory action of LLO on mucin exocytosis could be accounted for either by an unknown signaling system which remains to be determined or by direct action of LLO with the membrane vesicle components involved in the intracellular vesicular transport of mucins.

Formation of ring-shaped structures on erythrocyte membranes after treatment with botulinolysin, a thiol-activated hemolysin from Clostridium botulinum

Damage to erythrocyte membranes by botulinolysin (BLY) was studied by electron microscopy, which revealed ring-shaped structures with inner diameters and widths of approximately 32 and 6.7 nm, respectively. BLY bound to membranes at 0 degrees C, but subsequent treatment with glutaraldehyde prevented ring formation during further incubation at 37 degrees C. Zn2+ ions inhibited ring formation but not binding of BLY to membranes.

Role of free thiol groups in the biological activities of stonustoxin, a lethal factor from stonefish (Synanceja horrida) venom

Stonustoxin (SNTX) is a two-subunit protein purified from the venom of a stonefish, Synanceia horrida. It has potent lethal activity and is also a membrane pore-forming cytolysin. The role of thiol groups in the biological activities of SNTX was investigated. Both the hemolytic and lethal activities of SNTX were potentiated by the reducing agent, dithiothreitol (DTT). The hemolytic activity of SNTX was sensitive to the modification of thiol groups by 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB). The loss of haemolytic activity correlated with the number of thiol groups that were titrated with DTNB. Thiol modification of SNTX with DTNB also inhibited its lethality. These inhibitory effects of thiol modification could be reversed by reduction with DTT. It was also found that the haemolytic activity of SNTX could not be inhibited by cholesterol. These observations indicate that free thiol groups play an important role in the haemolytic activity and lethality of SNTX but unlike other thiol-activated cytolysins, SNTX was not inhibited by cholesterol. Thus, SNTX may represent a new class of cytolytic toxin.

The Arcanobacterium (Actinomyces) pyogenes hemolysin, pyolysin, is a novel member of the thiol-activated cytolysin family

Arcanobacterium (Actinomyces) pyogenes, an animal pathogen, produces a hemolytic exotoxin, pyolysin (PLO). The gene encoding PLO was cloned, and sequence analysis revealed an open reading frame of 1,605 bp encoding a protein of 57.9 kDa. PLO has 30 to 40% identity with the thiol-activated cytolysins (TACYs) of a number of gram-positive bacteria. The activity of PLO was found to be very similar to those of other TACYs, except that it was not thiol activated. The highly conserved TACY undecapeptide is divergent in PLO; in particular, the cysteine residue required for thiol activation has been replaced with alanine. However, mutagenesis of the alanine residue to cysteine did not confer thiol activation on PLO, suggesting a conformational difference in the undecapeptide region of this toxin. Specific antibodies against purified, recombinant PLO completely neutralized the hemolytic activity of A. pyogenes, suggesting that this organism produces a single hemolysin. Furthermore, these antibodies could passively protect mice against lethal challenge with A. pyogenes, suggesting that like other TACYs PLO is an important virulence factor in the pathogenesis of this organism.

A biotinylated perfringolysin O derivative: a new probe for detection of cell surface cholesterol

theta-Toxin is a cholesterol-binding, pore-forming cytolysin of Clostridium perfringens. To detect cell surface cholesterol, we prepared a theta-toxin derivative, BC theta by biotinylation of a protease-nicked theta-toxin, which has the same binding affinity for cholesterol as theta-toxin without cytolytic activity. Human erythrocytes, V79 cells and human umbilical vein endothelial cells (HUVEC), were stained with BC theta coupled with FITC-avidin, and then the cells were analyzed by either flow cytometry or laser confocal microscopy. The fluorescence intensity increased in both intact and briefly fixed cells when treated with BC theta. BC theta-treated V79 cells were stained by neither trypan blue nor propidium iodide, indicating that BC stained just the outer surface of the plasma membrane of vital cells. Treatment of the cells with digitonin, a cholesterol-sequestering reagent, decreased the fluorescence intensity to the background level, indicating that BC theta staining is specific for cholesterol. The fluorescence intensity of erythrocytes pre-permeabilized with a small amount of theta-toxin increased more than ten-fold, suggesting higher cholesterol contents in the inner layer of the plasma membrane. When cells were cultured with cholesterol-depleted medium, the fluorescence intensity stained by BC theta decreased remarkably in V79 cells, but did not change in HUVEC. This indicates that cell surface cholesterol may be provided in different ways with these two cell lines. These results suggest that BC theta can be a useful probe for visualizing cell surface cholesterol and for evaluating the effects of cellular events on the topology and distribution of cholesterol.

Immunologic epitope, gene, and immunity involved in pneumococcal glycoconjugate

Pneumococcal infection persists as a major cause of pneumonia, otitis media, and meningitis in infants. Children less than 2 years of age show the highest incidence of pneumococcal diseases. Production of monoclonal antibody (MAb) to polysaccharide (PS) and binding characteristics to PS epitopes were studied. Removal of the O-acetyl group from 9V PS by alkali hydrolysis resulted in a decreased binding with rabbit 9V antiserum (AS). However, the binding reaction with 9V MAb was less affected by the loss of O-acetyl content. Type 9V IgG MAb provided passive protection and enhanced the opsonophagocytic activity of polymorphonuclear (PMN) leukocytes to kill type 9V pneumococci. The pathogenecity of pneumococci is attributed to various virulence factors distributed on the cell surface, including capsular polysaccharide and protein antigens, for example, pneumolysin, autolysin, pneumococcal surface protein A (PspA), pneumococcal surface adhesion (PsaA), and hemin binding protein. Some of these protein antigens may be used as a component to combine with pneumococcal PS vaccine or as a carrier of conjugate vaccine. Clinical trials of pneumococcal conjugate vaccines showed that covalent linkage of capsular PS to protein carriers improved the immunogenicity of the PS. Development of glycoconjugate vaccine for selected pneumococcal types will help solve the problem of poor immunogenecity of PS vaccine in young children used for prevention of pneumococcal infection.

Molecular characterization of a major serotype M49 group A streptococcal DNase gene (sdaD)

Group A streptococci (GAS) express up to four types of secreted DNases. Although GAS infections are correlated with the production of anti-DNase B antibodies, the roles of DNases in the pathogenesis of GAS infections remain unclear. From a lambda library of serotype M49 strain CS101 GAS genome, a 2,147-bp fragment expressing DNase activity on an indicator agar was identified and sequenced. One 1,155-bp open reading frame (ORF) was identified in this fragment. This ORF was found to be 48% identical on the amino acid level to group C streptococcal DNase (Sdc). The regions of highest homology corresponded to amino acid residues that were also identified as part of the active site in staphylococcal nuclease. Transcription analysis revealed a specific 1.3-kb mRNA, which corresponded to the size predicted by the promoter and transcription termination signature sequences and indicated a monocistronic mode of transcription. Allelic replacement of the ORF rendered a M49 mutant devoid of extracellular DNase activity when cultured on indicator agar. Virulence parameters such as resistance to phagocytosis were not affected by the mutation. The sda gene was cloned and expressed in Escherichia coli as a thioredoxin fusion. By performing Ouchterlony immunodiffusion on the recombinant protein and by using protein preparations from culture supernatants of wild-type bacteria and the DNase mutant, the results of immunoreactivity with DNase type-specific polyclonal rabbit antisera classified the DNase as a type D enzyme. Fifty percent of patients with sera exhibiting high titers of antistreptolysin or anti-DNase B antibodies also had SdaD-reactive antibodies in comparison with <10% of serologically normal controls. While the value of recombinant SdaD for diagnostic purposes needs to be clarified, the isogenic DNase mutant pair of M49 should allow the significance of GAS DNase D as a bacterial virulence factor to be determined.

Contribution of individual tryptophan residues to the structure and activity of theta-toxin (perfringolysin O), a cholesterol-binding cytolysin

theta-Toxin (perfringolysin O), secreted by Clostridium perfringens, shares with other known thiol-activated toxins a conserved undecapeptide, ECTGLAWEWWR, located in the C-terminal region of the protein and containing the unique cysteine of the molecule. Single and double amino acid substitutions were created in the theta-toxin molecule to investigate the role of individual tryptophan residues in the lytic activity of theta-toxin. Wild-type and mutant theta-toxins were overproduced in Escherichia coli by means of a T7 RNA polymerase/promoter system and purified. The relative hemolytic activities of four mutant toxins, each with a Trp to Phe substitution outside the common Cys-containing region, were more than 60% that of wild-type theta-toxin. In contrast, mutant toxins with Phe replacements within the Cys-containing region (at Trp436, Trp438 or Trp439) showed significantly reduced hemolytic and erythrocyte-membrane-binding activities. The largest reduction in binding affinity, more than 100-fold, was observed for Trp438 mutant toxins. However, the mutants retain binding specificity for cholesterol and the ability to form arc-shaped and ring-shaped structures on membranes. These results indicate that the low hemolytic activities of these mutant toxins can be ascribed, at least in part, to reduced binding activities. With respect to protease susceptibility and far-ultraviolet circular-dichroism spectra, only the W436-->F mutant toxin, showed any considerable difference from wild-type toxin in secondary or higher-order structures, indicating that Trp436 is essential for maintenance of toxin structure.

Membrane-penetrating domain of streptolysin O identified by cysteine scanning mutagenesis

Streptolysin O (SLO), a polypeptide of 571 amino acids, belongs to a family of highly homologous toxins that bind to cell membranes containing cholesterol and then polymerize to form large transmembrane pores. A conserved region close to the C terminus contains the single cysteine residue of SLO and has been implicated in membrane binding, which has been the only clear assignment of function to a part of the sequence. We have used a cysteine-less active mutant of SLO to introduce single cysteine residues at 19 positions distributed throughout the sequence. The cysteines were derivatized with the polarity-sensitive fluorophore acrylodan, and the fluorescence emission of the label was examined at the different stages of SLO pore assembly. With several mutants, oligomerization on membranes was accompanied by emission blue-shifts, indicating movement of the label into a more hydrophobic environment. These effects were essentially confined to the range of amino acids 213-305. With oligomeric mutants L274C, S286C, and S305C, additional environmental alterations were induced when different nondenaturing detergents were used to dislodge the membrane lipids from the oligomers. The corresponding amino acid residues thus insert into the lipid bilayer during pore formation. Conversely, the spectra of oligomeric mutants A213C and T245C were not affected by detergents. Devoid of contact with the lipid bilayer, these amino acid residues probably participate in the interaction of SLO molecules within the oligomer.

Physical and genetic chromosomal map of an M type 1 strain of Streptococcus pyogenes

A physical map of the chromosome of an M type 1 strain of Streptococcus pyogenes was constructed following digestion with three different restriction enzymes, SmaI, SfiI, and SgrAI, and separation and analysis of fragments by pulsed-field gel electrophoresis. The genome size of this strain was estimated to be 1,920 kb. By employing Southern hybridization and PCR analysis, 36 genes were located on the map.

Intermedilysin, a novel cytotoxin specific for human cells secreted by Streptococcus intermedius UNS46 isolated from a human liver abscess

A novel cytotoxin (intermedilysin) specific for human cells was identified as a cytolytic factor of Streptococcus intermedius UNS46 isolated from a human liver abscess. Intermedilysin caused human cell death with membrane blebs. Intermedilysin was purified from UNS46 culture medium by means of gel filtration and hydrophobic chromatography. The purified toxin was resolved into major and minor bands of 54 and 53 kDa, respectively, by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. These proteins reacted with an antibody against intermedilysin. Five internal peptide fragments of intermedilysin were sequenced and found to have 42 to 71% homology with the thiol-activated cytotoxin pneumolysin. However, the action of intermedilysin differed from that of thiol-activated cytotoxins, especially in terms of a lack of activation by dithiothreitol and resistance to treatments with N-ethylmaleimide and 5,5'-dithio-bis-(2-nitrobenzoic acid), although cholesterol inhibited the toxin activity. Intermedilysin was potently hemolytic on human erythrocytes but was 100-fold less effective on chimpanzee and cynomolgus monkey erythrocytes. Intermedilysin was not hemolytic in nine other animal species tested. Since human erythrocytes treated with trypsin were far less sensitive to intermedilysin than were the intact cells, a cell membrane protein(s) may participate in the intermedilysin action. These data demonstrated that intermedilysin is distinguishable from all known bacterial cytolysins.

Serodiagnosis of listeriosis based upon detection of antibodies against recombinant truncated forms of listeriolysin O

Amino-terminal fragments of listeriolysin O (LLO) of 240 and 411 residues (fragments LLO240 and LLO411, respectively) were expressed in Escherichia coli as fusion polypeptides with maltose-binding protein (MBP) with the aim of producing specific antigens for use in serological tests. In Western blots (immunoblots) with crude bacterial extracts of the fusion polypeptides, the reactivities of MBP-LLO240 and MBP-LLO411 with anti-LLO antibody (ALLO)- and anti-streptolysin O antibody (ASLO)-positive human sera were first compared with that of the entire LLO (LLO530) also fused to MBP (MBP-LLO530). Sixteen of 17 (94.1%) ALLO-positive samples reacting with MBP-LLO530 also reacted with MBP-LLO411, whereas this proportion dropped to 11 of 17 (64.7%) with MBP-LLO240. Alternatively, 18 of 19 (94.7%) ASLO-positive samples giving an interpretable result reacted with MBP-LLO530, whereas 1 of 19 (5.3%) of these samples reacted with MBP-LLO240 or MBP-LLO411. The fusion polypeptide MBP-LLO411 was purified by maltose affinity chromatography and was further evaluated as a diagnostic antigen in a Western blot assay. Twenty-one of 21 (100%) serum samples obtained from patients with listeriosis and found to be positive for ALLO by a reference dot blot test reacted with MBP-LLO411, whereas 1 of 20 (5%) ASLO-positive serum samples and 1 of 100 (1%) serum samples from healthy adults were reactive. Thus, a polypeptide limited to the 411 amino-terminal residues of LLO is a specific and sensitive antigen for the detection of ALLO.

Neutralizing monoclonal antibodies against listeriolysin: mapping of epitopes involved in pore formation

Six different mouse monoclonal antibodies (MAbs) and a specific rabbit polygonal antibody were raised against listeriolysin. Four of the MAbs also recognized seeligeriolysin, and five cross-reacted with ivanolysin. The hemolytic activity could be neutralized by the polygonal antibody as well as by five of the MAbs. None of the neutralizing antibodies interfered with the binding of listeriolysin to the cellular membrane. The epitopes recognized by the MAbs were localized by using overlapping synthetic peptides between positions 59 and 279, a region hitherto not implicated in mediating hemolytic activity.

Expression of active streptolysin O in Escherichia coli as a maltose-binding-protein--streptolysin-O fusion protein. The N-terminal 70 amino acids are not required for hemolytic activity

Streptolysin 0 (SLO) is the prototype of a family of cytolysins that consists of proteins which bind to cholesterol and form very large transmembrane pores. Structure/function studies on the pore-forming cytolysin SLO have been complicated by the proteolytic inactivation of a substantial portion of recombinant SLO (rSLO) expressed in Escherichia coli. To overcome this problem, translational fusions between the E. coli maltose-binding protein (MBP) gene and SLO were constructed, using the vectors pMAL-p2 and pMAL-c2. MBP-SLO fusion proteins were degraded if secreted into the E. coli periplasm, but intact, soluble MBP-SLO fusion proteins were produced at high levels in the cytoplasm. Active SLO with the expected N-terminus was separated from the MBP carrier by cleavage with factor Xa. Cleavage with plasmin or trypsin also yielded active, but slightly smaller forms of SLO. Surprisingly, uncleaved MBP-SLO was also hemolytic and cytotoxic to human fibroblasts and keratinocytes. The MBP-SLO fusion protein displayed equal activities to SLO. Sucrose density gradient analyses showed that the fusion protein assembled into polymers, and no difference in structure was discerned compared with polymers formed by native SLO. These studies show that the N-terminal 70 residues of mature (secreted) SLO are not required for pore formation and that the N-terminus of the molecule is probably not inserted into the bilayer. In addition, they provide a simple means for producing mutants for structure/function studies and highly purified SLO for use as a permeabilising reagent in cell biology research.

Different forms of streptolysin O produced by Streptococcus pyogenes and by Escherichia coli expressing recombinant toxin: cleavage by streptococcal cysteine protease

To resolve apparent discrepancies in the literature, N-terminal sequences of the active high- and low-molecular-weight (high- and low-M(r)) forms of native streptolysin O (nSLO) purified from Streptococcus pyogenes culture supernatants and of the similar-size high- and low-M(r) forms of recombinant SLO (rSLO) found in the periplasm of Escherichia coli expressing a cloned slo gene were determined. The high-M(r) forms of nSLO and rSLO are identical, reflecting removal of a 31-residue signal peptide, but the similar-size low-M(r) forms are very different. Removal of C-terminal sequences by proteases in the E. coli periplasm produces an inactive low-M(r) form of rSLO. In contrast, an active low-M(r) form of nSLO is produced by proteolytic cleavage between the N-terminal residues Lys-77 and Leu-78, which was shown to correspond to an extremely sensitive cleavage site for the pyrogenic exotoxin B-derived streptococcal cysteine protease.