Purification and characterization of streptolysin O secreted by Streptococcus equisimilis (group C)

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.

A case of streptococcal toxic shock syndrome due to Group G streptococci identified as Streptococcus dysgalactiae subsp. equisimilis

A 79-year-old man with a 3-month history of lymphedema of the lower limbs, and diabetes mellitus, was admitted to our hospital for suspected deep venous thrombosis. Several hours after admission, leg pain and purpura-like skin color appeared. On the 2nd hospital day, he was referred to our department for possible acute occlusive peripheral artery disease (PAD) and skin necrosis with blisters; however, computed tomography with contrast showed no occlusive lesions. He had already developed shock and necrotizing deep soft-tissue infections of the left lower leg. Laboratory findings revealed renal dysfunction and coagulation system collapse. Soon after PAD was ruled out, clinical findings suggested necrotizing deep soft-tissue infections, shock state, disseminated intravascular coagulation, and multiple organ failure. These symptoms led to a high suspicion of the well-recognized streptococcal toxic shock syndrome (STSS). With a high suspicion of STSS, we detected Group G β-hemolytic streptococci (GGS) from samples aspirated from the leg bullae, and the species was identified as Streptococcus dysgalactiae subsp. equisimilis (SDSE) by 16S-ribosomal RNA sequencing. However, unfortunately, surgical debridement was impossible due to the broad area of skin change. Despite adequate antimicrobial therapy and intensive care, the patient died on the 3rd hospital day. The M-protein gene (emm) typing of the isolated SDSE was revealed to be stG6792. This type of SDSE is the most frequent cause of STSS due to GGS in Japan. We consider it to be crucial to rapidly distinguish STSS from acute occlusive PAD to achieve life-saving interventions in patients with severe soft-tissue infections.

[A case of acute septic osteomyelitis onset due to Streptococcus dysgalactiae subsp. equisimilis in an elderly diabetic patient]

Group C streptococci are increasingly causing invasive infections such as that we report here. A 70-year-old man being treated for diabetes and seen at the emergency room for neck pain and fever was hospitalized for possible sepsis. His temperature was 39.8 degrees C, regular pulse 101 bpm, and pain reinforced in flexing and cervical rotation. Streptococcus dysgalactiae subsp. equisimilis (SDSE) was cultured from blood. Neck pain gradually decreased with of 2 million units PCG 6 times/day. Magnetic resonance imaging (MRI) of the cervical spine showed high-intensity areas in fat-suppression imaging at C7, Thl and intervertebral disks plus enhancement around the vertebral body, yielding a diagnosis of cervicothoracic vertebral osteomyelitis. Antimicrobial intravenous therapy continuede 6 weeks. The man was discharged after 45 days without relapse.

Molecular emm genotyping and antibiotic susceptibility of Streptococcus dysgalactiae subsp. equisimilis isolated from invasive and non-invasive infections

To analyse the characteristics of infections caused by Streptococcus dysgalactiae subsp. equisimilis, clinical isolates (n=145) were collected at 11 medical institutions between September 2003 and October 2005. These isolates belonged to Lancefield group A (n=5), group C (n=18) or group G (n=122). Among all isolates, 42 strains were isolated from sterile samples such as blood, synovial fluid and tissue specimens from patients who were mostly over 50 years with invasive infections, and included seven cases of streptococcal toxic shock syndrome and necrotizing fasciitis. In contrast, the remaining 103 were isolated mainly from patients of all age groups with non-invasive infections such as pharyngotonsillitis. These isolates were classified into 25 types based on emm genotyping. A significant difference in emm types was observed between isolates from invasive and non-invasive infections (P<0.001): stG485, stG6792 and stG2078 predominated among isolates from invasive infections. A phylogenetic tree of complete open reading frames of emm genes in this organism showed high homology with those of Streptococcus pyogenes, but not with those of other streptococci. The presence of five different clones was estimated based on DNA profiles of isolates from invasive infections obtained by PFGE. Genes for resistance to macrolides [erm(A), three isolates; erm(B), five isolates; mef(A), seven isolates] and levofloxacin (mutations in gyrA and parC, four isolates) were identified in this organism. These results suggest the need for further nationwide surveillance of invasive infections caused by S. dysgalactiae subsp. equisimilis.

The MACPF/CDC family of pore-forming toxins

Pore-forming toxins (PFTs) are commonly associated with bacterial pathogenesis. In eukaryotes, however, PFTs operate in the immune system or are deployed for attacking prey (e.g. venoms). This review focuses upon two families of globular protein PFTs: the cholesterol-dependent cytolysins (CDCs) and the membrane attack complex/perforin superfamily (MACPF). CDCs are produced by Gram-positive bacteria and lyse or permeabilize host cells or intracellular organelles during infection. In eukaryotes, MACPF proteins have both lytic and non-lytic roles and function in immunity, invasion and development. The structure and molecular mechanism of several CDCs are relatively well characterized. Pore formation involves oligomerization and assembly of soluble monomers into a ring-shaped pre-pore which undergoes conformational change to insert into membranes, forming a large amphipathic transmembrane β-barrel. In contrast, the structure and mechanism of MACPF proteins has remained obscure. Recent crystallographic studies now reveal that although MACPF and CDCs are extremely divergent at the sequence level, they share a common fold. Together with biochemical studies, these structural data suggest that lytic MACPF proteins use a CDC-like mechanism of membrane disruption, and will help understand the roles these proteins play in immunity and development.

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.

Hypoplastic left heart syndrome: Rheumatic heart disease of the fetus?

Hypoplastic left heart syndrome (HLHS) accounts for nearly 25% of deaths among neonates with congenital heart disease. The essential feature of HLHS is a small left ventricle (LV) incapable of supporting the circulation. The etiology of HLHS is unknown. A hypothesis is proposed implicating an immune mechanism involving maternal antibodies produced in response to pharyngitis caused by group A beta-hemolytic streptococci (GABHS) ("strep throat"). After crossing the placenta, the antibodies injure the developing fetal heart, leading to HLHS either because of direct injury to the LV or secondary to reduced blood flow through affected aortic and mitral valves. Analogy is drawn to rheumatic heart disease (RHD), a known sequela of strep throat. In RHD a misdirected immune response originally intended for GABHS leads to cardiac injury through "molecular mimicry"; the normal heart antigens supposedly mimic the GABHS antigens. A similar pathogenesis is proposed for HLHS and related heart defects. HLHS may represent an extreme form of injury, while a milder insult may present as only mild aortic stenosis or a bicuspid aortic valve, conditions with wide prevalence among the general population. The injury may indeed superimpose on many other congenital heart defects, leading to a variable presentation of these other diseases. Beside remarkable likenesses between HLHS and RHD, the hypothesis is also supported by increasing evidence for the role of deleterious transplacental antibodies in the pathogenesis of other fetal diseases. Implications for other congenital heart diseases and the broader picture of global public health are discussed.

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.

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 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.

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.

Cloning and sequencing the streptolysin O genes of group C and group G streptococci

On the basis of the known streptolysin O (SLO) genomic sequence of Streptococcus pyogenes group A, we identified the SLO genes in some strains of group C and group G streptococci by the polymerase chain reaction procedure (PCR). The entire open reading frame region of these genes was cloned and analyzed. Their nucleotide sequence data showed that the defined SLO genes in group C and group G are almost identical to that of group A.