Subtilisin-like protease-1 secreted through type IV secretion system contributes to high virulence of Streptococcus suis 2

Streptococcus suis subtilisin-like serine proteases SspA-1 and SspA-2 interplay with complement C3a and C5a to facilitate bacterial immune evasion and infection

Streptococcus suis (S. suis), a significant zoonotic bacterial pathogen impacting swine and human, is associated with severe systemic diseases such as streptococcal toxic shock-like syndrome, meningitis, septicaemia, and abrupt fatality. The multifaceted roles of complement components C5a and C3a extend to orchestrating inflammatory cells recruitment, oxidative burst induction, and cytokines release. Despite the pivotal role of subtilisin-like serine proteases in S. suis pathogenicity, their involvement in immune evasion remains underexplored. In the present study, we identify two cell wall-anchored subtilisin-like serine proteases in S. suis, SspA-1 and SspA-2, as binding partners for C3a and C5a. Through Co-Immunoprecipitation, Enzyme-Linked Immunosorbent and Far-Western Blotting Assays, we validate their interactions with the aforementioned components. However, SspA-1 and SspA-2 have no cleavage activity against complement C3a and C5a performed by Cleavage assay. Chemotaxis assays reveal that recombinant SspA-1 and SspA-2 effectively attenuate monocyte chemotaxis towards C3a and C5a. Notably, the ΔsspA-1, ΔsspA-1, and ΔsspA-1/2 mutant strains exhibit compromised survival in blood, and resistance of opsonophagocytosis, alongside impaired survival in blood and in vivo colonization compared to the parental strain SC-19. Critical insights from the murine and Galleria mellonella larva infection models further underscore the significance of sspA-1 in altering mortality rates. Collectively, our findings indicate that SspA-1 and SspA-2 are novel binding proteins for C3a and C5a, thereby shedding light on their pivotal roles in S. suis immune evasion and the pathogenesis.

Streptococcus suis Serotype 2 Type IV Secretion Effector SspA-1 Induces Proinflammatory Cytokine Production via TLR2 Endosomal and Type I Interferon Signaling

The subtilisin-like protease-1 (SspA-1) plays an important role in the pathogenesis of a highly virulent strain of Streptococcus suis 2. However, the mechanism of SspA-1-triggered excessive inflammatory response is still unknown. In this study, we demonstrated that activation of type I IFN signaling is required for SspA-1-induced excessive proinflammatory cytokine production. Further experiments showed that the TLR2 endosomal pathway mediates SspA-1-induced type I IFN signaling and the inflammatory response. Finally, we mapped the major signaling components of the related pathway and found that the TIR adaptor proteins Mal, TRAM, and MyD88 and the downstream activation of IRF1 and IRF7 were involved in this pathway. These results explain the molecular mechanism by which SspA-1 triggers an excessive inflammatory response and reveal a novel effect of type I IFN in S. suis 2 infection, possibly providing further insights into the pathogenesis of this highly virulent S. suis 2 strain.

Genomic and phenotypic analysis of invasive Streptococcus suis isolated in Spain reveals genetic diversification and associated virulence traits

Streptococcus suis is a zoonotic pathogen that causes a major health problem in the pig production industry worldwide. Spain is one of the largest pig producers in the world. This work aimed to investigate the genetic and phenotypic features of invasive S. suis isolates recovered in Spain. A panel of 156 clinical isolates recovered from 13 Autonomous Communities, representing the major pig producers, were analysed. MLST and serotyping analysis revealed that most isolates (61.6%) were assigned to ST1 (26.3%), ST123 (18.6%), ST29 (9.6%), and ST3 (7.1%). Interestingly, 34 new STs were identified, indicating the emergence of novel genetic lineages. Serotypes 9 (27.6%) and 1 (21.8%) prevailed, followed by serotypes 7 (12.8%) and 2 (12.2%). Analysis of 13 virulence-associated genes showed significant associations between ST, serotype, virulence patterns, and clinical features, evidencing particular virulence traits associated with genetic clusters. The pangenome was generated, and the core genome was distributed in 7 Bayesian groups where each group included a variable set of over- and under-represented genes of different categories. The study provides comprehensive data and knowledge to improve the design of new vaccines, antimicrobial treatments, and bacterial typing approaches.

High selectivity of the hyperthermophilic subtilase propeptide domain toward inhibition of its cognate protease

Microbial extracellular subtilases are highly active proteolytic enzymes commonly used in commercial applications. These subtilases are synthesized in their inactive proform, which matures into the active protease under the control of the propeptide domain. In mesophilic bacterial prosubtilases, the propeptide functions as both an obligatory chaperone and an inhibitor of the subtilase catalytic domain. In contrast, the propeptides of hyperthermophilic archaeal prosubtilases act mainly as tight inhibitors and are not essential for subtilase folding. It is unclear whether this stronger inhibitory activity of hyperthermophilic propeptides results in their higher selectivity toward their cognate subtilases, in contrast to promiscuous mesophilic propeptides. Here, we showed that the propeptide of pernisine, a hyperthermostable archaeal subtilase, strongly interacts with and inhibits pernisine, but not the homologous subtilisin Carlsberg and proteinase K. Instead, the pernisine propeptide was readily degraded by subtilisin Carlsberg and proteinase K. In addition, the catalytic domain of unprocessed propernisine was also susceptible to degradation but became proteolytically stable after autoprocessing of propernisine into the inactive, noncovalent complex propeptide:pernisine. This allowed efficient transactivation of the autoprocessed complex propeptide:pernisine through degradation of pernisine propeptide by subtilisin Carlsberg and proteinase K at mesophilic temperature. Moreover, we demonstrated that active pernisine molecules are inhibited by the propeptide that is released after pernisine-catalyzed degradation of the unprocessed propernisine catalytic domain. This highlights the high inhibitory potency of the hyperthermophilic propeptide toward its cognate subtilase and its importance in regulating subtilase maturation, to prevent the degradation of the unprocessed subtilase precursors by the prematurely activated molecules. IMPORTANCE Many microorganisms secrete proteases into their environment to degrade protein substrates for their growth. The important group of these extracellular enzymes are subtilases, which are also widely used in practical applications. These subtilases are inhibited by their propeptide domain, which is degraded during the prosubtilase maturation process. Here, we showed that the propeptide of pernisine, a prion-degrading subtilase from the hyperthermophilic archaeon, strongly inhibits pernisine with extraordinarily high binding affinity. This interaction proved to be highly selective, as pernisine propeptide was rapidly degraded by mesophilic pernisine homologs. This in turn allowed rapid transactivation of propernisine by mesophilic subtilases at lower temperatures, which might simplify the procedures for preparation of active pernisine for commercial use. The results reported in this study suggest that the hyperthermophilic subtilase propeptide evolved to function as tight and selective regulator of maturation of the associated prosubtilase to prevent its premature activation under high temperatures.

Oligomerization and Adjuvant Activity of Peptides Derived from the VirB4-like ATPase of Clostridioides difficile

In a previous study, we demonstrated that the Clostridioides difficile VirB4-like ATPase forms oligomers in vitro. In the current investigation, to study the observed phenomenon in more detail, we prepared a library of VirB4-derived peptides (delVirB4s) fused to a carrier maltose-binding protein (MBP). Using gel chromatography and polyacrylamide gel electrophoresis, we found a set of overlapping fragments that contribute most significantly to protein aggregation, which were represented as water-soluble oligomers with molecular masses ranging from ~300 kD to several megadaltons. Membrane filtration experiments, sucrose gradient ultracentrifugation, and dynamic light scattering measurements indicated the size of the soluble complex to be 15-100 nm. It was sufficiently stable to withstand treatment with 1 M urea; however, it dissociated in a 6 M urea solution. As shown by the changes in GFP fluorescence and the circular dichroism spectra, the attachment of the delVirB4 peptide significantly altered the structure of the partner MBP. The immunization of mice with the hybrid consisting of the selected VirB4-derived peptide and MBP, GST, or GFP resulted in increased production of specific antibodies compared to the peptide-free carrier proteins, suggesting significant adjuvant activity of the VirB4 fragment. This feature could be useful for the development of new vaccines, especially in the case of "weak" antigens that are unable to elicit a strong immune response by themselves.

Riemerella anatipestifer T9SS Effector SspA Functions in Bacterial Virulence and Defending Natural Host Immunity

Riemerella anatipestifer is a major pathogenic agent of duck septicemic and exudative diseases. Recent studies have shown that the R. anatipestifer type IX secretion system (T9SS) is a crucial factor in bacterial virulence. The AS87_RS04190 protein was obviously missing from the secreted proteins of the T9SS mutant strain Yb2ΔgldM. A bioinformatic analysis indicated that the AS87_RS04190 protein contains a T9SS C-terminal domain sequence and encodes a putative subtilisin-like serine protease (SspA). To determine the role of the putative SspA protein in R. anatipestifer pathogenesis and proteolysis, we constructed two strains with an sspA mutation and complementation, respectively, and determined their median lethal doses, their bacterial loads in infected duck blood, and their adherence to and invasion of cells. Our results demonstrate that the SspA protein functions in bacterial virulence. It is also associated with the bacterial protease activity and has a conserved catalytic triad structure (Asp126, His158, and Ser410), which is necessary for protein function. The optimal reactive pH and temperature were determined to be 7.0 and 50°C, respectively, and K_m and V_max were determined to be 10.15 mM and 246.96 U/mg, respectively. The enzymatic activity of SspA is activated by Ca²⁺, Mg²⁺, and Mn²⁺ and inhibited by Cu²⁺ and EDTA. SspA degrades gelatin, fibrinogen, and bacitracin LL-37. These results demonstrate that SspA is an effector protein of T9SS and functions in R. anatipestifer virulence and its proteolysis of gelatin, fibrinogen, and bacitracin LL-37. IMPORTANCE In recent years, Riemerella anatipestifer T9SS has been reported to act as a virulence factor. However, the functions of the proteins secreted by R. anatipestifer T9SS are not entirely clear. In this study, a secreted subtilisin-like serine protease SspA was shown to be associated with R. anatipestifer virulence, host complement evasion, and degradation of gelatin, fibrinogen, and LL-37. The enzymatic activity of recombinant SspA was determined, and its K_m and V_max were 10.15 mM and 246.96 U/mg, respectively. Three conserved sites (Asp126, His158, and Ser410) are necessary for the protein's function. The median lethal dose of the sspA-deleted mutant strain was reduced >10,000-fold, indicating that SspA is an important virulence factor. In summary, we demonstrate that the R. anatipestifer AS87_RS04190 gene encodes an important T9SS effector, SspA, which plays an important role in bacterial virulence.

The Role of Streptococcal Cell-Envelope Proteases in Bacterial Evasion of the Innate Immune System

Bacteria possess the ability to evolve varied and ingenious strategies to outwit the host immune system, instigating an evolutionary arms race. Proteases are amongst the many weapons employed by bacteria, which specifically cleave and neutralize key signalling molecules required for a coordinated immune response. In this article, we focus on a family of S8 subtilisin-like serine proteases expressed as cell-envelope proteases (CEPs) by group A and group B streptococci. Two of these proteases known as Streptococcus pyogenes CEP (SpyCEP) and C5a peptidase cleave the chemokine CXCL8 and the complement fragment C5a, respectively. Both CXCL8 and C5a are potent neutrophil-recruiting chemokines, and by neutralizing their activity, streptococci evade a key defence mechanism of innate immunity. We review the mechanisms by which CXCL8 and C5a recruit neutrophils and the characterization of SpyCEP and C5a peptidase, including both in vitro and in vivo studies. Recently described structural insights into the function of this CEP family are also discussed. We conclude by examining the progress of prototypic vaccines incorporating SpyCEP and C5a peptidase in their preparation. Since streptococci-producing SpyCEP and C5a peptidase are responsible for a considerable global disease burden, targeting these proteases by vaccination strategies or by small-molecule antagonists should provide protection from and promote the resolution of streptococcal infections.

VirB4- and VirD4-Like ATPases, Components of a Putative Type 4C Secretion System in Clostridioides difficile

The type 4 secretion system (T4SS) represents a bacterial nanomachine capable of trans-cell wall transportation of proteins and DNA and has attracted intense interest due to its roles in the pathogenesis of infectious diseases. In the current investigation, we uncovered three distinct gene clusters in Clostridioides difficile strain 630 encoding proteins structurally related to components of the VirB4/D4 type 4C secretion system from Streptococcus suis strain 05ZYH33 and located within sequences of conjugative transposons (CTn). Phylogenic analysis revealed that VirB4- and VirD4-like proteins of the CTn4 locus, on the one hand, and those of the CTn2 and CTn5 loci, on the other hand, fit into separate clades, suggesting specific roles of identified secretion system variants in the physiology of C. difficile. Our further study on VirB4- and VirD4-like products encoded by CTn4 revealed that both proteins possess Mg²⁺-dependent ATPase activity, form oligomers (most likely hexamers) in aqueous solutions, and rely on potassium but not sodium ions for the highest catalytic rate. VirD4 binds nonspecifically to DNA and RNA. The DNA-binding activity of VirD4 strongly decreased with the W241A variant. Mutations in the nucleotide sequences encoding presumable Walker A and Walker B motifs decreased the stability of the oligomers and significantly but not completely attenuated the enzymatic activity of VirB4. In VirD4, substitutions of amino acid residues in the peptides reminiscent of Walker structural motifs neither attenuated the enzymatic activity of the protein nor influenced the oligomerization state of the ATPase. IMPORTANCE C. difficile is a Gram-positive, anaerobic, spore-forming bacterium that causes life-threatening colitis in humans. Major virulence factors of the microorganism include the toxins TcdA, TcdB, and CDT. However, other bacterial products, including a type 4C secretion system, have been hypothesized to contribute to the pathogenesis of the infection and are considered possible virulence factors of C. difficile. In the current paper, we describe the structural organization of putative T4SS machinery in C. difficile and characterize its VirB4- and VirD4-like components. Our studies, in addition to its significance for basic science, can potentially aid the development of antivirulence drugs suitable for the treatment of C. difficile infection.

Gene Analysis, Cloning, and Heterologous Expression of Protease from a Micromycete Aspergillus ochraceus Capable of Activating Protein C of Blood Plasma

Micromycetes are known to secrete numerous enzymes of biotechnological and medical potential. Fibrinolytic protease-activator of protein C (PAPC) of blood plasma from micromycete Aspergillus ochraceus VKM-F4104D was obtained in recombinant form utilising the bacterial expression system. This enzyme, which belongs to the proteinase-K-like proteases, is similar to the proteases encoded in the genomes of Aspergillus fumigatus ATCC MYA-4609, A. oryzae ATCC 42149 and A. flavus 28. Mature PAPC-4104 is 282 amino acids long, preceded by the 101-amino acid propeptide necessary for proper folding and maturation. The recombinant protease was identical to the native enzyme from micromycete in terms of its biological properties, including an ability to hydrolyse substrates of activated protein C (pGlu-Pro-Arg-pNA) and factor Xa (Z-D-Arg-Gly-Arg-pNA) in conjugant reactions with human blood plasma. Therefore, recombinant PAPC-4104 can potentially be used in medicine, veterinary science, diagnostics, and other applications.

Involvement of Various Enzymes in the Physiology and Pathogenesis of Streptococcus suis

Streptococcus suis causes severe infections in both swine and humans, making it a serious threat to the swine industry and public health. Insight into the physiology and pathogenesis of S. suis undoubtedly contributes to the control of its infection. During the infection process, a wide variety of virulence factors enable S. suis to colonize, invade, and spread in the host, thus causing localized infections and/or systemic diseases. Enzymes catalyze almost all aspects of metabolism in living organisms. Numerous enzymes have been characterized in extensive detail in S. suis, and have shown to be involved in the pathogenesis and/or physiology of this pathogen. In this review, we describe the progress in the study of some representative enzymes in S. suis, such as ATPases, immunoglobulin-degrading enzymes, and eukaryote-like serine/threonine kinase and phosphatase, and we highlight the important role of various enzymes in the physiology and pathogenesis of this pathogen. The controversies about the current understanding of certain enzymes are also discussed here. Additionally, we provide suggestions about future directions in the study of enzymes in S. suis.

Intranasal Vaccination With Multiple Virulence Factors Promotes Mucosal Clearance of Streptococcus suis Across Serotypes and Protects Against Meningitis in Mice

BACKGROUND

Streptococcus suis is an emerging zoonotic agent. Its natural habitat is the tonsils, which are the main portals of S. suis entry into the bloodstream of pigs. The remarkable variability of the bacteria and complex pathogenic mechanisms make the development of a vaccine a difficult task.

METHOD

Five conserved virulence factors involved in critical events of S. suis pathogenesis were combined and used as an intranasal vaccine (V5). The effect of V5 was investigated with intranasal and systemic challenge models.

RESULTS

V5 induced antibody and T-cell responses at the mucosal site and systemically. The immunity promoted clearance of S. suis from the nasopharynx independent of S. suis serotypes and reduced lethality after systemic challenge with S. suis serotype 2. Moreover, mice that survived sepsis from intravenous infection developed meningitis, whereas none of these mice showed neuropathological symptoms after V5 receipt.

CONCLUSION

Intranasal immunization with multiple conserved virulence factors decreases S. suis colonization at the nasopharynx across serotypes and inhibits the dissemination of the bacteria in the host. The protective mucosal immunity effects would potentially reduce the S. suis reservoir and prevent S. suis disease in pigs.

The Impact of SsPI-1 Deletion on Streptococcus suis Virulence

(1) Background: Streptococcus suis is an important zoonotic pathogen that infects pigs and can occasionally cause life-threatening systemic infections in humans. Two large-scale outbreaks of streptococcal toxic shock-like syndrome in China suggest that the pathogenicity of S. suis has been changing in recent years. Genetic analysis revealed the presence of a chromosomal pathogenicity island (PAI) designated SsPI-1 in Chinese epidemic S. suis strains. The purpose of this study is to define the role of SsPI-1 in the virulence of S. suis. (2) Methods: A SsPI-1 deletion mutant was compared to the wild-type strain regarding the ability to attach to epithelial cells, to cause host disease and mortality, and to stimulate host immune response in experimental infection of piglets. (3) Results: Deletion of SsPI-1 significantly reduces adherence of S. suis to epithelial cells and abolishes the lethality of the wild-type strain in piglets. The SsPI-1 mutant causes no significant pathological lesions and exhibits an impaired ability to induce proinflammatory cytokine production. (4) Conclusions: Deletion of the SsPI-1 PAI attenuates the virulence of this pathogen. We conclude that SsPI-1 is a critical contributor to the evolution of virulence in epidemic S. suis.

Comparative immunosecretome analysis of prevalent Streptococcus suis serotypes

Streptococcus suis is a major Gram-positive swine pathogen associated with a wide variety of diseases in pigs. The efforts made to develop vaccines against this pathogen have failed because of lack of common cross-reactive antigens against different serotypes. Nowadays the interest has moved to surface and secreted proteins, as they have the highest chances to raise an effective immune response because they are in direct contact with host cells and are really exposed and accessible to antibodies. In this work, we have performed a comparative immunosecretomic approach to identify a set of immunoreactive secreted proteins common to the most prevalent serotypes of S. suis. Among the 67 proteins identified, three (SSU0020, SSU0934, and SSU0215) were those predicted extracellular proteins most widely found within the studied serotypes. These immunoreactive proteins may be interesting targets for future vaccine development as they could provide possible cross-reactivity among different serotypes of this pathogen.

Type IV secretion in Gram-negative and Gram-positive bacteria

Type IV secretion systems (T4SSs) are versatile multiprotein nanomachines spanning the entire cell envelope in Gram-negative and Gram-positive bacteria. They play important roles through the contact-dependent secretion of effector molecules into eukaryotic hosts and conjugative transfer of mobile DNA elements as well as contact-independent exchange of DNA with the extracellular milieu. In the last few years, many details on the molecular mechanisms of T4SSs have been elucidated. Exciting structures of T4SS complexes from Escherichia coli plasmids R388 and pKM101, Helicobacter pylori and Legionella pneumophila have been solved. The structure of the F-pilus was also reported and surprisingly revealed a filament composed of pilin subunits in 1:1 stoichiometry with phospholipid molecules. Many new T4SSs have been identified and characterized, underscoring the structural and functional diversity of this secretion superfamily. Complex regulatory circuits also have been shown to control T4SS machine production in response to host cell physiological status or a quorum of bacterial recipient cells in the vicinity. Here, we summarize recent advances in our knowledge of 'paradigmatic' and emerging systems, and further explore how new basic insights are aiding in the design of strategies aimed at suppressing T4SS functions in bacterial infections and spread of antimicrobial resistances.

Functional analysis of BAS2108-2109 two component system: Evidence for protease regulation in Bacillus anthracis

BACKGROUND

Bacillus anthracis (BA) is a major bioterrorism concern which has evolved complex regulatory mechanisms for its virulence factors. Secreted proteases play an imperative role in the pathogenesis of BA, however their regulation remains elusive. Two component systems (TCS) are often employed by bacteria to sense and adapt to the environmental perturbations. In several pathogens, TCS are commonly associated with the regulation of virulence factors including proteases. The genome of BA encodes 41 TCS pairs, however, the role of any TCS in regulation of its proteases is not known.

PRINCIPAL FINDINGS

The study established BAS2108-2109 as a prototypical TCS where BAS2108 functions as a histidine kinase and BAS2109 as the response regulator. The expression of BAS2109 was found to be elevated under host simulated conditions and in pellicle forming cells. Electrophoretic mobility shift assay (EMSA) and lacZ reporter assay revealed positive autoregulation of the BAS2108-2109 operon by BAS2109. Collective analysis of ANS assay and EMSA demonstrated Lys167, Thr179 and Thr182 residues are crucial for the DNA binding activity of BAS2109. EMSA analysis further highlighted BAS2109 as the transcriptional regulator for different genes of BA, particularly proteases. Upregulation of proteases in BA overexpressing BAS2109 further strengthen its role in protease regulation.

SIGNIFICANCE

This is the first report to identify a TCS pair for its role in the regulation of proteases of BA. Importance of proteases in the pathogenesis of BA is well documented, therefore, studying the regulatory networks governing their expression will help in identification of new drug targets.

Pathogenic Streptococcus strains employ novel escape strategy to inhibit bacteriostatic effect mediated by mammalian peptidoglycan recognition protein

Pathogenic streptococcal species are responsible for some of the most lethal and prevalent animal and human infections. Previous reports have identified a candidate pathogenicity island (PAI) in two highly virulent clinical isolates of Streptococcus suis type 2, a causative agent of high-mortality streptococcal toxic shock syndrome. This PAI contains a type-IVC secretion system C subgroup (type-IVC secretion system) that is involved in the secretion of unknown pathogenic effectors that are responsible for streptococcal toxic shock syndrome caused by highly virulent strains of S. suis. Both virulence protein B4 and virulence protein D4 were demonstrated to be key components of this type-IVC secretion system. In this study, we identify a new PAI family across 3 streptococcal species; Streptococcus genomic island contains type-IV secretion system, which contains a genomic island type-IVC secretion system and a novel PPIase molecule, SP1. SP1 is shown to interact with a component of innate immunity, peptidoglycan recognition protein (PGLYRP-1) and to perturb the PGLYRP-1-mediated bacteriostatic effect by interacting with protein PGLYRP-1. Our study elucidates a novel mechanism by which bacteria escape by components of the innate immune system by secretion of the SP1 protein in pathogenic Streptococci, which then interacts with PGLYRP-1 from the host. Our results provide potential targets for the development of new antimicrobial drugs against bacteria with resistance to innate host immunity.

Mechanisms of Conjugative Transfer and Type IV Secretion-Mediated Effector Transport in Gram-Positive Bacteria

Conjugative DNA transfer is the most important means to transfer antibiotic resistance genes and virulence determinants encoded by plasmids, integrative conjugative elements (ICE), and pathogenicity islands among bacteria. In gram-positive bacteria, there exist two types of conjugative systems, (i) type IV secretion system (T4SS)-dependent ones, like those encoded by the Enterococcus, Streptococcus, Staphylococcus, Bacillus, and Clostridia mobile genetic elements and (ii) T4SS-independent ones, as those found on Streptomyces plasmids. Interestingly, very recently, on the Streptococcus suis genome, the first gram-positive T4SS not only involved in conjugative DNA transfer but also in effector translocation to the host was detected. Although no T4SS core complex structure from gram-positive bacteria is available, several structures from T4SS protein key factors from Enterococcus and Clostridia plasmids have been solved. In this chapter, we summarize the current knowledge on the molecular mechanisms and structure-function relationships of the diverse conjugation machineries and emerging research needs focused on combatting infections and spread of multiple resistant gram-positive pathogens.