Cooperative plasminogen recruitment to the surface of Streptococcus canis via M protein and enolase enhances bacterial survival

Genetic organization of an M protein trans-acting positive regulator (Mga) orthologue and its adjacent M-like protein (SCM) alleles in Streptococcus canis

OBJECTIVE

The purpose of this study was to identify the M protein trans-acting positive regulator (Mga) orthologue and its adjacent M-like protein (SCM) alleles in Streptococcus canis.

RESULTS

Using the 39 SCM allele isolates and polymerase chain reaction-based amplification and sequencing, we obtained the deduced Mga amino acid (AA) sequences. The 22 Mga sequences in whole-genome sequences were obtained by searching the National Collection of Type Cultures 12,191(T) Mga sequence into the database. The percentage identity to the type-strain Mga sequence was examined along with its size. The presence of the Mga-specific motifs was confirmed. Of the 62 strains, we identified 59 Mga sequences with an AA size of 509 (except for four different sizes). Percentage identity ranged from 96.66 to 100% with the confirmed Mga-specific motifs and diverse SCM allele populations. Our findings support the presence of an Mga orthologue and diverse SCM allele populations.

A distinct variant of the SzM protein of Streptococcus equi subsp. zooepidemicus recruits C1q independent of IgG binding and inhibits activation of the classical complement pathway

Streptococcus equi subsp. zooepidemicus (SEZ) is a major equine pathogen that causes pneumonia, abortion, and polyarthritis. It can also cause invasive infections in humans. SEZ expresses the M-like protein SzM, which recruits host proteins such as fibrinogen to the bacterial surface. Equine SEZ strain C2, which binds only comparably low amounts of human fibrinogen in comparison to human SEZ strain C33, was previously shown to proliferate in equine and human blood. As the expression of SzM_C2 was necessary for survival in blood, this study investigated the working hypothesis that SzM_C2 inhibits complement activation through a mechanism other than fibrinogen and non-immune immunoglobulin binding. Loss-of-function experiments showed that SEZ C2, but not C33, binds C1q via SzM in IgG-free human plasma. Furthermore, SzM C2 expression is necessary for recruiting purified human or equine C1q to the bacterial surface. Flow cytometry analysis demonstrated that SzM expression in SEZ C2 is crucial for the significant reduction of C3b labelling in human plasma. Addition of human plasma to immobilized rSzM_C2 and immobilized aggregated IgG led to binding of C1q, but only the latter activated the complement system, as shown by the detection of C4 deposition. Complement activation induced by aggregated IgG was significantly reduced if human plasma was pre-incubated with rSzM_C2. Furthermore, rSzM_C2, but not rSzM_C33, inhibited the activation of the classical complement pathway in human plasma, as determined in an erythrocyte lysis experiment. In conclusion, the immunoglobulin-independent binding of C1q to SzM_C2 is associated with complement inhibition.

Immunisation with the glycolytic enzyme enolase inhibits dissemination of Treponema pallidum in C57BL/6 mice

Treponema pallidum (T. pallidum), an obligate extracellular bacterium, is the causative agent of sexually transmitted bacterial diseases. In this study, the glycolytic enzyme enolase (Tp Eno) of T. pallidum were injected intramuscularly into C57BL/6 mice, resulting in higher levels of specific anti-Tp Eno antibodies and Tp Eno-specific splenocyte proliferation than those in the mice immunized with recombinant protein Tp Eno. Cytokine (IL-4, IL-6, IL-10, IFN-γ, and TNF-α) analysis of splenocytes showed that the Tp Eno could slightly trigger the Th1-biased immune response. Furthermore, immunization of mice with Tp Eno elicited a significant production of IFN-γ by CD4+ T-cells in the spleen. Subsequently, mice were inoculated intradermally (between the scapulae), intraperitoneally, intrarectally and via the corpora cavernosa with 2.5 × 106 organisms per site (1 × 107 total organisms). The bacterial organ burden detected in the blood, spleen, liver, testes or brain of immunized mice suggested that Tp Eno enhances protective immunity to inhibit T. pallidum colonization in distal tissues. Therefore, Tp Eno vaccination enhances Tp Eno-specific immunogenicity and provides protection against T. pallidum dissemination.

The type-2 Streptococcus canis M protein SCM-2 binds fibrinogen and facilitates antiphagocytic properties

Streptococcus canis is a zoonotic agent that causes severe invasive diseases in domestic animals and humans, but little is known about its pathogenesis and virulence mechanisms so far. SCM, the M-like protein expressed by S. canis, is considered one of the major virulence determinants. Here, we report on the two distinct groups of SCM. SCM-1 proteins were already described to interact with its ligands IgG and plasminogen as well as with itself and confer antiphagocytic capability of SCM-1 expressing bacterial isolates. In contrast, the function of SCM-2 type remained unclear to date. Using whole-genome sequencing and subsequent bioinformatics, FACS analysis, fluorescence microscopy and surface plasmon resonance spectrometry, we demonstrate that, although different in amino acid sequence, a selection of diverse SCM-2-type S. canis isolates, phylogenetically representing the full breadth of SCM-2 sequences, were able to bind fibrinogen. Using targeted mutagenesis of an SCM-2 isolate, we further demonstrated that this strain was significantly less able to survive in canine blood. With respect to similar studies showing a correlation between fibrinogen binding and survival in whole blood, we hypothesize that SCM-2 has an important contribution to the pathogenesis of S. canis in the host.

MPL36, a major plasminogen (PLG) receptor in pathogenic Leptospira, has an essential role during infection

Leptospirosis, a zoonosis with worldwide distribution, is caused by pathogenic spirochetes belonging to the genus Leptospira. Bacterial outer membrane proteins (OMPs), particularly those with surface-exposed regions, play crucial roles in pathogen dissemination and virulence mechanisms. Here we characterized the leptospiral Membrane Protein L36 (MPL36), a rare lipoprotein A (RlpA) homolog with a C-terminal Sporulation related (SPOR) domain, as an important virulence factor in pathogenic Leptospira. Our results confirmed that MPL36 is surface exposed and expressed during infection. Using recombinant MPL36 (rMPL36) we also confirmed previous findings of its high plasminogen (PLG)-binding ability determined by lysine residues of the C-terminal region of the protein, with ability to convert bound-PLG to active plasmin. Using Koch's molecular postulates, we determined that a mutant of mpl36 has a reduced PLG-binding ability, leading to a decreased capacity to adhere and translocate MDCK cell monolayers. Using recombinant protein and mutant strains, we determined that the MPL36-bound plasmin (PLA) can degrade fibrinogen. Finally, our mpl36 mutant had a significant attenuated phenotype in the hamster model for acute leptospirosis. Our data indicates that MPL36 is the major PLG binding protein in pathogenic Leptospira, and crucial to the pathogen's ability to attach and interact with host tissues during infection. The MPL36 characterization contributes to the expanding field of bacterial pathogens that explore PLG for their virulence, advancing the goal to close the knowledge gap regarding leptospiral pathogenesis while offering a novel potential candidate to improve diagnostic and prevention of this important zoonotic neglected disease.

Limiting Factors in Treatment Success of Biofilm-Forming Streptococci in the Case of Canine Infective Endocarditis Caused by Streptococcus canis

An 8-year-old male Rhodesian Ridgeback was presented with fever and severe thrombocytopenia. Clinical and laboratory examination, echocardiography, blood culture, and pathohistology revealed evidence of infective endocarditis, ischemic renal infarcts, and septic encephalitis. Treatment was started immediately but the dog's condition worsened, and the dog had to be euthanized. The causative Streptococcus canis strain was detected by blood culture and MALDI-TOF MS and analyzed using whole-genome sequencing and multilocus sequence typing. Antibiotic susceptibility testing did not detect any resistance. The affected heart valve was analyzed using FISH imaging, which showed a streptococcal biofilm on the heart valve. Bacteria in biofilms are recalcitrant to antibiotic treatment. Early diagnosis could be beneficial to treatment outcome. Treatment of endocarditis could be improved by researching the optimal dosage of antibiotics in conjunction with the use of biofilm-active drugs.

Enolase of Staphylococcus lugdunensis Is a Surface-Exposed Moonlighting Protein That Binds to Extracellular Matrix and the Plasminogen/Plasmin System

The coagulase-negative staphylococcal (CoNS) species Staphylococcus lugdunensis is unique in causing serious infections in humans that resemble those of Staphylococcus aureus rather than those of other CoNS species. The colonization and invasion of host tissue presupposes the presence of adherence factors, but only a few proteins mediating adhesion of S. lugdunensis to biotic surfaces are known yet. Here, we report on the functionality of the S. lugdunensis enolase (SlEno), which performs two distinct roles, first, as the metabolic enzyme of the glycolysis, and second, as an adherence factor to the extracellular matrix (ECM) of cells. Phylogenetic analyses of the SlEno confirmed their high conservation to enolases of other species and revealed a closer relationship to Staphylococcus epidermidis than to S. aureus. Using matrix-assisted laser desorption/ionization time of flight mass spectrometry and Western blot experiments, we identified SlEno to be located in the cytoplasm as well as on the cell surface of S. lugdunensis. Recombinantly generated and surface-associated SlEno showed the usual enolase activity by catalyzing the conversion of 2-phosphoglycerate to phosphoenolpyruvate but, in addition, also displayed strong binding to immobilized laminin, fibronectin, fibrinogen, and collagen type IV in a dose-dependent manner. We also showed a strong binding of SlEno to plasminogen (Plg) and observed a tissue plasminogen activator (tPA)-dependent conversion of Plg to plasmin (Pln) whereby the Plg activation significantly increased in the presence of SlEno. This interaction might be dependent on lysines of the SlEno protein as binding to Plg was inhibited by ε-aminocaproic acid. Furthermore, the enhanced activation of the Plg/Pln system by SlEno enabled S. lugdunensis to migrate through a fibrin matrix. This migration was about 10-fold higher than without exogenously added SlEno. Finally, we observed a significantly higher clearance of S. lugdunensis by freshly prepared granulocytes and in the presence of anti-SlEno antibodies. In conclusion, these data demonstrate for the first time a moonlighting function of the S. lugdunensis enolase, which is an underrated virulence factor for colonization and invasion of tissues. Hence, SlEno might be a potential vaccine candidate to prevent severe infections caused by this pathogen.

Endogenous and Borrowed Proteolytic Activity in the Borrelia

The Borrelia spp. are tick-borne pathogenic spirochetes that include the agents of Lyme disease and relapsing fever. As part of their life cycle, the spirochetes traffic between the tick vector and the vertebrate host, which requires significant physiological changes and remodeling of their outer membranes and proteome. This crucial proteome resculpting is carried out by a diverse set of proteases, adaptor proteins, and related chaperones. Despite its small genome, Borrelia burgdorferi has dedicated a large percentage of its genome to proteolysis, including a full complement of ATP-dependent proteases. Energy-driven proteolysis appears to be an important physiological feature of this dual-life-cycle bacterium. The proteolytic arsenal of Borrelia is strategically deployed for disposal of proteins no longer required as they move from one stage to another or are transferred from one host to another. Likewise, the Borrelia spp. are systemic organisms that need to break down and move through host tissues and barriers, and so their unique proteolytic resources, both endogenous and borrowed, make movement more feasible. Both the Lyme disease and relapsing fever Borrelia spp. bind plasminogen as well as numerous components of the mammalian plasminogen-activating system. This recruitment capacity endows the spirochetes with a borrowed proteolytic competency that can lead to increased invasiveness.

Streptococcal Infections in Marine Mammals

Marine mammals are sentinels for the marine ecosystem and threatened by numerous factors including infectious diseases. One of the most frequently isolated bacteria are beta-hemolytic streptococci. However, knowledge on ecology and epidemiology of streptococcal species in marine mammals is very limited. This review summarizes published reports on streptococcal species, which have been detected in marine mammals. Furthermore, we discuss streptococcal transmission between and adaptation to their marine mammalian hosts. We conclude that streptococci colonize and/or infect marine mammals very frequently, but in many cases, streptococci isolated from marine mammals have not been further identified. How these bacteria disseminate and adapt to their specific niches can only be speculated due to the lack of respective research. Considering the relevance of pathogenic streptococci for marine mammals as part of the marine ecosystem, it seems that they have been neglected and should receive scientific interest in the future.

Novel Models of Streptococcus canis Colonization and Disease Reveal Modest Contributions of M-Like (SCM) Protein

Streptococcus canis is a common colonizing bacterium of the urogenital tract of cats and dogs that can also cause invasive disease in these animal populations and in humans. Although the virulence mechanisms of S. canis are not well-characterized, an M-like protein, SCM, has recently identified been as a potential virulence factor. SCM is a surface-associated protein that binds to host plasminogen and IgGs suggesting its possible importance in host-pathogen interactions. In this study, we developed in vitro and ex vivo blood component models and murine models of S. canis vaginal colonization, systemic infection, and dermal infection to compare the virulence potential of the zoonotic S. canis vaginal isolate G361 and its isogenic SCM-deficient mutant (G361∆scm). We found that while S. canis establishes vaginal colonization and causes invasive disease in vivo, the contribution of the SCM protein to virulence phenotypes in these models is modest. We conclude that SCM is dispensable for invasive disease in murine models and for resistance to human blood components ex vivo, but may contribute to mucosal persistence, highlighting a potential contribution to the recently appreciated genetic diversity of SCM across strains and hosts.

Moonlighting Proteins

The single gene, single protein, single function hypothesis is increasingly becoming obsolete. Numerous studies have demonstrated that individual proteins can moonlight, meaning they can have multiple functions based on their cellular or developmental context. In this review, we discuss moonlighting proteins, highlighting the biological pathways where this phenomenon may be particularly relevant. In addition, we combine genetic, cell biological, and evolutionary perspectives so that we can better understand how, when, and why moonlighting proteins may take on multiple roles.

Novel diverse sequences of the Streptococcus canis M-like protein (SCM) gene and their prevalence in diseased companion animals: Association of their alleles with sequence types

OBJECTIVE

We aimed to determine novel alleles and their prevalence in Streptococcus canis M-like protein (SCM) and to elucidate association of their alleles with sequence types (STs)/clonal complexes (CCs) and antimicrobial resistance (AMR) phenotypes/genotypes.

METHODS

We amplified and sequenced scm, by using primers reported by Pinho recently, for 40 isolates in 2015 and 2017, in which the sequences could not be determined with conventional primers. Isolates, for which SCM alleles, STs, and AMR phenotypes/genotypes were previously determined, were included as controls. A phylogenetic tree of SCM amino acid sequences was constructed. Alleles, based on the tree positions with their prevalence, as well as STs/CCs and AMR phenotypes/genotypes were characterized.

RESULTS

Although one isolate possessed SCM allele type 1, 39 isolates had novel allele types 10-15, based on cluster analysis. The 11 and 12 allele types were firstly found in this study. We designated novel allele types as group II and non-novel allele types as group I. Prevalence of group II alleles was 29.9% and 16.2% in 2015 and 2017. Prevalent group II types were allele 10 (10.3%), allele 11 (2.7%), and allele 15 (3.3%) through both periods. There was a significant difference in distribution of STs/CCs between groups I/II SCM populations. We found significant differences in distribution of macrolide/lincosamide AMR genotype (7.7% vs. 26.8%) and AMR rates of fluoroquinolone (0% vs. 12.5%) between the two populations.

CONCLUSION

Our study presents group II scm sequences and their prevalence among diseased companion animals in Japan, with association of their alleles with STs.

Functional and structural basis of E. coli enolase inhibition by SF2312: a mimic of the carbanion intermediate

Many years ago, the natural secondary metabolite SF2312, produced by the actinomycete Micromonospora, was reported to display broad spectrum antibacterial properties against both Gram-positive and Gram-negative bacteria. Recent studies have revealed that SF2312, a natural phosphonic acid, functions as a potent inhibitor of human enolase. The mechanism of SF2312 inhibition of bacterial enolase and its role in bacterial growth and reproduction, however, have remained elusive. In this work, we detail a structural analysis of E. coli enolase bound to both SF2312 and its oxidized imide-form. Our studies support a model in which SF2312 acts as an analog of a high energy intermediate formed during the catalytic process. Biochemical, biophysical, computational and kinetic characterization of these compounds confirm that altering features characteristic of a putative carbanion (enolate) intermediate significantly reduces the potency of enzyme inhibition. When SF2312 is combined with fosfomycin in the presence of glucose-6 phosphate, significant synergy is observed. This suggests the two agents could be used as a potent combination, targeting distinct cellular mechanism for the treatment of bacterial infections. Together, our studies rationalize the structure-activity relationships for these phosphonates and validate enolase as a promising target for antibiotic discovery.

Homophilic protein interactions facilitate bacterial aggregation and IgG-dependent complex formation by the Streptococcus canis M protein SCM

Streptococcus canis is a zoonotic agent that causes serious invasive diseases in domestic animals and humans, but knowledge about its pathogenic potential and underlying virulence mechanisms is limited. Here, we report on the ability of certain S. canis isolates to form large bacterial aggregates when grown in liquid broth. Bacterial aggregation was attributed to the presence and the self-binding activity of SCM, the M protein of S. canis, as evaluated by bacterial sedimentation assays, immunofluorescence- and electron microscopic approaches. Using a variety of truncated recombinant SCM fragments, we demonstrated that homophilic SCM interactions occur via the N-terminal, but not the C-terminal part, of the mature M protein. Interestingly, when incubated in human plasma, SCM forms soluble protein complexes comprising its known ligands, immunoglobulin G (IgG) and plasminogen (Plg). Co-incubation studies with purified host proteins revealed that SCM-mediated complex formation is based on the interaction of SCM with itself and with IgG, but not with Plg or fibrinogen (Fbg), well-established constituents of M protein-mediated protein complexes in human-associated streptococci. Notably, these soluble, SCM-mediated plasma complexes harbored complement factor C1q, which can induce complement breakdown in the periphery and therefore represent another immune evasion mechanism of SCM.

Streptococcus canis Are a Single Population Infecting Multiple Animal Hosts Despite the Diversity of the Universally Present M-Like Protein SCM

Streptococcus canis is an animal pathogen which occasionally causes infections in humans. The S. canis M-like protein (SCM) encoded by the scm gene, is its best characterized virulence factor but previous studies suggested it could be absent in a substantial fraction of isolates. We studied the distribution and variability of the scm gene in 188 S. canis isolates recovered from companion animals (n = 152), wild animal species (n = 20), and humans (n = 14). Multilocus sequence typing, including the first characterization of wildlife isolates, showed that the same lineages are present in all animal hosts, raising the possibility of extensive circulation between species. Whole-genome analysis revealed that emm-like genes found previously in S. canis correspond to divergent scm genes, indicating that what was previously believed to correspond to two genes is in fact the same scm locus. We designed primers allowing for the first time the successful amplification of the scm gene in all isolates. Analysis of the scm sequences identified 12 distinct types, which could be divided into two clusters: group I (76%, n = 142) and group II (24%, n = 46) sharing little sequence similarity. The predicted group I SCM showed extensive similarity with each other outside of the N-terminal hypervariable region and a conserved IgG binding domain. This domain was absent from group II SCM variants found in isolates previously thought to lack the scm gene, which also showed greater amino acid variability. Further studies are necessary to elucidate the possible host interacting partners of the group II SCM variants and their role in virulence.

Prevalence and diversity of M-like protein (SCM) gene in Streptococcus canis isolates from diseased companion animals in Japan: Implication of SCM allele

Streptococcus canis (Sc)-origin M-like protein (SCM) binds to plasminogen and immunoglobulin G and facilitates anti-phagocytic properties. We aimed to determine the prevalence and diversity of the scm gene in Sc isolates from diseased companion animals in Japan and to propose potential SCM alleles of amino acid (AA) sequences. We collected β-hemolytic streptococci from diseased animals with host information nationwide in 2015 and 2017. After Sc identification and scm gene amplification and sequencing, the gene's prevalence and relationship between its presence and host information were determined. Furthermore, phylogenetic trees of AA sequences were constructed, and classification and distribution of SCM alleles based on variations of AA sequences were conducted. The scm detection rates were 70.6% (n = 48, 2015) and 82.9% (n = 97, 2017). There was a relationship between scm presence and Tokyo in 2015 and 2017. We found an association between scm detection and dogs in 2017 alone. Major sequence sizes were 1311 bp, 1308 bp, and 1305 bp. Using the phylogenetic trees of AA sequences, we confirmed shared positions of five identical sequence patterns in both periods. Nine SCM alleles were determined with six signal-peptide types. Most prevalent alleles were type 1, type 2, and type 4 in both periods. Our observations suggest prevalence and diversity of scm in animal-origin Sc isolates in Japan.

Protein moonlighting: what is it, and why is it important?

Members of the GroEL/HSP60 protein family have been studied for many years because of their critical roles as ATP-dependent molecular chaperones, so it might come as a surprise that some have important functions in ATP-poor conditions, for example, when secreted outside the cell. At least some members of each of the HSP10, HSP70, HSP90, HSP100 and HSP110 heat shock protein families are also 'moonlighting proteins'. Moonlighting proteins exhibit more than one physiologically relevant biochemical or biophysical function within one polypeptide chain. In this class of multifunctional proteins, the multiple functions are not due to gene fusions or multiple proteolytic fragments. Several hundred moonlighting proteins have been identified, and they include a diverse set of proteins with a large variety of functions. Some participate in multiple biochemical processes by using an active site pocket for catalysis and a different part of the protein's surface to interact with other proteins. Moonlighting proteins play a central role in many diseases, and the development of novel treatments would be aided by more information addressing current questions, for example, how some are targeted to multiple cellular locations and how a single function can be targeted by therapeutics without targeting a function not involved in disease.This article is part of the theme issue 'Heat shock proteins as modulators and therapeutic targets of chronic disease: an integrated perspective'.

Intracellular proteins moonlighting as bacterial adhesion factors

Pathogenic and commensal, or probiotic, bacteria employ adhesins on the cell surface to attach to and interact with the host. Dozens of the adhesins that play key roles in binding to host cells or extracellular matrix were originally identified as intracellular chaperones or enzymes in glycolysis or other central metabolic pathways. Proteins that have two very different functions, often in two different subcellular locations, are referred to as moonlighting proteins. The intracellular/surface moonlighting proteins do not contain signal sequences for secretion or known sequence motifs for binding to the cell surface, so in most cases is not known how these proteins are secreted or how they become attached to the cell surface. A secretion system in which a large portion of the pool of each protein remains inside the cell while some of the pool of the protein is partitioned to the cell surface has not been identified. This may involve a novel version of a known secretion system or it may involve a novel secretion system. Understanding the processes by which intracellular/cell surface moonlighting proteins are targeted to the cell surface could provide novel protein targets for the development of small molecules that block secretion and/or association with the cell surface and could serve as lead compounds for the development of novel antibacterial therapeutics.

Draft Genome Sequence of Streptococcus canis Clinical Strain TA4, Harboring the M-Like Protein Gene and Isolated in Japan from a Patient with Bacteremia

Streptococcus canis is an animal-origin β-hemolytic bacterium that can cause severe infections in animals and occasionally infects humans. Here, we report a draft genome sequence of an S. canis strain harboring the M-like protein gene. This strain was isolated from a patient with bacteremia (reported by Taniyama et al. [D. Taniyama, Y. Abe, T. Sakai, T. Kikuchi, and T. Takahashi, IDCases 7:48–52, 2017, https://doi.org/10.1016/j.idcr.2017.01.002]). The draft genome comprises 2,129,080 bp in 60 contigs.

Draft Genome Sequence of Zoonotic Streptococcus canis Isolate G361

Here, we report the draft genome sequence of an SCM-positive Streptococcus canis strain, G361, isolated from a vaginal swab of a 40-year-old woman. The draft genome comprises 2,045,931 bp in 62 contigs.

SCM, the M Protein of Streptococcus canis Binds Immunoglobulin G

The M protein of Streptococcus canis (SCM) is a virulence factor and serves as a surface-associated receptor with a particular affinity for mini-plasminogen, a cleavage product of the broad-spectrum serine protease plasmin. Here, we report that SCM has an additional high-affinity immunoglobulin G (IgG) binding activity. The ability of a particular S. canis isolate to bind to IgG significantly correlates with a scm-positive phenotype, suggesting a dominant role of SCM as an IgG receptor. Subsequent heterologous expression of SCM in non-IgG binding S. gordonii and Western Blot analysis with purified recombinant SCM proteins confirmed its IgG receptor function. As expected for a zoonotic agent, the SCM-IgG interaction is species-unspecific, with a particular affinity of SCM for IgGs derived from human, cats, dogs, horses, mice, and rabbits, but not from cows and goats. Similar to other streptococcal IgG-binding proteins, the interaction between SCM and IgG occurs via the conserved Fc domain and is, therefore, non-opsonic. Interestingly, the interaction between SCM and IgG-Fc on the bacterial surface specifically prevents opsonization by C1q, which might constitute another anti-phagocytic mechanism of SCM. Extensive binding analyses with a variety of different truncated SCM fragments defined a region of 52 amino acids located in the central part of the mature SCM protein which is important for IgG binding. This binding region is highly conserved among SCM proteins derived from different S. canis isolates but differs significantly from IgG-Fc receptors of S. pyogenes and S. dysgalactiae sub. equisimilis, respectively. In summary, we present an additional role of SCM in the pathogen-host interaction of S. canis. The detailed analysis of the SCM-IgG interaction should contribute to a better understanding of the complex roles of M proteins in streptococcal pathogenesis.

Human case of bacteremia caused by Streptococcus canis sequence type 9 harboring the scm gene

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This is the first reported human case of bacteremia caused by Streptococcus canis having scm gene encoding M-like protein.

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The identity was based on similarity of its 16S rRNA and detection of the specific cfg gene encoding a co-hemolysin.

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This isolate was confirmed to be sequence type 9.

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This strain didn’t have the erm(A), erm(B), or mef(A), macrolide resistance genes, but was not susceptible to azithromycin.

Streptococcus canis (Sc) is a zoonotic pathogen that is transferred mainly from companion animals to humans. One of the major virulence factors in Sc is the M-like protein encoded by the scm gene, which is involved in anti-phagocytic activities, as well as the recruitment of plasminogen to the bacterial surface in cooperation with enolase, and the consequent enhancement of bacterial transmigration and survival. This is the first reported human case of uncomplicated bacteremia following a dog bite, caused by Streptococcus canis harboring the scm gene. The similarity of the 16S rRNA from the infecting species to that of the Sc type strain, as well as the amplification of the species-specific cfg gene, encoding a co-hemolysin, was used to confirm the species identity. Furthermore, the isolate was confirmed as sequence type 9. The partial scm gene sequence harbored by the isolate was closely related to those of other two Sc strains. While this isolate did not possess the erm(A), erm(B), or mef(A), macrolide/lincosamide resistance genes, it was not susceptible to azithromycin: its susceptibility was intermediate. Even though human Sc bacteremia is rare, clinicians should be aware of this microorganism, as well as Pasteurella sp., Prevotella sp., and Capnocytophaga sp., when examining and treating patients with fever who maintain close contact with companion animals.

Pseudomonas aeruginosa Enolase Influences Bacterial Tolerance to Oxidative Stresses and Virulence

Pseudomonas aeruginosa is a Gram negative opportunistic pathogenic bacterium, which causes acute and chronic infections. Upon entering the host, bacteria alter global gene expression to adapt to host environment and avoid clearance by the host. Enolase is a glycolytic enzyme involved in carbon metabolism. It is also a component of RNA degradosome, which is involved in RNA processing and gene regulation. Here, we report that enolase is required for the virulence of P. aeruginosa in a murine acute pneumonia model. Mutation of enolase coding gene (eno) increased bacterial susceptibility to neutrophil mediated killing, which is due to reduced tolerance to oxidative stress. Catalases and alkyl hydroperoxide reductases play a major role in protecting the cell from oxidative damages. In the eno mutant, the expression levels of catalases (KatA and KatB) were similar as those in the wild type strain in the presence of H₂O₂, however, the expression levels of alkyl hydroperoxide reductases (AhpB and AhpC) were significantly reduced. Overexpression of ahpB but not ahpC in the eno mutant fully restored the bacterial resistance to H₂O₂ as well as neutrophil mediated killing, and partially restored bacterial virulence in the murine acute pneumonia model. Therefore, we have identified a novel role of enolase in the virulence of P. aeruginosa.

Enolase of Mycobacterium tuberculosis is a surface exposed plasminogen binding protein

BACKGROUND

Enolase, a glycolytic enzyme, has long been studied as an anchorless protein present on the surface of many pathogenic bacteria that aids in tissue remodeling and invasion by binding to host plasminogen.

METHODS

Anti-Mtb enolase antibodies in human sera were detected using ELISA. Immunoelectron microscopy, immunofluorescence microscopy and flow cytometry were used to show surface localization of Mtb enolase. SPR was used to determine the affinity of enolase-plasminogen interaction. Plasmin formation upon plasminogen binding to enolase and Mtb surface was measured by ELISA. Mice challenge and histopathological studies were undertaken to determine the protective efficacy of enolase immunization.

RESULTS

Enolase of Mtb is present on its surface and binds human plasminogen with high affinity. There was an average of 2-fold increase in antibody mediated recognition of Mtb enolase in human sera from TB patients with an active disease over control individuals. Substitution of C-terminal lysine to alanine in rEno decreased its binding affinity with human plasminogen by >2-folds. Enolase bound plasminogen showed urokinase mediated conversion into plasmin. Binding of plasminogen to the surface of Mtb and its conversion into fibrinolytic plasmin was significantly reduced in the presence of anti-rEno antibodies. Immunization with rEno also led to a significant decrease in lung CFU counts of mice upon infection with Mtb H37Rv.

CONCLUSIONS

Mtb enolase is a surface exposed plasminogen binding protein which upon immunization confers significant protection against Mtb challenge.

GENERAL SIGNIFICANCE

Plasminogen binding has been recognized for Mtb, however, proteins involved have not been characterized. We show here that Mtb enolase is a moonlighting plasminogen binding protein.

An overview of protein moonlighting in bacterial infection

We are rapidly returning to a world in which bacterial infections are a major health issue. Pathogenic bacteria are able to colonize and cause pathology due to the possession of virulence factors such as adhesins, invasins, evasins and toxins. These are generally specifically evolved proteins with selective actions. It is, therefore, surprising that most human bacterial pathogens employ moonlighting proteins as virulence factors. Currently, >90 bacterial species employ one or more moonlighting protein families to aid colonization and induce disease. These organisms employ 90 moonlighting bacterial protein families and these include enzymes of the glycolytic pathway, tricarboxylic acid (TCA) cycle, hexosemonophosphate shunt, glyoxylate cycle and a range of other metabolic enzymes, proteases, transporters and, also, molecular chaperones and protein-folding catalysts. These proteins have homologues in eukaryotes and only a proportion of the moonlighting proteins employed are solely bacterial in origin. Bacterial moonlighting proteins can be divided into those with single moonlighting functions and those with multiple additional biological actions. These proteins contribute significantly to the population of virulence factors employed by bacteria and some are obvious therapeutic targets. Where examined, bacterial moonlighting proteins bind to target ligands with high affinity. A major puzzle is the evolutionary mechanism(s) responsible for bacterial protein moonlighting and a growing number of highly homologous bacterial moonlighting proteins exhibit widely different moonlighting actions, suggesting a lack in our understanding of the mechanism of evolution of protein active sites.

Cloning and Characterization of Surface-Localized α-Enolase of Streptococcus iniae, an Effective Protective Antigen in Mice

Streptococcus iniae is a major fish pathogen that can also cause human bacteremia, cellulitis and meningitis. Screening for and identification of protective antigens plays an important role in developing therapies against S. iniae infections. In this study, we indicated that the α-enolase of S. iniae was not only distributed in the cytoplasm and associated to cell walls, but was also secreted to the bacterial cell surface. The functional identity of the purified recombinant α-enolase protein was verified by its ability to catalyze the conversion of 2-phosphoglycerate (2-PGE) to phosphoenolpyruvate (PEP), and both the recombinant and native proteins interacted with human plasminogen. The rabbit anti-rENO serum blockade assay shows that α-enolase participates in S. iniae adhesion to and invasion of BHK-21 cells. In addition, the recombinant α-enolase can confer effective protection against S. iniae infection in mice, which suggests that α-enolase has potential as a vaccine candidate in mammals. We conclude that S. iniae α-enolase is a moonlighting protein that also associates with the bacterial outer surface and functions as a protective antigen in mice.

Recent advances on plasmin inhibitors for the treatment of fibrinolysis-related disorders

Growing evidence suggests that plasmin is involved in a number of physiological processes in addition to its key role in fibrin cleavage. Plasmin inhibition is critical in preventing adverse consequences arising from plasmin overactivity, e.g., blood loss that may follow cardiac surgery. Aprotinin was widely used as an antifibrinolytic drug before its discontinuation in 2008. Tranexamic acid and ε-aminocaproic acid, two small molecule plasmin inhibitors, are currently used in the clinic. Several molecules have been designed utilizing covalent, but reversible, chemistry relying on reactive cyclohexanones, nitrile warheads, and reactive aldehyde peptidomimetics. Other major classes of plasmin inhibitors include the cyclic peptidomimetics and polypeptides of the Kunitz and Kazal-type. Allosteric inhibitors of plasmin have also been designed including small molecule lysine analogs that bind to plasmin's kringle domain(s) and sulfated glycosaminoglycan mimetics that bind to plasmin's catalytic domain. Plasmin inhibitors have also been explored for resolving other disease states including cell metastasis, cell proliferation, angiogenesis, and embryo implantation. This review highlights functional and structural aspects of plasmin inhibitors with the goal of advancing their design.

Detection and characterization of plasminogen receptors on clinical isolates of Trichosporon asahii

Trichosporon asahii is the major causative agent of deep-seated trichosporonosis. The virulence factors of this yeast pathogen remain uncharacterized. To investigate the pathogenicity of T. asahii, we focused on the interactions between surface molecules of the yeast and host biomolecules. We examined the ability of surface molecules to bind human plasminogen using clinical isolates of T. asahii. Living T. asahii cells accelerated the conversion of plasminogen to plasmin in a dose-dependent manner in the presence of tissue plasminogen activator. Extracts from cells using lithium chloride contained plasminogen-binding molecules based on surface plasmon resonance (SPR) analyses. In all strains tested, several of the fractions obtained using DEAE column chromatography bound and accelerated the conversion of plasminogen to plasmin. Based on far-Western blotting analyses, a common protein was identified within the four strains, which was identified as a hypothetical protein from genome analyses of T. asahii. blast searches suggested the protein might be heparinase, and heparinase activity was detected in the T. asahii extract. Furthermore, affinity chromatography using plasminogen as a ligand detected one protein band by SDS-PAGE, which was identified as thioredoxin-dependent peroxide reductase.SPR analyses suggested the presence of molecules on T. asahii cells that could bind plasminogen with differing affinities.

Interaction of surface molecules on Cryptococcus neoformans with plasminogen

Microbial pathogens are known to express molecules that interact with host proteins, leading to invasion and colonization. For example, some pathogenic microorganisms express proteins that bind to and enhance the activity of plasminogen. In this way, pathogens utilize the host fibrinolytic system to promote invasion. We found that triosephosphate isomerase (TPI), a glycolytic enzyme produced by Staphylococcus aureus, bound to mannooligosaccharides from the pathogenic capsulated fungus Cryptococcus neoformans and human plasminogen, suggesting that TPI is a moonlighting protein. Several C. neoformans surface proteins are thought to be plasminogen-binding proteins. Here, we examined the ability of surface polymers (including polysaccharides) to bind plasminogen. Heat-killed C. neoformans cells transformed plasminogen into plasmin in a dose-dependent manner in the presence of tissue plasminogen activator. Soluble polysaccharides were found to bind plasminogen based on surface plasmon resonance (SPR) analysis. Neutral polysaccharides fractionated using DEAE column chromatography bound and activated plasminogen. However, the fraction containing glucuronoxylomannan (the primary component of the capsule) did not activate plasminogen. In addition, binding between glucuronoxylomannan and plasminogen was weak. Components of the neutral polysaccharides were identified as mannose, galactose, glucose and xylose. In conclusion, neutral polysaccharides that may affect fibrinolysis were detected on the surface of C. neoformans.

Functional and immunological evaluation of two novel proteins of Leptospira spp

This work shows the production and characterization of two novel putative lipoproteins encoded by the genes LIC10645 and LIC10731 identified in the genome sequences of Leptospira interrogans. In silico conservation analysis indicated that the proteins are well conserved among pathogenic leptospiral serovars and species. Recombinant proteins were obtained in Escherichia coli BL21(DE3) Star pLysS strain, purified by metal-affinity chromatography, and used for characterization and immunological evaluations. Recombinant proteins were capable of eliciting a combination of humoral and cellular immune responses in animal models, and could be recognized by antibodies present in human serum samples. The recombinant proteins Lsa44 and Lsa45 were able to bind laminin, and were named Lsa44 and Lsa45 for leptospiral surface adhesins of 44 and 45 kDa, respectively. The attachment to laminin was dose-responsive with K D values of 108.21 and 250.38 nM for Lsa44 and Lsa45, respectively. Moreover, these proteins interact with plasminogen (PLG) with K D values of 53.56 and 36.80 nM, respectively. PLG bound to the recombinant proteins could be converted to plasmin (PLA) in the presence of an activator. Cellular localization assays suggested that the Lsa44 and Lsa45 were surface-exposed. These are versatile proteins capable of interacting with laminin and PLG/PLA, and hence could mediate bacterial adhesion and contribute to tissue penetration.

Interaction of streptococcal plasminogen binding proteins with the host fibrinolytic system

The ability to take advantage of plasminogen and its activated form plasmin is a common mechanism used by commensal as well as pathogenic bacteria in interaction with their respective host. Hence, a huge variety of plasminogen binding proteins and activation mechanisms exist. This review solely focuses on the genus Streptococcus and, in particular, on the so-called non-activating plasminogen binding proteins. Based on structural and functional differences, as well as on their mode of surface linkaging, three groups can be assigned: M-(like) proteins, surface displayed cytoplasmatic proteins with enzymatic activities (“moonlighting proteins”) and other surface proteins. Here, the plasminogen binding sites and the interaction mechanisms are compared. Recent findings on the functional consequences of these interactions on tissue degradation and immune evasion are summarized.

The interaction between bacterial enolase and plasminogen promotes adherence of Streptococcus pneumoniae to epithelial and endothelial cells

Binding and conversion of the plasma protein plasminogen is an important pathogenesis mechanism of the human pathogen Streptococcus pneumoniae. Once converted into plasmin, the proteolytic activity of this major fibrinolysis component promotes degradation of extracellular matrix and the dissolution of fibrin clots. Here, we present the exploitation of plasminogen-binding as a further pivotal strategy of pneumococci facilitating adherence to eukaryotic cells. Flow cytometric measurements demonstrated the immobilization of plasminogen on host cell surfaces of human alveolar type II pneumocytes (A549), nasopharyngeal epithelium (Detroit 562) and brain-derived endothelial cells (HBMEC). These host-derived cells were employed in cell culture infection analyses followed by confocal microscopy to monitor the plasminogen-mediated adherence. Results of these studies revealed that host cell-bound plasminogen promotes pneumococcal adherence to human epithelial and endothelial cells in dose-dependent manner, whereas pneumococcal internalization was not enhanced. As an opposed effect pneumococcal-bound plasminogen reduced attachment to the epithelial and endothelial cells, and increased the interaction with neutrophil granulocytes. Moreover, the surface-displayed enolase, which serves as major pneumococcal plasminogen receptor, was identified as a key factor for plasminogen-mediated bacterial attachment in infection analyses with S. pneumoniae enolase mutants.

Characterization and protective immunogenicity of the SzM protein of Streptococcus zooepidemicus NC78 from a clonal outbreak of equine respiratory disease

Streptococcus zooepidemicus of Lancefield group C is a highly variable tonsillar and mucosal commensal that usually is associated with opportunistic infections of the respiratory tract of vertebrate hosts. More-virulent clones have caused epizootics of severe respiratory disease in dogs and horses. The virulence factors of these strains are poorly understood. The antiphagocytic protein SeM is a major virulence factor and protective antigen of Streptococcus equi, a clonal biovar of an ancestral S. zooepidemicus strain. Although the genome of S. zooepidemicus strain H70, an equine isolate, contains a partial homolog (szm) of sem, expression of the gene has not been documented. We have identified and characterized SzM from an encapsulated S. zooepidemicus strain from an epizootic of equine respiratory disease in New Caledonia. The SzM protein of strain NC78 (SzM(NC78)) has a predicted predominantly alpha-helical fibrillar structure with an LPSTG cell surface anchor motif and resistance to hot acid. A putative binding site for plasminogen is present in the B repeat region, the sequence of which shares homology with repeats of the plasminogen binding proteins of human group C and G streptococci. Equine plasminogen is activated in a dose-dependent manner by recombinant SzM(NC78). Only 23.20 and 25.46% DNA homology is shared with SeM proteins of S. equi strains CF32 and 4047, respectively, and homology ranges from 19.60 to 54.70% for SzM proteins of other S. zooepidemicus strains. As expected, SzM(NC78) reacted with convalescent-phase sera from horses with respiratory disease associated with strains of S. zooepidemicus. SzM(NC78) resembles SeM in binding equine fibrinogen and eliciting strong protective antibody responses in mice. Sera of vaccinated mice opsonized S. zooepidemicus strains NC78 and W60, the SzM protein of which shared partial amino acid homology with SzM(NC78). We conclude that SzM is a protective antigen of NC78; it was strongly reactive with serum antibodies from horses during recovery from S. zooepidemicus-associated respiratory disease.