Identification of a novel gene cluster encoding staphylococcal exotoxin-like proteins: characterization of the prototypic gene and its protein product, SET1

Staphylococcal superantigen-like protein 3 triggers murine mast cell adhesion by binding to CD43 and augments mast cell activation

Staphylococcus aureus is a noteworthy pathogen in allergic diseases, as four staphylococcal exotoxins activate mast cells, a significant contributor to inflammation, in an IgE-independent manner. Although the adhesion of mast cells is an essential process for their immune responses, only a small number of exotoxins have been reported to affect the process. Here, we demonstrated that staphylococcal superantigen-like (SSL) 3, previously identified as a toll-like receptor 2 agonist, induced the adhesion of murine bone marrow-derived mast cells to culture substratum. SSL3-induced adhesion was mediated by fibronectin in an Arg-Gly-Asp (RGD) sequence-dependent manner, suggesting the integrins were involved in the process. Additionally, SSL3 was found to bind to an anti-adhesive surface protein CD43. SSL3 induced the adhesion of HEK293 cells expressing exogenous CD43, suggesting that CD43 is the target molecule for adhesion induced by SSL3. Evaluation of SSL3-derived mutants showed that the C-terminal region (253-326), specifically T285 and H307, are necessary to induce adhesion. SSL3 augmented the IL-13 production of mast cells in response to immunocomplex and SSL12. These findings reveal a novel function of SSL3, triggering cell adhesion and enhancing mast cell activation. This study would clarify the correlation between S. aureus and allergic diseases such as atopic dermatitis.

Staphylococcal superantigen-like protein 10 enhances the amyloidogenic biofilm formation in Staphylococcus aureus

Staphylococcus aureus is a highly infectious pathogen that represents a significant burden on the current healthcare system. Bacterial attachment to medical implants and host tissue, and the establishment of a mature biofilm, play an important role in chronic diseases such as endocarditis, osteomyelitis and wound infections. These biofilms decrease bacterial susceptibility to antibiotics and immune defences, making the infections challenging to treatment. S. aureus produces numerous exotoxins that contribute to the pathogenesis of the bacteria. In this study, we have identified a novel function of staphylococcal superantigen-like protein 10 (SSL10) in enhancing the formation of staphylococcal biofilms. Biofilm biomass is significantly increased when SSL10 is added exogenously to bacterial cultures, whereas SSL2 and SSL12 are found to be less active. Exogenously added SSL10 mask the surface charge of the bacterial cells and lowers their zeta potential, leading to the aggregation of the cells. Moreover, the biofilm formation by SSL10 is governed by amyloid aggregation, as evident from spectroscopic and microscopic studies. These findings thereby give the first overview of the SSL-mediated amyloid-based biofilm formation and further drive the future research in identifying potential molecules for developing new antibacterial therapies against Staphylococcus aureus.

Isolation and functional analysis of phage-displayed antibody fragments targeting the staphylococcal superantigen-like proteins

Staphylococcus aureus produces numerous virulence factors that manipulate the immune system, helping the bacteria avoid phagocytosis. In this study, we are investigating three immune evasion molecules called the staphylococcal superantigen-like proteins 1, 5, and 10 (SSL1, SSL5, and SSL10). All three SSLs inhibit vital host immune processes and contribute to S. aureus immune evasion. This study aimed to identify single-chain variable fragment (scFvs) antibodies from synthetic antibody phage libraries, which can recognize either of the three SSLs and could block the interaction between the SSLs and their respective human targets. The antibodies were isolated after three rounds of panning against SSL1, SSL5, and SSL10, and their ability to bind to the SSLs was studied using a time-resolved fluorescence-based immunoassay. We successfully obtained altogether 44 unique clones displaying binding activity to either SSL1, SSL5, or SSL10. The capability of the SSL-recognizing scFvs to inhibit the SSLs' function was tested in an MMP9 enzymatic activity assay, a P-selectin glycoprotein ligand 1 competitive binding assay, and an IgG1-mediated phagocytosis assay. We could show that one scFv was able to inhibit SSL1 and maintain MMP9 activity in a concentration-dependent manner. Finally, the structure of this inhibiting scFv was modeled and used to create putative scFv-SSL1-complex models by protein-protein docking. The complex models were subjected to a 100-ns molecular dynamics simulation to assess the possible binding mode of the antibody.

Identification of responsible amino acid residues in Staphylococcal superantigen-like 12 for the activation of mast cells

Staphylococcal superantigen-like 12 (SSL12) is reported to evoke the degranulation in murine mast cells. The allelic variant of SSL12 in the genome of reference strain NCTC8325 induced the degranulation of murine mast cells, that of MRSA252 strain did not, nevertheless relatively high sequence similarity (82%). To identify responsible amino acid residues of SSL12 for mast cell activation, we created a series of domain swap mutants and amino acid substitution mutants between the active and inactive variants. The mutants that harbored oligonucleotide/oligosaccharide binding (OB)-fold domain of the active variant activated mast cells. The replacement at position 56 (L56F) in the OB-fold domain diminished the mast cell stimulatory activity, and the combinatorial substitutions L56F/K92E, L56F/D95S, and L56F/S100V abolished the stimulatory activities of the mutant that harbored OB-fold domain of the active variant and the intact active variant. These indicate that the responsive elements of SSL12 for mast cell activation are in the OB-fold of SSL12, and L56 would be an essential amino acid residue for the activation of mast cells. The findings would contribute to the understanding of the molecular mechanism of SSL12 for mast cell activation and the development of toxoids preventing allergic inflammations associated with S. aureus. This article is protected by copyright. All rights reserved.

The hypersusceptible antibiotic screening strain Staphylococcus aureus SG511-Berlin harbors multiple mutations in regulatory genes

The genetic plasticity of Staphylococcus aureus has facilitated the evolution of many virulent and drug-resistant strains. Here we present the sequence of the 2.74 Mbp genome of S. aureus SG511-Berlin, which is frequently used for antibiotic screening. Although S. aureus SG511 and the related methicillin-resistant S. aureus MRSA252 share a high similarity in their core genomes, indicated by an average nucleotide identity (ANI) of 99.83%, the accessory genomes of these strains differed, as nearly no mobile elements and resistance determinants were identified in the genome of S. aureus SG511. Susceptibility testing showed that S. aureus SG511 was susceptible to most of the tested antibiotics of different classes. Intriguingly, and in contrast to the standard laboratory strain S. aureus HG001, S. aureus SG511 was even hyper-susceptible towards cell wall and membrane targeting agents, with the exception of the MurA-inhibitor fosfomycin. In depth comparative genome analysis revealed that, in addition to the loss of function mutation in the antibiotic sensor histidine kinase gene graS, further mutations had occurred in the lysyltransferase gene mprF, the structural giant protein gene ebh, and the regulator genes codY and saeR, which might contribute to antibiotic susceptibility. In addition, an insertion element in agrC abolishes Agr-activity in S. aureus SG511, and the spa and sarS genes, which encode the surface protein SpA and its transcriptional regulator, were deleted. Thus, the lack of mobile resistance genes together with multiple mutations affecting cell envelope morphology may render S. aureus SG511 hyper-susceptible towards most cell wall targeting agents.

[Modulation of Host Immune System by Staphylococcal Superantigen-like (SSL) Proteins]

Staphylococcus aureus is a common pathogen causing a wide range of infectious diseases in humans and animals. This bacterium secretes a variety of exoproteins, including toxins known as superantigens, such as toxic shock syndrome toxin-1 (TSST-1) and enterotoxins. Staphylococcal superantigen-like (SSL) proteins are a family of exoproteins showing structural similarities with superantigens but no superantigenic activity. This family is composed of 14 members (SSL1-SSL14), and recent studies have revealed that these members exhibit various immunomodulatory activities: e.g., inhibition of antibody and complement functions, impairment of leukocyte trafficking, modulation of receptor functions, inappropriate activation of immunocytes, and inhibition of blood coagulation. These activities have been proposed to contribute to immune evasion of the bacteria. The interactions between SSL proteins and their target molecules in the host immune system and the pathophysiological roles of SSL proteins in the bacterial infections are reviewed in this article.

Regulation of the Staphylococcal Superantigen-Like Protein 1 Gene of Community-Associated Methicillin-Resistant Staphylococcus aureus in Murine Abscesses

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) causes substantial skin and soft tissue infections annually in the United States and expresses numerous virulence factors, including a family of toxins known as the staphylococcal superantigen-like (SSL) proteins. Many of the SSL protein structures have been determined and implicated in immune system avoidance, but the full scope that these proteins play in different infection contexts remains unknown and continues to warrant investigation. Analysis of ssl gene regulation may provide valuable information related to the function of these proteins. To determine the transcriptional regulation of the ssl1 gene of CA-MRSA strain MW2, an ssl1 promoter::lux fusion was constructed and transformed into S.aureus strains RN6390 and Newman. Resulting strains were grown in a defined minimal medium (DSM) broth and nutrient-rich brain-heart infusion (BHI) broth and expression was determined by luminescence. Transcription of ssl1 was up-regulated and occurred earlier during growth in DSM broth compared to BHI broth suggesting expression is regulated by nutrient availability. RN6390 and Newman strains containing the ssl1::lux fusion were also used to analyze regulation in vivo using a mouse abscess model of infection. A marked increase in ssl1 transcription occurred early during infection, suggesting SSL1 is important during early stages of infection, perhaps to avoid the immune system.

Lack of correlation of virulence gene profiles of Staphylococcus aureus bacteremia isolates with mortality

PURPOSE

Whole genome sequencing (WGS) analysis of Staphylococcus aureus is increasingly used in clinical practice. Although bioinformatics tools used in WGS analysis readily define the S. aureus virulome, the clinical value of this type of analysis is unclear. Here, virulence genes in S. aureus bacteremia (SAB) isolates were evaluated by WGS, with superantigens (SAgs) further evaluated by conventional PCR and functional assays, and results correlated with mortality.

METHODS

152 SAB isolates collected throughout 2015 at a large Minnesota medical center were studied and associated clinical data analyzed. Virulence genes were identified from previously-reported WGS data (https://doi.org/10.1371/journal.pone.0179003). SAg genes sea, seb, sec, sed, see, seg, seh, sei, sej, and tst were also assessed by individual PCR assays. Mitogenicity of SAgs was assessed using an in vitro proliferation assay with splenocytes from HLA-DR3 transgenic mice.

RESULTS

Of the 152 SAB isolates studied, 106 (69%) were methicillin-susceptible S. aureus (MSSA). The number of deaths attributed and not attributed to SAB, and 30-day survivors were 24 (16%), 2 (1%), and 128 (83%), respectively. From WGS data, both MSSA and MRSA had high proportions of adhesion (>80%) and immune-evasion (>70%) genes. There was no difference in virulomes between survivor- and non-survivor-associated isolates. Although over 60% of SAB isolates produced functional SAgs, there were no differences in the distribution or prevalence of SAg genes between survivor- and non-survivor-associated isolates.

CONCLUSION

In this study of one year of SAB isolates from a large medical center, the S. aureus virulome, as assessed by WGS, and also for SAgs using individual PCRs and phenotypic characterization, did not impact mortality.

Interaction of host and Staphylococcus aureus protease-system regulates virulence and pathogenicity

Staphylococcus aureus causes various health care- and community-associated infections as well as certain chronic TH2 driven inflammatory diseases. It is a potent pathogen with serious virulence and associated high morbidity. Severe pathogenicity is accredited to the S. aureus secreted virulence factors such as proteases and host protease modulators. These virulence factors promote adhesion and invasion of bacteria through damage of tight junction barrier and keratinocytes. They inhibit activation and transmigration of various immune cells such as neutrophils (and neutrophil proteases) to evade opsono-phagocytosis and intracellular bacterial killing. Additionally, they protect the bacteria from extracellular killing by disrupting integrity of extracellular matrix. Platelet activation and agglutination is also impaired by these factors. They also block the classical as well as alternative pathways of complement activation and assist in spread of infection through blood and tissue. As these factors are exquisite factors of S. aureus mediated disease development, we have focused on review of diversification of various protease-system associated virulence factors, their structural building, diverse role in disease development and available therapeutic counter measures. This review summarises the role of protease-associated virulence factors during invasion and progression of disease.

Generation of a Monoclonal Antibody Against Staphylococcal Superantigen-Like Protein 5 (SSL5) That Discriminates SSL5 from Other SSL Proteins

Staphylococcus aureus secretes a family of exoproteins structurally homologous to bacterial superantigens, such as toxic shock syndrome toxin-1 (TSST-1), and those exoproteins are thus called staphylococcal superantigen-like proteins (SSLs). Recent studies have revealed that SSLs play roles in evasion of the host defense by disturbing host immune responses. We previously reported that staphylococcal superantigen-like protein 5 (SSL5; a member of the SSL family) inhibited matrix metalloproteinase-9 (MMP-9), which is crucial for leukocyte recruitment to sites of infection. In this study, we established a mouse hybridoma clone (30G5C) producing a monoclonal antibody specific for SSL5. In immunoblotting analysis using recombinant His-tagged SSL1 to SSL14 (His-SSLs), the antibody was found to specifically recognize SSL5 without crossreactivity with other His-SSLs. The antibody bound to the C-terminal region of SSL5 (β-grasp domain), but did not interfere with the binding of SSL5 to MMP-9, suggesting that this antibody is useful for identification of SSL5-producing S. aureus and screening for inhibitors of the SSL5/MMP-9 complex formation.

Phylogenetic distribution and expression of a penicillin-binding protein homologue, Ear and its significance in virulence of Staphylococcus aureus

PURPOSE

Staphylococcus aureus is an opportunistic human pathogen that can cause serious infections in humans. A plethora of known and putative virulence factors are produced by staphylococci that collectively orchestrate pathogenesis. Ear protein (Escherichia coli ampicillin resistance) in S. aureus is an exoprotein in COL strain, predicted to be a superantigen, and speculated to play roles in antibiotic resistance and virulence. The goal of this study was to determine if expression of ear is modulated by single nucleotide polymorphisms in its promoter and coding sequences and whether this gene plays roles in antibiotic resistance and virulence.

METHODOLOGY

Promoter, coding sequences and expression of the ear gene in clinical and carriage S. aureus strains with distinct genetic backgrounds were analysed. The JE2 strain and its isogenic ear mutant were used in a systemic infection mouse model to determine the competiveness of the ear mutant.Results/Key findings. The ear gene showed a variable expression, with USA300FPR3757 showing a high-level expression compared to many of the other strains tested including some showing negligible expression. Higher expression was associated with agr type 1 but not correlated with phylogenetic relatedness of the ear gene based upon single nucleotide polymorphisms in the promoter or coding regions suggesting a complex regulation. An isogenic JE2 (USA300 background) ear mutant showed no significant difference in its growth, antibiotic susceptibility or virulence in a mouse model.

CONCLUSION

Our data suggests that despite being highly expressed in a USA300 genetic background, Ear is not a significant contributor to virulence in that strain.

Identification of Set11 from Staphylococcus aureus Mu50 strain as a ortholog of SSL7 via bioinformatic analysis and determination of its possible targets from human serum using CNBr-pull down assays

OBJECTIVES

To identify and characterize staphylococcus exotoxin-like (SET) protein Set11 from Staphylococcus aureus Mu50 strain and its possible targets proteins from human blood/serum.

RESULTS

Set11 is a member of the staphylococcal superantigen-like (SSL) proteins (also called Staphylococcus exotoxin-like (SET) proteins) family that is found in staphylococcal strain Mu50. Its structure and function, however, remain unknown. We performed bioinformatics analysis of Set11: it had 90% sequence identity to SSL7 in NCTC 8325 strain, indicating Set11 is a SSL7 ortholog. SSL7 in ATCC 12598 strain binds complement C5 to inhibit complement system. To investigate if Set11 binds C5, we made the homology model of Set11 and the Set11-C5 complex model based on SSL7 and SSL7-C5 structures, respectively. Structural analysis and sequence alignment reveal that the residues in SSL7 involved in C5 binding are conserved in Set11, indicating C5 as the potential target for Set11. To identify new targets of Set11, we cloned, expressed and purified Set11 and performed CNBr-pull down combined mass spectrum assays using human blood and serum.

CONCLUSIONS

We identified Set11 as the ortholog of SSL7 and determined C5, fibronectin 1 isoform 3 preproprotein, albumin, alpha-1-microglobulin precursor and complement C3 processor as the potential target proteins for Set11, indicating new functions of Set11/SSL7.

Staphylococcus aureus Manipulates Innate Immunity through Own and Host-Expressed Proteases

Neutrophils, complement system and skin collectively represent the main elements of the innate immune system, the first line of defense of the host against many common microorganisms. Bacterial pathogens have evolved strategies to counteract all these defense activities. Specifically, Staphylococcus aureus, a major human pathogen, secretes a variety of immune evasion molecules including proteases, which cleave components of the innate immune system or disrupt the integrity of extracellular matrix and intercellular connections of tissues. Additionally, S. aureus secretes proteins that can activate host zymogens which, in turn, target specific defense components. Secreted proteins can also inhibit the anti-bacterial function of neutrophils or complement system proteases, potentiating S. aureus chances of survival. Here, we review the current understanding of these proteases and modulators of host proteases in the functioning of innate immunity and describe the importance of these mechanisms in the pathology of staphylococcal diseases.

Differences in humoral immune response between patients with or without nasal carriage of Staphylococcus aureus

The humoral immune response against 43 staphylococcal antigens was compared among hospitalized patients where none of them had any staphylococcal infection on the day of admission with or without nasal Staphylococcus aureus carriage. Fifty-nine carriers and 59 matched non-carriers were studied. The carriers harbored S. aureus of 35 different spa types, including three t037/ST239 methicillin-resistant S. aureus (MRSA) (5.1%). Among the 118 patients, 31 acquired S. aureus during hospitalization. In colonized and non-colonized patients, unique patterns of S. aureus-specific immune responses were observed. The mean fluorescence indices (MFIs) of antibodies against 36/43 (83.7%) antigens were seen to be elevated among carriers. The MFI among carriers with acquisition was significantly higher for staphylococcal superantigen-like protein 5 (SSL5, p = 0.028) when compared to carriers without acquisition. High antibody levels against staphylococcal enterotoxin A (SEA) among carriers illustrate its role as a superantigen in both infection and colonization. We also report a dynamic immune response in S. aureus-carrying patients against the recently reported formyl peptide receptor-like inhibitory (FLIPr)-like protein. In the current study, the dynamics of antibodies against staphylococcal antigens among carrier patients seem quite similar to non-carrier patients. To better understand the dynamic immunogenicity during S. aureus infection and colonization, artificial colonization studies and investigation of the changes in the levels of antibodies against other staphylococcal antigens are recommended.

Staphylococcal SSL5-induced platelet microparticles provoke proinflammatory responses via the CD40/TRAF6/NFκB signalling pathway in monocytes

Pathogens-induced platelet activation contributes to inflammation in cardiovascular diseases, but underlying mechanisms remain elusive. Staphylococcal superantigen-like protein 5 (SSL5) is a known activator of platelets. Here we examined whether SSL5 is implicated in Staphylococcus aureus (S. aureus)-induced inflammation and potential mechanisms involved. As expected, we show that SSL5 activates human platelets and induces generation of platelet microparticles (PMPs). Flow cytometry and scanning electron microscopy studies demonstrate that SSL5-induced PMPs (SSL5-PMPs) bind to monocytes, causing aggregate formation. In addition, SSL5-PMPs provoke monocyte expression and release of inflammatory mediators, including interleukin-1β (IL-1β), tumour necrosis factor-α (TNFα), monocyte chemoattractant protein-1 (MCP-1) and matrix metalloproteinase-9 (MMP-9) in a dose- and time-dependent manner. SSL5-PMPs also enhance MCP-1-induced monocyte migration. Blockade of CD40 and CD40 ligand (CD40L) interactions with neutralising antibodies significantly reduce monocyte release of inflammatory mediators and migration induced by SSL5-PMPs. SiRNA-mediated silencing of CD40 or TNF receptor (TNFR)-associated factor 6 (TRAF6) gene largely abrogates phosphorylation and nuclear translocation of NFκB (p65). In conclusion, SSL5 provokes the release of inflammatory mediators in monocytes, at least in part, via PMPs-mediated activation of the CD40/TRAF6/NFκB signalling pathway, though it normally inhibits leukocyte function. Our findings thus reveal a novel mechanism by which S. aureus induces inflammation.

Staphylococcal manipulation of host immune responses

Staphylococcus aureus, a bacterial commensal of the human nares and skin, is a frequent cause of soft tissue and bloodstream infections. A hallmark of staphylococcal infections is their frequent recurrence, even when treated with antibiotics and surgical intervention, which demonstrates the bacterium's ability to manipulate innate and adaptive immune responses. In this Review, we highlight how S. aureus virulence factors inhibit complement activation, block and destroy phagocytic cells and modify host B cell and T cell responses, and we discuss how these insights might be useful for the development of novel therapies against infections with antibiotic resistant strains such as methicillin-resistant S. aureus.

Detection of Staphylococcus Enterotoxin B (SEB) Using an Immunochromatographic Test Strip

Background:

Staphylococcus aureus is one of the most important microorganisms that causes various human diseases by secreting virulence factors known as staphylococcal super antigens (SAgs). Staphylococcal Enterotoxin B (SEB) is a bacterial antigen that is responsible for food poisoning in humans. Among SEB detection methods, a lateral flow device (LFD) is ideal for rapid immunochromatographic tests because it is easy to use, requires minimal time to produce results, and does not require personnel training.

Objectives:

In our laboratory, the production of an immunochromatographic test strip, for the detection of SEB using a sandwich assay and a competitive method, was described; the test can detect SEB with high sensitivity.

Materials and Methods:

The strip assays were compared with PCR, a valid method for detection. For PCR, a specific sequence for SEB production was detected using primers designed according to GenBank sequences.

Results:

In total, 80 food samples suspected of SEB contamination were assessed using the two methods. Fifty-four samples were contaminated based on the PCR technique and twenty-six of those were confirmed using the strip assay.

Conclusions:

The sensitivity of the sandwich method was approximately 10 ng/mL and that of the competitive method was approximately 250 ng/mL. In the LFD, a highly specific monoclonal antibody used for both the sandwich and competitive methods resulted in an increased sensitivity and accuracy for the detection of a minimal SEB concentration.

Structural basis for inhibition of TLR2 by staphylococcal superantigen-like protein 3 (SSL3)

Toll-like receptors (TLRs) are crucial in innate recognition of invading micro-organisms and their subsequent clearance. Bacteria are not passive bystanders and have evolved complex evasion mechanisms. Staphylococcus aureus secretes a potent TLR2 antagonist, staphylococcal superantigen-like protein 3 (SSL3), which prevents receptor stimulation by pathogen-associated lipopeptides. Here, we present crystal structures of SSL3 and its complex with TLR2. The structure reveals that formation of the specific inhibitory complex is predominantly mediated by hydrophobic contacts between SSL3 and TLR2 and does not involve interaction of TLR2-glycans with the conserved Lewis(X) binding site of SSL3. In the complex, SSL3 partially covers the entrance to the lipopeptide binding pocket in TLR2, reducing its size by ∼50%. We show that this is sufficient to inhibit binding of agonist Pam2CSK4 effectively, yet allows SSL3 to bind to an already formed TLR2-Pam2CSK4 complex. The binding site of SSL3 overlaps those of TLR2 dimerization partners TLR1 and TLR6 extensively. Combined, our data reveal a robust dual mechanism in which SSL3 interferes with TLR2 activation at two stages: by binding to TLR2, it blocks ligand binding and thus inhibits activation. Second, by interacting with an already formed TLR2-lipopeptide complex, it prevents TLR heterodimerization and downstream signaling.

Identification and Characterization of a Novel Staphylococcal Emetic Toxin

Staphylococcal enterotoxins (SEs) produced by Staphylococcus aureus have superantigenic and emetic activities, which cause toxic shock syndrome and staphylococcal food poisoning, respectively. Our previous study demonstrated that the sequence of SET has a low level of similarity to the sequences of other SEs and exhibits atypical bioactivities. Hence, we further explored whether there is an additional SET-related gene in S. aureus strains. One SET-like gene was found in the genome of S. aureus isolates that originated from a case of food poisoning, a human nasal swab, and a case of bovine mastitis. The deduced amino acid sequence of the SET-like gene showed 32% identity with the amino acid sequence of SET. The SET-like gene product was designated SElY. In the food poisoning and nasal swab isolates, mRNA encoding SElY was highly expressed in the early log phase of cultivation, whereas a high level of expression of this mRNA was found in the bovine mastitis isolate at the early stationary phase. To estimate whether SElY has both superantigenic and emetic activities, recombinant SElY was prepared. Cell proliferation and cytokine production were examined to assess the superantigenic activity of SElY. SElY exhibited superantigenic activity in human peripheral blood mononuclear cells but not in mouse splenocytes. In addition, SElY exhibited emetic activity in house musk shrews after intraperitoneal and oral administration. However, the stability of SElY against heating and pepsin and trypsin digestion was different from that of SET and SEA. From these results, we identified SElY to be a novel staphylococcal emetic toxin.

Regulation of Staphylococcal Superantigen-Like Gene, ssl8, Expression in Staphylococcus aureus strain, RN6390

Staphylococcal superantigen-like (SSL) proteins, which are encoded by a cluster of eleven ssl genes, contribute to the Staphylococcus aureus virulence. Recently we reported ssl8 expression profiles in seven clinically important strains-MW2, USA300FPR3757, MSSA476, Newman, RN6390, Mu50, and N315-and showed the differential expression of ssl8 in Newman, RN6390, and USA300FPR3757 strains, despite harboring identical allelic forms of ssl8, suggesting the roles for different regulatory elements for this gene in different S. aureus strains. In this communication, using RN6390, a common laboratory S. aureus strain and its isogenic knockout mutant strains of agr, sae, sarA, sigB, rot, and the agr-/sigB (-) double mutant, we showed that SarA and Rot are inducer and repressor, respectively, for ssl8 expression in RN6390. This is in contrast to the Newman strain, where ssl8 is positively regulated by Sae but negatively by Agr, indicating the variable expression of ssl8 in clinical strains is more likely due to strain-specific regulatory elements.

Staphylococcal Immune Evasion Proteins: Structure, Function, and Host Adaptation

Staphylococcus aureus is a successful human and animal pathogen. Its pathogenicity is linked to its ability to secrete a large amount of virulence factors. These secreted proteins interfere with many critical components of the immune system, both innate and adaptive, and hamper proper immune functioning. In recent years, numerous studies have been conducted in order to understand the molecular mechanism underlying the interaction of evasion molecules with the host immune system. Structural studies have fundamentally contributed to our understanding of the mechanisms of action of the individual factors. Furthermore, such studies revealed one of the most striking characteristics of the secreted immune evasion molecules: their conserved structure. Despite high-sequence variability, most immune evasion molecules belong to a small number of structural categories. Another remarkable characteristic is that S. aureus carries most of these virulence factors on mobile genetic elements (MGE) or ex-MGE in its accessory genome. Coevolution of pathogen and host has resulted in immune evasion molecules with a highly host-specific function and prevalence. In this review, we explore how these shared structures and genomic locations relate to function and host specificity. This is discussed in the context of therapeutic options for these immune evasion molecules in infectious as well as in inflammatory diseases.

Staphylococcal superantigen-like protein 10 (SSL10) inhibits blood coagulation by binding to prothrombin and factor Xa via their γ-carboxyglutamic acid (Gla) domain

The staphylococcal superantigen-like protein (SSL) family is composed of 14 exoproteins sharing structural similarity with superantigens but no superantigenic activity. Target proteins of four SSLs have been identified to be involved in host immune responses. However, the counterparts of other SSLs have been functionally uncharacterized. In this study, we have identified porcine plasma prothrombin as SSL10-binding protein by affinity purification using SSL10-conjugated Sepharose. The resin recovered the prodomain of prothrombin (fragment 1 + 2) as well as factor Xa in pull-down analysis. The equilibrium dissociation constant between SSL10 and prothrombin was 1.36 × 10(-7) M in surface plasmon resonance analysis. On the other hand, the resin failed to recover γ-carboxyglutamic acid (Gla) domain-less coagulation factors and prothrombin from warfarin-treated mice, suggesting that the Gla domain of the coagulation factors is essential for the interaction. SSL10 prolonged plasma clotting induced by the addition of Ca(2+) and factor Xa. SSL10 did not affect the protease activity of thrombin but inhibited the generation of thrombin activity in recalcified plasma. S. aureus produces coagulase that non-enzymatically activates prothrombin. SSL10 attenuated clotting induced by coagulase, but the inhibitory effect was weaker than that on physiological clotting, and SSL10 did not inhibit protease activity of staphylothrombin, the complex of prothrombin with coagulase. These results indicate that SSL10 inhibits blood coagulation by interfering with activation of coagulation cascade via binding to the Gla domain of coagulation factor but not by directly inhibiting thrombin activity. This is the first finding that the bacterial protein inhibits blood coagulation via targeting the Gla domain of coagulation factors.

Staphylococcal superantigen-like protein 10 binds to phosphatidylserine and apoptotic cells

Staphylococcal superantigen-like proteins (SSLs) are a family of exoproteins that have structural similarities to staphylococcal superantigens. Although SSLs do not have superantigenic activity, some of them have been reported to bind to host immune related molecules and they have been implicated in immune evasion by S. aureus. In this study, we showed that SSL10 is capable of binding to phospholipids. SSL10 bound to phosphatidylserine (PS) containing liposome, but not to phosphatidylcholine liposome. SSL10, but not SSL7, bound to PS containing liposome, suggesting that SSL10 specifically binds to PS. Analysis of PS binding ability among recombinant truncated SSL10 fragments revealed that the β-barrel in the N-terminal oligonucleotide/oligosaccharide-binding (OB)-fold domain contributes to PS binding capacity. Fluorescein isothiocyanate labeled OB-fold of SSL10 stained hydrogen peroxide treated Jurkat cells. Annexin V is widely utilized for detection of apoptosis. Unlike annexin V, the OB-fold domain of SSL10 also bound to apoptotic cells in the presence of EDTA, suggesting that the OB-fold of SSL10 recognizes PS and apoptotic cells in a Ca(2+) independent manner. These findings suggest SSL10 and its derived peptides may be a novel detection tool for apoptotic cells.

Evolutionary paths of streptococcal and staphylococcal superantigens

Background

Streptococcus pyogenes (GAS) harbors several superantigens (SAgs) in the prophage region of its genome, although speG and smez are not located in this region. The diversity of SAgs is thought to arise during horizontal transfer, but their evolutionary pathways have not yet been determined. We recently completed sequencing the entire genome of S. dysgalactiae subsp. equisimilis (SDSE), the closest relative of GAS. Although speG is the only SAg gene of SDSE, speG was present in only 50% of clinical SDSE strains and smez in none. In this study, we analyzed the evolutionary paths of streptococcal and staphylococcal SAgs.

Results

We compared the sequences of the 12–60 kb speG regions of nine SDSE strains, five speG⁺ and four speG^–. We found that the synteny of this region was highly conserved, whether or not the speG gene was present. Synteny analyses based on genome-wide comparisons of GAS and SDSE indicated that speG is the direct descendant of a common ancestor of streptococcal SAgs, whereas smez was deleted from SDSE after SDSE and GAS split from a common ancestor. Cumulative nucleotide skew analysis of SDSE genomes suggested that speG was located outside segments of steeper slopes than the stable region in the genome, whereas the region flanking smez was unstable, as expected from the results of GAS. We also detected a previously undescribed staphylococcal SAg gene, selW, and a staphylococcal SAg -like gene, ssl, in the core genomes of all Staphylococcus aureus strains sequenced. Amino acid substitution analyses, based on dN/dS window analysis of the products encoded by speG, selW and ssl suggested that all three genes have been subjected to strong positive selection. Evolutionary analysis based on the Bayesian Markov chain Monte Carlo method showed that each clade included at least one direct descendant.

Conclusions

Our findings reveal a plausible model for the comprehensive evolutionary pathway of streptococcal and staphylococcal SAgs.

Comparative genotypic and phenotypic characterisation of methicillin-resistant Staphylococcus aureus ST398 isolated from animals and humans

The high prevalence of methicillin-resistant Staphylococcus aureus (MRSA) ST398 among pigs in certain European countries and North America and its occurrence in other animal species raises a question concerning the molecular mechanisms mediating the success of this lineage. In this study a panel of S. aureus strains belonging to sequence type (ST) 5 (n = 4), ST8 (n = 5), ST15 (n = 5), ST22 (n = 8), clonal complex (CC) 30 (n = 8), CC97 (n = 8), CC130 (n = 4), CC151 (n = 4) and ST398 (n = 18) were screened by DNA microarray and PCR for the carriage of virulence and antimicrobial resistance genes. Isolates belonging to the same sequence type/clonal complex (ST/CC) were found to share similar virulence gene profiles. The ST398 lineage displayed the lowest content of virulence genes, which consisted mainly of genes detected among the majority or all of the analysed lineages. All MRSA ST398 isolates lacked accessory virulence genes that were detected in other ST/CC. In contrast to virulence genotype, the antimicrobial resistance genes profiles varied between isolates belonging to the same ST/CC and profile similarities could be observed for isolates from different lineages. MRSA ST398 isolates in particular displayed significant diversity and high content of antimicrobial resistance genes. This was comparable with certain MRSA belonging to other sequence types particularly the equine MRSA ST8. The apparent lack of significant virulence genes among MRSA ST398 strains, demonstrates that the lineage features a unique genetic background but no ST398-specific virulence markers could be identified.

Staphylococcal superantigen-like protein 3 binds to the Toll-like receptor 2 extracellular domain and inhibits cytokine production induced by Staphylococcus aureus, cell wall component, or lipopeptides in murine macrophages

Staphylococcal superantigen-like proteins (SSLs) are a family of exoproteins sharing structural similarity with superantigens, but no superantigenic activity. Corresponding host target proteins or receptors against a portion of SSLs in the family have been identified. In this study, we show that SSL3 specifically binds to Toll-like receptor 2 (TLR2) and inhibits the stimulation of macrophages by TLR2 ligands. An approximately 100-kDa protein was recovered by using recombinant His-tagged SSL3-conjugated Sepharose from the lysate of porcine spleen, and the protein was identified as porcine TLR2 by peptide mass fingerprinting analysis. The SSL3-conjugated Sepharose recovered human and mouse TLR2 but not TLR4 from human neutrophils and mouse macrophage RAW 264.7 cells, as well as a recombinant TLR2 extracellular domain chimera protein. The production levels of interleukin 12 (IL-12) from mouse macrophages treated with heat-killed Staphylococcus aureus and of tumor necrosis factor alpha (TNF-α) from RAW 264.7 cells induced by peptidoglycan or lipopeptide TLR2 ligands were strongly suppressed in the presence of SSL3. The mutation of consensus sialic acid-containing glycan-binding residues in SSL3 did not abrogate the binding ability to TLR2 or inhibitory activity on TLR2, indicating that the interaction of SSL3 with TLR2 was independent of the sialic acid-containing glycan-binding residues. These findings demonstrate that SSL3 is able to bind the extracellular domain of TLR2 and interfere with TLR2 function. The present study provides a novel mechanism of SSL3 in immune evasion of S. aureus via interfering with its recognition by innate immune cells.

Factors influencing the clinical outcome of methicillin-resistant Staphylococcus aureus bacteraemia

Methicillin-resistant Staphylococcus aureus (MRSA) bacteraemia is associated with high mortality due to a combination of host, pathogen and therapy related factors. This was a retrospective exploratory study to evaluate host, pathogen and therapy related factors influencing the clinical outcome of MRSA bacteraemia in a UK teaching hospital setting. Of the 38 consecutive episodes of MRSA bacteraemia over a 1-year period, 16 of 38 (40%) patients died at 1 month and 21/38 (55%) died at 6 months. Univariate analysis revealed age (p < 0.006), mean serum vancomycin level (p < 0.035), agr group I (p < 0.036) and set4-var2_11 gene (p < 0.036) at 1 month; and age (p < 0.004) and set4-var2_11 gene (p < 0.002) at 6 months as significant factors. However, there was no association between first trough vancomycin concentration and outcome at 1 month. Multivariate survival analysis from time of admission showed, for each one year increase in age, a patient is 1.121 (95% CI 1.006-1.250, p < 0.007) times more likely to die at any particular point in time, and patients with a mean serum vancomycin level of <10 mg/L, the odds ratio of adverse outcome is 16.129 (95% CI 2.398-111.111) compared to patients with a mean serum level >10 mg/L. A variety of host, pathogen, and therapy related factors influence the clinical outcome of MRSA bacteraemia.

GPVI and GPIbα mediate staphylococcal superantigen-like protein 5 (SSL5) induced platelet activation and direct toward glycans as potential inhibitors

Background

Staphylococcus aureus (S. aureus) is a common pathogen capable of causing life-threatening infections. Staphylococcal superantigen-like protein 5 (SSL5) has recently been shown to bind to platelet glycoproteins and induce platelet activation. This study investigates further the interaction between SSL5 and platelet glycoproteins. Moreover, using a glycan discovery approach, we aim to identify potential glycans to therapeutically target this interaction and prevent SSL5-induced effects.

Methodology/Principal Findings

In addition to platelet activation experiments, flow cytometry, immunoprecipitation, surface plasmon resonance and a glycan binding array, were used to identify specific SSL5 binding regions and mediators. We independently confirm SSL5 to interact with platelets via GPIbα and identify the sulphated-tyrosine residues as an important region for SSL5 binding. We also identify the novel direct interaction between SSL5 and the platelet collagen receptor GPVI. Together, these receptors offer one mechanistic explanation for the unique functional influences SSL5 exerts on platelets. A role for specific families of platelet glycans in mediating SSL5-platelet interactions was also discovered and used to identify and demonstrate effectiveness of potential glycan based inhibitors in vitro.

Conclusions/Significance

These findings further elucidate the functional interactions between SSL5 and platelets, including the novel finding of a role for the GPVI receptor. We demonstrate efficacy of possible glycan-based approaches to inhibit the SSL5-induced platelet activation. Our data warrant further work to prove SSL5-platelet effects in vivo.

Food poisoning and Staphylococcus aureus enterotoxins

Staphylococcus aureus produces a wide variety of toxins including staphylococcal enterotoxins (SEs; SEA to SEE, SEG to SEI, SER to SET) with demonstrated emetic activity, and staphylococcal-like (SEl) proteins, which are not emetic in a primate model (SElL and SElQ) or have yet to be tested (SElJ, SElK, SElM to SElP, SElU, SElU2 and SElV). SEs and SEls have been traditionally subdivided into classical (SEA to SEE) and new (SEG to SElU2) types. All possess superantigenic activity and are encoded by accessory genetic elements, including plasmids, prophages, pathogenicity islands, vSa genomic islands, or by genes located next to the staphylococcal cassette chromosome (SCC) implicated in methicillin resistance. SEs are a major cause of food poisoning, which typically occurs after ingestion of different foods, particularly processed meat and dairy products, contaminated with S. aureus by improper handling and subsequent storage at elevated temperatures. Symptoms are of rapid onset and include nausea and violent vomiting, with or without diarrhea. The illness is usually self-limiting and only occasionally it is severe enough to warrant hospitalization. SEA is the most common cause of staphylococcal food poisoning worldwide, but the involvement of other classical SEs has been also demonstrated. Of the new SE/SEls, only SEH have clearly been associated with food poisoning. However, genes encoding novel SEs as well as SEls with untested emetic activity are widely represented in S. aureus, and their role in pathogenesis may be underestimated.

Colonization and infection of the human host by staphylococci: adhesion, survival and immune evasion

The natural habitat of Staphylococcus aureus in humans is the moist squamous epithelium of the anterior nares. Several bacterial surface proteins are implicated in promoting adhesion to desquamated epithelial cells. Clumping factor B (ClfB) and iron-regulated surface determinant A both promote nasal colonization in rodent models, and in the case of ClfB, humans. One of the ligands involved in adhesion is cytokeratin 10. Reduction in nasal colonization can be achieved by active and passive immunization. S. aureus is well endowed with secreted and surface components that compromise innate immune responses, particularly the function of neutrophils. S. aureus secretes proteins that reduce migration of neutrophils from the bloodstream to the site of infection by impeding diapedesis and receptors for chemotactic molecules. Several secreted proteins interfere with complement C3 and C5 convertases, thus reducing the level of C3b opsonin and the chemotactic peptide C5a. Host proteases are recruited to the cell surface to enhance destruction of opsonic C3b and IgG. Surface components ClfA, protein A and polysaccharide capsule compromise the recognition of opsonins on the bacterial cell surface. If engulfed by neutrophils the intracellular bacterium can resist reactive oxygen intermediates, nitric oxide radicals, defensin peptides and bactericidal proteins. A prior infection by S. aureus does not induce complete protective immunity. This could be due to immunosuppression caused by expression of superantigen proteins that disrupt normal activation of T cells and B cells during antigen presentation. By studying the molecular pathogenesis of S. aureus infections markers might be found for investigating S. pseudintermedius infections of dogs.

Staphylococcal superantigen-like protein 5 inhibits matrix metalloproteinase 9 from human neutrophils

Staphylococcal superantigen-like proteins (SSLs) constitute a family of exoproteins exhibiting structural similarities to superantigens and enterotoxins but no superantigenic activity. In this article, we present evidence that SSL5 specifically binds to matrix metalloproteinase 9 (MMP-9) and inhibits its enzymatic activity. When human neutrophil cell lysate was applied to recombinant His-tagged SSL5 conjugated to Sepharose, the bound fraction gave a major band of approximately 100 kDa in SDS-polyacrylamide gel electrophoresis. This protein was identified as the proform of MMP-9 (proMMP-9) by peptide mass fingerprinting analysis. The recombinant SSL5-Sepharose also bound to proMMP-9 secreted by interleukin 8 (IL-8)-stimulated neutrophils and HT1080 fibrosarcoma cells. Surface plasmon resonance analysis revealed that recombinant SSL5 bound to proMMP-9 with rather high affinity (dissociation constant [K(D)] = 1.9 nM). Recombinant SSL5 was found to effectively inhibit MMP-9-catalyzed hydrolysis of gelatin and a synthetic fluorogenic peptide in a noncompetitive manner (K(i) = 0.097 nM), as assessed by zymography and the fluorescence quenching method. Finally, the transmigration of neutrophils across Matrigel basement membranes in response to N-formyl-methionyl-leucyl-phenylalanine (FMLP) was suppressed by the presence of recombinant SSL5. We discuss possible roles that SSL5 may play in immune evasion of staphylococci by inhibiting MMP and interfering with leukocyte trafficking.

Staphylococcal superantigen-like genes, ssl5 and ssl8, are positively regulated by Sae and negatively by Agr in the Newman strain

Some of the staphylococcal superantigen-like (SSL) proteins SSL5, SSL7, SSL9, and SSL11 act as immunomodulatory proteins in Staphylococcus aureus. However, little is known about their regulatory mechanisms. We determined the expression levels of ssl5 and ssl8 in seven clinically important S. aureus strains and their regulatory mechanisms in the Newman strain, which had the highest ssl5 and ssl8 expression. Independent comparisons of ssl5 or ssl8 coding and upstream sequences in these strains identified multiple haplotypes that did not correlate with the differential expression of ssl5 and ssl8, suggesting the role of additional regulatory elements. Using knockout mutant strains of known S. aureus global regulators such as Agr, Sae, and SigB in the Newman strain, we showed that both ssl5 and ssl8 were induced by Sae and repressed by Agr, suggesting that Sae and Agr are the positive and the negative regulators, respectively, of these two ssl genes. Moreover, we observed upregulation of sae in the agr mutant and upregulation of agr in the sae mutant compared with the isogenic Newman strain, suggesting that the Agr and Sae may be inhibiting each other. The SigB mutation did not affect ssl5 and ssl8 expression, but they were downregulated in the agr/sigB double mutant, indicating that SigB probably acts synergistically with Agr in their upregulation.

Membrane damage elicits an immunomodulatory program in Staphylococcus aureus

The Staphylococcus aureus HrtAB system is a hemin-regulated ABC transporter composed of an ATPase (HrtA) and a permease (HrtB) that protect S. aureus against hemin toxicity. S. aureus strains lacking hrtA exhibit liver-specific hyper-virulence and upon hemin exposure over-express and secrete immunomodulatory factors that interfere with neutrophil recruitment to the site of infection. It has been proposed that heme accumulation in strains lacking hrtAB is the signal which triggers S. aureus to elaborate this anti-neutrophil response. However, we report here that S. aureus strains expressing catalytically inactive HrtA do not elaborate the same secreted protein profile. This result indicates that the physical absence of HrtA is responsible for the increased expression of immunomodulatory factors, whereas deficiencies in the ATPase activity of HrtA do not contribute to this process. Furthermore, HrtB expression in strains lacking hrtA decreases membrane integrity consistent with dysregulated permease function. Based on these findings, we propose a model whereby hemin-mediated over-expression of HrtB in the absence of HrtA damages the staphylococcal membrane through pore formation. In turn, S. aureus senses this membrane damage, triggering the increased expression of immunomodulatory factors. In support of this model, wildtype S. aureus treated with anti-staphylococcal channel-forming peptides produce a secreted protein profile that mimics the effect of treating ΔhrtA with hemin. These results suggest that S. aureus senses membrane damage and elaborates a gene expression program that protects the organism from the innate immune response of the host.

Staphylococcus aureus infects almost every tissue within the human body utilizing a range of virulence factors to combat host defenses. The expression of these virulence factors is a tightly regulated process; however, the signals sensed by S. aureus during infection remain elusive. It has been hypothesized that heme toxicity is a signal sensed by S. aureus during infection. This hypothesis is based on the observation that S. aureus mutants which are incapable of relieving heme-toxicity due to inactivation of the ATPase HrtA elicit an immunomodulatory program that interferes with neutrophil recruitment to the site of infection. In keeping with this, S. aureus hrtA mutants exhibit liver-specific hypervirulence. Herein, we provide evidence for an alternative model to explain the hypervirulent phenotype of S. aureus ΔhrtA. We demonstrate that instead of accumulation of heme toxicity being the trigger for the observed immunomodulatory program, dysregulated pore formation caused by the HrtB permease triggers the anti-neutrophil response. In support of this model, over-expression of HrtB in wildtype S. aureus or exposing S. aureus to channel-forming antimicrobial peptides induces a similar immunomodulatory program. Our work provides evidence that S. aureus senses membrane damage and induces an immunomodulatory circuit that helps the pathogen evade immune-mediated clearance.

Staphylococcal superantigen-like protein 10 (SSL10) binds to human immunoglobulin G (IgG) and inhibits complement activation via the classical pathway

Staphylococcal superantigen-like (SSL) proteins are a family of exoproteins that share structural similarity with staphylococcal superantigens but exhibit no superantigenic activity. It was previously reported that two members (SSL5 and SSL7) bound to serum components and cell adhesion molecules involved in host immune response; however, the other family members have not been functionally characterized. In this study, we attempted to isolate SSL10-binding proteins from human serum and found that recombinant His-tagged SSL10 bound two major polypeptides of approximately 50 and approximately 25 kDa after affinity purification and SDS-polyacrylamide gel electrophoresis. These polypeptides were identified as heavy and light chains of human IgG by peptide mass fingerprinting analysis. The specific interaction between recombinant SSL10 and human IgG was confirmed by far Western blot analysis using immobilized SSL10 and pull-down analysis using SSL10-conjugated Sepharose. Surface plasmon resonance analysis revealed that the dissociation equilibrium constant for the interaction between human IgG and recombinant SSL10 was estimated to be 220 nM. We also found that recombinant SSL10 inhibited the binding of complement component C1q to IgG-Sepharose and hemolysis of IgG-sensitized sheep erythrocytes via the classical complement activation pathway. These results suggest that SSL10 may play a role in the evasion of Staphylococcus aureus from the host immune system via interfering complement activation.

Multiple genome comparison within a bacterial species reveals a unit of evolution spanning two adjacent genes in a tandem paralog cluster

It has been assumed that an open reading frame (ORF) represents a unit of gene evolution as well as a unit of gene expression and function. In the present work, we report a case in which a unit comprising the 3' region of an ORF linked to a downstream intergenic region that is in turn linked to the 5' region of a downstream ORF has been conserved, and has served as the unit of gene evolution. The genes are tandem paralogous genes from the bacterium Staphylococcus aureus, for which more than ten entire genomes have been sequenced. We compared these multiple genome sequences at a locus for the lpl (lipoprotein-like) cluster (encoding lipoprotein homologs presumably related to their host interaction) in the genomic island termed nuSaalpha. A highly conserved nucleotide sequence found within every lpl ORF is likely to provide a site for homologous recombination. Comparison of phylogenies of the 5'-variable region and the 3'-variable region within the same ORF revealed significant incongruence. In contrast, pairs of the 3'-variable region of an ORF and the 5'-variable region of the next downstream ORF gave more congruent phylogenies, with distinct groups of conserved pairs. The intergenic region seemed to have coevolved with the flanking variable regions. Multiple recombination events at the central conserved region appear to have caused various types of rearrangements among strains, shuffling the two variable regions in one ORF, but maintaining a conserved unit comprising the 3'-variable region, the intergenic region, and the 5'-variable region spanning adjacent ORFs. This result has strong impact on our understanding of gene evolution because most gene lineages underwent tandem duplication and then diversified. This work also illustrates the use of multiple genome sequences for high-resolution evolutionary analysis within the same species.

Identification and characterization of two novel staphylococcal enterotoxins, types S and T

In addition to two known staphylococcal enterotoxin-like genes (selj and selr), two novel genes coding for two superantigens, staphylococcal enterotoxins S and T (SES and SET), were identified in plasmid pF5, which is harbored by food poisoning-related Staphylococcus aureus strain Fukuoka 5. This strain was implicated in a food poisoning incident in Fukuoka City, Japan, in 1997. Recombinant SES (rSES) specifically stimulated human T cells in a T-cell receptor Vbeta9- and Vbeta16-specific manner in the presence of major histocompatibility complex (MHC) class II(+) antigen-presenting cells (APC). rSET also stimulated T cells in the presence of MHC class II(+) APC, although its Vbeta skewing was not found in reactive T cells. Subsequently, we examined the emetic activity of SES and SET. We also studied SElR to determine emetic activity in primates. This toxin was identified in previous studies but was not examined in terms of possession of emetic activity for primates. rSES induced emetic reactions in two of four monkeys at a dose of 100 microg/kg within 5 h of intragastric administration. In one monkey, rSET induced a delayed reaction (24 h postadministration) at a dose of 100 microg/kg, and in the other one, the reaction occurred 5 days postadministration. rSElR induced a reaction in two of six animals within 5 h at 100 microg/kg. On this basis, we speculate that the causative toxins of vomiting in the Fukuoka case are SES and SER. Additionally, SES, SER, and SET also induced emesis in house musk shrews as in the monkeys.

Molecular diagnostics of clinically important staphylococci

Bacterial species of the genus Staphylococcus known as important human and animal pathogens are the cause of a number of severe infectious diseases. Apart from the major pathogen Staphylococcus aureus, other species until recently considered to be nonpathogenic may also be involved in serious infections. Rapid and accurate identification of the disease-causing agent is therefore prerequisite for disease control and epidemiological surveillance. Modern methods for identification and typing of bacterial species are based on genome analysis and have many advantages compared to phenotypic methods. The genotypic methods currently used in molecular diagnostics of staphylococcal species, particularly of S. aureus, are reviewed. Attention is also paid to new molecular methods with the highest discriminatory power. Efforts made to achieve interlaboratory reproducibility of diagnostic methods are presented.

Assignment of Staphylococcus aureus isolates to clonal complexes based on microarray analysis and pattern recognition

A DNA microarray was designed for the rapid genotyping of Staphylococcus aureus. It covers 185 distinct genes and about 300 alleles thereof, including species-specific controls, accessory gene regulator (agr) alleles, genes encoding virulence factors, and microbial surface components recognizing adhesive matrix molecules, capsule type-specific genes, as well as resistance determinants. It was used to examine 100 clinical isolates and reference strains. Relationships of leukocidin and ssl/set (staphylococcal superantigen-like or exotoxin-like) genes were reviewed considering these experimental results as well as published sequences. A good correlation of overall hybridization pattern and multilocus sequence typing was found. Analysis of hybridization profiles thus allowed not only to assess virulence and drug resistance, but also to assign isolates to strains and to clonal complexes. Hybridization data were used to construct a split network tree and to analyse relationships between strains. Allelic variations of a number of genes indicate a division of S. aureus into three major branches that are not in accordance to agr group or capsule-type affiliations. Additionally, there are some isolated lineages, such as ST75, ST93, or ST152. These strains produce aberrant hybridization profiles, indicating that only a part of the gene pool of S. aureus has been investigated yet.

DNA microarray-based genotyping of methicillin-resistant Staphylococcus aureus strains from Eastern Saxony

A diagnostic microarray was used to characterise a collection of methicillin-resistant Staphylococcus aureus (MRSA) isolates from hospitals in the German region of Eastern Saxony. The most abundant epidemic MRSA (EMRSA) strains were ST5-MRSA II (Rhine-Hesse EMRSA, EMRSA-3), CC5/ST228-MRSA I (South German EMRSA), ST22-MRSA IV (Barnim EMRSA, EMRSA-15) and ST45-MRSA IV (Berlin EMRSA). Other strains were found only as sporadic isolates or in minor outbreaks. These strains included ST1-MRSA IV, ST8-MRSA IV (Hannover EMRSA and others), clonal group 5 strains carrying SCCmec type IV elements (Paediatric clone), ST45-MRSA V, CC8/ST239-MRSA III and ST398-MRSA V. Panton-Valentine leukocidin-positive MRSA isolates were still very rare. The predominant strain was ST80-MRSA IV, although increasing numbers of different strains have recently been detected (ST8-MRSA IV, ST30-MRSA IV and ST59-MRSA V). For more common MRSA strains, it was possible to detect variants that differed mainly in the carriage of additional resistance determinants and certain virulence-associated genes. Detection of such variants can sometimes allow epidemic strains to be resolved beyond spa types to a hospital-specific level, which is of significant value for epidemiological purposes.

Novel characteristics of community-acquired methicillin-resistant Staphylococcus aureus strains belonging to multilocus sequence type 59 in Taiwan

Community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) strains, which often produce Panton-Valentine leucocidin (PVL), are increasingly noted worldwide. In this study, we examined 42 MRSA strains (25 PVL-positive [PVL+] strains and 17 PVL-negative [PVL(-)] strains) isolated in Taiwan for their molecular characteristics. The PVL+ MRSA strains included CA-MRSA strains with multilocus sequence type (ST) 59 (major PVL+ MRSA in Taiwan), its variants, and worldwide CA-MRSA ST30 strains. The PVL(-) MRSA strains included the pandemic Hungarian MRSA ST239 strain, the Hungarian MRSA ST239 variant, MRSA ST59 (largely hospital-acquired MRSA strains) and its variants, the pandemic New York/Japan MRSA ST5 strain (Japanese type), and the MRSA ST8 strain. The major PVL+ CA-MRSA ST59 strain possessed a tetracycline resistance-conferring (tetK positive) penicillinase plasmid and a drug resistance gene cluster (a possible composite transposon) for multidrug resistance. Moreover, it carried a novel staphylococcal cassette chromosome mec (SCCmec) with two distinct ccrC genes (ccrC2-C8). This SCCmec (previously named SCCmec type V(T)) was tentatively designated SCCmec type VII. Sequencing of the PVL genes revealed the polymorphisms, and the PVL+ CA-MRSA ST59 strain possessed the ST59-specific PVL gene sequence. The data suggest that a significant amount of clonal spread is occurring in Taiwan and that the major PVL+ CA-MRSA ST59 Taiwan strain exhibits unique genetic characteristics, such as a novel SCCmec type and an ST59-specific PVL gene sequence.

A Staphylococcus aureus regulatory system that responds to host heme and modulates virulence

Staphylococcus aureus, a bacterium responsible for tremendous morbidity and mortality, exists as a harmless commensal in approximately 25% of humans. Identifying the molecular machinery activated upon infection is central to understanding staphylococcal pathogenesis. We describe the heme sensor system (HssRS) that responds to heme exposure and activates expression of the heme-regulated transporter (HrtAB). Inactivation of the Hss or Hrt systems leads to increased virulence in a vertebrate infection model, a phenotype that is associated with an inhibited innate immune response. We suggest that the coordinated activity of Hss and Hrt allows S. aureus to sense internal host tissues, resulting in tempered virulence to avoid excessive host tissue damage. Further, genomic analyses have identified orthologous Hss and Hrt systems in Bacillus anthracis, Listeria monocytogenes, and Enterococcus faecalis, suggesting a conserved regulatory system by which Gram-positive pathogens sense heme as a molecular marker of internal host tissue and modulate virulence.