Measurement of staphylococcal enterotoxin B in serum and culture supernatant with a capture enzyme-linked immunosorbent assay

T-cell receptor Vβ8 for detection of biologically active streptococcal pyrogenic exotoxin type C

Streptococcus pyogenes is an important human pathogen, commonly spread by airborne droplets but also by ingestion of contaminated food. Apart from causing infection, this pathogen produces 13 distinct types of streptococcal pyrogenic exotoxins (SPE). The current method for detection cannot distinguish between the biologically active form of SPE that has been reported to cause foodborne outbreaks and the inactivated toxin that poses no health risk. To measure the biological activity of SPE type C (SPE-C), one such toxin that was linked to foodborne outbreaks associated with milk and milk products, we developed a cell-based assay that can discern between biologically active and inactive SPE-C. To the best of our knowledge, this is the first showing that SPE-C activates T-cells expressing Vβ8. With this finding, we used a T-cell line natively expressing Vβ8 that was genetically engineered to also express the luciferase reporter gene under the regulation of nuclear factor of activated T-cells response element in combination with a B-cell line to present the recombinant SPE-C (rSPE-C) toxin via major histocompatibility complex (MHC) class II to the Vβ8 T-cell receptor (TCR) in an assay to detect and to discern between biologically active and inactive rSPE-C. By using this system, we demonstrated that SPE-C induced significant IL-2 secretion after 72 h and visible light emission after only 5 h, doubling by 24 h. We utilize this finding to assess the specificity of the assay and the effect of pasteurization on SPE-C activity. We observed no cross-reactivity with SPE-B and significant loss of SPE-C biological activity in spiked phosphate-buffered saline while SPE-C spiked into milk is heat stable. Once SPE-C has formed, it is infeasible to eliminate it from milk by thermal treatment.

A Higher Dose of Staphylococcus aureus Enterotoxin B Led to More Th1 and Lower Th2/Th1 Ratio in Th Cells

Exposure to Staphylococcus aureus enterotoxin B (SEB) is one of the causes of food poisoning and is associated with several immune diseases due to its superantigen capability. This study aimed to characterize the differentiations of naïve Th cells stimulated with different doses of SEB. The expression of T-bet, GATA-3, and Foxp3 or secretion of IFN-γ, IL-4, IL-5, IL-13, and IL-10 were evaluated in wild-type (WT) or DO11.10 CD4 T cells co-cultured with bone marrow dendritic cells (BMDCs). We found that the balance of Th1/Th2 could be dominated by the doses of SEB stimulation. A higher SEB dose could induce more Th1 and a lower Th2/Th1 ratio in Th cells co-cultured with BMDCs. This different tendency of Th cell differentiation induced by the SEB complements the existing knowledge about SEB acting as a superantigen to activate Th cells. Additionally, it is also helpful in managing the colonization of S. aureus and food contamination of SEB.

Molecular alterations induced by Yersinia pestis, dengue virus and Staphylococcal enterotoxin B under severe stress

BACKGROUND

Severe stress can have drastic and systemic effects with dire implications on the health and wellbeing of exposed individuals. Particularly, the effect of stress on the immune response to infection is of interest to public health because of its implications for vaccine efficacy and treatment strategies during stressful scenarios. Severe stress has previously been shown to cause an anergic state in the immune system that persists following exposure to a potent mitogen.

METHODS

Transcriptome and microRNA changes were characterized using blood samples collected from U.S. Army Ranger candidates immediately before and after training, followed by exposure to representative pathogenic agents: Yersinia pestis, dengue virus 2, and Staphylococcal enterotoxin B (SEB). We employed experimental and computational approaches to characterize altered gene expression, processes, pathways, and regulatory networks mediating the host's response towards severe stress; to assess the protective immunity status of the stressed host towards infection; and to identify pathogen-induced biomarkers under severe stress conditions.

RESULTS

We observed predicted inhibition of pathways significantly associated with lymphopoiesis, wound healing, inflammatory response, lymphocyte activation, apoptosis, and predicted activation of oxidative stress. Using weighted correlation network analyses, we demonstrated preservation of these pathways across infection and stress combinations. Regulatory networks comprising a common set of upstream regulators: transcription factors, microRNAs and post-translational regulators (kinases and phosphatases) may be drivers of molecular alterations leading to compromised protective immunity. Other sets of transcripts were persistently altered in both the pre- and post-stress conditions due to the host's response to each pathogenic agent, forming specific molecular signatures with the potential to distinguish infection from that of severe stress.

CONCLUSIONS

Our results suggest that severe stress alters molecules implicated in the development of leukopoietic stem cells, thereby leading to depletion of cellular and molecular repertoires of protective immunity. Suppressed molecules mediating membrane trafficking of recycling endosomes, membrane translocation and localization of the antigen processing mechanisms and cell adhesions indicate suboptimal antigen presentation, impaired formation of productive immunological synapses, and inhibited T-cell activations. These factors may collectively be responsible for compromised protective immunity (infection susceptibility, delayed wound healing, and poor vaccine response) observed in people under severe stress.

Development and evaluation of an IgY based silica matrix immunoassay platform for rapid onsite SEB detection

The present study involves immunoassay platform development based on a surface functionalized silica matrix for rapid onsite detection of Staphylococcal enterotoxin B (SEB).

Toxins as biological weapons for terror-characteristics, challenges and medical countermeasures: a mini-review

Toxins are hazardous biochemical compounds derived from bacteria, fungi, or plants. Some have mechanisms of action and physical properties that make them amenable for use as potential warfare agents. Currently, some toxins are classified as potential biological weapons, although they have several differences from classic living bio-terror pathogens and some similarities to manmade chemical warfare agents. This review focuses on category A and B bio-terror toxins recognized by the Centers for Disease Control and Prevention: Botulinum neurotoxin, staphylococcal enterotoxin B, Clostridium perfringens epsilon toxin, and ricin. Their derivation, pathogenesis, mechanism of action, associated clinical signs and symptoms, diagnosis, and treatment are discussed in detail. Given their expected covert use, the primary diagnostic challenge in toxin exposure is the early detection of morbidity clusters, apart from background morbidity, after a relatively short incubation period. For this reason, it is important that clinicians be familiar with the clinical manifestations of toxins and the appropriate methods of management and countermeasures.

Enzyme free detection of staphylococcal enterotoxin B (SEB) using ferrocene carboxylic acid labeled monoclonal antibodies: an electrochemical approach

We report a ferrocene based electrochemical immunosensor for staphylococcal enterotoxin B.

A Multiplex Assay for Detection of Staphylococcal and Streptococcal Exotoxins

Staphylococcal and streptococcal exotoxins, also known as superantigens, mediate a range of diseases including toxic shock syndrome, and they exacerbate skin, pulmonary and systemic infections caused by these organisms. When present in food sources they can cause enteric effects commonly known as food poisoning. A rapid, sensitive assay for the toxins would enable testing of clinical samples and improve surveillance of food sources. Here we developed a bead-based, two-color flow cytometry assay using single protein domains of the beta chain of T cell receptors engineered for high-affinity for staphylococcal (SEA, SEB and TSST-1) and streptococcal (SpeA and SpeC) toxins. Site-directed biotinylated forms of these high-affinity agents were used together with commercial, polyclonal, anti-toxin reagents to enable specific and sensitive detection with SD₅₀ values of 400 pg/ml (SEA), 3 pg/ml (SEB), 25 pg/ml (TSST-1), 6 ng/ml (SpeA), and 100 pg/ml (SpeC). These sensitivities were in the range of 4- to 80-fold higher than achieved with standard ELISAs using the same reagents. A multiplex format of the assay showed reduced sensitivity due to higher noise associated with the use of multiple polyclonal agents, but the sensitivities were still well within the range necessary for detection in food sources or for rapid detection of toxins in culture supernatants. For example, the assay specifically detected toxins in supernatants derived from cultures of Staphylococcus aureus. Thus, these reagents can be used for simultaneous detection of the toxins in food sources or culture supernatants of potential pathogenic strains of Staphylococcus aureus and Streptococcus pyogenes.

Stress-caused anergy of leukocytes towards Staphylococcal enterotoxin B and exposure transcriptome signatures

Leucocytes from soldiers exposed to battlefield-like stress (RASP: Rangers Assessment and Selection Program) were exposed in vitro to Staphylococcal enterotoxin B (SEB). We assayed SEB-induced regulation of gene expression, both in the presence and absence of severe stress, to generate two sets of gene profiles. One set of transcripts and microRNAs were specific to post-RASP SEB exposure, and another set were signatures of SEB exposure common to both the pre- and post-RASP leucocytes. Pathways and upstream regulatory analyses indicated that the post-RASP SEB-signature transcripts were manifestation of the anergic state of post-RASP leucocytes. These were further verified using expression-based predictions of cellular processes and literature searches. Specificity of the second set of transcripts to SEB exposure was verified using machine-learning algorithms on our and four other (Gene Expression Omnibus) data sets. Cell adhesion, coagulation, hypoxia and vascular endothelial growth factor-mediated vascular leakage were SEB-specific pathways even under the background of severe stress. Hsa-miR-155-3p was the top SEB exposure predictor in our data set, and C-X-C motif chemokine ligand 9 was SEB specific in all the analyzed data sets. The SEB-signature transcripts (which also showed distinct expression signatures from Yersinia pestis and dengue virus) may serve as potential biomarkers of SEB exposure even under the background of stress.

Relative efficiency of zinc sulfide (ZnS) quantum dots (QDs) based electrochemical and fluorescence immunoassay for the detection of Staphylococcal enterotoxin B (SEB)

In this paper an attempt was made to detect Staphylococcal enterotoxin B (SEB) both by electrochemical and fluorescence immunoassay methods using zinc sulphide (ZnS) QDs. Wet-chemical method was adopted for the preparation of fluorescent ZnS QDs (diameter ∼ 5–10 nm). These QDs were bioconjugated with monoclonal antibodies and then characterized by various method. A detection limit of 0.02 ng mL⁻¹ by fluorescence assay and 1.0 ng mL⁻¹ by electrochemical assay for SEB was achieved. While by sandwich ELISA it is possible to detect 0.24 ng mL⁻¹ only. The sensitivity of all techniques is very good, since the LD₅₀ of SEB is 20 ng kg⁻¹. Electrochemical assay is faster, need low-cost instrument, independent to the size of QDs and found to be one of the best alternative methods as compared to the other existing methods studied herein. The presented method could be expanded to the development of electrochemical and fluorescence biosensors for various agents for field and laboratory use.

xMAP-based analysis of three most prevalent staphylococcal toxins in Staphylococcus aureus cultures

Detection of staphylococcal toxins presents a great interest for medical diagnostics. Screening of clinical samples for the presence of several types of staphylococcal toxins using traditional methods-biological tests on animals or cell cultures as well as ELISA-is laborious. Multiplex detection methods would simplify testing. We have designed an xMAP-based assay to detect three staphylococcal toxins-enterotoxins A and B (SEA and SEB) and toxic shock syndrome toxin (TSST)-in cultural supernatants obtained from different strains of Staphylococcus aureus. The limits of detection of SEA, SEB, and TSST multiplex detection in S. aureus growth medium were 10, 1,000, and 5 pg/mL, respectively. Fifty-nine samples of S. aureus cultural supernatants were tested with the xMAP assay. The developed assay has proved highly effective detection of the natural toxins in the samples obtained due to bacterial cells cultivation. In prospect, the developed test system can be used in clinical diagnostics and in monitoring of foodstuffs and environmental objects.

Isotype switching increases efficacy of antibody protection against staphylococcal enterotoxin B-induced lethal shock and Staphylococcus aureus sepsis in mice

Staphylococcal enterotoxin B (SEB) is a potent toxin that is produced by Staphylococcus aureus strains and is classified as a category B select agent. We have previously shown that monoclonal antibody (MAb) 20B1, a murine anti-SEB IgG1, successfully treats SEB-induced lethal shock (SEBILS) and bacteremia that is caused by SEB-producing S. aureus. In this study, we have generated two isotype switch variants of the original IgG1 MAb 20B1, an IgG2a and IgG2b, both bearing the same variable region sequence, and compared their neutralizing and protective activity in in vitro and in vivo assays, respectively. All 3 isotypes demonstrated comparable affinity to SEB and comparable 50% inhibitory concentrations (IC50s) in T cell proliferation assays. In vivo, however, the IgG2a isotype variant of 20B1 exhibited significantly greater protection than IgG1 or IgG2b in murine SEB intoxication and S. aureus sepsis models. Protection was associated with downmodulation of inflammatory host response. Our data demonstrate that changing the isotype of already protective MAbs, without affecting their antigen specificity or sensitivity, can result in an enhancement of their protective ability. Isotype selection, therefore, should be carefully considered in the development of toxin-neutralizing MAbs and the design of antibody therapeutics.

IMPORTANCE

The purpose of this study was to enhance the protective efficacy of an existing, protective monoclonal antibody against staphylococcal enterotoxin B. Using two in vivo mouse models, our study demonstrates that the protective efficacy of a monoclonal antibody may be improved by inducing an isotype switch at the Fc region of an antibody, without altering the antigen specificity or sensitivity of the antibody. The development of therapeutic MAbs with higher efficacy may allow for the achievement of equal therapeutic benefit with a lower dosage. In turn, the use of lower doses may reduce the cost of these therapies, while reducing the potential for adverse side effects.

Detection and measurement of staphylococcal enterotoxin-like K (SEl-K) secretion by Staphylococcus aureus clinical isolates

Staphylococcal enterotoxin-like K (SEl-K) is a potent mitogen that elicits T-cell proliferation and cytokine production at very low concentrations. However, unlike the classical enterotoxins SEB and toxic shock syndrome toxin 1 (TSST-1), the gene for SEl-K is commonly present in more than half of all Staphylococcus aureus clinical isolates and is present in almost all USA300 community-acquired methicillin-resistant S. aureus (CA-MRSA) isolates. Sequencing of the sel-k gene in over 20 clinical isolates and comparative analysis with all 14 published sel-k sequences indicate that there are at least 6 variants of the sel-k gene, including one that is conserved among all examined USA300 strains. Additionally, we have developed a highly sensitive enzyme-linked immunosorbent assay (ELISA) that specifically detects and measures SEl-K protein in culture supernatants and biological fluids. Quantification of in vitro SEl-K secretion by various S. aureus isolates using this novel capture ELISA revealed detectable amounts of SEl-K secretion by all isolates, with the highest secretion levels being exhibited by MRSA strains that coexpress SEB. In vivo secretion was measured in a murine thigh abscess model, where similar levels of SEl-K accumulation were noted regardless of whether the infecting strain exhibited high or low secretion of SEl-K in vitro. We conclude that SEl-K is commonly expressed in the setting of staphylococcal infection, in significant amounts. SEl-K should be further explored as a target for passive immunotherapy against complicated S. aureus infection.

Staphylococcal Enterotoxin B-specific monoclonal antibody 20B1 successfully treats diverse Staphylococcus aureus infections

BACKGROUND

Methicillin-resistant Staphylococcus aureus (MRSA) has become a major health threat in the United States. Staphylococcal enterotoxin B (SEB) is a potent superantigen that contributes to its virulence. High mortality and frequent failure of therapy despite available antibiotics have stimulated research efforts to develop adjunctive therapies.

METHODS

Treatment benefits of SEB-specific monoclonal antibody (mAb) 20B1 were investigated in mice in sepsis, superficial skin, and deep-tissue infection models.

RESULTS

Mice challenged with a SEB-producing MRSA strain developed fatal sepsis, extensive tissue skin infection, and abscess-forming deep-seeded thigh muscle infection. Animals preimmunized against SEB or treated passively with mAb 20B1 exhibited enhanced survival in the sepsis model, whereas decrease of bacterial burden was observed in the superficial skin and deep-tissue models. mAb 20B1 bound to SEB in the infected tissue and decreased abscess formation and proinflammatory cytokine levels, lymphocyte proliferation, and neutrophil recruitment.

CONCLUSIONS

mAb 20B1, an SEB-neutralizing mAb, is effective against MRSA infection. mAb 20B1 protects against lethal sepsis and reduces skin tissue invasion and deep-abscess formation. The mAb penetrates well into the abscess and binds to SEB. It affects the outcome of S. aureus infection by modulating the host's proinflammatory immune response.

Generation, characterization, and epitope mapping of neutralizing and protective monoclonal antibodies against staphylococcal enterotoxin B-induced lethal shock

T-cell stimulating activity of Staphylococcal enterotoxin B (SEB) is an important factor in the pathogenesis of certain staphylococcal diseases including SEB mediated shock. SEB is one of the most potent superantigens known and treatment of SEB induced shock remains a challenge. We generated and characterized murine monoclonal antibodies (mAbs) to SEB in mice. We tested mAbs neutralize mitogenic effects of SEB in vitro and in vivo with T-cell proliferation assays and 2 murine models for SEB induced lethal shock (SEBILS). Epitope mapping suggests that all these mAbs recognize conformational epitopes that are destroyed by deleting the C terminus of the protein. Further site-directed mutagenesis identified potential residues involved in binding to SEB that differ between Methicillin resistant and sensitive Staphylococcus aureus strains. Only mAb 20B1 was effective as a monotherapy in treating SEBILS in HLA DR3 transgenic mice, which exhibit enhanced sensitivity to SEB. It is noteworthy that mAbs, 14G8 and 6D3 were not protective when given alone in the HLA DR3 mice but their efficacy of protection could be greatly enhanced when mAbs were co-administered simultaneously. Our data suggest combinations of defined mAbs may constitute a better treatment strategy and provide a new insight for the development of passive immunotherapy.

Highly sensitive microplate chemiluminescence enzyme immunoassay for the determination of staphylococcal enterotoxin B based on a pair of specific monoclonal antibodies and its application to various matrices

A highly specific and sensitive microplate chemiluminescent enzyme immunoassay (CLEIA) was established and validated for the detection of staphylococcal enterotoxin B (SEB). A pair of monoclonal antibodies (mAbs) that recognizes different epitopes of SEB was selected from 20 SEB-specific mAbs, and the experimental conditions were examined and optimized for the development of the CLEIA. This method exhibited high performance with a dynamic range of 0.01-5 ng/mL, and the measured limit of detection (LOD) was 0.01 ng/mL. Intra- and interassay coefficient variations were all lower than 13% at three concentrations (0.2, 0.4, and 2 ng/mL). For specificity studies, when this method was applied to test staphylococcal enterotoxins A, C1, and D, no cross-reactivity was observed. It has been successfully applied to the analysis of SEB in a variety of environmental, biological and humoral matrices such as sewage, tap water, river water, roast beef, peanut butter, cured ham, 10% nonfat dry milk, milk, orange juice, and human urine and serum. The aim of this article is to show that the highly sensitive, specific, and simple microplate CLEIA, based on a pair of highly specific monoclonal antibodies, has potential applications for quantifying SEB in public health and military reconnaissance.

Molecular screening of staphylococcal enterotoxin B gene in clinical isolates

OBJECTIVE

The role of staphylococcal enterotoxin B (SEB) in food poisoning is well known, however its role in other diseases remains to be explored. The aim of this study is the molecular screening and characterization of the SEB gene in clinically isolated strains.

MATERIALS AND METHODS

In this experimentally study, 300 Staphylococcus aureus (S. aureus) strains isolated from clinical samples were assayed. The isolated strains were confirmed by conventional bacteriological methods. Polymerase chain reaction (PCR) was used to determine the enterotoxin B (ent B) gene. Assessment of toxin production in all strains that contained the ent B gene was then performed. Finally, using specific antibody against SEB, a Western-blot was applied to confirm detection of enterotoxin B production.

RESULTS

RESULTS indicated that only 5% of the 300 clinically isolated S. aureus contained the ent B gene. All strains which contained the ent B gene produced a proteinous enterotoxin B. The results of sequence determination of the PCR product were compared with the gene bank database and 98% similarity was achieved. The results of the Western-blot confirmed that enterotoxin B was produced in strains that contained the ent B gene.

CONCLUSION

The results of this study indicate that 5% of clinically isolated S. aureus strains produce enterotoxin B. Considering that the enterotoxin B is an important superantigen, it is possible that a delay in diagnosis and lack of early proper treatment can cause an incidence of late complications, particularly in staphylococcal chronic infections. For this reason, it is suggested that in addition to detecting bacteria, an enterotoxin B detection test should be performed to control its toxigenicity.

Diverse enterotoxin gene profiles among clonal complexes of Staphylococcus aureus isolates from the Bronx, New York

Staphylococcal enterotoxins (SE) can cause toxin-mediated disease, and those that function as superantigens are implicated in the pathogenesis of allergic diseases. The prevalence of 19 enterotoxin genes was determined by PCR in clinical S. aureus strains derived from wounds (108) and blood (99). We performed spa typing and multilocus sequence typing (MLST) to determine clonal origin, and for selected strains staphylococcal enterotoxin B (SEB) production was measured by enzyme-linked immunosorbent assay. Strains carried a median of five SE genes. For most SE genes, the prevalence rates among methicillin-resistant and methicillin-sensitive S. aureus isolates, as well as wound- and blood-derived isolates, did not differ. At least one SE gene was detected in all except two S. aureus isolates (>99%). Complete egc clusters were found in only 11% of S. aureus isolates, whereas the combination of sed, sej, and ser was detected in 24% of clinical strains. S. aureus strains exhibited distinct combinations of SE genes, even if their pulsed-field gel electrophoresis and MLST patterns demonstrated clonality. USA300 strains also showed considerable variability in SE content, although they contained a lower number of SE genes (mean, 3). By contrast, SE content was unchanged in five pairs of serial isolates. SEB production by individual strains varied up to 200-fold, and even up to 15-fold in a pair of serial isolates. In conclusion, our results illustrate the genetic diversity of S. aureus strains with respect to enterotoxin genes and suggest that horizontal transfer of mobile genetic elements encoding virulence genes occurs frequently.