Characteristics of enterotoxin H-producing Staphylococcus aureus isolated from clinical cases and properties of the enterotoxin productivity

Highly Sensitive and Specific Detection of Staphylococcal Enterotoxins SEA, SEG, SEH, and SEI by Immunoassay

Staphylococcal food poisoning (SFP) is one of the most common foodborne diseases worldwide, resulting from the ingestion of staphylococcal enterotoxins (SEs), primarily SE type A (SEA), which is produced in food by enterotoxigenic strains of staphylococci, mainly S. aureus. Since newly identified SEs have been shown to have emetic properties and the genes encoding them have been found in food involved in poisoning outbreaks, it is necessary to have reliable tools to prove the presence of the toxins themselves, to clarify the role played by these non-classical SEs, and to precisely document SFP outbreaks. We have produced and characterized monoclonal antibodies directed specifically against SE type G, H or I (SEG, SEH or SEI respectively) or SEA. With these antibodies, we have developed, for each of these four targets, highly sensitive, specific, and reliable 3-h sandwich enzyme immunoassays that we evaluated for their suitability for SE detection in different matrices (bacterial cultures of S. aureus, contaminated food, human samples) for different purposes (strain characterization, food safety, biological threat detection, diagnosis). We also initiated and described for the first time the development of monoplex and quintuplex (SEA, SE type B (SEB), SEG, SEH, and SEI) lateral flow immunoassays for these new staphylococcal enterotoxins. The detection limits in buffer were under 10 pg/mL (0.4 pM) by enzyme immunoassays and at least 300 pg/mL (11 pM) by immunochromatography for all target toxins with no cross-reactivity observed. Spiking studies and/or bacterial supernatant analysis demonstrated the applicability of the developed methods, which could become reliable detection tools for the routine investigation of SEG, SEH, and SEI.

[Behavior of Staphylococcus aureus and Staphylococcal Enterotoxins A and Q in Scrambled Eggs]

Staphylococcal food poisoning (SFP) is caused by Staphylococcus aureus, and its typical symptom of vomiting is evoked by staphylococcal enterotoxins (SEs). SEs are classified as classical and new types. SEQ is a new-type enterotoxin predicted to have a high potential risk for SFP. To elucidate the correlation between the number of S. aureus cells and the production of SEs as well as classical and new-type enterotoxins in the food environment, the numbers of S. aureus strain cells carrying sea and seq genes and the production of SEA and SEQ protein were examined under 3 pHs values (pH 6.0, 7.0 and 8.0) and 2 NaCl concentrations (0.5 and 1.0%) conditions. The experiments were performed at 25℃, resembling the setting of scrambled eggs at room temperature after cooking. By 24 hr after incubation, the cell number in the scrambled egg was ≥10⁷/10 g, reaching 10⁹/10 g by 48 hr under all conditions. The productions of both SEA and SEQ were detected in the scrambled egg under all conditions by 48 h. SEQ was detected from 24 hr at all 3 pH values in the egg containing 1.0% NaCl, whereas in the egg containing 0.5% NaCl, it was detected from 24 hr at pH 6.0 and from 48 hr at other pHs. The SEQ production was consistently 100-1,000 times less than that of SEA. These results suggest that the new-type enterotoxin SEQ has the potential to evoke symptoms related to SFP following the consumption of egg products cooked under relative lower pH and water activity.

Molecular modelling and docking analysis of pleurocidin (an antimicrobial peptide) like peptides with enterotoxin H from Klebsilla pneumonia

Enterotoxin H is a key molecular target for replication and establishment of Klebsilla pneumonia in the host. Therefore, it is of interest to study the interaction of enterotoxin H with pleurocidin like peptides using molecular modelling (template PDB ID: 1YCE), Lig-Plot (ligand construction) and docking tools for therapeutic consideration. The hydrophobic pocket and the active site residues (Val 13, Met 16, Gly 25, Ala 25, and Ile 28) were identified using Cast P, Molegro and Sitehound tools. Docking results show that the pleurocidin like peptides interacts with the active sites of enterotoxin H with 300.96 docking score with optimal binding features.

Immunochemical assay with monoclonal antibodies for detection of staphylococcal enterotoxin H

Staphylococcal enterotoxins cause food poisoning of various degrees of severity. For milk and meat products, there is a high probability of contamination with staphylococcal enterotoxin H (SEH). In this regard specific and sensitive methods are required to be developed for its detection and monitoring. In this work, the gene seh was expressed and a preparation of recombinant toxin was obtained. Using hybridoma technology, a panel of high-affinity monoclonal antibodies (mAbs) to SEH was produced. The antibodies were characterized and shown to have no cross-reactivity towards the main staphylococcal enterotoxins (A, B, C1, D, E, G and I). Based on these mAbs, a method for specific and quantitative detection of SEH was developed in the format of sandwich enzyme immunoassay (linear range, 0.2–3 ng/ml). All the mAbs produced revealed SEH by immunoblotting. Immunochemical analysis of the culture fluids of staphylococcal isolates obtained from the milk of mastitis-infected cows by immunoblotting and sandwich enzyme immunoassay demonstrated the conformity of these methods. Using the developed method, the toxin was revealed in blood serum and liquid food products practically to 100%. From non-liquid foods, it was shown to be extracted to a maximum with a buffer of pH 4.0–4.5.

Basis of Virulence in Enterotoxin-Mediated Staphylococcal Food Poisoning

The Staphylococcus aureus enterotoxins are a superfamily of secreted virulence factors that share structural and functional similarities and possess potent superantigenic activity causing disruptions in adaptive immunity. The enterotoxins can be separated into two groups; the classical (SEA-SEE) and the newer (SEG-SElY and counting) enterotoxin groups. Many members from both these groups contribute to the pathogenesis of several serious human diseases, including toxic shock syndrome, pneumonia, and sepsis-related infections. Additionally, many members demonstrate emetic activity and are frequently responsible for food poisoning outbreaks. Due to their robust tolerance to denaturing, the enterotoxins retain activity in food contaminated previously with S. aureus. The genes encoding the enterotoxins are found mostly on a variety of different mobile genetic elements. Therefore, the presence of enterotoxins can vary widely among different S. aureus isolates. Additionally, the enterotoxins are regulated by multiple, and often overlapping, regulatory pathways, which are influenced by environmental factors. In this review, we also will focus on the newer enterotoxins (SEG-SElY), which matter for the role of S. aureus as an enteropathogen, and summarize our current knowledge on their prevalence in recent food poisoning outbreaks. Finally, we will review the current literature regarding the key elements that govern the complex regulation of enterotoxins, the molecular mechanisms underlying their enterotoxigenic, superantigenic, and immunomodulatory functions, and discuss how these activities may collectively contribute to the overall manifestation of staphylococcal food poisoning.

Food-Borne Outbreak Investigation and Molecular Typing: High Diversity of Staphylococcus aureus Strains and Importance of Toxin Detection

Staphylococcus aureus is an important aetiological agent of food intoxications in the European Union as it can cause gastro-enteritis through the production of various staphylococcal enterotoxins (SEs) in foods. Reported enterotoxin dose levels causing food-borne illness are scarce and varying. Three food poisoning outbreaks due to enterotoxin-producing S. aureus strains which occurred in 2013 in Belgium are described. The outbreaks occurred in an elderly home, at a barbecue event and in a kindergarten and involved 28, 18, and six cases, respectively. Various food leftovers contained coagulase positive staphylococci (CPS). Low levels of staphylococcal enterotoxins ranging between 0.015 ng/g and 0.019 ng/g for enterotoxin A (SEA), and corresponding to 0.132 ng/g for SEC were quantified in the food leftovers for two of the reported outbreaks. Molecular typing of human and food isolates using pulsed-field gel electrophoresis (PFGE) and enterotoxin gene typing, confirmed the link between patients and the suspected foodstuffs. This also demonstrated the high diversity of CPS isolates both in the cases and in healthy persons carrying enterotoxin genes encoding emetic SEs for which no detection methods currently exist. For one outbreak, the investigation pointed out to the food handler who transmitted the outbreak strain to the food. Tools to improve staphylococcal food poisoning (SFP) investigations are presented.

Detection of newly defined superantigenic toxin genes and coagulase gene polymorphism in Staphylococcus aureus isolates

The current study aims to use coagulase (coa) polymorphism gene to identify Staphylococcus aureus isolated from stool samples, evaluate the efficiency of these methods in discriminating variable strains, and compare these subtypes with antibiotypes. A total of 100 specimens were collected from patients in Babylon province, Iraq, between July 2016 and September 2016. Twenty S. aureus strains were isolated and identified using standard laboratory microbiological tests. The bacterial isolates were then examined by coa gene restriction fragment length polymorphism genotyping. Out of 20 isolates, coa gene types were classified, and the amplification products showed multiple size bands (500, 600, 700, 800, and 900-bp bands). Coa gene PCR restriction fragment length polymorphisms exhibited seven patterns that ranged from one to four fragments with AluI digestion. The results have demonstrated that many variants of the coa gene are present. At least one type of S. aureus newly described enterotoxin gene (staphylococcal enterotoxins) was harboring in all 20 (100%) of the isolates. The most frequently encountered gene were sei (100.%), seh (5%), seg (65%). Many S. aureus isolates carry at least one of the enterotoxin genes, and (95%) strains harbored more than one toxin gene coding.

High Prevalence and Properties of Enterotoxin-Producing Staphylococcus aureus ST5 Strains of Food Sources in China

Staphylococcus aureus with the ability of staphylococcal enterotoxins (SEs) production is one of the most common causes of bacterial foodborne outbreaks worldwide. In our study, 336 S. aureus isolates were recovered from 3476 food samples during 2010-2014. A total of 86 S. aureus isolates were proved to be enterotoxin-producing strains with PCR and enzyme-linked immunosorbent assay. In the 86 isolates, 20 STs were identified using multilocus sequence typing (MLST) and 20 isolates were typed as sequence type 5 (ST5), which was the most prevalent ST using MLST. There were six SE profiles and high carrier rates of sec (50%) and sed (75%) genes in the 20 S. aureus ST5 isolates. Additionally, 8 antibiotic resistance patterns were observed, and 10 multidrug-resistant isolates (50%) and 4 methicillin-resistant S. aureus isolates were identified. Our findings illustrate high prevalence of S. aureus ST5 isolates from food sources and diversity in SE profiles and antibiotic resistance patterns. These results indicate that great difference in the ability of obtaining SE production and antimicrobial resistance may exist between different genetic lineages of S. aureus strains.

Genotypic and phenotypic analysis of clinical isolates of Staphylococcus aureus revealed production patterns and hemolytic potentials unlinked to gene profiles and source

BACKGROUND

Nosocomial infections caused by the bacterial pathogen Staphylococcus aureus can lead to serious complications due to the varying presence of secreted toxins. Comparative studies of genomic information and production rates are needed to assess the pathogenic potential of isolated strains. Genotypic and phenotypic profiling of clinical and colonising isolates of S. aureus was used to characterise the release of exotoxins. Blood isolates were compared with colonisation strains to determine similarities and differences of single strains and clusters.

RESULTS

Fifty-one fresh isolates obtained from colonised individuals (n = 29) and S. aureus bacteremia (SAB) patients (n = 22) were investigated. The prevalence of genes encoding for three cytolysins (alpha/beta/gamma toxin) and twenty-four superantigens (SEA-SElX) was determined. Isolates exhibited eighteen distinct combinations of superantigens. Sequence analysis identified mutated open reading frames in hla in 13.7% of all strains, in selw (92.2%) and in selx (15.7%). All corrupted genes were associated with specific clonal complexes. Functional assessment of alpha toxin activity by a rabbit erythrocyte lysis assay revealed that supernatants lacking alpha toxin still displayed hemolysis. This was due to the presence of gamma toxin, as proven by inhibition experiments using antisera raised against the respective recombinant proteins. Alpha toxin, SEC, and TSST1 production was quantified by enzyme-linked immunosorbent assays on supernatants of all hla, sec, and tst positive isolates. Blood isolates and colonising strains showed comparable amounts of secreted proteins within a wide range. Agr types I to IV were identified, but did not allow a prediction of high or low production rates. In contrast, alpha toxin production rates between distinct clonal complexes clearly differed. Spa typing was performed and revealed thirty-two unique spa gene patterns and eight small clusters comprising nineteen isolates. Recognised spa-typing clusters displayed highly similar production rates.

CONCLUSION

Production rates of the three most prevalent exotoxins varied within both groups of blood isolates and colonising strains. By comparing genotypes and secretion, we found that identical complex gene patterns did not allow predictions of toxin production and function. However, identification of spa typing clusters was suitable to predict similar quantities of released exotoxins.

Prevalence of enterotoxins and toxin gene profiles of Staphylococcus aureus isolates recovered from a bakery involved in a second staphylococcal food poisoning occurrence

AIMS

The study objective was to characterize and analyse the distribution of enterotoxins and genes encoding enterotoxins in Staphylococcus aureus strains recovered from the 601 environment and ingredient samples obtained during multiple inspections of a bakery implicated in two separate staphylococcal food poisoning incidents.

METHODS AND RESULTS

Staphylococcus aureus isolates were evaluated using serological assays for identification of classical staphylococcal enterotoxins (SEs) SEA-SEE and polymerase chain reaction for the detection of newly described SE and SE-like enterotoxin genes seg-seu. Pulsed field gel electrophoresis identified thirteen pattern types. During these investigations, a total of 585 environmental swabs and 16 raw ingredient samples were collected by investigators, 85 of which were confirmed to contain Staph. aureus; of those isolates, 95·3% (81/85) harboured enterotoxin genes and 4·7% (4/85) carried newly described SE and SE-like enterotoxin genes in the absence of classical enterotoxins.

CONCLUSIONS

Our research demonstrates the prevalence and diversity of classical SEs and the probable underestimated impact of nonclassical SE and SE-like enterotoxins role in domestic staphylococcal food poisoning outbreaks.

SIGNIFICANCE AND IMPACT OF THE STUDY

Given the abundance of SEs and SE-like toxins, these findings illustrate the utilization of PCR for enterotoxin gene identification and its significance in outbreak investigations.

Temporal expression of staphylococcal enterotoxin h in comparison with accessory gene regulator-dependent and -independent enterotoxins

Using sandwich enzyme-linked immunosorbent assay (ELISA), the production of staphylococcal enterotoxin (SE) H was determined in 22 Staphylococcus aureus isolates bearing the seh gene. Samples of supernatants were taken at four time points corresponding to exponential phase (optical density at 600 nm [OD(600)] 0.3 to 0.6), late exponential phase (OD(600) 2 to 4), early stationary phase (OD(600) 4 to 6), and late stationary phase (OD(600) 7 to 12). In four isolates, SEH was detectable at a very low level at the first time point. In 18 isolates, the earliest SEH production was detected in the late exponential phase. For all isolates, there was an increase of SEH concentration with time. Western blot analysis revealed that SEH production, similar to SEA, started in the early exponential phase (OD(600) ∼ 0.5). Isolates with high SEH productivity, as measured by ELISA, demonstrated a higher seh transcription as well. sec transcription was induced in the stationary phase. An induction in the sea transcript was observed during mid- to late exponential phase. Expression profile of seh was similar to that of sea. We showed that the seh expression profile is similar to that of Agr-independent sea and not to that of Agr-dependent sec genes. SEH can be effectively expressed at low bacterial counts, meaning that even in an environment not favorable for S. aureus growth, seh-bearing strains can pose a risk for food safety.

The formation of Staphylococcus aureus enterotoxin in food environments and advances in risk assessment

The recent finding that the formation of staphylococcal enterotoxins in food is very different from that in cultures of pure Staphylococcus aureus sheds new light on, and brings into question, traditional microbial risk assessment methods based on planktonic liquid cultures. In fact, most bacteria in food appear to be associated with surfaces or tissues in various ways, and interaction with other bacteria through molecular signaling is prevalent. Nowadays it is well established that there are significant differences in the behavior of bacteria in the planktonic state and immobilized bacteria found in multicellular communities. Thus, in order to improve the production of high-quality, microbiologically safe food for human consumption, in situ data on enterotoxin formation in food environments are required to complement existing knowledge on the growth and survivability of S. aureus. This review focuses on enterotoxigenic S. aureus and describes recent findings related to enterotoxin formation in food environments, and ways in which risk assessment can take into account virulence behavior. An improved understanding of how environmental factors affect the expression of enterotoxins in foods will enable us to formulate new strategies for improved food safety.