Characterization and distribution of a new enterotoxin-related superantigen produced by Staphylococcus aureus

Molecular docking analysis of Enterotoxin I from Staphylococcus aureus with Nafcillin analogues

Staphylococcal enterotoxins (SEs) are liked with food poisoning and other related infections. Nafcillin is an antibiotic used to treat S. aureus. Therefore, it is of interest to study the molecular interactions of 25 nafcillin analogues with enterotoxin I using molecular docking analysis. The analysis shows optimal interaction features of Nafcillin analogues with Enterotoxin I from Staphylococcus aureus for further consideration.

Staphylococcal Enterotoxin C-An Update on SEC Variants, Their Structure and Properties, and Their Role in Foodborne Intoxications

Staphylococcal enterotoxins are the most common cause of foodborne intoxications (staphylococcal food poisoning) and cause a wide range of diseases. With at least six variants staphylococcal enterotoxin C (SEC) stands out as particularly diverse amongst the 25 known staphylococcal enterotoxins. Some variants present unique and even host-specific features. Here, we review the role of SEC in human and animal health with a particular focus on its role as a causative agent for foodborne intoxications. We highlight structural features unique to SEC and its variants, particularly, the emetic and superantigen activity, as well as the roles of SEC in mastitis and in dairy products. Information about the genetic organization as well as regulatory mechanisms including the accessory gene regulator and food-related stressors are provided.

Novel insights into the enterotoxigenic potential and genomic background of Staphylococcus aureus isolated from raw milk

In this study, 53 Staphylococcus (S.) aureus strains were typed by 16S-23S rDNA intergenic spacer region (ISR) typing and staphylococcal enterotoxin gene (SEg) typing for all the staphylococcal enterotoxin (se) and staphylococcal enterotoxin-like toxin (sel) genes known to date, revealing a higher discriminatory power than that of multi locus sequence typing. Six strains, one of each ISR- and SEg-type, were genome sequenced and the ability to produce some classical and new SEs when growing in milk was investigated. The manual analysis of the six genomes allowed us to confirm, correct and expand the results of common available genomic data pipelines such as VirulenceFinder. Moreover, it enabled us to (i) investigate the actual location of se and sel genes, even for genes such as selY, whose location (in the core genome) was so far unknown, (ii) find novel allelic variants of se and sel genes and pseudogenes, (iii) correctly annotate se and sel genes and pseudogenes, and (iv) discover a novel type of enterotoxin gene cluster (egc), i.e. the egc type 5 in strains 356P and 364P, while S. argenteus MSHR1132 harbored the egc type 6. Four of the six S. aureus strains produced sufficient amounts of SEA, SEC, SED and SEH in milk to cause staphylococcal food poisoning (SFP), with S. aureus 372 P being the highest producer of SED in milk found to date, producing as much as ca. 47,300 ng/mL and 49,200 ng/mL of SED, after 24 and 48 h of incubation in milk at 37 °C, respectively. S. aureus 372 P released a low amount of SER in milk, most likely because the seR gene was present as a pseudogene, putatively encoding only 51 amino acids. These findings confirm that not only the classical SEs, but also the new ones can represent a potential hazard for the consumers' health if produced in foods in sufficient amounts. Therefore, the detection of SEs in foods, especially if involved in SFP cases, should focus not only on classical, but also on all the new SEs and SEls known to date. Where reference methods are unavailable, the presence of the relevant genes, by using the conventional and real time PCR protocols we exhaustively provided herein, and their nucleotide sequences, should be investigated.

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.

Detection of the enterotoxigenic genes (sei,sej) in Staphylococcus aureus isolates from bovine mastitis milk in the West Azerbaijan of Iran

Staphylococcus aureus is a major causative pathogen of clinical and subclinical mastitis of dairy domestic ruminants. This organism produces a variety of extracellular toxins and virulence factors such as enterotoxin SEI and SEJ that contribute to its pathogenic potential. In this study 25 S. aureus isolates obtained from four dairy herds of Urmia region which is located in West Azerbaijan province in Iran. The tested isolates were identified on the basis of the cultural and biochemical properties, as well as amplification of the aroA gene which is specific for S. aureus. All isolates were also analyzed for the presence of the SEI (sei) and SEJ (sej) encoding genes using polymerase chain reaction (PCR). Seven positive isolates were detected for sei, but sej gene was not detected in any of the total number of 25 isolates. The present study revealed that the PCR amplification of the aroA gene could be used as a powerful tool for identification of S. aureus from the cases of bovine mastitis. Results of the present study also showed that the strains of S. aureus which cause mastitis can potentially produce enterotoxin SEI. Overall, our results suggest that it is of special importance to follow the presence of enterotoxin-producing S. aureus in other dairy products, especially for protecting the consumers from staphylococcal food poisoning.

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.

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.

Júnior J, Stamford TL, Mota RA, Sena MJD, Almeida AMD, Balbino VDQ, Leal-Balbino TC. Staphylococcal toxin genes in strains isolated from cows with subclinical mastitis

The present study was carried out in 11 dairy herds in four municipal districts of the rural area of the State of Pernambuco, Brazil. Out of 984 quarter milk (246 cows), 10 (1.0%) were positive for clinical mastitis, 562 (57.1%) for subclinical mastitis and 412 (41.9%) were negative. A total of 81 Staphylococcus spp. isolates were obtained from milk samples from the cows diagnosed with subclinical mastitis. From these, 53 (65.0%) were S. aureus, 16 (20.0%) coagulase-positive staphylococci (CPS) and 12 (15.0%) coagulase-negative staphylococci (CNS). The isolates were further investigated for the presence of toxin genes by multiplex and uniplex PCR. The main gene observed was seg followed by seh, sei and sej. The distribution of these observed genes among the isolates obtained from different areas showed a regional pattern for the SEs. The presence of toxin genes in the strains isolated from bovine milk demonstrates a potential problem for public health.

Molecular typing of nasal carriage isolates of Staphylococcus aureus from an Irish university student population based on toxin gene PCR, agr locus types and multiple locus, variable number tandem repeat analysis

Forty-eight Staphylococcus aureus isolates collected from a young, healthy, Irish university student population from 1995 to 2004 were screened for 16 enterotoxin (SE) and enterotoxin-like (SEl) genes (sea-see, seg-sei, selj-selo, selq, selu), and for the toxic shock toxin syndrome toxin-1 gene, tst. All of the isolates harboured at least one SE or SEl gene and 66.7 % possessed a classical SE gene (sea, seb, sec), the commonest being the seb gene. Most of the isolates (85.4 %) had a complete egc locus (selo, selm, sei, seln, seg). The intergenic sei-seln region of the egc locus was typed by PCR-RFLP in 34 isolates, 15 possessing pseudogenes psient1 and psient2 and 19 having the selu gene. The seh and sell genes, the selk-selq gene combination, and the tst gene were each found in <15 % of isolates. The agr genotype distribution was agr type III, 37.5 %; agr type I, 35.4 %; agr type II, 25 %; and agr type IV, 2.1 %. There was no association between SE-SEl genotype and agr type. All tst gene-positive isolates were of agr type III and harboured a classical SE gene. Multiple locus, variable number tandem repeat analysis (MLVA) produced 47 different patterns. While the sdr locus was present in all isolates, half of them lacked one or two of the sdr gene amplimers. Twenty isolates harboured the bbp gene, its presence being associated with agr type III, but not with the SE-SEl gene profile. The agr types of isolates were associated with MLVA subclusters. Selective MLST analysis revealed seven novel sequence types and a new aroE allele. Five clonal clusters (CCs), including CCs comprising major pandemic clones CC30, CC5 and CC22 and minor lineages CC6 and CC9, and three singletons were identified.

Biotyping of enterotoxigenic Staphylococcus aureus by enterotoxin gene cluster (egc) polymorphism and spa typing analyses

Thirty-five Staphylococcus aureus strains, including 10 reference strains and 25 strains recovered from clinical specimens and food samples, were analyzed by PCR REA (restriction endonucleases analysis) of the egc operon and spa typing. Nineteen spa types and seven different egc operons, including four putative new egc variants, were revealed. In 13 strains, allelic variants of sei and/or seg were found. By an analysis of their nucleotide sequence identities, a new homogeneous cluster of a sei variant, called the sei variant, was detected in six strains. In addition, the prototype sei was shown to be more polymorphic than assumed so far. Seven strains possessed the recently described seg variant, also exhibiting several nucleotide exchanges. spa typing was more effective than REA egc grouping as a typing technique. Since, in some cases, the REA typing method was able to discriminate strains showing the same spa type, it must be considered for PCR approaches involved in diagnostic procedures and may be useful for epidemiological studies. Hence, the polyphasic approach used in this study can be reliably and advantageously applied for typing egc-positive S. aureus strains.

Evaluation of molecular methods to determine enterotoxigenic status and molecular genotype of bovine, ovine, human and food isolates of Staphylococcus aureus

This study evaluated the use of PFGE and single enzyme AFLP techniques for the determination of the genetic relationships between Staphyloccocus aureus isolates from human, bovine, ovine and food related sources and reports the prevalence of 'classic' (sea to see) and 'new' (seg, seh, sei, sej, sem, sen and seo) staphylococcal enterotoxin (se) genes in 92 S. aureus strains. A sub-set of the se genotyping results was confirmed by ELISA and the presence of SE toxin determined in isolates from different sources. A 100% correlation was observed, between detection of enterotoxin genes sea-see and expression of corresponding enterotoxin proteins in vitro. The se genotyping data generated from 90 of the S. aureus isolates showed that many of the S. aureus strains producing identical se genotypes correlated with both AFLP and PFGE pattern types. However, single enzyme AFLP technique did not possess the discriminatory power of the PFGE method, but similar clonal relationships were observed by both techniques in many of the isolates tested. Results reported here include the first comprehensive study using a single enzyme AFLP technique to investigate the genetic background of S. aureus isolates from a wide distribution including animal, human and food related sources.

Pattern of enterotoxin genes seg, seh, sei and sej positive Staphylococcus aureus isolated from bovine mastitis

PCR detection of the genes encoding the newly described staphylococcal enterotoxins (SE) SEG, SEH, SEI and SEJ was carried out for 104 randomly selected Staphylococcus aureus field strains isolated from cases of bovine mastitis. Sixty-one (58.7%) isolates were positive for one or more of these novel enterotoxin genes. Thirty-six field strains were classified as carrier of seg, 22 of sei gene and 23 were positive for sej gene. None of the 104 investigated ruminant S. aureus strains carried the seh gene. Thirty-seven of these S. aureus strains showed a combination of genes encoding enterotoxin types SEA to SEE or toxic shock syndrome toxin 1 (TSST 1). Thirteen cultures harboured only one, 28 two, 12 three and 8 four enterotoxin genes. Among the 61 S. aureus field strains 14 (23.0%) were positive for the genes encoding SEJ and SED and 10 (16.4%) isolates for those encoding SEG and SEI. Isolates harbouring the sed/sej genes were further characterized by macrorestriction analysis and pulsed-field-gelelectrophoresis (Pfge). Macrorestriction analysis revealed six patterns. Nine of these14 S. aureus isolates (64.3%) exhibited two patterns with a high degree of relationship (>80%).

Systematic survey on the prevalence of genes coding for staphylococcal enterotoxins SElM, SElO, and SElN

Staphylococcus aureus remains a leading cause of food-poisoning with substantial impact on public health. Using a multiplex polymerase chain reaction-DNA enzyme immunoassay (PCR-DEIA), we studied the presence of genes encoding staphylococcal enterotoxin-like (SEl) superantigens sem, sen, and seo, associated with the enterotoxin gene cluster (egc), in 429 clinical Staphylococcus aureus isolates. 294 (68.5%) isolates tested positive for at least one of the three SEl genes. In contrast to the fixed gene combination seg/sei also located on egc, a substantial number of isolates (n = 108) were found to bear only one or two of the genes encoding SElM, SElN, and SElO. Regarding the origin of the S. aureus isolates, a significant difference (P = 0.022) was found for the possession of seo (61.2% of blood isolates versus 42.9% of nasal strains). Also sem (not significantly) was found more common in blood isolates (52.1% versus 40.5%). The survey of the newly described SEl genes sem-seo supports the concept that most clinical S. aureus isolates harbor subsets of pyrogenic toxin superantigens. The potential contribution of seo and sem to the pathogenic potential of S. aureus has to be further evaluated.

PCR detection of staphylococcal enterotoxin genes in Staphylococcus spp. strains isolated from meat and dairy products. Evidence for new variants of seG and seI in S. aureus AB-8802

AIMS

Evaluation of the occurrence of most known staphylococcal enterotoxin (SE) genes, egc (enterotoxin gene cluster) and TSST1 (toxic shock syndrome toxin 1) gene in both coagulase-positive (CPS) and coagulase-negative (CNS) staphylococcal strains isolated from meat and dairy products.

METHODS AND RESULTS

Specificity and reliability of the PCR detection methods used were ascertained by using nine reference strains of Staphylococcus (S. aureus) harbouring SE genes (seA to seE; seG, seH, seI, seM, seJ, seN and seO) and egc (containing the following sequence of genes: seO, seM, seI, phient1, phient2, seN and seG). Of 109 wild Staphylococcus spp. strains analysed, only 11 S. aureus strains were SE and/or TSST1 PCR-positive. The last 11 strains also appeared to harbour the egc. Restriction endonuclease analysis of part of the egc of both reference and wild strains showed that different variants of the egc exist. Moreover, nucleotide sequences of seG and seI indicate that the egc of the strain AB-8802 is characterized by the presence of variants of these enterotoxins (seGv and seIv).

CONCLUSIONS

The occurrence of SE genes in CNS and other non-S. aureus species isolated from Napoli-type salami, raw water buffalo milk and natural whey cultures used for mozzarella cheese manufacturing is very rare.

SIGNIFICANCE AND IMPACT OF THE STUDY

During this study it was shown that at least five different egc may exist in S. aureus. A thorough study of egc polymorphism should provide further insight into the phylogenetics of the egc.

Superantigens and nasal polyps

Nasal polyps represent an often severe T-cell-orchestrated eosinophilic upper airway disease with currently unknown pathogenesis, often associated with lower airway disease, such as asthma. Superantigens, predominantly derived from Staphylococcus aureus, are potent activators of T cells, induce the synthesis of IgE in B cells, and have direct effects on pro-inflammatory cells, such as eosinophils. IgE antibodies to S. aureus enterotoxins have been described in polyp tissue, linked to a local polyclonal IgE production and an aggravation of eosinophilic inflammation. Furthermore, such IgE antibodies have also been described in the sera of patients with asthma, and linked to severity of disease and steroid insensitivity. This review summarizes our current understanding of the possible role of S. aureus enterotoxins in chronic severe airway disease, such as nasal polyposis.

Identification of a new putative enterotoxin SEU encoded by the egc cluster of Staphylococcus aureus

AIMS

This paper reports on a new putative enterotoxin SEU encoded by the enterotoxin gene cluster egc from Staphylococcus aureus and on a real-time polymerase chain reaction (PCR) assay for detecting the seu gene.

METHODS AND RESULTS

PCR and sequencing revealed a new putative enterotoxin SEU encoded by some egc clusters. The seu gene resulted from sequence divergence in the psient1 and psient2 pseudogenes previously described in the egc cluster (Jarraud et al. [2001] Journal of Immunology166, 669). The presence of the seu gene was investigated in a collection of S. aureus strains by conventional PCR and by a specific 5'-nuclease PCR assay. Among the 24 strains harbouring the egc cluster, four tested positive for the seu gene.

CONCLUSIONS

The existence of the seu gene adds to the number of newly described se genes and underlines the need for a better understanding of their role in the pathogenesis of S. aureus.

SIGNIFICANCE AND IMPACT OF THE STUDY

A thorough study of the seu gene should provide further insight into the phylogenetics of the staphylococcal enterotoxins.

Detection and genotyping by real-time PCR of the staphylococcal enterotoxin genes sea to sej

This paper describes real-time fluorescence PCR assays for detecting and toxinotyping nine enterotoxin genes from Staphylococcus aureus. A universal set of primers allowed sea, seb, sec, sed, see, seg, seh, sei, sej enterotoxin genes from S. aureus to be detected in a single real-time PCR assay with the LightCycler (LC) instrument. Using the universal forward primer and a type-specific reverse primer, real-time PCR assays allowed the S. aureus enterotoxin genes to be specifically genotyped. A collection of S. aureus isolates (n=83) was detected and further characterised for sea, seb, sec, sed, see, seg, seh, sei, sej, using real-time PCR assays, and data were compared with those obtained by conventional block cycler PCR. Isolates were also tested for their ability to produce staphylococcal enterotoxins A, B, C and D by a commercial reversed passive latex agglutination (RPLA) test. Real-time PCR assays developed on the LightCycler system (LC-PCR) are a powerful tool for rapid detection and toxinotyping of the enterotoxin genes sea to sej from S. aureus. The work offers a very quick, reliable and specific alternative to conventional block cycler PCR assays to identify the enterotoxin profile of toxigenic S. aureus.

Prevalence of genes encoding pyrogenic toxin superantigens and exfoliative toxins among strains of Staphylococcus aureus isolated from blood and nasal specimens

A total of 429 different Staphylococcus aureus isolates encompassing 219 blood isolates and 210 isolates taken from anterior nares were systematically searched by two multiplex PCR-DNA enzyme immunoassays (PCR-DEIA) for exfoliative toxin (ET) genes eta and etb, as well as for the classical members of the pyrogenic toxin superantigen (PTSAg) gene family comprising the staphylococcal enterotoxin (SE) genes sea-see and the toxic shock syndrome toxin 1 gene tst. In addition, a third PCR-DEIA was established to investigate the possession of four recently described SE genes, viz. seg-sej. The most frequent PTSAg/ET genes amplified were seg and sei, which were found strictly in combination in 55.0% of the S. aureus isolates tested. Other frequently detected toxin genes were tst (20.3%), sea (15.9%), and sec (11.2%). Only five isolates harbored ET genes. Regarding the origin of the S. aureus isolates, a significant difference (P = 0.037) was found for the possession of the sed/sej gene combination (10.5% of blood isolates versus 3.3% of nasal strains). Overall, about half of S. aureus isolates tested harbored genes of the classical members of the PTSAg family and ETs (50.8%), whereas 73.0% of S. aureus isolates were toxin gene positive if the recently described SE genes were included. This notable higher prevalence indicates that the possession of PTSAg genes in particular seems to be a habitual feature of S. aureus. Moreover, mainly due to the fixed combinations of seg plus sei, as well as sed plus sej, the possession of multiple PTSAg genes (62.9%) is more frequent than assumed so far.

Staphylococcal enterotoxin genes of classical and new types detected by PCR in France

The PCR detection of the genes coding for the newly described staphylococcal enterotoxins (SE) G, H, I and J was carried out for 332 foodborne staphylococci, isolated from a variety of foods in France. The frequency of the Staphylococcus aureus strains harboring these genes was found to be very high (57%) and greater than that of the strains harboring "classical" SE genes as previously established. If one takes into account the newly described SE genes, in addition to the classical SE genes, the percentage of foodborne enterotoxigenic S. aureus strains doubles. The S. aureus biovars that were rarely or never enterotoxigenic (i.e., the poultry and bovine biovars) frequently become more potentially toxigenic, if taking into account the seg. seh, sei and sej genes. Furthermore, this work indicated a systematic association between the seg and sei genes and their high incidence among the S. aureus strains, which suggests that these two SE's could be an important phylogenetic link among the staphylococcal enterotoxins.