SEs belong to the broad family of pyrogenic toxin superantigens (SAgs; [3]). SAgs bypass conventional antigen recognition by interaction with major histocompatibility complex (MHC) class II molecules on the surface of antigen presenting cells, and with T-cell receptors (TCR) on specific T-cell subsets. Interaction typically occurs to the variable region of the TCR β chain (Vβ) but binding to the TCR Vα domain has been reported [21,25,29]. This leads to activation of a large number of T-cells followed by proliferation and massive release of chemokines and proinflammatory cytokines that may led to potentially lethal toxic shock syndrome [3]. However, staphylococcal enterotoxins have been proposed to be named according to their emetic activities [30]. Only SAgs that induce vomiting after oral administration in a primate model will be designated as SEs. Related toxins that lack emetic activity or have not been tested for it should be designated as staphylococcal enterotoxin-like (SEls) SAgs. Also, newly discovered toxins with more than 90% amino acid sequence identity with existing SEs or SEls should be designated as a numbered subtype. However, despite this consensus nomenclature some subtypes are still just called variants.

At the time of this review, the repertoire of S. aureus SEs/SEls comprised 22 members, excluding molecular variants: (i) the classical SEA, SEB, SEC (with the SEC1, SEC2 and SEC3, SEC ovine and SEC bovine variants), SED and SEE, which were discovered in studies of S. aureus strains involved in SFP outbreaks, and classified in distinct serological types [31,32,33,34,35]; and (ii) the new types of SEs (SEG, SEH, SEI, SER, SES, SET) and SEls (SElJ, SElK, SElL, SElM, SElN, SElO, SElP, SElQ, SElU, SElU2, and SElV) [28,36,37,38,39,40,41,42,43,44,45]. TSST-1, the toxic shock staphylococcal toxin, initially designated as SEF, lacks emetic activity [46,47].

SEs and SEls constitute a family of structurally related exoproteins that range in size from ~22 to 28 kDa (Table 1). Based on amino acid sequence comparisons, they have been distributed into four or five groups (Table 2), depending on the inclusion or not of SEH within group 1 [21,29,40,49]. The recently described SET is most related to a putative exotoxin from an S. aureus isolate involved in bovine mastitis, and to streptococcal pyrogenic toxin type K (SpeK) [40]. TSST-1, which is functionally a superantigen with no emetic activity, is more distant to SEs and SEls than to SSLs (staphylococcal superantigen-like proteins) [50]. The SSLs, first identified by screening staphylococcal genomes using two conserved amino acid motifs placed in the N-terminal and C-terminal domains of SAgs, are not mitogenic to T cells and do not bind MHC class II, although they display a wide array of activities targeting key elements of the innate and specific immunity, such as neutrophils, complement factor C5, and IgA [51,52,53,54,55,56].

The three-dimensional structures of TSST-1 [57,58] and several SEs and SEls [59,60,61,62,63,64,65,66,67,68,69] have been solved by crystallography (Table 1). The structures are remarkably conserved, although they interact differently with MHC class II molecules, and show different TCR specificity [70]. They are compact ellipsoidal proteins with two unequal domains separated by a shallow grove. The larger C-terminal domain is a β-grasp fold consisting of four- to five-strand β-sheet that packs against a highly conserved α-helix [71]. The smaller N-terminal domain consists of a mixed β-barrel with Greek-key topology, similar to the OB (oligosaccharide/oligonucleotide binding)-fold [72] also found in many other bacterial toxins (SSLs, streptococcal superantigens, nucleases and toxins of the AB₅ family, including cholera and pertussis toxins, and verotoxin) [29,50]. The two domains are stabilized by close packing and by a section of the N-terminus that extends over the top of the C-terminal domain. The N-terminal extension contributes substantially to the TCR-binding site, located in the cleft between the two protein domains, while the MHC class II binding site is in the OB-fold [29,50]. The top of the N-terminal domain usually contains a highly flexible disulfide loop, which has been implicated with emetic activity (see below).

Important efforts have been made to identify specific amino acids and domains within SEs which may be important for emesis, but results are still limited and controversial. Like TSST-1, SElL, and SElQ are nonemetic, while SEI displays weak emetic activity [38,41,42]. These toxins lack the disulfide loop characteristically found at the top of the N-terminal domain of other SEs. Nonetheless, the loop itself does not appear to be an absolute requirement for emesis, although it may stabilize a crucial conformation important for this activity [73]. Carboxymethylation of histidines on SEA or SEB generates proteins devoid of enterotoxicity, which still retain superantigenicity [75,76]. Analysis of the effects of carboxymethylation of each of the SEA histidines revealed that His61 is important for emesis, but not for T-cell proliferation [77]. Conversely, Leu48Gly and Phe44Ser mutant forms of SEA and SEB, respectively, do not bind MHC class II molecules or cause T-cell activation, but still provoke vomiting [78], hence separating emesis and superantigenicity as different functions of the proteins. Despite this, a high correlation exists between the two activities since, in most cases, genetic mutations resulting in a loss of superantigen activity also results in loss of emetic activity [78].

In contrast to the case of many other bacterial enterotoxins, specific cells and receptors in the digestive system have not been unequivocally linked to oral intoxication by a SE. It has been suggested that SEs stimulate the vagus nerve in the abdominal viscera, which transmits the signal to the vomiting center in the brain [79]. Supporting this idea, receptors on vagal afferent neurons are essential for SEA-triggered emesis [80], and capsaicin, a small molecular weight compound from chilli peppers that depletes peptidergic sensory nerve fibers, also diminishes SE effects in mammals [21]. In addition, SEs are able to penetrate the gut lining and activate local and systemic immune responses [81]. Release of inflammatory mediators (including histamine, leukotrienes, and neuroenteric peptide substance P) causes vomiting [82,83,84,85] and the emetic response can be eliminated by H2- and calcium channel-blockers, which also block the release of histamine [86]. Local immune system activation could also be responsible for the gastrointestinal damage associated with SE ingestion [87,88]. Inflammatory changes are observed in several regions of the gastrointestinal tract, but the most severe lesions appear in the stomach and the upper part of the small intestine [89]. The diarrhea sometimes associated with SEs intoxication may be due to the inhibition of water and electrolyte reabsorption in the small intestine [90,91]. In an attempt to link the two distinct activities of SEs, i.e., superantigenicity and enterotoxicity, it has been postulated that enterotoxin activity could facilitate transcitosis, enabling the toxin to enter the bloodstream and circulate through the body, thus allowing the interaction with antigen presenting- and T-cells that leads to superantigen activity [3,92]. In this way, circulation of SEs following ingestion of SEs as well as their spread from a S. aureus infection site, could have more profound effects upon the host versus if the toxin remains localized [21].

All se and sel genes are located on accessory genetic elements, including plasmids, prophages, S. aureus pathogenicity islands (SaPIs), genomic island vSa, or next to the staphylococcal cassette chromosome (SCC) elements (Table 1). Most of these are mobile genetic elements, and their spread among S. aureus isolates can modify their ability to cause disease and contribute to the evolution of this important pathogen.

Independently of their origin, enterotoxigenic S. aureus often differ in the number of mobile genetic elements and se/sel genes therein, as well as in the enterotoxins they produce. SEA, either alone or together with other SEs/SEls, is the enterotoxin most commonly reported in foods, and is also considered as the main cause of SFP, probably due to its extraordinarily high resistance to proteolytic enzymes [3,119,120]. The predominance of SEA is well documented in different countries. As relevant examples: (i) a comprehensive study of 359 outbreaks that occurred in the United Kingdom (UK) between 1969 and 1990 revealed that 79% of the S. aureus strains produced SEA [121]. Meat, poultry and their products, particularly ham and chicken, were the vehicle in 75% of the incidents. SEA was detected alone in 56.9% of the outbreaks and, in conjunction with SED, SEB, SEC or SEB and SED in a lower number of outbreaks (15.4, 3.4, 2.5 or 1.1%, respectively); (ii) SEA was also the enterotoxin most frequently found among 31 SFP outbreaks in France (69.7%), which were associated with a great variety of foods including milk products, different types of meat, and salads, between 1981 and 2002 [122]. In agreement with this, sea was the most common gene in the isolates tested, followed by sed, seg, sei and she; (iii) In Austria, an SFP outbreak that affected 40 children in 2007 was attributed to S. aureus isolates producing SEA and SED. Bovine milk products were identified as the source of the outbreak, and the cows, not the dairy owner, were the more likely reservoir of the SEs-producing S. aureus [123]; (iv) SEA was also the most common enterotoxin recovered from food poisoning outbreaks in USA (77.8% of all outbreaks) followed by SED and SEB [124]; (v) A study of S. aureus obtained from dairy products, responsible for 16 outbreaks in Brazil revealed that the most frequently encountered enterotoxin gene was sea followed by seb [125]. Finally, (vi) several studies have investigated the distribution of SEs and se/sel genes in S. aureus from foods and SFP outbreaks in Asian countries. Among strains recovered from patients associated with SFP outbreaks during 2001-2003 in Taiwan, sea was the most common gene, followed by seb and sec [13]. In Korea, about 90% of food poisoning isolates were reported to contain the sea gene [126]. SEA also was the most common SE associated to SFP in Japan [127]. In this country, an extensive outbreak that occurred in 2000 was attributed to low-fat milk containing SEA [128], while a recent outbreak (2009) was due to crepes containing SEA and SEC [129].

SEB, SEC or SED alone have been also implicated in SFP outbreaks through the world [121,122,125]. Interestingly, an outbreak, which affected three members of the same family in USA, was caused by coleslaw-containing SEC produced by a community-acquired methicillin resistant S. aureus from an asymptomatic food handler [130]. The fifth classical enterotoxin, SEE, has been infrequently reported in foods and food-producing animals, and its involvement in SFP outbreaks has only been demonstrated in rare occasions. However, six SFP outbreaks, which occurred in France at the end of 2009, were caused by SEE present in soft cheese made from unpasteurized milk. This enterotoxin has also been associated with outbreaks in USA and UK [33,121,131,132,133].

In contrast to classical SEs, the relationship between the novel SEs/SEls and SFP is not fully understood. Among them, SEG, SEH and SEI, SER, SES, and SET have shown to be emetic after oral administration in a primate model, while the emetic activity of SElL and SElP has only been demonstrated in rabbits and the small insectivore Suncus murinus, respectively [39,43]. The remaining SEls either lack emetic properties (SElQ), or have not been tested (SElJ, SElK, SElM, SElN, SElO, SElU, SElU2 and SElV). Moreover, commercial kits are not available for immunological detection of these SEs and SEls, although ELISA (enzyme-linked immunosorbent assay) has been described for SEH [134] and for SEG and SEI [135]. Of the new enterotoxins, only SEH-producing strains have clearly been involved in SFP outbreaks [134,136,137,138], but results from different researchers have shown the high incidence of genes encoding new SEs and SEls among food-borne S. aureus [131,139,140,141]. Mc Lauchlin et al. [131] revealed that 23 staphylococcal strains implicated in SFP outbreaks in UK, in which classical se genes were not detected, harbored one or more of the new se/sel genes, i.e., seg, seh, sei or selj. It is possible that the corresponding SEs might have been the cause of these outbreaks. The presence of egc genes was also shown in food-associated S. aureus from other countries [131,140,141,142,143,144], and newly described SE or SEl genes, particularly those belonging to the egc cluster, were more frequently detected in S. aureus strains isolated from raw pork and chicken meat in Korea than genes encoding classical SEs [145]. Despite this, egc-encoded SEs or SEls have not yet been directly implied in typical cases of SFP, although SEG and SEI have been reported as the cause of chronic diarrhea associated with severe but reversible enteropathy in two malnourished neonates [146].