Characterization of novel type C staphylococcal enterotoxins: biological and evolutionary implications

The type C staphylococcal enterotoxins (SEC) are a group of highly conserved proteins with significant immunological cross-reactivity. Although three antigenically distinct SEC subtypes (SEC1, SEC2, and SEC3) have been reported in the literature, we observed that the isoelectric points of SEC from several Staphylococcus aureus isolates are different from those of any of these three subtypes. This observation led us to propose that additional SEC molecular variants exist. For assessment of this possibility, the sec genes from representative human, animal, and food isolates were cloned and sequenced. The toxins encoded by the 18 isolates used in this study included five unique SEC proteins in addition to SEC1, SEC2, and SEC3. Six of the SEC proteins (including SEC1, SEC2, and SEC3) were produced by human and food isolates. Analysis of seven bovine and ovine isolates showed that isolates from each animal species produced a unique host-specific SEC. All of the SEC caused lymphocyte proliferation, although some of the toxins differed in their ability to stimulate cells from several animal species. An explanation for these results, which is supported by our phenotypic analysis of Sec+ staphylococcal isolates, is that toxin heterogeneity is due to selection for modified SEC sequences that facilitate the survival of S. aureus isolates in their respective hosts.