Characterization of the canine type c enterotoxin produced by Staphylococcus intermedius pyoderma isolates

The type C staphylococcal enterotoxins (SECs) are a group of highly conserved proteins with substantial antigenic cross-reactivity. Although Staphylococcus intermedius and coagulase-positive species of staphylococci are reported to produce SEC and other SEs, toxins produced by species other than Staphylococcus aureus have not been previously characterized. In this study we report the molecular, biological, and immunological properties of the canine SEC (SEC(canine)) expressed by pathogenic isolates of S. intermedius. The mature form of SEC(canine) has 239 amino acid residues and a pI of 7.0. Typical of the SEs, purified SEC(canine) induces an emetic response in monkeys and the proliferation of T cells in a Vβ-dependent manner. Although SEC(canine) has > 95% sequence identity to previously described SEC variants, its sequence is most related to SEC2 and SEC3. In contrast to the sequence similarity, the Vβ profile induced by SEC(canine) is typical of that induced by SEC1. This result is likely explained by the conservation of a cysteine residue at position 26 in SEC(canine); residues at this position have been previously shown to determine subtype-dependent differences in T-cell receptor interactions of other SEs. Overall, these results show that superantigen toxins produced by the multiple members of the genus Staphylococcus are highly conserved in respect to biological and structural properties. Further, the frequent association of SEC(canine) with pyoderma in dogs supports the notion that the toxins are important for staphylococcal survival and pathogenesis.