Staphylococcal superantigens promote bacterial persistence following postoperative surgical site infection

Staphylococcus aureus is a predominant cause of postoperative surgical site infections and persistent bacteremia. Here we describe a patient that following a total knee arthroplasty subsequently experienced three episodes of S. aureus bacteremia over a period of 4 months. The initial blood stream isolate (SAB-0429) was a clonal complex (CC) 5 and methicillin resistant S. aureus (MRSA), whereas two subsequent blood stream isolates (SAB-0485 and SAB-0495) were CC5 isolates but methicillin sensitive S. aureus (MSSA). The two latter isolates harbored a plasmid encoding three superantigen genes not present in the primary MRSA isolate. SAB-0485 and SAB-0495 expressed the plasmid encoded staphylococcal enterotoxin R (SER) exotoxin and demonstrated increased superantigen activity compared with SAB-0429. Compared to SAB-0429, the latter isolates also demonstrated an increased bacterial burden in a mouse bacteremia model that was dependent on increased IFNγ production. Curing of the plasmid from SAB-0485 reduced this virulence phenotype. These findings suggest that the superantigen exotoxins may provide a selective advantage in chronic postsurgical infections.