Staphylococcal Enterotoxin Type R Pseudogene Presence in Staphylococcus aureus Reference and Outbreak Strains

Novel staphylococcal enterotoxins (SEs) expressed by Staphylococcus aureus strains have been described throughout the years, among these being the SE protein SER. To further characterize this toxin, this research used 13 S. aureus strains previously determined to contain the SE type R (ser) gene. These S. aureus isolates were evaluated using serological assays for identification of SEA-SEE and PCR for the detection of newly described SE and SE-like enterotoxin genes seg-seu. PCR-based cloning was performed such that the ser gene could be ligated into the pTrc99A plasmid expression vector. Ligation products were used to transform Escherichia coli (DH10Br) strains so that the ser open reading frame (ORF) could be sequenced and expressed for further characterization. Four of the 13 S. aureus strains tested harbored a ser ORF that yielded a PCR-positive result, but contained a frameshift mutation that subsequently introduced a premature stop codon abrogating expression of a full-sized functional protein. In this study, 30% of the PCR-positive ser strains tested were found to carry genes that coded for a nonfunctional SER protein, a finding that clearly illustrates the limited effectiveness of PCR for reliably evaluating enterotoxin potential for ser and, perhaps, other enterotoxin types.