Comparative prevalence of superantigenic toxin genes in meticillin-resistant and meticillin-susceptible Staphylococcus aureus isolates

A total of 118 meticillin-resistant Staphylococcus aureus (MRSA) and 140 meticillin-susceptible S. aureus (MSSA) isolates from different patients in the same time period were comprehensively searched using a multiplex PCR for the classical and recently described superantigenic toxin gene family comprising the staphylococcal enterotoxin genes sea to ser and the toxic shock syndrome toxin 1 gene, tst-1. Both MRSA and MSSA isolates carried a number of superantigenic toxin genes, but the MRSA isolates harboured more superantigenic toxin genes than the MSSA isolates. The most frequent genotype of the MRSA isolates was sec, sell and tst-1 together with the gene combination seg, sei, selm, seln and selo, which was found strictly in combination in 69.5 % of the isolates tested. In contrast, possession of the sec, sell and tst-1 genes in MSSA isolates was significantly less than in MRSA (2.1 vs 77.1 %, respectively), although they also often contained the combination genes (25.0 %). This notable higher prevalence in MRSA isolates indicated that possession of the sec, sell and tst-1 genes in particular appeared to be a habitual feature of MRSA. Moreover, these were mainly due to the fixed combinations of the mobile genetic elements type I vSa4 encoding sec, sell and tst-1, and type I vSa beta encoding seg, sei, selm, seln and selo. Analysis of the relationship between toxin genotypes and the toxin gene-encoding profiles of mobile genetic elements has a possible role in determining superantigenic toxin genotypes in S. aureus.