MOLECULAR CHARACTERIZATION AND PHYLOGENETIC DISTRIBUTION OF THE STREPTOCOCCAL SUPERANTIGEN GENE (SSA) FROM STREPTOCOCCAL PYOGENES

A striking increase in the frequency and severity of invasive infectio ns caused by Streptococcus pyogenes has occurred in recent years. Amon g these diseases is streptococcal toxic-shock-like syndrome (TSLS), a condition characterized by fulminant soft-tissue destruction and multi organ failure. Streptococcal superantigen (SSA), a superantigen isolat ed from a TSLS-inducing, serotype M3 S. pyogenes strain, has recently been identified. We here describe the cloning, sequencing, and phyloge netic distribution of the SSA structural gene. The 783-bp open reading frame encodes a predicted 260-amino-acid protein that is similar in s ize to several other bacterial superantigens. The deduced sequence of the mature protein is 60.2% identical to that of staphylococcal entero toxin B but only 49% identical to that of streptococcal pyrogenic exot oxin A. Southern blot and PCR analysis of 138 group A streptococcal st rains representing 65 M protein serotypes and 15 nontypeable isolates identified ssa in 68 strains from 10 distinct clonal lineages. All ssa -positive clones expressed SSA. Of the two clones associated with TSLS , the ET 2-M3 lineage, but not the ET 1-M1 lineage, carried the SSA ge ne. Further analysis of the ET 2-M3 lineage found evidence for tempora l variation in ssa association. Contemporary ET 2-M3 disease isolates had ssa, but two older isolates of this clone recovered in 1910 and 19 20 lacked the gene. The clonal and temporal distribution patterns of s sa suggest a relatively recent acquisition of this superantigen-encodi ng gene by the ET 2-M3 lineage, perhaps by horizontal transfer and rec ombination.