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Background  The M type-specific surface protein antigens encoded by the 5' end of  emm  genes are targets of protective host immunity and attractive vaccine candidates against infection by  Streptococcus pyogenes , a global human pathogen. A history of genetic change in  emm  was evaluated for a worldwide collection of > 500  S. pyogenes  isolates that were defined for genetic background by multilocus sequence typing of housekeeping genes.    Results  Organisms were categorized by genotypes that roughly correspond to throat specialists, skin specialists, and generalists often recovered from infections at either tissue site. Recovery of distant clones sharing the same  emm  type was ~4-fold higher for skin specialists and generalists, as compared to throat specialists. Importantly,  emm  type was often a poor marker for clone. Recovery of clones that underwent recombinational replacement with a new  emm  type was most evident for the throat and skin specialists. The average ratio of nonsynonymous substitutions per nonsynonymous site (Ka) and synonymous substitutions per synonymous site (Ks) was 4.9, 1.5 and 1.3 for  emm  types of the throat specialist, skin specialist and generalist groups, respectively.    Conclusion  Data indicate that the relationships between  emm  type and genetic background differ among the three host tissue-related groups, and that the selection pressures acting on  emm  appear to be strongest for the throat specialists. Since positive selection is likely due in part to a protective host immune response, the findings may have important implications for vaccine design and vaccination strategies.

Relationships between emm and multilocus sequence types within a global collection of Streptococcus pyogenes