Rapid selection of complement-inhibiting protein variants in group A Streptococcus epidemic waves

Serotype M1 group A Streptococcus strains cause epidemic waves of human inf ections long thought to be mono- or pauciclonal. The gene encoding an extra cellular group A Streptococcus protein (streptococcal inhibitor of compleme nt) that inhibits human complement was sequenced in 1,132 M1 strains recove red from population-based surveillance of infections in Canada, Finland and the United States. Epidemic waves are composed of strains expressing a rem arkably heterogeneous array of variants of streptococcal inhibitor of compl ement that arise very rapidly by natural selection on mucosal surfaces. Thu s, our results enhance the understanding of pathogen population dynamics in epidemic waves and infectious disease reemergence.