Molecular population genetic evidence of horizontal spread of two alleles of the pyrogenic exotoxin C gene (speC) among pathogenic clones of Streptococcus pyogenes

It has recently been demonstrated that the bacteriophage-borne gene (speC) encoding pyrogenic exotoxin C is harbored by phylogenetic lineages representing virtually the entire breadth of genomic differentiation present in the species Streptococcus pyogenes (J. M. Musser, A. R. Hauser, M. H. Kim, P. M. Schlievert, K. Nelson, and R. K. Selander, Proc. Natl. Acad. Sci. USA 88:2668-2672, 1991). To determine whether the speC genes occurring in association with divergent chromosomal genotypes (clones) are identical or represent a group of allelic variants, we sequenced speC from 23 S. pyogenes strains representing 15 clones identified by multilocus enzyme electrophoresis. Two alleles of speC are present in natural populations, and each allele occurs in clones that are well differentiated in overall chromosomal character; in one case, isolates of a single clone had different speC alleles. We interpret these patterns of toxin allele-clone distribution as evidence of occasional episodes of speC horizontal dissemination, presumably by bacteriophage-mediated gene transfer and recombination.