Molecular Basis of Serotyping and the Underlying Genetic Organization of Streptococcus pyogenes

Excerpt

Historically, three serological typing schemes were used to classify Streptococcus pyogenes isolates. All are based on LPXTG-linked (or similar) surface proteins that exhibit high levels of antigenic heterogeneity. The serological targets include M proteins, T-antigens (which form pili), and serum opacity factor (SOF). More recently, the genetic basis for serological typing has been elucidated. Three emm pattern groupings, defined by 3' ends of emm and emm-like genes (encoding distinct peptidoglycan-spanning domains), display a strong correspondence with streptococci that tend to cause infection at the epithelium of the throat versus at the skin. Significant correlations extend to emm cluster groups (defined by surface-exposed, functional portions of M protein genes), and to T-antigen (FCT-region) and SOF genes as well. A deeper understanding of the genetic organization and population biology of this species was revealed through analysis of the genes that encode the serological targets. Horizontal transfer of serotype-related genes and the emergence of new strains/clones may be a result of selective pressures conferred by the host immune response. Furthermore, against a background of extensive lateral gene flow, the strong linkage observed among serotype-related genes may signify that M-proteins, T-antigens and/or SOF (or products of other linked genes) are direct determinants of host tissue site preferences of infection.