Whole Genome Association Study on Tissue Tropism Phenotypes in Group A Streptococcus

Group A Streptococcus (GAS) has a rich evolutionary history of horizontal transfer among its core genes. Yet, despite extensive genetic mixing, GAS strains have discrete ecological phenotypes. To further our understanding of the molecular basis for ecological phenotypes, comparative genomic hybridization of a set of 97 diverse strains to a GAS pan-genome microarray was undertaken, and the association of accessory genes with emm genotypes that define tissue tropisms for infection was determined. Of the 22 non-prophage, accessory gene regions (AGRs) identified, only three AGRs account for all statistically significant linkage disequilibrium among strains having the genotypic biomarkers for throat versus skin infection specialist. Networked evolution and population structure analysis of loci representing each of the AGRs reveals that most strains with the skin specialist and generalist biomarkers form discrete clusters, whereas strains with the throat specialist biomarker are highly diverse. To identify co-inherited and co-selected accessory genes, the strength of genetic associations was determined for all possible pair wise combinations of accessory genes among the 97 GAS strains. Accessory genes showing very strong associations provide the basis for an evolutionary model, which reveals that a major transition between many throat and skin specialist haplotypes correlates with the gain or loss of genes encoding fibronectin-binding proteins. This study employs a novel synthesis of tools to help delineate the major genetic changes associated with key adaptive shifts in an extensively recombined bacterial species. Diverse collection of 97 GAS strains compared to Alab49 reference genome representing the majority of amplicons on the GAS pan-genome microarray. Flip dye (technical replicates) performed for all comparisons. Ratios were determined by averaging across technical replicates.