Beta-haemolytic Streptococcus dysgalactiae subsp. equisimilis (SDSE) is most commonly classified into Lancefield groups C or G and is often considered non-pathogenic or only able to cause mild disease. However, a number of severe, invasive SDSE infections are reported annually in England and Wales. Furthermore, the PHE reference laboratory is receiving increasing numbers of SDSE isolates with the Lancefield group A antigen, with 86 atypical SDSE isolates referred from 2010 to 2017. The aim of this study was to characterise 25 atypical SDSE isolates from 2010-2017 using whole genome sequencing (WGS) and to compare these data with contemporaneous typical GAS and typical SDSE.
Phylogenetic analyses from WGS SNP data demonstrated that atypical SDSEs were more closely related to Lancefield groups C and G SDSE isolates compared to typical GAS isolates (Streptococcus pyogenes). MLST data derived from WGS indicated that 21/25 (84%) of the atypical SDSE isolates belonged to ST128, with single isolates of ST29 & ST134 identified and two isolates with novel STs, one of which is a single locus variant of ST128. eBurst analysis of the 21 ST128 isolates showed a founding population of eBurst group 4 indicating the atypical SDSE were an emerging clone. Interestingly, emm typing divided the isolates into 5 types, 14 (56%) STG495.0; 8 (32%) STG652.0; 2 (8%) STG245.0; and a single isolate of emm type STC46.1 and STG480. Phage sequence of 23/25 isolates were shown to contain the same incomplete phage sequence within their DNA (Streptococcus phage K13) according to PHAge Search Tool Enhanced Release.