Molecular characterisation of Streptococcus pyogenes (StrepA) non-invasive isolates during the 2022-23 UK upsurge

At the end of 2022 into early 2023 the UK Health Security Agency reported unusually high levels of scarlet fever and invasive disease caused by Streptococcus pyogenes (StrepA or group A Streptococcus ). During this time, we collected and genome sequenced 341 non-invasive throat and skin S. pyogenes isolates identified during routine clinical diagnostic testing in Sheffield, a large UK city. We compared the data with that obtained from a similar collection of 165 isolates from 2016-17. Numbers of throat-associated isolates collected peaked in early December 2022, reflecting the national scarlet fever upsurge, while skin infections peaked later in December. The most common emm -types in 2022-23 were emm 1 (28.7%), emm 12 (24.9%), and emm 22 (7.7%) in throat; and emm 1 (22%), emm 12 (10%), emm 76 (18%), and emm 49 (7%) in skin. Whilst all emm 1 isolates were the M1UK lineage, comparison with 2016-17 revealed diverse lineages in other emm -types, including emm 12, and emergent lineages within other types including a new acapsular emm 75 lineage, demonstrating that the upsurge was not completely driven by a single genotype. Analysis of the capsule locus predicted only 51% of throat isolates would produce capsule compared to 78% of skin isolates. 90% of throat isolates were also predicted to have high NADase and Streptolysin O (SLO) expression, based on the promoter sequence, compared to only 56% of skin isolates. Our study has highlighted the value in analysis of non-invasive isolates to characterise tissue tropisms, as well as changing strain diversity and emerging genomic features which may have implications for spillover into invasive disease and future S. pyogenes upsurges. Data summary All new genome sequence data is available on the NCBI short read archive under the bioproject PRJNA1062601 and individual accession numbers are listed in Supplementary Table 1 and Table 2 . Impact statement The human bacterial pathogen Streptococcus pyogenes , also known as group A Streptococcus or StrepA, caused a dramatic and sudden upsurge in scarlet fever in the UK at the end of 2022 into early 2023. We present molecular characterisation of this upsurge, through genome sequence analysis of throat, skin and other types of non-severe infection isolates collected by the microbiology diagnostic lab at the Northern General Hospital in Sheffield, England. We found that, whilst two strain types were the predominant cause of infections during the upsurge, other types had emerged or changed when compared to a similar collection from 2016-17. We also identified differences between throat-associated isolates and skin-associated isolates and highlighted important bacterial factors that might influence infection types. Isolates from non-severe throat/skin types of infections are rarely saved and therefore our knowledge of them is limited. However, here we demonstrate that study of such isolates may be key to understanding upsurges of more severe infections. ### Competing Interest Statement The authors have declared no competing interest.