Acquisition of toxin-encoding lysogenic bacteriophage elements enhances the virulence of pandemic <i>Streptococcus pyogenes</i> M1 _UK

Multiple countries have observed an alarming increase in scarlet fever cases, and invasive infections often associated with a new sublineage of <i>Streptococcus pyogenes</i> known as M1_UK. M1_UK strains express increased levels of the streptococcal pyrogenic exotoxin A (SpeA) superantigen, and here we compare the virulence characteristics of this sublineage with the circulating M1_global strain. We obtained contemporary Canadian M1_UK isolates, and genome sequencing revealed that some M1_UK strains had acquired additional DNAse- and superantigen-encoding prophage elements, as well as an isolate with a mutation in <i>covS</i>. Five <i>S</i>. <i>pyogenes</i> strains were chosen for functional experiments, including 5448 (M1_global strain), M1_UK350 (a "typical" M1_UK strain), M1_UK162 (M1_UK strain containing a mutation in the <i>covS</i> gene), M1_UK362_ΦSP1380.vir (M1_UK strain containing a prophage element encoding the <i>spd1</i>, <i>speC</i>, and <i>ssa</i> genes), and M1_UK155_Φ370.1 (M1_UK strain containing a prophage element encoding the <i>spd1</i> and <i>speC</i> genes). Exoprotein profiles demonstrated that all M1_UK background strains had enhanced production of the SpeA superantigen relative to <i>S. pyogenes</i> 5448. Furthermore, strains that had acquired the additional prophage elements showed enhanced activation for human T cells, although cytotoxic activity, adhesion capacity, and DNA degradation were not detectably different. Using a "humanized" superantigen-sensitive HLA-transgenic mouse infection model, the M1_UK162 <i>covS</i> mutant, and both M1_UK362_ΦSP1380.vir and M1_UK155_Φ370.1 strains each demonstrated increased severity during experimental skin infection compared to 5448 and M1_UK350. These findings indicate that circulating M1_UK background strains continue to acquire additional prophage-encoded virulence factors, or hypervirulent <i>covS</i> mutations, and that these genetic alterations may contribute to increase severity of human infections.