Acquisition of toxin-encoding lysogenic bacteriophage elements enhances the virulence of pandemic Streptococcus pyogenes M1UK

TL;DR

A new strain of Streptococcus pyogenes called M1UK is causing more severe infections due to genetic changes that boost toxin production and immune system activation.

Contribution

The study identifies how M1UK strains acquire prophage elements and mutations that enhance virulence compared to other S. pyogenes strains.

Findings

M1UK strains produce more of the SpeA superantigen than the M1global strain.

Prophage-encoded elements in M1UK strains increase human T cell activation and infection severity in mice.

M1UK strains with covS mutations or additional prophage elements show higher virulence in experimental models.

Abstract

Multiple countries have observed an alarming increase in scarlet fever cases, and invasive infections often associated with a new sublineage of Streptococcus pyogenes known as M1UK. M1UK 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 M1global strain. We obtained contemporary Canadian M1UK isolates, and genome sequencing revealed that some M1UK strains had acquired additional DNAse- and superantigen-encoding prophage elements, as well as an isolate with a mutation in covS. Five S. pyogenes strains were chosen for functional experiments, including 5448 (M1global strain), M1UK350 (a “typical” M1UK strain), M1UK162 (M1UK strain containing a mutation in the covS gene), M1UK362ΦSP1380.vir (M1UK strain containing a prophage element encoding the spd1, speC, and ssa genes), and M1UK155Φ370.1 (M1UK strain containing a prophage element encoding the spd1 and speC genes). Exoprotein profiles demonstrated that all M1UK background strains had enhanced production of the SpeA superantigen relative to S. pyogenes 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 M1UK162 covS mutant, and both M1UK362ΦSP1380.vir and M1UK155Φ370.1 strains each demonstrated increased severity during experimental skin infection compared to 5448 and M1UK350. These findings indicate that circulating M1UK background strains continue to acquire additional prophage-encoded virulence factors, or hypervirulent covS mutations, and that these genetic alterations may contribute to increase severity of human infections.

Figures