Characterization of a novel covS SNP identified in Australian group A Streptococcus isolates derived from the M1UK lineage

Abstract

Group A Streptococcus (GAS) is a human-adapted pathogen responsible for a variety of diseases. The GAS M1UK lineage has contributed significantly to the recently reported increases in scarlet fever and invasive infections. However, the basis for its evolutionary success is not yet fully understood. During the transition to systemic disease, the M1 serotype is known to give rise to spontaneous mutations in the control of virulence two-component regulatory system (CovRS) that confer a fitness advantage during invasive infections. Mutations that inactivate CovS function result in the derepression of key GAS virulence factors such as streptolysin O (SLO), a pore-forming toxin and major trigger of inflammasome/interleukin-1β-dependent inflammation. Conversely, expression of the streptococcal cysteine protease SpeB, which is required during initial stages of colonization and onset of invasive disease, is typically lost in such mutants. In this study, we identified and characterized a novel covS single nucleotide polymorphism detected in three separate invasive M1UK isolates. The resulting CovSAla318Val mutation caused a significant upregulation of SLO resulting in increased inflammasome activation in human THP-1 macrophages, indicating an enhanced inflammatory potential. Surprisingly, SpeB production was unaffected. Site-directed mutagenesis was performed to assess the impact of this mutation on virulence and global gene expression. We found that the CovSAla318Val mutation led to subtle, virulence-specific changes of the CovRS regulon compared to previously characterized covS mutations, highlighting an unappreciated level of complexity in CovRS-dependent gene regulation. Continued longitudinal surveillance is warranted to determine whether this novel covS mutation will expand in the M1UK lineage.