Characterization of a novel SNP identified in Australian group A isolates derived from the M1 lineage.

Group A (GAS) is a human-adapted pathogen responsible for a variety of diseases. The GAS M1 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 de-repression 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 single nucleotide polymorphism detected in three separate invasive M1 isolates. The resulting CovS 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 CovS mutation led to subtle, virulence-specific changes of the CovRS regulon compared to previously characterized mutations, highlighting an unappreciated level of complexity in CovRS-dependent gene regulation. Continued longitudinal surveillance is warranted to determine whether this novel mutation will expand in the M1 lineage.IMPORTANCEThe M1 lineage of GAS has contributed to a recent global upsurge in scarlet fever and invasive infections. Understanding how GAS can become more virulent is critical for infection control and identifying new treatment approaches. The two-component CovRS system, comprising the sensor kinase CovS and transcription factor CovR, is a central regulator of GAS virulence genes. In the M1 serotype, mutations are associated with an invasive phenotype. Such mutations have not been fully characterized in the M1 lineage. This study identified a novel mutation in invasive Australian M1 isolates that resulted in a more nuanced virulence gene regulation compared to previously characterized mutations. A representative isolate displayed upregulated SLO production and triggered amplified interleukin-1β secretion in infected human macrophages, indicating an enhanced inflammatory potential. These findings underscore the need for comprehensive analyses of mutants to fully elucidate their contribution to M1 virulence and persistence.