Evolutionary paths of streptococcal and staphylococcal superantigens

Area of Science:

• Microbiology
• Evolutionary Biology
• Genomics

Background:

• Streptococcus pyogenes (GAS) possesses superantigens (SAgs) primarily in its prophage regions.
• The evolutionary origins of SAgs are not fully understood, with horizontal transfer being a suspected mechanism.
• Streptococcus dysgalactiae subsp. equisimilis (SDSE), a close relative of GAS, has limited SAg diversity, with speG present in only 50% of strains and smez absent.

Purpose of the Study:

• To analyze the evolutionary pathways of streptococcal and staphylococcal SAgs.
• To investigate the genomic context and evolutionary history of speG and smez in SDSE.
• To identify and characterize novel SAg genes in Staphylococcus aureus.

Main Methods:

• Comparative sequence analysis of speG regions in SDSE strains.
• Genome-wide synteny comparisons between GAS and SDSE.
• Nucleotide skew analysis of SDSE genomes.
• Detection and analysis of SAg and SAg-like genes in Staphylococcus aureus.
• dN/dS window analysis and Bayesian Markov chain Monte Carlo methods for evolutionary analysis.

Main Results:

• The speG region in SDSE is syntenically conserved, regardless of speG presence.
• speG is identified as a direct descendant of a common streptococcal SAg ancestor, while smez was deleted from SDSE post-divergence from GAS.
• The region flanking smez in SDSE is unstable, similar to GAS.
• Novel staphylococcal SAg genes, selW and ssl, were found in Staphylococcus aureus core genomes.
• speG, selW, and ssl products show evidence of strong positive selection.

Conclusions:

• A comprehensive evolutionary model for streptococcal and staphylococcal SAgs is proposed.
• The study elucidates the distinct evolutionary trajectories of speG and smez within the Streptococcus genus.
• Evidence of positive selection suggests ongoing adaptation and diversification of SAgs in both Streptococcus and Staphylococcus species.