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Influence of recombination and niche separation on the population genetic structure of the pathogen Streptococcus
Awdhesh Kalia1, Brian G Spratt, Mark C Enright
1Department of Ecology and Evolutionary Biology, Yale University, New Haven, Connecticut 06520, USA.
Abstract:
The throat and skin of the human host are the principal reservoirs for the bacterial pathogen Streptococcus pyogenes. The emm locus encodes structurally heterogeneous surface fibrils that play numerous roles in virulence, depending on the strain. Isolates harboring the emm pattern A-C marker exhibit a strong tendency to cause throat infection, whereas emm pattern D strains are usually recovered from impetigo lesions; as a group, emm pattern E organisms fail to display obvious tissue tropisms. The peak incidence for streptococcal pharyngitis and impetigo varies with season and locale, leading to wide spatial and temporal distances between throat and skin strains. To assess any impact of niche separation on genetic variation, the extent of recombinational exchange between emm pattern A-C, D, and E subpopulations was evaluated. Analysis of nucleotide sequence data for internal portions of seven housekeeping loci from 212 isolates provides evidence of extensive recombination between strains belonging to different emm pattern subpopulations. Furthermore, no fixed nucleotide differences were found between emm pattern A-C and D strains. Thus, despite some niche separation created by distinct epidemiological trends and innate tissue tropisms there is little evidence for neutral gene divergence between throat and skin strains. Maintenance of a relationship between emm pattern and tissue tropism in the face of underlying recombination suggests that tissue tropism is associated with emm or a closely linked gene.
Insights
Streptococcus pyogenes strains causing throat or skin infections show extensive genetic exchange. Despite niche differences, genetic divergence is minimal, suggesting emm gene links tissue tropism.
Area of Science:
- Microbiology
- Genetics
- Epidemiology
Background:
- Streptococcus pyogenes is a significant human pathogen found in throat and skin reservoirs.
- The emm gene locus determines surface fibril structure and influences bacterial virulence and tissue tropism.
- Distinct emm patterns (A-C, D, E) are associated with specific infection sites like pharyngitis and impetigo.
Purpose of the Study:
- To investigate the genetic variation and recombination between Streptococcus pyogenes subpopulations from different host niches.
- To determine if niche separation impacts genetic divergence between throat and skin strains.
- To assess the relationship between emm gene patterns and tissue tropism.
Main Methods:
- Analysis of nucleotide sequences from seven housekeeping loci in 212 Streptococcus pyogenes isolates.
- Comparison of genetic data from strains belonging to emm pattern A-C, D, and E subpopulations.
- Evaluation of recombinational exchange and nucleotide differences between subpopulations.
Main Results:
- Evidence of extensive genetic recombination between different emm pattern subpopulations was found.
- No fixed nucleotide differences were identified between emm pattern A-C (throat) and D (skin) strains.
- Despite niche separation, significant genetic divergence between throat and skin strains was not observed.
Conclusions:
- Recombination occurs frequently between Streptococcus pyogenes subpopulations, limiting neutral gene divergence.
- The consistent association between emm pattern and tissue tropism suggests a strong link, potentially mediated by the emm gene itself or a closely linked gene.