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Genetic diversity and relationships among Streptococcus pyogenes strains expressing serotype M1 protein: recent
1Department of Pathology, Baylor College of Medicine, Houston, Texas 77030.
Abstract:
Chromosomal diversity and relationships among 126 Streptococcus pyogenes strains expressing M1 protein from 13 countries on five continents were analyzed by multilocus enzyme electrophoresis and restriction fragment profiling by pulsed-field gel electrophoresis. All isolates were studied for the presence of the gene encoding streptococcal pyrogenic exotoxin A by PCR. Strain subsets were also examined by automated DNA sequencing for allelic polymorphism in genes encoding M protein (emm), streptococcal pyrogenic exotoxin A (speA), streptokinase (ska), pyrogenic exotoxin B (interleukin-1 beta convertase) (speB), and C5a peptidase (scp). Seven distinct emm1 alleles that encode M proteins differing at one or more amino acids in the N-terminal variable region were identified. Although substantial levels of genetic diversity exist among M1-expressing organisms, most invasive disease episodes are caused by two subclones marked by distinctive multilocus enzyme electrophoretic profiles and pulsed-field gel electrophoresis restriction fragment length polymorphism (RFLP) types. One of these subclones (ET 1/RFLP pattern 1a) has the speA gene and was recovered worldwide. Identity of speA, emm1, speB, and ska alleles in virtually all isolates of ET 1/RFLP type 1a means that these organisms share a common ancestor and that global dispersion of this M1-expressing subclone has occurred very recently. The occurrence of the same emm and ska alleles in strains that are well differentiated in overall chromosomal character demonstrates that horizontal transfer and recombination play a fundamental role in diversifying natural populations of S. pyogenes.
Insights
Most invasive Streptococcus pyogenes infections are caused by two specific clones. One clone, carrying the speA gene, has spread globally, indicating recent emergence and horizontal gene transfer in bacterial evolution.
Area of Science:
- Microbiology
- Genetics
- Epidemiology
Background:
- Streptococcus pyogenes (M1 serotype) is a significant human pathogen.
- Understanding the genetic diversity and population structure of S. pyogenes is crucial for controlling invasive disease.
Purpose of the Study:
- To analyze the chromosomal diversity and relationships among 126 M1 Streptococcus pyogenes strains.
- To identify genetic markers associated with invasive disease and global dissemination.
Main Methods:
- Multilocus enzyme electrophoresis (MEE) and pulsed-field gel electrophoresis (PFGE) for chromosomal analysis.
- Polymerase chain reaction (PCR) to detect the speA gene.
- Automated DNA sequencing of emm, speA, ska, speB, and scp genes.
Main Results:
- Seven distinct emm1 alleles were identified, encoding M proteins with variations in the N-terminal region.
- Two major subclones of M1 S. pyogenes were identified, responsible for most invasive episodes.
- One subclone (ET 1/RFLP pattern 1a), possessing the speA gene, showed worldwide distribution.
- Identical alleles (speA, emm1, speB, ska) in the globally dispersed subclone suggest a recent common ancestor.
Conclusions:
- Despite overall genetic diversity, a few S. pyogenes subclones dominate invasive disease.
- The global spread of a specific M1 S. pyogenes subclone is a recent event.
- Horizontal gene transfer and recombination are key drivers of genetic diversity and adaptation in S. pyogenes populations.