Rapid molecular genetic subtyping of serotype M1 group A Streptococcus strains
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Published on: September 5, 2013
N Hoe1, K Nakashima, D Grigsby
1Baylor College of Medicine, Houston, Texas 77030, USA.
Abstract:
Serotype M1 group A Streptococcus, the most common cause of invasive disease in many case series, generally have resisted extensive molecular subtyping by standard techniques (e.g., multilocus enzyme electrophoresis, pulsed-field gel electrophoresis). We used automated sequencing of the sic gene encoding streptococcal inhibitor of complement and of a region of the chromosome with direct repeat sequences to unambiguously differentiate 30 M1 isolates recovered from 28 patients in Texas with invasive disease episodes temporally clustered and thought to represent an outbreak. Sequencing of the emm gene was less useful for M1 strain differentiation, and restriction fragment length polymorphism analysis with IS1548 or IS1562 as Southern hybridization probes did not provide epidemiologically useful subtyping information. Sequence polymorphism in the direct repeat region of the chromosome and IS1548 profiling data support the hypothesis that M1 organisms have two main evolutionary lineages marked by the presence or absence of the speA2 allele encoding streptococcal pyrogenic exotoxin A2.
Insights
Molecular subtyping of invasive Group A Streptococcus M1 isolates was challenging. Automated sequencing of the sic gene and direct repeat region successfully differentiated strains during a Texas outbreak, revealing two main evolutionary lineages.
Area of Science:
- Microbiology
- Molecular Biology
- Epidemiology
Background:
- Group A Streptococcus (GAS) serotype M1 is a common cause of invasive disease.
- Standard molecular subtyping techniques have limited success in differentiating M1 isolates.
- Accurate strain differentiation is crucial for tracking outbreaks and understanding disease transmission.
Purpose of the Study:
- To develop and apply novel molecular methods for unambiguous subtyping of invasive M1 GAS isolates.
- To investigate the genetic diversity and evolutionary lineages within M1 GAS strains.
- To characterize isolates from a temporally clustered invasive disease outbreak in Texas.
Main Methods:
- Automated sequencing of the sic gene (encoding streptococcal inhibitor of complement).
- Sequencing of a chromosomal direct repeat region.
- Comparison with emm gene sequencing and restriction fragment length polymorphism (RFLP) analysis using IS1548/IS1562 probes.
Main Results:
- Automated sequencing of the sic gene and direct repeat region successfully differentiated 30 M1 isolates from 28 patients.
- emm gene sequencing provided limited discriminatory power for M1 strains.
- RFLP analysis with IS1548/IS1562 probes was not epidemiologically useful for subtyping.
- Polymorphism in the direct repeat region and IS1548 profiling suggested two main evolutionary lineages of M1 GAS, distinguished by the presence or absence of the speA2 allele.
Conclusions:
- Automated sequencing of the sic gene and chromosomal direct repeats are effective methods for subtyping invasive M1 GAS.
- These methods were instrumental in characterizing isolates during a Texas invasive disease outbreak.
- Evidence supports the existence of two major evolutionary lineages within M1 GAS, linked to the speA2 allele.
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