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Modest Genetic Differences Seen in Streptococcus Strains

But study suggests these are associated with substantially different global gene expression

TUESDAY, Feb. 16 (HealthDay News) -- Group A Streptococcus strains from successive epidemics have relatively modest genetic differences but very different global gene expression, which may provide clues about their biology, according to a study published online Feb. 8 in the Proceedings of the National Academy of Sciences.

Stephen B. Beres, Ph.D., of the Methodist Hospital in Houston, and colleagues analyzed 344 serotype M3 group A Streptococcus strains from three successive epidemics in Ontario, Canada, sequencing the genome of 95 strains and analyzing 280 biallelic single nucleotide polymorphisms (SNPs) in all 344 strains.

The researchers found that, on average, strains differed by only 49 SNPs and 11 insertion-deletion events compared with the core genome. For any single strain, 10 percent of SNPs were strain specific, indicating that each strain had a unique genome sequence. Twenty-two genes, which included known virulence factors, appeared to be under selection based on a high frequency of mutations leading to amino acid changes. Analysis of global genome expression in four genetically representative strains showed considerable differences in global transcript patterns, despite only modest genetic differences.

"Until now, it has been a mystery why sometimes we see two opposing types of infection in patients who appear to have the same strain of flesh-eating bacteria," a co-author said in a statement. "In some cases, patients suffer from a devastating infection of tissue and muscle requiring extensive surgery. And, other patients present with a skin infection readily treated with antibiotics. Now, we understand in part why this happens."

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