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Group A streptococcus: Virulence factors and pathogenic mechanisms

Dennis L Stevens, MD, PhD
Amy Bryant, PhD
Section Editor
Daniel J Sexton, MD
Deputy Editor
Elinor L Baron, MD, DTMH


Group A Streptococcus (GAS), also known as Streptococcus pyogenes, cause a broad range of infections and complications. The interaction between the host and this important pathogen will be reviewed here. The wide variety of GAS infections are discussed separately. (See related topics.)


Streptococcus pyogenes is a facultative, gram-positive coccus that grows in chains. It causes an array of infections involving the respiratory tract and soft tissues ranging in severity from mild to severe. The only known reservoirs are the skin and mucous membranes of the human host. The pathogenic mechanisms underlying these infections are poorly understood, largely because each is the culmination of highly complex interactions between the human host defense mechanisms and specific virulence factors of the organism.


Group A Streptococcus (GAS) requires complex media containing blood products. It grows best in an environment of 10 percent carbon dioxide and produces pinpoint colonies on blood agar plates; these are surrounded by a zone of complete (beta) hemolysis.

Classification schemes — The Lancefield classification divides streptococci into types A through O based upon acid-extractable carbohydrate antigens of cell wall material [1]. GAS has also been subdivided based upon the surface expression of M and T antigens. Subtyping strains of GAS has proven invaluable for epidemiological studies. High-resolution genotyping provides more specific determination of relatedness among strains isolated in outbreaks of GAS infections [2]. Finally, rapid sequencing of the gene encoding M-protein provides a rapid, definitive comparison of M-typeable and M-nontypeable strains. (See 'M-proteins' below.)


A number of different constituents and products of group A Streptococcus (GAS) have been identified as virulence factors (figure 1).


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Literature review current through: Sep 2016. | This topic last updated: Oct 19, 2015.
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