Streptococcus pyogenes is a species of bacteria. Like most other streptococci, it is clinically important in human illness. It is an infrequent, but usually pathogenic, part of the skin flora. It is the predominant species harboring the Lancefield group A antigen, and is often called group A streptococcus (GAS). However, both Streptococcus dysgalactiae and the Streptococcus anginosus group can possess group A antigen. Group A streptococcal infection can cause illness, which typically produces small zones of beta-hemolysis, a complete destruction of red blood cells. (A zone size of 2–3 mm is typical). It is thus also called group A (beta-hemolytic) streptococcus (GABHS). Like other cocci, streptococci are round bacteria. The name is derived from the Greek meaning pus(pyo)-forming(genes) chain(Strepto) of berries (coccus), because streptococcal cells tend to link together in chains of round cells (see image) and a number of infections caused by it produce pus. Streptococci are catalase-negative and Gram-positive. S. pyogenes can be cultured on blood agar plates. Under ideal conditions, it has an incubation period of 1 to 3 days. An estimated 700 million GAS infections occur worldwide each year. While the overall mortality rate for these infections is 0.1%, over 650,000 of the cases are severe and invasive, and have a mortality rate of 25%. Early recognition and treatment are critical; diagnostic failure can result in sepsis and death.

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