The group A streptococci (GAS, Streptococcus pyogenes) are gram-positive, nonmotile, nonsporeforming coccus that occur in chains or in pairs of cells. Individual cells are round-to-ovoid cocci, 0.6-1.0 micrometer in diameter. They are catalase-negative aerotolerant anaerobe (facultative anaerobe) which require enriched medium containing blood in order to grow. Their capsule is composed of hyaluronic acid. GAS are important human pathogens which can cause a variety of diseases, ranging from mild infections to very severe invasive diseases. The M protein is a highly polymorphic cell-surface molecule that is antiphagocytic and forms the basis of a scheme commonly used to classify GAS strains. Serotype M3 strains cause a higher rate of lethal infections than strains of other M types. In addition, serotype M3 and other GAS strains can undergo rapid shifts in disease frequency and display epidemic behaviour. Strict human pathogen responsible for a wide variety of diseases, including pharyngitis, scarlet fever, impetigo, septicemia, toxic shock syndrome, flesh-eating disease, rheumatic fever and acute glomerulonephritis. Strain MGAS8232 is a serotype M18 organism isolated from a patient with acute rheumatic fever.