Streptococcus

The streptococci are classified as beta'hemolytic if complete hemolysis of the blood cells in the agar surrounding the colonies is observed, alpha'hemolytic if a characteristic greenish color is seen, or nonhemolytic if the blood cells are not affected. This chapter mainly focuses on the group A streptococci (S. pyogenes) that cause a large number of different disease syndromes. The group A streptococci are the causative agents of several serious diseases, such as necrotizing fasciitis, scarlet fever, sepsis, and a recently recognized toxic shock'like syndrome, as well as suppurative infections of the skin and throat, such as impetigo, erysipelas, and pharyngitis (strepthroat). A number of surface structures have been implicated as virulence factors of the group A streptococci. M protein is considered the major virulence factor of group A streptococci (S. pyogenes) because it protects the bacteria from phagocytosis by polymorphonuclear leukocytes. Infection by S. pyogenes causes production of antibodies that include those that react with M protein. The M protein or a derivative of it is being considered as a possible antistreptococcus vaccine. A greater understanding of the relation of the structure of the M protein to its function should help in the rational design of an antistreptococcal vaccine and should also increase our understanding of the way in which coiled'coil fibrous proteins can act.