The Streptococcus

Classification:
Lancefield simplified the classification of streptococci by showing that surface antigens (carbohydrates) extracted from the cell wall of streptococci could react with carbohydrate-specific antisera prepared in rabbits. For example, Lancefield’s group A are the S. pyogenes strains responsible for human diseases such as scarlet fever, streptococcal pharyngitis, erysipelas, puerperal sepsis, wound infections. N-acetylglucosamine, is the group A-specific carbohydrate, whereas N-acetylgalactosamine is the group C determinant. Extended studies of the cell wall carbohydrates of streptococci from several sources have shown at least 13 different serologic groups encompassing human and animal pathogens and commensals .

Group A streptococci may be subdivided into over 130 different types based on a variable surface protein, called M protein. M protein was used as a serologic determinant for early classification of group A strains, but recently, the sequence of the variable region of the M protein gene has been used instead with identical results (http://www.cdc.gov/ncidod/biotech/strep/protocol_emm-type.htm). M typing is used for epidemiologic purposes in studying the spread of streptococcal disease.

Serotyping based on the capsule has been used to differentiate different strains of group B streptococci. Currently, nine capsular serotypes have been described (Ia, Ib, II, III, IV, V, VI, VII and VIII). Throughout the world, serotype III strains are of particular importance, since they are responsible for the majority of infections, including neonatal meningitis.