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Dr. Lancefield (circa 1980) “typing” streptococci with a variety of M protein-specific antibodies mixed with a streptococcal extract. By visualizing the precipitin reactions in capillary pipets, group A streptococci were classified according to their expression of M antigens. See Swift, et al. 1943. Currently, the method used to identify the S. pyogenes serotype is to perform emm typing.  This is a PCR based sequencing that identifies the variable region of the M protein by molecular techniques.  For more details see: http://www.cdc.gov/ncidod/biotech/strep/protocol_emm-type.htm and http://www.cdc.gov/ncidod/biotech/strep/strepindex.htm

In 1918, Dr. Lancefield joined the Rockefeller Institute for Medical Research, commencing her studies of the hemolytic streptococci, known then as Streptococcus haemolyticus. Following in the path of Oswald Avery, who had previously developed a serum (or precipitation) system for differentiating among types of pneumococci, Lancefield used similar methods to classify S. haemolyticus into groups according to antigens composed of carbohydrate. She also demonstrated that one of these groups, group A streptococci (S. pyogenes), was specific to humans and human disease, including pharyngitis (“strep throat”), scarlet fever, rheumatic fever, nephritis and impetigo. Group B streptococci were subsequently shown to be associated with neonatal disease. Dr. Lancefield further elaborated on the extensive variety of the group A streptococci, demonstrating that different serotypes were the result of antigenic variation of a cell surface protein, which she named M protein. In demonstrating the basis of antigenic specificity, she offered an explanation of M protein’s role in the bacterium’s mechanism of both surviving in the context of the human host and causing disease.

For more reading on the subject: see Dr. Lancefield’s bibliography (1918-1979) and her streptococcal strain collection.