Streptococci are classified into the alpha, beta, or gamma groups, according to their action on blood cells. Streptococci of the alpha group (e.g., the viridans and S. pneumoniae ) cause some destruction (hemolysis) of red blood cells. The beta group are more destructive of red blood cells; they also produce toxic substances that affect white blood cells and the clotting properties of blood. Members of these two groups are sometimes called hemolytic (red blood cell–destroying) streptococci. The beta-hemolytic streptococci are often further classified into lettered groups, called Lancefield groups for R. C. Lancefield, the scientist who originated the scheme in the 1930s. Group A hemolytic streptococci are responsible for most human streptococcal disease; group B hemolytic streptococci can cause serious problems in newborns. The gamma group, or nonhemolytic group, does not affect red blood cells. Enterococci (usually harmless bacteria that inhabit the intestines) and lactococci (bacteria used in starter cultures in the production of fermented dairy products) used to be considered a part of the Streptococcus genus but are now placed in their own genera.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.