There are a wide variety of species included in the genus Streptococcus , many of which are innocuous to humans. There are those, however, which are important agents of infection. Particularly significant as human pathogens are S. pyogenes, S. pneumoniae, and Enterococcus. Several others produce disease in humans, though they are more commonly associated with lower animals. The members of the Streptococci are gram-positive organisms that usually form pairs or chains .
Further classification is quite complex because of their wide variation, distribution, and effects.
In 1937, Sherman separated the genus according to physiological and growth characteristics , especially with regards to temperature limitations on growth. This categorization has become somewhat obsolete as relationships between species have been shown to overlap. The Streptococcus may also be designated according to the type of hemolysis they produce on blood agar. Lancefield developed a serological classification based on the presence of C substance , an extractable carbohydrate found in the cell wall. At present, there are at least 20 groups , each of which is separable from the others by serology.