A genus of spherical Gram-positive bacteria occurring widely in nature, typically as chains or pairs of cells. Many are saprotrophic and exist as usually harmless commensals inhabiting the skin, mucous membranes, and intestine of humans and animals. Others are parasites, some of which cause diseases. There are several systems of classifying streptococcal species. The Lancefield system, devised by US microbiologist Rebecca Lancefield (1895–1981) in the 1920s, identifies groups serologically according to unique carbohydrate antigens in their cell wall. There are 20 Lancefield groups, designated A to V; the common pathogen S. pyogenes, responsible for ‘strep throat’ and scarlet fever, is a group A streptococcus, while S. agalactiae is group B, and so on. However, some species lack a Lancefield antigenic marker, notably S. pneumoniae, which is responsible for pneumococcal infections ranging from bronchitis to septicaemia and meningitis. Another classification system depends on the pattern of breakdown (haemolysis) of blood cells when the bacterium is incubated on blood agar. Alpha-haemolytic bacteria, e.g. S. pneumoniae, cause only a partial haemolysis, whereas beta-haemolytic bacteria, e.g. S. pyogenes, lyse the blood cells completely, forming a clear zone around the bacterial colonies.