A large genus of spherical or ovoid bacteria that are characteristically arranged in pairs or in chains resembling strings of beads. Many of the streptococci that constitute part of the normal flora of the mouth, throat, intestine, and skin are harmless commensal forms; other streptococci are highly pathogenic. The cells are gram-positive and can grow either anaerobically or aerobically, although they cannot utilize oxygen for metabolic reactions. Glucose and other carbohydrates serve as sources of carbon and energy for growth. All members of the genus lack the enzyme catalase. Streptococci can be isolated from humans and other animals.
Streptococcus pyogenes is well known for its participation in many serious infections. It is a common cause of throat infection, which may be followed by more serious complications such as rheumatic fever, glomerulonephritis, and scarlet fever. Other beta-hemolytic streptococci participate in similar types of infection, but they are usually not associated with rheumatic fever and glomerulonephritis. Group B streptococci, which are usually beta-hemolytic, cause serious infections in newborns (such as meningitis) as well as in adults. Among the alpha-hemolytic and nonhemolytic streptococci, S. pneumoniae is an important cause of pneumonia and other respiratory infections. Vaccines that protect against infection by the most prevalent capsular serotypes are available. The viridans streptococci comprise a number of species commonly isolated from the mouth and throat. Although normally of low virulence, these streptococci are capable of causing serious infections (endocarditis, abcesses).
a spherical bacterium (0.6–1 microns in diameter) that reproduces by cell division in one plane, resulting in a chain of cells of different lengths. Streptococci are nonsporeforming, nonmotile, and gram-positive and grow well on agarculture mediums. They ferment sugars and alcohols. Some streptococcal species are of practical importance; for example, Streptococcus lactis, which ferments lactose to form lactic acid, is used to obtain clabber and other sour-milk products. Some species form the polysaccharide dextran, which is used to produce a blood substitute. Pathogenic streptococci may cause tonsillitis, erysipelas, suppuration, and blood infections. Some are capable of dissolving erythrocytes (hemolysis).