streptococcus, (genus Streptococcus), group of spheroidal bacteria belonging to the family Streptococcaceae. The term streptococcus (“twisted berry”) refers to the bacteria’s characteristic grouping in chains that resemble a string of beads. Streptococci are microbiologically characterized as gram-positive and nonmotile.

Streptococcus contains a variety of species, some of which cause disease in humans and animals, while others are important in the manufacture of certain fermented products. Streptococcus pyogenes, often referred to as group A streptococcus bacteria, can cause rheumatic fever, impetigo, scarlet fever, puerperal fever, streptococcal toxic shock syndrome, strep throat, tonsillitis, and other upper respiratory infections. Necrotizing fasciitis, a rapidly spreading infection of the skin and underlying tissue caused by S. pyogenes, has been popularly referred to as the “flesh-eating disease.” Streptococcus agalactiae, or group B streptococcus bacteria, can cause infections of the bladder and uterus in pregnant women; in newborn infants infection with the bacterium may result in sepsis (blood poisoning), meningitis (inflammation of the membranes covering the brain and spinal cord), or pneumonia.

Streptococcus pneumoniae, also called pneumococcus, is an important human pathogen that causes pneumonia, sinusitis, otitis media, and meningitis. Fecal (enterococcal) species occur in great numbers in the bowel and can cause urinary tract infections and endocarditis. S. mutans, belonging to the viridans species, inhabits the mouth and contributes to tooth decay. Among the lactic species, S. lactis and S. cremoris are used in commercial starters for the production of butter, cultured buttermilk, and certain cheeses.

Streptococci generally are classified by the type of carbohydrate contained in the cell wall, a system called the Lancefield classification.