STREPTOCOCCUS

plural Streptococci, any member of a genus (Streptococcus) of spheroidal bacteria in the family Streptococcaceae. The term streptococcus (twisted berry) refers to the bacteria's characteristic grouping in chains resembling a string of beads. 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. Streptococci are microbiologically characterized as gram-positive, nonmotile, facultative anaerobes (not requiring oxygen). Their cells are spherical to ovoid, less than 2 micrometres in diameter, and often occur in pairs or chains. Species of streptococci are classified into four groupspyogenic, viridans, fecal (enterococcus), and lacticprimarily on the basis of the organisms' ability to cause hemolysis (rupture of red blood cells) when grown on blood agar. Pyogenic (pus-forming), or hemolytic, species of streptococci cause rheumatic fever, scarlet fever, erysipelas, and strep throat, tonsillitis, and other upper respiratory infections. Viridans and fecal species occur in great numbers in the throat and bowel, respectively, but rarely cause disease. 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 can also be classified by the type of carbohydrate contained in the cell wall, a system called the Lancefield classification.