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The group B Streptococcus (GBS) initially was isolated by Nocard in 1887 and described as Streptococcus agalactiae, a cause of bovine mastitis. The organism is an encapsulated gram-positive diplococcus that usually produces a narrow zone of beta-hemolysis on blood agar. Most strains are resistant to bacitracin. The streptococci were classified serologically in the 1930s by Lancefield, based on cell wall polysaccharides. The GBS organisms are differentiated further by type-specific capsular polysaccharides and protein antigens. Current serotypes include: Ia, Ib, Ia/c, II, III, IV, V, VI, VII, and VIII.

Since the mid-1960s, GBS has become the major cause of bacterial infections in the perinatal period, including bacteremia, amnionitis, endometritis, and urinary tract infection in pregnant women as well as focal and systemic infections in newborns. It is a relatively rare cause of infection in older children and nonpregnant adults. GBS early-onset...

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