Group B streptococci (Streptococcus agalactiae) are gram-positive, aerobic diplococci that typically produce a narrow zone of beta hemolysis on 5% sheep blood agar. These organisms are divided into 9 serotypes on the basis of capsular polysaccharides (Ia, Ib, II, and III through VIII). Serotypes Ia, Ib, II, III, and V account for approximately 95% of cases in infants in the United States. Serotype III is the predominant cause of early-onset meningitis and most late-onset infections in infants. Pilus-like structures are important virulence factors and potential vaccine candidates.