Group B streptococcus (GBS), or Streptococcus agalactiae, is a gram-positive bacterium commonly found in the genital and gastrointestinal tracts of humans and less frequently in the upper respiratory tracts of children and adults. This bacterium poses significant health risks, particularly to neonates, young infants, pregnant women, and those with certain medical conditions. GBS can colonize the gastrointestinal and vaginal tracts of up to one-third of pregnant individuals, resulting in various infections, including asymptomatic bacteriuria, urinary tract infections, chorioamnionitis, postpartum endometritis, pneumonia, puerperal sepsis, and bacteremia. Although GBS accounts for a small percentage of urinary tract infections and peripartum bacteremia during pregnancy, it can occasionally lead to more severe maternal sequelae, such as meningitis and endocarditis. However, invasive maternal GBS infections pose the most significant risks to the neonate, including bacteremia, sepsis, and death.

Vertical transmission of GBS during vaginal birth can result in early-onset GBS disease (GBS-EOD) in newborns. Maternal colonization of GBS in the gastrointestinal tract and vagina is the primary risk factor for GBS-EOD in neonates. According to the Centers for Disease Control and Prevention (CDC), approximately 0.23 cases per 1000 live births are diagnosed with early-onset GBS in the United States. Preventive measures include correct specimen collection, nucleic acid amplification testing (NAAT) for GBS identification, and specific regimens for mothers with premature rupture of membranes, preterm labor, or penicillin allergy, along with coordination between obstetrics and pediatrics.

The American College of Obstetricians and Gynecologists (ACOG) recommends universal GBS screening at 36 to 37 6/7 weeks of gestation, with positive cases receiving appropriate intrapartum antibiotics. Penicillin is the preferred antibiotic, with alternatives available for individuals allergic to penicillin. Effective prevention of neonatal GBS disease depends on proper screening, timely antibiotic administration, and coordination with pediatric care clinicians. Before the widespread use of maternal intrapartum chemoprophylaxis, the incidence of early-onset GBS was much higher. In Europe and the United States, neonatal GBS-EOD rates have declined markedly due to guidelines for maternal GBS screening and intrapartum antibiotic prophylaxis (IAP). Universal third-trimester screening and IAP have significantly reduced neonatal GBS infections, although these measures may have unintended consequences for mothers and infants. Currently, an estimated 31% of individuals in the United States are administered antibiotics for intrapartum GBS prophylaxis.

Despite CDC recommendations for routine GBS screening and intrapartum antibiotics, GBS remains a leading cause of early-onset neonatal sepsis in the United States. Furthermore, up to 40% of individuals who test positive for GBS during prenatal care may test negative at delivery, leading to significant overtreatment and increasing antibiotic resistance. Therefore, evolving therapeutics to prevent GBS colonization at delivery are still being investigated, including probiotic interventions and maternal vaccines.