What is Group B Streptococcus (GBS)?

Group B Streptococcus (GBS): Overview and Clinical Significance

Group B Streptococcus (GBS), or Streptococcus agalactiae, is a gram-positive bacterium that causes invasive disease primarily in infants, pregnant women, and adults with underlying medical conditions, with the highest disease burden and mortality among young infants. 1

Epidemiology and Disease Burden

• GBS is the leading infectious cause of neonatal morbidity and mortality in the United States 2
• Incidence has declined from 1.7 cases per 1,000 live births in the early 1990s to 0.34-0.37 cases per 1,000 live births in recent years 2
• Approximately 1,200 cases of early-onset invasive disease occur annually in the US 2
• Case-fatality rates have improved significantly:
  • Declined from 50% in the 1970s to 4-6% in recent years 2
  • Higher mortality in preterm infants (20% overall, up to 30% for ≤33 weeks' gestation) 2
  • 2-3% for full-term infants 2
  • 15-32% for adults with invasive disease 2, 1

Disease Classification

GBS infections in newborns are classified by timing of onset:

1. Early-onset disease (EOD):

   • Occurs within first 7 days of life, typically within 24-48 hours 2, 1
   • Primarily presents as sepsis and pneumonia; less commonly meningitis 2
   • Acquired vertically from colonized mothers 2
   • Approximately 70% of cases occur in term infants 2

2. Late-onset disease (LOD):

   • Occurs between 7 days and 3 months of age 2, 1
   • More commonly presents as meningitis compared to early-onset disease 1
   • Prevention strategies for EOD do not prevent late-onset disease 2

Pathogenesis and Transmission

• Vertical transmission occurs when:
  • GBS ascends from vagina to amniotic fluid after labor onset or membrane rupture
  • Bacteria invade through intact membranes
  • Infants are exposed during passage through birth canal 2, 1
• Without intervention, approximately 1-2% of infants born to GBS-colonized mothers develop early-onset disease 1, 3
• Colonized mothers are >25 times more likely to deliver infants with early-onset disease 1

Risk Factors

Major risk factors for GBS disease include:

• Maternal GBS colonization: 10-30% of pregnant women are colonized in vagina or rectum 1
• Obstetric factors:
  • Prolonged rupture of membranes (especially >18 hours) 1
  • Preterm labor (<37 weeks gestation) 1
  • Intrapartum fever (>38°C) 1
  • GBS bacteriuria during current pregnancy 1
• Other factors:
  • Previous child with invasive GBS disease 1
  • Young maternal age (<20 years) 1
  • Black race 1, 4
  • Low maternal anticapsular antibodies against GBS 1
  • Advanced age, diabetes mellitus, and immunocompromising conditions (for adult disease) 1

Clinical Presentation

• Early-onset disease: Respiratory distress, apnea, or other sepsis signs within first 24-48 hours 2
• Common manifestations: Sepsis, pneumonia, meningitis, osteomyelitis, septic arthritis 1
• In pregnant/postpartum women: Urinary tract infection, amnionitis, endometritis, wound infection 2
• In non-pregnant adults: Skin/soft tissue infection, bacteremia, genitourinary infection, pneumonia 2

Prevention Strategies

The CDC recommends one of two prevention strategies:

1. Universal screening-based approach (preferred):

   • Screen all pregnant women at 36 0/7-37 6/7 weeks gestation 3
   • Provide intrapartum antibiotic prophylaxis (IAP) to:
     • Women with positive GBS cultures
     • Women with GBS bacteriuria during current pregnancy
     • Women with history of previous infant with GBS disease
     • Women with unknown GBS status who develop risk factors 2, 1

2. Risk factor-based approach:

   • Provide IAP to women who develop risk factors during labor/delivery:
     • Preterm delivery (<37 weeks)
     • Prolonged rupture of membranes (≥18 hours)
     • Intrapartum fever (≥38°C)
     • GBS bacteriuria during current pregnancy
     • Previous infant with GBS disease 2

Antibiotic Prophylaxis

• First-line: Penicillin G (preferred) or ampicillin 1
• Even 2 hours of antibiotic exposure reduces GBS colony counts and decreases neonatal sepsis risk 3
• Obstetric interventions should not be delayed solely to provide 4 hours of antibiotics 3

Prevention Challenges and Future Directions

• Despite screening and IAP, GBS disease persists 5
• Adherence to protocols remains suboptimal 6
• Vaccine development continues as a potential future prevention strategy 7

Key Points for Clinical Practice

• GBS remains a significant cause of neonatal morbidity and mortality despite prevention efforts
• Universal screening at 36-37 weeks and appropriate IAP are the cornerstones of prevention
• Proper specimen collection (2 cm into vagina and 1 cm into anus) is critical 7
• Patients can perform their own swabs effectively 7
• Patients with GBS in urine should receive IAP regardless of colony count 7
• GBS-positive women with PPROM after 34 weeks are not candidates for expectant management due to higher infectious complications 7

Effective prevention requires coordinated efforts among prenatal, obstetric, and pediatric care providers, with continued vigilance to reduce the burden of this serious infection.