Why Penicillin G is the First-Line Treatment for Group B Streptococcus
Penicillin G remains the agent of choice for treating Group B Streptococcus (GBS) infections, including during labor, because it has a narrow spectrum of antimicrobial activity that effectively targets GBS while minimizing the risk of developing antibiotic resistance. 1
Effectiveness Against GBS
- GBS isolates have consistently demonstrated complete susceptibility to penicillin G, with no confirmed resistance reported to date 1, 2
- Penicillin G acts by inhibiting cell wall peptidoglycan biosynthesis, rendering the bacterial cell osmotically unstable 2
- GBS is classified as "extremely susceptible" to penicillin G, making it highly effective even against aerobic strains 2
Advantages Over Alternative Antibiotics
- While ampicillin is an acceptable alternative with demonstrated efficacy, penicillin G is preferred due to its narrower spectrum of activity 1
- The narrower spectrum of penicillin G makes it less likely to select for resistant organisms in the mother or newborn 1
- Clinical trials have demonstrated that intravenous penicillin G during labor reduces the risk of early-onset neonatal GBS infection from 4.7% to 0.4% 3
Recommended Dosing for GBS Prophylaxis
- For intrapartum GBS prophylaxis: penicillin G, 5 million units IV initial dose, then 2.5 million units IV every 4 hours until delivery 1
- Intravenous administration is the only recommended route for GBS prophylaxis because it achieves higher intraamniotic concentrations 1
- While intramuscular administration has been evaluated, it is insufficient as sole therapy 4
Safety Profile
- Anaphylaxis associated with penicillin G occurs in approximately 4 per 10,000 to 4 per 100,000 recipients 1
- Despite this risk, the benefits of preventing GBS disease greatly outweigh the risks of anaphylaxis 1
- Studies show that intrapartum exposure to penicillin for GBS treatment does not increase the risk of penicillin allergy in children 5
Alternative Options for Penicillin-Allergic Patients
- For patients with non-severe penicillin allergy: cefazolin, 2g IV initial dose, then 1g IV every 8 hours until delivery 1
- For patients with severe penicillin allergy (history of anaphylaxis, angioedema, respiratory distress, or urticaria):
Antimicrobial Resistance Considerations
- Increasing resistance to clindamycin and erythromycin has been observed among GBS isolates (7-25% for erythromycin and 3-15% for clindamycin) 1, 7
- Some studies report even higher resistance rates: 28% for clindamycin and 30% for erythromycin 7
- The high co-resistance rate (92%) between erythromycin and clindamycin emphasizes the importance of susceptibility testing when these alternatives are needed 7
Impact on Public Health
- Implementation of GBS prevention strategies using penicillin G has reduced early-onset GBS disease incidence by more than 80% in the United States 8
- From 1994 to 2010, an estimated 70,000 cases of early-onset GBS disease were prevented in the United States 8
Penicillin G remains highly effective against GBS despite decades of use, with no documented resistance. Its narrow spectrum, proven efficacy, and favorable safety profile make it the optimal first-line treatment for GBS infections, particularly for intrapartum prophylaxis to prevent neonatal GBS disease.