Is penicillin or amoxicillin more effective for treating Group B streptococcal (strep B) infections?

Penicillin vs Amoxicillin for Group B Streptococcal Infections

Both penicillin and amoxicillin are equally effective against Group B Streptococcus (GBS), but penicillin G is preferred for treatment due to its narrower spectrum, while amoxicillin is acceptable and often preferred for prophylaxis in children due to better palatability. 1, 2

Treatment Context: Intrapartum Prophylaxis vs Active Infection

For Intrapartum Prophylaxis (Preventing Neonatal GBS Disease)

Penicillin G is the first-line agent for intrapartum chemoprophylaxis: 5 million units IV initial dose, then 2.5 million units IV every 4 hours until delivery 1. The rationale for preferring penicillin G over ampicillin/amoxicillin in this setting includes:

• Narrower spectrum of action reduces selection pressure for resistant organisms 3
• Excellent placental transfer and fetal tissue levels 3
• Lower cost and proven efficacy 1

Ampicillin is an acceptable alternative (2 g IV initial dose, then 1 g IV every 4 hours until delivery) but is considered second-line due to its broader spectrum 1.

For Active GBS Infections (Neonatal Sepsis, Meningitis, Soft Tissue)

Penicillin remains the drug of choice for treating established GBS infections 4, 5. Key evidence supporting this:

• 100% of GBS isolates remain susceptible to penicillin across multiple surveillance studies from 1992-1998 4, 5
• No documented penicillin resistance has ever been reported in GBS worldwide 2
• For necrotizing fasciitis or severe soft tissue infections caused by streptococci (including GBS), penicillin 2-4 million units IV every 4-6 hours plus clindamycin is recommended 1

Practical Considerations

When Amoxicillin May Be Preferred

• Pediatric oral therapy: Amoxicillin suspension has better taste acceptance than penicillin V in children 2, 6
• Once-daily dosing: Amoxicillin 50 mg/kg/day (maximum 1000 mg) once daily improves adherence for strep throat, though this applies to Group A Streptococcus, not GBS 2

Resistance Patterns to Consider

While penicillin/amoxicillin resistance is non-existent, alternative agents show concerning resistance:

• Erythromycin resistance: 20.2% of GBS isolates 4
• Clindamycin resistance: 6.9% overall, with geographic variation (California 12%, Florida 2.1%) 4
• This matters for penicillin-allergic patients requiring alternative prophylaxis 1

Common Pitfalls to Avoid

• Do not use broad-spectrum agents like Augmentin (amoxicillin-clavulanate) for uncomplicated GBS—the clavulanate adds no benefit since GBS doesn't produce beta-lactamase 7, 6
• Do not substitute cephalosporins as first-line therapy when penicillin/amoxicillin are appropriate—reserve these for penicillin-allergic patients 1
• For penicillin-allergic women at high risk for anaphylaxis, use clindamycin or erythromycin only if susceptibility testing confirms sensitivity; otherwise use vancomycin 1

Algorithm for Selection

1. Is the patient penicillin-allergic?

   • No → Use penicillin G (IV) or amoxicillin (oral for children)
   • Yes → Assess anaphylaxis risk and obtain susceptibility testing 1

2. What is the clinical scenario?

   • Intrapartum prophylaxis → Penicillin G preferred 1
   • Active neonatal infection → Penicillin G (often with gentamicin initially) 1
   • Oral therapy in children → Amoxicillin for palatability 2

3. Is compliance a concern?

   • Yes → Consider IM benzathine penicillin G for prophylaxis 1
   • No → Proceed with standard IV or oral regimens 1, 2