What is the dosage of amoxicillin for group B strep (Streptococcus agalactiae) treatment?

Amoxicillin for Group B Streptococcus Treatment

Amoxicillin is not the first-line treatment for Group B Streptococcus (GBS); penicillin G is the preferred agent due to its narrow spectrum of activity, with ampicillin as an acceptable alternative. 1

First-Line Treatment Options for GBS

• Penicillin G is the recommended first-line treatment for GBS infections with the following dosing regimen: 5 million units IV initial dose, followed by 2.5-3.0 million units IV every 4 hours until delivery 1
• Ampicillin is an acceptable alternative with the following dosing: 2 g IV initial dose, then 1 g IV every 4 hours until delivery 1
• All GBS isolates remain susceptible to penicillin and ampicillin worldwide, making these the most reliable treatment options 2

Treatment for Penicillin-Allergic Patients

For patients NOT at high risk for anaphylaxis:

• Cefazolin: 2 g IV initial dose, then 1 g IV every 8 hours until delivery 1

For patients at high risk for anaphylaxis (history of anaphylaxis, angioedema, respiratory distress, or urticaria following penicillin/cephalosporin administration):

• If GBS isolate is susceptible to clindamycin and erythromycin:
  • Clindamycin: 900 mg IV every 8 hours until delivery 1, 3
• If susceptibility testing is not available or isolate is resistant:
  • Vancomycin: 1 g IV every 12 hours until delivery 1, 3

Important Considerations for Antibiotic Selection

• Erythromycin is no longer recommended for GBS prophylaxis due to increasing resistance rates (up to 20.2% in studies) 1, 2
• Clindamycin resistance rates have increased to 3-15% in the US, necessitating susceptibility testing before use 1, 2
• Resistance patterns vary by geographic region, with some areas showing higher resistance rates to both erythromycin and clindamycin 2
• Testing for inducible clindamycin resistance is necessary for isolates that are susceptible to clindamycin but resistant to erythromycin 1

Clinical Pearls and Pitfalls

• Penicillin is preferred over ampicillin due to its narrower spectrum of activity, which may reduce selection for resistant organisms 1
• Susceptibility testing should always be performed for GBS isolates from penicillin-allergic patients at high risk for anaphylaxis 3
• The risk of anaphylaxis with penicillin administration is estimated at 4/10,000 to 4/100,000 recipients, which is far outweighed by the benefits of preventing GBS disease 1
• Intravenous administration is the only recommended route for GBS treatment/prophylaxis due to higher concentrations achieved 1
• For GBS urinary tract infections specifically, nitrofurantoin may be an effective option as studies show good sensitivity patterns 4

Antimicrobial Resistance Patterns

• While all GBS isolates remain susceptible to penicillin and ampicillin, some studies have noted decreased sensitivity in certain isolates, highlighting the importance of monitoring susceptibility patterns 4
• Resistance to multiple antibiotics is becoming more common, with some studies showing 35% of clinical isolates resistant to 6 of 12 antibiotics tested 4
• Type V GBS strains show higher resistance to erythromycin and clindamycin compared to other serotypes 2