What is the best antibiotic for group B streptococcus (GBS)?

Best Antibiotic for Group B Streptococcus

Penicillin G is the best antibiotic for Group B Streptococcus infections, with a recommended dosing regimen of 5 million units IV initially, followed by 2.5-3.0 million units IV every 4 hours. 1

First-Line Treatment

• Penicillin G is superior to ampicillin because it has a narrower spectrum of activity, which reduces selection pressure for resistant organisms while maintaining excellent activity against GBS 1, 2
• The dosing range of 2.5-3.0 million units every 4 hours achieves adequate drug levels in fetal circulation and amniotic fluid while avoiding neurotoxicity 3
• All GBS isolates worldwide remain 100% susceptible to penicillin, making resistance a non-issue 4
• Penicillin G is bactericidal during active bacterial multiplication by inhibiting cell wall peptidoglycan synthesis 5

Alternative for Non-Allergic Patients

• Ampicillin is an acceptable alternative with dosing of 2 g IV initially, then 1 g IV every 4 hours 1
• However, ampicillin's broader spectrum makes it less preferable than penicillin G when both options are available 1, 2

Treatment Algorithm for Penicillin-Allergic Patients

Step 1: Assess Allergy Severity

• High-risk for anaphylaxis is defined as: history of anaphylaxis, angioedema, respiratory distress, or urticaria following penicillin or cephalosporin administration 3, 4
• Verify the allergy history carefully, as true penicillin allergy occurs in far fewer patients than reported 4

Step 2: Non-Severe Penicillin Allergy

• Use cefazolin: 2 g IV initially, then 1 g IV every 8 hours 1, 6
• Cross-reactivity between penicillins and cephalosporins occurs in only approximately 10% of penicillin-allergic patients 4
• GBS isolates remain highly susceptible to cefazolin with consistently low minimum inhibitory concentrations 6

Step 3: Severe Penicillin Allergy

• Obtain susceptibility testing for clindamycin and erythromycin immediately 4, 6
• If susceptible to both clindamycin and erythromycin: Use clindamycin 900 mg IV every 8 hours 1, 6
• If resistant or susceptibility unknown: Use vancomycin 1 g IV every 12 hours 1, 6
• Never use cefazolin in severe penicillin allergy due to cross-reactivity risk 4

Critical Considerations

• Erythromycin is no longer acceptable for GBS treatment due to resistance rates up to 20.2% 3, 1
• Clindamycin resistance has increased to 3-15% in the US, making susceptibility testing mandatory before use 1
• Test for inducible clindamycin resistance in isolates susceptible to clindamycin but resistant to erythromycin 1
• The risk of penicillin-induced anaphylaxis is 4/10,000 to 4/100,000, which is far outweighed by benefits of preventing GBS disease 1

Important Pitfalls to Avoid

• Only use IV administration - oral or intramuscular routes do not achieve adequate concentrations for GBS treatment 1
• Vancomycin should be reserved for cases with no other options due to concerns about promoting antimicrobial resistance 6
• Always obtain susceptibility testing when treating penicillin-allergic patients, as empiric therapy may fail 4, 6
• Consider infectious disease consultation for complicated cases or limited treatment options 4