What is the best antibiotic for group B strep (Streptococcus agalactiae) treatment?

Best Antibiotic for Group B Streptococcus

Penicillin G (5 million units IV initially, then 2.5-3 million units IV every 4 hours) is the best antibiotic for Group B Streptococcus treatment due to its narrow spectrum of activity, universal susceptibility, and proven efficacy. 1

First-Line Treatment

• Penicillin G is the preferred agent because of its narrow spectrum, which minimizes selection pressure for antibiotic-resistant organisms 1, 2
• All GBS isolates worldwide remain universally susceptible to penicillin, with no documented resistance 3, 4
• Ampicillin (2 g IV initially, then 1 g IV every 4 hours) is an acceptable alternative but has a broader spectrum than penicillin G 1

Treatment for Penicillin-Allergic Patients

The approach depends on the severity of the allergy:

Non-Severe Penicillin Allergy (No History of Anaphylaxis, Angioedema, Respiratory Distress, or Urticaria):

• Cefazolin is the preferred alternative (2 g IV initially, then 1 g IV every 8 hours) 1, 3, 5
• Cross-reactivity between penicillins and cephalosporins occurs in approximately 10% of patients with penicillin allergy 3, 5
• GBS isolates remain highly susceptible to cefazolin 3, 5

Severe Penicillin Allergy (High Risk for Anaphylaxis):

• Obtain susceptibility testing for clindamycin and erythromycin on the GBS isolate 1, 3, 5
• If susceptible to both: Use clindamycin 900 mg IV every 8 hours 1, 3, 5
• If resistant or susceptibility unknown: Use vancomycin 1 g IV every 12 hours 1, 3, 5
• Erythromycin is no longer recommended due to increasing resistance rates (21-30% in recent studies) 3, 6, 4

Critical Clinical Considerations

Resistance Patterns:

• Clindamycin resistance ranges from 4-28% depending on geographic location 6, 4
• Erythromycin resistance ranges from 21-30% 6, 4
• Co-resistance to both clindamycin and erythromycin occurs in 92% of erythromycin-resistant strains 6
• Susceptibility testing is essential when using clindamycin or erythromycin for penicillin-allergic patients 1, 3, 6

Important Pitfalls to Avoid:

• Do NOT use oral antibiotics to treat GBS colonization outside of active labor or infection, as this is ineffective in eliminating carriage and may promote resistance 1, 7
• Do NOT discontinue aminoglycosides prematurely when treating invasive GBS disease, as combination therapy with ampicillin/penicillin plus gentamicin shows synergistic killing 8, 9
• Do NOT use erythromycin as empiric therapy without susceptibility testing due to high resistance rates 3, 6
• Vancomycin should be reserved for cases where no other options exist to minimize promoting antimicrobial resistance 5

Special Populations:

Pregnant Women:

• GBS bacteriuria at any concentration during pregnancy requires both treatment of the UTI and intrapartum prophylaxis during labor 7
• Women with GBS colonization detected at 35-37 weeks require intrapartum prophylaxis during labor, not antepartum treatment 1, 7
• Planned cesarean delivery before labor onset and membrane rupture does not require routine prophylaxis 1

Neonates with Invasive Disease:

• Combination therapy with ampicillin plus gentamicin is recommended for suspected sepsis to provide synergistic killing and broader coverage 1, 9
• Lumbar puncture should be performed if sepsis is suspected and the patient is stable enough to tolerate the procedure 1