Treatment of Group B Streptococcus (GBS) Infection with Penicillin Susceptibility
For a patient with greater than 100,000 CFU/mL of Group B Streptococcus that is susceptible to penicillin, use Penicillin G as first-line therapy, with ampicillin as an equally effective alternative. 1
First-Line Treatment Options
Penicillin G remains the gold standard for GBS treatment given the universal susceptibility of GBS to beta-lactam antibiotics, with no documented resistance worldwide. 1, 2, 3, 4
Recommended Dosing Regimens
The specific dosing depends on the clinical context:
For intrapartum prophylaxis (preventing neonatal disease): Penicillin G 5 million units IV initial dose, then 2.5-3.0 million units IV every 4 hours until delivery 5
For treatment of established infection: Penicillin G 200,000-300,000 units/kg/day IV divided every 4 hours (up to 12-24 million units daily in adults) 5
Alternative beta-lactam option: Ampicillin 2 g IV initial dose, then 1 g IV every 4 hours 5
Why Penicillin G is the Definitive Choice
All clinical studies confirm 100% susceptibility of GBS to penicillin G, ampicillin, cefazolin, cefotaxime, and vancomycin. 2, 3 This universal susceptibility has been documented across multiple surveillance studies spanning decades, with no emergence of resistance. 1, 4
- Research from 1996-2003 involving 3,813 invasive GBS isolates showed all were sensitive to penicillin and ampicillin 2
- A 2020 pediatric study confirmed all GBS isolates remained sensitive to penicillin (MICs 0.06-2.0 μg/mL) 4
- Even rare GBS isolates with reduced susceptibility to certain cephalosporins remain fully susceptible to penicillin G and ampicillin 6
Alternative Agents (Only for Penicillin Allergy)
If the patient has a documented penicillin allergy, the choice of alternative agent depends on the severity of the allergic history:
For Non-Severe Penicillin Allergy
- Cefazolin 2 g IV initial dose, then 1 g IV every 8 hours is preferred because it achieves effective tissue concentrations 5
For High-Risk Anaphylaxis History
(History of anaphylaxis, angioedema, respiratory distress, or urticaria to penicillin or cephalosporin) 5
You must obtain susceptibility testing results before choosing between clindamycin and vancomycin: 5
If GBS is susceptible to both clindamycin AND erythromycin: Use clindamycin 900 mg IV every 8 hours 5
If GBS is susceptible to clindamycin but resistant to erythromycin: Clindamycin can only be used if D-zone testing for inducible resistance is negative 5
If susceptibility unknown or clindamycin resistance detected: Use vancomycin 1 g IV every 12 hours 5
Critical Pitfalls to Avoid
Never use erythromycin as an alternative agent for penicillin-allergic patients at high risk for anaphylaxis - this recommendation was explicitly removed from CDC guidelines due to increasing resistance rates (25.6% to 32.8% resistance documented). 5, 2
Do not assume clindamycin susceptibility without testing. Clindamycin resistance in GBS has increased from 10.5% to 15.0%, and inducible resistance may not be detected without D-zone testing. 5, 1, 2 Approximately 12.7% of invasive GBS isolates show clindamycin resistance. 2
Gentamicin should never be used as monotherapy - all GBS isolates show resistance to gentamicin when used alone. 3 It may only be used in combination with ampicillin for synergy in specific situations like enterococcal-type infections. 5
Why Susceptibility Testing is Not Routinely Performed
The laboratory note stating "susceptibility testing not routinely performed" is standard practice because beta-hemolytic streptococci, including GBS, are predictably susceptible to penicillin. 1, 2, 3, 4
However, susceptibility testing MUST be performed if:
- The patient has a penicillin allergy with high risk for anaphylaxis 5
- Alternative agents (clindamycin, erythromycin) are being considered 5, 1
- The clinician contacts the laboratory within 3 days to request it 5
Testing should include both clindamycin and erythromycin susceptibility, plus D-zone testing to detect inducible clindamycin resistance if the isolate is erythromycin-resistant. 5, 1