Understanding Group B Strep and Prophylaxis
Group B Streptococcus (GBS), or Streptococcus agalactiae, is a common bacterium that colonizes the gastrointestinal and genitourinary tracts. In most adults, colonization is asymptomatic, but it can cause severe, invasive infections in vulnerable populations, including newborns, pregnant women, and the elderly.
For newborns, the primary risk is early-onset GBS disease, which occurs when the bacteria is transmitted from a colonized mother during childbirth. To prevent this, healthcare providers administer intrapartum antibiotic prophylaxis (IAP) to at-risk pregnant women. This is not a treatment for the mother's colonization but a preventive measure for the baby during labor.
The American College of Obstetricians and Gynecologists (ACOG) and the Centers for Disease Control and Prevention (CDC) recommend universal screening for GBS colonization via vaginal-rectal culture between 36 and 37 weeks of gestation. IAP is indicated for women with a positive screening result, a previous infant with invasive GBS disease, or GBS bacteriuria during their current pregnancy.
First-Line Antibiotics for GBS
For patients with no penicillin allergy, the preferred antibiotics for intrapartum prophylaxis are beta-lactams, which work by inhibiting bacterial cell wall synthesis.
- Penicillin G: The agent of choice for GBS prophylaxis due to its high efficacy, narrow spectrum of activity, and low likelihood of inducing resistance.
- Ampicillin: An acceptable alternative to penicillin.
Administering IV antibiotics during labor is critical, ideally for a certain duration before delivery, to ensure adequate drug levels in the mother and fetus. Oral antibiotics before labor are not effective for preventing transmission during delivery, as the bacteria can recolonize quickly.
Managing Penicillin Allergies
For patients with a history of penicillin allergy, the choice of antibiotic depends on the severity and type of the reaction. This requires careful consideration to select an effective and safe alternative.
- For non-severe penicillin allergy: If the patient's history does not include anaphylaxis, angioedema, respiratory distress, or urticaria, cefazolin is the recommended alternative. Cefazolin is a first-generation cephalosporin with low cross-reactivity with penicillin and proven efficacy against GBS.
- For severe penicillin allergy: If a high-risk allergy (e.g., anaphylaxis, angioedema) is documented, the approach is more complex. Because of rising resistance, susceptibility testing of the GBS isolate is critical before administering certain alternatives.
- Clindamycin: A viable option, but only if susceptibility testing confirms the GBS isolate is susceptible. Clindamycin resistance rates are high in some regions and have been increasing over time, making it an unreliable empiric choice.
- Vancomycin: The recommended choice if the isolate is resistant to clindamycin, or if the susceptibility is unknown and the allergy is severe. Vancomycin resistance in GBS is very rare.
Consideration of Penicillin Allergy Testing
Penicillin allergy testing is a safe and increasingly recommended procedure during pregnancy. For women with a reported allergy, testing can determine if a true type I hypersensitivity exists. In many cases, it reveals that the allergy is not present, allowing for the use of first-line beta-lactam antibiotics, which have superior efficacy and a narrower spectrum compared to alternatives like clindamycin or vancomycin.
Comparative Antibiotic Chart for GBS Prophylaxis
| Antibiotic | Standard Use | Penicillin Allergy (non-severe) | Penicillin Allergy (severe) | Key Considerations | Resistance Trends |
|---|---|---|---|---|---|
| Penicillin G | First-line choice for prophylaxis in non-allergic patients. | N/A | N/A | Most effective with narrowest spectrum. Given IV during labor. | Very low resistance. |
| Ampicillin | Acceptable alternative to penicillin for prophylaxis in non-allergic patients. | N/A | N/A | Also a beta-lactam, slightly broader spectrum than penicillin. | Very low resistance. |
| Cefazolin | N/A | First-line choice for prophylaxis. | N/A | First-generation cephalosporin; low risk of cross-reactivity. | Very low resistance. |
| Clindamycin | N/A | N/A | Used if GBS isolate is known to be susceptible. | Requires susceptibility testing before use due to resistance. | Increasing resistance, varying regionally. |
| Vancomycin | N/A | N/A | Used if clindamycin resistance is confirmed or unknown. | Last resort due to risk of resistance and toxicity; infused slowly IV. | Very low resistance. |
Treating GBS Infections in Newborns and Other Populations
While intrapartum prophylaxis is highly effective, some newborns still develop early-onset GBS disease and require treatment.
- Empiric Therapy for Neonates: For presumptive sepsis in newborns, empiric treatment typically involves ampicillin combined with an aminoglycoside (like gentamicin). This combination covers GBS and other potential pathogens, such as E. coli.
- Confirmed Neonatal GBS Infection: Once GBS is confirmed, treatment can be narrowed to penicillin G or ampicillin. For meningitis, a longer course of IV therapy is required.
- Adult GBS Urinary Tract Infections (UTIs): Treatment depends on whether the patient is pregnant. In pregnant women, any GBS bacteriuria warrants treatment during pregnancy and again with IAP during labor. For non-pregnant adults with uncomplicated GBS UTIs, oral options like amoxicillin or cephalexin are often used.
The Evolving Landscape of Antibiotic Resistance
Continuous monitoring of antibiotic resistance trends is vital for ensuring effective treatment. Recent studies indicate that while penicillin and other beta-lactams maintain excellent activity against GBS, resistance to alternative agents is a growing concern.
- High Macrolide and Lincosamide Resistance: Resistance to erythromycin and clindamycin has increased significantly over the last two decades. Erythromycin is no longer considered a reliable option for GBS prophylaxis. The high and regionally variable resistance to clindamycin underscores the importance of pre-treatment susceptibility testing for allergic patients.
- Fluoroquinolone Resistance: Resistance to fluoroquinolones (e.g., levofloxacin) has also been on the rise.
- Reliable First-Line Agents: The consistent susceptibility of GBS to penicillin, ampicillin, and cefazolin makes them the best choices when there is no severe allergy. This highlights why penicillin allergy testing is so valuable, as it allows for the use of these proven agents.
Conclusion
For the prevention of early-onset GBS disease during labor, intravenous penicillin G or ampicillin is the best antibiotic choice for women without a severe beta-lactam allergy. When a non-severe penicillin allergy is present, cefazolin is the recommended alternative. For women with a severe, high-risk penicillin allergy, the treatment becomes more complex, requiring susceptibility testing for clindamycin or defaulting to vancomycin if resistance is present or unknown. Ultimately, the best course of action is determined by a combination of established clinical guidelines, allergy history, susceptibility testing, and individual patient factors.
For further guidance on specific cases, healthcare providers should consult the latest guidelines from the CDC and ACOG and consider infectious disease consultation, especially in the context of increasing resistance to alternative antibiotics.
