The Standard of Care for GBS: Penicillin First
The established guidelines for treating Group B Streptococcus (GBS) infections, especially for preventing early-onset neonatal disease, prioritize beta-lactam antibiotics. Penicillin G is the first-line agent, and ampicillin is a common alternative, due to GBS's historically consistent susceptibility to these drugs. These antibiotics work by disrupting the bacteria's cell wall, leading to bacterial death.
The administration of these antibiotics is critical for intrapartum prophylaxis, where IV antibiotics are given during labor to colonized mothers. This timing is essential because the bacteria can regrow rapidly, and treatment must occur during labor to be effective.
The Problem with Azithromycin and Rising Resistance
For a patient with a penicillin allergy, alternatives must be considered. In the past, macrolide antibiotics like erythromycin and azithromycin were used. However, their reliability has significantly declined due to widespread and increasing resistance.
Mechanisms of Resistance
Resistance to azithromycin and other macrolides in GBS is a significant concern. The primary mechanisms include modification of the ribosomal target site, often mediated by erm genes, and the presence of efflux pumps, like those encoded by mef(A), which actively remove the antibiotic from the bacterial cell. Mutations in ribosomal RNA can also contribute to resistance. Global resistance rates to azithromycin can be high, impacting its effectiveness.
When Might Azithromycin Be Considered?
Azithromycin's use for GBS is limited and typically reserved for specific, monitored situations. It may be considered in combination therapy in certain obstetric scenarios, such as preterm prelabor rupture of membranes (PPROM). Research also explores its potential immunomodulatory effects in severe GBS sepsis, but this is not standard clinical practice.
A Comparison of GBS Treatment Options
The following table compares the main antibiotic classes used for GBS infections.
| Antibiotic Class | Mechanism of Action | Common Examples | GBS Resistance Rates | Role in GBS Treatment |
|---|---|---|---|---|
| Beta-Lactams | Inhibits bacterial cell wall synthesis | Penicillin G, Ampicillin | Consistently low (<5%) | First-line therapy. High efficacy. Standard for intrapartum prophylaxis. |
| Macrolides | Inhibits bacterial protein synthesis by binding to the 50S ribosomal subunit | Azithromycin, Erythromycin | Variable and often high (>30%) | Limited or unreliable use. Not a first-line agent. May be used in specific, combination therapies in some obstetric cases. |
| Lincosamides | Inhibits bacterial protein synthesis by binding to the 50S ribosomal subunit | Clindamycin | Variable and increasing (up to 40%) | Alternative for penicillin-allergic patients, but requires susceptibility testing due to common resistance. |
| Glycopeptides | Inhibits bacterial cell wall synthesis | Vancomycin | Very low (<5%) | Alternative for severe penicillin allergy or clindamycin-resistant strains. Typically reserved for high-risk situations. |
The Role of Susceptibility Testing
Due to increasing resistance in macrolides and lincosamides, susceptibility testing is vital for penicillin-allergic patients. Guidelines recommend testing for clindamycin susceptibility for pregnant women with a severe penicillin allergy who are GBS-positive. If susceptible, clindamycin is used; if resistant, vancomycin is the recommended alternative. This approach ensures effective treatment.
Conclusion: Azithromycin's Declining Role
Azithromycin is generally not a reliable primary treatment for GBS due to high and increasing resistance rates. Penicillin and ampicillin are the preferred treatments. For penicillin-allergic patients, clindamycin or vancomycin are alternatives, but susceptibility testing is essential. The challenge of antibiotic resistance underscores the need for ongoing surveillance and new prevention methods like vaccines.
For more information on the evolving landscape of antibiotic resistance, see the {Link: CIDRAP website https://www.cidrap.umn.edu/antimicrobial-stewardship/data-reveal-high-global-variability-antibiotic-resistance-group-b-strep}.
