Understanding What Antibiotics are Susceptible to Group B Strep

October 12, 2025 •

4 min read

According to the CDC, intrapartum antibiotic prophylaxis has dramatically reduced the incidence of early-onset neonatal Group B Strep (GBS) disease. However, understanding exactly what antibiotics are susceptible to group B strep is crucial, especially as resistance patterns change over time.

Quick Summary

This article details the antibiotics used to treat Group B Strep (GBS), starting with the universally effective first-line options. It explores alternative treatments for patients with penicillin allergies and highlights the growing issue of antibiotic resistance in GBS strains, particularly among macrolides and lincosamides. The importance of susceptibility testing for proper treatment selection is also addressed.

Key Points

Penicillin is First-Line: Penicillin and ampicillin are the most effective and widely used antibiotics for treating GBS, with very low resistance rates.
Alternatives Exist for Allergies: For patients with penicillin allergies, the alternative antibiotic depends on the allergy's severity, with cefazolin for non-severe cases and clindamycin or vancomycin for severe ones.
Clindamycin Resistance is Rising: Resistance to clindamycin is increasing globally, making susceptibility testing of the GBS isolate mandatory for patients with a severe penicillin allergy.
Erythromycin is Often Ineffective: High rates of resistance have made macrolide antibiotics like erythromycin unreliable for GBS treatment, especially as an empirical choice.
Susceptibility Testing is Crucial: For penicillin-allergic patients, laboratory testing of the GBS isolate is key to selecting an appropriate and effective alternative antibiotic.
Vancomycin for High-Risk Cases: Vancomycin is the fallback option for patients with severe penicillin allergies when GBS isolates show resistance to clindamycin.

First-Line Antibiotics: Penicillin and Ampicillin

For decades, penicillin has remained the cornerstone of therapy for Group B Streptococcus (GBS) infections due to its consistent efficacy and targeted action. Penicillin and its derivative, ampicillin, are beta-lactam antibiotics that work by disrupting the bacterial cell wall synthesis, leading to cell death.

Penicillin G: This is the preferred agent for intrapartum antibiotic prophylaxis (IAP) in pregnant women who test positive for GBS and do not have a history of severe penicillin allergy. It is typically administered intravenously during labor to reduce the risk of transmission to the newborn.
Ampicillin: An acceptable alternative to penicillin, ampicillin can also be used for IAP and demonstrates universal susceptibility against GBS. However, it is considered less preferred than penicillin due to its broader antimicrobial activity.

Alternative Antibiotics for Penicillin Allergies

When a patient has a history of penicillin allergy, the treatment approach depends on the severity of the allergic reaction. The use of alternative antibiotics is guided by specific clinical recommendations and, ideally, laboratory testing of the GBS isolate.

Cephalosporins (for non-severe allergies)

For individuals with a documented penicillin allergy that is not severe (e.g., no anaphylaxis, angioedema, or respiratory distress), a first-generation cephalosporin like cefazolin is the recommended alternative. Cefazolin is a beta-lactam antibiotic that offers a high degree of GBS susceptibility while carrying a low risk of cross-reactivity with penicillin.

Clindamycin and Vancomycin (for severe allergies)

For patients with a severe, IgE-mediated penicillin allergy, clindamycin or vancomycin are the primary alternatives. However, their use requires careful consideration due to rising resistance rates.

Clindamycin: This antibiotic is recommended only if the GBS isolate is known to be susceptible to it. Susceptibility testing is crucial because resistance to clindamycin among GBS strains is increasingly common and varies by geographic region. Clinicians must be aware of inducible resistance mechanisms that can be detected via a D-test in a lab setting.
Vancomycin: This is the antibiotic of choice for women with a severe penicillin allergy whose GBS isolate is resistant to clindamycin or if susceptibility results are unavailable. It is a potent antibiotic generally reserved for serious infections or when other options are not suitable.

Rising Antibiotic Resistance in Group B Strep

While GBS remains universally susceptible to penicillin and ampicillin in most regions, the landscape of resistance to alternative agents is a growing concern. Numerous studies and meta-analyses have documented increasing resistance over time, particularly for macrolides and lincosamides.

Macrolides (like Erythromycin): Resistance to erythromycin is widespread, making it an unreliable empirical choice for prophylaxis. Resistance mechanisms often involve genetic modifications that confer resistance to multiple drug classes.
Lincosamides (like Clindamycin): Resistance to clindamycin is also increasing, which complicates the selection of an appropriate alternative for penicillin-allergic patients. In many areas, the resistance rate is high enough that clindamycin cannot be relied upon without prior susceptibility testing.
Tetracycline: Resistance to tetracycline is extremely high in GBS and this class of antibiotics is not used for treatment.

Comparison of Antibiotics for GBS


Antibiotic Class	Examples	Typical Use Case	Susceptibility to GBS	Resistance Concerns	Allergy Considerations
Beta-Lactams (Penicillins)	Penicillin G, Ampicillin	First-line treatment and prophylaxis, especially in pregnant women.	High, near universal.	Historically low, but rare cases of reduced susceptibility reported.	Requires alternatives for individuals with penicillin allergies.
Beta-Lactams (Cephalosporins)	Cefazolin, Ceftriaxone	Alternative for non-severe penicillin allergies.	High.	Low for GBS.	Typically safe for non-severe penicillin allergies; low risk of cross-reactivity.
Lincosamides	Clindamycin	Alternative for severe penicillin allergies, if susceptible.	Varies widely by region; decreasing over time.	Increasing global resistance, requires susceptibility testing.	Used for severe allergies, but resistance is a major limitation.
Glycopeptides	Vancomycin	Alternative for severe penicillin allergies and clindamycin resistance.	High.	Historically rare, but surveillance is ongoing.	Used for severe allergies, particularly when clindamycin resistance is present.
Macrolides	Erythromycin, Azithromycin	No longer recommended empirically due to high resistance rates.	Varies widely by region; low and decreasing.	High and increasing resistance.	Should not be used without confirmed susceptibility due to resistance risk.

The Critical Role of Susceptibility Testing

Given the rise in GBS resistance to macrolides and lincosamides, susceptibility testing is more important than ever for selecting an appropriate antibiotic. Testing is particularly vital for pregnant women who have a penicillin allergy. The Centers for Disease Control and Prevention (CDC) and other health organizations recommend testing the GBS isolate for susceptibility to clindamycin and erythromycin in such cases.

Susceptibility tests, such as the Kirby-Bauer disk diffusion method or the D-test to detect inducible resistance, provide clinicians with the data needed to make informed decisions and ensure that an effective antibiotic is selected. This targeted approach helps to preserve the effectiveness of antibiotics and improves patient outcomes.

Conclusion

While penicillin and ampicillin remain the gold standard and most consistently effective treatment for GBS infections, the evolving landscape of antibiotic resistance necessitates a careful approach to alternative therapies. For patients with penicillin allergies, selecting the correct antibiotic requires distinguishing between non-severe and severe reactions. First-generation cephalosporins like cefazolin are generally safe and effective for non-severe allergies. However, for severe allergies, the choice between clindamycin and vancomycin must be guided by recent susceptibility testing due to prevalent and rising resistance to clindamycin. Continued surveillance of antibiotic resistance patterns is essential to ensure that preventative and treatment strategies for Group B Strep remain effective in protecting vulnerable populations, especially newborns.

For more information on GBS prevention and guidelines, consult the Centers for Disease Control and Prevention (CDC) at https://www.cdc.gov/group-b-strep/.

Frequently Asked Questions

Penicillin is considered the most reliable antibiotic for treating Group B Strep (GBS) due to its long history of effectiveness and the consistently low rates of resistance observed in GBS strains.

For a pregnant woman with a penicillin allergy, the choice of antibiotic depends on the severity of her allergic reaction. For non-severe allergies, a cephalosporin like cefazolin is used. For severe allergies, clindamycin or vancomycin are options, but susceptibility testing on the GBS isolate is necessary.

While resistance to first-line antibiotics like penicillin is very rare, resistance to alternative antibiotics, particularly macrolides (e.g., erythromycin) and lincosamides (e.g., clindamycin), is common and continues to rise.

Susceptibility testing is necessary for clindamycin because of the high and variable rates of resistance observed in Group B Strep. Relying on it empirically without testing risks treatment failure, especially in patients with severe penicillin allergies.

Yes, vancomycin is highly effective against GBS and is used for patients with severe penicillin allergies when clindamycin resistance is present or unknown.

Intravenous (IV) antibiotics during labor are most effective for preventing transmission to the baby. However, for a GBS urinary tract infection found during pregnancy, oral antibiotics like penicillin or amoxicillin may be started immediately, with IV antibiotics still administered during labor.

Research into alternative therapies, such as GBS vaccines, has shown some promise, but more studies are needed to determine their effectiveness and safety. Probiotics have been studied but did not effectively eliminate maternal GBS colonization.