Antibiotic Coverage for Streptococcus agalactiae (Group B Streptococcus)
Penicillin G is the definitive first-line antibiotic for Streptococcus agalactiae, with universal susceptibility documented worldwide and no reported resistance. 1, 2, 3
First-Line Treatment
Penicillin G remains the gold standard due to its narrow spectrum of activity, proven efficacy, and complete absence of resistance among all GBS isolates globally. 1, 2, 4
Ampicillin is an acceptable alternative with equivalent efficacy, though it has broader spectrum activity which may promote more collateral resistance. 1, 2, 4
All beta-lactam antibiotics (penicillin, ampicillin, cephalosporins) demonstrate excellent activity against GBS, with 100% susceptibility rates maintained across decades of surveillance. 4, 5, 6
Alternative Regimens for Penicillin Allergy
Low-Risk Penicillin Allergy (No Anaphylaxis History)
Cefazolin is the preferred alternative for patients without history of anaphylaxis, angioedema, respiratory distress, or urticaria to penicillins or cephalosporins. 7, 1, 2
Cross-reactivity between penicillins and cephalosporins occurs in approximately 10% of patients, making risk stratification essential. 2
First-generation cephalosporins maintain near-universal GBS susceptibility, with only 1% resistance documented. 4
High-Risk Penicillin Allergy (Anaphylaxis History)
Clindamycin requires susceptibility testing before use, as resistance rates range from 3-15% and are increasing over time. 7, 1, 2, 5
If the isolate is susceptible to both clindamycin and erythromycin, clindamycin is the preferred agent. 7, 1, 2
D-zone testing must be performed on isolates that are erythromycin-resistant but clindamycin-susceptible to detect inducible clindamycin resistance. 7, 1
Vancomycin should be reserved for cases where the isolate is resistant to clindamycin, susceptibility is unknown, or no other options exist. 7, 1, 2
Erythromycin is no longer recommended due to resistance rates of 7-21% and increasing macrolide resistance trends. 7, 4, 5
Context-Specific Considerations
Pregnancy & Neonatal Prevention
Intrapartum prophylaxis during labor (not antepartum treatment) is the standard approach for GBS-colonized pregnant women, using penicillin G 5 million units IV initially, then 2.5-3.0 million units IV every 4 hours until delivery. 7, 1
Pregnant women with GBS bacteriuria at any concentration during any trimester require both immediate treatment of the UTI and mandatory intrapartum IV prophylaxis during labor, regardless of prior treatment. 7, 1
At least 4 hours of antibiotic administration before delivery achieves 78-89% reduction in early-onset neonatal GBS disease. 1, 2
Severe Infections & Synergy
Combination therapy with ampicillin plus gentamicin is recommended for neonatal sepsis to provide synergistic bactericidal activity and broader coverage. 2, 8, 9
Gentamicin alone is ineffective against GBS (all strains are resistant), but when combined with beta-lactams, it provides enhanced killing against both tolerant and non-tolerant strains. 8, 4
Approximately 22.7% of GBS strains demonstrate tolerance to penicillin/ampicillin alone, making combination therapy valuable in severe infections. 8
Critical Pitfalls to Avoid
Never treat asymptomatic GBS colonization (vaginal or urethral) with oral or IV antibiotics outside of active labor or documented infection—this does not eliminate carriage, promotes resistance, and provides no clinical benefit. 1, 2
Do not assume erythromycin or clindamycin susceptibility without testing in penicillin-allergic patients, as resistance is increasing and can exceed 12% for erythromycin and 7% for clindamycin. 7, 5
Gentamicin monotherapy is never appropriate for GBS infections, as all strains are intrinsically resistant; it must be combined with a beta-lactam for synergistic effect. 8, 4
Cefoxitin resistance has been reported among GBS isolates, so only first-generation cephalosporins (cefazolin) should be used as alternatives. 1