What is the recommended antibiotic regimen for Streptococcus agalactiae (group B Streptococcus) pharyngitis, including first‑line therapy, alternatives for penicillin allergy, and treatment of severe or invasive disease?

Treatment of Group B Streptococcal Pharyngitis

For Group B Streptococcus (GBS) pharyngitis—particularly in immunocompromised or chronically ill adults—penicillin G, clindamycin, or erythromycin are the most active agents, with mean minimal inhibitory concentrations (MIC) of 0.06 μg/ml or less, and clindamycin or erythromycin should be considered when oral beta-lactams fail. 1

Critical Distinction: GBS vs. GAS Pharyngitis

• Group B Streptococcus pharyngitis is rare and occurs predominantly in immunocompromised or chronically ill adults, not in the typical healthy patient with acute pharyngitis. 1
• The vast majority of bacterial pharyngitis is caused by Group A Streptococcus (GAS), not Group B Streptococcus. 2, 3
• If you are treating routine streptococcal pharyngitis in an otherwise healthy patient, you are almost certainly treating GAS, not GBS—confirm the organism with throat culture before assuming GBS. 3

First-Line Treatment for Confirmed GBS Pharyngitis

• Penicillin G is the most active agent against GBS with a mean MIC of 0.06 μg/ml or less, making it the theoretical first choice. 1
• However, oral beta-lactams (penicillin V, ampicillin) may be suboptimal for GBS pharyngitis in compromised patients due to higher minimal bactericidal concentrations (MBC) of GBS compared to GAS, inadequate penetration into pharyngeal tissues, or host factors. 1
• When oral penicillin or ampicillin therapy produces a poor response in confirmed GBS pharyngitis, switch to clindamycin or erythromycin, both of which have mean MICs of 0.06 μg/ml or less and may achieve better clinical outcomes. 1

Alternative Regimens for Treatment Failure or Penicillin Allergy

• Clindamycin is highly effective against GBS (mean MIC ≤0.06 μg/ml) and should be used when oral beta-lactams fail or in patients with immediate penicillin allergy. 1
• Erythromycin is equally active against GBS (mean MIC ≤0.06 μg/ml) and represents another alternative for penicillin-allergic patients or treatment failures. 1
• Rifampin and cefaclor are least active against GBS with mean MICs of 0.71 μg/ml or more, making them poor choices for GBS pharyngitis. 1
• Ampicillin has intermediate activity against GBS and may be less reliable than penicillin G, clindamycin, or erythromycin. 1

Dosing Recommendations (Extrapolated from GAS Guidelines)

• For non-immediate penicillin allergy: First-generation cephalosporins such as cephalexin 500 mg orally twice daily for 10 days are safe and effective, with only 0.1% cross-reactivity risk in delayed reactions. 4
• For immediate/anaphylactic penicillin allergy: Clindamycin 300 mg orally three times daily for 10 days is preferred, avoiding all beta-lactams due to up to 10% cross-reactivity risk. 4
• Azithromycin 500 mg once daily for 5 days is an alternative, but macrolide resistance is 5–8% in the United States, making clindamycin more reliable. 4

Treatment Duration

• A full 10-day course is mandatory for all antibiotics except azithromycin to achieve maximal bacterial eradication and prevent potential complications. 4
• Shortening the course by even a few days increases treatment failure rates, even though GBS pharyngitis does not carry the same rheumatic fever risk as GAS. 4

Common Pitfalls to Avoid

• Do not assume all streptococcal pharyngitis is GAS—obtain throat culture to confirm the organism, especially in immunocompromised patients or those with atypical presentations. 1, 3
• Do not continue oral penicillin or ampicillin if the patient shows poor clinical response—switch to clindamycin or erythromycin, which are more reliably active against GBS. 1
• Do not use rifampin or cefaclor as first-line agents for GBS pharyngitis due to inferior in vitro activity (mean MIC ≥0.71 μg/ml). 1
• Do not prescribe cephalosporins to patients with immediate/anaphylactic penicillin reactions due to 10% cross-reactivity risk. 4

Adjunctive Symptomatic Management

• Acetaminophen or NSAIDs (ibuprofen) should be offered for moderate to severe symptoms or high fever, with strong evidence for pain and inflammation reduction. 4
• Avoid aspirin in children due to the risk of Reye syndrome. 4
• Corticosteroids are not recommended as adjunctive therapy. 4

When to Suspect GBS Pharyngitis

• Consider GBS pharyngitis in immunocompromised adults, patients with underlying chronic diseases, or those who fail standard GAS therapy and have culture-confirmed GBS. 1
• GBS pharyngitis is infrequently recognized and should prompt evaluation for underlying immunocompromise or chronic illness. 1