Group B Strep
Summary
Group B Streptococcus (Streptococcus agalactiae) is a gram-positive, catalase-negative, beta-hemolytic diplococcus that is bacitracin-resistant and hippurate-hydrolysis positive. It colonizes the vaginal/rectal tract of ~10-30% of pregnant women and is a leading cause of neonatal sepsis, pneumonia, and meningitis. Universal screening at 35-37 weeks gestation with intrapartum antibiotic prophylaxis (typically penicillin) prevents vertical transmission.
Detail
Streptococcus agalactiae is classified by the Lancefield grouping system as Group B Strep (GBS) based on its cell wall polysaccharide antigen. Microbiologically, it is a gram-positive coccus arranged in chains, catalase-negative (distinguishing it from Staphylococcus), and beta-hemolytic on blood agar. Key lab identifiers include resistance to bacitracin (differentiating it from Group A Strep, which is bacitracin-sensitive) and a positive hippurate hydrolysis test and CAMP test (enhanced hemolysis when grown near Staphylococcus aureus).
Clinical significance centers on perinatal infection. GBS colonizes the maternal genitourinary and gastrointestinal tract asymptomatically in a significant portion of women. Vertical transmission during labor and delivery can cause early-onset neonatal disease (within first week of life), presenting as sepsis, pneumonia, or meningitis, particularly in premature or low-birth-weight infants. Late-onset disease (1 week to 3 months) often presents as bacteremia or meningitis and may be acquired postnatally.
Risk factors for neonatal GBS disease include maternal GBS colonization, preterm labor, prolonged rupture of membranes (>18 hours), maternal fever, and previous infant with GBS disease. Prevention is a major focus of obstetric care: universal vaginal-rectal swab screening for GBS colonization is performed at 35-37 weeks gestation. Women who test positive, have unknown status with risk factors, or have GBS bacteriuria during pregnancy receive intrapartum antibiotic prophylaxis (IAP), with penicillin G as first-line, ampicillin as an alternative, and cefazolin or vancomycin used in penicillin-allergic patients depending on allergy severity and resistance patterns.
Beyond neonates, GBS can also cause invasive disease in adults, particularly the elderly, diabetics, and immunocompromised patients, manifesting as skin/soft tissue infections, urinary tract infections, bacteremia, and endocarditis.
Treatment for confirmed GBS infection is penicillin or ampicillin, as GBS remains highly susceptible to beta-lactams, unlike some other streptococcal species that show increasing resistance.
Sources
- First Aid for the USMLE Step 1
- Sherris Medical Microbiology
- CDC Guidelines for Prevention of Perinatal Group B Streptococcal Disease
- Harrison's Principles of Internal Medicine
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