Streptococcus agalactiae
One-Sentence Definition
Streptococcus agalactiae (group B Streptococcus, GBS) is a Gram-positive coccus that colonizes the genital/GI tract and is the classic cause of early- and late-onset neonatal sepsis and meningitis, also causing disease in adults with comorbidity.
Taxonomy & Morphology
- Gram-positive cocci in chains; Lancefield group B
- β-hemolytic (usually); CAMP-test positive historically
- Capsular serotypes Ia, Ib, II–IX — vaccine development targets
Virulence Highlights
- Polysaccharide Capsule — antiphagocytic
- β-hemolysin/cytolysin; pili for adhesion
- Sialylated capsule mimics host glycans
Clinical Syndromes
- Early-onset neonatal disease (≤7 days): pneumonia/sepsis from peripartum transmission
- Late-onset (7–89 days): bacteremia/meningitis
- Adults: bacteremia, soft-tissue infection, UTI, endocarditis (diabetes, elderly)
Diagnosis
- Maternal rectovaginal screening culture / NAAT (prevention programs)
- Blood/CSF culture in neonates; MALDI for ID
- Serotyping / WGS in surveillance
Treatment Notes (conceptual)
- Intrapartum antibiotic prophylaxis (IAP) when indicated — usually penicillin/ampicillin
- Invasive disease: penicillin G (± aminoglycoside early for synergy in severe cases — protocol-dependent)
- Clindamycin only if susceptibility confirmed (inducible MLS resistance)
AMR
- Penicillin generally reliable; rising macrolide/clindamycin resistance matters for IAP alternatives
- Monitor via AST when using non-β-lactam prophylaxis
Genomics & Computational Notes
- Capsule switching and CC17 hypervirulent neonatal lineage tracked by WGS
- Useful links: Population Structure and Clustering · Vaccination (maternal GBS vaccine pipeline)
Related Notes
- Bacterial Meningitis · Streptococcus pyogenes · Streptococcus pneumoniae · Capsule
- MOCs: MOC - Bacteriology · MOC - Public Health & Epidemiology · MOC - Diseases by System
Active Recall
- Early-onset vs late-onset GBS — transmission difference?
- Why confirm clindamycin susceptibility before using it for IAP?
- What is the purpose of late-pregnancy GBS screening?