Streptococcus agalactiae (GBS): Introduction, Pathogenicity, Lab Diagnosis, Treatment, and Keynotes

Introduction

Streptococcus agalactiae, also known as Group B Streptococcus (GBS), is a Gram-positive, beta-hemolytic coccus arranged in chains. It is a major cause of neonatal sepsis, meningitis, and infections in pregnant women, and an emerging pathogen in immunocompromised adults.

GBS commonly colonizes the gastrointestinal and genitourinary tracts.

Pathogenicity

Virulence Factors

• Capsular polysaccharide – prevents phagocytosis
• Beta-hemolysin/cytolysin – tissue destruction
• C5a peptidase – inhibits complement activation
• Surface proteins – adherence and invasion
• Hyaluronidase – tissue penetration

Diseases Caused

Neonates

• Early-onset disease (0–6 days): Sepsis, pneumonia, respiratory distress
• Late-onset disease (7–89 days): Meningitis, bacteremia

Pregnant Women

• Urinary tract infections
• Chorioamnionitis
• Postpartum endometritis

Adults (Elderly/Immunocompromised)

• Skin and soft-tissue infections
• Bacteremia
• Osteomyelitis
• Pneumonia

Lab Diagnosis

Specimen

• Vaginal/rectal swab (screening in pregnancy)
• Blood, CSF (neonatal sepsis)
• Urine
• Wound/soft tissue samples

Microscopy

• Gram-positive cocci in chains

• Beta-hemolytic colonies on blood agar (may be narrow)

Culture Characteristics

• Blood agar: Grey-white, narrow beta-hemolysis
• CAMP Test: Positive (arrowhead enhancement with S. aureus)
• Hippurate hydrolysis: Positive
• Latex agglutination: Lancefield Group B antigen detected

Automated/Advanced Methods

• MALDI-TOF MS – rapid identification
• PCR-based screening for pregnant women
• VITEK 2 / BD Phoenix – ID and susceptibility

Treatment

First-Line

• Penicillin G (drug of choice)
• Ampicillin also effective

In Penicillin Allergy

• Cefazolin (if mild allergy)
• Clindamycin or Erythromycin (only if strain is sensitive; resistance common)
• Vancomycin (for severe allergy or resistant strains)

Neonatal Management

• Empirical therapy: Ampicillin + Gentamicin
• Definitive therapy adjusted based on culture results

Keynotes

1. Leading cause of neonatal sepsis and meningitis.
2. Pregnant women must be screened at 35–37 weeks for GBS colonization.
3. CAMP test and hippurate hydrolysis are classic identification tests.
4. Penicillin remains highly effective.
5. Rising antimicrobial resistance is noted for clindamycin and erythromycin.
6. Early detection and intrapartum prophylaxis significantly reduce neonatal disease.

Further Readings

https://www.ncbi.nlm.nih.gov/books/NBK553143/
https://pmc.ncbi.nlm.nih.gov/articles/PMC7520794/
https://www.preprints.org/manuscript/202503.0224/v1/download
https://www.sciencedirect.com/topics/medicine-and-dentistry/streptococcus-agalactiae
https://www.oncotarget.com/article/23551/text/
https://www.mayoclinic.org/diseases-conditions/group-b-strep/diagnosis-treatment/drc-20351735