Streptococcus agalactiae (GBS): Introduction, Pathogenicity, Lab

Introduction

Table of Contents

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.

Fig. Streptococcus agalactiae (GBS) growth on blood agar after 48 hours of incubation

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

Gram-positive cocci in chains Streptococcus agalactiae (GBS) — Fig. Gram-positive cocci in chains of Streptococcus agalactiae (GBS)

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

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