Why Streptococcus agalactiae is Called Group B Streptococcus
Streptococcus agalactiae is called Group B Streptococcus (GBS) because it possesses the Lancefield Group B carbohydrate antigen on its cell wall, which is a specific peptidoglycan-anchored polysaccharide that defines this species taxonomically. 1
The Lancefield Classification System
The Lancefield classification system, developed by Rebecca Lancefield in 1930, categorizes beta-hemolytic streptococci based on the carbohydrate antigens present in their cell walls. This system uses letters (A, B, C, etc.) to distinguish between different streptococcal species:
- Group A: Streptococcus pyogenes
- Group B: Streptococcus agalactiae
- Other groups: Various other streptococcal species
The Group B Carbohydrate (GBC)
The Group B carbohydrate is a complex cell wall component with several important characteristics:
- It is a peptidoglycan-anchored polysaccharide 1
- It functions similarly to teichoic acids found in other gram-positive bacteria 1
- It plays an essential role in cell wall biogenesis 1
- It is the major teichoic acid-like polymer in S. agalactiae's cell wall 2
Functional Importance of the Group B Carbohydrate
Research has demonstrated that the Group B carbohydrate is not merely a taxonomic marker but serves critical functions:
- Essential for normal cell growth and division 1
- Required for proper peptidoglycan synthesis 1
- Involved in proper cell separation during division 1
- Necessary for maintaining normal cell morphology 1
When the GBC is absent (through genetic manipulation of the gbcO gene that initiates its synthesis), S. agalactiae exhibits:
- Significant growth defects
- Abnormal cell division
- Defective cell separation
- Aberrant cell morphology
- Inability to properly synthesize peptidoglycan 1
Clinical Significance of Group B Streptococcus
Understanding the classification of S. agalactiae as Group B Streptococcus has important clinical implications:
GBS is a leading cause of neonatal sepsis, pneumonia, and meningitis 3
It colonizes approximately 25% of women's vaginal tracts 4
It causes invasive disease primarily in:
Early-onset GBS disease has declined from 1.7 cases per 1,000 live births in the early 1990s to 0.34-0.37 cases per 1,000 live births in recent years due to prevention efforts 3
The classification of S. agalactiae as Group B Streptococcus allows for rapid identification in clinical settings, which is crucial for appropriate management of infections, especially in pregnant women and newborns.