How People Become Group B Streptococcus (GBS) Positive
Group B Streptococcus colonization occurs when the bacteria naturally inhabit the gastrointestinal tract and subsequently spread to colonize the vaginal and rectal areas in approximately 10-30% of pregnant women, representing asymptomatic carriage rather than active infection. 1, 2
Primary Mechanism of Colonization
The gastrointestinal tract serves as the natural reservoir and primary source for GBS, from which the bacteria then colonize the vaginal tract 2. This is not an "infection" in the traditional sense but rather asymptomatic colonization where GBS exists as a member of the normal gastrointestinal and/or vaginal flora 3.
Colonization Patterns
GBS colonization can manifest in three distinct patterns 4:
- Transient colonization: Temporary presence that resolves spontaneously
- Intermittent colonization: Comes and goes over time
- Persistent colonization: Continuous presence
This variability explains why screening is performed at 36 0/7 to 37 6/7 weeks of gestation rather than earlier in pregnancy, as colonization status can change 5.
Transmission and Spread
Approximately 50% of women colonized with GBS will transmit the bacteria to their newborns during labor or after rupture of membranes, though vertical transmission is the primary concern rather than how adults acquire colonization 5. The bacteria ascend from the vagina to the amniotic fluid after onset of labor or membrane rupture 1.
Risk Factors for Heavy Colonization
GBS bacteriuria during pregnancy (occurring in 2-7% of pregnancies) serves as a marker for heavy genital tract colonization and indicates substantial bacterial load 1, 2. Women with heavy colonization have higher risk for transmitting GBS to their infants 1.
Important Clinical Distinctions
GBS colonization is asymptomatic in the vast majority of carriers - approximately 18% of women globally carry GBS without any symptoms 3. The bacteria only become pathogenic when they establish in vulnerable host niches beyond the lower genital tract 3.
Who Gets Screened
Universal screening between 36 0/7 and 37 6/7 weeks of gestation is recommended for all pregnant women using vaginal-rectal cultures, as this timing best predicts colonization status at delivery 5. The exception is women with GBS bacteriuria detected at any point during pregnancy, who do not require screening since they automatically qualify for intrapartum antibiotic prophylaxis 2.
Key Clinical Pitfall
Do not confuse GBS colonization with GBS infection - colonization represents asymptomatic carriage in the gastrointestinal and vaginal tracts, while infection occurs when GBS invades normally sterile sites causing urinary tract infections, chorioamnionitis, or neonatal sepsis 2, 6. Most colonized women never develop infection and remain asymptomatic carriers 3.