Group B Streptococcus (GBS) is a common bacterium naturally found in many individuals. While often harmless, GBS can sometimes lead to serious infections, particularly in newborns. Understanding how GBS is acquired and transmitted provides important insights into its impact on health.
Understanding Group B Streptococcus
Group B Streptococcus (Streptococcus agalactiae) is a bacterium frequently found in the human digestive tract and, for women, in the lower genital tract. Between 10% and 35% of healthy adults carry GBS, including 10% to 25% of healthy adult women. Many individuals who carry GBS are “colonized,” meaning the bacteria are present without causing symptoms or illness.
An “infection” occurs when the bacteria multiply or spread to areas where they cause illness. Most colonized individuals remain unaware of its presence because they do not experience symptoms. This widespread colonization underscores why GBS is a frequent topic of discussion in public health.
Primary Transmission Routes: Mother to Infant
The primary way GBS is acquired is through vertical transmission from a colonized pregnant person to their baby, typically during childbirth. As the baby passes through the birth canal, they can inhale or ingest GBS bacteria from the mother’s vagina or rectum. The bacteria can also travel upward into the uterus if the membranes rupture.
In rare instances, GBS can cross intact membranes and infect the baby before labor. This direct exposure during birth is the primary reason for early-onset GBS disease in newborns, manifesting within the first week of life, often within 24 hours of birth. Without preventative measures, approximately 50% of babies born to GBS-positive mothers become colonized, and 1% to 2% of these infants develop GBS disease.
Other Ways GBS Can Be Acquired
Beyond mother-to-infant transmission, GBS colonization in non-pregnant adults typically occurs when the bacteria, naturally residing in the gut, spread to the genital or urinary tracts. GBS is not classified as a sexually transmitted infection (STI). It is not generally transmitted through food, water, or casual contact like coughing or sneezing.
While direct physical contact, such as hand-holding or kissing, can transmit GBS, clinical infections from these routes are uncommon. Research suggests a link between specific GBS types and consumption of certain foods, such as fish, or fecal-oral transmission. GBS infections in adults, often originating from their own colonization, are more common in older individuals or those with weakened immune systems and underlying medical conditions.
Factors That Increase GBS Acquisition Risk
Several factors can increase the risk of GBS colonization or transmission to a newborn. For women, the anatomical proximity of the anus to the vagina can contribute to GBS colonization. Underlying health conditions such as diabetes are also associated with a higher prevalence of GBS colonization in pregnant women.
Factors that increase the risk of GBS transmission from a colonized mother to her infant during birth include:
Testing positive for GBS bacteria late in pregnancy.
Premature birth before 37 weeks.
Prolonged rupture of membranes (water breaks 18 or more hours before delivery).
Maternal fever of 100.4°F (38°C) or higher during labor.
History of a previous infant with GBS disease.
GBS detected in the urine during the current pregnancy.