Group B Streptococcus (GBS), known scientifically as Streptococcus agalactiae, is a common bacterium that lives naturally in the human body, often without causing illness. Approximately one in five pregnant women worldwide carry this microbe, making it a common finding during routine screenings. The primary health concern associated with GBS is the risk of transmission to a newborn during labor and delivery, which can lead to serious neonatal infections. Management focuses on preventing disease transmission rather than achieving permanent eradication of the organism.
Understanding Group B Strep Colonization
GBS is difficult to permanently remove because it is a commensal organism, a natural resident of the body’s microbial community. It primarily colonizes the gastrointestinal tract and can spread to the genitourinary tract. This state, called colonization or carriage, is distinct from an active infection because it causes no symptoms in healthy adults.
Colonization is often transient or intermittent, meaning the bacteria’s presence can fluctuate. Since the gastrointestinal tract acts as a persistent reservoir, temporary courses of oral antibiotics are generally ineffective. These treatments fail to eliminate the bacteria from the bowel, leading to the rapid re-establishment of GBS. Therefore, the goal is to manage the risk to newborns, not to eradicate the harmless carrier state.
Screening and Prevention During Pregnancy
The most important strategy for managing GBS colonization is the rigorous screening and prevention protocol used during pregnancy to protect the infant. Current guidelines recommend that all pregnant individuals undergo screening for GBS colonization late in the third trimester, specifically between 36 weeks and 37 weeks and six days of gestation. This involves a simple swab of the lower vagina and the rectum, with samples cultured in a laboratory.
The timing of this screening is crucial because it aims to predict the colonization status close to the time of delivery, which is the period of highest transmission risk. A positive culture result means the individual is colonized and is considered at risk for passing the bacteria to the baby during birth. However, a positive screen does not mean the mother has an active infection or that the baby will definitely become ill.
For those who screen positive, the preventative strategy is Intrapartum Antibiotic Prophylaxis (IAP), which involves administering intravenous (IV) antibiotics during labor. Penicillin is the preferred antibiotic because GBS is highly susceptible to it, though alternatives are used for those with allergies. IAP is timed to ensure high drug levels are present in the bloodstream and surrounding tissues during delivery.
IAP is highly effective, reducing the bacterial load in the genital tract and preventing mother-to-newborn transmission. This preventative measure is credited with dramatically reducing the incidence of early-onset neonatal GBS disease. Antibiotics are not given before labor because they would fail to permanently clear colonization and increase the risk of antibiotic resistance.
Indications for Intrapartum Antibiotic Prophylaxis (IAP)
IAP is indicated if the pregnant individual:
- Screens positive for GBS colonization.
- Had GBS detected in their urine during the current pregnancy.
- Previously delivered an infant with invasive GBS disease.
IAP is also given if the GBS status is unknown and any of the following risk factors are present:
- Labor starts before 37 weeks of gestation.
- Rupture of membranes lasts 18 hours or longer.
- Maternal fever occurs during labor.
Treating Active GBS Infections
While colonization is managed with prevention, active GBS infections, which are serious, require definitive treatment with antibiotics. GBS can cause invasive disease in newborns and non-pregnant adults, especially those with underlying health conditions. It is a leading cause of sepsis and meningitis in newborns.
If a newborn develops an infection, treatment is aggressive and prompt, consisting of high-dose intravenous antibiotics, typically penicillin or ampicillin, sometimes combined with an aminoglycoside like gentamicin. For meningitis cases, the IV antibiotic course may be extended to 14 days to ensure the infection is fully cleared.
In non-pregnant adults, GBS can cause invasive infections, including bloodstream, soft tissue, and urinary tract infections. These active disease states are treated with a specific, often prolonged, course of systemic antibiotics, such as penicillin or ampicillin. For deep-seated infections involving bone or extensive soft tissue, antibiotic therapy may be paired with surgical intervention.