Overview
Streptococcus agalactiae — better known as group B strep (GBS) — is a bacterium that quietly lives in the gut and genital tract of many healthy adults. For most people it is simply part of the microbiome and causes no symptoms at all. Its reputation comes from one critical exception: GBS can pass from mother to baby during birth, and it remains the most common cause of early-onset sepsis in newborns.[1]
A global systematic review of nearly 300,000 pregnant women estimated that 18% carry GBS, with regional variation from about 11% to 35%.[2] Because carriage is so common and usually silent, pregnancy care in the United States relies on universal screening rather than symptoms.
GBS is also an increasingly recognized cause of invasive disease in non-pregnant adults, particularly older adults and people with chronic illness.[3]
Classification
- Phylum: Bacillota (formerly Firmicutes)
- Family: Streptococcaceae
- Genus: Streptococcus
- Species: S. agalactiae
"Group B" refers to the Lancefield classification, which sorts streptococci by a carbohydrate antigen on their cell surface. GBS is distinct from group A strep (Streptococcus pyogenes), which causes strep throat. GBS strains are further divided into serotypes by their capsule. Serotypes I–V account for about 98% of colonizing strains worldwide, and serotype III, which is strongly associated with invasive disease, makes up roughly a quarter.[2]
Where GBS Lives
GBS colonizes both the gastrointestinal and genitourinary tracts, and colonization of these sites is the primary risk factor for newborn disease.[4] The intestine acts as a reservoir from which the vaginal tract can be recolonized, which is why screening uses a combined vaginal-rectal swab rather than a vaginal sample alone.
Colonization is often intermittent, coming and going over weeks to months. That variability is why testing is timed close to delivery, when the result best reflects a woman's status during labor.
GBS in Pregnancy and Newborns
About half of colonized women pass GBS to their newborns, usually during labor or after the membranes rupture. Without preventive antibiotics, 1–2% of those exposed newborns develop early-onset GBS disease. Additional risk factors include birth before 37 weeks, very low birth weight, prolonged rupture of membranes, and intra-amniotic infection.[4]
Screening and prevention
The American College of Obstetricians and Gynecologists recommends universal GBS screening between 36 0/7 and 37 6/7 weeks of gestation. Women who test positive should receive intrapartum antibiotic prophylaxis unless a prelabor cesarean birth is performed with intact membranes.[4] The American Academy of Pediatrics joins this recommendation, noting that intrapartum prophylaxis is the only currently available effective strategy for preventing early-onset disease. There is currently no effective way to prevent late-onset disease, which occurs after the first week of life.[1]
Penicillin G remains the mainstay of treatment. However, resistance to alternative non-beta-lactam antibiotics used in penicillin-allergic patients, including clindamycin and erythromycin, has been increasing, which is one reason allergy status is assessed carefully before delivery.[3]
Can Probiotics Reduce GBS Colonization?
Because GBS shares its niche with the vaginal Lactobacillus community, researchers have asked whether probiotics could reduce colonization before delivery. The evidence so far is mixed.
| Trial | Probiotic | Result |
|---|---|---|
| Ho et al., 2016 (n=99) | Oral L. rhamnosus GR-1 + L. reuteri RC-14 | 42.9% of women converted from GBS-positive to negative vs 18.0% on placebo (p=0.007)[5] |
| Farr et al., 2020 (n=82 randomized) | Oral four-strain Lactobacillus supplement, twice daily for 14 days | No significant difference: 63.6% vs 77.8% remained GBS-positive (p=0.24)[6] |
The strains in the positive trial, Lactobacillus rhamnosus GR-1 and Lactobacillus reuteri RC-14, are the same pair with the strongest evidence for vaginal microbiome support in bacterial vaginosis. Still, two small trials with conflicting results are not enough to change practice. Probiotics are not a substitute for screening or intrapartum antibiotics. If you are interested in probiotics during pregnancy, discuss them with your provider and see our pregnancy and infant microbiome protocol.
GBS in Adults
Most colonized adults never develop illness. However, invasive GBS disease in non-pregnant adults has been increasing,[3] and it can present as bloodstream infection (sepsis), pneumonia, skin and soft-tissue infection, bone and joint infection, or urinary tract infection. Older age and chronic conditions such as diabetes raise the risk.
When to See a Clinician
During pregnancy, follow your provider's screening schedule and tell your care team if you go into labor early or your water breaks before you have a result. In newborns, seek urgent care for fever or low temperature, poor feeding, unusual sleepiness, irritability, or breathing difficulty, particularly in the first weeks of life. Adults with fever, spreading skin redness, or signs of a urinary or chest infection should also be evaluated.
This page is for educational purposes and is not medical advice. Follow your obstetric and pediatric providers' guidance on GBS screening and prevention.