1. Overview
Group B Streptococcus (GBS), scientifically known as Streptococcus agalactiae, is a Gram-positive bacterium that commonly colonizes the gastrointestinal tract, urinary tract, and genital area of adults. While GBS rarely causes illness in healthy adults, it is a leading cause of severe neonatal infections, including pneumonia, meningitis, and sepsis, and is associated with significant neonatal morbidity and mortality.
In women, GBS colonization is most frequently found in the vagina and rectum. During pregnancy, colonized women can transmit the bacteria to their infants during labor and delivery, making routine screening an essential component of antenatal care.
2. Need for Screening
GBS screening is performed primarily to prevent vertical transmission of the organism from mother to newborn. A pregnant woman colonized with GBS may appear asymptomatic, yet transmission during childbirth can lead to early-onset neonatal disease, which may be life-threatening.
Routine screening allows early identification of colonized mothers so that appropriate intrapartum antibiotic prophylaxis (IAP) can be administered. In addition to maternal screening, testing may also be performed in newborns showing signs of infection.
3. When to Get Tested and Sample Collection
Screening is recommended when a pregnant woman is between 35 and 37 weeks of gestation. Testing is also indicated in cases of preterm labor or preterm premature rupture of membranes.
Specimen collection involves a single swab obtained from both the vagina and rectum of the pregnant woman. A urine sample collected at any time during pregnancy may also be used to detect significant GBS bacteriuria, which indicates heavy colonization.
After collection, swabs should be immediately placed into Amies transport medium or an equivalent medium and transported to the laboratory within 24 hours. If processing is delayed, specimens should be refrigerated at 4–8°C to preserve bacterial viability.
4. Culture and Identification
For optimal detection, vaginal–rectal swabs are inoculated into a selective enrichment broth, typically Todd-Hewitt broth, and incubated for 18–24 hours at 35–37°C under ambient or 5% CO₂ conditions.
Following enrichment, the broth culture is plated onto blood agar or Columbia agar with colistin and nalidixic acid.
GBS colonies typically appear as large (>0.5 mm), grey to white, translucent colonies, with a narrow zone of beta-hemolysis or sometimes non-hemolytic growth after 24 hours.
Identification of isolates may be performed using several acceptable phenotypic and automated methods, including:
- Catalase test
- Modified CAMP test
- Latex agglutination
- Pyrrolidonyl arylamidase (PYR) test
- Automated systems such as MALDI-TOF or Vitek 2 Compact
5. Reporting and Result Interpretation
If no GBS colonies are observed after 24 hours, cultures are incubated further and re-examined at 48 hours. If still negative, the sample is reported as GBS-negative.
Presumptive identification of GBS is based on catalase-negative isolates that produce a positive CAMP reaction.
A positive screening result indicates that GBS is currently present in the vagina and/or rectum and that the mother is at risk of transmitting the organism to her infant during delivery. Importantly, colonization does not indicate active disease.
6. Prevention and Clinical Management
According to clinical recommendations highlighted in the presentation, antibiotic therapy is administered during labor, not before labor, in women who test positive for GBS.
The administration of intrapartum antibiotic prophylaxis significantly reduces neonatal exposure by lowering the maternal vaginal bacterial load at the time of delivery. It also provides passive protection to the newborn by ensuring effective antibiotic levels in the amniotic fluid and neonatal circulation during early life.
Penicillin is the first-line agent, with alternatives selected based on allergy status and susceptibility testing when required.
7. Importance of Screening Programs
Universal screening between 35–37 weeks of gestation, combined with timely intrapartum antibiotic prophylaxis, has been shown to markedly reduce early-onset GBS disease in newborns. Proper specimen collection, culture techniques, accurate reporting, and adherence to treatment protocols are critical components of an effective GBS prevention strategy.
Overall, GBS screening represents a simple, cost-effective, and life-saving intervention in prenatal care, protecting both maternal and neonatal health.
