Group B Streptococcus (GBS, Streptococcus agalactiae) is a beta-haemolytic bacterium that lives harmlessly in the vagina, rectum, or urinary tract of about 10 to 30% of healthy adults. Most carriers have no symptoms. GBS becomes clinically important during pregnancy (where it can cause neonatal sepsis), in urinary tract infections, and as invasive disease in elderly or immunocompromised adults.
Medically reviewed by Dr. Qiao Yufei, MD
This guide covers what GBS is, why it matters in pregnancy, antenatal screening at 35–37 weeks, intrapartum antibiotic prophylaxis, and how testing works at Mediway Medical Centre. GBS sits in our broader STDs in Singapore overview because of its genitourinary location, although it is not strictly a sexually transmitted infection.
Group B Streptococcus (GBS), also called Streptococcus agalactiae, is a Gram-positive, beta-haemolytic bacterium classified into the Lancefield Group B by its cell-wall carbohydrate. The "beta-haemolytic" label refers to the lab pattern of complete red-cell destruction on a blood agar plate, which helps distinguish GBS from other streptococcal species.
GBS is part of the normal genital and gut flora in 10 to 30% of healthy adults. Most colonised people have no symptoms and never need treatment. Per the US Centers for Disease Control and Prevention (CDC), GBS becomes a clinical concern in three specific settings:
"Streptococcus" is a large genus with many species. Common ones include Group A Streptococcus (S. pyogenes: strep throat, scarlet fever), Streptococcus pneumoniae (pneumonia, meningitis), and Group B Streptococcus (S. agalactiae: the genitourinary species this page covers). They are all beta-haemolytic in lab terms but cause different infections at different anatomical sites.
Most people with GBS colonisation never need treatment. The bacterium becomes clinically important in three specific contexts:
The most important setting. Maternal GBS colonisation can pass to the baby during childbirth, causing neonatal sepsis, pneumonia, or meningitis. Antenatal screening at 35–37 weeks plus intrapartum antibiotics reduce this risk by around 80%.
GBS in urine (GBS bacteriuria) causes UTI symptoms (burning urination, frequency, urgency). In pregnancy, GBS bacteriuria signals heavy colonisation and warrants antibiotic treatment plus intrapartum prophylaxis.
Older adults, people with diabetes, and immunocompromised patients can develop serious GBS infections: skin and soft tissue infections, pneumonia, bone and joint infections, sepsis, or meningitis. Treatment is typically penicillin or amoxicillin.
GBS is part of the normal flora in many healthy people. It is not classified as a sexually transmitted infection in the strict sense, but its presence in the genital tract means partner-to-partner sharing can occur. Key transmission and risk routes:
The symptoms depend on the type of infection. Most people with GBS colonisation have no symptoms at all.
GBS can cause skin and soft tissue infections (cellulitis), bone and joint infections, pneumonia, endocarditis, and bloodstream infection (sepsis), particularly in older or immunocompromised adults. These present with the symptoms typical of those infections, plus systemic signs (fever, malaise).
GBS testing is selected based on the clinical setting:
At Mediway Medical Centre, antenatal GBS screening swabs and urine cultures are routinely available. Hospital-based testing is required for blood and CSF cultures.
GBS remains susceptible to penicillin, which is the first-line antibiotic in almost all settings. Treatment depends on the type of infection and the clinical context:
Generally not treated in non-pregnant adults. Asymptomatic GBS in the vagina or rectum is part of the normal flora and antibiotics are not usually indicated.
Treated with IV penicillin or ampicillin, often combined with an aminoglycoside (gentamicin) for severe infections. Course length varies by site (10 to 14 days for sepsis, 4 to 6 weeks for endocarditis or bone/joint infection). These cases are managed in hospital.
GBS colonisation in adults cannot really be prevented (it is normal flora), but neonatal GBS disease is highly preventable through routine antenatal care.
All pregnant women are offered a vaginal-rectal swab at 35–37 weeks of pregnancy. GBS-positive women receive intrapartum antibiotic prophylaxis during labour.
GBS in urine at any time during pregnancy indicates heavy colonisation and is treated as a positive screen: the woman receives treatment for the UTI plus intrapartum antibiotic prophylaxis without further screening.
No GBS vaccine is currently licensed, but several maternal-vaccination candidates are in late-stage clinical trials and may become available within the next few years. A vaccine would simplify prevention by giving immunity rather than relying on intrapartum antibiotics.
Most asymptomatic GBS colonisation is harmless and does not need treatment. Untreated GBS in clinically important settings can cause serious harm:
The combination of routine antenatal screening and intrapartum prophylaxis has dramatically reduced neonatal GBS disease in countries where it is universal. Singapore follows similar protocols in obstetric care.
Group B Streptococcus (GBS, Streptococcus agalactiae) is a beta-haemolytic Gram-positive bacterium that lives harmlessly in the vagina, rectum, or urinary tract of about 10 to 30% of healthy adults. Most carriers have no symptoms. GBS becomes clinically important in three settings: pregnancy (where it can cause neonatal sepsis), urinary tract infections, and invasive disease in elderly or immunocompromised adults.
No, GBS is not classified as a sexually transmitted infection in the strict sense. It is part of the normal genital and gut flora in many healthy people. However, it is included in our STD cluster because it shares the genitourinary anatomical site with other STIs and because of its critical importance in pregnancy and neonatal infection.
Yes. Beta-haemolytic strep in urine (GBS bacteriuria) is treated with antibiotics, typically penicillin, amoxicillin, or cephalexin. Treatment is particularly important in pregnancy because GBS bacteriuria signals heavy colonisation and increases the risk of preterm labour and neonatal infection. Pregnant women with GBS bacteriuria also receive intrapartum antibiotic prophylaxis during labour.
Antenatal GBS screening is performed at 35 to 37 weeks of pregnancy by taking a vaginal-rectal swab and culturing it for GBS. A positive screen identifies women who should receive intrapartum antibiotic prophylaxis (IV penicillin or alternative) during labour to prevent neonatal GBS infection. GBS detected in urine at any time during pregnancy also indicates antibiotic prophylaxis without the need for further screening.
Intrapartum antibiotic prophylaxis is the giving of intravenous antibiotics (typically penicillin G) to GBS-positive mothers during labour, starting at least 4 hours before delivery if possible. IAP reduces neonatal early-onset GBS disease by around 80%. Alternatives such as cefazolin, clindamycin, or vancomycin are used for women with penicillin allergy.
Beta-haemolytic refers to a pattern of complete red blood cell destruction (haemolysis) on a blood agar plate in the lab. Group B Streptococcus is a beta-haemolytic streptococcus (the lab characteristic helps distinguish it from other strep species). Group A Streptococcus (Streptococcus pyogenes, the cause of strep throat) is also beta-haemolytic but causes different infections.
Yes, though much less commonly. GBS can cause urinary tract infections, skin and soft tissue infections, bone and joint infections, and invasive disease (pneumonia, sepsis, meningitis) particularly in older adults, people with diabetes, and those with weakened immunity. Treatment is typically penicillin or amoxicillin, with course length depending on the type of infection.
References: CDC: Group B Strep · CDC: GBS clinical overview · WHO · HealthHub Singapore
This page is for educational purposes only and does not constitute medical advice, diagnosis, or treatment. Consult a qualified healthcare professional for diagnosis and treatment of suspected Group B Streptococcus infection, particularly during pregnancy.