Streptococcus agalactiae (Group B Strep)

last authored: Dec 2011, Lorraine Rutter
last reviewed: Dec 2011, Dianna Verhulst




Streptococcus agalactiae, also known as group B Streptococcus (GBS), is a bacteria that commonly is found in the lower gastrointestinal and female genital tracts. Asymptomatic colonization is as high as 50% (Schuchat, 1998). GBS typically is harmless in healthy adults, but is very pathogenic in newborns, potentially causing pneumonia, sepsis, or meningitis. Neonatal (newborn) infections caused by GBS are rare but more common in the setting of prematurity and prolonged rupture of the membranes. GBS is also a known cause of postpartum infection and can also affect adults whose immune systems are compromised by age or chronic disease.


Given the severity of neonatal infections, pregnant mothers are advised to be screened for GBS infections. Mothers who are positive, whose status is not known, or who have other risk factors are usually treated with intrapartum antiobiotics to reduce the risk of infant infection.

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The Case of Ruth W

Ruth is a 34 year old woman pregnant for the first time. Her friend's infant contracted GBS infection, and Ruth has some questions about this.

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Up to 50% of healthy American adults carry GBS in their lower intestinal tract (Schuchat, 1998), and vaginal colonization rates range from 12–27% of women in north Africa, India, the Middle East, Pakistan, Thailand, Saudi Arabia and the United States (WHO, 2005). GBS can also live in the mammary glands of ruminants such as cows, causing mastitis and affecting milk quality (Keefe, 1997).

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Classification and Characteristics

GBS are gram positive cocci forming diplococci or chains. They are beta-hemolytic on blood agar plates.

The main virulence factor of GBS is its polysaccharide capsule, which prevents it from being opsonized and phagocytosed.

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Transmission and Infection

One of the main sources of infection is during delivery; transmission rates are as high as 40-70%. Transmission can also occur across the placenta or via breask milk (Johri et al, 2006).

Antibiotics are very helpful in reducing these rates. Pregnant woman who tests positive for group B strep and gets antibiotics during labor has only a 1 in 4,000 chance of delivering a baby with group B strep disease, compared to a 1 in 200 chance if she does not get antibiotics during labor.

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Clinical Manifesations


Any illness in a baby who is born to a GBS positive woman over the first 10 days of life should be aggressively investigated and treated. Mortality is 5%, but infants who survive can also have seizures, deafness, and developmental delay.

Early onset (up to 7 days postpartum) disease can include:

Babies are normally born healthy. Signs of infection can include:

Late onset disease, occuring between 7-90 days, typically includes bacteremia and meningitis.





Pregnant women can also be affected, leading to UTI, endometriosis, and chorioamnionitis.

Other adult populations that may be infected include the elderly and those with chronic illness such as diabetes, liver disease, or cancer. Infections can include:

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All expectant mothers should be screened at 35-37 weeks with vaginal and anorectal swabs. The specimen should be processed within 24 hours. Swabs may be processed by direct plating, or by growth on enriched culture medium (the latter to increase sensitivity).

Results should be communicated to the mother.

If a mother receives insufficent dosing during labour, the baby should be monitored carefully for 24-48 hours. A CBC and blood culture is sometimes done.

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The goal of prophylaxis is to reduce the burden of bacteria and the chances of ascending infection. Women should be treated prophylactically if they meet the following:

Antibiotic treatments must be started at least 4 hours prior to delivery for sufficient coverage. IV penicillin q4h is typically used. Alternatives in cases of allergy include cefazolin or clindamycin.


There is no apparent benefit of treating during the antepartum period, nor of using antibiotic washes of the vagina.


Efforts are underway to develop a vaccine for women of childbearing age.





In infants or adults, treatment options include:

Sensitivities are important to obtain, given rates of resistance.

Surgical intervention or debridement may be necessary for significant infections.

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Resources and References

Johri AK et al. 2006. Group B Streptococcus: global incidence and vaccine development. Nat Rev Microbiol. 4(12): 932–942.

Keefe GP. 1997. Streptococcus agalactiae mastitis: a review. Can Vet J. 38(7):429-37.

Nandyal RR. 2008. Update on group B streptococcal infections: perinatal and neonatal periods. J Perinat Neonatal Nurs. 22(3):230-7 Centers for Disease Control and Prevention (CDC).

Prevention of perinatal group B streptococcal disease: a public health perspective. Centers for Disease Control and Prevention. MMWR Recomm Rep. May 31 1996;45:1-24

WHO. 2005. State of the art of vaccine research and development: Initiative for Vaccine Research.

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