What is Streptococcus Group B?

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  • Written By: Caitlin Kenney
  • Edited By: Bronwyn Harris
  • Last Modified Date: 06 March 2020
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Streptococcus group B, more commonly phrased group B streptococcus or group B strep, is bacterium that occurs naturally in the lower reproductive tracts and intestines of healthy adults, but can cause a life threatening infection in newborn babies. Illness from streptococcus group B can also affect older people and people with chronic liver problems or diabetes. Most adults do not have to worry about group B strep, but pregnant women should be screened for the bacterium to manage the risks during pregnancy with antibiotics.

The group B strep bacterium is scientifically called Streptococcus agalactiae. Streptococcus is the genus, a classification of spherically shaped, gram-positive bacteria that appear in pairs or chains, and agalactiae is the species. The colloquial name group B strep comes from the presence of the Lancefield group B antigen on the bacterium. An antigen is a molecule that the body recognizes as a foreign substance and ignites an immune reaction.


Though the bacteria is present and harmless in many healthy adults, streptococcus group B infections can cause serious problems for newborn babies because they lack the antibody to fight the infection. The bacterium colonizes in the lower genital tract of the pregnant woman and it can be transmitted to the baby either in utero or during delivery. While transmission is fairly common in women who carry the bacteria, it is far less common that the baby actually develops the disease. Fever during labor, early labor or membrane rupture, a urinary tract infection with group B strep, and history of previous children with group B strep increase the risks of the baby becoming infected. Doctors routinely screen the mother for the bacteria around 35-37 weeks into the pregnancy, so they are usually prepared to fight transmission using antibiotics.

A streptococcus group B infection that appears in the first twelve hours after birth is called early-onset group B strep disease, and is the more common and dangerous form of group B strep infection. Signs of the disease include poor feeding, limpness, and fever. Late-onset group B strep disease can set in around a week to several months after birth and may be marked by seizures, lethargy, fussiness, respiratory distress, abnormal heart rate and blood pressure, and fever. The baby can be diagnosed with the disease through an analysis of the blood and spinal fluids.

Streptococcus group B can cause life-threatening complications in infants, including pneumonia; bacteremia, or infection of the blood; and meningitis. Intravenous antibiotics are used to treat the infant, as well as other treatments that may be needed to support the baby as it fights infection. It is important to take the infant to get medical attention at the first sign of infection.

Older adults, adults with chronic medical conditions, and pregnant women can also develop dangerous group B strep. Symptoms can include bacteremia, pneumonia, urinary tract infections, or skin infections. The disease is treated with either oral or intravenous antibiotics, depending on the extent of the infection.


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