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Group B Streptococcus (GBS)

Streptococci are a genus of gram-positive, spherical bacteria that appear in chains. There are over 30 recognized species within this genus.  Streptococci are subdivided into 5 main pathogenic groups commonly found in humans (A, B, C, D and G). This subdivision is based on the antigenic differences of the carbohydrates in the bacterial cell wall. Group B Streptococci (GBS)contain one species, Streptococcus agalactiae.

GBS is part of the normal gastrointestinal flora of many humans.  GBS can also be found in genitourinary tract of up to 30% of women which can be transferred to a neonate passing through the birth canal and can cause serious infections of the newborn. This is a common problem and as many as 1.8 cases per 1000 live births will be affected by group B streptococcal infection. Infection with GBS can cause instances of stillbirth. Newborn GBS disease is separated into early-onset disease that occurs up to 7 days after birth and late-onset disease which occurs on days 7–90. Early-onset septicemia usually results in pneumonia, while late-onset septicemia is often accompanied by meningitis. Hearing loss can be a long-term complication of GBS meningitis.

Transmission

GBS can be transferred to a neonate passing through the birth canal and can cause serious neonatal infection.   In the elderly or persons with compromised immune systems, septicemia or other serious infections can occur when the bacteria enter the blood from the GI tract.

Diagnosis

Bacterial culture methods are used to identify the presence of GBS in the female GU tract or in the blood or CSF of a patient with septicemia or meningitis.  GBS  are identified on culture plates by the presence of group B Lancefield antigen, by its ability to hydrolyze sodium hippurate, and by its  sensitivity to bile which causes GBS bacteria to lyse.  Culture results may take 2-3 days so physicians must treat patients presumptively while awaiting culture results. Sensitive molecular tests, such as PCR, detect GBS DNA may play an increasing important role in detecting the presence of GBS.

Prevention and Treatment

There is currently no vaccine available for GBS. However, vaccination of adolescent women is potentially an effective strategy to prevent newborn disease and efforts are ongoing to develop such a vaccine. Prevention of newborn GBS in the United States is based on screening all pregnant women for the presence of GBS in their GU tract followed by prophylactic antibiotics for women who test positive. There has been a significant reduction in early-onset infection since this approach was adopted.

Recommended Reading

CDC Website: http://www.cdc.gov/groupbstrep/index.html

American Pregnancy Association: http://americanpregnancy.org/pregnancycomplications/groupbstrepinfection.html

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