Group B Strep and Pregnancy

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This artistic recreation, based on scanning electron microscopy (SEM), depicts a three-dimensional (3D), computer-generated image, of a group of Gram-positive, Streptococcus agalactiae (group B Streptococcus) bacteria. Photo Credit: Alissa Eckert, who is a medical illustrator at the CDC.

By Shawn Ruiz

Group B Strep (GBS), also known as Streptococcus agalactiae, is a Gram-positive, beta-hemolytic, catalase-negative, facultative anaerobe that is a normal component of the gastrointestinal and genitourinary tracts[1]. In fact, GBS colonizes the gastrointestinal and genitourinary tracts of up to 50% of healthy adults[2]. Most healthy adults who are colonized by GBS will not experience any symptoms or GBS-related infections. While the bacteria is usually harmless in healthy adults, it is a major cause of meningitis, pneumonia, and and sepsis in neonates[3]. Moreover, GBS is the leading infectious cause of neonatal mortality and morbidity in the United States; between four and six percent of babies who develop GBS disease die[4][5]. GBS causes both early onset (<7 days old) and late onset (7-90 days old) infections in neonates[4]. The main risk factor for an early-onset GBS infection in a neonate is colonization of a birthing person's genital tract with Group B strep during labor[4]. About one in four pregnant individuals carry GBS in their body[5]. If the bacteria is present in a pregnant person, it can be directly transferred to their baby in a multitude of ways. For example, GBS can travel from the vagina into the amniotic fluid where the baby can ingest it. The baby can also come into contact with the bacteria as they make their way down the birth canal[3]. In the early 1990s, the early GBS infection rate was 1.7 cases per 1,000 births[3]. In an effort to decrease this infection rate, the American Congress of Obstetricians and Gynecologists and the American Academy of Pediatrics recommended screening pregnant individuals for GBS before they go into labor[3]. As a result, it is now common practice to screen pregnant individuals for GBS at some point between 35 and 37 weeks of pregnancy[5]. Pregnant people who test positive for GBS are treated with intravenous antibiotics during labor[5]. Penicillin and ampicillin are the recommended antibiotics for intrapartum GBS prophylaxis[6]. If a pregnant person tests positive for GBS and they are treated with antibiotics during labor, the risk of their neonate developing a serious, life-threatening GBS infection drops by 80% [3]. Early GBS infection rates in the United States have significantly dropped (0.25 cases per 1,000 births) since these preventative measures went into effect around 1995[3]. While intrapartum prophylaxis to prevent GBS transmission from the birthing individual to their neonate during labor and delivery has proven to be effective, this preventative measure does not target in utero infections that occur earlier in pregnancy, and little is known about the mechanisms that result in the infection of the amniotic cavity[7]. In utero GBS infections have devastating effects, including preterm birth and mortality in both the pregnant person and their baby[7]. That said, the it is critical that researchers and public health officials work toward understanding exactly how GBS infects the amniotic cavity. Finally, new preventative therapies are needed before the widespread use of intrapartum antibiotics results in bacterial (GBS) resistance to, and thus the ineffectiveness of, the current antibiotic treatments[7].

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Authored for BIOL 238 Microbiology, taught by Joan Slonczewski, 2021, Kenyon College.

  1. [“Streptococcus Agalactiae.” Wikipedia, Wikimedia Foundation, 24 Mar. 2021,]
  2. [Johri, Atul Kumar, et al. “Group B Streptococcus: Global Incidence and Vaccine Development.” Nature Reviews Microbiology, vol. 4, no. 12, 2006, pp. 932–942., doi:10.1038/nrmicro1552.]
  3. 3.0 3.1 3.2 3.3 3.4 3.5 [Dekker, Rebecca. “The Evidence on: Group B Strep.” Evidence Based Birth , Evidence Based Birth , 17 July 2017,]
  4. 4.0 4.1 4.2 [Morgan, John A. “Group B Streptococcus And Pregnancy.” StatPearls [Internet]., U.S. National Library of Medicine, 29 Jan. 2021,,in%20term%20infants%5B2%5D.]
  5. 5.0 5.1 5.2 5.3 [“Group B Strep: Fast Facts and Statistics.” Centers for Disease Control and Prevention, Centers for Disease Control and Prevention, 11 June 2020,]
  6. [Horsley, Elizabeth. “CDC Updates Guidelines for the Prevention of Perinatal GBS Disease.” American Family Physician, The American Academy of Family Physicians Foundation , 1 May 2011,,units%20intravenously%20every%20four%20hours.]
  7. 7.0 7.1 7.2 [Whidbey, Christopher, et al. “A Hemolytic Pigment of Group B Streptococcus Allows Bacterial Penetration of Human Placenta.” Journal of Experimental Medicine, vol. 210, no. 6, 2013, pp. 1265–1281., doi:10.1084/jem.20122753.]