The Double Life of Group B Streptococcus: Asymptomatic Colonizer and Potent Pathogen

Abstract

Group B streptococcus (GBS) is a β-hemolytic gram-positive bacterium that colonizes the lower genital tract of approximately 18% of women globally as an asymptomatic member of the gastrointestinal and/or vaginal flora. If established in other host niches, however, GBS is highly pathogenic. During pregnancy, ascending GBS infection from the vagina to the intrauterine space is associated with preterm birth, stillbirth, and fetal injury. In addition, vertical transmission of GBS during or after birth results in life-threatening neonatal infections, including pneumonia, sepsis, and meningitis. Although the mechanisms by which GBS traffics from the lower genital tract to vulnerable host niches are not well understood, recent advances have revealed that many of the same bacterial factors that promote asymptomatic vaginal carriage also facilitate dissemination and virulence. Furthermore, highly pathogenic GBS strains have acquired unique factors that enhance survival in invasive niches. Several host factors also exist that either subdue GBS upon vaginal colonization or alternatively permit invasive infection. This review summarizes the GBS and host factors involved in GBS's state as both an asymptomatic colonizer and an invasive pathogen. Gaining a better understanding of these mechanisms is key to overcoming the challenges associated with vaccine development and identification of novel strategies to mitigate GBS virulence.

Keywords: colonization and virulence; group B streptococcus; host–pathogen interaction; immune response; signal transduction systems.

Figure 1.. Clinical Pathways in Maternal Group B Streptococcus (GBS) Colonization.

GBS is associated with several perinatal outcomes. In the case of adequate prophylaxis, most mothers and babies are healthy with normal lifespan. Virulent ascending GBS is associated with significant morbidity and mortality, some of which is not preventable with intrapartum prophylaxis. With adequate treatment of ascending infection, normal and healthy outcomes may be achieved. Figure adapted from Lawn, et al, 2017 [175].

Figure 2.. Host and bacterial factors that contribute to GBS’s status as either an asymptomatic colonizer or invasive pathogen.

GBS typically colonizes the gastrointestinal/vaginal tract asymptomatically but is highly pathogenic in other host compartments. The host responses to GBS are multifaceted and can promote asymptomatic colonization and clearance or alternatively permit invasive infection and disease. Many of the bacterial factors that promote colonization are also involved in dissemination and tissue damage. GBS tightly regulates the expression of these factors using signal transduction systems, which sense and respond to variations in the external environment.