Streptococcus agalactiae, commonly known as Group B Streptococcus (GBS), is a Gram-positive bacterium that frequently colonizes the human body. It often resides harmlessly in the gastrointestinal and genitourinary tracts. While GBS typically causes no issues for healthy adults, it is a major cause of serious infection in newborns, often acquired during the birthing process.
Defining Group B Streptococcus and Colonization
Group B Streptococcus is a microscopic organism that lives as a commensal, coexisting with its host without causing disease. It is a common part of the natural microbiota in the lower intestinal tract and can subsequently be found in the vagina. GBS colonization is not a sexually transmitted infection, and the presence of the bacteria is generally asymptomatic in adults.
The status of colonization is transient, meaning the presence of the bacteria can fluctuate. Studies indicate that GBS colonization occurs in up to 35% of healthy women at any given time. Globally, the prevalence among pregnant women is estimated to be around 18%.
Colonization is the carriage of the organism and is distinctly different from an active infection, where the bacteria invades the body’s tissues. For most adults, a positive GBS test indicates carriage, which typically does not require treatment outside of pregnancy.
Screening and Risk Assessment During Pregnancy
Screening for GBS is a standard part of prenatal care because a pregnant woman’s colonization status is the primary risk factor for passing the infection to her baby. The procedure involves collecting a sample using a cotton-tipped swab from both the lower vagina and the rectum, which is then cultured in a laboratory.
The optimal time for this universal screening is late in the third trimester, typically between 36 0/7 and 37 6/7 weeks of gestation. Testing during this window provides the most accurate prediction of the woman’s status at the time of delivery, reflecting the transient nature of GBS colonization. If a woman tests positive, or if her status is unknown and certain risk factors arise, preventive measures are initiated during labor.
Risk factors for transmission include the onset of labor or rupture of membranes before 37 weeks gestation, a prolonged period of ruptured membranes exceeding 18 hours, or the development of a maternal fever equal to or greater than \(100.4^\circ\text{F}\) (\(38.0^\circ\text{C}\)) during labor. Mothers who have had a previous infant develop GBS disease, or who have GBS detected in their urine during the current pregnancy, are also considered at high risk.
Health Consequences in Newborns
GBS infection in newborns can manifest in two main forms, distinguished by the timing of onset. Early-Onset GBS Disease (EOD) occurs within the first week of life, typically presenting within the first 24 hours after birth. This infection is usually acquired when the infant is exposed to the bacteria while passing through the birth canal.
EOD commonly leads to severe conditions such as sepsis, pneumonia, and meningitis. Signs in a newborn can be subtle and non-specific, including unusual lethargy, difficulty feeding, or poor muscle tone. Respiratory distress, such as rapid breathing, grunting sounds, or periods of apnea, are also concerning indicators.
The second type is Late-Onset GBS Disease (LOD), diagnosed between seven days and three months of age. This form is less directly linked to maternal colonization at birth and is thought to be acquired from external sources. LOD frequently presents as meningitis, occurring in about one-third of cases.
Symptoms of LOD may include fever, vomiting, a high-pitched cry, or a blank, staring expression. While the mortality rate for LOD is lower than for EOD, survivors of GBS meningitis face a significant risk of long-term neurological complications, including hearing loss, developmental delays, or seizure disorders.
Prevention and Treatment Protocols
The primary method used to prevent EOD in newborns is Intrapartum Antibiotic Prophylaxis (IAP). This intervention involves administering intravenous antibiotics to the mother during labor to reduce the bacterial load and minimize the infant’s exposure during delivery. IAP is recommended for all women who have tested positive for GBS colonization, as well as those with an unknown GBS status who develop specific risk factors during labor.
Penicillin G is the preferred antibiotic for IAP, with an initial dose followed by maintenance doses administered every four hours until delivery. This protocol works best when the antibiotic has been circulating in the mother’s system for at least four hours before the birth. Alternative antibiotics, such as Cefazolin, Clindamycin, or Vancomycin, are available for women who report a penicillin allergy.
If a newborn develops signs of GBS infection, a diagnostic work-up, including blood cultures, is performed. Treatment for an infected infant involves the immediate initiation of empiric antibiotics, typically a combination of Ampicillin and Gentamicin.