Microbiology
Summary
Streptococcus agalactiae, also known as group B strep (GBS) is a gram-positive bacterium that causes serious infections in newborns. Distinct from group A strep (Strep pyogenes), differentiating GBS becomes crucial in clinical practice. Group B strep is positive for hippurate test and has a polysaccharide capsule. It is also CAMP test positive.
Group B strep is beta-hemolytic and bacitracin-resistant, features that further help in its identification. This bacterium is the leading cause of meningitis in neonates and can also cause sepsis and pneumonia in newborns. To prevent GBS infections in babies, pregnant women have their vagina and rectum swabbed for the bacteria at 35 weeks, and if positive, receive intrapartum penicillin as a prophylactic measure.
Lesson Outline
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FAQs
Streptococcus agalactiae, or group B strep (GBS) is a gram-positive, bacitracin-resistant, beta-hemolytic bacterium that commonly colonizes the gastrointestinal and genitourinary tracts. It is a significant cause of neonatal infections, including sepsis, pneumonia, and meningitis, and can also cause urinary tract infections, chorioamnionitis, and postpartum infections in adults. Prenatal screening and intrapartum penicillin prophylaxis can significantly reduce the risk of GBS transmission to newborns during delivery.
On blood agar, GBS exhibits beta-hemolysis (complete hemolysis) and is bacitracin-resistant. The CAMP test is a crucial confirmatory test used to identify GBS. In the CAMP test, GBS is streaked perpendicular to a Staphylococcus aureus strain on blood agar. A positive CAMP test exhibits enhanced beta-hemolysis in the shape of an arrowhead at the intersection of the bacterial streaks, indicating the presence of GBS.
GBS can cause neonatal meningitis when newborns are exposed to the bacteria during passage through the birth canal of a colonized mother. Invasive GBS infection may occur in utero or during delivery, leading to bacteremia and invasion of the central nervous system. Risk factors for neonatal meningitis caused by GBS include premature birth, prolonged rupture of membranes, maternal GBS colonization, and intra-amniotic infection or chorioamnionitis. Additionally, infants born to mothers with a previous GBS-infected infant are also at increased risk for meningitis.
Intrapartum penicillin is administered to GBS-colonized pregnant women during labor to prevent the transmission and subsequent early-onset GBS infection in newborns. Penicillin is the antibiotic of choice because of its narrow spectrum, low toxicity, and proven efficacy in eradicating GBS from the maternal genital tract. Administering intrapartum penicillin prophylaxis has significantly reduced the incidence of early-onset GBS infections in newborns, particularly those with risk factors such as prematurity and prolonged rupture of membranes.
As part of prenatal care, it is recommended that pregnant women be screened for GBS colonization between 35 and 37 weeks of gestation. The screening involves obtaining vaginal and rectal swab specimens, which are cultured to detect the presence of GBS. If the culture results are positive for GBS, the pregnant woman is considered colonized and should receive intrapartum antibiotic prophylaxis (such as penicillin) during labor to prevent transmission of GBS to the newborn. Prenatal GBS screening and prophylactic antibiotic treatment have been successful in substantially reducing the incidence of early-onset GBS infection in newborns.
