What are the sources of infection and management for a patient with a positive blood culture for Group B Streptococcus (GBS)?

Sources of Infection and Management for Group B Streptococcus (GBS) Bacteremia

A single positive blood culture for Group B Streptococcus (GBS) should be considered a true infection requiring prompt treatment, as GBS is a recognized pathogen that requires only one positive culture to be considered significant. 1

Common Sources of GBS Bacteremia

• Urinary tract infections: GBS bacteriuria during pregnancy is considered a surrogate for heavy maternal colonization and is associated with higher risk for invasive disease. Women with symptomatic or asymptomatic GBS urinary tract infection should be treated according to current standards of care. 2

• Genital tract colonization: The gastrointestinal tract serves as the primary reservoir for GBS and is the likely source of vaginal colonization. Approximately 10-30% of pregnant women are colonized with GBS in the vagina or rectum. 2, 3

• Chorioamnionitis: Women with intrapartum fever and signs of chorioamnionitis should be treated with broad-spectrum intravenous antibiotics that include coverage for GBS. 4

• Neonatal transmission: GBS can be acquired vertically through exposure from the vagina of a colonized woman, particularly when GBS ascends from the vagina to the amniotic fluid after onset of labor or rupture of membranes. 2

• Invasive GBS disease: Previous delivery of an infant with invasive GBS disease is a risk factor for recurrent invasive disease in subsequent deliveries. 2

Diagnostic Approach

• For pregnant women, screening for GBS colonization should be performed at 35-37 weeks' gestation with cultures taken from one swab first to the vagina and then to the rectum. 4

• Culture techniques that maximize the likelihood of GBS recovery are required for accurate diagnosis, including use of selective broth medium and proper specimen collection. 2

• Specimen labels should clearly identify that specimens are for group B streptococcal culture, and if susceptibility testing is ordered for penicillin-allergic patients, this should be specified. 2

Management of GBS Bacteremia

First-line Treatment

• Penicillin G is the preferred agent due to its narrow spectrum of activity: 5 million units intravenously initial dose, then 2.5 million units intravenously every 4 hours until delivery (for pregnant patients) or until completion of treatment course. 2

• Ampicillin is an alternative regimen: 2 g intravenously initial dose, then 1 g intravenously every 4 hours. 2, 5

For Penicillin-Allergic Patients

• For patients not at high risk for anaphylaxis: Cefazolin, 2 g intravenously initial dose, then 1 g intravenously every 8 hours. 2

• For patients at high risk for anaphylaxis with susceptible isolates: Clindamycin, 900 mg intravenously every 8 hours; OR Erythromycin, 500 mg intravenously every 6 hours. 2, 1

• If susceptibility testing is not possible or isolates are resistant: Vancomycin, 1 g intravenously every 12 hours. 2

Special Considerations for Pregnant Women

• Women with GBS bacteriuria during pregnancy should receive intrapartum antibiotic prophylaxis regardless of the level of colony-forming units. 4

• Women who have previously given birth to an infant with invasive GBS disease should receive intrapartum chemoprophylaxis; prenatal culture-based screening is not necessary for these women. 2

• Women with threatened preterm delivery (<37 weeks' gestation) should be assessed for need for intrapartum prophylaxis to prevent perinatal GBS disease. 2

Management of Close Contacts

• Close contacts of patients with invasive GBS infections should be monitored closely. 2

• Unlike Group A Streptococcus, routine culture or treatment of asymptomatic household contacts is not generally recommended for GBS infections unless there are specific risk factors. 2

Common Pitfalls and Caveats

• Failure to recognize the significance of a single positive blood culture for GBS, which unlike common contaminants, should be considered a true infection. 1

• Inadequate antibiotic duration or inappropriate antibiotic selection, particularly in penicillin-allergic patients. 2

• Not considering GBS as a cause of bacteremia in non-pregnant adults, especially those with underlying conditions. 2

• Failing to investigate for potential sources of infection such as endocarditis, osteomyelitis, or soft tissue infections when GBS bacteremia is identified. 5

• Not recognizing increasing resistance to clindamycin and erythromycin among GBS isolates, which necessitates susceptibility testing for penicillin-allergic patients. 2