Group B Streptococcal Disease

Group B Streptococcus (GBS), also known as Streptococcus agalactiae, is the leading cause of newborn infection; presentations of GBS include bacteremia, meningitis, neonatal sepsis, pneumonia, or other focal infections.  In newborns, GBS is transmitted vertically from a colonized parent to an infant before or during labor due to the rupture of membranes. In adults, GBS infections of the lower respiratory tract and gastrointestinal tract are common and generally asymptomatic. However, adults with conditions such as diabetes or liver disease may be at an increased risk for developing symptomatic group B streptococcal disease.  Laboratory testing for GBS is recommended for antepartum screening of pregnant individuals to inform potential intrapartum antibiotic prophylaxis and prevent early-onset disease in neonates.    Additionally, diagnostic testing is useful in symptomatic patients. Culture and nucleic acid amplification testing (NAAT) are the most common types of tests performed to evaluate GBS.  For more information about the recommended laboratory testing for pregnant individuals, see the ARUP Consult Pregnancy and Prenatal Laboratory Testing topic.

Quick Answers for Clinicians

What are the screening recommendations for group B Streptococcus?

Universal screening for group B Streptococcus (GBS) is recommended for all pregnant individuals at 36 0/7 to 37 6/7 weeks of gestation unless intrapartum antibiotic prophylaxis for GBS is indicated due to existing risk factors.   

How does group B Streptococcus present clinically?

In neonates, group B Streptococcus (GBS) syndromes include both early-onset disease (EOD), which occurs in neonates <7 days old, and late-onset disease (LOD), which occurs between 7 and 90 days of age. Both EOD and LOD can manifest as bacteremia, meningitis, neonatal sepsis, and pneumonia. In adults, severe GBS infections may cause sepsis, meningitis, pneumonia, bone and joint infections, or skin and soft tissue infections. Pregnant individuals may experience amnionitis, urinary tract infections, stillbirth, or bloodstream infections, including sepsis. 

How is group B Streptococcus transmitted?

Group B Streptococcus (GBS) bacteria do not spread through food, water, or sexual contact, and adult transmission is not well understood. In neonates, vertical transmission from an infected parent generally occurs during birth or after the rupture of membranes. 

Which patients are at an increased risk of group B streptococcal disease?

In newborns, the risk of group B streptococcal disease is higher among infants born to individuals with vaginal-rectal colonization with group B Streptococcus (GBS) during the intrapartum period; individuals with fever or prolonged rupture of membranes who have previously birthed an infant with GBS infection; or individuals who are experiencing preterm delivery. Adults with chronic illnesses (eg, diabetes, liver disease, or cardiovascular disease) and pregnant individuals are at risk of group B streptococcal disease. GBS infection rates are also elevated among African American and elderly populations.

Indications for Testing

According to the American College of Obstetricians and Gynecologists (ACOG), the American Academy of Pediatrics (AAP), and the American Society for Microbiology (ASM), universal screening for GBS is recommended for all pregnant individuals at 36 0/7 to 37 6/7 weeks of gestation unless intrapartum antibiotic prophylaxis for GBS is indicated due to existing risk factors.   

Diagnostic laboratory testing may be useful for patients who exhibit signs and symptoms of GBS infection including bacteremia without a focus, meningitis, bone and joint infections, or other soft tissue infections. Because GBS symptoms are often nonspecific, testing should be informed by clinical evaluation.

Laboratory Testing

Detailed recommendations for the laboratory detection and identification of GBS in pregnant individuals, including specimen collection and storage information, can be found in the ASM’s Guidelines for the Detection and Identification of Group B Streptococcus. 

Screening

ASM recommends screening pregnant individuals for colonization with GBS at 36 0/7 to 37 6/7 weeks of gestation with a single swab of the lower vagina and then rectum. Swab specimens are incubated in selective enrichment broth before testing by agar plating (culture) or NAAT.

Diagnosis

Culture is the most common laboratory method for the identification of GBS. However, culture requires multiple days (long turnaround time), so it may not be appropriate when results are needed quickly (eg, in cases of preterm birth). NAAT has a faster turnaround time and may be useful in some situations. It is important that NAAT be performed on specimens incubated in enrichment broth to maximize sensitivity and specificity.  In other individuals with group B streptococcal disease, diagnosis is confirmed when GBS is isolated from a normally sterile body site such as blood or cerebrospinal fluid (CSF).

Treatment-Related Testing

Because penicillin is the preferred agent for intrapartum prophylaxis, antimicrobial susceptibility testing should be performed on all GBS isolates from pregnant individuals with a severe penicillin allergy to inform selection of an alternate antibiotic treatment. 

ARUP Laboratory Tests

Recommended Testing

Use to detect genital and rectal GBS in pregnant individuals

Additional Testing

Identify bacteria in wounds

Anaerobe culture is recommended for body fluids, tissue, and deep wounds/surgical cultures; refer to anaerobe culture and gram stain

Anaerobe culture is NOT included with this order

Use to rapidly detect a panel of common viruses, bacteria, and fungi associated with meningitis and encephalitis

Do NOT use as a replacement for CSF bacterial and/or fungal culture and cryptococcal antigen testing in at-risk patients

A negative result does not exclude a diagnosis of meningitis or encephalitis due to infection

Medical Experts

Contributor

Fisher

Mark A. Fisher, PhD, D(ABMM)
Associate Professor of Pathology (Clinical), University of Utah
Medical Director, Bacteriology, Special Microbiology, and Antimicrobial Susceptibility Testing, ARUP Laboratories
Contributor

References