Group B streptococcus testing

July 22, 2015

Streptococcus agalactiae, also known as Group B strep (GBS), is a Gram-positive colonizing bacterium that can cause life-threatening complications for newborns when it is passed to them from their mothers. This bacterium has been found in as many as 30 percent of healthy women1, to whom it poses little danger. According to the American Pregnancy Association, Group B strep is found in as many as one in every 2,000 children born in the United States each year.2

Women in the last few weeks of pregnancy are frequently tested for GBS due to the possibility of transmitting the bacteria to a newborn baby. Through screening, clinicians identify whether the baby may be at potential risk. Clinical laboratory professionals frequently test microbiology samples specifically for Streptococcus agalactiae. By utilizing established screening testing of late-term mothers, laboratory professionals assist obstetrical physicians in identifying potential threats to newborns, as well as their mothers.

The threat of Group B strep

Group B strep infection does not pose a considerable threat to the mother, but the organism may profoundly impact the health of a newborn. Presentation of GBS infection in the mother is typically found in the vaginal tract and secondarily in the rectum or intestinal tract. The most common treatment is penicillin or ampicillin; in some cases increased resistance to antibiotics have been noted. In addition, a recent study found “increased odds of MRSA carriage in GBS positive women.”1 The CDC recommends vaginal GBS screening of all women in the late stages of pregnancy.2 (Non-pregnant adults who are immunocompromised due to cancer, diabetes, HIV, or increased age are also at-risk for GBS infection development.3)

The purpose of screening pregnant women from 35 to 37 weeks gestation is to identify possible GBS infection for the mother and also to give an indication of the possibility that the newborn could be infected at the time of birth. Infection can occur either while the child is still in the placenta or during birth. Onset of neonatal disease is characterized by several possible health complications; however, the most common pathogenesis is meningitis. Meningitis in a newborn is obviously a concern; symptoms can include pneumonia, sepsis, lack of appetite, vomiting, nausea, and torpor.4 Meningitis and sepsis in newborns can result in an increased mortality rate depending on complications and the child’s susceptibility to antibiotic resistance.

According to Chen et al, prior to the introduction of GBS screening in pregnant mothers, “GBS was responsible for substantial perinatal morbidity and mortality, with as many as one to three in 1,000 neonates affected.”3 Development of symptoms of GBS infection during the first six days of life is called early onset; development of symptoms between seven days and three months is called late onset.5 The addition of antibiotics to GBS-colonized women prior to the child’s birth has led to a decrease in mortality rates, from >50 percent in the 1970s to <10 percent by the 1990s.”3 However, GBS remains a significant cause of both neonatal bacterial meningitis and sepsis.”3 Testing by the clinical laboratory thus remains critical to bacterial identification and early prevention of transmission to the newborn child.

GBS can be isolated from any clinical sample and can be indicated with different disease states (such as bacteremia or endocarditis), but urine samples and vaginal swabs are submitted to rule out GBS infections before they pose a risk to babies. Urine cultures are cultured on blood/maconkey agar plates, as is standard for urine cultures. Vaginal swabs are, according to Bailey and Scott’s Diagnostic Microbiology, placed in “Todd-Hewitt broth with antimicrobials (gentamicin, nalidixic acid, or colistin and nalidixic acid) to suppress the growth of vaginal flora and allow growth of [GBS] following subculture to blood agar.”6 An example of a Todd-Hewitt Broth is Lim Broth. If GBS is only suspected after birth, physicians will usually elect to collect a blood culture to see if the baby’s bacteremia is due to a GBS infection.

Laboratory testing approaches

Testing has developed into multiple avenues, ranging from traditional testing methods to newer molecular approaches.

Many labs use traditional testing methods to identify GBS. After growing in a selective enrichment media such as Lim Broth, samples are plated onto blood agar. Urines and blood cultures will be plated onto blood agar. Colonies will exhibit beta hemolysis and need to be differentiated from other beta-hemolytic organisms.

GBS looks different from the other beta-hemolytic organisms. It has the least amount of hemolysis (small zone of hemolysis around each colony). Gram stains will show GBS as gram-positive cocci in pairs and chains. Once colonies are isolated, different tests can be used for identification of GBS. Hippurate hydrolysis can be used, with GBS hydrolyzing sodium hippurate. The Christie-Atkins-Munch-Petersen (CAMP) test can also be used. It detects the production of enhanced hemolysis which occurs when ß-lysin and the hemolysins of GBS come in contact, making an arrow-shaped hemolysis pattern. The CDC recommends that “the most accurate way to identify GBS is to demonstrate that the bacteria in question has the Lancefield Group B antigen on the surface of the bacteria.”7 To detect antigens, Bailey and Scott’s says that latex agglutination procedures appear to be the most sensitive and specific.6 These kits are easy to use and will differentiate GBS from other species of streptococcus.

Next-generation testing methods may improve organism recovery and decrease identification turnaround times. With the molecular field growing at an exponential rate, laboratory professionals are confronted with a plethora of new tests. Many vendors are either releasing or plan to release some sort of molecular test for GBS. Their manufacturers claim that these tests are simple to use and offer increased specificity and sensitivity. A review of some of the tests shows that the initial specimen still must be incubated in Lim Broth. After incubation, one can utilize genetic detection methods to identify the presence of GBS in as little as an hour. Traditionally, laboratories would have to plate the swab that was immersed in Lim Broth to a plate, incubate, and test the next day. A quicker identification will aid with treatments and potentially save lives and reduce costs. Clinical laboratory scientists will need to stay abreast of  their options.  


  1. Parriott, AM, Brown AM, Onyebuch AA. Predischarge postpartum methicillin resistant Staphylococcus aureus infection and Group B streptococcus carriage at the individual and hospital levels. Infectious Diseases in Obstetrics and Gynecology. Accessed May 27, 2015.
  2. American Pregnancy Association. Group B Strep Infection. Accessed May 27, 2015.
  3. Chen VL, Avci FY, Kasper DL. A maternal vaccine against Group B streptococcus: past, present and future. Vaccine. 2013 Aug 28;31 Suppl 4:D13-9. doi: 10.1016/j.vaccine.2012.12.080.
  4. Baby Center. Meningitis.
  5. Medline Plus. Group B Streptococcal Septicemia of the Newborn.  Accessed May 27, 2015.
  6. Forbes BA, Sahm DF, Weissfield A. Bailey and Scott’s Diagnostic Microbiology, 11th ed.  St. Louis, MO: Mosby Inc. 2002. 302-315.
  7. Group B Strep (GBS).Centers for Disease Control and Prevention  Accessed May 27, 2015.

Photo 108389214 © Ivan Shidlovski |
Photo 293671412 © Nataliagh |
Photo 198092184 © Skypixel |