Mostly harmless in adults, Streptococcus agalactiae, also known as Group B strep (GBS), can be deadly in infants. The infection can cause sepsis, pneumonia, bacteremia and meningitis in neonates.
Laboratory professionals play a part in diagnosing GBS by testing pregnant women during weeks 35-37 of their gestation period and can aid clinicians by sharing their results.
The Effects of GBS
GBS colonizes in the gastrointestinal or urogenital tract of 10-30 percent of all pregnant women, explained Paul D. Fey, PhD, D(ABMM), professor and medical director of Clinical Microbiology, University of Nebraska Medical Center, Omaha. "If the mother is colonized and the baby passes through the birth canal, approximately 50% of the babies become colonized with GBS, and one to two percent will have the disease," he added.
Neonates with early onset GBS will become ill within 12-20 hours of birth, Fey explained. "The babies can also get a late onset disease in which they acquire GBS from a visitor to the hospital or their mother, for example," he said.
When early onset GBS appears in newborns, it can look like other types of illnesses. "The neonates typically present with problems that are indistinguishable from other bacterial infections, such as poor feeding, temperature, difficulty breathing, lethargy and hypotension," explained Patricia M. Tille, PhD, MT(ASCP), program director, South Dakota State University, Brookings, SD. Infants with late onset GBS, usually appearing when the babies are anywhere from seven days to three months old, can also present with cellulitis, osteomyelitis or septic arthritis, she added. Both types of GBS in infants can cause meningitis, a disease which can lead to permanent neurological damage.
Recently, GBS has even been diagnosed among infants older than three months old, Tille noted. "This is often associated with infants who are preterm, low birth weight, or have an additional congenital disease or immune deficiency requiring longer than normal hospitalization following birth," she said.
Testing for GBS
To make sure babies don't have to suffer from the ill effects of GBS, pregnant women get tested with anal and vaginal cultures. "First, the patient's swab is grown in broth, called LIM broth (BD, Franklin Lakes, NJ)," explained Fey. "Then it is plated on blood agar plates to isolate GBS."
A newer method includes growing the swab in Carrot broth (Hardy Diagnostics, Santa Monica, Calif.), he added. "If GBS is present in the swab, an orange color forms in the broth. This can be reported as positive for GBS," Fey told ADVANCE. In addition, a polymerase chain reaction (PCR) test from Cepheid (Sunnyvale, Calif.) is very rapid and can produce a result within the same day, he said. "PCR tests offer high levels of sensitivity, along with the advantage of providing results faster than conventional cultures," reported Leonard Levin, MS, MBA, RM(NRCM), M(ASCP), microbiology supervisor, Robert Wood Johnson University Hospital, Hamilton, NJ.
"However, molecular assays must be interpreted with care and often growth of the organism is still required in order to determine antibiotic susceptibility," Tille cautioned. She noted that the CDC recommends a simultaneous culture when using molecular assays.
GBS can also be identified by a nucleic acid test from Hologic Gen-Probe (San Diego), said Linda Garringer, BS, MT(ASCP), chief microbiologist, Erie County Health Department, Buffalo, NY.
Laboratory professionals can also test a baby who may have GBS by standard blood culture or cerebrospinal fluid culture, Fey said.
After receiving the test results, a colonized mother is given antibiotics, such as ampicillin (as a broad spectrum coverage until GBS is identified, Tille said) or penicillin (once GBS has been identified). "Intrapartum antibiotic treatment has demonstrated to decrease the incidence of early-onset sepsis in neonates and reduced the risk of other invasive infections," Tille explained. "Intrapartum therapy is recommended if the mother is colonized, if there is history of a previous birth resulting in GBS infection, and if there is no information available as to the result of any screening test or prenatal verification of the mother as negative for colonization."
In most cases, this can clear up the GBS danger for the neonates. If a patient is allergic to penicillin, the organism must be cultured for antimicrobial susceptibility testing. "In this case, we test for erythromycin or clindamycin resistance as this is the second line antibiotic," Fey noted. "There is a fair amount of resistance to erythromycin and clindamycin and susceptibility testing should always be performed in these cases."
In the Future
Hopefully one day pregnant women won't have to worry about being colonized with GBS at all. Studies are examining a possible GBS vaccine, Fey noted.
But in the meantime, GBS is a real danger for almost one in four mothers-to-be. "GBS remains a viable healthcare concern and will continue to drive research in order to identify ways to accurately detect colonization and infection," Tille stressed.
And, as in any detection of disease, the entire healthcare team needs to work together for the health of mothers and their babies. "Any laboratory method that is crucial to the health of mom and baby should have close communication with infectious disease, clinicians and laboratory personnel to be effective," Garringer said. "Too often important information, such as pregnancy, patient's age, week of pregnancy or previous colonization is not reported on the laboratory requisition."
Amanda Koehler is a freelance writer.