The CDC and other health organizations recognize July as International Group B Streptococcus (Group B Strep, GBS) month. In many ways, Group B Strep seems somewhat harmless, remaining asymptomatic for most healthy adults who carry the bacteria. Yet the same factors that make it innocuous for most carriers make it even more challenging to protect those who are most vulnerable: newborn babies and adults with risk factors such as diabetes, cardiovascular disease, obesity, and cancer. Group B Strep is a leading cause of sepsis, meningitis, and pneumonia in neonates, with around 1980 babies contracting GBS disease in the United States each year, according to the CDC. Approximately one in four pregnant women carry GBS. About 10 cases in every 100,000 non-pregnant adults each year have serious Group B Strep disease.
Challenges of Group B Strep
Group B Strep presents several challenges for diagnosis and treatment. Because Group B Strep is asymptomatic for most healthy adults, only routine testing can detect GBS and prevent vertical transmission. GBS is a “fast acting,” transient bacteria, which means that colonization can quickly come and go without any signs. An expecting mother could therefore test negative but test positive shortly after. Although it is found in the gastrointestinal, urinary, and lower reproductive tracts, is generally not considered a sexually transmitted infection (STI). The means of transmission remain unknown, so patient history offers no indications of a possible infection.
GBS testing can have a high rate of false negatives, especially when plated directly without enrichment (see below for enrichment guidelines). Specimen handling and storage can also significantly impact organism viability and detection accuracy. Although some health organizations like the CDC recommend consistent testing for all expecting mothers, other organizations base GBS testing guidelines on risk factors, such as prior detection during pregnancy. Health organizations like the CDC typically recommend that testing occur in the third trimester (week 36-37) and that antibiotic prophylaxis occur during, rather than prior to, delivery.
Accurate antepartum detection of GBS
Common causes of false negatives for GBS include: incomplete sample collection, premature screening, and improper testing methods (such as incorrect media usage). The following CDC guidelines for GBS testing can help optimize specimen collection, sample handling and storage, and organism detection.
To increase detection accuracy, vaginal-rectal swabs should be inoculated into a selective enrichment broth media and incubated for 18-24 hours at 35-37°C in ambient air or 5% CO2 conditions for both culture-based and molecular methods. Subculturing after enrichment is not necessary. Although some molecular assays are designed for testing without enrichment, the use of enrichment broth prior to testing can nevertheless increase accuracy. If your laboratory opts for inoculating agar culture plates in addition to enrichment broths, inoculate enrichment broth first.
Agar plates with selective agents may inhibit growth of other natural flora, thereby promoting GBS growth. Media must be capable of detecting non-hemolytic strains of GBS, which account for around 5% of all infections. Chromogenic media may help facilitate fast GBS detection through the color change of colonies. However, non-hemolytic strains of GBS do not produce the identifying color changes on chromogenic media. Identification of candidate isolates from chromogenic agars may therefore be required. Hemolysis-enhancing media can be used to detect isolates that would be non-hemolytic on standard blood agar plates.
One of the most common causes of false negatives is incomplete sample collection (usually vaginal swabbing without rectal swabbing). Both steps are critical for accurate results.
Testing methods & detection of antibiotic resistance
When properly performed, including an enrichment step, both culture and molecular methods offer a high degree of accuracy. By reducing sample handling, molecular testing may minimize human error, as well as improve the turn-around time of test results. However, culture-based methods are still being used, especially when Nucleic Acid Amplification Tests (NAAT) are invalid or produce indeterminate results, requiring repeat testing or reflex to culture. And while NAAT may dramatically reduce hands-on sample processing, they are not capable of detecting antibiotic resistance. According to the CDC, more than 40% of GBS infections show resistance to Clindamycin, a common antibiotic for patients with severe penicillin allergy. Molecular testing thus cannot completely replace culture-based detection. Laboratories relying on NAAT for primary detection should reflex positive specimens requiring Antibiotic Susceptibility Testing (AST) for subculture to appropriate agar media.
Optimal accuracy for GBS screening
Antepartum GBS screening is crucial for protecting neonates from sepsis, meningitis, and other serious risks associated with Group B Strep disease. Microbiologics helps ensure accurate GBS detection with user-friendly controls for culture-based and molecular diagnostics.
“Antibiotic Resistance Threats in the United States.” CDC. https://www.cdc.gov/drugresistance/pdf/threats-report/gbs-508.pdf
“Group B Strep (GBS).” CDC. https://www.cdc.gov/groupbstrep/index.html
Rosa-Fraile, Manuel and Barbara Spellerberg. “Reliable Detection of Group B Streptococcus in the Clinical Laboratory.” Journal of Clinical Microbiology. Sep. 2017. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5648696/.
Sayers, Jennifer L. “Group B strep: questions and answers.” MLO. 24 Apr. 2017. https://www.mlo-online.com/home/article/13009069/group-b-strep-questions-and-answers