Group B Strep Awareness Month

by | Clinical, Molecular

Publish Date: June 26, 2023

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 ASM 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% COconditions for both culture-based and molecular methods. In addition to enrichment broth, some laboratories may choose to simultaneously directly inoculate agar culture plates as well. This can help decrease time to detection instead of first incubating in enrichment broth followed by playing to agar media. Although some molecular assays are designed for testing without enrichment, the use of enrichment broth prior to testing can nevertheless increase accuracy.


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.

Sample collection

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.

Learn More



“Antibiotic Resistance Threats in the United States.” CDC

“Group B Strep (GBS).” CDC

Rosa-Fraile, Manuel and Barbara Spellerberg. “Reliable Detection of Group B Streptococcus in the Clinical Laboratory.” Journal of Clinical Microbiology. Sep. 2017.

Sayers, Jennifer L. “Group B strep: questions and answers.” MLO. 24 Apr. 2017.

Filkins L, Hauser J, Robinson-Dunn B, Tibbetts R, Boyanton B, Revell P. Published 10 March 2020. Updated 23 July 2021. Guidelines for the Detection and Identification of Group B Streptococcus. American Society for Microbiology.

Written by Microbiologics

Microbiologics is the foremost biological products and services provider, focused on protecting the health and safety of people around the world. For over 50 years, they have been partnering with healthcare and life science laboratories, manufacturers and suppliers across the globe to co-create and provide biological control materials, assay services and consulting for microbiology, molecular diagnostics and virology. Headquartered in Saint Cloud, Minnesota, the company has additional facilities in California, Kentucky and Michigan. Microbiologics has advanced industry standards for biological controls by adopting the most rigorous testing and quality standards including ISO 13485, ISO/IEC 17025, and ISO 17034. The company has an extensive distribution network spanning over 150 countries. For more information, visit

You May Also Like


Share This