Streptococcus agalactiae (group B Streptococcus)

Streptococcus agalactiae (group B Streptococcus)

Source: CDC

Group B Streptococcus (group B strep) or S. agalactiae is a species of bacterium that causes illness in people of all ages. Also known as GBS, this bacterium is a common cause of severe infections in newborns during the first week of life. More recently, experts recognized the increasing impact invasive GBS disease has on adults.

PCR Serotyping

GBS are currently divided into ten serotypes based on type-specific capsular antigens and are designated as Ia, Ib, II, III, IV, V, VI, VII, VIII, and IX. There are multiple methods to perform GBS serotyping:

  • The Lancefield precipitation test – considered the standard method for GBS serotype determination
  • Latex agglutination method – serotypes GBS isolates phenotypically; several kits are commercially available
  • Real-time PCR1
  • Conventional PCR – a recommended and validated method2
  • Whole-genome sequencing3

Real-Time PCR

You can use a molecular approach using a real-time PCR assay targeting the cfb gene (CAMP factor) for detecting S. agalactiae in singleplex format or in triplex format along with other streptococcal species, S. pyogenes and S. suis. This assay(s) is useful for detection directly from clinical specimens when culture is negative or not available. Primer/probe sequences are detailed in the resources below. For additional information contact


1Breeding KM, Ragipani B, Lee KD, Malik M, Randis TM, Ratner AJ. Real-time PCR-based serotyping of Streptococcus agalactiae. Sci Rep. 2016;2(6):38523.

2 Imperi M, Pataracchia M, Alfarone G, Baldassarri L, Orefici G, Creti R. A multiplex PCR assay for the direct identification of the capsular type (Ia to IX) of Streptococcus agalactiae. J Microbiol Methods. 2010;80(2):212–4.

3Metcalf BJ, Chochua S, Gertz RE Jr, et al. Short-read whole genome sequencing for determination of antimicrobial resistance mechanisms and capsular serotypes of current invasive Streptococcus agalactiae recovered in the USA. Clin Microbiol Infect. 2017;pii: S1198-743X(17):30118–0.