page_title: Streptococcus agalactiae
section_title: Screening for GBS colonization
hierarchical_section_title: Streptococcus agalactiae / Prevention of neonatal infection / Screening for GBS colonization
caption_attribution_description: Streptococcus agalactiae (Group B Streptococcus) on ChromID CPS chromogenic agar. Isolate obtained from a urine sample from a 32 year old full term (39 weeks) asymptomatic pregnant female.
context_page_description: Streptococcus agalactiae is a gram-positive coccus with a tendency to form chains. It is a beta-hemolytic, catalase-negative, and facultative anaerobe.
  S. agalactiae is the species designation for the only species of streptococci belonging to the group B of the Rebecca Lancefield classification of streptococci. GBS is surrounded by a bacterial capsule composed of polysaccharides. The species is subclassified into ten serotypes depending on the immunologic reactivity of their polysaccharide capsule. This is why the plural term group B streptococci and the singular term group B streptococcus are both commonly encountered.
  In general, GBS is a harmless commensal bacterium being part of the human microbiota colonizing the gastrointestinal and genitourinary tract of up to 30% of healthy human adults. Nevertheless, GBS can cause severe invasive infections especially in newborns, the elderly, and people with compromised immune systems.
  S. agalactiae is also a common veterinary pathogen, because it can cause bovine mastitis in dairy cows. The species name agalactiae meaning "of no milk", alludes to this.
context_section_description: Though the GBS colonization status of women can change during pregnancy, cultures to detect GBS carried out ≤5 weeks before delivery predict quite accurately the GBS carrier status at delivery. In contrast, if the prenatal culture is performed more than five weeks before delivery it is unreliable for predicting accurately the GBS carrier status at delivery.
  The clinical specimens recommended for culture of GBS at 36–37 weeks’ gestation (32–34 weeks of gestation for women with twins) are swabs collected from lower vagina and rectum through the anal sphincter. Following the recommendations of the Centers for Disease Control and Prevention of United States  (CDC) these swabs should be placed into a non-nutritive transport medium and later inoculated into a selective enrichment broth, Todd Hewitt broth with selective antibiotics  (enrichment culture).
  After incubation the enrichment broth is subcultured to blood agar plates and GBS like colonies are identified by the CAMP test or using latex agglutination with GBS antisera. After incubation the enrichment broth can also be subcultured to granada medium agar where GBS grows as pink-red colonies or to chromogenic agars, where GBS grows as colored colonies. GBS-like colonies that develop in chromogenic media should be confirmed as GBS using additional reliable tests to avoid mis-identification.
  Nucleic acid amplification tests (NAAT) such as polymerase chain reaction (PCR) and DNA hybridization probes have been developed for identifying GBS directly from recto-vaginal samples, but they still cannot replace antenatal culture for the most accurate detection of GBS carriers.
caption: Streptococcus agalactiae colonies in chromogenic medium (ChromID CPS chromogenic agar)
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