Group B Streptococcus Vaginal–Rectal Culture Screen


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Subject: Group B Streptococcus Vaginal–Rectal Culture Screen

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  • Group B Streptococcus (GBS) infection is the leading cause of early-onset neonatal sepsis. Bacteremia, multiorgan disease, and meningitis are all possible manifestations of neonatal GBS infection. Maternal GBS carriage in the GU or GI tract is the primary risk factor for neonatal infection. The CDC and relevant professional organizations have recommended screening pregnant women for GU and GI carriage of GBS and using culture results as primary guidance for the use of intrapartum antimicrobial prophylaxis for prevention of neonatal infection.

Special Collection and Transport Instructions

  • Swab specimens should be collected at 35–37 weeks' gestation.

  • Swab the lower vagina/vaginal introitus, followed by the rectum (i.e., through the anal sphincter). Two swabs or a single swab may be used. If two swabs are used, they should be submitted to the laboratory together as a single specimen.

  • Transport to the laboratory within 24 hours in a nonnutritive transport medium; transport and store the specimen at 4°C prior to laboratory processing. Note if the patient is at risk for anaphylaxis to penicillin, an indication for susceptibility testing for GBS-positive specimens.


  • Enrichment culture: Swabs are inoculated into selective broth medium, like Todd-Hewitt broth with gentamicin (8 μg/mL) and nalidixic acid (15 μg/mL) [TransVag broth], or colistin (10 μg/mL) and nalidixic acid (15 μg/mL) [Lim broth]. Commercially available enriched chromogenic broths may be used for the enrichment culture. Incubate broth cultures for 18–24 hours at 35–37°C in room air or 5% CO2.

  • For TransVag and Lim broth, subculture to an appropriate agar medium (e.g., SBA, CNA, GBS chromogenic agar). Examine subculture plates for colonies suggestive of GBS and perform confirmatory testing.

  • Process chromogenic broth according to the manufacturer's instructions.

  • Broth enrichment is the most common method. Alternative testing of the broth enrichment culture includes specific latex agglutination or nucleic acid probe or PCR.

  • Susceptibility testing should be performed on GBS isolates for patients with significant penicillin allergy; D-testing for inducible clindamycin resistance should be performed on isolates that are clindamycin sensitive but erythromycin resistant by routine testing.

  • Direct plating of swabs may be performed in urgent circumstances, like when an unscreened woman presents in active labor, but enrichment culture should also be performed to ensure optimal sensitivity.

  • History of early-onset neonatal GBS infection in a prior pregnancy and GBS bacteriuria or UTI at any time during a current pregnancy are indications for intrapartum prophylaxis, regardless of screening test results; screening is not recommended for these patients.


  • Expected results: 10–30% of pregnant women are vaginal or rectal carriers of GBS. Carriage may be transient, intermittent, or persistant.

  • Positive results: In the absence of intrapartum prophylaxis, 1–2% of infants born to GBS-colonized mothers develop early-onset neonatal infection.

  • Negative results: The risk of early-onset GBS neonatal infection is markedly reduced but not eliminated in patients with negative vaginal–rectal carriage screens.


  • GBS carriage may be intermittent and may fall below the level of detection at the time of screening culture. Optimal detection depends on the quality of specimen collection. Both rectal and lower vaginal specimens should be submitted. Specimens unacceptable for culture: cervical, perianal, perirectal, and perineal specimens; a speculum should not be used for collection. Nonhemolytic GBS may be missed if colonies with consistent morphology are not ruled out by additional specific testing.

Suggested Reading

Prevention of Perinatal Group B Streptococcal Disease. Revised Guidelines from CDC, 2010. MMWR November 19,  2010;59(RR-10).