Comparison of culture with two different qPCR assays for detection of rectovaginal carriage of Streptococcus agalactiae (group B streptococci) in pregnant women

Development of rapid and sensitive detection methods for group B streptococci (GBS) in pregnant women remains useful in order to adequately identify pregnant women at risk of transferring GBS to their neonate. This study compared the CDC recommended sampling and culture method with two qPCR methods for detecting GBS colonization. For a total of 100 pregnant women at 35-37 weeks of gestation, one rectovaginal ESwab each was collected. Eswab medium was inoculated into Lim broth, incubated for 24 h and plated onto chromID™ Strepto B agar (ChromAgar). DNA was extracted with the bioMérieux easyMAG platform, either directly from the rectovaginal ESwab or from Lim broth enrichment culture. Two different qPCR formats were compared, i.e. the hydrolysis probe format (Taqman, Roche) targeting the sip gene and the hybridization probe format (Hybprobe, Roche) targeting the cfb gene. Both qPCR techniques identified 33% of the women as GBS-positive. Only one culture-positive sample was qPCR-negative. QPCR directly on the sample significantly increased the number of women found to be GBS-positive (27%) compared to culture (22%). Moreover, the sensitivity of qPCR after Lim broth enrichment (33%) was again significantly higher than qPCR after DNA extraction directly from the rectovaginal swabs (27%).In conclusion, for prenatal screening of GBS from rectovaginal samples of pregnant women, our results are in accordance with CDC guidelines, which suggest using qPCR after Lim broth enrichment in addition to conventional (culture-based) detection. qPCR after Lim broth enrichment further increased the percentage of GBS-positive women, as detected by direct qPCR, from 27 to 33%, although the bacterial inoculum was low for these subjects.