Evaluation of Trans-Vag Broth, Colistin-Nalidixic Agar, and CHROMagar StrepB for Detection of Group B Streptococcus in Vaginal and Rectal Swabs from Pregnant Women in South Africa

Abstract

d Maternal vaginal colonization with group B streptococcus (GBS) is a major risk factor for invasive GBS infection in newborns.The CDC-recommended method for detecting GBS colonization is to culture vaginal and rectal swabs in a selective broth followed by subculture on blood agar or a selective medium. A high incidence of antimicrobial resistance in the fecal microflora can compromise the recovery of GBS from the selective broth. Here, we compared CHROMagar StrepB (CA), Columbia colistinnalidixic agar (CNA), and Trans-Vag selective broth enrichment for the isolation of GBS from 130 vaginal and 130 rectal swabs from pregnant women. The swabs were randomized for plating first on either CA or CNA, and they then were inoculated in Trans-Vag broth. GBS was cultured from 37.7% of the vaginal swabs and 33.1% of the rectal swabs. There were no differences in the detection rates for the vaginal swabs between CA (31.5%), CNA (26.2%), and the selective broth (30.0%). The sensitivities in relation to a composite score were 83.7%, 69.4%, and 79.6%, respectively. However, recovery of GBS from the rectal swabs was significantly higher from CA (29.2%; P < 0.0001) and CNA (23.8%; P ‫؍‬ 0.002) than from the selective broth (9.2%). The sensitivities were 88.4%, 72.1%, and 27.9%, respectively. The order of plating on the solid medium was significant (P ‫؍‬ 0.003), with GBS detection rates of 30.8% and 24.6% when swabs were plated first and second, respectively. These findings show that a selective broth is not suitable for the recovery of GBS from rectal swabs in settings such as ours, due to masking of the GBS colonies by persistent microflora. Infection by group B streptococcus (GBS) is one of the most common infections in newborns (1). Maternal colonization has been found to be a major risk factor for invasive GBS disease within 6 days of birth. Approximately 10 to 40% of pregnant women are colonized with GBS in either the vagina, the rectum, or both areas (2). The rate of peripartum transmission of GBS to newborns of colonized women is approximately 50%, after which 1 to 2% of these newborns develop invasive GBS infection in the first week of life (3, 4). The Centers for Disease Control and Prevention (CDC) has recommended that all pregnant women be screened for rectovaginal carriage of GBS at 35 to 37 weeks' gestation to identify women who should receive intrapartum antimicrobial prophylaxis (IAP). When successfully implemented, IAP targeted at GBS-colonized pregnant women has reduced the incidence of invasive GBS disease by 86 to 89% (5-7). The sensitivities of screening methods for the identification of maternal carriage of GBS depends on the timing of specimen collection, the source of the specimen, and the culture technique used. Optimally, specimens should be collected as close to delivery as possible. The use of vaginal and rectal swab specimens has been shown to yield higher GBS culture-positivity rates than vaginal swabs alone or cervical specimens (8-10). The current CDC recommendation for the isolation of GBS from v...

Occurrence and detection method evaluation of group B streptococcus from prenatal vaginal specimen in Northwest China

“…Comparison of SBA, CNA and SCB methods revealed that SCB was the most reliable. The sensitivity of CNA was similar to previous reports [ 6 , 9 ] as was the sensitivity of SCB [ 3 ]. SCB is more expensive than SBA or CNA but processing time is reduced.…”

Risk factors for Group B Streptococcus colonisation and disease in Gambian women and their infants

“…Our study has several limitations. First, our results are limited by the sensitivity of detection of GBS on selective media, estimated at 85% 30 and by the fact that we identified the serotype by conventional PCR. This may imply that we have underestimated low levels of colonisation in infants where DNA was below the limit of detection.…”

Natural acquired humoral immunity against serotype-specific group B Streptococcus rectovaginal colonization acquisition in pregnant women

“…We tried to optimize the comparability of our assay to that of other laboratories by calibrating our in-house reference (Polygam) with reference sera from C. J. Baker, who has published widely in the field. Secondly, the sensitivity of detection of GBS on selective media is estimated at 85% and mainly identifies the dominant colonizing serotype, with lower-density co-colonizing serotypes likely being missed [23]. Therefore, it is possible that we overestimated new acquisition through missing colonization episodes not detectable by standard culture methods at visit 1, and we similarly may also have missed some new acquisitions.…”

“…Samples were collected by trained study nurses with rayon-tipped swabs that were placed into Amies transport medium without charcoal (Transwab Amies; Medical Wire, UK). For GBS isolation, swabs were inoculated onto CHROMagar StrepB (CA; Media Mage, South Africa) as previously described [23]. Serotyping was performed by the latex agglutination method with specific antisera (Statens Serum Institute, Sweden) to serotypes Ia, Ib and II to IX CPS antigens, as previously described [24].…”