Temporal Characterization of Carrot Broth-Enhanced Real-Time PCR as an Alternative Means for Rapid Detection of Streptococcus agalactiae from Prenatal Anorectal and Vaginal Screenings

Abstract

ABSTRACT Analysis of overnight carrot broth culture using the BD GeneOhm StrepB assay (carrot broth-enhanced PCR) yields increased sensitivity compared to that of carrot broth culture alone for the detection of Streptococcus agalactiae . We investigated the prospect of reducing the carrot broth incubation time prior to PCR performance. In vitro experimentation demonstrated that carrot broth-enhanced PCR nominally detected 10 CFU S. agalactiae after 4 h of carrot broth incubation with competitive flora. Detection rates improved with inocula of 100 and 1,000 CFU S. agalactiae , with the majority of these aliquots demonstrating detection after 2 h of carrot broth incubation. Carrot broth was prospectively inoculated with clinical vaginal/anorectal swabs, with 500-μl aliquots collected. Early aliquots from 227 specimens were subjected to carrot broth-enhanced PCR (early-aliquot carrot broth-enhanced PCR) in instances of subsequent positive carrot broth culture or positive overnight clinical carrot broth-enhanced PCR. The S. agalactiae detection rate by early-aliquot carrot broth-enhanced PCR (66.1%) exceeded that observed for 227 remnant swabs retrospectively tested by direct swab PCR (56.4%; P = 0.03). Early-aliquot carrot broth-enhanced PCR detection rate differences were most pronounced in aliquots from 83 carrot broth aliquots collected after 6 h (84.3%) compared to detection rates from either direct swab PCR of these samples (51.8%; P < 0.0002) or early-aliquot carrot broth-enhanced PCR of 144 carrot broth aliquots collected after fewer than 6 h of incubation (55.6%; P < 0.0002). Enhanced sensitivity of early-aliquot carrot broth-enhanced PCR versus direct swab PCR suggests that this assay could serve as a surrogate rapid detection method facilitating the prevention of group B streptococcal disease.