False-positive detection of Group B Streptococcus (GBS) in chromogenic media due to presence of Enterococcus faecalis in High Vaginal Swabs

Vaginal colonization of Group B Streptococcus (GBS) is associated with preterm births and neonatal sepsis. Thus, routine screening of GBS in prenatal care is recommended. Chromogenic media are useful in rapid and sensitive screening for GBS. herein, we evaluated the performance of Carrot broth for the detection of GBS in vaginal swabs of pregnant women. In all 20/201 (9.9%) vaginal swab samples were positive in the carrot broth. 17/20 (85%) and 19/20 (95%) samples yielded colonies on Blood agar and Crome agar respectively. However, 16s rRNA sequencing revealed that none of the carrot broth positive cultures had sequence similarities to the Enterococcus faecalis and not GBS. Furthermore, Enterococcus faecalis was detected by PCR in DNA isolated from the corresponding uncultured vaginal swabs samples, while GBS could be detected by PCR only in 4 samples. Thus carrot broth-based culture can lead to false-positive detection due to the presence of Enterococcus faecalis.