Increased recovery of Streptococcus agalactiae from vaginal-rectal swabs using a selective enrichment broth medium

The aim of this study was to clarify whether the detection rate of group B Streptococcus (GBS) in screening tests for pregnant women at 35-37 weeks of gestation increases using the selective enrichment media available in Japan.

Vaginal-rectal swabs were collected from pregnant women at 35-37 weeks of gestation from nine obstetric medical institutions in the Nagasaki Prefecture. The collected swabs were inoculated into selective enrichment media and directly plated onto blood agar to investigate the differences in the isolation frequencies of GBS.

We collected vaginal-rectal swabs from 116 individuals and successfully isolated GBS from 22 individuals (19.0%) using a selective enrichment broth. When the swabs were directly plated onto blood agar for isolation, GBS was not isolated in nine (40.9%) out of the 22 positive samples. Among these nine samples, six (66.7%) showed reduced pigment production in at least one of the selective enrichment broths that detected GBS based on pigment production.

In a GBS screening test for pregnant women, use of a selective enrichment broth resulted in an approximately 1.7-fold increase in the number of positive samples compared to the direct plating method on blood agar. Although not described in the American Society for Microbiology guidelines, RambaQUICK GBS broth and Poamedia semisolid medium for GBS, both available in Japan, detected GBS significantly more effectively than the direct plating method on blood agar. This finding supports the utility of selective enrichment broths for the GBS screening of pregnant women.