[Evaluation of the new chromogenic medium StrepB Select for screening of group B Streptococcus in pregnant women]

To compare and evaluate the new chromogenic StrepB Select (BioRad) medium to the Granada (bioMérieux) and Columbia horse blood agar plates (CH-BAP) for screening of Group B Streptococcus (GBS) vaginal colonization in pregnant women.

One hundred and ninety vaginal swabs were processed and the three media inoculated. All plates were examined after 18-24h incubation at 37 degrees C and reincubated for an additional 24h. All suspected colonies were identified as GBS by using a commercial Lancefield group-specific latex agglutination test.

GBS were isolated in 32 samples (16.8%) by at least one medium. After 24h of incubation, GBS were recovered on CH-BAP in 16 samples (50%) compared to 28 (87.5%) for StrepB Select, and 27 (84.5%) for Granada. After 48h of incubation, 30 (93.7%) out of the 32 GBS vaginal carriers were positive with CH-BAP, StrepB Select and Granada. Other streptococcal and enterococcal species gave GBS-like colonies on StrepB Select medium implying the use of other tests to confirm GBS identification. We demonstrated that direct agglutination of GBS isolated all media with Pastorex StrepB accurately identified GBS. The two StrepB Select false-negative results corresponded to a very low colonisation rate. Two Granada false-negative results corresponded to non-haemolytic and non-pigmented GBS strains which were correctly identified on StrepB Select. Both selective media inhibit the growth of associated saprophytic flora.

The use of the new StrepB Select chromogenic medium in routine laboratories may therefore markedly facilitate the rapid and accurate detection of GBS in vaginal samples.