Evaluation of the group B differential agar for the detection of group B streptococci from vaginal specimens

Background Group B streptococci (GBS) are the leading cause of severe perinatal
infections. Most current guidelines for the prevention of GBS perinatal disease are
based on prenatal screening culture for vaginal GBS colonisation. Use of selective
and differential media could improve the sensitivity of these cultures.
Objective To evaluate the GBS-Differential Agar (GBSDA) recently formulated by
Becton Dickinson for the selective growth and production of orange colonies of b-
hemolytic (b-H) GBS.
Methods 283 vaginal swabs (VAG) collected from pregnant women were inoculated
in selective Lim broth. After overnight incubation, Lim broth were subcultured on
GBSDA, on Granada agar (Biomedics, Spain) and on Columbia blood agar (BA). To
evaluate the stability, 99 isolates of GBS (REF) from adult or neonatal infections
(Belgian GBS reference laboratory collection) were cultured on GBSDA and Granada
at their limit of expiration, and on BA. GBSDA and Granada were incubated
anaerobically and BA aerobically + 7% CO2, at 35°C, 24 to 48 h. Positive and
negative control strains (GBS ; E. faecalis) were cultured with each run. Specific
identification of colonies suggestive of GBS (pale to dark orange on GBSDA and
Granada, b-H on BA) was performed.
Results b-H GBS were recovered from 63 VAG (22.3 %): 62 were easily identified
after overnight incubation on GBSDA and 63 on Granada without requiring any
subculture. All GBS were also recovered from BA however it was after many
subcultures. All orange colonies were confirmed as GBS. Among REF, 3 strains
were non hemolytic ; they grew but were not differentiated as orange colonies on
GBSDA or Granada. 96 REF were b-H, 94 (97.9%) produced orange to very dark
orange colonies on GBSDA, 2 produced white colonies, and on Granada, 74 (77.1
%) produced pale to dark orange colonies and 22 white to white-orange colonies.
Conclusion 1) GBSDA and Granada: a) very high sensitivity and specificity for the
detection of b-H GBS, in a single step b) Results available within 48 h after
inoculation in Lim broth, low workload 2) Excellent stability up to expiration date for
GBSDA 3) Non hemolytic GBS: grown but not differentiated on GBSDA or Granada.