Objective: To evaluate group B streptococcus (GBS) detection in an in vitro setting, using a low and controlled
inoculum from swabs directly inoculated into a selective medium, as compared to delayed inoculation following
a period in a commercial Amies transport medium with charcoal (Venturi Transystem™
Study design: Clinical isolates of GBS (n = 103), were inoculated into the Amies transport medium with charcoal
in a concentration of 100 colony-forming units (cfu)/ml (10 cfu/swab). Swabs were then transferred to an
enrichment broth (NPC) at time intervals of 0, 2, 4, 6 and 24 hours. Broths were then incubated for 18–24 hours
at 35°C in air, before being transferred to New Granada Medium Modified (NGM) for GBS detection and incubated
for a further 18–24 hours at 35°C in air. If the characteristic orange pigmented colonies were observed after
this period, the specimen was recorded as + (1–10 colonies) or ++ (more than 10 colonies).
Results: Overall 92.2% (95/103) of isolates were detected in all tubes and at all times. An additional two isolates
were non-hemolytic, non-pigment forming GBS. Of note, 3.9% (4/103) were negative until 2 hours delayed
inoculation and 1.9% (2/103) gave inconsistent results, likely due to the low inoculum used.
Conclusion: Delayed inoculation into selective enrichment broth following a period in transport medium,
even with a low inoculum, gave a similar and acceptable GBS detection rate to direct inoculation. Hence,
Amies transport medium with charcoal is an appropriate transport medium to use, where it is not practical for
clinical specimens to be directly inoculated into selective enrichment broth and as endorsed in the Centers
for Diseases Control (CDC) Guidelines, 2002.