Introduction:  Group B Streptococcus (GBS) is the most common cause of early onset neonatal sepsis in developed countries with 15-25% of women being asymptomatic carriers of the organism. It is recommended that an enrichment media be used to selectively screen for GBS when performing antenatal screening.

The aim of the study was to evaluate a chromogenic enrichment broth (Carrot broth)and compare the isolation rate to direct agar plate inoculation. 

Method:  150 low vaginal/perianal antenatal screening swabs for GBS were inoculated  onto Biomerieux chromID® Strepto B agar and into Strep B Carrot Broth™. Directly inoculated plates were incubated and read at 24 and 48hrs. Positive cultures were confirmed using the Vitek MS.

Carrot broth tubes were incubated at 35°C for 24hrs. The development of any orange colour in the broth was a positive result. All negative broths were subcultured to chromID®StreptoB agar, incubated and plates read at 24 and 48hrs. All positive and negative Carrot Broth tubes were confirmed using the Illumigene Group B Streptococcus (GBS) Assay ( based on DNA amplification). This was considered the “gold standard” against which true positive and true negative results were determined.

Results:  107 results were concordant negatives while 24 were concordant positives.

19 results were discordant. Of these 17 were Carrot broth positive and direct culture negative, while 2 samples were both broth and direct culture negative but positive using Illumigene GBS. 

80% of GBS positive samples showed a colour change in the Carrot Broth in less than 24hrs.

Conclusion:  Using a broth enrichment step does increase the isolation of GBS from screening swabs.

In 2012 our laboratory processed a total of 8035 samples for GBS. Based on the study figures potentially 908 false negative results were issued.

Positive Carrot Broth results can be available in as little as 6hrs and due to the chromogenic nature of the media  no additional work-up is required. This study also showed that Carrot broth enrichment was almost as sensitive as a molecular method at just a fraction of the cost.