Comparison of two Chromogenic Media: Brilliance Group B and ChromID STRB for the screening of Group B streptococci from an antenatal clinic in Sydney, Australia

Microbiological methods for Group B Streptococci (GBS) screen have been greatly improved by the use of selective chromogenic substrates in chromogenic culture media. In this study we to compared and assessed the performance of two commercial chromogenic agars, Brilliance Group B (ThermoFisher, Australia) and ChromID STRB (bioMerieux, Australia) in the detection and identification of GBS (Streptococcus agalactiae). 163 antenatal low vaginal swabs were sent to the laboratory from pregnant women. The sensitivity and specificity were 100% and 91% for Brilliance Group B (BGB) and 97% and 100% for ChromID STRB. The predictive negative value for the detection of GBS on BGB was 100% and the positive predicted value was 73%. Chrom IDSTRB had a negative predicted value of 99% and a positive predicted value 100%. While both plates performed well in the screening detection of GBS, both plate media required the integration of MALDI-TOF MS (Matrix-Assisted Laser Desorption/Ionization, Mass Spectrometry, Bruker Australia) in the identification after isolation of positive-coloured colonies as indicated by the manufacturers. MALDI-TOF MS allowed accurate detection of GBS from both chromogenic plates, after prior-treatment with formic acid.