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

In this study we compared and assessed the agreement of two commercial chromogenic agars, Brilliance Group B (ThermoFisher, Australia) and ChromID STRB (bioMerieux, Australia) in the detection and presumptive identification of GBS (Streptococcus agalactiae) by colour and later confirmed by MALDI-TOF MS (Matrix-Assisted Laser Desorption/Ionization, Mass Spectrometry, Bruker Australia). The sensitivity and specificity based on colour were 100% and 91% for Brilliance Group B (BGB) and 97% and 100% for ChromID STRB. The PNV (predictive negative value) for GBS on BGB was 100% and the PPV (positive predicted value) was 73%. Chrom IDSTRB had a NPV of 99% and a PPV of 100%. While both plates compared well in accuracy in the presumptive identification of GBS by colour, both plate media required the integration of MALDI-TOF MS for confirmatory identification. MALDI-TOF MS allowed accurate detection of GBS from both chromogenic plates with a score >= 2.0 after prior-treatment with formic acid.