Objectives
Screening for Group B Streptococcus (GBS) in ante-natal patients is important to prevent post-partum neonatal GBS sepsis and meningitis. Use of chromogenic agar, enrichment broth, or a molecular method, may offer improved laboratory detection of GBS.
Methods
From August-October 2013 a total of 276 de-identified genital swabs from ante-natal patients were used to evaluate GBS detection methods. A suspension was created in 1 mL of saline and 50 μL inoculated onto one plate each of: Brilliance GBS (Thermofisher), ChromID StreptoB (bioMerieux), StrepB Select (Bio-Rad), CHROMagar StrepB (CHROMagar, Dutec), CNA and Islam agar. 50 μL was also inoculated into StrepB Carrot Broth (Hardy, Bioline) and LIM Broth. Broths were incubated at 35°C overnight then subcultured onto CHROMagar StrepB. Plates were incubated at 35°C for up to 24h and examined for typical colonies of GBS with identification confirmed by MALDI-TOF (Bruker). For the first 224 samples, the incubated LIM broth was stored at 4°C until tested by Illumigene GBS assay (Meridian, Bioline).
Results
50 samples were positive for GBS by any culture method (50/275, 18.2%). Results by direct culture were similar for each agar: Brilliance GBS 41/50 (82%), ChromID StreptoB 39/50 (78%), StrepB Select 42/50 (84%), CHROMagar StrepB 41/50 (82%), and Islam Agar 41/50 (82%). Orange pigment in Carrot Broth indicated GBS after 24h in 41/50 (82%). Culture using enrichment broth showed more positives using LIM broth (45/50, 90%) than Carrot Broth (41/50, 82%). Five additional GBS-positives (15.6%) were obtained after LIM broth compared to direct culture on CHROMagar StrepB. Of 224 samples tested by Illumigene, 61 samples tested positive for GBS (27%).
Conclusions
Chromogenic GBS agars produced similar recovery of GBS on direct culture compared to culture on Islam and CNA agar. LIM broth enrichment increased detection of GBS. The illumigene assay detected the highest number of GBS positives.