Optimizing culture-based detection of Group B Streptococcus in pregnant women: a comparative evaluation of Granada™ and CHROMID™ StrepB

Streptococcus agalactiae colonises between 10-30% of pregnant women and can cause serious neonatal infections that lead to significant mortality. Culture-based screening for group B Streptococcus (GBS) late in pregnancy guides intrapartum antibiotic prophylaxis, but may fail to detect some carriers. We compared three culture strategies for GBS detection.

Retrospective single-centre study of 530 vagino-rectal samples from 508 pregnant women screened at 37 weeks of gestation (October 2024-April 2025). Samples were inoculated into Todd-Hewitt broth and plated directly onto Granada™ agar (GRAN1); after enrichment, the broth was subcultured onto Granada™ agar (GRAN2) and a chromogenic medium (STRB). Presumptive colonies were confirmed by MALDI-TOF MS. Diagnostic performance was assessed against a composite reference standard (confirmed GBS on any strategy), and methods were compared using Cohen's kappa and McNemar's exact test.

GBS was detected in 90 samples (17.0%). Positivity rates were 15.7% for GRAN1, 16.2% for GRAN2 and 16.8% for STRB, with sensitivities of 92.2%, 95.6% and 98.9%, respectively. Agreement was almost perfect (κ = 0.944-0.979), with no statistically significant differences between methods (McNemar's exact test, p ≥ 0.070). The GRAN1-STRB combination detected all positive samples, providing presumptive results within 24 h at a culture media cost of €5.17 per sample.

Differences between culture strategies were small. Combining direct plating onto Granada™ agar with post-enrichment chromogenic subculture detected all GBS-positive samples and provided presumptive results within 24 h. This approach may be a useful alternative for culture-based GBS screening.