Group B <italic>Streptococcus</italic> and Vaginal Microbiome Dynamics in Pregnancy.

This study aimed to compare the vaginal microbiome of group B Streptococcus (GBS)-positive and -negative pregnant women in selected tertiary care hospitals in Sri Lanka. Fifteen GBS-positive and fifteen GBS-negative women were included in the study. The composition of vaginal microbiome was profiled by sequencing 16S rRNA V3-V4 region. Microbiome diversity and variation were evaluated. Alpha diversity measured by Shannon diversity and Chao 1 index differed significantly between GBS-positive and GBS-negative groups (Chao 1, p = 0.0003; Shannon, p = 0.0003), while beta diversity pairwise permutational multivariate analysis of variance (p = 0.435) showed no significant difference, indicating similar overall bacterial compositions. Gardnerella and Prevotella were more abundant in GBS-positive group, while majority of the GBS-negative group was dominated by Lactobacillus (87.9%). Lactobacillus iners was the most abundant species in both groups. Lactobacillus gallinarum, reported for the first time in Sri Lanka, was the second most common Lactobacillus species. Linear discriminant analysis effect size identified no taxa enriched in GBS-negative group, whereas GBS-positive group showed significantly higher relative abundance with several taxa with Bifidobacterium showed the highest linear discriminant analysis score (4.459). Methylocystis, though of low biological relevance, was significantly abundant in GBS-positive group (p = 0.025). Lactobacillus spp. were reduced in GBS-positive women, with higher abundance of other bacterial taxa suggesting that disruption of a Lactobacillus dominant vaginal microbiome may facilitate GBS colonization and potentially increase the risk of neonatal infection.