Probiotics in pregnancy and group B streptococcus colonization: A multicentric, randomized, placebo-controlled, double-blind study with a focus on vaginal microbioma

Objective: To evaluate the feasibility and effects of the use of probiotics in pregnancy, starting in the third trimester, on rectovaginal colonization of group B streptococcus (GBS) in women at low obstetric risk. Methods: A multicentre, randomized, placebo-controlled, double-blind, parallel-group study was conducted in three tertiary hospitals in northern Italy and included low-risk pregnant women. The intervention consisted of oral administration of two capsules of probiotics or placebo from 30 weeks of pregnancy until 37 weeks of pregnancy. The primary outcome was GBS colonization, evaluated with rectovaginal swabs. In a subgroup, selected at random, changes in the vaginal microbiome after treatment administration were evaluated using 16S Metagenomic Sequencing Library Preparation sequencing and analysis. Results: In total, 267 pregnant women were randomized to receive probiotics (n = 133) or placebo (n = 134). The two groups were similar at baseline. After treatment, no differences were found in the rates of positive rectovaginal swabs (p = 0.24) and antibiotic administration (p = 0.27). Only one case of postpartum fever (>38 °C) was found in the placebo group. Labour and delivery outcomes and neonatal outcomes were similar in both groups. Analysis of the vaginal microbiota showed that the relative abundance of Lactobacillus spp. was not modified significantly by the probiotics, but the relative abundance of Gardnerella spp. decreased significantly (3.6 ± 7.9 vs 5.5 ± 10.2; p = 0.03). Interestingly, the relative abundance of Lactobacillus spp. reduced significantly in women who subsequently presented with partial rupture of membranes (46.9 ± 43.6 vs 77.7 ± 24.9; p = 0.02). Conclusion: Although the clinical outcomes were unaffected, administration of probiotics led to favourable changes in vaginal microbiota. It remains to be established how this effect could be translated into clinical advantage.