Abstract
Background/Objectives: Early-life gut colonization is crucial for immune system development and metabolic programming. Lactobacillus reuteri has been investigated for its capacity to modulate neonatal gut microbiota, but evidence regarding maternal supplementation during lactation remains limited. This study aimed to evaluate the effect of maternal supplementation with L. reuteri SGL 01 on the microbial composition of breast milk and neonatal feces over the first month of life. Methods: This is an exploratory, prospective, open-label randomized clinical trial. Lactating mothers of full-term and exclusively breastfed infants were randomized to receive either daily L. reuteri SGL 01 (1 × 10(9) CFU) for 30 days or no supplementation. Quantitative real-time PCR was used to assess Bifidobacterium spp., Lactobacillus spp., Clostridium spp., and the Bacteroides fragilis group in maternal milk and neonatal feces at baseline (T0) and after 30 days (T1). Results: Twenty-seven mother-infant dyads completed the study (15 supplemented, 12 controls). No significant changes in breast milk microbiota composition were observed across any of the bacterial taxa following maternal supplementation. In contrast, neonatal fecal samples from the supplemented group showed significant increases in Bifidobacterium spp. (p < 0.001), Lactobacillus spp. (p = 0.029), and Clostridium spp. (p = 0.003) at T1. No significant microbial changes were observed in the control group, except for a slight reduction in Clostridium spp. (p = 0.046). Conclusions: Maternal supplementation with L. reuteri SGL 01 did not modify breast milk microbiota but was associated with a modulation of neonatal gut colonization, including an increased abundance of beneficial taxa such as Bifidobacterium, suggesting potential indirect maternal-to-infant microbial effects.