Changes in bacterial and metabolic profiles in breastfed infants during dietary transition to solids.

BACKGROUND: Early-life microbiota plays a crucial role in immune and metabolic development, yet its maturation during the key dietary transition from human milk to solid food remains poorly characterized. METHODS: In this pilot study, we aimed to characterize changes in the oral and fecal bacteriomes of breastfed infants throughout this period. Oral swabs and fecal samples from twelve mother-infant pairs (breastfed infants) from the CELSPAC:TNG cohort were collected at five time points from two to twelve months postpartum and analyzed using 16S rRNA amplicon sequencing. Whole metabolome profiling of infants' fecal and mothers' human milk samples was conducted using NMR spectroscopy. RESULTS: Continual changes in relative abundances of several bacterial genera in oral (particularly Gemella and Streptococcus) and fecal (particularly Blautia and Intestinibacter) samples, as well as in the metabolome (including concentrations of short-chain fatty acids), were observed throughout the first year of life. Cross-modal omics analysis revealed a strong negative relationship between the concentration of β-hydroxybutyrate and the genus Bifidobacterium in infants' fecal samples. CONCLUSION: This study provides a detailed description of the changes in oral and fecal bacteriomes throughout the first year of infants' lives and associates these changes with changes in the fecal metabolome, including short-chain fatty acids. IMPACT: The infant gut bacteriome undergoes a marked compositional shift during the transition to solid food, including increases in relative abundances of genera Blautia and Intestinibacter. In oral samples, relative abundances of genera Gemella and Streptococcus were decreasing, while those of Porphyromonas, Granulicatella, and Neisseria were increasing. These shifts are associated with longitudinal changes in infants' fecal metabolome. A strong negative association between Bifidobacterium and β-hydroxybutyrate highlights a key relationship between early-life taxa and metabolic outputs.