SRP343443 PRJNA774819 Highly specialized carbohydrate metabolism capability in Bifidobacterium strain associated with intestinal barrier maturation in early preterm infants Leaky gut, or high intestinal barrier permeability, is common in preterm newborns. The role of microbiota in this process remains largely uncharacterized. We employed both short and long read sequencing of the 16S rRNA gene and metagenomes to characterize the intestinal microbiome of a longitudinal cohort of 113 preterm infants born between 24 to 33 weeks of gestation. Enabled by enhanced taxonomic resolution, we found significantly increased abundance of Bifidobacterium breve and a diet rich in mothers breastmilk to be associated with intestinal barrier maturation during the first week of life. We combined these factors using genome resolved metagenomics and identified a highly specialized genetic capability of the Bifidobacterium strains to assimilate human milk oligosaccharides and host derived glycoproteins. Our study revealed mechanistic roles of breastmilk feeding and intestinal microbial colonization in postnatal intestinal barrier maturation; these observations are critical towards advancing therapeutics to prevent and treat hyperpermeable gut-associated conditions, including necrotizing enterocolitis. human gut metagenome