(A) Schematic of the experimental design. Samples collected from preterm infants were cultured to create individualized microbial communities. The communities were used to colonize pregnant germ-free dams on E15–17. Pups acquired bacteria from the mothers in the perinatal period, followed by repeated exposure to topically applied bacteria on the mother’s fur on P2 and P4. Fecal samples were collected for 16S rRNA gene sequencing in postnatal week 2. Final weight, length, and tissues were collected on P20–21. (B) Principal coordinates analysis of 16S rRNA gene sequencing of donor infant fecal microbiotas (squares) and mouse pup fecal microbiotas (triangles) based on Jensen-Shannon divergence. Circles represent the centroid for each pair. Each color represents a unique preterm infant-mouse pair (gray shades, appropriate growth subjects; red/orange shades, poor growth subjects). (C) Relative abundances of bacterial species (or the lowest taxonomic assignment for sequences not resolved to the species level) in preterm infant and neonatal mouse fecal samples, pooled by group. (D) Mean (SE) Jensen-Shannon divergence between matched donor preterm infants and recipient mouse sample pairs and mismatched pairs. (E) Proportion of reads in the preterm infant fecal microbiota representing ASVs that were identified in the corresponding recipient mouse fecal samples (dark blue), not identified in the recipient mouse feces but identified by 16S rRNA sequencing of cultured communities (blue-gray), or not identified in recipient mouse feces or culture collections (gray). Bars represent individual preterm infants. (F) Taxa with higher relative abundance in the microbiotas of pups in the poor growth (red) or appropriate growth (gray) groups using generalized linear models. Figure panels represent samples from 10 preterm infants and 25 mouse litters.