Barking up the microbiome tree: a multi-body-site characterization of maternal postpartum and pup early colonization dynamics

Early-life microbial colonization is critical for shaping host development, yet how different maternal microbial reservoirs contribute to colonization of distinct offspring body sites remains poorly understood. Here, we explored microbial colonization and maturation of oral and rectal microbiota in puppies during the first postnatal week and assessed the contributions of maternal oral, vaginal, rectal, and milk microbiota to early colonization. We collected 505 samples from 33 dams and their litters at day 1 and day 8 postpartum and characterized microbial communities using 16S rRNA gene sequencing. We found that the pup oral microbiome on day 1 closely resembled maternal milk and vaginal microbiota. In contrast, the pup rectal microbiome was initially distinct from all maternal body sites, suggesting contributions from additional unmeasured sources. By day 8, both oral and rectal microbiomes exhibited signs of early maturation, with decreased relative abundance of opportunistic taxa and increased resemblance to maternal profiles. In dams, the vaginal microbiome showed the largest postpartum shift, while the rectal microbiome exhibited a smaller but significant change, and milk and oral microbiomes remained stable. Our results reveal that maternal contributions to early microbiome assembly are body-site specific, with oral and rectal microbiomes following distinct developmental trajectories during the first week of life, consistent with a dynamic colonization process shaped by both maternal and likely additional non-maternal sources. The observed patterns parallel those reported in humans, supporting the value of dogs as a comparative model for studying early-life microbiome colonization and maternal-offspring transmission in mammals.