Breastfeeding and maternal microbial reservoirs are associated with infant gut phageome development during the first year of life.

The early-life assembly of the gut microbiome is a critical determinant of long-term health, yet the role of bacteriophages in this process remains poorly understood. Here we longitudinally profile the gut phageome of 34 mother-infant dyads during the first year of life with metagenomics and a phage-centered computational workflow. Phage diversity increases with age, paralleling bacterial diversification but following distinct ecological trajectories. Shared viral populations across maternal and infant samples are consistent with potential maternal contribution, although not indicative of direct transmission. Feeding-related variables are consistently associated with phageome composition at specific developmental stages. No single factor fully explains phageome variation, supporting a multifactorial model involving maternal, environmental, and dietary inputs. Together, these findings provide a framework for understanding early-life phage ecology, positioning phages as active participants in early microbiome assembly and supporting the inclusion of viral components in microbiome-based strategies aimed at promoting healthy ecosystem development in infancy.