Delivery mode impacts gut bacteriophage colonization during infancy.

Background: Cesarean section delivery is associated with altered early-life bacterial colonization and later adverse inflammatory and immune health outcomes. Although gut bacteriophages can alter gut microbiome composition and impact host immune responses, little is known about how delivery mode impacts bacteriophage colonization over time. To begin to address this we examined how delivery mode affected bacteriophage colonization over the first two years of life. Results: Shotgun metagenomic sequencing was conducted on 272 serial stool samples from 55 infants, collected at 1-2 days of life and 2, 6, 12 and 24 months. 33/55 (60%) infants were born by vaginal delivery. DNA viruses were identified, and by host inference, 94% of the viral sequences were found to be bacteriophages. Alpha diversity of the virome was increased in vaginally delivered infants compared to cesarean section delivered infants at 2 months (Shannon index, p=0.022). Beta diversity significantly differed by delivery mode at 2, 6, and 12 months when stratified by peripartum antibiotic use (Bray-Curtis dissimilarity, all p<0.05). Significant differentially abundant predicted bacteriophage hosts by delivery mode were seen at all time points. Moreover, there were differences in predicted bacteriophage functional gene abundances up to 24 months by delivery mode. Many of the functions considered to play a role in host response were increased in vaginal delivery. Conclusions: Clear differences in bacteriophage composition and function were seen by delivery mode over the first two years of life. Given that phages are known to affect host immune response, our results suggest that future investigation into how delivery mode may lead to adverse inflammatory outcomes should not only include bacterial microbial colonization but also the potential role of bacteriophages and transkingdom interactions.