Assembly-based analysis of the infant gut microbiome reveals novel ubiquitous plasmids

Little is known about the role of mobile genetic elements in natural ecosystems such as the infant gut microbiome. Here, we conduct the most comprehensive longitudinal study of the infant plasmidome to date by analyzing monthly fecal samples from 12 infants from birth to one year of age. We employ an assembly-based bioinformatic pipeline for the reconstruction and identification of full-length plasmids, including a novel approach for assigning putative plasmid hosts. We then investigated plasmid content and dynamics in the infant gut microbiome. After assembly and identification, we identified 620 unique circular plasmids in the infant cohort, including a number of novel sequences. Independent assembly of the same plasmids in several samples and infants helped corroborate the authenticity of the plasmids. Among the observed plasmids was the recently described ubiquitous and abundant Bacteroides plasmid pBI143. Overall, the genus Bacteroides had the highest plasmid carriage, while the highest plasmid diversity was observed in Clostridium, including 5 previously unknown widespread plasmids. Lastly, we leveraged the longitudinal nature of our dataset to investigate contemporaneous correlations between temporal variations in plasmid abundances and species dynamics. This enabled us to link co-residing plasmids and tightly linked plasmid-taxon pairs within each infant. These insights into plasmid ecology help us understand determinants driving plasmid distribution in complex microbial communities.