The role of plasmids in the gut microbiome during the first year of life

Plasmids are extrachromosomal self-replicating genetic elements that play a key role in bacterial ecology and evolution by shuttling diverse host-beneficial traits between bacteria. However, our understanding of plasmids is still limited, particularly in the human gut microbiota, and little is known about how they are acquired and become established in infants. In this study, we explored a longitudinal fecal metagenomic dataset obtained from 98 Swedish children who were followed during their first year of life. For this, we developed a bioinformatics pipeline for the complete sequence assembly and annotation of plasmids, together with the identification of plasmid contigs. We found that gut plasmids in these children were extremely diverse, particularly in the first four months of life, and this diversity decreased with maturation of the gut microbiota. Members of genera Bacteroides and Bifidobacterium were identified as major hosts of transmissible plasmids and important hubs of horizontal gene transfer in the early human gut microbiota. Additionally, we discovered that plasmids played a substantial role in expanding the gene repertoires of their bacterial hosts: approximately a quarter of unannotated plasmid genes were found only on plasmids and not on chromosomes. Together, our results provide the first characterization of the early acquisition and development of plasmids in the infant gut microbiome. Their diversity and abundance in the first months of life could benefit a variable and rapidly proliferating microbiota by providing increased adaptability in a highly competitive environment.