Hybrid, ultra-deep metagenomic sequencing enables genomic and functional characterization of low-abundance species in the human gut microbiome.

A large number of microbial genomes have already been identified from the human gut microbiome, but the understanding of the role of the low-abundance species at the individual level remains challenging, largely due to the relatively shallow sequencing depth used in most studies. To improve genome assembling performance, a HiSeq-PacBio hybrid, ultra-deep metagenomic sequencing approach was used to reconstruct metagenomic-assembled genomes (MAGs) from 12 fecal samples. Such approach combined third-generation sequencing with ultra-deep second-generation sequencing to improve the sequencing coverage of the low-abundance subpopulation in the gut microbiome. Our study generated a total of 44 megabase-scale scaffolds, achieving four single-scaffolds of complete (circularized, no gaps) MAGs (CMAGs) that were the first circular genomes of their species. Moreover, 475 high-quality MAGs were assembled across all samples. Among them, 234 MAGs were currently uncultured, including 24 MAGs that were not found in any public genome database. Additionally, 287 and 77 MAGs were classified as low-abundance (0.1-1%) and extra-low-abundance (<0.1%) gut species in each individual, respectively. Our results also revealed individual-specific genomic features in the MAG profiles, including microbial genome growth rate, selective pressure, and frequency of chromosomal mobile genetic elements. Finally, thousands of extrachromosomal mobile genetic elements were identified from the metagenomic data, including 5097 bacteriophages and 79 novel plasmid genomes. Overall, our strategy represents an important step toward comprehensive genomic and functional characterization of the human gut microbiome at an individual level.