CRISPR-Cas immune repertoires as an ecological record of bacterial interactions with mobile genetic elements in the human gut

Bacteria in the human gut influence host physiology and disease risk, but their ecology is strongly shaped by mobile genetic elements (MGEs) such as phages and plasmids. Past interactions between bacteria and MGEs can be inferred from CRISPR-Cas cassettes, which contain short DNA fragments derived from invading elements. To lay the groundwork for research on the impact of such interactions on the human host, we constructed an extended microbiome resource comprising 1.7 K prokaryotic mOTUs, 19.5 K viral vOTUs, and 24.2 K plasmid PTUs, using fecal shotgun metagenomes from 1034 adults over 55 y of age residing in South-East Norway. We also recovered 74.2 K unique CRISPR-Cas cassettes to map past bacteria-MGE interactions and assessed their associations with the human diet and lifestyle factors. CRISPR-Cas spacers, and which viruses and plasmids they targeted, varied substantially within bacterial species, but were predominantly directed towards cohort-specific MGEs. Moreover, bacteria were more likely to target MGEs present in the same sample, consistent with local exposure. Plasmid MGEs were more often targeted by Type II CRISPR-Cas cassettes, whereas viruses were more likely to be targeted by Type I CRISPR-Cas cassettes. Bacteria also shared more targets within taxonomic families than across families, where mobilizable plasmids were more frequent among the targets. CRISPR-Cas cassettes mirrored microbiome associations to human demographic and lifestyle factors and enabled the recovery of dairy-associated B. animalis. Together, this research provides a large-scale resource and a structured analysis of bacteria-MGE interactions in the gut microbiome and their contribution to microbial ecosystem dynamics.