Comparative Genomics Guides Elucidation of Vitamin B12 Biosynthesis in Novel Human-Associated Akkermansia Strains.

Akkermansia muciniphila is a mucin-degrading bacterium found in the gut of most humans and is considered a "next-generation probiotic." However, knowledge of the genomic and physiological diversity of human-associated Akkermansia sp. strains is limited. Here, we reconstructed 35 metagenome-assembled genomes and combined them with 40 publicly available genomes for comparative genomic analysis. We identified at least four species-level phylogroups (AmI to AmIV), with distinct functional potentials. Most notably, we identified genes for cobalamin (vitamin B12) biosynthesis within the AmII and AmIII phylogroups. To verify these predictions, 10 Akkermansia strains were isolated from adults and screened for vitamin B12 biosynthesis genes via PCR. Two AmII strains were positive for the presence of cobalamin biosynthesis genes, while all 9 AmI strains tested were negative. To demonstrate vitamin B12 biosynthesis, we measured the production of acetate, succinate, and propionate in the presence and absence of vitamin supplementation in representative strains of the AmI and AmII phylogroups, since cobalamin is an essential cofactor in propionate metabolism. Results showed that the AmII strain produced acetate and propionate in the absence of supplementation, which is indicative of vitamin B12 biosynthesis. In contrast, acetate and succinate were the main fermentation products for the AmI strains when vitamin B12 was not supplied in the culture medium. Lastly, two bioassays were used to confirm vitamin B12 production by the AmII phylogroup. This novel physiological trait of human-associated Akkermansia strains may affect how these bacteria interact with the human host and other members of the human gut microbiome.IMPORTANCE There is significant interest in the therapeutic and probiotic potential of the common gut bacterium Akkermansia muciniphila However, knowledge of both the genomic and physiological diversity of this bacterial lineage is limited. Using a combination of genomic, molecular biological, and traditional microbiological approaches, we identified at least four species-level phylogroups with differing functional potentials that affect how these bacteria interact with both their human host and other members of the human gut microbiome. Specifically, we identified and isolated Akkermansia strains that were able to synthesize vitamin B12 The ability to synthesize this important cofactor broadens the physiological capabilities of human-associated Akkermansia strains, fundamentally altering our understanding of how this important bacterial lineage may affect human health.