Metagenomic Identification and Characterization of Novel Vitamin B12 Synthesizers from the Rumen of Beef Cattle Fed High-Lipid Inclusion Diets.

Beef production in intensive systems requires optimal nutrition to maximize growth and profitability. While triglycerides contain twice the energy per unit weight compared to polysaccharides, they are not nearly as commonly used as a supplemental source of energy compared to starch, largely in part due to their negative effects on rumen physiology when their inclusion levels are too high. To gain further insights into the response of rumen microbial communities to elevated dietary lipid levels, we took advantage of rumen samples collected as part of a previously published study that tested high inclusion (4% and 8%) of tallow or linseed oil in beef cattle as part of a 5 × 5 Latin square design, with corn used as a base dietary ingredient. Using a 16S rRNA gene-based profiling approach, two uncharacterized candidate rumen bacterial Operational Taxonomic Units (OTUs), referred to as Bt-995 and Bt-1367, were found to be in higher abundance in rumen samples collected from steers when they were fed diets with higher inclusion of linseed oil. Using a metagenomics approach to assemble contigs corresponding to genomic regions of these OTUs, various predicted metabolic functions were found to be shared. Consistent with the dietary treatments of the original animal study, functions associated with starch utilization and triglyceride metabolism were identified. Unexpectedly, however, contig sets from both OTUs also encoded genes predicted to be involved in vitamin B12 biosynthesis, as well as ethanolamine utilization, a function that is dependent on vitamin B12 as a co-factor. Together, these results indicate that vitamin B12-related functions may provide an advantage to rumen bacteria under conditions of high dietary triglyceride inclusion.