Metagenomics-Based Analysis of Candidate Lactate Utilizers from the Rumen of Beef Cattle

In ruminant livestock production, ruminal acidosis is an unintended consequence of the elevated dietary intake of starch-rich feedstuffs. The transition from a state of subacute acidosis (SARA) to acute acidosis is due in large part to the accumulation of lactate in the rumen, which is a consequence of the inability of lactate utilizers to compensate for the increased production of lactate. In this report, we present the 16S rRNA gene-based identification of two bacterial operational taxonomic units (OTUs), Bt-01708_Bf (89.0% identical to Butyrivibrio fibrisolvens) and Bt-01899_Ap (95.3% identical to Anaerococcus prevotii), that were enriched from rumen fluid cultures in which only lactate was provided as an exogenous substrate. Analyses of in-silico-predicted proteomes from metagenomics-assembled contigs assigned to these candidate ruminal bacterial species (Bt-01708_Bf: 1270 annotated coding sequences, 1365 hypothetical coding sequences; Bt-01899_Ap: 871 annotated coding sequences, 1343 hypothetical coding sequences) revealed genes encoding lactate dehydrogenase, a putative lactate transporter, as well as pathways for the production of short chain fatty acids (formate, acetate and butyrate) and for the synthesis of glycogen. In contrast to these shared functions, each OTU also exhibited distinct features, such as the potential for the utilization of a diversified set of small molecules as substrates (Bt-01708_Bf: malate, quinate, taurine and polyamines) or for the utilization of starch (Bt-01899_Ap: alpha-amylase enzymes). Together, these results will contribute to the continued characterization of ruminal bacterial species that can metabolize lactate into distinct subgroups based on other metabolic capabilities.