Enrichment of Candidate Formate and Acetaldehyde-Producing Ruminal Bacteria in Response to Corn-Based Diets

In ruminant production systems, feeding diets with easily digestible and energy-dense compounds during the finishing phase is a common strategy to ensure optimal growth and performance. As ruminal microbial communities are responsible for fermenting feed into nutrients that the animal host can absorb and use, concentrate-based diets favor specialists that can utilize starch, as well as other cross-feeding species that benefit from their metabolic activities and their end products. Currently, a gradual increase in the proportion of concentrate in rations remains the best strategy towards developing resilience to undesirable end products, thus mitigating their adverse effects on the host. In an effort to gain further insights into the mechanisms involved in the adaptation of ruminal microbial communities to starch-rich diets, we used a 16S rRNA gene-based approach to identify eight highly represented species-level Operational Taxonomic Units (OTUs) that were more abundant in the rumen of finishing lambs fed a whole corn-based diet for 56 (D56, n = 7) or 63 days (D63, n = 8) compared to samples collected from lambs on a forage diet (p < 0.05) that were harvested prior to the dietary transition (D0, n = 4). To assess their metabolic capabilities, a metagenomics-based approach was used for OTUs corresponding to uncharacterized bacterial species, while publicly available genomes were analyzed as a proxy for OTUs closely related to known bacterial species. As anticipated, the potential to hydrolyze starch and metabolize glucose into pyruvate was found in all datasets analyzed, but more diverse capabilities were observed for other functions, such as the types of end products synthesized. For instance, lactate was not the only undesirable microbial end product predicted, as the potential to generate formate, acetaldehyde and ethanol was also found across datasets. While the impact of these compounds on the rumen remains poorly explored, ongoing research on how they can affect gut health in humans indicates that they may have undesirable effects. Together, these and other results from this study provide further insight into how ruminal bacterial communities in finishing lambs adapt to starch-based ingredients and how they can in turn affect the ruminal environment.