Abstract
The ability of ruminants to utilize cellulosic biomass is a result of the metabolic activities of symbiotic microbial communities that reside in the rumen. To gain further insight into this complex microbial ecosystem, a selection-based batch culturing approach was used to identify candidate cellulose-utilizing bacterial consortia. Prior to culturing with cellulose, rumen contents sampled from three beef cows maintained on a forage diet shared 252 Operational Taxonomic Units (OTUs), accounting for 41.6–50.0% of bacterial 16S rRNA gene sequences in their respective samples. Despite this high level of overlap, only one OTU was enriched in cellulose-supplemented cultures from all rumen samples. Otherwise, each set of replicate cellulose supplemented cultures originating from a sampled rumen environment was found to have a distinct bacterial composition. Two of the seven most enriched OTUs were closely matched to well-established rumen cellulose utilizers (Ruminococcus flavefaciens and Fibrobacter succinogenes), while the others did not show high nucleotide sequence identity to currently defined bacterial species. The latter were affiliated to Prevotella (1 OTU), Ruminococcaceae (3 OTUs), and the candidate phylum Saccharibacteria (1 OTU), respectively. While further investigations will be necessary to elucidate the metabolic function(s) of each enriched OTU, these results together further support cellulose utilization as a ruminal metabolic trait shared across vast phylogenetic distances, and that the rumen is an environment conducive to the selection of a broad range of microbial adaptations for the digestion of plant structural polysaccharides.
Highlights
As the main component of plant cell walls and sink for photosynthesis products, cellulose represents one of the most abundant organic polymers and reserve of monosaccharides on earth
Given the vast diversity in rumen bacterial community assembly reported to date and the high level of functional redundancy anticipated amongst microorganisms in this ecosystem [15], we aimed to compare the composition of rumen bacteria among individuals of a beef cattle herd maintained on a forage diet, and identify previously uncharacterized utilizers of cellulose
While a number of cellulose-utilizing rumen bacterial species have been isolated over the years, metagenomics studies have indicated that the vast majority remain to be characterized [10,11,14,30]
Summary
As the main component of plant cell walls and sink for photosynthesis products, cellulose represents one of the most abundant organic polymers and reserve of monosaccharides on earth. Ruminants represent a highly successful biological group that has thrived across a wide range of habitats, and played a central role throughout human history [4]. Even to this day, the ability of domesticated ruminants to transform inedible plant biomass into protein-rich products that can be consumed by humans is expected to greatly contribute in meeting the demands of a rapidly growing and urbanizing global population [5].
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