A mechanistic overview of ruminal fibre digestion.

Ruminants have co-evolved with symbiotic rumen microbiota, which readily convert ingested plant fibres into the nutrients they need to sustain their growth and maintenance. Fibre degradation within the rumen microbiome has been attributed to a limited number of cultivable representatives, which has restricted our ability to understand the different enzymatic machineries that exist. However, via a combination of culturing, meta-omics, bioinformatics, biochemistry and enzymology, we are beginning to expand our insight into the different fibre-digesting strategies that rumen microbiota employ. We discuss findings from studies on well-known Ruminococcus , Fibrobacter and Prevotella isolates, as well as those from poorly understood and as-yet uncultured Bacteroidetes lineages. Collectively, these approaches have revealed new mechanistic information related to the hydrolytic capacity of cellulosomes, free enzymes, outer membrane vesicles, polysaccharide utilization loci and large multi-modular enzymes, which are generating deeper insights into the intricate microbial networks that engage in ruminal fibre digestion.