Identification and characterisation of rumen bacteria with prominent roles in the ruminal metabolism of forages : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy (Microbiology and Genetics) at Massey University, Palmerston North, New Zealand
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Date
2022
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Massey University
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Abstract
This thesis documents the characterisation of two groups of rumen bacteria that are both prominent in forage-fed ruminants, with the aim to better understand their roles in ruminal metabolism. The first group, referred to as the R-7 group, has in recent years been shown to be one of the most abundant rumen bacterial groups, though the few isolated representative strains available were uncharacterised. Two strains of the group included in the Hungate1000 culture collection, R-7 and WTE2008, were selected for characterisation. To facilitate phylogenetic analyses of this group, the complete genomes of an additional three previously isolated R-7 group strains were sequenced. Genomic, phylogenetic and phenotypic characterisation of R-7 and WTE2008 demonstrated that despite their 16S rRNA gene sequences sharing 98.6-99.0% nucleotide identity, their genome-wide average nucleotide identity of 84% assigned them as separate species of a novel genus and family of the proposed order ‘Christensenellales’ using the Genome Taxonomy Database. Phenotypic characterisation showed that the strains were identical in morphology, and both possessed the ability to degrade plant cell wall polysaccharides xylan and pectin, but not cellulose. Acetate, ethanol, hydrogen and lactate were produced by both strains, though R-7 produced greater amounts of hydrogen than WTE2008, which instead produced more lactate. Based on these analyses, it is proposed that R-7 and WTE2008 belong to separate species (Aristaeella gen. nov. hokkaidonensis sp. nov. and Aristaeella lactis sp. nov., respectively) of a newly proposed family (Aristaeellaceae fam. nov.).
The second bacterial group of interest, due to their dominant role in ruminal propionate production, was the Prevotella 1 group, following analyses of metatranscriptome datasets of rumen microbial communities of lucerne-fed sheep for dominant community members that express propionate pathway genes from succinate. Screening of 14 strains spanning the diversity of Prevotella 1 found that all except one P. brevis strain produced propionate in a cobalamin (vitamin B12)-dependent manner. To better understand the pathway and regulation of propionate production from succinate, a comparative multi-omics approach was used to test the hypothesis that propionate production is regulated by a cobalamin-binding riboswitch. Scanning of a completed genome assembly of Prevotella ruminicola KHP1 identified four ‘cobalamin’ family riboswitches. However, the riboswitches were not in close proximity to genes putatively involved in converting succinate to propionate, nor were these genes arranged in a single operon. Comparative genomics of the 14 screened strains found that all strains possessed all homologues of candidate propionate pathway genes identified in the KHP1 genome. However, the 13 propionate-producing strains possessed a putative transporter and three subunits encoding a putative methylmalonyl-CoA decarboxylase upstream but antisense to two genes encoding methylmalonyl-CoA mutase subunits, whereas the non-producing strain did not. Comparative transcriptomics and proteomics of KHP1 cultures in the presence and absence of cobalamin demonstrated that some gene candidates were upregulated by cobalamin at the transcriptome level, including co-located genes annotated as phosphate butyryltransferase and butyrate kinase, despite the strain not producing butyrate, suggesting that propionate production may occur via propionyl phosphate. However, only both subunits of methylmalonyl-CoA mutase showed greater transcript and protein abundances in the presence of cobalamin. These results show that while some propionate pathway candidate genes were differentially expressed between cobalamin treatments, they did not appear to be under direct control of a cobalamin-binding riboswitch. This study has contributed to our understanding of the roles of both Aristaeellaceae fam. nov. and Prevotella 1 in ruminal metabolism.
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Foigures 1.2, 1.4, 1.5 & 1.6 are re-used with permission.
Keywords
Rumen, Microbiology, Metabolism, Ruminants, Feeding and feeds, Forage plants