The rumen epimural bacterial community and its role in rumen development.

In the first 4 months of life, the young calf goes through a development from monogastric to ruminant, which requires morphological and functional development of the rumen. The rumen microbiome plays a profound role
in this functional development. In this study, the rumen bacterial community of calves at the age of 16 weeks was investigated. Fifteen male Holstein Friesian calves were euthanized and the bacterial composition of both
rumen fluid and rumen wall samples were analyzed by 16SrRNA amplicon sequencing (V3-V4 region, Illumina technology). Rumen fluid and rumen wall samples showed separate clustering of the bacterial composition
on the NMDS distance matrix. The Chao1 diversity metrics showed a significant higher species richness in rumen fluid samples (psample types. In rumen fluid samples, Bacteroidetes was the most dominant phylum, followed by Firmicutes and Proteobacteria. In epimural samples however, Firmicutes was the most dominant phylum, followed by
Bacteroidetes and Campylobacterota, which is in contrast with adult Holstein cattle where Proteobacteria were described as the most dominant phylum in epimural samples (De Mulder et al., 2017, Pacifico et al., 2021).This difference between the present and these previous studies could be explained by the fact that Campylobacterota is a newly proposed phylum, and forms a combination of Epsilonproteobacteria (formerly included in Proteobacteria) and Desulfurellales. Campylobacter, a member of the phylum Campylobacterota,
is described as a common genus in the rumen epimural fraction, with a role in protein metabolism and oxygen scavenging.