Host and dietary interactions with the rumen microbiome can affect the efficacy of supplements, and their effect on the composition of the bacterial population is still unknown. A 16S rRNA metagenomic approach and Next-Generation Sequencing (NGS) technology were used to investigate the bacterial microbiome composition in the liquid fraction of the rumen content collected via stomach tubing. To investigate biodiversity, samples were taken from three groups of four lactating dairy cows given a supplement of either 50 g of potato protein (Ctrl group), or 50 g of lyophilized Saccharomyces cerevisiae (LY group) or 50 g of dried S. cerevisiae (DY group) in a potato protein support. Rumen samples were collected after 15 days of dietary treatments and milk production was similar between the three groups. Taxonomic distribution analysis revealed a prevalence of the Firmicutes phylum in all cows (79.76%) and a significantly (P < 0.05) higher presence of the genus Bacillus in the DY group. Volatile fattyacid concentration was not significantly different between groups, possibly because of relatively high inter-animal variability or limited effect of the treatments or both, and the correlation analysis with bacterial taxa showed significant associations, in particular between many Firmicutes genera and butyrate. Limited differences were observed between dietary treatments, but the lack of microbiome data before yeast administration does not allow to draw firm conclusions on the effect of dietary treatments.Keywords: rumen bacteria community, Saccharomyces cerevisiae, Next-Generation Sequencing, 16S rRNA, dairy cows Implications Next-Generation Sequencing technology offers the opportunity to gather information regarding the rumen microbiome by comparing repository databases of rumen bacteria 16S rRNA gene sequences generated through different experiments. The high extent of sequencing attained through this research provides valuable information on the microbial biodiversity of the liquid phase of the rumen and represents a contribution to research in rumen microbiology and dairy feed supplements. However, the effectiveness of yeast supplements to modify the rumen bacterial microbiome requires further investigation.