Kav, A.B., Department of Ruminant Science, Institute of Animal Sciences, Volcani Center, Bet Dagan 50250, Israel, Department of Molecular Microbiology and Biotechnology, George S. Wise Faculty of Life Science, Tel Aviv University, Ramat-Aviv 69978, Israel
Sasson, G., Department of Ruminant Science, Institute of Animal Sciences, Volcani Center, Bet Dagan 50250, Israel
Jami, E., Department of Ruminant Science, Institute of Animal Sciences, Volcani Center, Bet Dagan 50250, Israel, Department of Molecular Microbiology and Biotechnology, George S. Wise Faculty of Life Science, Tel Aviv University, Ramat-Aviv 69978, Israel
Doron-Faigenboim, A., Department of Ruminant Science, Institute of Animal Sciences, Volcani Center, Bet Dagan 50250, Israel
Benhar, I., Department of Molecular Microbiology and Biotechnology, George S. Wise Faculty of Life Science, Tel Aviv University, Ramat-Aviv 69978, Israel
Mizrahi, I., Department of Ruminant Science, Institute of Animal Sciences, Volcani Center, Bet Dagan 50250, Israel

Plasmids are self-replicating genetic elements capable of mobilization between different hosts. Plasmids often serve as mediators of lateral gene transfer, a process considered to be a strong and sculpting evolutionary force in microbial environments. Our aim was to characterize the overall plasmid population in the environment of the bovine rumen, which houses a complex and dense microbiota that holds enormous significance for humans. We developed a procedure for the isolation of total rumen plasmid DNA, termed rumen plasmidome, and subjected it to deep sequencing using the Illumina paired-end protocol and analysis using public and custom-made bioinformatics tools. A large number of plasmidome contigs aligned with plasmids of rumen bacteria isolated from different locations and at various time points, suggesting that not only the bacterial taxa, but also their plasmids, are defined by the ecological niche. The bacterial phylum distribution of the plasmidome was different from that of the rumen bacterial taxa. Nevertheless, both shared a dominance of the phyla Firmicutes, Bacteroidetes, and Proteobacteria. Evidently, the rumen plasmidome is of a highly mosaic nature that can cross phyla. Interestingly, when we compared the functional profile of the rumen plasmidome to two plasmid databases and two recently published rumen metagenomes, it became apparent that the rumen plasmidome codes for functions, which are enriched in the rumen ecological niche and could confer advantages to their hosts, suggesting that the functional profiles of mobile genetic elements are associated with their environment, as has been previously implied for viruses.