Rumen epithelium-attached bacteria, known as epimural bacteria, are the most distinct from the other bacterial microbes and may be more influential to host metabolic activity (Liu et al., 2016). Epimural bacteria are often facultative anaerobes and can produce urease, suggesting that they help maintain rumen anaerobiosis, and yet do not contribute directly to ruminal digestion (Millen et al., 2016). Finally, the eukaryote-associated microbes are attached to eukaryotes present in the rumen, such as protozoa and fungi (Miron et al., 2001).