**3. Effects of EOs on rumen microbiota**

The activity of EOs can alter the rumen microbiota. Early studies on the effects of dietary EOs focused mostly on the effect of EOs on feed utilization efficiency and ruminant performance. There have been only a limited number of studies conducted on the impact of EOs on rumen microbiota. These effects typically involve regulating *Essential Oils in Animal Diets to Improve the Fatty Acids Composition of Meat and Milk… DOI: http://dx.doi.org/10.5772/intechopen.114045*

ruminal fermentation processes thus, it affects the final products. Benchaar et al. [23] reported that the addition of EOs affected the bacteria responsible for ammonium production, the proteolytic bacteria *Provotella*, and the methane-producing archaea. EOs can reduce ruminal ammonia-N production, as they can inhibit ammoniaproducing bacteria *Clostridium sticklandii*, *Peptostreptococcus anaerobius*, *lostridium sticklandii,* and *Peptostreptococcus anaerobius* and reduce the decomposition of amino acids in the rumen [24].

Depending on the type of EOs, EOs may have inhibitory effects on rumen microbiota with a decrease in rumen microbial abundance, and stimulatory effects with an increase in rumen microbial abundance [25]. EOs addition can modulate rumen fermentation by altering volatile fatty acid concentration and decreasing methane emissions by broadly altering the rumen bacterial community [26]. In beef cattle, the addition of EOs increased the relative abundance of *Parabacteroides distasonis* and *Bacteroides thetaiotaomicron* bacteria, which is associated with increased propionate concentration in the rumen [27]. Additionally, *in vitro* studies conducted by Zhou et al. [26] revealed that the addition of EOs reduced the abundance of *Succiniclasticum* bacteria, which converts the succinate to propionate, indicating a negative impact on propionate concentration. EOs can effectively reduce the presence of methanogens in the rumen, and may also have a negative impact on the relative abundance of *Ruminococcus flavefaciens*, *Ruminococcus* albus, and *Fibrobacter succinogenes* [28] and *Ruminococcus* flavefaciens in dairy goats [29]. In beef cattle, EOs have been found to increase the relative abundance of the bacterial family Succinivibrionaceae [26–30]. This increase is strongly negatively correlated with the relative abundance of Methanobacteriaceae microorganisms [31].

On the other hand, adding EOs to ruminant diets can have varying effects on the rumen microbiota. The effects are dependent on the components of the EO, which should be obtained from the same plant species, these components can also vary depending on the geographical location and the season of harvest [32]. Furthermore, the impact of the same EO on the rumen microbiota can vary depending on the dosage used. The impacts of EOs on rumen microbiota seems also more affected by ruminal pH. The pH level can affect the dissociation of EO molecules and their final form. Some studies suggest that the undissociated hydrophobic form of the active EO molecules is more effective in antimicrobial activity as it dissolves better in the bacterial membrane's lipid bilayer, this effect has been observed in low rumen pH conditions [33, 34].
