**4. Effect of EOs on the microbiota operating the biohydrogenation of fatty acids**

Altering rumen populations with EOs may also have an effect on some rumen microorganisms responsible for the biohydrogenation of PUFA, which causes a modification of the rumen FA profile that passes into the milk and meat of ruminants [35]. The use of EO supplements suitable for ruminants must be well planned in terms of the type of EOs and dosage. A proper choice has a positive effect on feed efficiency, digestion, rumen fermentation, and meat and milk production, while an inappropriate choice can lead to negative effects, including reducing feeding efficiency and increasing methane production and resistance of rumen bacteria to the effects of EOs. The antibacterial properties of EOs may cause bacteria to become resistant to EOs. However, very few studies have researched bacterial resistance to EOs. EOs are a mixture of chemical compounds that have antimicrobial activities [8], making it hard

for bacteria to develop resistance mechanisms [36]. However, it has been found that Staphylococcus aureus has developed resistance to EOs [37, 38].
