*3.2.1 Using EOs to mitigate BRD bacterial pathogens*

Although antimicrobial activity of EOs against human respiratory bacterial pathogens has been well documented, limited information is available with respect to the effects of EOs against bovine respiratory pathogens. We have recently published data showing that EOs inhibit the BRD bacterial pathogens *M. haemolytica*, *P. multocida* and *H. somni* [16]. The EOs of ajowan, thyme and cinnamon leaf completely or partially inhibited these BRD pathogens in both vapor and liquid phases [16, 17]. These EOs did not display any noticeable cytotoxicity to bovine turbinate cells of the upper respiratory tract [17] and also exhibited minimal antimicrobial activity on six commensal *Lactobacillus* strains that were isolated from the nasal pharynx of a healthy feedlot cattle [17]. This suggests that EOs will have limited negative effects on the commensal bacterial community within the bovine respiratory tract, when they are administered to target pathogens. In addition, Kissels et al. [71] evaluated four different EO components, including carvacrol, thymol, transanethole and 1,8-cineole, as antibacterial agents or as adjuvants for the antibiotics doxycycline and tilmicosin against *M. haemolytica* and *P. multocida*. Carvacrol and thymol inhibited the growth of both of these tested pathogens with MIC values ranged from 0.63 to 2.50 mM. These two EO compounds also displayed an additive effect when one of them was combined with tilmicosin. In addition, combination of thymol with dexycycline displayed synergetic effect against tested BRD pathogens. These studies demonstrated that EOs can be used to control bovine respiratory pathogens in feedlot cattle. Volatile nature of EO makes the EO more promising

therapy for the control of bovine respiratory pathogen in the upper respiratory tract as it makes suitable to intranasal administration via nasal spray [72]. However, further research in terms of the effect of EOs on the respiratory commensal microbiota of cattle and the cytotoxicity of EOs on lower respiratory tract is needed.
